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```An automobile, autocar, motor car or car is a wheeledmotor vehicle used
for transporting passengers, which also carries its own engine or motor. Most definitions of the
term specify that automobiles are designed to run primarily on roads, to have seating for one to
eight people, to typically have four wheels, and to be constructed principally for the transport of
people rather than goods.[3] Propulsion is a means of creating force leading to movement.
Look up propulsion in
Wiktionary, the free
dictionary.
A propulsion system has a source of mechanical power (some type of engine or motor,
muscles), and some means of using this power to generate force, such as wheel and
axles, propellers, apropulsive nozzle, wings, fins or legs.
Other components such as clutches,gearboxes and so forth may be needed to connect the power
source to the force generating component.
Ground propulsion is any mechanism for propellingsolid bodies along the ground, usually for
the purposes oftransportation. The propulsion system often consists of a combination of
an engine or motor, a gearbox
Automotive engineering is an applied science that includes elements of Mechanical
engineering, Electrical engineering, Electronic Engineering, Software Engineeringand Safety
engineering as applied to the design, manufacture and operation
of automobiles, buses and trucks and their respective engineering subsystems.and wheel and
axles in standard applications. n automobiles, the wheelbase is the horizontal distance between
the center of the front wheel and the center of the rear wheel. At equilibrium, the total torque of
the forces acting on the car is zero, and thus the wheelbase is related to the force on each pair of
tires by the following formula:
where
is the force on the front tires,
wheelbase,
is the force on the rear tires,
is the
is the distance from the center of gravity (CG) to the rear wheels,
is
the distance from the center of gravity to the front wheels (
+
= ),
is
the mass of the car, and is the gravity constant. So, for example, when one loads the
truck with heavy goods, the center of gravity shifts rearward and the force on the rear tire
increases causing it to sink to the extent that depends on the stiffness of thesuspension.
If the automobile is accelerating or decelerating, extra torque is placed on the rear or
fronttire respectively, and the equation relating the wheelbase, height above the ground
of the CG, and the force on each pair of tires becomes:
where
is the force on the front tires,
is the force on the rear tires,
is
the distance from the CG to the rear wheels,
is the distance from the CG to
the front wheels,
is the wheelbase,
is the mass of the car, is the
acceleration of gravity (approx. 9.8 m/s2),
is the height of the CG above the
ground, is the acceleration (or deceleration if the value is negative). So, as is
common experience, when the automobile accelerates, the rear usually sinks
and the front rises depending on the suspension. Likewise, when braking the
front noses down and the rear rises.:[1]
Because of the effect the wheelbase has on the weight distribution of the
vehicle, wheelbase dimensions are crucial to the balance and steering of the
automobile. For example, a car with a much greater weight load on the rear
tends to understeer due to the lack of the load (force) on the front tires and
therefore the grip (friction) from them. This is why it is crucial, when towing a
single-axle caravan, to distribute the caravan's weight so that down-thrust on
the tow-hook is about 100 pounds force (400 N). Likewise, a car
may oversteer or even "spin out" if there is too much force on the front tires and
not enough on the rear tires. Also, when turning there is lateral torque placed
upon the tires which imparts a turning force that depends upon the length of the
tire distances from the CG. Thus, in a car with a short wheelbase, the short
lever arm from the CG to the rear wheel will result in a greater lateral forceon
the rear tire which means greater acceleration and less time for the driver to
adjust and prevent a spin out or worse.
Wheelbases provide the basis for one of the most common vehicle size
class systems.
The axle track in automobiles and otherwheeled vehicles which have two or more wheels on
an axle, is the distance between the centreline of two roadwheels on the same axle, each on the
other side of the vehicle. In a case of the axle with dual wheels, the centerline in the middle of the
dual wheel is used for the axle track specification.
In a car, or any vehicle, with two axles, this will be expressed as "front track" and "rear track".
However the front wheels and/or rear wheels on either side of a vehicle do not necessarely have
to be mounted on the same axle for the distance that they are apart to be called the
Front wheel drive
Front-wheel-drive layouts are those in which the front wheels of the vehicle are driven. The most
popular layout used in cars today is the front-engine, front-wheel drive, with the engine in front of
the front axle, driving the front wheels. This layout is typically chosen for its compact packaging;
since the engine and driven wheels are on the same side of the vehicle, there is no need for a
central tunnel through the passenger compartment to accommodate a prop-shaftbetween the
engine and the driven wheels.
As the steered wheels are also the driven wheels, FF (front-engine, front-wheel-drive layout) cars
are generally considered superior to FR (front-engine, rear-wheel-drive layout) cars in conditions
such as snow, mud or wet tarmac. The weight of the engine over the driven wheels also improves
grip in such conditions. However, powerful cars rarely use the FF layout because weight
transference under acceleration reduces the weight on the front wheels and reduces
their traction, putting a limit on the amount of torque which can be utilized. Electronic traction
control can avoid wheelspin but largely negates the benefit of extra torque/power.
A transverse engine (also known as "east-west") is commonly used in FF designs, in contrast to
FR which uses a longitudinal engine. The FF layout also restricts the size of the engine that can
be placed in modern engine compartments, as FF configurations usually have Inline4 and V6 engines, while longer engines such as Inline-6 and 90° V8 will rarely fit. This is another
reason luxury/sports cars almost never use the FF layout. Exceptions do exist, such as the Volvo
S80 (FWD/4WD) which uses transversely mounted inline 6 and V8 engines, and the Ford Taurus,
available with a 60° V8 and all-wheel drive.
Most Audis are FF layout cars, but with longitudinal engines, such as the Audi A4 and Audi A6,
however "FrontTrak" front-wheel-drive models are only entry-level trims in the United States and
Canada; most Audis usually come with "quattro" all-wheel drive. The Audi A3 is a FF layout car
with a traverse engine mounting, as it does not share a platform with more expensive offerings in
the marque.
Characteristics
Front-wheel drive gives more interior space since the powertrain is a single unit contained in the
engine compartment of the vehicle and there is no need to devote interior space for
a driveshaft tunnel or reardifferential, increasing the volume available for passengers and cargo.
[1]
There are some exceptions to this as rear engine designs do not take away interior space
(see Porsche 911, and Volkswagen Beetle). It also has fewer components overall and thus lower
weight.[1] The direct connection between engine and transaxle reduces the mass and
mechanical inertia of the drivetrain compared to a rear-wheel-drive vehicle with a similar engine
and transmission, allowing greater fuel e
conomy.[1] In front-wheeldrive cars the mass of the drivetrain is placed over the driven wheels and thus moves the center
of gravity farther forward than a comparable rear-wheel-drive layout, improving traction and
directional stability on wet, snowy, or icy surfaces.[1][2][3] Front-wheel-drive cars, with a front weight
bias, tend toundersteer at the limit, which according to, for instance, Saab engineer Gunnar
Larsson, is easier since it makes instinct correct in avoiding terminal oversteer, and less prone to
result in fishtailing or a spin.[3][4]
According to a sales brochure for the 1989 Lotus Elan, the ride and handling engineers
at Lotus found that "for a given vehicle weight, power and tire size, a front-wheel-drive car was
always faster over a given section of road."[5] However, this may only apply for cars with moderate
power-to-weight ratio.[2][6][7] According to road test with two Dodge Daytonas, one FWD and one
RWD, the road layout is also important for what configuration is the fastest. [3]
Weight shifting limits the acceleration of a front-wheel-drive vehicle. During heavy acceleration,
weight is shifted to the back, improving traction at the rear wheels at the expense of the front
driving wheels; consequently, most racing cars are rear-wheel drive for acceleration. However,
since front-wheel-drive cars have the weight of the engine over the driving wheels, the problem
only applies in extreme conditions in which case the car understeers. On snow, ice, and sand,
rear-wheel drive loses its traction advantage to front or all-wheel-drive vehicles which have
greater weight over the driven wheels. Rear-wheel-drive cars with rear engine or mid
engine configuration retain traction over the driven wheels, although fishtailing remains an issue
on hard acceleration while in a turn. Some rear engine cars (e.g. Porsche 911) can suffer from
reduced steering ability under heavy acceleration, since the engine is outside the wheelbase and
at the opposite end of the car from the wheels doing the steering. A rear-wheel-drive car's center
of gravity is shifted rearward when heavily loaded with passengers or cargo, which may cause
unpredictable handling behavior.[4]
On FR cars, the long driveshaft adds to drivetrain elasticity. [4]

Interior space: Since the powertrain is a single unit contained in the engine compartment
of the vehicle, there is no need to devote interior space for a driveshaft tunnel or
rear differential, increasing the volume available for passengers and cargo. [1]
Instead, the tunnel may be used to route the exhaust system pipes.
Weight: Fewer components usually means lower weight.
Improved fuel efficiency due to less weight.[8]
Cost: Fewer material components and less installation complexity overall. However, the
considerable MSRP differential between a FF and FR car cannot be attributed to layout
alone. The difference is more probably explained by production volumes as most rear-wheel
cars are usually in the sports/performance/luxury categories (which tend to be more upscale
and/or have more powerful engines), while the FF configuration is typically in mass-produced
mainstream cars. Few modern "family" cars have rear-wheel drive as of 2009, so a direct
cost comparison is not necessarily possible. A contrast could be somewhat drawn between
the Audi A4 FrontTrak (which has an FF layout and front-wheel drive) and a rear-wheeldrive BMW 3-Series (which is FR), both which are in the compact executive car classification.
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[1]
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Improved drivetrain efficiency: the direct connection between engine and transaxle
reduce the mass and mechanical inertia of the drivetrain compared to a rear-wheel-drive
vehicle with a similar engine and transmission, allowing greater fuel economy.[1]
Assembly efficiency: the powertrain can often be assembled and installed as a unit,
which allows more efficient production.[citation needed]
Placing the mass of the drivetrain over the driven wheels moves the centre of
gravity farther forward than a comparable rear-wheel-drive layout, improving traction and
directional stability on wet, snowy, or icy surfaces.[1][2][3]
Predictable handling characteristics: front-wheel-drive cars, with a front weight bias, tend
toundersteer at the limit, which (according to e.g. SAAB engineer Gunnar Larsson) is easier
since it makes instinct correct in avoiding terminal oversteer, and less prone to result
in fishtailing or a spin.[3][4]
A skilled driver can control the movement of the car even while skidding by steering,
throttling and pulling the hand brake (given that the hand brake operates the rear wheels as
in most cases, with some Citroen and Saab models being notable exceptions).
It is easier to correct trailing-throttle or trailing-brake oversteer.[3]
The wheelbase can be extended without building a longer driveshaft (as with rear-wheeldriven cars).

Front-engine front-wheel-drive layouts are "nose heavy" with more weight distribution
forward, which makes them prone to understeer, especially in high horsepower applications.
Enthusiast driver aids, such as active front differential, active steering, and ultra-quick
electrically-adjustable shocks, can negate the understeer problem and allow the car to
perform as well as a front-engine rear-wheel-drive car. These trick differentials, which are
found on the 2009 Acura TL SH-AWD and 2010Audi S4 3.0 TFSI quattro, and 2011 Audi
RS5 4.2 FSI quattro, are heavy, complex, and expensive. [9][10]. While these aids do tame front
end plow, cars fitted with these systems are still at a disadvantage when track tested [11].
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Torque steer is the tendency for some front-wheel-drive cars to pull to the left or right
under hard acceleration. It is a result of the offset between the point about which the wheel
steers (it is aligned with the points where the wheel is connected to the steering
mechanisms) and the centroid of itscontact patch. The tractive force acts through the
centroid of the contact patch, and the offset of the steering point means that a turning
moment about the axis of steering is generated. In an ideal situation, the left and right wheels
would generate equal and opposite moments, canceling each other out; however, in reality,
this is less likely to happen. Torque steer can be addressed by using a longitudinal layout,
equal length drive shafts, half shafts, a multilink suspension or centre-point steering
geometry.[12][13][14][15][16][17][18]
In a vehicle, the weight shifts back during acceleration, giving more traction to the rear
wheels. This is one of the main reasons why nearly all racing cars are rear-wheel drive.
However, since front-wheel-drive cars have the weight of the engine over the driving wheels,
the problem only applies in extreme conditions such as attempting to accelerate up a wet hill
or attempting to beat another RWD car off the line.
In some towing situations, front-wheel-drive cars can be at a traction disadvantage since
there will be less weight on the driving wheels. Because of this, the weight that the vehicle is
rated to safely tow is likely to be less than that of a rear-wheel-drive or four-wheel-drive
vehicle of the same size and power.
Traction can be reduced while attempting to climb a slope in slippery conditions such as
Due to geometry and packaging constraints, the CV joints (constant-velocity joints)
attached to the wheel hub have a tendency to wear out much earlier than the universal
joints typically used in their rear-wheel-drive counterparts (although rear-wheel-drive vehicles
with independent rear suspensionalso employ CV joints and half-shafts). The significantly
shorter drive axles on a front-wheel-drive car causes the joint to flex through a much wider
degree of motion, compounded by additional stress and angles of steering, while the CV
joints of a rear-wheel-drive car regularly see angles and wear of less than half that of frontwheel-drive vehicles.
Turning circle — FF layouts almost always use a Transverse engine ("east-west")
installation, which limits the amount by which the front wheels can turn, thus increasing the
turning circle of a front-wheel-drive car compared to a rear-wheel-drive one with the same
wheelbase. A notable example is the original Mini. It is widely misconceived that this
limitation is due to a limit on the angle at which a CV joint can be operated, but this is easily
disproved by considering the turning circle of car models that use a longitudinal FF or F4
layout from Audi and (prior to 1992) Saab
The FF transverse engine layout (also known as "east-west") restricts the size of the
engine that can be placed in modern engine compartments, so it is rarely adopted by
powerful luxury and sports cars. FF configurations can usually only accommodate Inline4 and V6 engines, while longer engines such as Inline-6 and 90° big-bore V8 will rarely fit,
though there are exceptions. One way around this problem is using a staggered engine.
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It makes heavier use of the front tires (i.e. accelerating, braking, and turning), causing
more wear in the front than in a rear-wheel-drive layout.
Under extreme braking (like for instance in a panic stop), the already front heavy layout
further reduces traction to the rear wheels. This results in disproportionate gripping forces
focused at the front while the rear does not have enough weight to effectivly use its brakes.
Because the rear tire's capabilities in braking were not very high, a significant number of
cheaper front drive vehicles used drum brakes in the rear even today.
The steering 'feel' is more numbed than a RWD car. This is due to the extra weight of
drive shafts and CV join components that increase unsprung weight. Combined with torque
steer, determining how much lateral traction is actually available is more difficult if not
impossible especially during high performance driving.
Rear-wheel-drive
layouts
Rear-wheel drive (RWD) typically places the engine in the front of the vehicle and the driven
wheels are located at the rear, a configuration known as front-engine, rear-wheel drive layout (FR
layout). The front mid-engine, rear mid-engine andrear engine layouts are also used. This was
the traditional automobile layout for most of the 20th century. [19] Nearly
all motorcycles and bicyclesuse rear-wheel drive, either by driveshaft, chain, orbelt, since the
front wheel is turned for steering, and it would be very difficult and cumbersome to "bend" the
drive mechanism around the turn of the front wheel. A relatively rare exception is with the 'moving
bottom bracket' type of recumbent bicycle, where the entire drivetrain, including pedals and chain,
pivot with the steering front wheel.
Characteristics
The vast majority of rear-wheel-drive vehicles use a longitudinally-mounted engine in the front of
the vehicle, driving the rear wheels via a driveshaftlinked via a differential between the rear axles.
Some FR layout vehicles place the gearbox at the rear, though most attach it to the engine at the
front.
The FR layout is often chosen for its simple design and good handling characteristics. Placing the
drive wheels at the rear allows ample room for the transmission in the center of the vehicle and
avoids the mechanical complexities associated with transmitting power to the front wheels. For
performance-oriented vehicles, the FR layout is more suitable than front-wheel-drive designs,
especially with engines that exceed 200 horsepower. This is because weight transfers to the rear
of the vehicle during acceleration, which loads the rear wheels and increases their grip.
result of the longitudinal orientation of the drivetrain, as compared to the FF layout (front-engine,
front-wheel drive). Powerful engines such as the Inline-6 and 90° big-bore V8 are usually too long
to fit in a FFtransverse engine ("east-west") layout; the FF configuration can typically
accommodate at the maximum an Inline-4 or V6. This is another reason luxury/sports cars almost
never use the FF layout.

Even weight distribution — The layout of a rear-wheel-drive car is much closer to an even
fore-and-aft weight distribution than a front-wheel-drive car, as more of the engine can lie
between the front and rear wheels (in the case of a mid engine layout, the entire engine), and
the transmission is moved much farther back.[20]

Weight transfer during acceleration — During heavy acceleration, weight is placed on the
rear, or driving wheels, which improves traction.
No torque steer[21] (unless it's an all-wheel steer with an offset differential).
Steering radius — As no complicated drive shaft joints are required at the front wheels, it
is possible to turn them further than would be possible using front-wheel drive, resulting in a
smaller steering radius for a given wheelbase.
Better handling at the hands of an expert — the more even weight distribution and weight
transfer improve the handling of the car. The front and rear tires are placed under more even
loads, which allows for more grip while cornering.[22]
Better braking — the more even weight distribution helps prevent lockup from the rear
wheels becoming unloaded under heavy braking.[22]
Towing — Rear-wheel drive puts the wheels which are pulling the load closer to the point
where a trailer articulates, helping steering, especially for large loads. [23]
Serviceability — Drivetrain components on a rear-wheel-drive vehicle are modular and do
not involve packing as many parts into as small a space as does front-wheel drive, thus
requiring less disassembly or specialized tools in order to service the vehicle. [citation needed]
Robustness — due to geometry and packaging constraints, the universal joints attached
to the wheel hub have a tendency to wear out much later than the CV joints typically used in
front-wheel-drive counterparts. The significantly shorter drive axles on a front-wheel-drive car
causes the joint to flex through a much wider degree of motion, compounded by additional
stress and angles of steering, while the CV joints of a rear-wheel-drive car regularly see
angles and wear of less than half that of front-wheel-drive vehicles. [citation needed]
Can accommodate more powerful engines as a result of the longitudinal orientation of the
drivetrain, such as the Inline-6, 90° big-bore V8, V10 and V12 making the FR a common
configuration for luxury and sports cars. These engines are usually too long to fit in a
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FF transverse engine ("east-west") layout; the FF configuration can typically accommodate at
the maximum an Inline-4 or V6.
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Under heavy acceleration (as in racing), oversteer and fishtailing may occur as the rear
wheels break free and spin. The corrective action is to let off the throttle (this is what traction
control automatically does for RWD vehicles).

On snow, ice and sand, rear-wheel drive loses its traction advantage to front- or allwheel-drive vehicles, which have greater weight on the driven wheels. This issue is
particularly noticeable on pickup trucks, as the weight of the engine and cab will significantly
shift the weight from the rear to the front wheels. Rear-wheel-drive cars with rear
engine or mid engine configuration do not suffer from this, although fishtailing remains an
issue. To correct this situation, owners of RWD vehicles can buy sandbags to load in the
back of the vehicle (either in the bed, or trunk) in order to increase the weight over the rear
axel, however speeds should be restricted to city limits or less.
Some rear engine cars (e.g. Porsche 911) can suffer from reduced steering ability under
heavy acceleration, because the engine is outside the wheelbase and at the opposite end of
the car from the wheels doing the steering although the engine weight over the rear wheels
provides outstanding traction and grip during acceleration.
Decreased interior space — Though individual designs vary greatly, rear-wheel-drive
vehicles may have: Less front leg room as the transmission tunnel takes up a space between
the driver and front passenger, less leg room for center rear passengers (due to the tunnel
needed for the drive shaft), and sometimes less boot space (since there is also more
hardware that must be placed underneath the boot). Rear engine designs (such as
the Porsche 911 and Volkswagen Beetle) do not inherently take away interior space.
Increased weight — The components of a rear-wheel-drive vehicle's power train are less
complex, but they are larger. The driveshaft adds weight. There is extra sheet metal to form
the transmission tunnel. There is a rear axle or rear half-shafts, which are typically longer
than those in a front-wheel-drive car. A rear-wheel-drive car will weigh slightly more than a
comparable front-wheel-drive car (but less than four-wheel drive).
Rear biased weight distribution when loaded — A rear-wheel-drive car's center of gravity
is shifted rearward when heavily loaded with passengers or cargo, which may cause
unpredictable handling behavior at the hands of an inexperienced driver [4]. It needs to be
noted that rear engined cars are by their very nature, rear weight biased.
Higher initial purchase price — Modern rear-wheel-drive vehicles are typically more
expensive to purchase than comparable front-wheel-drive vehicles. Part of this can be
explained by the added cost of materials and increased complex assembly of FR layouts, as
the powertrain is not one compact unit. However, the difference is more probably explained
by production volumes as most rear-wheel cars are usually in the sports/performance/luxury
categories (which tend to be more upscale and/or have more powerful engines), while the FF
configuration is typically in mass-produced mainstream cars.
The possibility of a slight loss in the mechanical efficiency of the drivetrain (approximately
17% coastdown losses between engine flywheel and road wheels compared to 15% for front-
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wheel drive — however these losses are highly dependent on the individual transmission).
[citation needed]
Cars with rear engine or mid engine configuration and a transverse engine layout
do not suffer from this.
The long driveshaft (on front engine cars) adds to drivetrain elasticity. [4] The driveshaft
must also be extended for cars with a stretched wheelbase (e.
ain article: Four-wheel drive
Most 4WD layouts are front-engined and are derivatives of earlier front-engined, two-wheel-drive
designs. They fall into two major categories:

Front-engine, rear-wheel drive derived 4WD systems, standard in most sport utility
vehicles and in passenger cars, (usually referred to “front engine, rear-wheel drive/four-wheel
drive”), forerunners of today's models include the Jensen FF, AMC Eagle and MercedesBenz W124 with the 4Maticsystem and Suzuki Grand Vitara with/without 4 mode transfer
case.

Transverse and longitudinal engined 4WD systems derived almost exclusively from frontengined, front-drive layouts, fitted to luxury, sporting and heavy duty segments, for example
the transverse-engined Mitsubishi 3000GT VR-4 and Toyota RAV4 and the longitudinalengined Audi Quattro and most of the Subaru line.
For a full explanation of 4WD engineering considerations, see the main article on four-wheel drive
In terms of handling, traction and performance, 4WD systems generally have most of the
advantages of both front-wheel drive and rear-wheel drive. Some unique benefits are:

Traction is nearly doubled compared to a two-wheel-drive layout. Given sufficient power,
this results in unparalleled acceleration and driveability on surfaces with less than ideal grip,
and superior engine braking on loose surfaces. The development of 4WD systems for high
performance cars was stimulated primarily by rallying.

Handling characteristics in normal conditions can be configured to emulate FWD or
RWD, or some mixture, even to switch between these behaviours according to circumstance.
However, at the limit of grip, a well balanced 4WD configuration will not degenerate into
either understeer or oversteer, but instead break traction of all 4 wheels at the same time into
a four-wheel drift. Combined with modern electronic driving aids, this flexibility allows
production car engineers a wide range of freedom in selecting handling characteristics that
will allow a 4WD car to be driven more safely at higher speeds by inexpert motorists than
2WD designs.

4WD systems require more machinery and complex transmission components, and so
increase the manufacturing cost of the vehicle and complexity of maintenance procedures
and repairs compared to 2WD designs

4WD systems increase power-train mass, rotational inertia and power transmission
losses, resulting in a reduction in performance in ideal dry conditions and increased fuel
consumption compared to 2WD designs
The handbrake cannot be used to induce over-steer for maneuvering purposes, as the
drivetrain couples the front and rear axles together. To overcome this limitation, some
custom prepared stage rally cars have a special mechanism added to the transmission to
disconnect the rear drive if the handbrake is applied while the car is moving.

he first FR car was an 1895 Panhard model, so this layout was known as the "Système Panhard"
in the early years. Most American cars used the FR layout until the mid 1980s. The Oil crisis of
the 1970s and the success of small FF cars like the Mini, Volkswagen Golf, Toyota Tercel,
After the Arab Oil Embargo of 1973 and the 1979 fuel crises, a majority of American FR vehicles
(station wagons, luxury sedans) were phased out for the FF layout — this trend would spawn the
SUV/van conversion market. Throughout the 1980s and 1990s, most American companies set as
a priority the eventual removal of rear-wheel drive from their mainstream and luxury lineup.
[26]
Chrysler went 100% FF by 1990 and GM's American production went entirely FF by 1997
except the Corvette and Camaro. Ford's full-size cars (the Ford Crown Victoria, Mercury Grand
Marquis, and Lincoln Town Car) have always been FR,[27] as was theFord Mustang[28] and Lincoln
LS. In 2008 Hyundai introduced its own rear-wheel-drive car, the Hyundai Genesis.
In Australia, FR cars have remained popular throughout this period, with the Holden
Commodore andFord Falcon having consistently strong sales. In Europe, front-wheel drive was
popularized by small cars like the Mini, Renault 5 and Volkswagen Golf and adopted for virtually
all mainstream cars.
Upscale marques like Mercedes-Benz, BMW, and Jaguar remained mostly independent of this
trend, and retained a lineup mostly or entirely made up of FR cars. [29] Japanese mainstream
marques such as Toyota and Nissan became mostly or entirely FF early on, while reserving for
their latterly-conceived luxury divisions (Lexus and Infiniti, respectively) a mostly FR lineup. While
many automakers lost sight of the true sports car, Mazda introduced the highly
successful Miata roadster in 1990, a true 2-seater sports car using the traditional FR layout which
led to other compaines such as General Motors to produce a FR sports car based on
their Kappa platform.
Currently most cars are FF, including virtually all front-engined economy cars, though FR cars are
making a return as an alternative to large sport-utility vehicles. In North America, GM returned to
production of the FR luxury car with the 2003 Cadillac CTS, and with the removal of the DTS,
[30]
Cadillac will be entirely FR (with four-wheel drive available as an option on several models) by
2010, and the 2010 Camaro returns as a FR sports car. Chrysler returned its full-size cars to this
layout with the Chrysler 300 and related models.[31][32] Despite Ford's 2011 discontinuation of the
rear-wheel drivePanther Platform cars, they are seeking to develop a new FR replacement.
[33]
Nissan is also bringing back the Silvia to their line-up, Mazda is said to be releasing a new
rotary-powered FR car in their RX line-up by 2010 and Toyota has announced the FT-86, an
affordable RWD car which is the successor to the AE86. Hyundai introduced their affordable
RWD car being the 2009 Hyundai Genesis and 2010Hyundai Genesis Coupe
In the 21st century, with solutions to the engineering complexities of 4WD being widely
understood, and consumer demand for increasing performance in production cars, front-engined
4WD layouts are rapidly becoming more common, and most major manufacturers now offer 4WD
options on at least some models. Manufacturers with a notable expertise and history in producing
4WD performance cars are Audi and Subaru.
See
also
ChasisIn the case of vehicles, the term chassis means the frameplus the "running gear"
like engine, transmission,driveshaft, differential, and suspension. A body (sometimes referred to
as "coachwork"), which is usually not necessary for integrity of the structure, is built on the
chassis to complete the vehicle. For commercial vehicles chassis consists of an assembly of all
the essential parts of a truck (without the body) to be ready for operation on the road. [1]The design
of a pleasure car chassis will be different than one for commercial vehicles because of the
heavier loads and constant work use.[2] Commercial vehicle manufacturers sell “chassis only”,
“cowl and chassis”, as well as "chassis cab" versions that can be outfitted with specialized
bodies. These include motor homes, fire engines, ambulances, box trucks, etc.
In particular applications, such as school buses, a government agency like National Highway
Traffic Safety Administration (NHTSA) in the U.S. defines the design standards of chassis and
body conversions.[3]
Monocoque (/ˈmɒnɵkɒk/ or /ˈmɒnɵkoʊk/) is a construction technique that supports structural
load by using an object's external skin, as opposed to using an internal frame ortruss that is then
covered with a non-load-bearing skin orcoachwork. The term is also used to indicate a form of
vehicle construction in which the body and chassis form a single unit.
The word monocoque comes from the Greek for single (mono) and French for shell (coque).
[1]
The technique may also be called structural skin or stressed skin. A semi-monocoque differs
in having longerons and stringers.[2]Most car bodies are not true monocoques, instead modern
cars use unitary construction which is also known as unit body, unibody, or Body Frame
Integral construction.[3] This uses a system of box sections, bulkheads and tubes to provide most
of the strength of the vehicle, to which the s
a type of construction (as of a fuselage) in which the outer skin carries all or a
major part of the stresses
2
: a type of vehicle construction (as of an automobile) in which the body is integral w
efinition of MONOCOQUE
1
: a type of construction (as of a fuselage) in which the outer skin carries all or a
major part of the stresses
2
: a type of vehicle construction (as of an automobile) in which the body is integral
with the chassis
Originally called laminated or carriage spring,[citation needed] a leaf spring is a simple form ofspring,
commonly used for the suspension in wheeledvehicles. It is also one of the oldest forms of
springing, dating back to medieval times.
An advantage of a leaf spring over a helical spring is that the end of the leaf spring may be
guided along a definite path.
Sometimes referred to as a semi-elliptical spring orcart spring, it takes the form of a
slender arc-shaped length of spring steel of rectangular cross-section. The center of the arc
provides location for the axle, whiletie holes are provided at either end for attaching to the vehicle
body. For very heavy vehicles, a leaf spring can be made from several leaves stacked on top of
each other in several layers, often with progressively shorter leaves. Leaf springs can serve
locating and to some extent damping as well as springing functions. While the interleaf friction
provides a damping action, it is not well controlled and results in stiction in the motion of the
suspension. For this reason manufacturers have experimented with mono-leaf springs.
A leaf spring can either be attached directly to theframe at both ends or attached directly at one
end, usually the front, with the other end attached through a shackle, a short swinging arm. The
shackle takes up the tendency of the leaf spring to elongate when compressed and thus makes
for softer springiness. Some springs terminated in a concave end, called aspoon end (seldom
used now), to carry a swivelling member.
Contents
[hide]
1 History
2 Characteristics
3 Manufacturing process
4 Use by Blacksmiths
6 References
History
There were a variety of leaf springs, usually employing the word "elliptical". "Elliptical" or "full
elliptical" leaf springs referred to two circular arcs linked at their tips. This was joined to the frame
at the top center of the upper arc, the bottom center was joined to the "live" suspension
components, such as a solid front axle. Additional suspension components, such as trailing arms,
would be needed for this design, but not for "semi-elliptical" leaf springs as used in theHotchkiss
drive. That employed the lower arc, hence its name. "Quarter-elliptic" springs often had the
thickest part of the stack of leaves stuck into the rear end of the side pieces of a short ladder
frame, with the free end attached to the differential, as in the Austin Seven of the 1920s. As an
example of non-elliptic leaf springs, the Ford Model T had multiple leaf springs over its differential
that were curved in the shape of a yoke. As a substitute for dampers (shock absorbers), some
manufacturers laid non-metallic sheets in between the metal leaves, such as wood.
Leaf springs were very common on automobiles, right up to the 1970s in Europe and Japan and
late 70's in America when the move to front-wheel drive, and more
sophisticated suspension designs sawautomobile manufacturers use coil springs instead. Today
leaf springs are still used in heavy commercial vehicles such as vans and trucks, SUVs,
widely over the vehicle's chassis, whereas coil springs transfer it to a single point. Unlike coil
springs, leaf springs also locate the rear axle, eliminating the need for trailing arms and
a Panhard rod, thereby saving cost and weight in a simple live axle rear suspension.
A more modern implementation is the parabolic leaf spring. This design is characterised by fewer
leaves whose thickness varies from centre to ends following a parabolic curve. In this design,
inter-leaf friction is unwanted, and therefore there is only contact between the springs at the ends
and at the centre where the axle is connected. Spacers prevent contact at other points. Aside
from a weight saving, the main advantage of parabolic springs is their greater flexibility, which
translates into vehicleride quality that approaches that of coil springs. There is a trade-off in the
form of reduced load carrying capability, however. The characteristic of parabolic springs is better
riding comfort and not as "stiff" as conventional "multi-leaf springs". It is widely used on buses for
better comfort. A further development by the British GKN company and by Chevrolet with the
Corvette amongst others, is the move to composite plastic leaf springs.
Typically when used in automobile suspension the leaf both supports an axle and locates/
partially locates the axle. This can lead to handling issues (such as 'axle tramp'), as the flexible
nature of the spring makes precise control of the unsprung mass of the axle difficult. Some
suspension designs which use leaf springs do not use the leaf to locate the axle and do not have
this drawback. The Fiat 128's rear suspension is an example.
A Coil spring, also known as a helical spring, is a mechanical device, which is typically used to
store energy and subsequently release it, to absorb shock, or to maintain a force between
contacting surfaces. They are made of an elastic material formed into the shape of a helixwhich
returns to its natural length when unloaded.
Coil springs are a special type of torsion spring: the material of the spring acts in torsion when the
spring is compressed or extended.
Metal coil springs are made by winding a wire around a shaped former - a cylinder is used to form
cylindrical coil springs.
A torsion bar suspension, also known as a torsion spring suspension or torsion beam
suspension, is a general term for any vehicle suspension that uses a torsion bar as its main
weight bearing spring. One end of a long metal bar is attached firmly to the vehicle chassis; the
opposite end terminates in a lever, the torsion key, mounted perpendicular to the bar, that is
attached to a suspension arm, a spindle, or the axle. Vertical motion of the wheel causes the bar
to twist around its axis and is resisted by the bar's torsionresistance. The effective spring rate of
the bar is determined by its length, cross section, shape and material.
orsion bar suspensions are used on combat vehicles or tanks like the T-72, Leopard 1, Leopard 2
and Abrams (many tanks from late in World War II used this suspension), and on trucks
and SUVs from Ford,Dodge, GM, Mitsubishi, Mazda, Nissan, Isuzu andToyota. Manufacturers
change the torsion bar or key to adjust the ride height, usually to compensate for heavier or
lighter engines. While the ride height may be adjusted by turning the adjuster bolts on the stock
torsion key, rotating the stock key too far can bend the adjusting bolt and (more importantly) place
the shock piston outside its standard travel. Over-rotating the torsion bars can also cause the
suspension to hit the bump-stop prematurely, causing a harsh ride. Aftermarket forged-metal
torsion key kits use relocked adjuster keys to prevent over-rotation, and shock brackets to keep
the piston travel in the stock range.
The main advantages of a torsion bar suspension are durability, easy adjustability of ride height,
and small profile along the width of the vehicle. It takes up less of the vehicle's interior volume
compared tocoil springs. A disadvantage is that torsion bars, unlike coil springs, usually cannot
provide a progressive spring rate. In most torsion bar systems, ride height (and therefore many
handling features) may be changed by simply adjusting bolts that connect the torsion bars to
the steering knuckles. In most cars with this type of suspension, swapping torsion bars for a
different spring rate is usually an easy task.
Leveling
Some vehicles use torsion bars to provide automatic levelling, using a motor to pre-stress the
bars to provide greater resistance to load and, in some cases (depending on the speed with
which the motors can act), to respond to changes in road conditions. Height adjustable
suspension has been used to implement a wheel-change mode where the vehicle is raised on
three wheels so that the remaining wheel is lifted off the ground without the aid of a jack.
History
A dashpot is a mechanical device, a damper which
resists motion via viscousfriction. The resulting force is proportional to the velocity, but acts in
the opposite direction,[2] slowing the motion and absorbing energy. It is commonly used in
conjunction with a spring (which acts to resist displacement). Theprocess and instrumentation
[1]
diagram (P&ID) symbol for a dashpot is
.
Contents
[hide]
1 Types
2 Applications
3 Viscoelasticity
4 References
Types
Two common types of dashpots exist - linear and rotary. Linear dashpots are generally specified
by stroke (amount of linear displacement) and damping coefficient (force per velocity). Rotary
dashpots will have damping coefficients in torque per angular velocity.
A less common type of dashpot is an eddy current damper, which uses a large magnet inside a
tube constructed of a non-magnetic but conducting material (such as aluminium orcopper). Like a
common viscous damper, the eddy current damper produces a resistive force proportional to
velocity.[3][4][5][6]
Dashpots frequently use a one-way mechanical bypass to permit fast unrestricted motion in one
direction and slow motion using the dashpot in the opposite direction. This permits, for example, a
door to be opened quickly without added resistance, but then to close slowly using the dashpot.
For hydraulic dashpots this unrestricted motion is accomplished using a one-way check-valve that
allows fluid to bypass the dashpot fluid constriction. Non-hydraulic dashpots may use a ratcheting
gear to permit free motion in one direction.
Applications
Dashpot in a Zenith-Strombergcarburetor
A dashpot is a common component in a door closer to prevent it from slamming shut. A spring
applies force to close the door and the dashpot, implemented by requiring fluid to flow through a
narrow channel between reservoirs (often with a size adjustable by a screw), slows down the
motion of the door.
Consumer electronics often use dashpots where it is undesirable for a media access door or
control panel to suddenly pop open when the door latch is released. The dashpot slows the
sudden movement down into a steady and gentle movement until the access door has opened all
the way under spring tension.
Dashpots are commonly used in dampers and shock absorbers. The hydraulic cylinder in an
automobile shock absorber is a dashpot. They are also used on carburettors, where the return of
the throttle lever is cushioned right before the throttle fully closes and then is allowed to fully close
slowly to reduce emissions of sudden deceleration (compared to deceleration without a dashpot)
Large forces and high speeds can be controlled by dashpots. For example, they are used to
arrest thesteam catapults on aircraft carrier decks.
Relays can be made to have a long delay by utilizing a piston filled with fluid that is allowed to
escape slowly.
Some high energy motor starter contactors have used the dashpot. The Allen West type, for
example, uses a hydraulic piston. The rod of the piston is part of the built in 'over current'
function. The current sensing coil and rod act as a solenoid. The rod is the plunger in the over
current coil. During over current the plunger moves up and releases the dropout lever cutting
power to the motor. During motor start, current in-rush time is short in comparison with dashpot
action and so start current cannot pull the plunger as quick. The damping of the current coil
plunger prevents the motor start current from unlatching the dropout lever. On three phase units
there is one dashpot for each of the three contactors so excess current in either phase with drop
all three contactors as the dropout lever encompasses all contactors.
Viscoelasticity
n automobiles, a double wishbone (orupper and lower A-arm) suspension is anindependent
suspension design using two (occasionally parallel) wishbone-shaped arms to locate the wheel.
Each wishbone or arm has two mounting points to the chassis and one joint at the knuckle.
The shock absorberand coil spring mount to the wishbones to control vertical movement. Double
wishbone designs allow the engineer to carefully control the motion of the wheel throughout
suspension travel, controlling such parameters as camber angle, caster angle,toe pattern, roll
center height, scrub radius, scuff and more.
Contents
[hide]
1 Implementation
3 Uses
5 References
Implementation
Double wishbone suspension
The double-wishbone suspension can also be referred to as "double A-arms," though the arms
themselves can be A-shaped, L-shaped, or even a single bar linkage. A single wishbone or Aarm can also be used in various other suspension types, such asMacPherson strut and Chapman
strut. The upper arm is usually shorter to induce negative camber as the suspension jounces
(rises), and often this arrangement is titled an "SLA" or "short long arms" suspension. When the
vehicle is in a turn, body roll results in positive camber gain on the lightly loaded inside wheel,
while the heavily loaded outer wheel gains negative camber.
Between the outboard end of the arms is a knuckle with a spindle (the kingpin), hub, or upright
which carries the wheel bearing and wheel.
To resist fore-aft loads such as acceleration and braking, the arms require two bushings or ball
joints at the body.
At the knuckle end, single ball joints are typically used, in which case the steering loads have to
be taken via a steering arm, and the wishbones look A- or L-shaped. An L-shaped arm is
generally preferred on passenger vehicles because it allows a better compromise of handling and
comfort to be tuned in. The bushing inline with the wheel can be kept relatively stiff to effectively
handle cornering loads while the off-line joint can be softer to allow the wheel to recess under
fore-aft impact loads. For a rear suspension, a pair of joints can be used at both ends of the arm,
making them more H-shaped in plan view. Alternatively, a fixed-length driveshaft can perform the
function of a wishbone as long as the shape of the other wishbone provides control of the upright.
This arrangement has been successfully used in the Jaguar IRS. In elevation view, the
suspension is a 4-bar link, and it is easy to work out the camber gain (see camber angle) and
other parameters for a given set of bushing or ball-joint locations. The various bushings or ball
joints do not have to be on horizontal axes, parallel to the vehicle centre line. If they are set at an
angle, then antidive and antisquat geometry can be dialed in.
In many racing cars, the springs and dampers are relocated inside the bodywork. The suspension
uses a bellcrank to transfer the forces at the knuckle end of the suspension to the internal spring
and damper. This is then known as a "push rod" if bump travel "pushes" on the rod (and
subsequently the rod must be joined to the bottom of the upright and angled upward). As the
wheel rises, the push rod compresses the internal spring via a pivot or pivoting system. The
opposite arrangement, a "pull rod," will pull on the rod during bump travel, and the rod must be
attached to the top of the upright, angled downward. Locating the spring and damper inboard
increases the total mass of the suspension, but reduces the unsprung mass, and also allows the
designer to make the suspension more aerodynamic.
The advantage of a double wishbone suspension is that it is fairly easy to work out the effect of
moving each joint, so the kinematics of the suspension can be tuned easily and wheel motion can
be optimized. It is also easy to work out the loads that different parts will be subjected to which
allows more optimized lightweight parts to be designed. They also provide increasing negative
camber gain all the way to full jounce travel, unlike the MacPherson strut, which provides
negative camber gain only at the beginning of jounce travel and then reverses into positive
camber gain at high jounce amounts.
The disadvantage is that it is slightly more complex than other systems like a MacPherson strut.
Due to the increased number of components within the suspension setup it takes much longer to
service and is heavier than an equivalent MacPherson design.
Uses
The double wishbone suspension was introduced in the 1930s. French carmaker Citroën used it
since 1934 in their Rosalie and Traction Avant models. Packard Motor Car Company of Detroit,
Michiganused it on the Packard One-Twenty from 1935.[citation needed], and advertised it as a safety
feature. Prior to the dominance of front wheel drive in the 1980s, many everyday cars used
double wishbone front-suspension systems, or a variation on it. Since that time, the MacPherson
strut has become almost ubiquitous, as it is simpler and cheaper to manufacture. In most cases,
a MacPherson strut requires less space to engineer into a chassis design, and in front-wheeldrive layouts, can allow for more room in the engine bay. A good example of this is observed in
the Honda Civic, which changed its front-suspension design from a double wishbone to a
MacPherson strut after the year 2000 model.
Double wishbones are usually considered to have superior dynamic characteristics as well as
load-handling capabilities, and are still found on higher performance vehicles. Examples of makes
in which double wishbones can be found include Alfa Romeo, Honda and Mercedes-Benz. Short
long arms suspension, a type of double wishbone suspension, is very common on front
suspensions for medium-to-large cars such as the Honda Accord, Peugeot 407, or Mazda
6/Atenza, and is very common on sports cars and racing cars.
See
also
The MacPherson strut is a type of car suspensionsystem which uses the axis of a telescopic
damper as the upper steering pivot. It is widely used in modern vehicles and named afterEarle S.
MacPherson, who developed the design.
Contents
[hide]
1 History
2 Design
5 References
History
Earle S. MacPhersondeveloped the design of the strut in 1949 partially based on designs created
by Guido Fornaca of FIAT in the mid-1920s.[1] It is possible the MacPherson was inspired by the
suspension on the French Cottin-Desgouttes that used the same design, but with leaf springs.
Cottin-Desgouttes front suspension was in turn inspired by J. Walter Christie's 1904 design and
he was inspired by plants.[2]
The first car to feature MacPherson struts was the 1949 Ford Vedette,[3] and it was also adopted
in the 1951 Ford Consul and later Zephyr. MacPherson originally created the design for use at all
four wheels (Mitsubishi Starion, for example), but in practice it is more commonly used for the
front suspension only, where it provides a steering pivot (kingpin) as well as a suspension
mounting for the wheel.
Design
MacPherson struts consist of a wishbone or a substantial compression link stabilized by a
secondary link which provides a bottom mounting point for the hub or axle of the wheel. This
lower arm system provides both lateral and longitudinal location of the wheel. The upper part of
the hub is rigidly fixed to the inner part of the strut proper, the outer part of which extends
upwards directly to a mounting in the body shell of the vehicle.
To be really successful, the MacPherson strut required the introduction of unibody
(or monocoque) construction, because it needs a substantial vertical space and a strong top
mount, which unibodies can provide, while benefiting them by distributing stresses. [4] The strut will
usually carry both the coilspring on which the body is suspended and the shock absorber, which
is usually in the form of a cartridge mounted within the strut (see coilover). The strut also usually
has a steering arm built into the lower inner portion. The whole assembly is very simple and can
be preassembled into a unit; also by eliminating the upper control arm, it allows for more width in
the engine compartment, which is useful for smaller cars, particularly with transverse-mounted
engines such as most front wheel drive vehicles have. It can be further simplified, if needed, by
substituting an anti-roll bar (torsion bar) for the radius arm.[4] For those reasons, it has become
almost ubiquitous with low cost manufacturers. Furthermore, it offers an easy method to set
suspension geometry.[5]
Although it is a popular choice, due to its simplicity and low manufacturing cost, the design has a
few disadvantages in the quality of ride and the handling of the car. Geometric analysis shows it
cannot allow vertical movement of the wheel without some degree of either camber
angle change, sideways movement, or both. It is not generally considered to give as good
handling as a double wishbone suspension, because it allows the engineers less freedom to
choose camber change and roll center.
Another drawback is that it tends to transmit noise and vibration from the road directly into the
body shell, giving higher noise levels and a "harsh" feeling to the ride compared with double
wishbones, requiring manufacturers to add extra noise reduction or cancellation and isolation
mechanisms.
Despite these drawbacks, the MacPherson strut setup is still used on high performance cars such
as the Porsche 911, several Mercedes-Benz models and nearly all current BMWs (including the
new Mini but excluding the 2007 X5,[6] 2009 7-series, 2011 5-series and 5-series GT).
The Porsche 911 up until the 1989 model year (964) use MacPherson strut designs that do not
have coil springs, using a torsion bar suspension instead.
See
also
The basic aim of steering is to ensure that the wheels are pointing in the desired directions. This
is typically achieved by a series of linkages, rods, pivots and gears. One of the fundamental
concepts is that of caster angle- each wheel is steered with a pivot point ahead of the wheel; this
makes the steering tend to be self-centering towards the direction of travel.
The steering linkages connecting the steering box and the wheels usually conforms to a variation
of Ackermann steering geometry, to account for the fact that in a turn, the inner wheel is actually
travelling a path of smaller radius than the outer wheel, so that the degree of toe suitable for
driving in a straight path is not suitable for turns. The angle the wheels make with the vertical
plane also influences steering dynamics (see camber angle) as do the tires.
Rack
and pinion, recirculating ball, worm and sector
Rack and pinion steering mechanism: 1 Steering wheel; 2 Steering column; 3 Rack and pinion; 4 Tie rod; 5
Kingpin.
Rack and pinion unit mounted in the cockpit of an Ariel Atom sports car chassis. For most high volume production,
this is usually mounted on the other side of this panel
Many modern cars use rack and pinion steering mechanisms, where the steering wheel turns the
pinion gear; the pinion moves the rack, which is a linear gear that meshes with the pinion,
converting circular motion into linear motion along the transverse axis of the car (side to side
motion). This motion applies steering torque to the swivel pin ball joints that replaced previously
used kingpins of the stub axle of the steered wheels via tie rods and a short lever arm called the
steering arm.
The rack and pinion design has the advantages of a large degree of feedback and direct steering
"feel". A disadvantage is that it is not adjustable, so that when it does wear and develop lash, the
only cure is replacement.
Older designs often use the recirculating ballmechanism, which is still found on trucks and utility
vehicles. This is a variation on the older worm and sector design; the steering column turns a
large screw (the "worm gear") which meshes with a sector of a gear, causing it to rotate about its
axis as the worm gear is turned; an arm attached to the axis of the sector moves the Pitman arm,
which is connected to the steering linkage and thus steers the wheels. The recirculating ball
version of this apparatus reduces the considerable friction by placing large ball bearings between
the teeth of the worm and those of the screw; at either end of the apparatus the balls exit from
between the two pieces into a channel internal to the box which connects them with the other end
of the apparatus, thus they are "recirculated".
The recirculating ball mechanism has the advantage of a much greater mechanical advantage, so
that it was found on larger, heavier vehicles while the rack and pinion was originally limited to
smaller and lighter ones; due to the almost universal adoption of power steering, however, this is
cars. The recirculating ball design also has a perceptible lash, or "dead spot" on center, where a
minute turn of the steering wheel in either direction does not move the steering apparatus; this is
easily adjustable via a screw on the end of the steering box to account for wear, but it cannot be
entirely eliminated because it will create excessive internal forces at other positions and the
mechanism will wear very rapidly. This design is still in use in trucks and other large vehicles,
where rapidity of steering and direct feel are less important than robustness, maintainability, and
The worm and sector was an older design, used for example in Willys and Chrysler vehicles, and
the Ford Falcon (1960s).[1]
Other systems for steering exist, but are uncommon on road vehicles. Children's toys and gokartsoften use a very direct linkage in the form of a bellcrank (also commonly known as a Pitman
arm) attached directly between the steering column and the steering arms, and the use of cableoperated steering linkages (e.g. the Capstan and Bowstring mechanism) is also found on some
home-built vehicles such as soapbox cars and recumbent tricycles.
Power
steering
Main article: Power steering
Power steering helps the driver of a vehicle to steer by directing some of the its power to assist in
swivelling the steered roadwheels about their steering axes. As vehicles have become heavier
and switched to front wheel drive, particularly using negative offset geometry, along with
increases in tire width and diameter, the effort needed to turn the wheels about their steering axis
has increased, often to the point where major physical exertion would be needed were it not for
power assistance. To alleviate this auto makers have developed power steering systems: or more
correctly power-assisted steering—on road going vehicles there has to be a mechanical linkage
as a fail safe. There are two types of power steering systems; hydraulic and electric/electronic. A
hydraulic-electric hybrid system is also possible.
A hydraulic power steering (HPS) uses hydraulic pressure supplied by an engine-driven pump to
assist the motion of turning the steering wheel. Electric power steering (EPS) is more efficient
than the hydraulic power steering, since the electric power steering motor only needs to provide
assistance when the steering wheel is turned, whereas the hydraulic pump must run constantly.
In EPS, the amount of assistance is easily tunable to the vehicle type, road speed, and even
driver preference. An added benefit is the elimination of environmental hazard posed by leakage
and disposal of hydraulic power steering fluid. In addition, electrical assistance is not lost when
the engine fails or stalls, whereas hydraulic assistance stops working if the engine stops, making
the steering doubly heavy as the driver must now turn not only the very heavy steering—without
any help—but also the power-assistance system itself.
Speed
Sensitive Steering
An outgrowth of power steering is speed sensitive steering, where the steering is heavily assisted
at low speed and lightly assisted at high speed. The auto makers perceive that motorists might
need to make large steering inputs while manoeuvering for parking, but not while traveling at high
speed. The first vehicle with this feature was the Citroën SM with its Diravi layout[citation needed],
although rather than altering the amount of assistance as in modern power steering systems, it
altered the pressure on a centring cam which made the steering wheel try to "spring" back to the
straight-ahead position. Modern speed-sensitive power steering systems reduce the mechanical
or electrical assistance as the vehicle speed increases, giving a more direct feel. This feature is
Four-wheel
steering
Speed-dependent four-wheel steering.
Early example of four-wheel steering. 1910 photograph of 80 hp Caldwell Vale tractor in action.
1937 Mercedes-Benz Type G 5 with four-wheel steering.
Sierra Denali with Quadrasteer, rear steering angle.
Articulated Arnhem trolleybusdemonstrating its four-wheel steering on front and rear axles (2006).
Heavy transport trailer with all-wheel steering remote controlled by a steersman walking at the rear of the trailer
(2008).
2007 Liebherr-Bauma telescopic handler using crab steering.
Hamm DV70 tandem roller using crab steering to cover maximum road surface (2010).
Agricultural slurry applicator using crab steering to minimise soil compaction (2009).
Four-wheel steering (or all-wheel steering) is a system employed by some vehicles to improve
steering response, increase vehicle stability while maneuvering at high speed, or to
decrease turning radius at low speed.
Active four-wheel steering
In an active four-wheel steering system, all four wheels turn at the same time when the driver
steers. In most active four-wheel steering systems, the rear wheels are steered by a computer
and actuators. The rear wheels generally cannot turn as far as the front wheels. There can be
controls to switch off the rear steer and options to steer only the rear wheel independent of the
front wheels. At low speed (e.g. parking) the rear wheels turn opposite of the front wheels,
reducing the turning radius by up to twenty-five percent, sometimes critical for large trucks or
tractors and vehicles with trailers, while at higher speeds both front and rear wheels turn alike
(electronically controlled), so that the vehicle may change position with less yaw, enhancing
straight-line stability. The "Snaking effect" experienced duringmotorway drives while towing
a travel trailer is thus largely nullified.[dubious – discuss]
Four-wheel steering found its most widespread use in monster trucks, where maneuverability in
small arenas is critical, and it is also popular in large farm vehicles and trucks. Some of the
modern European Intercity buses also utilize four-wheel steering to assist maneuverability in bus
terminals, and also to improve road stability.
Previously, Honda had four-wheel steering as an option in their 1987–2000 Prelude and Honda
Ascot Innova models (1992–1996). Mazda also offered four-wheel steering on the 626 andMX6 in
1988. General Motors offered Delphi's Quadrasteer in their
consumer Silverado/Sierra and Suburban/Yukon. However, only 16,500 vehicles have been sold
with this system since its introduction in 2002 through 2004. Due to this low demand, GM
discontinued the technology at the end of the 2005 model year. [2] Nissan/Infiniti offer several
versions of theirHICAS system as standard or as an option in much of their line-up. A new "Active
Drive" system is introduced on the 2008 version of the Renault Laguna line. It was designed as
one of several measures to increase security and stability. The Active Drive should lower the
effects of under steer and decrease the chances of spinning by diverting part of the G-forces
generated in a turn from the front to the rear tires. At low speeds the turning circle can be
tightened so parking and maneuvering is easier.
Production cars with active four wheel steering
Gearbox" redirects here. For the video game developer, see Gearbox Software.
5-speed gearbox + reverse, the 1600 Volkswagen Golf (2009).
A machine consists of a power source and a power transmission system, which provides controlled
application of the power. Merriam-Webster defines transmission as an assembly of parts including the
speed-changing gears and the propeller shaft by which the power is transmitted from an engine to a
live axle.[1] Often transmissionrefers simply to the gearbox that uses gearsand gear trains to
provide speed and torqueconversions from a rotating power source to another device. [2][3]
In British English, the term transmission refers to the whole drive train, including gearbox, clutch, prop
shaft (for rear-wheel drive), differential and final drive shafts. In American English, however, the
distinction is made that a gearbox is any device which converts speed and torque, whereas a
transmission is a type of gearbox that can be "shifted" to dynamically change the speed-torque ratio
such as in a vehicle.
The most common use is in motor vehicles, where the transmission adapts the output of the internal
combustion engine to the drive wheels. Such engines need to operate at a relatively high rotational
speed, which is inappropriate for starting, stopping, and slower travel. The transmission reduces the
higher engine speed to the slower wheel speed, increasing torque in the process. Transmissions are
also used on pedal bicycles, fixed machines, and anywhere else where rotational speed and torque
Often, a transmission will have multiple gear ratios (or simply "gears"), with the ability to switch
between them as speed varies. This switching may be done manually (by the operator), or
automatically. Directional (forward and reverse) control may also be provided. Single-ratio
transmissions also exist, which simply change the speed and torque (and sometimes direction) of
motor output.
In motor vehicles, the transmission will generally be connected to the crankshaft of the engine. The
output of the transmission is transmitted via driveshaft to one or more differentials, which in turn, drive
the wheels. While a differential may also provide gear reduction, its primary purpose is to permit the
wheels at either end of an axle to rotate at different speeds (essential to avoid wheel slippage on
turns) as it changes the direction of rotation.
Conventional gear/belt transmissions are not the only mechanism for speed/torque adaptation.
Alternative mechanisms include torque converters and power transformation (for example, dieselelectric transmission and hydraulic drive system). Hybrid configurations also exist.
Contents
[hide]
1 Explanation
2 Uses
3 Simple
4 Multi-ratio systems
o
4.1 Automotive basics
o
4.2 Manual
o
4.3 Non-synchronous
o
4.4 Automatic
o
4.5 Semi-automatic
o
4.6 Bicycle gearing
5 Uncommon types
o
5.1 Dual clutch transmission
o
5.2 Continuously variable
o
5.3 Infinitely variable
o
5.4 Electric variable
6 Non-direct
o
6.1 Electric
o
6.2 Hydrostatic
o
6.3 Hydrodynamic
8 References
Explanation
Transmission types
Manual

Sequential manual

Non-synchronous

Preselector
Automatic

Manumatic
Semi-automatic

Electrohydraulic

Dual clutch

Saxomat
Continuously variable
Bicycle gearing

Derailleur gears

Hub gears

V

T

E
Interior view of Pantigo Windmill, looking up into cap from floor -- cap rack, brake wheel, brake and wallower.
Pantigo Windmill is located on James Lane, East Hampton, Suffolk County, Long Island, New York.
Early transmissions included the right-angle drives and other gearing in windmills, horse-powered
devices, and steam engines, in support of pumping,milling, and hoisting.
Most modern gearboxes are used to increase torque while reducing the speed of a prime mover output
shaft (e.g. a motor crankshaft). This means that the output shaft of a gearbox will rotate at a slower
rate than the input shaft, and this reduction in speed will produce amechanical advantage, causing an
increase in torque. A gearbox can be set up to do the opposite and provide an increase in shaft speed
with a reduction of torque. Some of the simplest gearboxes merely change the physical direction in
which power is transmitted.
Many typical automobile transmissions include the ability to select one of several different gear ratios.
In this case, most of the gear ratios (often simply called "gears") are used to slow down the output
speed of the engine and increase torque. However, the highest gears may be "overdrive" types that
increase the output speed.
Uses
Gearboxes have found use in a wide variety of different—often stationary—applications, such as wind
turbines.
Transmissions are also used in agricultural, industrial, construction, mining and automotive equipment.
In addition to ordinary transmission equipped with gears, such equipment makes extensive use of the
hydrostatic drive and electrical adjustable-speed drives.
Simple
The main gearbox and rotor of a Bristol Sycamore helicopter
The simplest transmissions, often called gearboxes to reflect their simplicity (although complex
systems are also called gearboxes in the vernacular), provide gear reduction (or, more rarely, an
increase in speed), sometimes in conjunction with a right-angle change in direction of the shaft
(typically in helicopters, see picture). These are often used on PTO-powered agricultural equipment,
since the axial PTO shaft is at odds with the usual need for the driven shaft, which is either vertical (as
with rotary mowers), or horizontally extending from one side of the implement to another (as
with manure spreaders, flail mowers, and forage wagons). More complex equipment, such
as silagechoppers and snowblowers, have drives with outputs in more than one direction.
The gearbox in a wind turbine converts the slow, high-torque rotation of the turbine into much faster
rotation of the electrical generator. These are much larger and more complicated than the PTO
gearboxes in farm equipment. They weigh several tons and typically contain three stages to achieve
an overall gear ratio from 40:1 to over 100:1, depending on the size of the turbine.
(For aerodynamic and structural reasons, larger turbines have to turn more slowly, but the generators
all have to rotate at similar speeds of several thousand rpm.) The first stage of the gearbox is usually a
planetary gear, for compactness, and to distribute the enormous torque of the turbine over more teeth
of the low-speed shaft.[4] Durability of these gearboxes has been a serious problem for a long time. [5]
Regardless of where they are used, these simple transmissions all share an important feature: thegear
ratio cannot be changed during use. It is fixed at the time the transmission is constructed.
For transmission types that overcome this issue, see Continuously Variable Transmission, also known
as CVT.
Multi-ratio
systems
Tractor transmission with 16 forward and 8 backward gears
Amphicar gearbox cutaway w/optional shift for water going propellers
Many applications require the availability of multiple gear ratios. Often, this is to ease the starting and
stopping of a mechanical system, though another important need is that of maintaining good fuel
efficiency.
Automotive
basics
The need for a transmission in an automobile is a consequence of the characteristics of the internal
combustion engine. Engines typically operate over a range of 600 to about 7000 revolutions per
minute (though this varies, and is typically less for diesel engines), while the car's wheels rotate
between 0 rpm and around 1800 rpm.
Furthermore, the engine provides its highest torque and power outputs unevenly across the rev range
resulting in atorque band and a power band. Often the greatest torque is required when the vehicle is
moving from rest or traveling slowly, while maximum power is needed at high speed. Therefore, a
system that transforms the engine's output so that it can supply high torque at low speeds, but also
operate at highway speeds with the motor still operating within its limits, is required. Transmissions
perform this transformation.
A diagram comparing the power and torque bands of a "torquey" engine versus a "peaky" one
The dynamics of a car vary with speed: at low speeds, acceleration is limited by the inertia of vehicular
gross mass; while at cruising or maximum speeds wind resistance is the dominant barrier.
Many transmissions and gears used inautomotive and truck applications are contained in a cast
iron case, though more frequently aluminium is used for lower weight especially in cars. There are
usually three shafts: a mainshaft, a countershaft, and an idler shaft.
The mainshaft extends outside the case in both directions: the input shaft towards the engine, and the
output shaft towards the rear axle (on rear wheel drive cars- front wheel drives generally have the
engine and transmission mounted transversely, the differential being part of the transmission
assembly.) The shaft is suspended by the main bearings, and is split towards the input end. At the
point of the split, a pilot bearing holds the shafts together. The gears and clutches ride on the
mainshaft, the gears being free to turn relative to the mainshaft except when engaged by the clutches.
Types of automobile transmissions include manual, automatic or semi-automatic transmission.
Manual
Main article: Manual transmission
Manual transmission come in two basic types:

a simple but rugged sliding-mesh or unsynchronized / nonsynchronous system, where straight-cut spur gear sets are spinning
freely, and must be synchronized by the operator matching engine
revs to road speed, to avoid noisy and damaging "gear clash",

and the now common constant-mesh gearboxes which can include
non-synchronised, orsynchronized / synchromesh systems, where
typically diagonal cut helical (or sometimes either straight-cut,
or double-helical) gear sets are constantly "meshed" together, and
a dog clutch is used for changing gears. On synchromesh boxes,
friction cones or "synchro-rings" are used in addition to the dog clutch
to closely match the rotational speeds of the two sides of the
(declutched) transmission before making a full mechanical
engagement.
The former type was standard in many vintage cars (alongside e.g. epicyclic and multi-clutch systems)
before the development of constant-mesh manuals and hydraulic-epicyclic automatics, older heavyduty trucks, and can still be found in use in some agricultural equipment. The latter is the modern
standard for on- and off-road transport manual and semi-automatic transmission, although it may be
found in many forms; e.g., non-synchronised straight-cut in racetrack or super-heavy-duty applications,
non-synchro helical in the majority of heavy trucks and motorcycles and in certain classic cars (e.g. the
Fiat 500), and partly or fully synchronised helical in almost all modern manual-shift passenger cars and
light trucks.
Manual transmissions are the most common type outside North America and Australia. They are
cheaper, lighter, usually give better performance, and fuel efficiency (although automatic transmissions
with torque converter lockup and advanced electronic controls can provide similar results). It is
customary for new drivers to learn, and be tested, on a car with a manual gear change.
In Malaysiaand Denmark all cars used for testing (and because of that, virtually all those used for
instruction as well) have a manual transmission. In Japan, the
Philippines, Germany, Poland, Italy, Israel, theNetherlands, Belgium, New Zealand, Austria, Bulgaria,
the UK,[6][7] Ireland,[7] Sweden, Norway,Estonia, France, Spain, Switzerland, the Australian states
of Victoria, Western Australia and Queensland, Finland and Lithuania, a test pass using an automatic
car does not entitle the driver to use a manual car on the public road; a test with a manual car is
required.[citation needed] Manual transmissions are much more common than automatic transmissions
in Asia, Africa, South Americaand Europe.
Manual transmissions can include both synchronized and unsynchronized gearing. For example,
reverse gear is usually unsynchronised, as the drive is only expected to engage it when the vehicle is
at a standstill. Many older (up to 1970s) cars also lacked syncro on first gear (for various reasons cost, typically "shorter" overall gearing, engines typically having more low-end torque, the extreme
wear which would be placed on a frequently used 1st gear synchroniser...), meaning it also could only
be used for moving away from a stop unless the driver became adept at double-declutching and had a
particular need to regularly downshift into the lowest gear.
Some manual transmissions have an extremely low ratio for first gear, which is referred to as a
"creeper gear" or "granny gear". Such gears are usually not synchronized. This feature is common on
pickup trucks tailored to trailer-towing, farming, or construction-site work. During normal on-road use,
the truck is usually driven without using the creeper gear at all, and second gear is used from a
standing start. Some off-road vehicles, most particularly the Willys Jeep and its descendents, also had
transmissions with "granny first"s either as standard or an option, but this function is now more often
provided for by a low-range transfer gearbox attached to a normal fully synchronised transmission.
Non-synchronous
Main article: Non-synchronous transmissions
There are commercial applications engineered with designs taking into account that the gear shifting
will be done by an experienced operator. They are a manual transmission, but are known as nonsynchronized transmissions. Dependent on country of operation, many local, regional, and national
laws govern the operation of these types of vehicles (see Commercial Driver's License). This class
may include commercial, military, agricultural, or engineering vehicles. Some of these may use
combinations of types for multi-purpose functions. An example would be a power take-off (PTO) gear.
The non-synchronous transmission type requires an understanding of gear range, torque, engine
power, and multi-functional clutch and shifter functions. Also see Double-clutching, and Clutchbrakesections of the main article.
Automatic
Main article: Automatic transmission
Epicyclic gearing or planetary gearing as used in an automatic transmission.
Most modern North American and Australian and some European and Japanese cars have
an automatic transmission that will select an appropriate gear ratio without any operator intervention.
They primarily usehydraulics to select gears, depending on pressure exerted by fluid within the
transmission assembly. Rather than using a clutch to engage the transmission, a fluid flywheel,
or torque converter is placed in between the engine and transmission. It is possible for the driver to
control the number of gears in use or select reverse, though precise control of which gear is in use
may or may not be possible.
Automatic transmissions are easy to use. However, in the past, automatic transmissions of this type
have had a number of problems; they were complex and expensive, sometimes had reliability
problems (which sometimes caused more expenses in repair), have often been less fuel-efficient than
their manual counterparts (due to "slippage" in the torque converter), and their shift time was slower
than a manual making them uncompetitive for racing. With the advancement of modern automatic
transmissions this has changed.[citation needed]
Attempts to improve the fuel efficiency of automatic transmissions include the use of torque
converterswhich lock up beyond a certain speed, or in the higher gear ratios, eliminating power loss,
and overdrive gears which automatically actuate above certain speeds; in older transmissions both
technologies could sometimes become intrusive, when conditions are such that they repeatedly cut in
and out as speed and such load factors as grade or wind vary slightly. Current computerized
transmissions possess very complex programming to both maximize fuel efficiency and eliminate any
intrusiveness, and we are at a point in technological advancement where automatics are beginning to
outperform manuals in both performance and efficiency. [citation needed]. This is due mainly to electronic
advances rather than mechanical ones although improvements in CVT technology and the use of
automatic clutches have also helped. The 2012 model of the Honda Jazz sold in the UK actually
claims marginally better fuel consumption for the CVT version than the manual version.
For certain applications, the slippage inherent in automatic transmissions can be advantageous; for
instance, in drag racing, the automatic transmission allows the car to be stopped with the engine at a
high rpm (the "stall speed") to allow for a very quick launch when the brakes are released; in fact, a
common modification is to increase the stall speed of the transmission. This is even more
advantageous for turbocharged engines, where the turbocharger needs to be kept spinning at high
rpm by a large flow of exhaust in order to keep the boost pressure up and eliminate the turbo lag that
occurs when the engine is idling and the throttle is suddenly opened.
Semi-automatic
Main article: Semi-automatic transmission
A hybrid form of transmission where the an integrated control system handles manipulation of
theclutch automatically, but the driver can still - and may be required to - take manual control of gear
selection. This is sometimes called a "clutchless manual," or "automated manual" transmission. Many
of these transmissions allow the driver to fully delegate gear shifting choice to the control system,
which then effectively acts as if it was a regular automatic transmission. They are generally designed
using manual transmission "internals", and when used in passenger cars, have synchromesh operated
helical constant mesh gear sets.
Early semi-automatic systems used a variety of mechanical and hydraulic systems - including
centrifugal clutches, torque converters, electro-mechanical (and even electrostatic) and servo/solenoid
controlled clutches - and control schemes - automatic declutching when moving the gearstick, preselector controls, centrifugal clutches with drum-sequential shift requiring the driver to lift the throttle for
a successful shift, etc. - and some were little more than regular lock-up torque converter automatics
with manual gear selection.
Most modern implementations, however, tend to be standard or slightly modified manual transmissions
(and very occasionally modified automatics, even including a few cases of CVTs with "fake" fixed gear
ratios), with servo-controlled clutching and shifting under command of the central engine computer.
These are intended to be a combined replacement option both for more expensive and less efficient
"normal" automatic systems, and for drivers who prefer manual shift but are no longer able to operate
a clutch, and users are encouraged to leave the shift lever in fully automatic "Drive" most of the time,
only engaging manual-sequential mode for sporty driving or when otherwise strictly necessary.
Specific types of this transmission include: Easytronic, Tiptronic and Geartronic, as well as the
systems used as standard in all ICE-powered Smart-MCC vehicles, and on geared step-through
scooters such as the Honda Cub or Suzuki Address.
A dual-clutch transmission uses two sets of internals which are alternately used, each with its own
clutch, so that a "gearchange" actually only consists of one clutch engaging as the other disengages,
making for a supposedly "seamless" shift with no break in (or jarring reuptake of) power transmission.
Each clutch's attached shaft carries half of the total input gear complement (with a shared output
shaft), including synchronised dog clutch systems that pre-select which of its set of ratios is most likely
to be needed at the next shift, under command of a computerised control system.
Specific types of this transmission include: Direct-Shift Gearbox.
There are also sequential transmissions which use the rotation of a drum to switch gears, much like
those of a typical fully manual motorcycle.[8] These can be designed with a manual or automatic clutch
system, and may be found both in automobiles (particularly track and rally racing cars), motorcycles
(typically light "step-thru" type city utility bikes, e.g. the Honda Cub) and quadbikes (often with a
separately engaged reversing gear), the latter two normally using a scooter-style centrifugal clutch.
Bicycle
gearing
Shimano XT rear derailleur on amountain bike
Main articles: Bicycle gearing, Derailleur gears, and Hub gear
Bicycles usually have a system for selecting different gear ratios. There are two main types: derailleur
gears and hub gears. The derailleur type is the most common, and the most visible,
usingsprocket gears. Typically there are several gears available on the rear sprocket assembly,
attached to the rear wheel. A few more sprockets are usually added to the front assembly as well.
Multiplying the number of sprocket gears in front by the number to the rear gives the number of gear
ratios, often called "speeds".
Hub gears use epicyclic gearing and are enclosed within the axle of the rear wheel. Because of the
small space, they typically offer fewer different speeds, although at least one has reached 14 gear
ratios and Fallbrook Technologies manufactures a transmission with technically infinite ratios.[9]
Causes for failure of bicycle gearing include: worn teeth, damage caused by a faulty chain, damage
due to thermal expansion, broken teeth due to excessive pedaling force, interference by foreign
objects, and loss of lubrication due to negligence.
Uncommon
Dual
types
clutch transmission
Main article: Dual clutch transmission
This arrangement is also sometimes known as a direct shift gearbox or powershift gearbox. It seeks to
combine the advantages of a conventional manual shift with the qualities of a modern automatic
transmission by providing different clutches for odd and even speed selector gears. When changing
gear, the engine torque is transferred from one gear to the other continuously, so providing gentle,
smooth gear changes without either losing power or jerking the vehicle. Gear selection may be
manual, automatic (depending on throttle/speed sensors), or a 'sports' version combining both options.
Continuously
variable
Main article: Continuously variable transmission
The Continuously Variable Transmission (CVT) is a transmission in which the ratio of the rotational
speeds of two shafts, as the input shaft and output shaft of a vehicle or other machine, can be varied
continuously within a given range, providing an infinite number of possible ratios. The CVT allows the
relationship between the speed of the engine and the speed of the wheels to be selected within a
continuous range. This can provide even better fuel economy if the engine is constantly running at a
single speed. The transmission is in theory capable of a better user experience, without the rise and
fall in speed of an engine, and the jerk felt when poorly changing gears.
CVTs are increasingly found on small cars, and especially high-gas-milage or hybrids vehicles. On
these platforms the torque is limited because the electric motor can provide torque without changing
the speed of the engine. By leaving the engine running at the rate that generates the best gas milage
for the given operating conditions, overall milage can be improved over a system with a smaller
number of fixed gears, where the system may be operating at peak efficiency only for a small range of
speeds. CVTs are rare on other platforms, especially high-torque applications, as they are generally
constructed using rubber belts or similar devices that are subject to slippage at high torque.
Infinitely
variable
The IVT is a specific type of CVT that includes not only an infinite number of gear ratios, but an
infiniterange as well. This is a turn of phrase, it actually refers to CVTs that are able to include a "zero
ratio", where the input shaft can turn without any motion of the output shaft while remaining in gear.
Zero output implies infinite ratios, as any "high gear" ratio is an infinite number of times higher than the
zero "low gear".
Most (if not all) IVTs result from the combination of a CVT with an epicyclic gear system with a fixed
ratio. The combination of the fixed ratio of the epicyclic gear with a specific matching ratio in the CVT
side results in zero output. For instance, consider a transmission with an epicyclic gear set to 1:-1 gear
ratio; a 1:1 reverse gear. When the CVT side is set to 1:1 the two ratios add up to zero output. The IVT
is always engaged, even during its zero output. When the CVT is set to higher values it operates
conventionally, with increasing forward ratios.
In practice, the epicyclic gear may be set to the lowest possible ratio of the CVT, if reversing is not
needed or is handled through other means. Reversing can be incorporated by setting the epicyclic
gear ratio somewhat higher than the lowest ratio of the CVT, providing a range of reverse ratios.
Electric
variable
The Electric Variable Transmission (EVT) combines a transmission with an electric motor to provide
the illusion of a single CVT. In the common implementation, a gasoline engine is connected to a
traditional transmission, which is in turn connected to an epicyclic gear system's planet carrier. An
electric motor/generator is connected to the central "sun" gear, which is normally un-driven in typical
epicyclic systems. Both sources of power can be fed into the transmission's output at the same time,
splitting power between them. In common examples, between ¼ and ½ of the engine's power can be
fed into the sun gear. Depending on the implementation, the transmission in front of the epicyclic
system may be greatly simplified, or eliminated completely. EVTs are capable of continuously
modulating output/input speed ratios like mechanical CVTs, but offer the distinct benefit of being able
to also apply power from two different sources to one output, as well as potentially reducing overall
complexity dramatically.
In typical implementations, the gear ratio of the transmission and epicyclic system are set to the ratio
of the common driving conditions, say highway speed for a car, or city speeds for a bus. When the
drivers presses on the gas, the associated electronics interprets the pedal position and immediately
sets the gasoline engine to the RPM that provides the best gas milage for that setting. As the gear
ratio is normally set far from the maximum torque point, this set-up would normally result in very poor
acceleration. Unlike gasoline engines, electric motors offer efficient torque across a wide selection of
RPM, and are especially effective at low settings where the gasoline engine is inefficient. By varying
the electrical load or supply on the motor attached to the sun gear, additional torque can be provided
to make up for the low torque output from the engine. As the vehicle accelerates, the power to the
motor is reduced and eventually ended, providing the illusion of a CVT.
The canonical example of the EVT is Toyota's Hybrid Synergy Drive. This implementation has no
conventional transmission, and the sun gear always receives 28% of the torque from the engine. This
power can be used to operate any electrical loads in the vehicle, recharging the batteries, powering
the entertainment system, or running the air conditioning. Any residual power is then fed back into a
second motor that powers the output of the drivetrain directly. At highway speeds this additional
generator/motor pathway is less efficient than simply powering the wheels directly. However, during
acceleration, the electrical path is much more efficient than engine operating so far from its torque
point.[10] GM uses a similar system in the Allison Bus hybrid powertrains and the Tahoe and Yukon
pick-up trucks, but these use a two-speed transmission in front of the epicyclic system, and the sun
gear receives close to half the total power.
Non-direct
Electric
Electric transmissions convert the mechanical power of the engine(s) to electricity with electric
generators and convert it back to mechanical power with electric motors. Electrical or
electronicadjustable-speed drive control systems are used to control the speed and torque of the
motors. If the generators are driven by turbines, such arrangements are called turbo-electric. Likewise
Diesel-electric arrangements are used on many railway locomotives, ships, large mining trucks, and
some bulldozers. In these cases, each driven wheel is equipped with its own electric motor, which can
be fed varying electrical power to provide any required torque or power output for each wheel
independently. This produces a much simpler solution for multiple driven wheels in very large vehicles,
where drive shafts would be much larger or heavier than the electrical cable that can provide the same
amount of power. It also improves the ability to allow different wheels to run at different speeds, which
is useful for steered wheels in large construction vehicles.
Hydrostatic
Hydrostatic transmissions transmit all power hydraulically, using the
components of hydraulic machinery. They are similar to electrical
transmissions, but hydraulic fluid as the power distribution system
rather than electricity.
The transmission input drive is a central hydraulic pump and final
drive unit(s) is/are a hydraulic motor, or hydraulic cylinder
(see: swashplate). Both components can be placed physically far
apart on the machine, being connected only by flexible hoses.
Hydrostatic drive systems are used on excavators, lawn tractors,
forklifts, winch drive systems, heavy lift equipment, agricultural
machinery, earth-moving equipment, etc. An arrangement for motorvehicle transmission was probably used on the FergusonF1 P99 racing car in about 1961.
The Human Friendly Transmission of the Honda DN-01 is
hydrostatic.
Hydrodynamic
If the hydraulic pump and/or hydraulic motor make use of
the hydrodynamic effects of the fluid flow, i.e. pressure due to a
change in the fluid's momentum as it flows through vanes in a
turbine. The pump and motor usually consist of rotating vanes without
seals and are typically placed in close proximity. The transmission
ratio can be made to vary by means of additional rotating vanes, an
effect similar to varying the pitch of an airplane propeller.
The torque converter in most automotive automatic transmissions is,
in itself, a hydrodynamic transmission. Hydrodynamic transmissions
are used in many passenger rail vehicles, those that are not using
electrical transmissions. In this application the advantage of smooth
power delivery may outweigh the reduced efficiency caused by
turbulence energy losses in the fluid.
See
also

Chain drive

Hydraulic transmission

Manual transmission

Motorcycle transmission

Transfer case
References
1.
^ http://www.merriam-webster.com/dictionary/transmission Mer
riam-Webster definition oftransmission
2.
^ J. J. Uicker, G. R. Pennock, and J. E. Shigley, 2003, Theory
of Machines and Mechanisms, Oxford University Press, New
York.
3.
^ B. Paul, 1979, Kinematics and Dynamics of Planar
Machinery, Prentice Hall.
4.
^ Stiesdal, Henrik (August 1999), The wind turbine:
Components and operation, retrieved 2009-10-06.
5.
^ Musial, W.; Butterfield, S.; McNiff, B. (May 2007), National
Renewable Energy
Laboratory,http://www.nrel.gov/wind/pdfs/41548.pdf.
6.
^ Practical Driving Test FAQs
7.
^ a b Graduated Licensing: Is it what it's meant to be?
8.
^ [1] Howstuffworks.com
9.
^ Rohloff 14-speed hub
10. ^ "The Prius 'Continuously Variable Transmission'"
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Transfer case
Inside of a 231 New Process Gear transfer case. Part time/Manual, shift on the fly
A transfer case is a part of a four-wheel-drive system found in four-wheel-drive and all-wheeldrive vehicles. The transfer case is connected to the transmission and also to the front and
rear axles by means of drive shafts. It is also referred to as a "transfer gearcase", "transfer
gearbox","transfer box" or "jockey box".
Contents
[hide]
1 Functions
2 Types
o
2.1 Drive type

2.1.1 Gear-driven

2.1.2 Chain-driven
o
2.2 Housing type

2.2.1 Married

2.2.2Divorced/independent
o
2.3 Transfer case shift type

2.3.1 M.S.O.F.

2.3.2 E.S.O.F.
4 References
Functions

The transfer case receives power from the transmission and sends it to both the front and rear
axles. This can be done with a set of gears, but the majority of transfer cases manufactured today
are chain driven.[1] On some vehicles, such as four-wheel-drive trucks or vehicles intended for offroad use, this feature is controlled by the driver. The driver can put the transfer case into either
"two-wheel-drive" or "four-wheel-drive" mode. This is sometimes accomplished by means of
ashifter, similar to that in a manual transmission. On some vehicles this may be electronically
operated by a switch instead. Some vehicles, such as all-wheel-drive sports cars, have transfer
cases that are not selectable. Such a transfer case is permanently "locked" into all-wheel-drive
mode.

An on-road, transfer case synchronizes the difference between the rotation of the front and
rear wheels,[2] in much the same way the differential acts on a given axle. This is necessary,
because the front and rear tires never turn at the same speed when front and rear tire sizes differ.

Transfer cases designed for off-road use can mechanically lock the front and rear axles when
needed[3] (e.g. when one of the axles is on a slippery surfaces or stuck in mud, whereas the other
has better traction). This is the equivalent to the differential lock.

The transfer case may contain one or more sets of low range gears (generally for off-road
vehicles). Low range gears are engaged with a shifter or electronic switch. On many transfer
cases, this shifter is the same as the one that selects 2WD or 4WD operation. Low range gears
slow down the vehicle and increase the torque available at the axles. Low-range gears are used
during slow-speed or extreme off road maneuvers, such as rockcrawling, or when pulling a heavy
load. This feature is often absent on all-wheel-drive cars. Some very large vehicles, such as
heavy equipment or military trucks, have more than one low-range gear.
Types
Those used on "part-time four-wheel-drive" system off-road vehicles such as trucks, truggies, rockcrawling vehicles, and some military vehicles generally allow the driver to select 2WD versus 4WD as
well as high versus low gear ranges. Those used in sports cars are usually "transparent" to the driver;
there is no shifter or select lever.
Drive
type
Gear-driven
There are two different types of "internal workings" found in most transfer cases. Gear-driven transfer
cases can use sets of gears to drive either the front or both the front and rear driveshafts. These are
generally strong, heavy units that are used in large trucks, but there are currently several gear drive
cases in production for passenger cars. [citation needed]
Chain-driven
Chain-driven transfer cases use a chain to drive most often only one axle, but can drive both axles.
Chain-driven transfer cases are quieter and lighter than gear-driven ones. They are used in vehicles
such as compact trucks, full size trucks, Jeeps and SUVs. Some off-road driving enthusiasts modify
their vehicles to use gear-driven transfer cases, accepting the additional weight and noise to gain the
extra strength they generally provide.
Housing
type
Married
Transfer cases are also classified as either "divorced"/"independent" or "married". "Married" transfer
cases are bolted directly to the transmission. Sometimes a "married" transfer case is an integral part of
the transmission and the two components share the same housing, as is commonly found on
recentSubaru products and some other four-wheel-drive cars.
Divorced/independent
An "independent" transfer case is completely separate from the transmission; it is bolted to the
transmission output shaft and a short driveshaft travels from the transfer case to the front and rear
differentials. Independent transfer cases are used on very long wheelbase vehicles, such as
commercial trucks or military trucks.
Transfer
case shift type
M.S.O.F.
Manual Shift On-the-Fly transfer cases have a selector lever on the driver's side floor transmission
hump and may also have either two sealed automatic front axle locking hubs or two manual front axle
hub selectors of "LOCK" and "UNLOCK" or "FREE". To engage the four-wheel-drive system the
vehicle must be moving at a lower speeds, the speed at which 4x4 can be engaged depends on the
vehicle. This is only for the four-wheel-drive high setting. To engage the four-wheel-drive low setting,
the vehicle must be stopped and the transmission must be shifted to neutral, then the four-wheel-drive
low can be selected.
E.S.O.F.
Electronic Shift On-the-Fly (ESOF) transfer cases have a dash-mounted selector switch or buttons with
front sealed automatic locking axle hubs or drive flanges. Some models also have what is calledselec
trac, which has a slider switch on the center console. Unlike the manual transfer case, this system has
a transfer case motor. To engage the four-wheel-drive system the vehicle must be moving at a lower
speeds, the speed at which 4x4 can be engaged depends on the vehicle. To engage the four-wheeldrive low setting, the vehicle must be stopped and the transmission must be shifted to neutral, then the
four-wheel-drive low can be selected.
See
also

AMC/Jeep Transmissions

Jeep four wheel drive systems
References
1. ^ http://www.morsetec.com/drive.html#hyvo BorgWarner MorseTec chain drive
2. ^ http://www.cdxetextbook.com/trans/finalDrives/allWheel/transfercasediff.html Transfer case
differential action
3.
Manual transmission
these issues on the talk page.
2010)
standards. (May 2011)
Transmission types
Manual

Sequential manual

Non-synchronous

Preselector
Automatic
Manumatic

Semi-automatic

Electrohydraulic

Dual clutch

Saxomat
Continuously variable
Bicycle gearing

Derailleur gears

Hub gears

V

T

E
A floor-mounted gear stick in a modern passenger car with a manual transmission
A manual transmission, also known as a manual gearbox or standard transmission (informally,
a manual, 5 speed, or the number of forward gears said with the word speed following i.e.: 4 speed
with overdrive, 4 speed, 5 speed, 6 speed or standard, stick-shift, straight shift, or straight, (U.S.)) is a
type of transmission used in motor vehicle applications. It generally uses a driver-operated clutch,
typically operated by a foot pedal (automobile) or hand lever (motorcycle), for
regulating torque transfer from the internal combustion engine to the transmission; and a gear stick,
either operated by foot (as in a motorcycle) or by hand (as on an automobile).
A conventional manual transmission is frequently the base equipment in a car; other options include
automated transmissions such as an automatic transmission (often a manumatic), a semi-automatic
transmission, or acontinuously variable transmission (CVT).
Contents
[hide]
1 Overview
2 Unsynchronized transmission
3 Synchronized transmission
4 Internals
o
4.1 Shafts
o
4.2 Dog clutch
o
4.3 Synchromesh
o
4.4 Reverse
o
4.5 Design variations

4.5.1 Ratio count

4.5.2 Gear ratios

4.5.3 External overdrive

4.5.4 Shaft and gear configuration
5 Clutch
6 Gear shift types
o
6.1 Floor-mounted shifter
6.1.1 "Four on the floor"

o
6.2 Column-mounted shifter
o
6.3 Console-mounted shifter
o
6.4 Sequential manual
o
6.5 Semi-manual
7 Benefits
o
7.1 Fuel economy
o
7.2 Longevity and cost
o
7.3 Lubrication
o
7.4 Performance and control
o
7.5 Engine braking
8 Drawbacks
o
8.1 Complexity and learning curve
o
8.2 Shifting speed
o
8.3 Ease of use
o
8.4 Stopping on hills
9 Applications and popularity
10 Truck transmissions
11 Maintenance
13 References
Overview
Manual transmissions often feature a driver-operated clutch and a movable gear stick. Most
automobile manual transmissions allow the driver to select any forward gear ratio ("gear") at any time,
but some, such as those commonly mounted on motorcycles and some types of racing cars, only allow
the driver to select the next-higher or next-lower gear. This type of transmission is sometimes called
asequential manual transmission. Sequential transmissions are commonly used in auto racing for their
ability to make quick shifts.[citation needed]
Manual transmissions are characterized by gear ratios that are selectable by locking selected gear
pairs to the output shaft inside the transmission. Conversely, most automatic
transmissions featureepicyclic (planetary) gearing controlled by brake bands and/or clutch packs to
select gear ratio.Automatic transmissions that allow the driver to manually select the current gear are
calledManumatics. A manual-style transmission operated by computer is often called
an automatedtransmission rather than an automatic.
Contemporary automobile manual transmissions typically use four to six forward gears and one
reverse gear, although automobile manual transmissions have been built with as few as two and as
many as eight gears. Transmission for heavy trucks and other heavy equipment usually have at least 9
gears so the transmission can offer both a wide range of gears and close gear ratios to keep the
engine running in the power band. Some heavy vehicle transmissions have dozens of gears, but many
are duplicates, introduced as an accident of combining gear sets, or introduced to simplify shifting.
Some manuals are referred to by the number of forward gears they offer (e.g., 5-speed) as a way of
distinguishing between automatic or other available manual transmissions. Similarly, a 5-speed
automatic transmission is referred to as a "5-speed automatic."
Unsynchronized
transmission
Main article: Non-synchronous transmission
The earliest form of a manual transmission is thought to have been invented by Louis-René Panhard
and Emile Levassor in the late 19th century. This type of transmission offered multiple gear ratios and,
in most cases, reverse. The gears were typically engaged by sliding them on their shafts (hence the
phrase shifting gears), which required careful timing and throttle manipulation when shifting, so the
gears would be spinning at roughly the same speed when engaged; otherwise, the teeth would refuse
to mesh. These transmissions are called sliding mesh transmissions or sometimes crash boxes,
because of the difficulty in changing gears and the loud grinding sound that often accompanied. Newer
manual transmissions on cars have all gears mesh at all times and are referred to as constantmeshtransmissions, with "synchro-mesh" being a further refinement of the constant mesh principle.
In both types, a particular gear combination can only be engaged when the two parts to engage (either
gears or clutches) are at the same speed. To shift to a higher gear, the transmission is put in neutral
and the engine allowed to slow down until the transmission parts for the next gear are at a proper
speed to engage. The vehicle also slows while in neutral and that slows other transmission parts, so
the time in neutral depends on the grade, wind, and other such factors. To shift to a lower gear, the
transmission is put in neutral and the throttle is used to speed up the engine and thus the relevant
transmission parts, to match speeds for engaging the next lower gear. For both upshifts and
downshifts, the clutch is released (engaged) while in neutral. Some drivers use the clutch only for
starting from a stop, and shifts are done without the clutch. Other drivers will depress (disengage) the
clutch, shift to neutral, then engage the clutch momentarily to force transmission parts to match the
engine speed, then depress the clutch again to shift to the next gear, a process called double
clutching. Double clutching is easier to get smooth, as speeds that are close but not quite matched
need to speed up or slow down only transmission parts, whereas with the clutch engaged to the
engine, mismatched speeds are fighting the rotational inertia and power of the engine.
Even though automobile and light truck transmissions are now almost universally synchronised,
transmissions for heavy trucks and machinery, motorcycles, and for dedicated racing are usually
not.Non-synchronized transmission designs are used for several reasons. The friction material, such
asbrass, in synchronizers is more prone to wear and breakage than gears, which are forged steel, and
the simplicity of the mechanism improves reliability and reduces cost. In addition, the process of
shifting a synchromesh transmission is slower than that of shifting a non-synchromesh transmission.
For racing of production-based transmissions, sometimes half the teeth (or dogs) on the synchros are
removed to speed the shifting process, at the expense of greater wear.
Heavy duty trucks often use unsynchronized transmissions, though military trucks usually have
synchronized transmissions, allowing untrained personnel to operate them in emergencies. In the
United States, traffic safety rules refer to non-synchronous transmissions in classes of
largercommercial motor vehicles. In Europe, heavy duty trucks use synchronized gearboxes as
standard.
Similarly, most modern motorcycles use unsynchronized transmissions: their low gear inertias and
higher strengths mean that forcing the gears to alter speed is not damaging, and the pedal operated
selector on modern motorcycles, with no neutral position between gears (except, commonly, 1st and
2nd), is not conducive to having the long shift time of a synchronized gearbox. On bikes with a 1-N-2(3-4...) transmission, it is necessary either to stop, slow right down, or synchronize gear speeds by
blipping the throttle when shifting from 2nd into 1st.
Synchronized
transmission
Top and side view of a typical manual transmission, in this case a FordToploader, used in cars with external floor
shifters.
Most modern manual-transmission vehicles are fitted with a synchronized gear box. Transmission
gears are always in mesh and rotating, but gears on one shaft can freely rotate or be locked to the
shaft. The locking mechanism for a gear consists of a collar (or dog collar) on the shaft which is able to
slide sideways so that teeth (ordogs) on its inner surface bridge two circular rings with teeth on their
outer circumference: one attached to the gear, one to the shaft. When the rings are bridged by the
collar, that particular gear is rotationally locked to the shaft and determines the output speed of the
transmission. The gearshift lever manipulates the collars using a set of linkages, so arranged so that
one collar may be permitted to lock only one gear at any one time; when "shifting gears", the locking
collar from one gear is disengaged before that of another is engaged. One collar often serves for two
gears; sliding in one direction selects one transmission speed, in the other direction selects another.
In a synchromesh gearbox, to correctly match the speed of the gear to that of the shaft as the gear is
engaged the collar initially applies a force to a cone-shaped brass clutch attached to the gear, which
brings the speeds to match prior to the collar locking into place. The collar is prevented from bridging
the locking rings when the speeds are mismatched by synchro rings (also called blocker rings or baulk
rings, the latter being spelled balk in the U.S.). The synchro ring rotates slightly due to the frictional
torque from the cone clutch. In this position, the dog clutch is prevented from engaging. The brass
clutch ring gradually causes parts to spin at the same speed. When they do spin the same speed,
there is no more torque from the cone clutch and the dog clutch is allowed to fall in to engagement. In
a modern gearbox, the action of all of these components is so smooth and fast it is hardly noticed.
The modern cone system was developed by Porsche and introduced in the 1952 Porsche 356; cone
synchronisers were called Porsche-type for many years after this. In the early 1950s, only the secondthird shift was synchromesh in most cars, requiring only a single synchro and a simple linkage; drivers'
manuals in cars suggested that if the driver needed to shift from second to first, it was best to come to
a complete stop then shift into first and start up again. With continuing sophistication of mechanical
development, fully synchromesh transmissions with three speeds, then four, and then five, became
universal by the 1980s. Many modern manual transmission cars, especially sports cars, now offer six
speeds. The 2012 Porsche 911 offers a seven-speed manual transmission, with the seventh gear
intended for cruising- top speed being attained on sixth.
Reverse gear is usually not synchromesh, as there is only one reverse gear in the normal automotive
transmission and changing gears into reverse while moving is not required - and often highly
undesirable, particularly at high forward speed. Additionally, the usual method of providing reverse,
with an idler gear sliding into place to bridge what would otherwise be two mismatched forward gears,
is necessarily similar to the operation of a crash box. Among the cars that have synchromesh in
reverse are the 1995-2000 Ford Contour and Mercury Mystique, '00-'05 Chevrolet Cavalier, Mercedes
190 2.3-16, the V6 equipped Alfa Romeo GTV/Spider (916),[1] certain Chrysler, Jeep, and GM products
which use the New Venture NV3500 and NV3550 units, the European Ford Sierra and
Granada/Scorpio equipped with the MT75 gearbox, the Volvo 850, and almost
all Lamborghinis and BMWs.
Internals
Shafts
Like other transmissions, a manual transmission has several shafts with various gears and other
components attached to them. Typically, a rear-wheel-drive transmission has three shafts: an input
shaft, a countershaft and an output shaft. The countershaft is sometimes called a layshaft.
In a rear-wheel-drive transmission, the input and output shaft lie along the same line, and may in fact
be combined into a single shaft within the transmission. This single shaft is called a mainshaft. The
input and output ends of this combined shaft rotate independently, at different speeds, which is
possible because one piece slides into a hollow bore in the other piece, where it is supported by a
bearing. Sometimes the term mainshaft refers to just the input shaft or just the output shaft, rather than
the entire assembly.
In many transmissions the input and output components of the mainshaft can be locked together to
create a 1:1 gear ratio, causing the power flow to bypass the countershaft. The mainshaft then
behaves like a single, solid shaft: a situation referred to as direct drive.
Even in transmissions that do not feature direct drive, it's an advantage for the input and output to lie
along the same line, because this reduces the amount of torsion that the transmission case has to
bear.
Under one possible design, the transmission's input shaft has just one pinion gear, which drives the
countershaft. Along the countershaft are mounted gears of various sizes, which rotate when the input
shaft rotates. These gears correspond to the forward speeds and reverse. Each of the forward gears
on the countershaft is permanently meshed with a corresponding gear on the output shaft. However,
these driven gears are not rigidly attached to the output shaft: although the shaft runs through them,
they spin independently of it, which is made possible by bearings in their hubs. Reverse is typically
implemented differently; see the section on Reverse.
Most front-wheel-drive transmissions for transverse engine mounting are designed differently. For one
thing, they have an integral final drive and differential. For another, they usually have only two shafts;
input and countershaft, sometimes called input and output. The input shaft runs the whole length of the
gearbox, and there is no separate input pinion. At the end of the second (counter/output) shaft is a
pinion gear that mates with the ring gear on the differential.
Front-wheel and rear-wheel-drive transmissions operate similarly. When the transmission is put in
neutral and the clutch is disengaged, the input shaft, clutch disk and countershaft can continue to
rotate under their own inertia. In this state, the engine, the input shaft and clutch, and the output shaft
all rotate independently.
Dog
clutch
Dog clutches. The gear-like teeth ("dogs", right-side images) engage and disengage with each other.
Among many different types of clutches, a dog clutch provides non-slip coupling of two rotating
members. It is not at all suited to intentional slipping, in contrast with the foot-operated friction clutch of
a manual-transmission car.
The gear selector does not engage or disengage the actual gear teeth which are permanently meshed.
Rather, the action of the gear selector is to lock one of the freely spinning gears to the shaft that runs
through its hub. The shaft then spins together with that gear. The output shaft's speed relative to the
countershaft is determined by the ratio of the two gears: the one permanently attached to the
countershaft, and that gear's mate which is now locked to the output shaft.
Locking the output shaft with a gear is achieved by means of a dog clutch selector. The dog clutch is a
sliding selector mechanism which is splined to the output shaft, meaning that its hub has teeth that fit
into slots (splines) on the shaft, forcing that shaft to rotate with it. However, the splines allow the
selector to move back and forth on the shaft, which happens when it is pushed by a selector fork that
is linked to the gear lever. The fork does not rotate, so it is attached to a collar bearing on the selector.
The selector is typically symmetric: it slides between two gears and has a synchromesh and teeth on
each side in order to lock either gear to the shaft.
Synchromesh
Synchronizer rings
If the teeth, the so-called dog teeth, make contact with the gear, but the two parts are spinning at
different speeds, the teeth will fail to engage and a loud grinding sound will be heard as they clatter
together. For this reason, a modern dog clutch in an automobile has a synchronizer mechanism
or synchromesh, which consists of a cone clutch and blocking ring. Before the teeth can engage, the
cone clutch engages first, which brings the selector and gear to the same speed using friction.
Moreover, until synchronization occurs, the teeth are prevented from making contact, because further
motion of the selector is prevented by ablocker (or baulk) ring. When synchronization occurs, friction
on the blocker ring is relieved and it twists slightly, bringing into alignment certain grooves and notches
that allow further passage of the selector which brings the teeth together. Of course, the exact design
of the synchronizer varies from manufacturer to manufacturer.
The synchronizer[2] has to overcome the momentum of the entire input shaft and clutch disk when it is
changing shaft rpm to match the new gear ratio. It can be abused by exposure to the momentum and
power of the engine itself, which is what happens when attempts are made to select a gear without
fully disengaging the clutch. This causes extra wear on the rings and sleeves, reducing their service
life. When an experimenting driver tries to "match the revs" on a synchronized transmission and force
it into gear without using the clutch, the synchronizer will make up for any discrepancy in RPM. The
success in engaging the gear without clutching can deceive the driver into thinking that the RPM of the
layshaft and transmission were actually exactly matched. Nevertheless, approximate rev.
matchingwith clutching can decrease the general change between layshaft and transmission and
decrease synchro wear.
Reverse
The previous discussion normally applies only to the forward gears. The implementation of the reverse
gear is usually different, implemented in the following way to reduce the cost of the transmission.
Reverse is also a pair of gears: one gear on the countershaft and one on the output shaft. However,
whereas all the forward gears are always meshed together, there is a gap between the reverse gears.
Moreover, they are both attached to their shafts: neither one rotates freely about the shaft. When
reverse is selected a small gear, called an idler gear or reverse idler, is slid between them. The idler
has teeth which mesh with both gears, and thus it couples these gears together and reverses the
direction of rotation without changing the gear ratio.
In other words, when reverse gear is selected, it is in fact actual gear teeth that are being meshed,
with no aid from a synchronization mechanism. For this reason, the output shaft must not be rotating
when reverse is selected: the car must be stopped. In order that reverse can be selected without
grinding even if the input shaft is spinning inertially, there may be a mechanism to stop the input shaft
from spinning. The driver brings the vehicle to a stop, and selects reverse. As that selection is made,
some mechanism in the transmission stops the input shaft. Both gears are stopped and the idler can
be inserted between them. There is a clear description of such a mechanism in the Honda Civic 19961998 Service Manual, which refers to it as a "noise reduction system":
Whenever the clutch pedal is depressed to shift into reverse, the mainshaft continues to rotate
because of its inertia. The resulting speed difference between mainshaft and reverse idler gear
produces gear noise [grinding]. The reverse gear noise reduction system employs a cam plate which
was added to the reverse shift holder. When shifting into reverse, the 5th/reverse shift piece,
connected to the shift lever, rotates the cam plate. This causes the 5th synchro set to stop the rotating
mainshaft.
—(13-4)
A reverse gear implemented this way makes a loud whining sound, which is not normally heard in the
forward gears. The teeth on the forward gears of most consumer automobiles are helically cut.
Whenhelical gears rotate, there is constant contact between gears, which results in quiet operation. In
spite of all forward gears being always meshed, they do not make a sound that can be easily heard
above the engine noise. By contrast, most reverse gears are spur gears, meaning that they have
straight teeth, in order to allow for the sliding engagement of the idler, which is difficult with helical
gears. The teeth of spur gears clatter together when the gears spin, generating a characteristic whine.
Attempting to select reverse while the vehicle is moving forward causes severe gear wear (except in
transmissions with synchromesh on the reverse gear). However, most manual transmissions have a
gate that locks out reverse directly from 5th gear to help prevent this. In order to engage reverse from
5th, the shift lever has to be moved to the center position between 3rd and 4th, then back over and
into reverse. Another widespread solution places reverse to the left of 1st gear, instead of behind the
5th (where you might expect to find a 6th gear). Similarly, many newer six-speed manual
transmissions have a collar under the shift knob which must be lifted to engage reverse to also help
prevent this.
It is clear that the spur gear design of reverse gear represents some compromises (less robust,
unsynchronized engagement and loud noise) which are acceptable due to the relatively small amount
of driving that takes place in reverse. The gearbox of the classic SAAB 900 is a notable example of a
gearbox with a helical reverse gear engaged in the same unsynchronized manner as the spur gears
described above. Its strange design allows reverse to share cogs with first gear, and is exceptionally
quiet, but results in difficult engagement and unreliable operation. However, many modern
transmissions now include a reverse gear synchronizer and helical gearing.
Design
variations
Ratio count
Until the mid-1970s, cars were generally equipped with 3-speed transmissions as standard equipment.
4-speed units began to appear on volume-production models in the 1950s and gained popularity in the
1960s; some exotics had 5-speeds. In the 1970s, as fuel prices rose and fuel economy became an
important selling feature, 4-speed transmissions with an overdrive 4th gear or 5-speeds were offered
in mass market automobiles and even compact pickup trucks, pioneered by Toyota (who advertised
the fact by giving each model the suffix SR5 as it acquired the fifth speed). 6-speed transmissions
started to emerge in high-performance vehicles in the early 1990s.
Today, mass market automotive manual transmissions are nearly all at least 5-speed. [citation
needed]
Recently Porsche announced the next-generation 911 will be available with a 7-speed manual
transmission, the first of its kind for a normal automobile[3][4] with the first six gear ratios the same as
the 6-speed gearbox and the 7th gear being of a higher ratio.
It has been widely anticipated that for Electric Vehicles (EV's) clutches and multi-speed gearboxes
would not be required, as electric motors can drive the vehicle both forward and reverse from zero
speed and typically operate over a wider speed range than combustion engines. Elimination of the
gearbox represents a significant reduction in powertrain weight and complexity, and also removes a
notable source of parasitic losses. The majority of first-generation consumer EV's have therefore been
single-speed. However, current trends indicates that multi-speed gearboxes are likely to return for
many future EV's, since this allows the use of smaller, lower torque motors running at higher speeds to
achieve both greater torque at the wheels for low speed tractive effort, and higher top road speed.
Modest efficiency gains are also possible by reducing the proportion of the time that the motor(s)
operate very low speeds where efficiency is lower. The wider speed range of motors means that
number of ratios required is lower than for combustion engine vehicles, with two to four speed designs
emerging as the optimum depending on application.
Initially the Tesla Roadster was intended to have a purpose-built two-speed manual transmission [5] but
this gearbox proved to be problematic and was later replaced with a fixed-ratio transmission.
Gear ratios
The slowest gears (designated '1' or low gear) in most automotive applications allow for three to four
engine rotations for each output revolution (3:1). "High" gear in a three or four speed manual
transmission allows the output shaft to spin at the same speed as the engine (1:1). Five and six
speeds are often 'overdrive' with the engine turning less than a full turn for each revolution of the
output shaft (0.8:1, for example).
External overdrive
Main article: Overdrive
In the 1950s, 1960s, and 1970s, fuel-efficient highway cruising with low engine speed was in some
cases enabled on cars equipped with 3- or 4-speed transmissions by means of a
separate overdriveunit in or behind the rear housing of the transmission. This was actuated either
manually while in high gear by throwing a switch or pressing a button on the gearstick knob or on the
steering column, or automatically by momentarily lifting the foot from the accelerator with the car
travelling above a certain road speed. Automatic overdrives were disengaged by flooring the
accelerator, and a lockout control was provided to enable the driver to disable overdrive and operate
the transmission as a normal (non-overdrive) transmission. [6]
Shaft and gear configuration
On a conventional rear-drive transmission, there are three basic shafts; the input, the output, and the
countershaft. The input and output together are called the mainshaft, since they are joined inside the
transmission so they appear to be a single shaft, although they rotate totally independently of each
other. The input length of this shaft is much shorter than the output shaft. Parallel to the mainshaft is
the countershaft. There are a number of gears fixed along the countershaft, and matching gears along
the output shaft, although these are not fixed, and rotate independently of the output shaft. There are
sliding dog collars, or dog clutches, between the gears on the output shaft, and to engage a gear to
the shaft, the collar slides into the space between the shaft and the inside space of the gear, thus
rotating the shaft as well. One collar is usually mounted between two gears, and slides both ways to
engage one or the other gears, so on a four-speed there would be two collars. A front-drive
transmission is basically the same, but may be simplified. There often are two shafts, the input and the
output, but depending on the direction of rotation of the engine, three may be required. Rather than the
input shaft driving the countershaft with a pinion gear, the input shaft takes over the countershaft's job,
and the output shaft runs parallel to it. The gears are positioned and engaged just as they are on the
countershaft and output shaft of a rear-drive. This merely eliminates one major component, the pinion
gear. Part of the reason that the input and output are in-line on a rear drive unit is to relieve torsional
stress on the transmission and mountings, but this isn't an issue in a front-drive as the gearbox is
integrated into the transaxle.
The basic process is not universal. The fixed and free gears can be mounted on either the input or
output shaft, or both.
The distribution of the shifters is also a matter of design; it need not be the case that all of the freerotating gears with selectors are on one shaft, and the permanently splined gears on the other. For
instance a five-speed transmission might have the first-to-second selectors on the countershaft, but
the third-to-fourth selector and the fifth selector on the mainshaft, which is the configuration in the
1998Honda Civic. This means that when the car is stopped and idling in neutral with the clutch
engaged and the input shaft spinning, the third, fourth and fifth gear pairs do not rotate.
In some transmission designs (Volvo 850 and V/S70 series, for example) there are actually two
countershafts, both driving an output pinion meshing with the front-wheel-drive transaxle's ring gear.
This allows the transmission designer to make the transmission narrower, since each countershaft
need only be half as long as a traditional countershaft with four gears and two shifters.
Clutch
Main article: Clutch
In all vehicles using a transmission (virtually all modern vehicles), a coupling device is used to
separate the engine and transmission when necessary. This is because an internal-combustion engine
must continue to run when in use, although a few modern cars with automatic transmissions shut off
the engine at a stoplight. The clutch accomplishes this in manual transmissions. Without it, the engine
and tires would at all times be inextricably linked, and any time the vehicle stopped the engine would
stall. Without the clutch, changing gears would be very difficult, even with the vehicle moving already:
deselecting a gear while the transmission is under load requires considerable force (and risks
significant damage). As well, selecting a gear requires the revolution speed of the engine to be held at
a very precise value which depends on the vehicle speed and desired gear – the speeds inside the
transmission have to match. In a car the clutch is usually operated by a pedal; on a motorcycle, a lever
on the left handlebar serves the purpose.

When the clutch pedal is fully depressed, the clutch is fully disengaged, and no torque is
transferred from the engine to the transmission (and by extension to the drive wheels). In this
uncoupled state it is possible to select gears or to stop the car without stopping the engine.

When the clutch pedal is fully released, the clutch is fully engaged and all of the engine's
torque is transferred. In this coupled state, the clutch does not slip, but rather acts as rigid
coupling to transmit power to the gearbox.

Between these extremes of engagement and disengagement the clutch slips to varying
degrees. When slipping it still transmits torque despite the difference in speeds between the
engine crankshaft and the transmission input. Because this torque is transmitted by means of
friction rather than direct mechanical contact, considerable power is wasted as heat (which is
dissipated by the clutch). Properly applied, slip allows the vehicle to be started from a standstill,
and when it is already moving, allows the engine rotation to gradually adjust to a newly selected
gear ratio.

Learning to use the clutch efficiently requires the development of muscle memory and a level
of coordination.

A rider of a highly tuned motocross or off-road motorcycle may "hit" or "fan" the clutch when
exiting corners to assist the engine in revving to the point where it delivers the most power.
The clutch is typically disengaged by a thrust bearing that makes contact with pressure petals on the
clutch ring plate and pushes them inward to release the clutch pad friction. Normally the bearing
remains retracted away from the petals and does not spin. However, the bearing can be "burned out"
and damaged by using the clutch pedal as a foot rest, which causes the bearing to spin continuously
from touching the clutch plates.
Gear
shift types
Floor-mounted
shifter
Main article: Gear stick
A gear stick
In most vehicles with manual transmission, gears are selected by manipulating a lever called a gear
stick, shift stick, gearshift, gear lever, gear selector, or shifterconnected to the transmission via
linkage or cables and mounted on the floor, dashboard, or steering column. Moving the lever forward,
backward, left, and right into specific positions selects particular gears.
A sample layout of a four-speed transmission is shown below. N marks neutral, the position wherein
no gears are engaged and the engine is decoupled from the vehicle's drive wheels. The entire
horizontal line is a neutral position, though the shifter is usually spring-loaded so it will return to the
centre of the N position if not moved to another gear. The R marks reverse, the gear position used for
moving the vehicle rearward.
This layout is called the shift pattern. Because of the shift quadrants, the basic arrangement is often
called an H-pattern. The shift pattern is usually molded or printed on or near the gear knob. While the
layout for gears one through four is nearly universal, the location of reverse is not. Depending on the
particular transmission design, reverse may be located at the upper left extent of the shift pattern, at
the lower left, at the lower right, or at the upper right. There is often a mechanism that allows selection
of reverse only from the neutral position, or a reverse lockout that must be released by depressing the
spring-loaded gear knob or lifting a spring-loaded collar on the shift stick, to reduce the likelihood of
"Four on the floor"
Four-speed transmissions with floor-mounted shifters were sometimes referred to as "four on the floor"
during the period when the steering column was the more common shifter location. The latter, often
being the standard non-performance transmission, usually had only three forward speeds and was
referred to as "three on the tree."
Most front-engined, rear-wheel drive cars have a transmission that sits between the driver and the
front passenger seat. Floor-mounted shifters are often connected directly to the transmission. Frontwheel drive and rear-engined cars often require a mechanical linkage to connect the shifter to the
transmission.
Column-mounted
shifter
Column mounted gear shift lever in aSaab 96
Some cars have a gear lever mounted on the steering column of the car. A 3-speed column shifter,
which came to be popularly known as a "Three on the Tree", began appearing in America in the late
1930s and became common during the 1940s and 1950s. If a U.S. vehicle was equipped with
overdrive, it was very likely to be a Borg-Warner type, operated by briefly backing off the gas when
above 28 mph to enable, and momentarily flooring the gas pedal to return to normal gear. The control
simply disables overdrive for such situations as parking on a hill or preventing unwanted shifting into
overdrive.[citation needed]
Later,[vague] European and Japanese models began to have 4-speed column shifters with this shift
pattern:
A majority of North American-spec vehicles for USA and Canada had a 3-speed column-mounted
shifter - the first generation Chevrolet/GMC vans of 1964-70 vintage had an ultra-rare 4-speed column
shifter. The column-mounted manual shifter disappeared in North America by the mid 1980s, last
appearing in the 1987 Chevrolet pickup truck. Outside North America, the column-mounted shifter
remained in production. All Toyota Crown and Nissan Cedric taxis in Hong Kong had the 4-speed
column shift until 1999 when automatic transmissions were first offered. Since the late 1980s or early
1990s,[vague] a 5-speed column shifter has been offered in some vans sold in Asia and Europe, such
as Toyota Hiace and Mitsubishi L400.
Column shifters are mechanically similar to floor shifters, although shifting occurs in a vertical plane
instead of a horizontal one. Because the shifter is further away from the transmission, and the
movements at the shifter and at the transmission are in different planes, column shifters require more
complicated linkage than floor shifters. Advantages of a column shifter are the ability to switch
between the two most commonly used gears—second and third—without letting go of the steering
wheel, and the lack of interference with passenger seating space in vehicles equipped with a bench
seat.
Console-mounted
shifter
Newer small cars and MPVs, like the Suzuki MR Wagon, the Fiat Multipla, the Toyota Matrix,
thePontiac Vibe, the Chrysler RT platform cars and the Honda Civic Si EP3 may feature a manual
orautomatic transmission gear shifter located on the vehicle's instrument panel, similar to the mid1950sChryslers. Console-mounted shifters are similar to floor-mounted gear shifters in that most of the
ones used in modern cars operate on a horizontal plane and can be mounted to the vehicle's
transmission in much the same way a floor-mounted shifter can. However, because of the location of
the gear shifter in comparison to the locations of the column shifter and the floor shifter, as well as the
positioning of the shifter to the rest of the controls on the panel often require that the gearshift be
mounted in a space that does not feature a lot of controls integral to the vehicle's operation or
frequently used controls, such as those for the car stereo or car air conditioning, to help prevent
accidental activation or driver confusion, especially in right-hand drive cars.
More and more small cars and vans from manufacturers such as Suzuki, Honda, and Volkswagen are
featuring console shifters in that they free up space on the floor for other car features such as storage
compartments without requiring that the gear shift be mounted on the steering column. Also, the basic
location of the gear shift in comparison to the column shifter makes console shifters easier to operate
than column shifters.
Sequential
manual
Main article: Sequential manual transmission
Some transmissions do not allow the driver to arbitrarily select any gear. Instead, the driver may only
ever select the next-lowest or next-highest gear ratio. Sequential transmissions often incorporate a
synchro-less dog-clutch engagement mechanism (instead of the synchromesh dog clutch common on
H-pattern automotive transmissions), in which case the clutch is only necessary when selecting first or
reverse gear from neutral, and most gear changes can be performed without the clutch. However,
sequential shifting and synchro-less engagement are not inherently linked, though they often occur
together due to the environment(s) in which these transmissions are used, such as racing cars and
motorcycles.
Sequential transmissions are generally controlled by a forward-backward lever, foot pedal, or set of
paddles mounted behind the steering wheel. In some cases, these are connected mechanically to the
transmission. In many modern examples, these controls are attached to sensors which instruct a
transmission computer to perform a shift—many of these systems can be switched into an automatic
mode, where the computer controls the timing of shifts, much like an automatic transmission.
Motorcycles typically employ sequential transmissions, although the shift pattern is modified slightly for
safety reasons. In a motorcycle the gears are usually shifted with the left foot pedal, the layout being
this:
The gear shift lever on a 2003 Suzuki SV650Smotorcycle.
6-5-4-3-2N1
The pedal goes one step–both up and down–from the center, before it reaches its limit and has to be
allowed to move back to the center position. Thus, changing multiple gears in one direction is
accomplished by repeatedly pumping the pedal, either up or down. Although neutral is listed as being
between first and second gears for this type of transmission, it "feels" more like first and second gear
are just "further away" from each other than any other two sequential gears. Because this can lead to
difficulty in finding neutral for inexperienced riders most motorcycles have a neutral indicator light on
the instrument panel to help find neutral. The reason neutral does not actually have its own spot in the
sequence is to make it quicker to shift from first to second when moving. Neutral can be accidentally
shifted into, though most high end, newer model motorcycles have means of avoiding this. [citation
needed]
The reason for having neutral between the first and second gears instead of at the bottom is that
when stopped, the rider can just click down repeatedly and know that they will end up in first and not
neutral. This allows a rider to quickly move his bike from a standstill in an emergency situation. This
may also help on a steep hill on which high torque is required. It could be disadvantageous or even
dangerous to attempt to be in first without realizing it, then try for a lower gear, only to get neutral.
On motorcycles used on race tracks, the shifting pattern is often reversed, that is, the rider clicks down
to upshift. This usage pattern increases the ground clearance by placing the rider's foot above the shift
lever when the rider is most likely to need it, namely when leaning over and exiting a tight turn.
The shift pattern for most underbone motorcycles with an automatic centrifugal clutch is also modified
for two key reasons - to enable the less-experienced riders to shift the gears without problems of
"finding" neutral, and also due to the greater force needed to "lift" the gearshift lever (because the
gearshift pedal of an underbone motorcycle also operates the clutch). The gearshift lever of an
underbone motorcycle has two ends. The rider clicks down the front end with the left toe all the way to
the top gear and clicks down the rear end with the heel all the way down to neutral. Some underbone
models such as the Honda Wave have a "rotary" shift pattern, which means that the rider can shift
directly to neutral from the top gear, but for safety reasons this is only possible when the motorcycle is
stationary. Some models also have gear position indicators for all gear positions at the instrument
panel.
Semi-manual
Some new transmissions (Alfa Romeo's Selespeed gearbox and BMW's Sequential Manual
Gearbox(SMG) for example) are conventional manual transmissions with a computerized control
mechanism. These transmissions feature independently selectable gears but do not have
a clutch pedal. Instead, the transmission computer controls a servo which disengages the clutch when
necessary.
These transmissions vary from sequential transmissions in that they still allow nonsequential shifts:
BMWs SMG system, for example, can shift from 6th gear directly to 4th gear.
An early version of this type of transmission was the Autostick, which was used in the Volkswagen
Beetle and Karmann Ghia from 1967 to 1976, where the clutch was disengaged by servo when the
driver pushed downward slightly on the gear shift lever. This was a 3-speed unit.
In the case of the early second generation Saab 900, a 'Sensonic' option was available where gears
were shifted with a conventional shifter, but the clutch is controlled by a computer.
See semi-automatic transmission for more examples.
Benefits
Fuel
economy
The manual transmission couples the engine to the transmission with a rigid clutch instead of
thetorque converter on an automatic transmission or the v-belt of a continuously variable transmission,
[7]
which slip by nature. Manual transmissions also lack the parasitic power consumption of the
automatic transmission's hydraulic pump. Because of this, manual transmissions generally offer
betterfuel economy than automatic or continuously variable transmissions; however the disparity has
been somewhat offset with the introduction of locking torque converters on automatic transmissions.
[8]
Increased fuel economy with a properly operated manual transmission vehicle versus an equivalent
automatic transmission vehicle can range from 5% to about 15% depending on driving conditions and
style of driving.[9] The lack of control over downshifting under load in an automatic transmission,
coupled with a typical vehicle engine's greater efficiency under higher load, can enable additional fuel
gains from a manual transmission by allowing the operator to keep the engine performing under a
more efficient load/RPM combination. This is especially true for older models, as advances like
variable valve timing allow better performance over a broader RPM range. In recognition of this, many
current models (2010 and on) come with manual modes, or overrides on automatic models, although
the degree of control varies greatly by the manufacturer. Also, manual transmissions do not require
active cooling and because they are, mechanically, much simpler than automatic transmissions, they
generally weigh less than comparable automatics, which can improve economy in stop-and-go traffic.
[8]
However this gap in economy is being rapidly being closed, and many mid to higher end model
automatic cars now get better economy than their standard spec counterparts [citation needed]. This is in part
due to the increasing impact of computers co-ordinating multiple systems, particularly in hydrid models
in which the engine and drive motors must be managed, a feat impossible with standard
transmissions.
Longevity
and cost
Because manual transmissions are mechanically simpler, are more easily manufactured, and have
fewer moving parts than automatic transmissions, they require less maintenance and are easier to
repair. Typically, there are no electrical components, pumps and cooling mechanisms (in the manual
transmission), other than an internal switch to activate reversing lighting. Manual transmissions are far
less popular in the United States than in other world markets. The price of a new car with a manual
transmission will commonly be lower than the same car with an automatic transmission.
Lubrication
Most manual transmissions rely on splash lubrication although some five speed Rover gearboxes did
incorporate an oil pump. The problem with splash lubrication is that it is speed dependent. There are
centrifugal effects, hydrodynamic effects and effects from the gears working as pumps. If a gearbox is
fitted with Perspex windows and run on a test rig these effects can be observed. As the gearbox is run
through its rev range, the oil jets will switch over and move around. Research on the Austin Maxi 1500
gearbox showed that one of the ball races was running dry at 80 miles per hour (130 km/h), the speed
that much of the United Kingdom's motorway traffic runs at. The solution was to alter the casting to
include a small projection that would intercept the main oil jet that was present at 80 mph and disperse
it. This small modification enabled the later Maxi 1750 gearbox to be relatively trouble free. Four speed
gearboxes seldom show these problems because at top speed (and maximum power) they are
basically a solid shaft and the gears are not transmitting power.
Performance
and control
Manual transmissions generally offer a wider selection of gear ratios. Many vehicles offer a 5-speed or
6-speed manual, whereas the automatic option would typically be a 4-speed. This is generally due to
the increased space available inside a manual transmission compared with an automatic, since the
latter requires extra components for self-shifting, such as torque converters and pumps. However,
automatic transmissions are now adding more speeds as the technology matures. ZF currently makes
7- and 8-speed automatic transmissions. The increased number gears allows for better use of the
engine's power band, allowing increased fuel economy, by staying in the most fuel-efficient part of the
power band, or higher performance, by staying closer to the engine's peak power. However, a manual
transmission has more space to put in more speeds, as the 991 Generation of the Porsche 911 has a
7- speed manual transmission, which is a first for a production vehicle.
Engine
braking
In contrast to most manual gearboxes, most automatic transmissions have far less effective engine
braking. This means that the engine does not slow the car as effectively when the automatic
transmission driver releases the engine speed control. This leads to more usage of the brakes in cars
with automatic transmissions, bringing shorter brake life. Brakes are also more likely to overheat in
hilly or mountainous areas, causing reduced braking ability brake fade and the potential for complete
failure with the automatic transmission vehicle.
Drawbacks
Complexity
and learning curve
For most people, there is a slight learning curve with a manual transmission, which may be intimidating
and unappealing for an inexperienced driver. Because the driver must develop a feel for properly
engaging the clutch, an inexperienced driver will often stall the engine. Most drivers can learn how to
drive a car with a manual transmission in as little as an hour, although it may take weeks before it
becomes "second nature." Additionally, if an inexperienced driver selects the wrong gear by mistake,
damage to mechanical components and even loss of control may occur.
Shifting
speed
Some automatic transmissions can shift ratios faster than a manual gear change can be
accomplished, due to the time required for the average driver to push the clutch pedal to the floor and
move the gearstick from one position to another. This is especially true in regards to dual clutch
transmissions, which are specialized computer-controlled manual transmissions. Even though some
automatic transmissions and semi-automatic transmissions can shift faster, many purists still prefer a
regular manual transmission.
Ease
of use
Because manual transmissions require the operation of an extra pedal, and keeping the car in the
correct gear at all times, they require a bit more concentration, especially in heavy traffic situations.
The automatic transmissions, on the other hand, simply require the driver to speed up or slow down as
needed, with the car doing the work of choosing the correct gear. Manual transmissions also place a
greater workload on the driver in heavy traffic situations, when the driver must operate the clutch pedal
quite often. Because the clutch pedal can require a substantial amount of force, especially on large
trucks, and the long pedal travel compared to the brake or accelerator requires moving the entire leg,
not just the foot near the ankle, a manual transmission can cause fatigue, and is more difficult for weak
or injured people to drive. Additionally, because automatic transmissions can be driven with only one
foot, people with one leg that is missing or impaired can still drive, unlike the manual transmission that
requires the use of two feet at once. Likewise, manual transmissions require the driver to remove one
hand periodically from the steering wheel while the vehicle is in motion, which can be difficult or
impossible to do safely for people with a missing or impaired arm, and requires increased coordination,
even for those with full use of both hands.
Stopping
on hills
The clutch experiences most of its wear in first gear because moving the car from a standstill involves
a great deal of friction at the clutch. When accelerating from a standstill on an incline, this problem is
made worse because the amount of work needed to overcome the acceleration of gravity causes the
clutch to heat up considerably more. For this reason, stop-and-go driving and hills tend to have an
effect on the clutches to a certain degree. Automatic transmissions are better suited for these
applications because they have a hydraulic torque converter which is externally cooled, unlike a clutch.
Torque converters also do not have a friction material that rubs off over time like a clutch. Some
automatics even lock the output shaft so that the car cannot roll backwards when beginning to
accelerate up an incline. To reduce wear in these applications, some manual transmissions will have a
very low, "granny" gear which provides the leverage to move the vehicle easily at very low speeds.
This reduces wear at the clutch because the transmission requires less input torque. However the
issue ofhandling stops on hills is easy to learn, and due to the driving of common cars (unlike trucks
and lorries) of low importance.
Many drivers use the parking brake to prevent the car from rolling backward when starting to
accelerate up a steep hill. This saves precious clutch life. Many modern cars have "hill assist" features.
The vehicle's computer applies just enough brake pressure to prevent the car from rolling backwards.
This allows the driver to start normally with no additional effort, even on steep hills.
Applications
and popularity
Many types of automobiles are equipped with manual transmissions.
Sports cars are also often equipped with manual transmissions because they offer more direct driver
involvement and better performance. Off-road vehicles and trucks often feature manual transmissions
because they allow direct gear selection and are often more rugged than their automatic counterparts.
Conversely, manual transmissions are no longer popular in many classes of cars sold in North
America, Australia and some parts of Asia, although they remain dominant in Europe, Asia and
developing countries. Nearly all cars are available with an automatic transmission option, and family
cars and large trucks sold in the US are predominantly fitted with automatics, however in some cases if
a buyer wishes he/she can have the car fitted with a manual transmission at the factory. In Europe
most cars are sold with manual transmissions. Most luxury cars are only available with an automatic
transmission. In most cases where both transmissions are available for a given car, automatics are an
at cost option, but in some cases the reverse is true. Some cars, such as rental cars and taxis, are
nearly universally equipped with automatic transmissions in countries such as the US, but the opposite
is true in Europe.[10] As of 2008, 75.2% of vehicles made in Western Europe were equipped with
manual transmission, versus 16.1% with automatic and 8.7% with other.[11]
In some places (for example New Zealand (for the second-phase Restricted licence, but not the final
Full licence), Belgium, China, Estonia, Dominican
Republic, Finland, France, Germany, Ireland, Israel,Jordan, Netherlands, Norway, Philippines, Poland,
Singapore, Slovenia, South Africa, South Korea,Spain, Sri Lanka, Sweden, Turkey, U.A.E and
the UK), when a driver takes the licensing road test using an automatic transmission, the resulting
license is restricted to the use of automatic transmissions. This treatment of the manual transmission
skill seems to maintain the widespread use of the manual transmission. As many new drivers worry
that their restricted licence will become an obstacle for them where most cars have manual
transmissions, they make the effort to learn with manual transmissions and obtain full licences. Some
other countries (such
as Greece, India, Italy,Pakistan, Malaysia, Serbia, Brazil, Russia, Ukraine and Denmark) go even
further, whereby the licence is granted only when a test is passed on a manual transmission. In
Denmark and Brazil drivers are allowed to take the test on an automatic if they are handicapped, but
with such a licence they will not be allowed to drive a car with a manual transmission.
Truck
transmissions
these issues on the talk page.
This section does not cite any references or sources.(February 2009)
This section may require cleanup to meet Wikipedia'squality
standards. (February 2009)
This section may contain original research. (February 2009)
Some trucks have transmissions that look and behave like ordinary car transmissions - these
transmissions are used on lighter trucks, typically have up to 6 gears, and usually have synchromesh.
For trucks needing more gears, the standard "H" pattern can get very complicated, so additional
controls are used to select additional gears. The "H" pattern is retained, then an additional control
selects among alternatives. In older trucks, the control is often a separate lever mounted on the floor
or more recently a pneumatic switch mounted on the "H" lever; in newer trucks the control is often an
electrical switch mounted on the "H" lever. Multi-control transmissions are built in much higher power
ratings, but rarely use synchromesh.
There are several common alternatives for the shifting pattern. Usual types are:
Range transmissions use an "H" pattern through a narrow range of gears, then a "range"

control shifts the "H" pattern between high and low ranges. For example, an 8-speed range
transmission has an H shift pattern with four gears. The first through fourth gears are accessed
when low range is selected. To access the fifth through eighth gears, the range selector is moved
to high range, and the gear lever again shifted through the first through fourth gear positions. In
high range, the first gear position becomes fifth, the second gear position becomes sixth, and so
on.
Splitter transmissions use an "H" pattern with a wide range of gears, and the other selector

splits each sequential gear position in two: First gear is in first position/low split, second gear is in
first position/high split, third gear is in second position/low split, fourth gear is in second
position/high split, and so on.
Range-Splitter transmissions combine range-splitting and gear-splitting. This allows even

more gear ratios. Both a range selector and a splitter selector are provided.
Although there are many gear positions, shifting through gears usually follows a regular pattern. For
example, a series of upshifts might use "move to splitter direct; move to splitter overdrive; move shift
lever to No. 2 and move splitter to underdrive; move splitter to direct; move splitter to overdrive; move
shift lever to No. 3 and move splitter to underdrive"; and so on. In older trucks using floor-mounted
levers, a bigger problem is common gear shifts require the drivers to move their hands between shift
levers in a single shift, and without synchromesh, shifts must be carefully timed or the transmission will
not engage. For this reason, some splitter transmissions have an additional "under under" range, so
when the splitter is already in "under" it can be quickly downshifted again, without the delay of a double
shift.
Today's truck transmissions are most commonly "range-splitter". The most common 13-speed has a
standard H pattern, and the pattern from left upper corner is as follows: R, down to L, over and up to 1,
down to 2, up and over to 3, down to 4. The "butterfly" range lever in the center front of the knob is
flipped up to high range while in 4th, then shifted back to 1. The 1 through 4 positions of the knob are
repeated. Also, each can be split using the thumb-actuated under-overdrive lever on the left side of the
knob while in high range. The "thumb" lever is not available in low range, except in 18 speeds; 1
through 4 in low range can be split using the thumb lever and L can be split with the "Butterfly" lever. L
cannot be split using the thumb lever in either the 13- or 18-speed. The 9-speed transmission is
basically a 13-speed without the under-overdrive thumb lever.
Truck transmissions use many physical layouts. For example, the output of an N-speed transmission
may drive an M-speed secondary transmission, giving a total of N*M gear combinations; for example a
4-speed main box and 3-speed splitter gives 12 ratios. Transmissions may be in separate cases with a
shaft in between; in separate cases bolted together; or all in one case, using the same lubricating oil.
The second transmission is often called a "Brownie" or "Brownie box" after a popular brand. With a
third transmission, gears are multiplied yet again, giving greater range or closer spacing. Some trucks
thus have dozens of gear positions, although most are duplicates. Sometimes a secondary
transmission is integrated with the differential in the rear axle, called a "two-speed rear end." Twospeed differentials are always splitters. In newer transmissions, there may be two countershafts, so
each main shaft gear can be driven from one or the other countershaft; this allows construction with
short and robust countershafts, while still allowing many gear combinations inside a single gear case.
Heavy-duty transmissions are almost always non-synchromesh. One argument is synchromesh adds
weight that could be payload, is one more thing to fail, and drivers spend thousands of hours driving so
can take the time to learn to drive efficiently with a non-synchromesh transmission. Heavy-duty trucks
driven frequently in city traffic, such as cement mixers, need to be shifted very often and in stop-andgo traffic. Since few heavy-duty transmissions have synchromesh, automatic transmissions are
commonly used instead, despite their increased weight, cost, and loss of efficiency.
Heavy trucks are usually powered with diesel engines. Diesel truck engines from the 1970s and earlier
tend to have a narrow power band, so need many close-spaced gears. Starting with the
1968Maxidyne, diesel truck engines have increasingly used turbochargers and electronic controls that
widen the power band, allowing fewer and fewer gear ratios. A transmission with fewer ratios is lighter
and may be more efficient due to fewer transmissions in series. Fewer shifts also makes the truck
more drivable. As of 2005, fleet operators often use 9,10,13 or 18-speed transmissions, but automated
manual and semi-automatic transmissions are becoming more common on heavy vehicles, as they
can improve efficiency and drivability, reduce the barrier to entry for new drivers, and may improve
safety by allowing the driver to concentrate on road conditions.
Maintenance
Because clutches use changes in friction to modulate the transfer of torque between engine and
transmission, they are subject to wear in everyday use. A very good clutch, when used by an expert
driver, can last hundreds of thousands of kilometres (or miles). Weak clutches, abrupt downshifting,
inexperienced drivers, and aggressive driving can lead to more frequent repair or replacement.
Manual transmissions are lubricated with gear oil or engine oil in some cars, which must be changed
periodically in some cars, although not as frequently as the automatic transmission fluid in a vehicle so
equipped. (Some manufacturers specify that changing the gear oil is never necessary except after
transmission work or to rectify a leak.)
Gear oil has a characteristic aroma due to the addition of sulfur-bearing anti-wear compounds. These
compounds are used to reduce the high sliding friction by the helical gear cut of the teeth (this cut
eliminates the characteristic whine of straight cut spur gears). On motorcycles with "wet" clutches
(clutch is bathed in engine oil), there is usually nothing separating the lower part of the engine from the
transmission, so the same oil lubricates both the engine and transmission. The original Mini placed the
gearbox in the oil sump below the engine, thus using the same oil for both. The clutch was however a
fairly conventional dry plate clutch.

Reverse

Neutral

First gear

Second gear

Third gear

Fourth gear
See
also
Wikimedia Commons has
media related to: Manual
transmission

Automatic transmission

Diesel-electric transmission

Freewheel

Gear ratio

Hydraulic transmission

Transmission (mechanics)

Borg-Warner T-56

Non-synchronous transmission

Overdrive

Preselector gearbox

References
1.
^ "Buyers Guide Alfa Romeo Spider & GTV 916". Alfisti.net. Retrieved 2010-10-16.
2.
^ "Synchronizers; graphic illustration of how they work". Retrieved 2007-07-18.
3.
^ "Porsche 911: The latest intel including renderings!". Germancarblog.com. 2011-07-04.
Retrieved 2011-09-01.
4.
^ "2012 Porsche 911 will feature a 7-speed manual transmission". Worldcarfans.com.
Retrieved 2011-09-01.
5.
^ "2007 Tesla roadster". Supercars.net. 2006-07-19. Retrieved 2011-09-01.
6.
^ "The Borg-Warner Overdrive Transmission Explained". FORDification.com. Retrieved
2012-04-22.
7.
^ An Investigation into The Loss Mechanisms associated with a Pushing Metal V-Belt
Continuously Variable Transmission, Sam Akehurst, 2001, PhD Thesis, University of Bath.
8.
^ a b "U.S. Department of Energy vehicle fuel economy website". Fueleconomy.gov.
Retrieved 2010-10-16.
9.
^ An Overview of Current Automatic, Manual and Continuously Variable Transmission
Efficiencies and Their Projected Future Improvements, Kluger and Long, SAE 1999-01-1259
10.
^ "Rick Steve's Europe: Driving in Europe". Ricksteves.com. Retrieved 2010-10-16.
11.
^ "Why Dual Clutch Technology Will Be Big Business". Dctfacts.com. Retrieved 2010-0207.
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n a ground vehicle with asuspension, the unsprung weight (or the unsprung mass) is the mass
of the suspension, wheels or tracks(as applicable), and other components directly connected to
them, rather than supported by the suspension. (The mass of the body and other components
supported by the suspension is the sprung mass.) Unsprung weight includes the mass of
components such as the wheel axles, wheel bearings, wheel hubs, tires, and a portion of the
weight of driveshafts, springs, shock absorbers, and suspension links. Even if the
vehicle's brakes are mounted outboard (i.e., within the wheel), their weight is still considered part
of the unsprung weight.
Effects
of unsprung weight
The unsprung weight of a wheel controls a trade-off between a wheel's bump-following ability and
its vibration isolation. Bumps and surface imperfections in the road cause tire compression—
which induces a force on the unsprung weight. The unsprung weight then responds to this force
with movement of its own. The amount of movement, for short bumps, is inversely proportional to
the weight - a lighter wheel which readily moves in response to road bumps will have more grip
and more constant grip when tracking over an imperfect road. For this reason, lighter wheels are
sought especially for high-performance applications. In contrast, a heavier wheel which moves
less will not absorb as much vibration; the irregularities of the road surface will transfer to the
cabin through the geometry of the suspension and hence ride quality and road noise are
deteriorated. For longer bumps that the wheels follow, greater unsprung mass causes more
energy to be absorbed by the wheels and makes the ride worse.
Pneumatic or elastic tires help by providing some springing for most of the (otherwise) unsprung
mass, but the damping that can be included in the tires is limited by considerations of fuel
economy and overheating. The shock absorbers, if any, damp the spring motion also and must
be less stiff than would optimally damp the wheel bounce. So the wheels execute some vibrations
after each bump before coming to rest. On dirt roads and perhaps on some softly paved roads,
these motions form small bumps, known as corrugations, washboarding or "corduroy" because
they resemble smaller versions of the bumps in roads made of logs. These cause sustained
wheel bounce in subsequent vehicles, enlarging the bumps.
High unsprung weight also exacerbates wheel control issues under hard acceleration or braking.
If the vehicle does not have adequate wheel location in the vertical plane (such as a rear-wheel
drive car withHotchkiss drive, a live axle supported by simple leaf springs), vertical forces exerted
by acceleration or hard braking combined with high unsprung mass can lead to severe wheel
hop, compromising traction and steering control.
As mentioned above, there is a positive effect of unsprung mass. High frequency road
irregularities, such as the gravel in an asphalt or concrete road surface, are isolated from the
body more completely because the tires and springs act as separate filter stages, with the
unsprung weight tending to uncouple them. Likewise, sound and vibration isolation is improved
(at the expense of handling), in production automobiles, by the use of rubber bushings between
the frame and suspension, by any flexibility in the frame or body work, and by the flexibility of the
seats.
Unsprung
weight and vehicle design
Unsprung weight is largely a function of the design of a vehicle's suspension and the materials
used in the construction of suspension components. Beam axle suspensions, in which wheels on
opposite sides are connected as a rigid unit, generally have greater unsprung weight
than independent suspension systems, in which the wheels are suspended and allowed to move
separately. Heavy components such as the differential can be made part of the sprung weight by
connecting them directly to the body (as in a de Dion tube rear suspension). Lightweight
materials, such as aluminum,plastic, carbon fiber, and/or hollow components can provide further
weight reductions at the expense of greater cost and/or fragility.
Inboard brakes can significantly reduce unsprung weight, but put more load on half axles and
(constant velocity) universal joints, and require space that may not be easily accommodated. If
located next to a differential or transaxle, waste heat from the brakes may overheat the
differential or vice versa, particularly in hard use, such as motor racing. They also make anti-dive
suspension characteristics harder to achieve as the moment created by the act of braking is not
reacted on the suspension arms. Jaguar's patented independent rear suspension (IRS) further
reduced unsprung weight by replacing the upper wishbone arms of the suspension with the drive
shafts as well as mounting the brakes inboard in some versions.
Scooter-type motorcycles use an integrated engine-gearbox-final drive system that pivots as part
of the rear suspension and hence is partly unsprung. This arrangement is linked to the use of
quite small wheels, further impacting the reputation for road-holding.
rom Wikipedia, the free encyclopedia
Unsourced material may be challenged and removed. (December 2009)
In a vehicle with a suspension, such as an automobile, motorcycle or a tank, sprung
mass (or sprung weight) is the portion of the vehicle's total mass that is supported above the
suspension, including in most applications approximately half of the weight of the suspension
itself. The sprung weight typically includes the body, frame, the internal components, passengers,
and cargo, but does not include the mass of the components suspended below the suspension
components (including the wheels, wheel bearings, brake rotors, calipers, and/or Continuous
tracks (Also called caterpillar tracks), if any), which are part of the vehicle's unsprung weight.
The larger the ratio of sprung weight to unsprung weight, the less the body and vehicle occupants
are affected by bumps, dips, and other surface imperfections such as small bridges. However, a
large sprung weight to unsprung weight ratio can also be deleterious to vehicle control. [citation needed]
A brake is a mechanical device which inhibits motion. Its opposite component is a clutch. The
Most commonly brakes use friction to convert kinetic energy into heat, though other methods of
energy conversion may be employed. For example regenerative braking converts much of the
energy to electrical energy, which may be stored for later use. Other methods convert kinetic
energy into potential energy in such stored forms as pressurized air or pressurized oil.Eddy
current brakes use magnetic fields to convert kinetic energy into electric current in the brake disc,
fin, or rail, which is converted into heat. Still other braking methods even transform kinetic
energy into different forms, for example by transferring the energy to a rotating flywheel.
Brakes are generally applied to rotating axles or wheels, but may also take other forms such as
the surface of a moving fluid (flaps deployed into water or air). Some vehicles use a combination
of braking mechanisms, such as drag racing cars with both wheel brakes and a parachute, or
airplanes with both wheel brakes and drag flaps raised into the air during landing.
Since kinetic energy increases quadratically with velocity (
), an object moving at
10 m/s has 100 times as much energy as one of the same mass moving at 1 m/s, and
consequently the theoretical braking distance, when braking at the traction limit, is 100 times as
long. In practice, fast vehicles usually have significant air drag, and energy lost to air drag rises
quickly with speed.
Almost all wheeled vehicles have a brake of some sort. Even baggage carts and shopping
carts may have them for use on a moving ramp. Most fixed-wing aircraft are fitted with wheel
brakes on theundercarriage. Some aircraft also feature air brakes designed to reduce their speed
in flight. Notable examples include gliders and some World War II-era aircraft, primarily
some fighter aircraft and manydive bombers of the era. These allow the aircraft to maintain a safe
speed in a steep descent. TheSaab B 17 dive bomber used the deployed undercarriage as an air
brake.
Friction brakes on automobiles store braking heat in the drum brake or disc brake while braking
then conduct it to the air gradually. When traveling downhill some vehicles can use their engines
to brake.
When the brake pedal of a modern vehicle with hydraulic brakes is pushed, ultimately
a piston pushes the brake pad against the brake disc which slows the wheel down. On the brake
drum it is similar as the cylinder pushes the brake shoes against the drum which also slows the
wheel down.
Contents
[hide]
1 Types
2 Characteristics
o
2.1 Brake boost
3 Noise
4 Inefficiency
6 References
Types
Brakes may be broadly described as using friction, pumping, or electromagnetics. One brake may
use several principles: for example, a pump may pass fluid through an orifice to create friction:

Frictional brakes are most common and can be divided broadly into "shoe" or "pad"
brakes, using an explicit wear surface, and hydrodynamic brakes, such as parachutes, which
use friction in a working fluid and do not explicitly wear.Typically the term "friction brake" is
used to mean pad/shoe brakes and excludes hydrodynamic brakes, even though
hydrodynamic brakes use friction.
Friction (pad/shoe) brakes are often rotating devices with a stationary pad and a rotating
wear surface. Common configurations include shoes that contract to rub on the outside of a
rotating drum, such as a band brake; a rotating drum with shoes that expand to rub the inside
of a drum, commonly called a "drum brake", although other drum configurations are possible;
and pads that pinch a rotating disc, commonly called a "disc brake". Other brake
configurations are used, but less often. For example, PCC trolley brakes include a flat shoe
which is clamped to the rail with an electromagnet; the Murphy brake pinches a rotating
drum, and the Ausco Lambert disc brake uses a hollow disc (two parallel discs with a
structural bridge) with shoes that sit between the disc surfaces and expand laterally.

Pumping brakes are often used where a pump is already part of the machinery. For
example, an internal-combustion piston motor can have the fuel supply stopped, and then
internal pumping losses of the engine create some braking. Some engines use a valve
override called a Jake braketo greatly increase pumping losses. Pumping brakes can dump
energy as heat, or can be regenerative brakes that recharge a pressure reservoir called
a hydraulic accumulator.

Electromagnetic brakes are likewise often used where an electric motor is already part
of the machinery. For example, many hybrid gasoline/electric vehicles use the electric motor
as a generator to charge electric batteries and also as a regenerative brake. Some
diesel/electric railroad locomotives use the electric motors to generate electricity which is
then sent to a resistor bank and dumped as heat. Some vehicles, such as some transit
buses, do not already have an electric motor but use a secondary "retarder" brake that is
effectively a generator with an internal short-circuit. Related types of such a brake are eddy
current brakes, and electro-mechanical brakes (which actually are magnetically driven friction
brakes, but nowadays are often just called “electromagnetic brakes” as well).
Characteristics
Brakes are often described according to several characteristics including:

Peak force – The peak force is the maximum decelerating effect that can be obtained.
The peak force is often greater than the traction limit of the tires, in which case the brake can
cause a wheel skid.

Continuous power dissipation – Brakes typically get hot in use, and fail when the
temperature gets too high. The greatest amount of power (energy per unit time) that can be
dissipated through the brake without failure is the continuous power dissipation. Continuous
power dissipation often depends on e.g., the temperature and speed of ambient cooling air.








Fade – As a brake heats, it may become less effective, called brake fade. Some designs
are inherently prone to fade, while other designs are relatively immune. Further, use
considerations, such as cooling, often have a big effect on fade.
Smoothness – A brake that is grabby, pulses, has chatter, or otherwise exerts varying
brake force may lead to skids. For example, railroad wheels have little traction, and friction
brakes without an anti-skid mechanism often lead to skids, which increases maintenance
costs and leads to a "thump thump" feeling for riders inside.
Power – Brakes are often described as "powerful" when a small human application force
leads to a braking force that is higher than typical for other brakes in the same class. This
notion of "powerful" does not relate to continuous power dissipation, and may be confusing in
that a brake may be "powerful" and brake strongly with a gentle brake application, yet have
lower (worse) peak force than a less "powerful" brake.
Pedal feel – Brake pedal feel encompasses subjective perception of brake power output
as a function of pedal travel. Pedal travel is influenced by the fluid displacement of the brake
and other factors.
Drag – Brakes have varied amount of drag in the off-brake condition depending on
design of the system to accommodate total system compliance and deformation that exists
under braking with ability to retract friction material from the rubbing surface in the off-brake
condition.
Durability – Friction brakes have wear surfaces that must be renewed periodically. Wear
surfaces include the brake shoes or pads, and also the brake disc or drum. There may be
tradeoffs, for example a wear surface that generates high peak force may also wear quickly.
Weight – Brakes are often "added weight" in that they serve no other function. Further,
brakes are often mounted on wheels, and unsprung weight can significantly hurt traction in
some circumstances. "Weight" may mean the brake itself, or may include additional support
structure.
Noise – Brakes usually create some minor noise when applied, but often create squeal
or grinding noises that are quite loud.
Brake
boost
Most modern vehicles use a vacuum assisted brake system that greatly increases the force
applied to the vehicle's brakes by its operator.[1] This additional force is supplied by the manifold
vacuumgenerated by air flow being obstructed by the throttle on a running engine. This force is
greatly reduced when the engine is running at fully open throttle, as the difference between
ambient air pressure and manifold (absolute) air pressure is reduced, and therefore available
vacuum is diminished. However, brakes are rarely applied at full throttle; the driver takes the right
foot off the gas pedal and moves it to the brake pedal - unless left-foot braking is used.
Because of low vacuum at high RPM, reports of unintended acceleration are often accompanied
by complaints of failed or weakened brakes, as the high-revving engine, having an open throttle,
is unable to provide enough vacuum to power the brake booster. This problem is exacerbated in
vehicles equipped with automatic transmissions as the vehicle will automatically downshift upon
application of the brakes, thereby increasing the torque delivered to the driven-wheels in contact
Noise
Brake lever on a horse-drawn hearse
Although ideally a brake would convert all the kinetic energy into heat, in practice a significant
amount may be converted into acoustic energy instead, contributing tonoise pollution.
For road vehicles, the noise produced varies significantly with tire construction, road surface, and
the magnitude of the deceleration.[2] Noise can be caused by different things. These are signs that
there may be issues with brakes wearing out over time.
Inefficiency
A significant amount of energy is always lost while braking, even with regenerative braking which
is not perfectly efficient. Therefore a good metric of efficient energy use while driving is to note
how much one is braking. If the majority of deceleration is from unavoidable friction instead of
braking, one is squeezing out most of the service from the vehicle. Minimizing brake use is one of
the fuel economy-maximizing behaviors.
While energy is always lost during a brake event, a secondary factor that influences efficiency is
"off-brake drag", or drag that occurs when the brake is not intentionally actuated. After a braking
event, hydraulic pressure drops in the system, allowing the brake caliper pistons to retract.
However, this retraction must accommodate all compliance in the system (under pressure) as
well as thermal distortion of components like the brake disc or the brake system will drag until the
contact with the disc, for example, knocks the pads and pistons back from the rubbing surface.
During this time, there can be significant brake drag. This brake drag can lead to significant
parasitic power loss, thus impact fuel economy and vehicle performance.
See
also
A drum brake is a brake in which the friction is caused by a set of shoes or pads that press
against a rotating drum-shaped part called a brake drum.
The term "drum brake" usually means a brake in which shoes press on the inner surface of the
drum. When shoes press on the outside of the drum, it is usually called aclasp brake. Where the
drum is pinched between two shoes, similar to a conventional disk brake, it is sometimes called a
"pinch drum brake", although such brakes are relatively rare. A related type of brake uses a
flexible belt or "band" wrapping around the outside of a drum, called aband brake.
Contents
[hide]
1 History
2 Components
o
2.1 Back plate
o
2.2 Brake drum
o
2.3 Wheel cylinder
o
2.4 Brake shoe
o
3 In operation
o
3.1 Normal braking
o
o
3.3 Emergency brake
4 Self-applying characteristic
5 Drum brake designs
7 As a tailshaft parking/emergency brake
9 Re-arcing
10 Use in music
12 References
History
The modern automobile drum brake was invented in 1902 by Louis Renault, whose unique
genius inspired him to use woven asbestos lining for the drum brakes lining as there were no
other alternatives that dissipated heat like the asbestos lining, though a less-sophisticated drum
brake had been used by Maybach a year earlier. In the first drum brakes, the shoes were
mechanically operated with levers and rods or cables. From the mid-1930s the shoes were
operated with oil pressure in a small wheel cylinder and pistons (as in the picture), though some
vehicles continued with purely-mechanical systems for decades. Some designs have two
wheel cylinders.
The shoes in drum brakes are subject to wear and the brakes needed to be adjusted regularly
until the introduction of self-adjusting drum brakes in the 1950s. In the 1960s and 1970s brake
drums on the front wheels of cars were gradually replaced with disc brakes and now practically all
cars use disc brakes on the front wheels, with many offering disc brakes on all wheels. However,
drum brakes are still often used for handbrakes as it has proven very difficult to design a disc
brake suitable for holding a car when it is not in use. Moreover, it is very easy to fit a drum
handbrake inside a disc brake so that one unit serves as both service brake and handbrake.
Early type brake shoes contained asbestos. When working on brake systems of older cars, care
must be taken not to inhale any dust present in the brake assembly. The United States Federal
Government began to regulate asbestos production, and brake manufacturers had to switch to
non-asbestos linings. Owners initially complained of poor braking with the replacements;
however, technology eventually advanced to compensate. A majority of daily-driven older
vehicles have been fitted with asbestos-free linings. Many other countries also limit the use of
asbestos in brakes.
Components
Some of the major components of the drum brake assembly are the back plate, the brake drum
and shoe, the wheel cylinder, and various springs and pins.
Back
plate
The back plate serves as the base on which all the components are assembled. It attaches to the
axle and forms a solid surface for the wheel cylinder, brake shoes and assorted hardware. Since
all the braking operations exert pressure on the back plate, it needs to be very strong and wearresistant. Levers for emergency or parking brakes, and automatic brake-shoe adjuster were also
Back plate made in the pressing shop.
Brake
drum
The brake drum is generally made of a special type of cast iron which is heat-conductive and
wear-resistant. It is positioned very close to the brake shoe without actually touching it, and
rotates with the wheel and axle. As the lining is pushed against the inner surface of the drum,
friction heat can reach as high as 600 °F (316 °C).
Wheel
cylinder
One wheel cylinder is used for each wheel. Two pistons operate the shoes, one at each end of
the wheel cylinder. When hydraulic pressure from the master cylinder acts upon the piston cup,
the pistons are pushed toward the shoes, forcing them against the drum. When the brakes are
not being applied, the piston is returned to its original position by the force of the brake shoe
return springs. The parts of the wheel cylinder are as follows:
Cut-away section of a wheel cylinder.
Brake
shoe
Brake shoes are typically made of two pieces of sheet steel welded together. The friction material
is either rivetted to the lining table or attached with adhesive. The crescent-shaped piece is called
the Web and contains holes and slots in different shapes for return springs, hold-down hardware,
parking brake linkage and self-adjusting components. All the application force of the wheel
cylinder is applied through the web to the lining table and brake lining. The edge of the lining table
generally has three “V"-shaped notches or tabs on each side called nibs. The nibs rest against
the support pads of the backing plate to which the shoes are installed. Each brake assembly has
two shoes, a primary and secondary. The primary shoe is located toward the front of the vehicle
and has the lining positioned differently than the secondary shoe. Quite often the two shoes are
interchangeable, so close inspection for any variation is important.
Brake shoe assembly
Linings must be resistant against heat and wear and have a high friction coefficient unaffected by
fluctuations in temperature and humidity. Materials which make up the brake shoe include, friction
modifiers (which can include can include graphite and cashew nut shells), powdered metal such
as lead, zinc, brass, aluminium and other metals that resist heat fade, binders, curing agents and
fillers such as rubber chips to reduce brake noise.
Automatic
The self-adjuster is used to adjust the distance between the brake shoe and the drum
automatically as brake shoes wear.
Sectional layout showing the push rods, nut adjuster and lever pawl.
In
operation
Normal
braking
When the brakes are applied, brake fluid is forced under pressure from the master cylinder into
the wheel cylinder, which in turn pushes the brake shoes into contact with the machined surface
on the inside of the drum. This rubbing action reduces the rotation of the brake drum, which is
coupled to the wheel. Hence the speed of the vehicle is reduced. When the pressure is released,
return springs pull the shoes back to their rest position.
Automatic
As the brake linings wear, the shoes must travel a greater distance to reach the drum. When the
distance reaches a certain point, a self-adjusting mechanism automatically reacts by adjusting the
rest position of the shoes so that they are closer to the drum. Here, the adjusting lever rocks
that it unscrews a little bit when it turns, lengthening to fill in the gap. When the brake shoes wear
a little more, the adjuster can advance again, so it always keeps the shoes close to the drum.
Emergency
brake
The parking brake (emergency brake) system controls the brakes through a series of steel cables
that are connected to either a hand lever or a foot pedal. The idea is that the system is fully
mechanical and completely bypasses the hydraulic system so that the vehicle can be brought to a
stop even if there is a total brake failure. Here the cable pulls on a lever mounted in the brake and
is directly connected to the brake shoes. This has the effect of bypassing the wheel cylinder and
controlling the brakes directly.
Self-applying
characteristic
Drum brakes have a natural "self-applying" characteristic, better known as "selfenergizing." [1] The rotation of the drum can drag either one or both of the shoes into the friction
surface, causing the brakes to bite harder, which increases the force holding them together. This
increases the stopping power without any additional effort being expended by the driver, but it
does make it harder for the driver to modulate the brake's sensitivity. It also makes the brake
more sensitive to brake fade, as a decrease in brake friction also reduces the amount of brake
assist.
Disc brakes exhibit no self-applying effect because the hydraulic pressure acting on the pads is
perpendicular to the direction of rotation of the disc. [1] Disc brake systems usually have servo
assistance ("Brake Booster") to lessen the driver's pedal effort, but some disc braked cars
(notably race cars) and smaller brakes for motorcycles, etc., do not need to use servos. [1]
Note: In most designs, the "self applying" effect only occurs on one shoe. While this shoe is
further forced into the drum surface by a moment due to friction, the opposite effect is happening
on the other shoe. The friction force is trying to rotate it away from the drum. The forces are
different on each brake shoe resulting in one shoe wearing faster. It is possible to design a twoshoe drum brake where both shoes are self-applying (having separate actuators and pivoted at
opposite ends), but these are very uncommon in practice.
Drum
brake designs
Rendering of a drum brake
Rear drum brakes are typically of a leading/trailing design (for non-servo systems), or
primary/secondary (for duo servo systems) the shoes being moved by a single doubleacting hydraulic cylinder and hinged at the same point.[1] In this design, one of the brake shoes
will always experience the self-applying effect, irrespective of whether the vehicle is moving
forwards or backwards.[1] This is particularly useful on the rear brakes, where the parking brake
(handbrake or footbrake) must exert enough force to stop the vehicle from travelling backwards
and hold it on a slope. Provided the contact area of the brake shoes is large enough, which isn't
always the case, the self-applying effect can securely hold a vehicle when the weight is
transferred to the rear brakes due to the incline of a slope or the reverse direction of motion. A
further advantage of using a single hydraulic cylinder on the rear is that the opposite pivot may be
made in the form of a double-lobed cam that is rotated by the action of the parking brake system.
Front drum brakes may be of either design in practice, but the twin leading design is more
effective.[1]This design uses two actuating cylinders arranged so that both shoes will utilize the
self-applying characteristic when the vehicle is moving forwards. [1] The brake shoes pivot at
opposite points to each other.[1] This gives the maximum possible braking when moving forwards,
but is not so effective when the vehicle is traveling in reverse. [1]
The optimum arrangement of twin leading front brakes with leading/trailing brakes on the rear
allows for more braking force to be deployed at the front of the vehicle when it is moving
forwards, with less at the rear. This helps to prevent the rear wheels locking up, but still provides
adequate braking at the rear when it is needed.[1]
The brake drum itself is frequently made of cast iron, although some vehicles have
used aluminumdrums, particularly for front-wheel applications. Aluminum conducts heat better
than cast iron, which improves heat dissipation and reduces fade. Aluminum drums are also
lighter than iron drums, which reduces unsprung weight. Because aluminum wears more easily
than iron, aluminum drums will frequently have an iron or steel liner on the inner surface of the
drum, bonded or riveted to the aluminum outer shell.
Drum brakes are used in most heavy duty trucks, some medium and light duty trucks, and few
cars, dirt bikes, and ATVs. Drum brakes are often applied to the rear wheels since most of the
stopping force is generated by the front brakes of the vehicle and therefore the heat generated in
the rear is significantly less. Drum brakes allow simple incorporation of a parking brake.
Drum brakes are also occasionally fitted as the parking (and emergency) brake even when the
rear wheels use disk brakes as the main brakes. The vast majority of rear disc braking systems
use a parking brake in which the piston in the caliper is actuated by a cam or screw. This
compresses the pads against the rotor. However, this type of system becomes much more
complicated when the rear disc brakes use fixed, multi-piston calipers. In this situation, a small
drum is usually fitted within or as part of the brake disk. This type of brake is also known as a
banksia brake.
In hybrid vehicle applications, wear on braking systems is greatly reduced by energy recovering
motor-generators (see regenerative braking), so some hybrid vehicles such as the GMC Yukon
hybrid andToyota Prius (except the third generation) use drum brakes.
Disc brakes rely on pliability of caliper seals and slight runout to release pads, leading to drag,
fuel mileage loss, and disc scoring. Drum brake return springs give more positive action and,
adjusted correctly, often have less drag when released.
Certain heavier duty drum brake systems compensate for load when determining wheel cylinder
pressure; a feature rare when disks are employed (Hydropneumatic suspension systems as
employed on Citroen vehicles adjust brake pressure depending on load regardless of if drum or
disks are used). One such vehicle is the Jeep Comanche. The Comanche can automatically send
more pressure to the rear drums depending on the size of the load.
Due to the fact that a drum brakes friction contact area is at the circumference of the brake, a
drum brake can provide more braking force than an equal diameter disc brake. The increased
friction contact area of drum brake shoes on the drum allows drum brake shoes to last longer
than disc brake pads used in a brake system of similar dimensions and braking force. Drum
brakes retain heat and are more complex than disc brakes but are often the more economical and
powerful brake type to use in rear brake applications due to the low heat generation of rear
brakes, a drum brakes self applying nature, large friction surface contact area, and long life wear
characteristics(%life used/kW of braking power).
As
a tailshaft parking/emergency brake
Drum brakes have also been incorporated on the transmission tailshaft as parking brakes (e.g.
Chryslers through 1956), with the an advantage that it is completely independent of the service
brakes, but having a severe disadvantage in that when used with a bumper jack (common in that
era) on the rear (without proper wheel blocks) the differential's action can allow the vehicle to roll
off the jack.
Drum brakes, like most other types, are designed to convert kinetic energy into heat by friction.
[1]
This heat is intended to be further transferred to the surrounding air, but can just as easily be
transferred into other components of the braking system.
Brake drums have to be large to cope with the massive forces that are involved, and they must be
able to absorb and dissipate a lot of heat. Heat transfer to atmosphere can be aided by
incorporatingcooling fins onto the drum. However, excessive heating can occur due to heavy or
repeated braking, which can cause the drum to distort, leading to vibration under braking.
The other consequence of overheating is brake fade.[1] This is due to one of several processes or
more usually an accumulation of all of them.
1. When the drums are heated by hard braking, the diameter of the drum increases slightly
due tothermal expansion, this means the brakes shoes have to move farther and the
brake pedal has to be depressed further.
2. The properties of the friction material can change if heated, resulting in less friction. This
can be a much larger problem with drum brakes than disk brakes, since the shoes are
inside the drum and not exposed to cooling ambient air. The loss of friction is usually
only temporary and the material regains its efficiency when cooled, [1] but if the surface
overheats to the point where it becomes glazed the reduction in braking efficiency is
more permanent. Surface glazing can be worn away with further use of the brakes, but
that takes time.
3. Excessive heating of the brake drums can cause the brake fluid to vaporize, which
reduces thehydraulic pressure being applied to the brake shoes.[1] Therefore less
deceleration is achieved for a given amount of pressure on the pedal. The effect is
worsened by poor maintenance. If the brake fluid is old and has absorbed moisture it
thus has a lower boiling point and brake fade occurs sooner.[1]
Brake fade is not always due to the effects of overheating. If water gets between the friction
surfaces and the drum, it acts as a lubricant and reduces braking efficiency. [1] The water tends to
stay there until it is heated sufficiently to vaporize, at which point braking efficiency is fully
restored. All friction braking systems have a maximum theoretical rate of energy conversion.
Once that rate has been reached, applying greater pedal pressure will not result in a change of
this rate, and indeed the effects mentioned can substantially reduce it. Ultimately this is what
brake fade is, regardless of the mechanism of its causes.
Disc brakes are not immune to any of these processes, but they deal with heat and water more
effectively than drums.
Drum brakes can be grabby if the drum surface gets light rust or if the brake is cold and damp,
giving the pad material greater friction. Grabbing can be so severe that the tires skid and continue
to skid even when the pedal is released. Grab is the opposite of fade: when the pad friction goes
up, the self-assisting nature of the brakes causes application force to go up. If the pad friction and
self-amplification are high enough, the brake will stay on due to self-application even when the
external application force is released.
While disk brake rotors can be machined to clean up the friction surface (i.e. 'turning'), the same
generally cannot be done with brake drums. Machining the friction surface of a brake drum
increases the diameter, which would require oversized shoes in order to maintain proper contact
with the drum. However, since oversized shoes are generally unavailable for most applications,
worn and/or damaged drums generally must be replaced.
Another disadvantage of drum brakes is their relative complexity. A person must have a general
understanding of how drum brakes work and take simple steps to ensure the brakes are
reassembled correctly when doing work on drum brakes. And, as a result of this increased
complexity (compared to disk brakes), maintenance of drum brakes is generally more timeconsuming. Also, the greater number of parts results in a greater number of failure modes
compared to disk brakes. Springs can break from fatigue if not replaced along with worn brake
shoes. And the drum and shoes can become damaged from scoring if various components (such
as broken springs or self-adjusters) break and become loose inside the drum.
Also, drum brakes do not apply immediately when the wheel cylinders are pressurized because
the force of the return springs has to be overcome before the shoes start to move towards the
drum. This means that the very common hybrid disc/drum systems would only brake with the
discs on light pedal pressure unless extra hardware is added. In practice, a "proportioning valve"
is added to such cars, which serves to apply the drums slightly before the discs. If the
proportioning valve is left out or functions improperly, the result is a car that stops only with the
front discs during careful stops, which are most of the stops encountered in daily driving. Such a
car would show "brake dive" (the car tilts forward during braking) and very fast wear of front disc
brake pad linings. In motorcycle applications "linked brakes" are essentially the same type of
system.
Re-arcing
Before 1984, it was common to re-arc brake shoes to match the arc within brake drums. This
practice, however, was controversial as it removed friction material from the brakes and caused a
reduction in the life of the shoes as well as created hazardous asbestos dust. Current design
theory is to use shoes for the proper diameter drum, and to simply replace the brake drum when
necessary, rather than perform the re-arcing procedure.
Use
in music
A brake drum can be very effective in modern concert and film music to provide a non-pitched
metal sound similar to an anvil. Some have more resonance than others, and the best method of
producing the clearest sound is to hang the drum with nylon cord or to place it on foam. Other
methods include mounting the brake drum on a snare drum stand. Either way, the brake drum is
struck with hammers or sticks of various weight.
It is also commonly used in steelpan ensembles, where it is called "the iron."
See
also

Balancing machine

Brake bleeding

Brake lining

Hydraulic disc brakes
References
1.
^ a b c d e f g h i j k l m n o p q The AA Book of the car, 1976
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Motorcycle components
A brake shoe is the part of a braking system which carries the brake lining in the drum
brakes used on automobiles, or the brake block in train brakes and bicycle brakes.
Contents
[hide]
1 Automobile drum brake
3 Bicycle rim brake
4 Cataloguing
5 References
Automobile
drum brake
The brake shoe carries the brake lining, which is riveted or glued to the shoe. When the brake is
applied, the shoe moves and presses the lining against the inside of the drum.
The friction between lining and drum provides the braking effort. Energy is dissipated as heat.
Modern cars have disc brakes all round, or discs at the front and drums at the rear. An advantage
of discs is that they can dissipate heat more quickly than drums so there is less risk of
overheating.
The reason for retaining drums at the rear is that a drum is more effective than a disc as
a parking brake.
Railway
The brake shoe carries the brake block. The block was originally made of wood but is now
usually cast iron. When the brake is applied, the shoe moves and presses the block against the
tread of the wheel. As well as providing braking effort this also "scrubs" the wheel and keeps it
clean. Tread brakes on passenger trains have now largely been superseded by disc brakes.
Bicycle
rim brake
This comprises a pair of rectangular open boxes which are mounted on the brake calipers of
a bicycleand that hold the brake blocks which rub on the rim of a bicycle wheel to slow the bicycle
down or stop it.
Brake lining
Unsourced material may be challenged and removed. (January 2007)
Drum shoes with linings
Brake linings are the consumable surfaces in brake systems, such as drum brakes and disc
brakes used in transport vehicles.
Contents
[hide]
1 History
2 Structure and function
3 Maintenance
4 Cataloguing
5 References
History
Brake linings were invented by Bertha Benz (the wife of Karl Benz who invented the first patented
automobile) during her historic first long distance car trip in the world in August 1888. The first
asbestos brake linings were developed in 1902 by Herbert Frood. [1]
Structure
and function
Brake linings are composed of a relatively soft but tough and heat-resistant material with a
highcoefficient of dynamic friction (and ideally an identical coefficient of static friction) typically
mounted to a solid metal backing using high-temperature adhesives or rivets. The complete assembly
(including lining and backing) is then often called a brake pad or brake shoe. The dynamic friction
coefficient "µ" for most standard brake pads is usually in the range of 0.35 to 0.42. This means that a
force of 1000 Newtons (or pounds) on the pad will give a resulting brake force close to 400 Newtons
(or pounds). There are some racing pads that have a very high µ of 0.55 to 0.62 with excellent high
temperature behaviour. These pads have high iron content and will usually outperform any other pad
used with iron discs. Unfortunately nothing comes for free, and these high µ pads wear fast and also
wear down the discs at a rather fast rate. However they are a very cost effective alternative to more
exotic/expensive materials.
In this view of an automobile disc brake, the brake pad is the black material held by the red metal component
(the brake caliper). The brake lining is that part of the brake pad which actually contacts the metal disc when the
brake is engaged.
Using a typical bicycle brake as an example, the backing would be the metal shell which provides
mechanical support, and the lining would be the rubbery portion which contacts the rims when the
brakes are applied. In most modern vehicular applications the system is conceptually identical, except
the rims would be replaced with solid steel (or sometimes exotic metal) disc. Furthermore, a
metaltang is usually incorporated into the pad assembly. The tang contacts the rotors when the linings
are worn out, causing an annoying noise designed to alert the motorist that brake servicing is required.
Since the lining is the portion of the braking system which converts the vehicle's kinetic energy into
heat, the lining must be capable of surviving high temperatures without excessive wear (leading to
frequent replacement) oroutgassing (which causes brake fade, a decrease in the stopping power of
the brake).
Due to its efficacy, chrysotile asbestos was often a component in brake linings. However, studies such
as a 1989 National Institutes of Health item showed an uncommonly high proportion of brake
mechanics were afflicted with pleural and peritoneal mesothelioma, both of which are linked
tochrysotile and asbestos exposure.[2] Public health authorities generally recommend against inhaling
brake dust,[3] chrysotile has been banned in many developed countries, such as Australia in late 2003,
[4]
and Chrysotile has been progressively replaced in most brake linings and pads by other fibers such
as the synthetic aramids.
Maintenance
When the lining is worn out, the backing or rivets will contact the rotors or drums during braking, often
causing damage requiring remachining or replacement of the drums or rotors. An annoying squeal
caused by the warning tang is the typical alert that the pads need to be replaced; if the squeal is
ignored for too long, drum or rotor damage (usually accompanied by an unpleasant grinding sound or
sensation) will be the typical result.
The lining may also become contaminated by oil or leaked brake fluid. Typical symptoms will be brake
chatter, where the pads vibrate as the lining grabs and releases the rotor's surface. The solution is to
repair the source of the contamination and replace the damaged pads.
In the automotive repair industry, many consumers purchase brake pads with a lifetime warranty.
These pads use a much harder lining than traditional brake pads and tend to cause excessive wear of
the much more expensive rotors or drums. For that reason, consumers should ensure that the new
brake pads installed are those specified or supplied by the vehicle's manufacturer. Relined brake pads
are usually inexpensive and perfectly acceptable, with new lining material attached to reconditioned
(cleaned, inspected and painted) backing assemblies.
Brake pads must always be replaced simultaneously on both ends of a vehicle's axle, as the different
lining thicknesses (and possibly material types) will cause uneven braking, making the vehicle pull in
the direction of the more effective brake. For most vehicles, replacing pads (and therefore linings) is
very easy, requiring a minimum of tools and time — the linings are designed to be consumable and
should therefore be easy to service.
Brake linings can also be found just about everywhere there are braking systems, from elevator safety
brakes to spindle brakes inside a VCR. The form and materials are frequently different, but the
principle is the same.
king brake
"Hand brake" redirects here. It is not to be confused with HandBrake.
improve it by rewriting it in an encyclopedic style. (March 2011)
2009)
Hand brake lever from a Geo Storm.
Brake warning light. The light is turned on, indicating that the brake is engaged.
In cars, the parking brake, also called hand brake,emergency brake, or e-brake, is a
latching brake usually used to keep the vehicle stationary. It is sometimes also used to prevent a
vehicle from rolling when the operator needs both feet to operate the clutch and throttle pedals.
Automobile hand brakes usually consist of a cable directly connected to the brake mechanism on one
end and to a lever or foot pedal at the driver's position. The mechanism is often a handoperated lever (hence the hand brake name), on the floor on either side of the driver, or a pull handle
located below and near the steering wheel column, or a (foot-operated) pedal located far apart from
the other pedals.
Although sometimes known as an emergency brake, using it in any emergency where the footbrake is
still operational is likely to badly upset the brake balance of the car and vastly increase the likelihood of
loss of control of the vehicle, for example by initiating a rear-wheel skid. Additionally, the stopping force
provided by using the handbrake is small and would not significantly aid in stopping the vehicle. The
parking brake operates only on the rear wheels, which have reduced traction while braking. The
emergency brake is instead intended for use in case of mechanical failure where the regular footbrake
is inoperable or compromised. Modern brake systems are typically very reliable and equipped
with dual-circuit hydraulics and low brake fluid sensor systems, meaning the handbrake is rarely used
to stop a moving vehicle.
The most common use for a parking brake is to keep the vehicle motionless when it is parked. Parking
brakes have a ratchet locking mechanism that will keep them engaged until a release button is
pressed. On vehicles with automatic transmissions, this is usually used in concert with a parking
pawlin the transmission. Automotive safety experts[who?] recommend the use of both systems to
immobilize a parked car, and the use of both systems is required by law in some places [citation needed], yet
many individuals use only the "Park" position on the automatic transmission and not the parking brake.
It's similar with manual transmission cars: They are recommended always to be left with the handbrake
engaged, in concert with their lowest gear (usually either first or reverse). The use of both systems is
also required by law in some jurisdictions. However, when parking on level ground, many people either
only engage the handbrake (gear lever in neutral), or only select a gear (handbrake released). If
parking on a hill with only one system results in the car rolling and damaging the car or other property,
insurance companies in some countries, for example in Germany, aren’t required to pay for the
damages.
Contents
[hide]
1 Types of brakes
2 Large vehicles
o
2.1 Electric parking brake
o
2.2 Jacking
5 References
Types
of brakes
The hand brake lever in a Saab 9-5automobile
School buses which are equipped with a hydraulic brake system will have a hand brake lever to the left
of the driver (in left hand drive buses) near the floor. It is operated by pushing the lever down with
one's hand to apply the brake, and pulling it upwards to release it. However, this has been known to
cause severe back problems in drivers who do this regularly, [citation needed] and many choose to push it up
with their feet.
Some cars with automatic transmissions are fitted with automatically releasing parking brakes. Later
models require the foot brake to be depressed before the car's transmission can be moved from park.
When reverse or drive is selected, the parking brake automatically releases. Earlier models would
release the parking brake when the gear selector was placed in a forward or reverse gear without
requiring any input on the brake pedal at all. These earlier automatic release systems were a safety
hazard, since there would be no protection against accidentally knocking the transmission into gear.
In cars with rear drum brakes, the emergency brake cable usually actuates these drums mechanically
with much less force than is available through the hydraulic system. In cars with rear disc brakes, the
emergency brake either actuates the disc calipers (again, with much less force) or a small drum brake
housed within the hub assembly.
Hudson automobiles used an unusual hybrid hydraulic-mechanical dual-brake system which operated
the rear brakes through the otherwise conventional mechanical emergency-brake system when a
failure of the hydraulic system allowed the pedal to travel beyond its normal limit. [1]
A number of production vehicles, light and medium duty trucks, and motor homes have been made
with a separate drum brake on the transmission output shaft; called a driveline parking brake. This has
an advantage of being completely independent of other braking systems. This is effective as long as
the drive train is intact — propeller shaft, differential, and axle shafts. In many vehicles, this type of
parking brake is operated by either a foot pedal or a hydraulic cylinder controlled by the transmission
gear selector, or by both.
Large
vehicles
Large vehicles are usually fitted with power operated or power assisted handbrakes. Power assisted
handbrakes are usually found on large vans as well as some older heavy vehicles. These operate in
the same way as a conventional handbrake, but pulling the lever will operate a valve that allows air or
hydraulic pressure or vacuum into a cylinder which applies force to the brake shoes and makes
applying the handbrake easier. When releasing the handbrake, the same mechanism also provides
assistance to the driver in disengaging the ratchet. Particularly on commercial vehicles with air
operated brakes, this has the added benefit of making it much harder or even impossible to release
the parking brake when insufficient air pressure is available to operate the brakes. A reservoir or
accumulator is usually provided so a limited amount of power assistance is available with the engine
off. Power operated handbrakes are fitted to heavy commercial vehicles with air brakes, such as trucks
and buses. These usually are spring applied, with air pressure being used to hold the brake off and
powerful springs holding the brakes on. In most cases, a small lever in the cab is connected to a valve
which can admit air to the parking brake cylinders to release the parking brake, or release the air to
apply the brake. On some modern vehicles the valve is operated electrically from a lever or button in
the cab. The system is relatively safe since if air pressure is lost the springs will apply the brakes. Also,
the system prevents the parking brake being released if there is insufficient air pressure to apply the
foot brake. A disadvantage to this system is that if a vehicle requires towing and can not provide its
own air supply, an external supply must be provided to allow the parking brake to be released, or the
brake shoes must be manually wound off against the springs.
Electric
parking brake
A recent variation is the electric parking brake. First installed in the 2001 Renault Vel Satis, electric
brakes have since appeared in a number of vehicles.
Two variations are available: In the more-traditional "cable-pulling" type, an electric motor simply pulls
the emergency brake cable rather than a mechanical handle in the cabin. A more complex unit uses
two computer-controlled motors attached to the rear brake calipers to activate it.
It is expected that these systems will incorporate other features in the future. BMW, Renault, Subaru
and VW already have a system where the emergency brake initiates when the car stops and then
goes off as soon as the gas pedal is pressed preventing the car from rolling. The new feature is called
a hill hold. The vehicle operator can easily turn off the system.
Jacking
It is important to know which wheels are providing the braking action when lifting the car with a jack.
Typically the rear wheels are the ones that are stopped with parking brakes. The Alfasud, Saab
99s,Pre-Facelift 900's, the Citroën Xantia and most early Subarus applied the handbrake force to the
front wheels, which makes them notable exceptions. If one lifts the braking wheels off the ground then
the car can move and fall off the jack. This is why makers recommend that jacking be conducted on
level ground and with chocks immobilizing the wheels that remain on the ground.
hand brakes
Virtually all railroad rolling stock is equipped with manually operated mechanical hand brake devices
that set and release the brakes. Most of these involve a chain linked to the brake rigging, most often at
the brake cylinder, that when tightened pull the piston out against the releasing springs, thus applying
the brakes on the car (if there is only one brake cylinder per car) or bogie (if there is more than once
cylinder per car). Newer locomotives have electric systems that simply place an electric motor in place
of the chain winding mechanism. This brake acts independent of the action of the automatic air brakes,
which function collectively when coupled in a train and are under the control of the locomotive
engineer.
Manual hand brakes serve to keep a piece of rolling stock stationary after it has been spotted in a rail
Before the development of locomotive-actuated train braking systems in the late 19th century,
designated railroad employees known as brakemen would move about the tops of cars, setting hand
brakes in an effort to stop the train in a timely manner. This process was imprecise and extremely
dangerous. Many brakemen lost life and limb as a result of falling from a moving train, icy and wet
conditions often adding to the hazards involved in negotiating the top of a swaying boxcar.[2] In the
U.S., an 1893 federal law, the Railroad Safety Appliance Act, required automatic brakes on all
See
also
Master cylinder
A master cylinder from a Geo Storm
In automotive engineering, the master cylinder is a control device that converts nonhydraulic pressure (commonly from a driver's foot) into hydraulic pressure. This device controls slave
cylinders located at the other end of thehydraulic system.
As piston(s) move along the bore of the master cylinder, this movement is transferred through the
hydraulic fluid, to result in a movement of the slave cylinder(s). The hydraulic pressure created by
moving a piston (inside the bore of the master cylinder) toward the slave cylinder(s) compresses the
fluid evenly, but by varying the comparative surface-area of the master cylinder and/or each slave
cylinder, one can vary the amount of force and displacement applied to each slave cylinder, relative to
the amount of force and displacement applied to the master cylinder.
Vehicle
applications
The most common vehicle uses of master cylinders are in brake and clutch systems. In brake systems,
the operated devices are cylinders inside of brake calipers and/or drum brakes; these cylinders may
be called wheel cylinders or slave cylinders, and they push the brake pads towards a surface that
rotates with the wheel (this surface is typically either a drum, or a disc, a.k.a. a rotor) until the
stationary brake pad(s) create friction against that rotating surface (typically the rotating surface is
metal or ceramic/carbon, for their ability to withstand heat and friction without wearing-down rapidly). In
the clutch system, the device which the master cylinder operates is called the slave cylinder; it moves
the throw out bearing until the high-friction material on the transmission's clutch disengages from the
engine's metal (or ceramic/carbon) flywheel. For hydraulic brakes or clutches alike, flexible highpressure hoses or inflexible hard-walled metal tubing may be used; but the flexible variety of tubing is
needed for at least a short length adjacent to each wheel, whenever the wheel can move relative to
the car's chassis (this is the case on any car with steering and other suspension movements; some
drag racers and go-karts have no rear suspension, as the rear axle is welded to the chassis, and some
antique cars also have no rear suspension movement).
A reservoir above each master cylinder supplies the master cylinder with enough brake fluid to avoid
air from entering the master cylinder (even the typical clutch uses brake fluid, but it may also be
referred to as "clutch fluid" in a clutch application). Most modern light trucks and passenger cars have
one master cylinder for the brakes which contains two pistons; but many racing vehicles, as well as
some classic and antique cars, have two separate master cylinders, each with only one piston (much
like hydraulic clutches typically have only 1 piston per master cylinder). Each piston in a master
cylinder operates a brake circuit, and for modern light trucks and passenger cars, usually a brake
circuit leads to a brake caliper or shoe on only two of the vehicle's wheels, and the other brake circuit
provides brake-pressure to power the other two brakes. For safety, this is done so that usually only
two wheels lose their braking ability at the same time; it results in longer stopping distances and should
be fixed immediately, but at least gives some braking ability, which is preferable to
The Master Cylinder
Here is where you'll find the master cylinder:
Brake Image Gallery
Master cylinder location. See more brake pictures.
In the figure below, the plastic tank you see is the brake-fluid reservoir, the master cylinder's brake-fluid
source. The electrical connection is a sensor that triggers a warning light when the brake fluid gets low.
The master cylinder, reservoir and sensor
As you'll see here, there are two pistons and two springs inside the cylinder.
Diagram of master cylinder
The Master Cylinder in Action
When you press the brake pedal, it pushes on the primary piston through a linkage. Pressure builds in the
cylinder and lines as the brake pedal is depressed further. The pressure between the primary
and secondary piston forces the secondary piston to compress the fluid in its circuit. If the brakes are
operating properly, the pressure will be the same in both circuits.
If there is a leak in one of the circuits, that circuit will not be able to maintain pressure. Here you can see
what happens when one of the circuits develops a leak.
Master cylinder with leak
When the first circuit leaks, the pressure between the primary and secondary cylinders is lost. This causes
the primary cylinder to contact the secondary cylinder. Now the master cylinder behaves as if it has only one
piston. The second circuit will function normally, but you can see from the animation that the driver will have
to press the pedal further to activate it. Since only two wheels have pressure, the braking power will be
severely reduced.
The ABS System
The theory behind anti-lock brakes is simple. A skidding wheel(where the tire contact patch is
sliding relative to the road) has less traction than a non-skidding wheel. If you have been stuck
on ice, you know that if your wheels are spinning you have no traction. This is because the
contact patch is sliding relative to the ice (seeBrakes: How Friction Works for more). By keeping
the wheels from skidding while you slow down, anti-lock brakes benefit you in two ways: You'll
stop faster, and you'll be able to steer while you stop.
There are four main components to an ABS system:
•
Speed sensors
•
Pump
•
Valves
•
Controller
Speed Sensors
The anti-lock braking system needs some way of knowing when a wheel is about to lock up. The
speed sensors, which are located at each wheel, or in some cases in the differential, provide this
information.
Valves
There is a valve in the brake line of each brake controlled by the ABS. On some systems, the
valve has three positions:
•
In position one, the valve is open; pressure from the master cylinder is passed right
through to the brake.
•
In position two, the valve blocks the line, isolating that brake from the master cylinder.
This prevents the pressure from rising further should the driver push the brake pedal harder.
•
In position three, the valve releases some of the pressure from the brake.
Pump
Since the valve is able to release pressure from the brakes, there has to be some way to put that
pressure back. That is what the pump does; when a valve reduces the pressure in a line, the
pump is there to get the pressure back up.
Controller
The controller is a computer in the car. It watches the speed sensors and controls the valves.
ABS at Work
There are many different variations and control algorithms for ABS systems. We will discuss how
one of the simpler systems works.
The controller monitors the speed sensors at all times. It is looking for decelerations in the wheel
that are out of the ordinary. Right before a wheel locks up, it will experience a rapid deceleration.
If left unchecked, the wheel would stop much more quickly than any car could. It might take a car
five seconds to stop from 60 mph (96.6 kph) under ideal conditions, but a wheel that locks up
could stop spinning in less than a second.
The ABS controller knows that such a rapid deceleration is impossible, so it reduces
the pressure to that brake until it sees an acceleration, then it increases the pressure until it sees
the deceleration again. It can do this very quickly, before the tire can actually significantly change
speed. The result is that the tire slows down at the same rate as the car, with the brakes keeping
the tires very near the point at which they will start to lock up. This gives the system maximum
braking power.
When the ABS system is in operation you will feel a pulsing in the brake pedal; this comes from
the rapid opening and closing of the valves. Some ABS systems can cycle up to 15 times per
second.
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MORE TO EXPLORE
Disc Brake Basics
Here is the location of the disc brakes in a car:
The main components of a disc brake are:
•
•
The caliper, which contains a piston
•
The rotor, which is mounted to the hub
Parts of a disc brake
The disc brake is a lot like the brakes on a bicycle. Bicycle brakes have a caliper, which squeezes
the brake pads against the wheel. In a disc brake, the brake pads squeeze the rotorinstead of
the wheel, and the force is transmitted hydraulically instead of through a cable. Frictionbetween
the pads and the disc slows the disc down.
A moving car has a certain amount of kinetic energy, and the brakes have to remove this energy
from the car in order to stop it. How do the brakes do this? Each time you stop your car, your
brakes convert the kinetic energy to heat generated by the friction between the pads and the disc.
Most car disc brakes are vented.
Disc brake vents
Vented disc brakes have a set of vanes, between the two sides of the disc, that pumps air
through the disc to provide cooling.
Drum brakes work on the same principle as disc brakes: Shoes press against a spinning
surface. In this system, that surface is called a drum.
Many cars have drum brakes on the rear wheels and disc brakes on the front. Drum brakes have
more parts than disc brakes and are harder to service, but they are less expensive to
manufacture, and they easily incorporate an emergency brake mechanism.
In this edition of HowStuffWorks, we will learn exactly how a drum brake system works, examine
the emergency brake setup and find out what kind of servicing drum brakes need.
The Vacuum Booster
The vacuum booster is a metal canister that contains a clever valve and a diaphragm. A rod
going through the center of the canister connects to the master cylinder's piston on one side and
to the pedal linkage on the other.
Another key part of the power brakes is the check valve.
The photo above shows the check valve, which is a one-way valvethat only allows air to be
suckedout of the vacuum booster. If theengine is turned off, or if a leak forms in a vacuum hose,
the check valve makes sure that air does not enter the vacuum booster. This is important
because the vacuum booster has to be able to provide enough boost for a driver to make several
stops in the event that the engine stops running -- you certainly don't want to lose brake function if
you run out of gas on the highway. In the next section, we'll see how the booster works (and
check out a cool animation!).
If you've ever opened the hood of your car, you've probably seen thebrake booster. It's the
round, black cannister located at the back of the engine compartment on the driver's side of the
car.
Back in the day, when most cars had drum brakes, power brakes were not really necessary -drum brakes naturally provide some of their own power assist. Since most cars today have disc
brakes, at least on the front wheels, they need power brakes. Without this device, a lot of drivers
would have very tired legs.
The brake booster uses vacuumfrom the engine to multiply the force that your foot applies to
themaster cylinder. In this article, we'll see what's inside the black cannister that provides power
braking.
HIT THE BRAKES!
The vacuum booster is a very simple, elegant design. The device needs a vacuum source to
operate. Ingasoline-powered cars, the engine provides a vacuum suitable for the boosters. In
fact, if you hook a hose to a certain part of an engine, you can suck some of the air out of the
container, producing a partial vacuum. Because diesel engines don't produce a vacuum, dieselpowered vehicles must use a separate vacuum pump.
On cars with a vacuum booster, the brake pedal pushes a rod that passes through the booster
into the master cylinder, actuating the master-cylinder piston. The engine creates a partial
vacuum inside the vacuum booster on both sides of the diaphragm. When you hit the brake
pedal, the rod cracks open a valve, allowing air to enter the booster on one side of the diaphragm
while sealing off the vacuum. This increases pressure on that side of the diaphragm so that it
helps to push the rod, which in turn pushes the piston in the master cylinder.
As the brake pedal is released, the valve seals off the outside air supply while reopening the
vacuum valve. This restores vacuum to both sides of the diaphragm, allowing everything to return
to its original position.
Print
Cite
Feedback
Clutch
For other uses, see Clutch (disambiguation).
Unsourced material may be challenged and removed. (May 2012)
Rear side of a Ford V6 engine, looking at the clutch housing on the flywheel
Single, dry, clutch friction disc. Thesplined hub is attached to the disc with springs to damp chatter.
A clutch is a mechanical device that provides for thetransmission of power (and therefore usually
motion) from one component (the driving member) to another (the driven member) when engaged, but
can be disengaged.
Clutches are used whenever the transmission of power or motion needs to be controlled either in
amount or over time (e.g., electric screwdrivers limit how much torque is transmitted through use of a
clutch; clutches control whether automobiles transmit engine power to the wheels).
In the simplest application, clutches are employed in devices which have two rotating shafts (drive
shaft or line shaft). In these devices, one shaft is typically attached to a motor or other power unit (the
driving member) while the other shaft (the driven member) provides output power for work to be done.
In a torque-controlled drill, for instance, one shaft is driven by a motor and the other drives a drill
chuck. The clutch connects the two shafts so that they may be locked together and spin at the same
speed (engaged), locked together but spinning at different speeds (slipping), or unlocked and spinning
at different speeds (disengaged).
Contents
[hide]
1 Friction clutches
o
1.1 Multiple plate clutch
o
1.2 Wet vs. dry
o
1.3 Centrifugal
o
1.4 Cone clutch
o
1.5 Torque limiter
2 Major types by application
o
2.1 Vehicular (general)

2.1.1 Automobile powertrain

2.1.2 Motorcycles

2.1.3 Automobile non-powertrain
3 Other clutches and applications
o
3.1 Specialty clutches and applications

3.1.1 Single-revolution clutch


3.1.3 Kickback clutch-brakes
5 Notes
6 References
Friction
clutches
A friction clutch
Friction clutches are by far the most well-known type of clutches.
Materials
Various materials have been used for the disc friction facings, including asbestos in the past. Modern
clutches typically use a compound organic resin with copper wire facing or a ceramicmaterial. A typical
coefficient of friction used on a friction disc surface is 0.35 for organic and 0.25 for ceramic. Ceramic
materials are typically used in heavy applications such as trucks carrying large loads or racing, though
the harder ceramic materials increase flywheel and pressure plate wear.
Push/Pull
Friction disk clutches generally are classified as push type or pull type depending on the location of the
pressure plate fulcrum points. In a pull type clutch, the action of pressing the pedal pulls the release
bearing, pulling on the diaphragm spring and disengaging the vehicle drive. The opposite is true with a
push type, the release bearing is pushed into the clutch disengaging the vehicle drive. In this instance,
the release bearing can be known as a thrust bearing (as per the image above).
Clutch pads are attached to the frictional pads, part of the clutch. They are most commonly made of
rubber but have been known to be made of asbestos. Clutch pads usually last about 100,000 miles
(160,000 km) depending on how vigorously the car is driven.
Dampers
In addition to the damped disc centres which reduce driveline vibration, pre-dampers may be used to
reduce gear rattle at idle by changing the natural frequency of the disc. These weaker springs are
compressed solely by the radial vibrations from an idling engine. They are fully compressed and no
longer in use once drive is taken up by the main damper springs.
Mercedes truck examples: A clamp load of 33 kN is normal for a single plate 430. The 400 Twin
application offers a clamp load of a mere 23 kN. Bursts speeds are typically around 5,000 rpm with the
weakest point being the facing rivet.
Manufacturing
Modern clutch development focuses its attention on the simplification of the overall assembly and/or
manufacturing method. For example drive straps are now commonly employed to transfer torque as
well as lift the pressure plate upon disengagement of vehicle drive. With regards to the manufacture of
diaphragm springs, heat treatment is crucial. Laser welding is becoming more common as a method of
attaching the drive plate to the disc ring with the laser typically being between 2-3KW and a feed rate
1m/minute.
Multiple
plate clutch
This type of clutch has several driving members interleaved or "stacked" with several driven members.
It is used in race cars including F1, IndyCar, World Rally and even most club
racing, motorcycles,automatic transmissions and in some diesel locomotives with mechanical
transmissions. It is also used in some electronically controlled all-wheel drive systems.
Wet
vs. dry
A wet clutch is immersed in a cooling lubricating fluid which also keeps the surfaces clean and gives
smoother performance and longer life. Wet clutches, however, tend to lose some energy to the liquid.
Since the surfaces of a wet clutch can be slippery (as with a motorcycle clutch bathed in engine oil),
stacking multiple clutch discs can compensate for the lower coefficient of friction and so eliminate
slippage under power when fully engaged.
The Hele-Shaw clutch was a wet clutch that relied entirely on viscous effects, rather than on friction.
A dry clutch, as the name implies, is not bathed in fluid and should be, literally, dry.
Centrifugal
A centrifugal clutch is used in some vehicles (e.g., Mopeds) and also in other applications where the
speed of the engine defines the state of the clutch, for example, in a chainsaw. This clutch system
employs centrifugal force to automatically engage the clutch when the engine rpm rises above a
threshold and to automatically disengage the clutch when the engine rpm falls low enough. The
system involves a clutch shoe or shoes attached to the driven shaft, rotating inside a clutch bell
attached to the output shaft. The shoe(s) are held inwards by springs until centrifugal force overcomes
the spring tension and the shoe(s) make contact with the bell, driving the output. In the case of a
chainsaw this allows the chain to remain stationary whilst the engine is idling; once the throttle is
pressed and the engine speed rises, the centrifugal clutch engages and the cutting chain moves.
SeeSaxomat and Variomatic.
Cone
clutch
As the name implies, a cone clutch has conical friction surfaces. The cone's taper means that a given
amount of movement of the actuator makes the surfaces approach (or recede) much more slowly than
in a disc clutch. As well, a given amount of actuating force creates more pressure on the mating
surfaces.
Torque
limiter
Also known as a slip clutch or safety clutch, this device allows a rotating shaft to slip when higher than
normal resistance is encountered on a machine. An example of a safety clutch is the one mounted on
the driving shaft of a large grass mower. The clutch will yield if the blades hit a rock, stump, or other
immobile object. Motor-driven mechanical calculators had these between the drive motor and gear
train, to limit damage when the mechanism jammed, as motors used in such calculators had high stall
torque and were capable of causing damage to the mechanism if torque wasn't limited.

Carefully-designed types operate, but continue to transmit maximum permitted torque, in such
tools as controlled-torque screwdrivers.

Many safety clutches are not friction clutches, but belong to the interference clutch family, of
which the dog clutch (see below) is the best-known.
Major
types by application
Vehicular
(general)
There are different designs of vehicle clutch but most are based on one or more friction discs pressed
tightly together or against a flywheel using springs. The friction material varies in composition
depending on many considerations such as whether the clutch is "dry" or "wet". Friction discs once
contained asbestos but this has been largely eliminated. Clutches found in heavy duty applications
such as trucks and competition cars use ceramic clutches that have a greatly increased friction
coefficient. However, these have a "grabby" action generally considered unsuitable for passenger cars.
The spring pressure is released when the clutch pedal is depressed thus either pushing or pulling the
diaphragm of the pressure plate, depending on type. However, raising the engine speed too high while
engaging the clutch will cause excessive clutch plate wear. Engaging the clutch abruptly when the
engine is turning at high speed causes a harsh, jerky start. This kind of start is necessary and
desirable in drag racing and other competitions, where speed is more important than comfort.
Automobile powertrain
This plastic pilot shaft guide tool is used to align the clutch disk as the spring-loaded pressure plate is installed.
The transmission's drive splines and pilot shaft have a complementary shape. A number of such devices fit various
makes and models of drivetrains.
In a modern car with a manual transmission the clutch is operated by the left-most pedal using
a hydraulic or cableconnection from the pedal to the clutch mechanism. On older cars the clutch might
be operated by a mechanical linkage. Even though the clutch may physically be located very close to
the pedal, such remote means of actuation are necessary to eliminate the effect of vibrations and
slight engine movement, engine mountings being flexible by design. With a rigid mechanical linkage,
smooth engagement would be near-impossible because engine movement inevitably occurs as the
drive is "taken up."
The default state of the clutch is engaged - that is the connection between engine and gearbox is
always "on" unless the driver presses the pedal and disengages it. If the engine is running with clutch
engaged and the transmission in neutral, the engine spins the input shaft of the transmission, but no
power is transmitted to the wheels.
The clutch is located between the engine and the gearbox, as disengaging it is required to change
gear. Although the gearbox does not stop rotating during a gear change, there is no torque transmitted
through it, thus less friction between gears and their engagement dogs. The output shaft of the
gearbox is permanently connected to the final drive, then the wheels, and so both always rotate
together, at a fixed speed ratio. With the clutch disengaged, the gearbox input shaft is free to change
its speed as the internal ratio is changed. Any resulting difference in speed between the engine and
gearbox is evened out as the clutch slips slightly during re-engagement.
Clutches in typical cars are mounted directly to the face of the engine's flywheel, as this already
provides a convenient large diameter steel disk that can act as one driving plate of the clutch. Some
racing clutches use small multi-plate disk packs that are not part of the flywheel. Both clutch and
flywheel are enclosed in a conical bellhousing, which (in a rear-wheel drive car) usually forms the main
mounting for the gearbox.
A few cars, notably the Alfa Romeo Alfetta, Porsche 924, and Chevrolet Corvette (since 1997), sought
a more even weight distribution between front and back [note 1] by placing the weight of the transmission
at the rear of the car, combined with the rear axle to form a transaxle. The propeller shaft between
front and rear rotates continuously as long as the engine is running, even if the clutch is disengaged or
the transmission in neutral.
Motorcycles
Motorcycles typically employ a wet clutch with the clutch riding in the same oil as the transmission.
These clutches are usually made up of a stack of alternating plain steel and friction plates. Some of
the plates have lugs on their inner diameters locking them to the engine crankshaft, while the other
plates have lugs on their outer diameters that lock them to a basket which turns the transmission input
shaft. The plates are forced together by a set of coil springs or a diaphragm spring plate when the
clutch is engaged.
On most motorcycles the clutch is operated by the clutch lever located on the left handlebar. No
pressure on the lever means that the clutch plates are engaged (driving), while pulling the lever back
towards the rider will disengage the clutch plates through cable or hydraulic actuation, allowing the
rider to shift gears or coast.
Racing motorcycles often use slipper clutches to eliminate the effects of engine braking which, being
applied only to the rear wheel, can lead to instability.
Automobile non-powertrain
There are other clutches found in a car. For example, a belt-driven engine cooling fan may have a
clutch that is heat-activated. The driving and driven members are separated by a silicone-based fluid
and a valve controlled by a bimetallic spring. When the temperature is low, the spring winds and
closes the valve, which allows the fan to spin at about 20% to 30% of the shaft speed. As the
temperature of the spring rises, it unwinds and opens the valve, allowing fluid past the valve which
allows the fan to spin at about 60% to 90% of shaft speed.
Other clutches such as for an air conditioning compressor electronically-engaged clutches using
magnetic force to couple the driving member to the driven member.
Other
clutches and applications
Belt clutch: Used on agricultural equipment and some piston-engine-driven helicopters.

Engine power is transmitted via a set of vee-belts that are slack when the engine is idling, but by
means of a tensioner pulley can be tightened to increase friction between the belts and the
sheaves.
Dog clutch: Utilized in automobile manual transmissions mentioned above. Positive

engagement, non-slip. Typically used where slipping is not acceptable. Partial engagement under
any significant load tends to be destructive.
Hydraulic clutch: The driving and driven members are not in physical contact; coupling is

hydrodynamic.
Electromagnetic clutch: Typically a clutch that is engaged by an electromagnet that is an

integral part of the clutch assembly. However, magnetic particle clutches have magnetically
influenced particles contained in a chamber between driving and driven members which upon
application ofdirect current causes the particles to clump together and adhere to the operating
surfaces. Engagement and slippage are notably smooth.
Overrunning clutch or freewheel: If some external force makes the driven member rotate

faster than the driver, the clutch effectively disengages. Examples include:

Borg-Warner overdrive transmissions in cars

Ratchet: typical bicycles have these so that the rider can stop pedaling and coast

An oscillating member where this clutch can then convert the oscillations into
intermittent linear or rotational motion of the complimentary member; others use ratchets with
the pawl mounted on a moving member

The winding knob of a camera employs a (silent) wrap-spring type as a clutch in
winding and as a brake in preventing it from being turned backwards.

The rotor drive train in helicopters uses a freewheeling clutch to disengage the rotors
from the engine in the event of engine failure, allowing the craft to safely descend by
autorotation.

Wrap-spring clutches: These have a helical spring wound with square-cross-section wire. In
simple form the spring is fastened at one end to the driven member; its other end is unattached.
The spring fits closely around a cylindrical driving member. If the driving member rotates in the
direction that would unwind the spring the spring expands minutely and slips although with some
drag. Rotating the driving member the other way makes the spring wrap itself tightly around the
driving surface and the clutch locks up.
Specialty
clutches and applications
Single-revolution clutch
When inactive it is disengaged and the driven member is stationary. When "tripped", it locks up solidly
(typically in a few to tens of milliseconds) and rotates the driven member just one full turn. If the trip
mechanism is operated when the clutch would otherwise disengage the clutch remains engaged.
Variants include half-revolution (and other fractional-revolution) types. These were an essential part of
printing telegraphs such as teleprinter page printers, as well as electric typewriters, notably the IBM
Selectric. They were also found in motor-driven mechanical calculators; the Marchant had several of
them. They are also used in farm machinery and industry. Typically, these were a variety of dog clutch.
Single-revolution clutches in teleprinters were of this type. Basically the spring was kept expanded
(details below) and mostly out of contact with the driving sleeve, but nevertheless close to it. One end
of the spring was attached to a sleeve surrounding the spring. The other end of the spring was
attached to the driven member inside which the drive shaft could rotate freely. The sleeve had a
projecting tooth, like a ratchet tooth. A spring-loaded pawl pressed against the sleeve and kept it from
rotating. The wrap spring's torque kept the sleeve's tooth pressing against the pawl. To engage the
clutch, an electromagnet attracted the pawl away from the sleeve. The wrap spring's torque rotated the
sleeve which permitted the spring to contract and wrap tightly around the driving sleeve. Load torque
tightened the wrap so it did not slip once engaged. If the pawl were held away from the sleeve the
clutch would continue to drive the load without slipping. When the clutch was to disengage power was
disconnected from the electromagnet and the pawl moved close to the sleeve. When the sleeve's tooth
contacted the pawl the sleeve and the load's inertia unwrapped the spring to disengage the clutch.
Considering that the drive motors in some of these (such as teleprinters for news wire services) ran 24
hours a day for years the spring could not be allowed to stay in close contact with the driving cylinder;
wear would be excessive. The other end of the spring was fastened to a thick disc attached to the
driven member. When the clutch locked up the driven mechanism coasted and its inertia rotated the
disc until a tooth on it engaged a pawl that kept it from reversing. Together with the restraint at the
other end of the spring created by the trip pawl and sleeve tooth, this kept the spring expanded to
minimize contact with the driving cylinder. These clutches were lubricated with conventional oil, but the
wrap was so effective that the lubricant did not defeat the grip. These clutches had long operating
lives, cycling for tens, maybe hundreds of millions of cycles without need of maintenance other than
occasional lubrication with recommended oil.
These superseded wrap-spring single-revolution clutches in page printers, such as teleprinters,
including the Teletype Model 28 and its successors, using the same design principles. As well, the IBM
Selectric typewriter had several of them. These were typically disc-shaped assemblies mounted on the
drive shaft. Inside the hollow disc-shaped housing were two or three freely-floating pawls arranged so
that when the clutch was tripped, the load torque on the first pawl to engage created force to keep the
second pawl engaged, which in turn kept the third one engaged. The clutch did not slip once locked
up. This sequence happened quite fast, on the order of milliseconds. The first pawl had a projection
that engaged a trip lever. If the lever engaged the pawl, the clutch was disengaged. When the trip
lever moved out of the way the first pawl engaged, creating the cascaded lockup just described. As the
clutch rotated it would stay locked up if the trip lever were out of the way, but if the trip lever engaged
the clutch would quickly unlock.
Kickback clutch-brakes
These mechanisms were found in some types of synchronous-motor-driven electric clocks. Many
different types of synchronous clock motors were used, including the pre-World War II Hammond
manual-start clocks. Some types of self-starting synchronous motors always started when power was
applied, but in detail, their behavior was chaotic and they were equally likely to start rotating in the
wrong direction. Coupled to the rotor by one (or possibly two) stages of reduction gearing was a wrapspring clutch-brake. The spring did not rotate. One end was fixed; the other was free. It rode freely but
closely on the rotating member, part of the clock's gear train. The clutch-brake locked up when rotated
backwards, but also had some spring action. The inertia of the rotor going backwards engaged the
clutch and "wound" the spring. As it "unwound", it re-started the motor in the correct direction. Some
designs had no explicit spring as such; it was simply a compliant mechanism. The mechanism was
lubricated; wear did not seem to be a problem.
See
also
Centrifugal clutch
Talbot cars 'Traffic Clutch' of the 1930s
A centrifugal clutch is a clutch that uses centrifugal forceto connect two concentric shafts, with the
driving shaft nested inside the driven shaft.
The input of the clutch is connected to the enginecrankshaft while the output may drive a shaft, chain,
or belt. As engine revolutions per minute increase, weighted arms in the clutch swing outward and
force the clutch to engage. The most common types have friction pads or shoes radially mounted that
engage the inside of the rim of a housing. On the center shaft there are an assorted number of
extension springs, which connect to a clutch shoe. When the center shaft spins fast enough, the
springs extend causing the clutch shoes to engage the friction face. It can be compared to a drum
brake in reverse. This type can be found on most home built karts, lawn and garden equipment, fuelpowered model cars and low power chainsaws. Another type used in racing karts has friction and
clutch disks stacked together like a motorcycle clutch. The weighted arms force these disks together
and engage the clutch.
When the engine reaches a certain speed, the clutch activates, working somewhat like a continuously
variable transmission. As the load increases, the speed drops, disengaging the clutch, letting the
speed rise again and reengaging the clutch. If tuned properly, the clutch will tend to keep the speed at
or near the torque peak of the engine. This results in a fair bit of waste heat, but over a broad range of
speeds it is much more useful than a direct drive in many applications.
A chainsaw clutch. The chain wraps around a sprocket behind the clutch that turns with the outer drum.
Centrifugal clutches are often used in mopeds, underbones,lawnmowers, go-karts, chainsaws,
and mini bikes to

keep the internal combustion engine from stalling when the output shaft is slowed or stopped
abruptly

disengage loads when starting and idling.
Thomas Fogarty, who is also credited with inventing theballoon catheter, is credited with inventing a
centrifugal clutch in the 1940s although automobiles were already being manufactured with centrifugal
clutches as early as 1936.[1]
Contents
[hide]
3 Dry fluid centrifugal clutch
5 References

No kind of control mechanism is necessary

It is cheaper than other clutches.

Prevents the internal combustion engine from stalling when the output shaft is slowed or
stopped abruptly therefore decreases the engine braking force.

Since it involves friction and slipping between driver and driven parts there is loss of power.

As it involves slipping, therefore it is not desirable in cases where there is heavy load or in
high torque requirements.
Dry
fluid centrifugal clutch
See http://contentdm.lib.byu.edu/ETD/image/etd223.pdf for a basic description of Dry Fluid Centrifugal
Clutch design and operating principle. A device such as this was reportedly used in the
Taylor Aerocar roadable airplane of the late 1940s and early 1950s.
See
also
Cone clutch
article's layout. (January 2012)
Click [show] on right for more details.[show]
Schematic drawing of a cone clutch:
1. Cones: female cone (green), male cone(blue)
2. Shaft: male cone is sliding on splines
3. Friction material: usually on female cone, here on male cone
4. Spring: brings the male cone back after using the clutch control
5. Clutch control: separating both cones by pressing
6. Rotating direction: both direction of the axis are possible
A cone clutch serves the same purpose as a disk or plateclutch. However, instead of mating two
spinning disks, the cone clutch uses two conical surfaces to transmit torque by friction.
The cone clutch transfers a higher torque than plate or disk clutches of the same size due to the
wedging action and increased surface area. Cone clutches are generally now only used in low
peripheral speed applications although they were once common in automobiles and other
combustion engine transmissions.
They are usually now confined to very specialist transmissions in racing, rallying, or in extreme
off-road vehicles, although they are common in power boats. This is because the clutch does not
have to be pushed in all the way and the gears will be changed quicker. Small cone clutches are
used in synchronizer mechanisms in manual transmissions.
External
Dog clutch
Unsourced material may be challenged and removed. (December 2006)
Dog clutch used to drive the rotating platter in a microwave oven.
A dog clutch is a type of clutch that couples two rotating shafts or other rotating components not
by friction but by interference. The two parts of the clutch are designed such that one will push the
other, causing both to rotate at the same speed and will never slip.
Dog clutches are used where slip is undesirable and/or the clutch is not used to control torque. Without
slippage, dog clutches are not affected by wear in the same way that friction clutches are.
Dog clutches are used inside manual automotive transmissions to lock different gears to the rotating
input and output shafts. A synchromesharrangement ensures smooth engagement by matching the
shaft speeds before the dog clutch is allowed to engage.
A good example of a simple dog clutch can be found in a Sturmey-Archer bicycle hub gear, where a
sliding cross-shaped clutch is used to lock the driver assembly to different parts of the planetary
geartrain.
Fluid coupling
Daimler car fluid flywheel of the 1930s
see Viscous coupling unit.
A fluid coupling is a hydrodynamic device used to transmit rotating mechanical power. [1] It has
been used inautomobile transmissions as an alternative to a mechanicalclutch. It also has
widespread application in marine and industrial machine drives, where variable speed operation
and/or controlled start-up without shock loading of the power transmission system is essential.
Contents
[hide]
1 History
2 Overview
o
2.1 Stall speed
o
2.2 Slip
o
2.3 Hydraulic fluid
o
2.4 Hydrodynamic braking
3 Applications
o
3.1 Industrial
o
3.2 Rail transportation
o
3.3 Automotive
o
3.4 Aviation
4 Calculations
5 Manufacture
7 References and notes
o
7.1 Notes
o
7.2 References
History
The fluid coupling originates from the work of Dr. Hermann Föttinger, who was the chief
designer at the AG Vulcan Works in Stettin.[2] His patents from 1905 covered both fluid couplings
and torque converters.
Dr Bauer of the Vulcan-Werke collaborated with English engineer Harold Sinclair of Hydraulic
Coupling Patents Limited to adapt the Föttinger coupling to vehicle transmission. Following
Sinclair's discussions with the London General Omnibus Company begun in October 1926 and
trials on an Associated Daimler bus chassis Percy Martin of Daimler decided to apply the principle
to a private car.[3]
In 1930 Harold Sinclair, working with The Daimler Company of Coventry, England, devised a
transmission system using a fluid coupling and Wilson self-changing gearbox for buses in an
attempt to mitigate the lurching he had experienced while riding on London buses during the
1920s.[2] These couplings are described as constructed under Vulcan-Sinclair and Daimler
patents.
In 1939 General Motors Corporation introduced Hydramatic drive, the first fully automatic
automotive transmission system installed in a mass produced automobile. [2] The Hydramatic
employed a fluid coupling.
The first Diesel locomotives using fluid couplings were also produced in the 1930s [4]
Overview
A fluid coupling consists of three components, plus the hydraulic fluid:

The housing, also known as the shell[5] (which must have an oil tight seal around the drive
shafts), contains the fluid and turbines.

Two turbines (fan like components):
One connected to the input shaft; known as the pump or impellor,[5] primary

wheel[5] input turbine
The other connected to the output shaft, known as the turbine, output

turbine, secondary wheel[5] or runner
The driving turbine, known as the 'pump', (or driving torus[note 1]) is rotated by the prime mover,
which is typically an internal combustion engine or electric motor. The impellor's motion imparts
both outwards linear and rotational motion to the fluid.
The hydraulic fluid is directed by the 'pump' whose shape forces the flow in the direction of the
'output turbine' (or driven torus[note 1]). Here, any difference in the angular velocities of 'input stage'
and 'output stage' result in a net force on the 'output turbine' causing a torque; thus causing it to
rotate in the same direction as the pump.
The motion of the fluid is effectively toroidal - travelling in one direction on paths that can be
visualised as being on the surface of a torus:

If there is a difference between input and output angular velocities the motion has a
component which is circular (i.e. round the rings formed by sections of the torus)

If the input and output stages have identical angular velocities there is no net centripetal force
- and the motion of the fluid is circular and co-axial with the axis of rotation (i.e. round the
edges of a torus), there is no flow of fluid from one turbine to the other.
Stall
speed
An important characteristic of a fluid coupling is its stall speed. The stall speed is defined as the
highest speed at which the pump can turn when the output turbine is locked and maximum input
power is applied. Under stall conditions all of the engine's power would be dissipated in the fluid
coupling as heat, possibly leading to damage.
Step-circuit coupling
A modification to the simple fluid coupling is the step-circuit coupling which was formerly
manufactured as the "STC coupling" by the Fluidrive Engineering Company.
The STC coupling contains a reservoir to which some, but not all, of the oil gravitates when the
output shaft is stalled. This reduces the "drag" on the input shaft, resulting in reduced fuel
consumption when idling and a reduction in the vehicle's tendency to "creep".
When the output shaft begins to rotate, the oil is thrown out of the reservoir by centrifugal force,
and returns to the main body of the coupling, so that normal power transmission is restored. [6]
Slip
A fluid coupling cannot develop output torque when the input and output angular velocities are
identical.[7] Hence a fluid coupling cannot achieve 100 percent power transmission efficiency. Due
to slippage that will occur in any fluid coupling under load, some power will always be lost in fluid
friction and turbulence, and dissipated as heat.
The very best efficiency a fluid coupling can achieve is 94 percent, that is for every 100
revolutions input, there will be 94 revolutions output. Like other fluid dynamical devices, its
efficiency tends to increase gradually with increasing scale, as measured by the Reynolds
number.
Hydraulic
fluid
As a fluid coupling operates kinetically, low viscosity fluids are preferred.[7] Generally speaking,
multi-grade motor oils or automatic transmission fluids are used. Increasing density of the fluid
increases the amount of torque that can be transmitted at a given input speed. [8]
Hydrodynamic
braking
Fluid couplings can also act as hydrodynamic brakes, dissipating rotational energy as heat
through frictional forces (both viscous and fluid/container). When a fluid coupling is used for
braking it is also known as a retarder.[5]
Applications
Industrial
Fluid couplings are used in many industrial application involving rotational power, [9][10] especially in
Rail
transportation
Fluid couplings are found in some Diesel locomotives as part of the power transmission
system. Self-Changing Gears made semi-automatic transmissions for British Rail,
and Voith manufacture turbo-transmissions for railcars and diesel multiple units which contain
various combinations of fluid couplings and torque converters.
Automotive
Fluid couplings were used in a variety of early semi-automatic transmissions and automatic
transmissions. Since the late 1940s, the hydrodynamic torque converter has replaced the fluid
coupling in automotive applications.
In automotive applications, the pump typically is connected to the flywheel of the engine—in fact,
the coupling's enclosure may be part of the flywheel proper, and thus is turned by the
engine's crankshaft. The turbine is connected to the input shaft of the transmission. While the
transmission is in gear, as engine speed increases torque is transferred from the engine to the
input shaft by the motion of the fluid, propelling the vehicle. In this regard, the behavior of the fluid
coupling strongly resembles that of a mechanical clutch driving a manual transmission.
Fluid flywheels, as distinct from torque converters, are best known for their use in Daimler cars in
conjunction with a Wilson pre-selector gearbox. Daimler used these throughout their range of
luxury cars, until switching to automatic gearboxes with the 1958 Majestic. Daimler and Alvis were
both also known for their military vehicles and armored cars, some of which also used the
combination of pre-selector gearbox and fluid flywheel.
Aviation
The most prominent use of fluid couplings in aeronautical applications was in the Wright turbocompound reciprocating engine, in which three power recovery turbines extracted approximately
20 percent of the energy or about 500 horsepower (370 kW) from the engine's exhaust gases and
then, using three fluid couplings and gearing, converted low-torque high-speed turbine rotation to
low-speed, high-torque output to drive the propeller.
Calculations
Generally speaking, the power transmitting capability of a given fluid coupling is strongly related to
pump speed, a characteristic that generally works well with applications where the applied load
doesn't fluctuate to a great degree. The torque transmitting capacity of any hydrodynamic coupling
can be described by the expression
is the impeller speed, and
, where
is the mass density of the fluid,
is the impeller diameter.[11] In the case of automotive applications,
is only an approximation. Stop-
and-go driving will tend to operate the coupling in its least efficient range, causing an adverse
effect on fuel economy.
Manufacture
Fluid couplings are relatively simple components to produce. For example, the turbines can be
aluminum castings or steel stampings, and the housing can also be a casting or made from
stamped or forged steel.
Manufacturers of industrial fluid couplings include Voith, Transfluid,[12] TwinDisc,[13] Siemens,
[14]
PARAG,[15] Fluidomat,[16] and Reuland Electric.[17]
See
also

Torque converter

Water brake
References
and notes
Notes
1.
^ a b A General Motors term
References
Electromagnetic clutch
Unsourced material may be challenged and removed. (August 2011)
Electromagnetic clutches operate electrically, but transmit torque mechanically. This is why they
used to be referred to as electro-mechanical clutches. Over the years, EM became known as
electromagnetic versus electro mechanical, referring more about their actuation method versus
physical operation. Since the clutches started becoming popular over 60 years ago, the variety of
applications and clutch designs has increased dramatically, but the basic operation remains the same.
Single-face clutches make up approximately 90% of all electromagnetic clutch sales.
The electromagnetic clutch is most suitable for remote operation since no linkages are required to
control its engagement. It has fast, smooth operation. However, because energy dissipates as heat in
the electromagnetic actuator every time the clutch is engaged, there is a risk of overheating.
Consequently the maximum operating temperature of the clutch is limited by the temperature rating of
the insulation of the electromagnet. This is a major limitation. Another disadvantage is higher initial
cost.
Contents
[hide]
1 Friction-plate clutch
o
1.1 How it works

1.1.1 Engagement

1.1.2 Disengagement

1.1.3 Cycling
2 Applications
o
2.1 Machinery
o
2.2 Automobiles
o
2.3 Locomotives
3 Other types of electromagnetic clutches
o
3.1 Multiple disk clutches
o
3.2 Electromagnetic tooth clutches
o
3.3 Electromagnetic particle clutches
o
3.4 Hysteresis-powered clutch
5 References
Friction-plate
clutch
Main article: Friction-plate electromagnetic couplings
A friction-plate clutch uses a single plate friction surface to engage the input and output members of
the clutch.
How
it works
Engagement
When the clutch is required to actuate, current flows through the electromagnet, which produces a
magnetic field. The rotor portion of the clutch becomes magnetized and sets up a magnetic loop that
attracts the armature. The armature is pulled against the rotor and a frictional force is generated at
contact. Within a relatively short time, the load is accelerated to match the speed of the rotor, thereby
engaging the armature and the output hub of the clutch. In most instances, the rotor is constantly
rotating with the input all the time.
Disengagement
When current is removed from the clutch, the armature is free to turn with the shaft. In most designs,
springs hold the armature away from the rotor surface when power is released, creating a small air
gap.
Cycling
Cycling is achieved by interrupting the current through the electromagnet. Slippage normally occurs
only during acceleration. When the clutch is fully engaged, there is no relative slip, assuming the clutch
is sized properly, and thus torque transfer is 100% efficient.
Applications
Machinery
This type of clutch is used in some lawnmowers, copy machines, and conveyor drives. Other
applications include packaging machinery, printing machinery, food processing machinery, and factory
automation.
Automobiles
When the electromagnetic clutch is used in automobiles, there may be a clutch release switch inside
the gear lever. The driver operates the switch by holding the gear lever to change the gear, thus
cutting off current to the electromagnet and disengaging the clutch. With this mechanism, there is no
need to depress the clutch pedal. Alternatively, the switch may be replaced by a touch
sensor or proximity sensor which senses the presence of the hand near the lever and cuts off the
current. The advantages of using this type of clutch for automobiles are that complicated linkages are
not required to actuate the clutch, and the driver needs to apply a considerably reduced force to
operate the clutch. It is a type of semi-automatic transmission.
Electromagnetic clutches are also often found in AWD systems, and are used to vary the amount of
power sent to individual wheels or axles.
A smaller electromagnetic clutch connects the air conditioning compressor to a pulley driven by the
crankshaft, allowing the compressor to cycle on only when needed.
Locomotives
Electromagnetic clutches have been used on diesel locomotives, e.g. by Hohenzollern Locomotive
Works.
Other
types of electromagnetic clutches
Multiple
disk clutches
Multiple disk clutch
Introduction – Multiple disk clutches are used to deliver extremely high torque in a relatively small
space. These clutches can be used dry or wet (oil bath). Running the clutches in an oil bath also
greatly increases the heat dissipation capability, which makes them ideally suited for multiple speed
gear boxes and machine tool applications.
How it works – Multiple disk clutches operate via an electrical actuation but transmit torque
mechanically. When current is applied through the clutch coil, the coil becomes an electromagnet and
produces magnetic lines of flux. These lines of flux are transferred through the small air gap between
the field and the rotor. The rotor portion of the clutch becomes magnetized and sets up a magnetic
loop, which attracts both the armature and friction disks. The attraction of the armature compresses
(squeezes) the friction disks, transferring the torque from the in inner driver to the out disks. The output
disks are connected to a gear, coupling, or pulley via drive cup. The clutch slips until the input and
output RPMs are matched. This happens relatively quickly typically (.2 - 2 sec).
When the current is removed from the clutch, the armature is free to turn with the shaft. Springs hold
the friction disks away from each other, so there is no contact when the clutch is not engaged, creating
a minimal amount of drag.
Electromagnetic
tooth clutches
Electromagnetic tooth clutch
Introduction – Of all the electromagnetic clutches, the tooth clutches provide the greatest amount of
torque in the smallest overall size. Because torque is transmitted without any slippage, clutches are
ideal for multi stage machines where timing is critical such as multi stage printing presses. Sometimes,
exact timing needs to be kept, so tooth clutches can be made with a single position option which
means that they will only engage at a specific degree mark. They can be used in dry or wet (oil bath)
applications, so they are very well suited for gear box type drives.
They should not be used in high speed applications or applications that have engagement speeds over
50 rpm otherwise damage to the clutch teeth would occur when trying to engage the clutch.
How it works – Electromagnetic tooth clutches operate via an electric actuation but transmit torque
mechanically. When current flows through the clutch coil, the coil becomes an electromagnet and
produces magnetic lines of flux. This flux is then transferred through the small gap between the field
and the rotor. The rotor portion of the clutch becomes magnetized and sets up a magnetic loop, which
attracts the armature teeth to the rotor teeth. In most instances, the rotor is consistently rotating with
the input (driver). As soon as the clutch armature and rotor are engaged, lock up is 100%.
When current is removed from the clutch field, the armature is free to turn with the shaft. Springs hold
the armature away from the rotor surface when power is released, creating a small air gap and
providing complete disengagement from input to output.
Electromagnetic
particle clutches
Main article: magnetic particle clutch
Electromagnetic particle clutch
Introduction – Magnetic particle clutches are unique in their design, from other electro-mechanical
clutches because of the wide operating torque range available. Like a standard, single face clutch,
torque to voltage is almost linear. However, in a magnetic particle clutch torque can be controlled very
accurately. This makes these units ideally suited for tension control applications, such as wire winding,
foil, film, and tape tension control. Because of their fast response, they can also be used in high cycle
application, such as card readers, sorting machines, and labeling equipment.
How it works – Magnetic particles (very similar to iron filings) are located in the powder cavity. When
current flows through the coil, the magnetic flux that is created tries to bind the particles together,
almost like a magnetic particle slush. As the current is increased, the magnetic field builds,
strengthening the binding of the particles. The clutch rotor passes through the bound particles, causing
drag between the input and the output during rotation. Depending upon the output torque requirement,
the output and input may lock at 100% transfer.
When current is removed from the clutch, the input is almost free to turn with the shaft. Because the
magnetic particles remain in the cavity, all magnetic particle clutches have some minimum drag.
Hysteresis-powered
Hysteresis powered clutch
clutch
Introduction – Electrical hysteresis units have an extremely high torque range. Since these units can
be controlled remotely, they are ideal for testing applications where varying torque is required. Since
drag torque is minimal, these units offer the widest available torque range of any electromagnetic
product. Most applications involving powered hysteresis units are in test stand requirements. Since all
torque is transmitted magnetically, there is no contact, so no wear occurs to any of the torque transfer
components providing for extremely long life.
How it works – When the current is applied, it creates magnetic flux. This passes into the rotor portion
of the field. The hysteresis disk physically passes through the rotor, without touching it. These disks
have the ability to become magnetized depending upon the strength of the flux (this dissipates as flux
is removed). This means, as the rotor rotates, magnetic drag between the rotor and the hysteresis disk
takes place causing rotation. In a sense, the hysteresis disk is pulled after the rotor. Depending upon
the output torque required, this pull eventually can match the input speed, giving a 100% lockup.
When current is removed from the clutch, the armature is free to turn and no relative force is
transmitted between either member. Therefore, the only torque seen between the input and the output
is bearing drag.
See

also
Electromagnetic brake
If you drive a manual transmission car, you may be surprised to find out that it has more than one clutch.
And it turns out that folks with automatic transmission cars have clutches, too. In fact, there are clutches in
many things you probably see or use every day: Many cordless drills have a clutch, chain saws have a
centrifugal clutch and even some yo-yos have a clutch.
In this article, you'll learn why you need a clutch, how the clutch in your car works and find out some
interesting, and perhaps surprising, places where clutches can be found.
Clutches are useful in devices that have two rotating shafts. In these devices, one of the shafts is typically
driven by a motor or pulley, and the other shaft drives another device. In a drill, for instance, one shaft is
driven by a motor and the other drives a drill chuck. The clutch connects the two shafts so that they can
either be locked together and spin at the same speed, or be decoupled and spin at different speeds.
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In a car, you need a clutch because the engine spins all the time, but the car's wheels do not. In order for a
car to stop without killing the engine, the wheels need to be disconnected from the engine somehow. The
clutch allows us to smoothly engage a spinning engine to a non-spinning transmission by controlling the
slippage between them.
To understand how a clutch works, it helps to know a little bit aboutfriction, which is a measure of how hard
it is to slide one object over another. Friction is caused by the peaks and valleys that are part of every
surface -- even very smooth surfaces still have microscopic peaks and valleys. The larger these peaks and
valleys are, the harder it is to slide the object. You can learn more about friction in How Brakes Work.
A clutch works because of friction between a clutch plate and a flywheel. We'll look at how these parts work
together in the next section.
Fly Wheels, Clutch Plates and Friction
In a car's clutch, a flywheel connects to the engine, and a clutch plate connects to the transmission. You
can see what this looks like in the figure below.
Exploded view of a clutch
When your foot is off the pedal, the springs push the pressure plate against the clutch disc, which in turn
presses against the flywheel. This locks the engine to the transmission input shaft, causing them to spin at
the same speed.
Photo courtesy Carolina Mustang
Pressure plate
The amount of force the clutch can hold depends on the friction between the clutch plate and the flywheel,
and how much force the spring puts on the pressure plate. The friction force in the clutch works just like the
blocks described in the friction section of How Brakes Work, except that the spring presses on the clutch
plate instead of weight pressing the block into the ground.
How a clutch engages and releases
When the clutch pedal is pressed, a cable or hydraulic piston pushes on the release fork, which presses the
throw-out bearing against the middle of the diaphragm spring. As the middle of the diaphragm spring is
pushed in, a series of pins near the outside of the spring causes the spring to pull the pressure plate away
from the clutch disc (see below). This releases the clutch from the spinning engine.
Photo courtesy Carolina Mustang
Clutch plate
Note the springs in the clutch plate. These springs help to isolate the transmission from the shock of the
clutch engaging.
This design usually works pretty well, but it does have a few drawbacks. We'll look at common clutch
problems and other uses for clutches in the following sections.
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Rack-and-pinion Steering
Rack-and-pinion steering is quickly becoming the most common type of steering on cars, small
trucks and SUVs. It is actually a pretty simple mechanism. A rack-and-pinion gearset is enclosed
in a metal tube, with each end of the rack protruding from the tube. A rod, called a tie rod,
connects to each end of the rack.
The pinion gear is attached to the steering shaft. When you turn the steering wheel, the gear
spins, moving the rack. The tie rod at each end of the rack connects to the steering arm on
the spindle (see diagram above).
The rack-and-pinion gearset does two things:
•
It converts the rotational motion of the steering wheel into the linear motion needed to
turn the wheels.
•
It provides a gear reduction, making it easier to turn the wheels.
On most cars, it takes three to four complete revolutions of the steering wheel to make the wheels
turn from lock to lock (from far left to far right).
The steering ratio is the ratio of how far you turn the steering wheel to how far the wheels turn.
For instance, if one complete revolution (360 degrees) of the steering wheel results in the wheels
of the car turning 20 degrees, then the steering ratio is 360 divided by 20, or 18:1. A higher ratio
means that you have to turn the steering wheel more to get the wheels to turn a given distance.
However, less effort is required because of the higher gear ratio.
Generally, lighter, sportier cars have lower steering ratios than larger cars and trucks. The lower
ratio gives the steering a quicker response -- you don't have to turn the steering wheel as much to
get the wheels to turn a given distance -- which is a desirable trait in sports cars. These smaller
cars are light enough that even with the lower ratio, the effort required to turn the steering wheel
is not excessive.
Some cars have variable-ratio steering, which uses a rack-and-pinion gearset that has a
different tooth pitch (number of teeth per inch) in the center than it has on the outside. This makes
the car respond quickly when starting a turn (the rack is near the center), and also reduces effort
near the wheel's turning limits.
Power Rack-and-pinion
When the rack-and-pinion is in a power-steering system, the rack has a slightly different design.
Part of the rack contains a cylinder with a piston in the middle. The piston is connected to the
rack. There are two fluid ports, one on either side of the piston. Supplying higher-pressure fluid to
one side of the piston forces the piston to move, which in turn moves the rack, providing the
power assist.
We'll check out the components that provide the high-pressure fluid, as well as decide which side
of the rack to supply it to, later in the article. First, let's take a look at another type of steering.
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Recirculating-ball Steering
Recirculating-ball steering is used on many trucks and SUVs today. The linkage that turns the
wheels is slightly different than on a rack-and-pinion system.
The recirculating-ball steering gear contains a worm gear. You can image the gear in two parts.
The first part is a block of metal with a threaded hole in it. This block has gear teeth cut into the
outside of it, which engage a gear that moves the pitman arm (see diagram above). The steering
wheel connects to a threaded rod, similar to a bolt, that sticks into the hole in the block. When the
steering wheel turns, it turns the bolt. Instead of twisting further into the block the way a regular
bolt would, this bolt is held fixed so that when it spins, it moves the block, which moves the gear
that turns the wheels.
Instead of the bolt directly engaging the threads in the block, all of the threads are filled with ball
bearings that recirculate through the gear as it turns. The balls actually serve two purposes: First,
they reduce friction and wear in the gear; second, they reduce slop in the gear. Slop would be felt
when you change the direction of the steering wheel -- without the balls in the steering gear, the
teeth would come out of contact with each other for a moment, making the steering wheel feel
loose.
Power steering in a recirculating-ball system works similarly to a rack-and-pinion system. Assist is
provided by supplying higher-pressure fluid to one side of the block.
Now let's take a look at the other components that make up a power-steering system.
Power Steering
There are a couple of key components in power steeringin addition to the rack-and-pinion or
recirculating-ball mechanism.
Pump
The hydraulic power for the steering is provided by a rotary-vane pump (see diagram below).
This pump is driven by the car's engine via a belt and pulley. It contains a set of retractable vanes
that spin inside an oval chamber.
As the vanes spin, they pull hydraulic fluid from the return line at low pressure and force it into the
outlet at high pressure. The amount of flow provided by the pump depends on the car's engine
speed. The pump must be designed to provide adequate flow when the engine is idling. As a
result, the pump moves much more fluid than necessary when the engine is running at faster
speeds.
The pump contains a pressure-relief valve to make sure that the pressure does not get too high,
especially at high engine speeds when so much fluid is being pumped.
Rotary Valve
A power-steering system should assist the driver only when he is exerting force on the steering
wheel (such as when starting a turn). When the driver is not exerting force (such as when driving
in a straight line), the system shouldn't provide any assist. The device that senses the force on
the steering wheel is called the rotary valve.
The key to the rotary valve is a torsion bar. The torsion bar is a thin rod of metal that twists
when torque is applied to it. The top of the bar is connected to the steering wheel, and the bottom
of the bar is connected to the pinion or worm gear (which turns the wheels), so the amount of
torque in the torsion bar is equal to the amount of torque the driver is using to turn the wheels.
The more torque the driver uses to turn the wheels, the more the bar twists.
The input from the steering shaft forms the inner part of a spool-valve assembly. It also
connects to the top end of the torsion bar. The bottom of the torsion bar connects to the outer
part of the spool valve. The torsion bar also turns the output of the steering gear, connecting to
either the pinion gear or the worm gear depending on which type of steering the car has.
The Future of Power Steering
Since the power-steering pump on most cars today runs constantly, pumping fluid all the time, it
wastes horsepower. This wasted power translates into wasted fuel.
You can expect to see several innovations that will improve fuel economy. One of the coolest
ideas on the drawing board is the "steer-by-wire" or "drive-by-wire" system. These systems would
completely eliminate the mechanical connection between the steering wheel and the steering,
replacing it with a purely electronic control system. Essentially, the steering wheel would work like
the one you can buy for your home computer to play games. It would contain sensors that tell the
car what the driver is doing with the wheel, and have some motors in it to provide the driver with
feedback on what the car is doing. The output of these sensors would be used to control a
motorized steering system. This would free up space in the engine compartment by eliminating
the steering shaft. It would also reduce vibration inside the car.
General Motors has introduced a concept car, the Hy-wire, that features this type of driving
system. One of the most exciting things about the drive-by-wire system in the GM Hy-wire is that
you can fine-tune vehicle handling without changing anything in the car's mechanical components
-- all it takes to adjust the steering is some new computer software. In future drive-by-wire
vehicles, you will most likely be able to configure the controls exactly to your liking by pressing a
few buttons, just like you might adjust the seat position in a car today. It would also be possible in
this sort of system to store distinct control preferences for each driver in the family.
Differential (mechanical device)
For other uses, see Differential.
Unsourced material may be challenged and removed. (July 2009)
A differential is a device, usually, but not necessarily, employing gears, which is connected to the
outside world by three shafts, through which it transmits torque and rotation. The gears or other
components make the three shafts rotate in such a way that
theangular velocities of the three shafts, and
equal, so
and
is proportional to the sum (or average) of
, where , , and
are
are constants. Often, but not always,
and
are
and . Except in some special-purpose
differentials, there are no other limitations on the rotational speeds of the shafts. Any of the shafts can
be used to input rotation, and the other(s) to output it. See animation here of a simple differential in
which
and
are equal. The shaft rotating at speed
is at the bottom-right of the image.
In automobiles and other wheeled vehicles, a differential allows the driving roadwheels to rotate at
different speeds. This is necessary when the vehicle turns, making the wheel that is travelling around
the outside of the turning curve roll farther and faster than the other. The engine is connected to the
shaft rotating at angular velocity . The driving wheels are connected to the other two shafts, and
and
are equal. If the engine is running at a constant speed, the rotational speed of each driving
wheel can vary, but the sum (or average) of the two wheels' speeds can not change. An increase in
the speed of one wheel must be balanced by an equal decrease in the speed of the other.
It may seem illogical that the speed of one input shaft can determine the speeds of two output shafts,
which are allowed to vary. Logically, the number of inputs should be at least as great as the number of
outputs. However, the system has another constraint. The ratio of the speeds of the two driving wheels
equals the ratio of the radii of the paths around which the two wheels are rolling, which is determined
by the track-width of the vehicle (the distance between the driving wheels) and the radius of the turn.
Thus the system does not have one input and two independent outputs. It has two inputs and two
outputs.
A different automotive application of differentials is in epicyclic gearing. A gearbox is constructed out of
several differentials. In each differential, one shaft is connected to the engine (through a clutch or
functionally similar device), another to the driving wheels (through another differential as described
above), and the third shaft can be braked so its angular velocity is zero. (The braked component may
not be a shaft, but something that plays an equivalent role.) When one shaft is braked, the gear ratio
between the engine and wheels is determined by the value(s) of
and/or
for that differential, which
reflect the numbers of teeth on its gears. Several differentials, with different gear ratios, are
permanently connected in parallel with each other, but only one of them has one shaft braked so it can
not rotate, so only that differential transmits power from the engine to the wheels. (If the transmission
is in "neutral" or "park", none of the shafts is braked.) Shifting gears simply involves releasing the
braked shaft of one differential and braking the appropriate shaft on another. This is a much simpler
operation to do automatically than engaging and disengaging gears in a conventional gearbox.
Epicyclic gearing is almost always used in automatic transmissions, and is nowadays also used in
some hybrid and electric vehicles.
Non-automotive uses of differentials include performing analog arithmetic. Two of the differential's
three shafts are made to rotate through angles that represent (are proportional to) two numbers, and
the angle of the third shaft's rotation represents the sum or difference of the two input numbers.
Anequation clock that used a differential for addition, made in 1720, is the earliest device definitely
known to have used a differential for any purpose.[1] In the 20th Century, large assemblies of many
differentials were used as analog computers, calculating, for example, the direction in which a gun
should be aimed. However, the development of electronic digital computers has made these uses of
differentials obsolete.[2] Practically all the differentials that are now made are used in automobiles and
Contents
[hide]
1 Purpose
2 Functional description
3 History
4 Loss of traction
5 Traction-aiding devices
6 Epicyclic differential
7 Spur-gear differential
8 Non-automotive applications
9 Active differentials
10 Automobiles without differentials
12 References and footnotes
Purpose
A vehicle's wheels rotate at different speeds, mainly when turning corners. The differential is designed
to drive a pair of wheels while allowing them to rotate at different speeds. In vehicles without a
differential, such as karts, both driving wheels are forced to rotate at the same speed, usually on a
common axle driven by a simple chain-drive mechanism. When cornering, the inner wheel needs to
travel a shorter distance than the outer wheel, so with no differential, the result is the inner wheel
spinning and/or the outer wheel dragging, and this results in difficult and unpredictable handling,
damage to tires and roads, and strain on (or possible failure of) the entire drivetrain.
Functional
description
Input torque is applied to the ring gear (blue), which turns the entire carrier (blue). The carrier is connected to both
the sun gears (red and yellow) only through the planet gear (green). Torque is transmitted to the sun gears through
the planet gear. The planet gear revolves around the axis of the carrier, driving the sun gears. If the resistance at
both wheels is equal, the planet gear revolves without spinning about its own axis, and both wheels turn at the
same rate.
If the left sun gear (red) encounters resistance, the planet gear (green) spins as well as revolving, allowing the left
sun gear to slow down, with an equal speeding up of the right sun gear (yellow).
The following description of a differential applies to a "traditional" rear-wheel-drive car or truck with an
"open" or limited slip differential combined with a reduction gearset:
Torque is supplied from the engine, via thetransmission, to a drive shaft (British term: 'propeller shaft',
commonly and informally abbreviated to 'prop-shaft'), which runs to the final drive unit that contains the
differential. A spiral bevel pinion gear takes its drive from the end of the propeller shaft, and is encased
within the housing of the final drive unit. This meshes with the large spiral bevel ring gear, known as
the crown wheel. The crown wheel and pinion may mesh in hypoid orientation, not shown. The crown
wheel gear is attached to the differential carrier or cage, which contains the 'sun' and 'planet' wheels or
gears, which are a cluster of four opposed bevel gears in perpendicular plane, so each bevel gear
meshes with two neighbours, and rotates counter to the third, that it faces and does not mesh with.
The two sun wheel gears are aligned on the same axis as the crown wheel gear, and drive the
axle half shaftsconnected to the vehicle's driven wheels. The other two planet gears are aligned on a
perpendicular axis which changes orientation with the ring gear's rotation. In the two figures shown
above, only one planet gear (green) is illustrated, however, most automotive applications contain two
opposing planet gears. Other differential designs employ different numbers of planet gears, depending
on durability requirements. As the differential carrier rotates, the changing axis orientation of the planet
gears imparts the motion of the ring gear to the motion of the sun gears by pushing on them rather
than turning against them (that is, the same teeth stay in the same mesh or contact position), but
because the planet gears are not restricted from turning against each other, within that motion, the sun
gears can counter-rotate relative to the ring gear and to each other under the same force (in which
case the same teeth do not stay in contact).
Thus, for example, if the car is making a turn to the right, the main crown wheel may make 10 full
rotations. During that time, the left wheel will make more rotations because it has further to travel, and
the right wheel will make fewer rotations as it has less distance to travel. The sun gears (which drive
the axle half-shafts) will rotate in opposite directions relative to the ring gear by, say, 2 full turns each
(4 full turns relative to each other), resulting in the left wheel making 12 rotations, and the right wheel
making 8 rotations.
The rotation of the crown wheel gear is always the average of the rotations of the side sun gears. This
is why, if the driven roadwheels are lifted clear of the ground with the engine off, and the drive shaft is
held (say leaving the transmission 'in gear', preventing the ring gear from turning inside the
differential), manually rotating one driven roadwheel causes the opposite roadwheel to rotate in the
opposite direction by the same amount.
When the vehicle is traveling in a straight line, there will be no differential movement of the planetary
system of gears other than the minute movements necessary to compensate for slight differences in
wheel diameter, undulations in the road (which make for a longer or shorter wheel path), etc.
History
There are many claims to the invention of the differential gear but it is possible that it was known, at
least in some places, in ancient times. Some historical milestones of the differential include:

1050 BC–771 BC: The Book of Song (which itself was written between 502 and 557 A.D.)
makes the assertion that the South Pointing Chariot, which may have used a differential gear, was
invented during the Western Zhou Dynasty in China.[citation needed]

150 - 100 BC: Hypothesized use, now discredited, in the Greek Antikythera mechanism

30 BC - 20 BC: Differential gear systems possibly used in China
South Pointing Chariot model

227–239 AD: Despite doubts from fellow ministers at court, Ma Jun from the Kingdom of
Wei in China invents the first historically verifiable South Pointing Chariot, which providedcardinal
direction as a non-magnetic, mechanized compass. Some such chariots may have used
differential gears.

658, 666 AD: two Chinese Buddhist monks and engineers create South Pointing Chariots
for Emperor Tenji of Japan.

1027, 1107 AD: Documented Chinese reproductions of the South Pointing Chariot by Yan Su
and then Wu Deren, which described in detail the mechanical functions and gear ratios of the
device much more so than earlier Chinese records.

1720: Joseph Williamson uses a differential gear in a clock.

1810: Rudolph Ackermann of Germany invents a four-wheel steering system for carriages,
which some later writers mistakenly report as a differential.

1827: modern automotive differential patented by watchmaker Onésiphore Pecqueur (1792–
1852) of the Conservatoire des Arts et Métiers in France for use on a steam cart. (Sources:
Britannica Online and[3])

1832: Richard Roberts of England patents 'gear of compensation', a differential for road
locomotives.

1876: James Starley of Coventry invents chain-drive differential for use on bicycles; invention
later used on automobiles by Karl Benz.

1897: first use of differential on an Australian steam car by David Shearer.

1958: Vernon Gleasman patents the Torsen dual-drive differential, a type of limited slip
differentialthat relies solely on the action of gearing instead of a combination of clutches and
gears.
Note: The Antikythera mechanism (150 BC–100 BC), discovered on an ancient shipwreck near
theGreek island of Antikythera, was once suggested to have employed a differential gear. This has
since been disproved. Other possible uses of differentials prior to Joseph Williamson's clock of 1720
are hypothetical.
Loss
of traction
One undesirable side effect of a conventional differential is that it can limit traction under less than
ideal conditions. The amount of traction required to propel the vehicle at any given moment depends
on the load at that instant—how heavy the vehicle is, how much drag and friction there is, the gradient
of the road, the vehicle's momentum, and so on.
The torque applied to each driving wheel is a result of the engine, transmission and drive axles
applying a twisting force against the resistance of the traction at that roadwheel. In lower gears and
thus at lower speeds, and unless the load is exceptionally high, the drivetrain can supply as much
torque as necessary, so the limiting factor becomes the traction under each wheel. It is therefore
convenient to define traction as the amount of torque that can be generated between the tire and the
road surface, before the wheel starts to slip. If the torque applied to one of the drive wheels exceeds
the threshold of traction, then that wheel will spin, and thus only provide torque at each other driven
wheel limited by the sliding friction at the slipping wheel. The reduced nett traction may still be enough
to propel the vehicle.
A conventional "open" (non-locked or otherwise traction-aided) differential always supplies close to
equal (because of internal friction) torque to each side. [4] To illustrate how this can limit torque applied
to the driving wheels, imagine a simple rear-wheel drive vehicle, with one rear roadwheel on asphalt
with good grip, and the other on a patch of slippery ice. It takes very little torque to spin the side on
slippery ice, and because a differential splits torque equally to each side, the torque that is applied to
the side that is on asphalt is limited to this amount.[5][6]
Based on the load, gradient, et cetera, the vehicle requires a certain amount of torque applied to the
drive wheels to move forward. Since an open differential limits total torque applied to both drive wheels
to the amount utilized by the lower traction wheel multiplied by a factor of 2, when one wheel is on a
slippery surface, the total torque applied to the driving wheels may be lower than the minimum torque
required for vehicle propulsion.[4]
A proposed way to distribute the power to the wheels, is to use the concept of gearless differential, of
which a review has been reported by Provatidis, [7] but the various configurations seem to correspond
either to the "sliding pins and cams" type, such as the ZF B-70 available for early VWs, or are a
variation of the ball differential.
Many newer vehicles feature traction control, which partially mitigates the poor traction characteristics
of an open differential by using the anti-lock braking system to limit or stop the slippage of the low
traction wheel, increasing the torque that can be applied to both wheels. While not as effective in
propelling a vehicle under poor traction conditions as a traction-aided differential, it is better than a
simple mechanical open differential with no electronic traction assistance.
Traction-aiding
devices
This section may contain original research. Please improve
consisting only of original research may be removed. (July 2009)
ARB, air-locking differential
A cutaway drawing of a car's rearaxle, showing the crown wheel andpinion of the final drive, and the smaller
differential gears
A cutaway view of an automotive final drive unit which contains the differential
There are various devices for getting more usable traction from vehicles with differentials.

One solution is the Positive Traction (Posi), the most well-known of which is the clutch-type.
With this differential, the sun gears are coupled to the carrier via a multi-disc clutch which allows
extra torque to be sent to the wheel with higher resistance than available at the other driven road
wheel when the limit of friction is reached at that other wheel. Below the limit of friction more
torque goes to the slower (inside) wheel.

A limited slip differential (LSD) or anti-spin is another type of traction aiding device that uses a
mechanical system that activates under centrifugal force to positively lock the left and right spider
gears together when one wheel spins a certain amount faster than the other. This type behaves
as an open differential unless one wheel begins to spin and exceeds that threshold. While
positraction units can be of varying strength, some of them with high enough friction to cause an
inside tire to spin or outside tire to drag in turns like a spooled differential, the LSD will remain
open unless enough torque is applied to cause one wheel to lose traction and spin, at which point
it will engage. A LSD can use clutches like a posi when engaged, or may also be a solid
mechanical connection like a locker or spool. It is called limited slip because it does just that; it
limits the amount that one wheel can "slip" (spin).

A locking differential, such as ones using differential gears in normal use but using air or
electrically controlled mechanical system, which when locked allow no difference in speed
between the two wheels on the axle. They employ a mechanism for allowing the axles to be
locked relative to each other, causing both wheels to turn at the same speed regardless of which
has more traction; this is equivalent to effectively bypassing the differential gears entirely. Other
locking systems may not even use differential gears but instead drive one wheel or both
depending on torque value and direction. Automatic mechanical lockers do allow for some
differentiation under certain load conditions, while a selectable locker typically couples both axles
with a solid mechanical connection like a spool when engaged.

A high-friction 'Automatic Torque Biasing' (ATB) differential, such as the Torsen differential,
where the friction is between the gear teeth rather than at added clutches. This applies more
torque to the driven roadwheel with highest resistance (grip or traction) than is available at the
other driven roadwheel when the limit of friction is reached at that other wheel. When tested with
the wheels off the ground, if one wheel is rotated with the differential case held, the other wheel
will still rotate in the opposite direction as for an open differential but there will be some frictional
losses and the torque will be distributed at other than 50/50. Although marketed as being "torquesensing", it functions the same as a limited-slip differential. 3D Animation of a Torsen Differential

A very high-friction differential, such as the ZF "sliding pins and cams" type, so that there is
locking from very high internal friction. When tested with the wheels off the ground with torque
applied to one wheel it will lock, but it is still possible for the differential action to occur in use,
albeit with considerable frictional losses, and with the road loads at each wheel in opposite
directions rather than the same (acting with a "locking and releasing" action rather than a
distributed torque).

Electronic traction control systems usually use the anti-lock braking system (ABS) roadwheel
speed sensors to detect a spinning roadwheel, and apply the brake to that wheel. This
progressively raises the reaction torque at that roadwheel, and the differential compensates by
transmitting more torque through the other roadwheel—the one with better traction.
In Volkswagen Group vehicles, this specific function is called 'Electronic Differential Lock' (EDL).

A spool is just what it sounds like. It may replace the spider gears within the differential
carrier, or the entire carrier. A spool locks both axle shafts together 100% for maximum traction.
This is typically only used in drag racing applications, where the vehicle is to be driven in a
straight line while applying tremendous torque to both wheels.

In a four-wheel drive vehicle, a viscous coupling unit can replace a centre differential entirely,
or be used to limit slip in a conventional 'open' differential. It works on the principle of allowing the
two output shafts to counter-rotate relative to each other, by way of a system of slotted plates that
operate within a viscous fluid, often silicone. The fluid allows slow relative movements of the
shafts, such as those caused by cornering, but will strongly resist high-speed movements, such as
those caused by a single wheel spinning. This system is similar to a limited slip differential.
A four-wheel drive (4WD) vehicle will have at least two differentials (one in each axle for each pair of
driven roadwheels), and possibly a centre differential to apportion torque between the front and rear
axles. In some cases (e.g. Lancia Delta Integrale, Porsche 964 Carrera 4 of 1989[8]) the centre
differential is an epicyclic differential (see below) to divide the torque asymmetrically, but at a fixed rate
between the front and rear axle. Other methods utilise an 'Automatic Torque Biasing' (ATB) centre
differential, such as a Torsen—which is what Audi use in their quattro cars (with longitudinal engines).
4WD vehicles without a centre differential should not be driven on dry, paved roads in four-wheel drive
mode, as small differences in rotational speed between the front and rear wheels cause a torque to be
applied across the transmission. This phenomenon is known as "wind-up", and can cause
considerable damage to the transmission or drive train. On loose surfaces these differences are
absorbed by the tire slippage on the road surface.
A transfer case may also incorporate a centre differential, allowing the drive shafts to spin at different
speeds. This permits the four-wheel drive vehicle to drive on paved surfaces without experiencing
"wind-up".
Epicyclic
differential
Epicyclic gearing is used here to apportion torque asymmetrically. The input shaft is the green hollow one, the
yellow is the low torque output, and the pink is the high torque output. The force applied in the yellow and the pink
gears is the same, but since the arm of the pink one is 2× to 3× as big, the torque will be 2× to 3× as high.
An epicyclic differential uses epicyclic gearing to split and apportion torque asymmetrically between
the front and rear axles. An epicyclic differential is at the heart of the Toyota Priusautomotive drive
train, where it interconnects the engine, motor-generators, and the drive wheels (which have a second
differential for splitting torque as usual). It has the advantage of being relatively compact along the
length of its axis (that is, the sun gear shaft).
Epicyclic gears are also called planetary gears because the axes of the planet gears revolve around
the common axis of the sun and ring gears that they mesh with and roll between. In the image, the
yellow shaft carries the sun gear which is almost hidden. The blue gears are called planet gears and
the pink gear is the ring gear or annulus.
Spur-gear
differential
This is another type of differential that was used in some early automobiles, more recently
theOldsmobile Toronado, as well as other non-automotive applications. It consists of spur gears only.
A spur-gear differential has two equal-sized spur gears, one for each half-shaft, with a space between
them. Instead of the Bevel gear, also known as a miter gear, assembly (the "spider") at the centre of
the differential, there is a rotating carrier on the same axis as the two shafts. Torque from a prime
mover or transmission, such as the drive shaft of a car, rotates this carrier.
Mounted in this carrier are one or more pairs of identical pinions, generally longer than their diameters,
and typically smaller than the spur gears on the individual half-shafts. Each pinion pair rotates freely
on pins supported by the carrier. Furthermore, the pinions pairs are displaced axially, such that they
mesh only for the part of their length between the two spur gears, and rotate in opposite directions.
The remaining length of a given pinion meshes with the nearer spur gear on its axle. Therefore, each
pinion couples that spur gear to the other pinion, and in turn, the other spur gear, so that when the
drive shaft rotates the carrier, its relationship to the gears for the individual wheel axles is the same as
that in a bevel-gear differential.
Non-automotive
applications
The oldest known example of a differential was once thought to be in the Antikythera mechanism. It
was supposed to have used such a train to produce the difference between two inputs, one input
related to the position of the sun on the zodiac, and the other input related to the position of
the moonon the zodiac; the output of the differential gave a quantity related to the moon's phase. It has
now been proven that the assumption of the existence of a differential gearing arrangement was
incorrect.[9][original research?]
Chinese south-pointing chariots may also have been very early applications of differentials. The
chariot had a pointer which constantly pointed to the south, no matter how the chariot turned as it
travelled. It could therefore be used as a type of compass. It is widely thought that a differential
mechanism responded to any difference between the speeds of rotation of the two wheels of the
chariot, and turned the pointer appropriately. However, the mechanism was not precise enough, and,
after a few miles of travel, the dial could have very well been pointing in the complete opposite
direction.
The earliest definitely verified use of a differential was in a clock made by Joseph Williamson in 1720.
It employed a differential to add the Equation of Time to local mean time, as determined by the clock
mechanism, to produce solar time, which would have been the same as the reading of a sundial.
During the 18th Century, sundials were considered to show the "correct" time, so an ordinary clock
would frequently have to be readjusted, even if it worked perfectly, because of seasonal variations in
the Equation of Time. Williamson's and other equation clocks showed sundial time without needing
readjustment. Nowadays, we consider clocks to be "correct" and sundials usually incorrect, so many
sundials carry instructions about how to use their readings to obtain clock time.
In the first half of the twentieth century, mechanical analog computers, called differential analyzers,
were constructed that used differential gear trains to perform addition and subtraction. The U.S. Navy
Mk.1 gun fire control computer used about 160 differentials of the bevel-gear type.
A differential gear train can be used to allow a difference between two input axles. Mills often used
such gears to apply torque in the required axis. Differentials are also used in this way in watchmaking
to link two separate regulating systems with the aim of averaging out errors. Greubel Forsey use a
differential to link two double tourbillon systems in their Quadruple Differential Tourbillon.
Active
differentials
A relatively new technology is the electronically-controlled 'active differential'. An electronic control
unit(ECU) uses inputs from multiple sensors, including yaw rate, steering input angle, and lateral
acceleration—and adjusts the distribution of torque to compensate for undesirable handling
behaviours like understeer. Active differentials used to play a large role in the World Rally
Championship, but in the 2006 season the FIA has limited the use of active differentials only to those
drivers who have not competed in the World Rally Championship in the last five years.
Fully integrated active differentials are used on the Ferrari F430, Mitsubishi Lancer Evolution, and on
the rear wheels in the Acura RL. A version manufactured by ZF is also being offered on the latest Audi
S4 and Audi A4.[10]
The second constraint of the differential is passive—it is actuated by the friction kinematics chain
through the ground. The difference in torque on the roadwheels and tires (caused by turns or bumpy
ground) drives the second degree of freedom, (overcoming the torque of inner friction) to equalise the
driving torque on the tires. The sensitivity of the differential depends on the inner friction through the
second degree of freedom. All of the differentials (so called “active” and “passive”) use clutches and
brakes for restricting the second degree of freedom, so all suffer from the same disadvantage—
decreased sensitivity to a dynamically changing environment. The sensitivity of the ECU controlled
differential is also limited by the time delay caused by sensors and the response time of the actuators.
Automobiles
without differentials
Although the vast majority of automobiles in the developed world use differentials, there are a few that
do not. Several different types exist:

Vehicles with a single driving wheel. Besides motorcycles, which are generally not classified
as automobiles, this group includes most three-wheeled cars. These were quite common in
Europe in the mid-20th Century, but have now become rare there. They are still common in some
areas of the developing world, such as India. Some early four-wheeled cars also had only one
driving wheel to avoid the need for a differential. However, this arrangement led to many
problems. The system was unbalanced, the driving wheel would easily spin, etc.. Because of
these problems, few such vehicles were made.

Vehicles using two freewheels. A freewheel, as used on a pedal bicycle for example, allows a
road wheel to rotate faster than the mechanism that drives it, allowing a cyclist to stop pedalling
while going downhill. Some early automobiles had the engine driving two freewheels, one for each
driving road wheel. When the vehicle turned, the engine would continue to drive the wheel on the
inside of the curve, but the wheel on the outside was permitted to rotate faster by its freewheel.
Thus, while turning, the vehicle had only one driving wheel. Driving in reverse is also impossible
as is engine braking due to the freewheels.

Vehicles with continuously variable transmissions, such as the DAF Daffodil. The Daffodil, and
other similar vehicles which were made until the 1970s by the Dutch company DAF, had a type of
transmission that used an arrangement of belts and pulleys to provide an infinite number of gear
ratios. The engine drove two separate transmissions which ran the two driving wheels. When the
vehicle turned, the two wheels could rotate at different speeds, making the two transmissions shift
to different gear ratios, thus functionally substituting for a differential. The slower moving wheel
received more driving torque than the faster one, so the system had limited-slip characteristics.
The duplication also provided redundancy. If one belt broke, the vehicle could still be driven.

Light vehicles with closely spaced rear wheels, such as the Isetta and Opperman Unicar, or
very low mass vehicles.

Vehicles with separate motors for the driving wheels. Electric cars can have a separate motor
for each driving wheel, eliminating the need for a differential, but usually with some form of
gearing at each motor to get the large wheel torques necessary. Hybrid vehicles in which the final
drive is electric can be configured similarly.
See
also

Ball differential

Limited slip differential

Locking differential

Whippletree (mechanism), which evenly divides linear force as a differential divides torque.

Aron's electricity meter, an early electricity meter, relying on the use of a mechanical
differential.

Equation clock. One design uses a differential to add mean (clock) time and the equation of
time to get solar (sundial) time.

Torque Vectoring
References
and footnotes
mited-slip differential
(Redirected from Limited slip differential)
Cone-type LSD
A limited-slip differential (LSD) is a type of automotivedifferential gear arrangement that allows for some
difference in angular velocity of the output shafts, but imposes a mechanical bound on the disparity.
In an automobile, such limited-slip differentials are sometimes used in place of a standard differential, where
they convey certain dynamic advantages, at the expense of greater complexity. A slang term for a limited-slip
differential is Posi, named after GM's "Posi-Traction" unit which was built by Eaton. [1]
Contents
[hide]
1 Early history
2 Benefits
3 Basic principle of operation
o
3.1 Torque split during operation
4 Types
o
4.1 Fixed value
o
4.2 Torque sensitivity
4.2.1 Clutch, cone-type LSD

4.2.1.1 2-, 1-, and 1.5-way LSD

4.2.2 Geared LSD

o
4.3 Speed sensitivity

4.3.1 Viscous

4.3.2 Gerotor pump
o
4.4 Electronic
o
4.5 Electronic systems: brake-based
o
4.6 Other related final drives
5 Factory names
6 References
Early
history
In 1932, Ferdinand Porsche designed a Grand Prix racing car for the Auto Union company. The high power of
the design caused one of the rear wheels to experience excessive wheel spin at any speed up to 100 mph
(160 km/h). In 1935, Porsche commissioned the engineering firm ZF to design a limited-slip differential that
would perform better.[citation needed] The ZF "sliding pins and cams" became available, [2] and one example was the
Type B-70 for early VWs.
Benefits
The main advantage of a limited-slip differential is shown by considering the case of a standard (or "open")
differential in off-roading situations where one wheel has no contact with the ground. In such a case, with a
standard differential, the non-contacting wheel will receive 100% of the power while the contacting wheel will
remain stationary. The torque transmitted will be equal at both wheels, therefore will not exceed the threshold of
torque needed to move the wheel with grip. In this situation, a limited-slip differential prevents 100% of the
power from being allocated to one wheel, and thereby keeping both wheels in powered rotation.
Basic
principle of operation
Automotive limited-slip differentials all contain a few basic elements. First, all have a gear train that, like an
open differential, allows the outputs to spin at different speeds while holding the average speed of the two
outputs to be equal to the input speed.
Second, all have some sort of mechanism that applies a torque internal to the differential that resists the relative
motion of the output shafts. In simple terms this means they have some mechanism which resists a speed
difference between the outputs by creating a resisting torque between either the two outputs or the outputs and
the differential housing. There are many mechanisms used to create this resisting torque. The type of limited-slip
differential typically gets its name from the design of this resisting mechanism. Examples include viscous and
clutch-based LSDs. The amount of limiting torque provided by these mechanisms varies by design and is
discussed later in the article.
Torque
split during operation
An open differential has a fixed torque split between the input and outputs. In most cases the relationship is:

Trq out_1 = Trq out_2 , where 1 and 2 are typically the left and right drive wheels.

Trq in = Trq out_1 + Trq out_2 .
Thus the wheels always see the same torque even when spinning at different speeds, including the case where
one is stationary. Note, the torque split can be unequal, though 50:50 is typical.
A limited-slip differential has a more complex torque split and should be considered in the case when the outputs
are spinning the same speed and when spinning at different speeds. The torque difference between the two axles
is called Trq d .[3] (In this work it is called Trq f for torque friction[4]). Trq d is the difference in torque delivered to
the left and right wheel. The magnitude of Trq d comes from the slip limiting mechanism in the differential and
may be a function of input torque as in the case of a gear differential or the difference in the output speeds as in
the case of a viscous differential.
The torque delivered to the outputs is

Trq 1 = ½ Trq in + ½ Trq d for the slower output

Trq 2 = ½ Trq in – ½ Trq d for the faster output
When traveling in a straight line where one wheel starts to slip and spin faster than the wheel with traction,
torque is reduced to the slipping wheel (Trq 2 ) and provided to the slower wheel (Trq 1 ).
In the case when the vehicle is turning and neither wheel is slipping the inside wheel will be turning slower than
the outside wheel. In this case the inside wheel will receive more torque than the outside wheel which can result
in understeer.[4]
When both wheels are spinning at the same speed the torque distribution to each wheel is

Trq (1 or 2) = ½ Trq in ±(½ Trq d ) while

Trq 1 +Trq 2 =Trq in .
This means the maximum torque to either wheel is statically indeterminate but is in the range of ½ Trqin ±( ½
Trq d ).
Types
Several types of LSD are commonly used on passenger cars.

Fixed value

Torque sensitive

Speed sensitive

Electronically controlled
Fixed
value
In this differential the maximum torque difference between the two outputs, Trq d , is a fixed value at all times
regardless of torque input to the differential or speed difference between the two outputs.Typically this
differential used spring loaded clutch assemblies.
Torque
sensitivity
This category includes helical gear limited-slip differentials and clutch, cone (an alternative type of clutch) where
the engagement force of the clutch is a function of the input torque applied to the differential (as the engine
applies more torque the clutches grip harder and Trq d increases).
ZF LSD – clutch stack visible on left
ZF LSD – spider pinion shaft ramps visible
Torque sensing LSDs respond to driveshaft torque, so that the more driveshaft input torque present, the harder
the clutches, cones or gears are pressed together, and thus the more closely the drive wheels are coupled to each
other. Some include spring loading to provide some small torque so that with little or no input torque (trailing
throttle/gearbox in neutral/main clutch depressed) the drive wheels are minimally coupled. The amount of
preload (hence static coupling) on the clutches or cones are affected by the general condition (wear) and by how
Clutch, cone-type LSD
The clutch type has a stack of thin clutch-discs, half of which are coupled to one of the drive shafts, the other half
of which are coupled to the spider gear carrier. The clutch stacks may be present on both drive shafts, or on only
one. If on only one, the remaining drive shaft is linked to the clutched drive shaft through the spider gears. In a
cone type the clutches are replaced by a pair of cones which are pressed together achieving the same effect.
One method for creating the clamping force is the use of a cam-ramp assembly such as used in a Salisbury/ramp
style LSD. The spider gears mount on the pinion cross shaft which rests in angled cutouts forming cammed
ramps. The cammed ramps are not necessarily symmetrical. If the ramps are symmetrical, the LSD is 2 way. If
they are saw toothed (i.e. one side of the ramp is vertical), the LSD is 1 way. If both sides are sloped, but are
asymmetric, the LSD is 1.5 way. (See the discussion of 2, 1.5 and 1 way below)
An alternative is to use the natural separation force of the gear teeth to load the clutch. An example is the center
differential of the 2011 Audi Quattro RS 5.[5]
As the input torque of the driveshaft tries to turn the differential center, internal pressure rings (adjoining the
clutch stack) are forced sideways by the pinion cross shaft trying to climb the ramp, which compresses the clutch
stack. The more the clutch stack is compressed, the more coupled the wheels are. The mating of the vertical ramp
(80–85 C° in practice to avoid chipping) surfaces in a one-way LSD on overrun produces no cam effect or
corresponding clutch stack compression.
2-, 1-, and 1.5-way LSD
previously stated, the coupling is proportional to the input torque. With no load, the coupling is reduced to the
static coupling. The behavior on overrun (particularly sudden throttle release) determines whether the LSD is 1
way, 1.5 way, or 2 way.
A 2-way differential will have the same limiting torque Trq d in both the forward and reverse directions. This
means the differential will provide some level of limiting under engine braking.
A 1-way differential will provide its limiting action in only one direction. When torque is applied in the opposite
direction it behaves like an open differential. In the case of a FWD car it is argued to be safer than a 2-way
differential.[6] The argument is if there is no additional coupling on overrun, i.e. a 1-way LSD as soon as the
driver lifts the throttle, the LSD unlocks and behaves somewhat like a conventional open differential. This is also
the best for FWD cars, as it allows the car to turn in on throttle release, instead of ploughing forward. [6]
A 1.5-way differential refers to one where the forward and reverse limiting torques, Trq d_fwd, d_rev , are different
but neither is zero as in the case of the 1-way LSD. This type of differential is common in racing cars where a
strong limiting torque can aid stability under engine braking.
Geared LSD
Audi Quattro Torsen Differential
Geared, torque-sensitive mechanical limited-slip differentials use helical gears or worm gears rather than the
beveled spider gears of the clutch based differentials. As torque is applied to the gears they are pushed against
the walls of the differential housing which creates friction. The friction resists the relative movement of the
outputs and creates the limiting torque Trq d .
Examples include:

Torsen differential based upon the Dual-Drive Differential invented by Vernon Gleasman in 1958, then
later sold to Gleason Corporation, who started marketing it in 1982;

Quaife differential, sold under the name Automatic Torque Biasing Differential (ATB), covered by
European Patent No. 130806A2.

Eaton Corporation differential, sold under the nameEaton Detroit Truetrac.
Speed
sensitivity
Speed-sensitive differentials limit the torque difference between the outputs, Trq d , based on the difference in
speed between the two output shafts. Thus for small output speed differences the differential’s behavior may be
very close to an open differential. As the speed difference increase the limiting torque increases. This results in
different dynamic behavior as compared to a torque sensitive differential.
Viscous
Nissan 240SX Viscous LSD
The viscous type is generally simpler because it relies on hydrodynamic friction from fluids with
high viscosity.Silicone-based oils are often used. Here, a cylindrical chamber of fluid filled with a stack of
perforated discs rotates with the normal motion of the output shafts. The inside surface of the chamber is coupled
to one of the driveshafts, and the outside coupled to the differential carrier. Half of the discs are connected to the
inner, the other half to the outer, alternating inner/outer in the stack. Differential motion forces the interleaved
discs to move through the fluid against each other. In some viscous couplings when speed is maintained the fluid
will accumulate heat due to friction. This heat will cause the fluid to expand, and expand the coupler causing the
discs to be pulled together resulting in a non-viscous plate to plate friction and a dramatic drop in speed
difference. This is known as the hump phenomenon and it allows the side of the coupler to gently lock. In
contrast to the mechanical type, the limiting action is much softer and more proportional to the slip, and so is
easier to cope with for the average driver. New Process Gear used a viscous coupling of the Ferguson style in
several of their transfer cases including those used in theAMC Eagle.
Viscous LSDs are less efficient than mechanical types, that is, they "lose" some power. In particular, any
sustained load which overheats the silicone results in sudden permanent loss of the differential effect. [7] They do
have the virtue of failing gracefully, reverting to semi-open differential behavior. Typically a visco-differential
that has covered 60,000 miles (97,000 km) or more will be functioning largely as an open differential; [citation
needed]
this is a known weakness of the original Mazda MX-5(a.k.a. Miata) sports car. The silicone oil is factory
sealed in a separate chamber from the gear oil surrounding the rest of the differential. This is not serviceable and
when the differential's behavior deteriorates, the VLSD center is replaced.
Gerotor pump
This works by hydraulically compressing a clutch pack. The gerotor pump uses the housing to drive the outer
side of the pump and one axle shaft to drive the other. When there is differential wheel rotation, the pump
pressurizes its working fluid into the clutch pack area. This provides a clamp load for frictional resistance to
transfer torque to the higher traction wheel. The pump-based systems have a lower and upper limits on applied
pressure, and internal damping to avoid hysteresis. The newest gerotor pump based system has computer
regulated output for more versatility and no oscillation.
Electronic
An electronic limited-slip differential will typically have a planetary or bevel gear set similar to that of an open
differential and a clutch pack similar to that in a torque sensitive or gerotor pump based differential. In the
electronic unit the clamping force on the clutch is controlled externally by a computer or other controller. This
allows the control of the differential’s limiting torque, Trq d , to be controlled as part of a total chassis
management system. An example of this type of differential is Subaru’s DCCD used in the 2011 Subaru WRX
STi.[8] Another example is the Porsche PSD system used on the Porsche 928.
Electronic
systems: brake-based
These systems are alternatives to a traditional limited-slip differential. The systems use an open differential
paired with various chassis sensors such as speed sensors, anti-lock braking system(ABS)
sensors, accelerometers, and microcomputers to electronically monitor wheel slip and vehicle motion. When the
chassis control system determines a wheel is slipping the computer applies the brakes to that wheel. A significant
difference between the limited-slip differential systems listed above and this brake based system is the brake
based systems do not inherently send the greater torque to the slower wheel.
BMW's electronic limited-slip differential used on the 2012 535i is an example of such a system.SAAB XWD
(Haldex Generation 4) with eLSD is another example that uses electrically controlled brakes and differentials to
distribute torque.
Other
related final drives

Spool

Locking differential

differentials & limited slip differentials (lsds)
Firstly, some definitions...
A differential is a mechanical device which allows a flexible division of drive
between wheels to allow cornering
A limited slip differential (LSD) is a device which automatically reduces the loss
of drive which can result from spinning wheels on one side of an axle.
Spinning wheels are most likely to result from cornering while on the gas, pulling
away from a stand still or accelerating in a car with lots of power
Now a little more depth...
differentials - an introduction
Before understanding why a limited slip or locking differentials are important,
first we'll briefly need to touch on why we need a differential in the first place. In
simple terms, a differential is a device which allows for the differences in wheel
speed which naturally occurs when a car turns a corner.
As you can see in Diagram 1, the inside and outside wheels of a car turn in
different radius corners, and thus need to rotate at different speeds (with the
outside wheels travelling faster). However at least two of the wheels will also
need to be linked to allow the car to be two wheel drive. Consider a front wheel
drive car with the two front wheels linked together with no flexibility, such as
with a solid axle between them.
Diagram 1: A car driving in a circle
In this case a certain amount of tension would build up when cornering as the
outside wheel tries to rotate quicker that the inside wheel (due to the bigger arc
it must go through). Eventually this tension would relive itself with a wheel
skipping over the surface, or with a drive shaft snapping. This situation is
obviously not a good one, so differentials where invented (see Diagram 2).
Diagram 2: A basic differential positioned between two driven wheels
As you can see, a diff is essentially a combination of cogs which work together to
turn the wheels. It looks complicated, but it uses simple mechanics to allow the
two wheels to rotate at different rates.
Diagram 3: A close up of a differential
how a differential works:
The drive from the engine rotates the large yellow crown wheel (1), which is
attached to the smaller blue cogs (2). These planetary gears can rotate freely,
but work together to turn the green side gears, which are connected to the half
shafts (3). If one wheel needs to rotate faster than the other, the green cogs
permit this to happen. Simple really!
limited slip differentials
Differentials work by allowing a flexible distribution of drive between the wheels
on an axle, which allows for the different rates of rotation while cornering.
However this flexibility is also the differential's weakness, as it will always allow
drive to 'escape' via the easiest route. So if you are turning a corner while hard
on the gas in a powerful car, you can find that the inside wheel starts spinning
(due to the weight transfer leading to less grip), and you lose the ability to put
power down on the road via the outside wheel. This isn't good, especially if
you're trying to put in a good time on the track, and this is why the limited slip
differential (LSD) was invented. The differentials shown in the diagrams above
are known as 'open' diffs which means they have no mechanism to prevent this
drive loss. The first LSDs connected the two half shafts together with a clutch
pack allowing a limited amount of clutch slip between each side of the axle. This
allowed for the relatively small differences in rotation while cornering, but
prevented violent wheelspin from just one of the wheels which could lead to loss
of drive.
types of limited slip differential
Today there are a variety of differentials which can reduce unwanted wheelspin
on one side of an axle, which is prevented using either viscous, mechanical,
hydraulic and electronic systems. A simplified example of a clutch type LSD is
illustrated in Diagram 4 below. Many race bred cars have LSDs fitted as
standard, especially powerful front wheel drive cars which are more prone to
wheelspin while pulling out of a corner.
Diagram 4: A clutch type LSD
In Diagram 4 above, the simple open differential has been fitted with a clutch
(1). This clutch prevents the two blue side gears from freely rotating
independently which can help in the occasions when drive loss would be an
issue, however there is enough flexibility in the system to allow small differences
such as when cornering. Clutch packs such as these as usually held together by
a spring, which automatically keeps the clutch tight even when it has worn
down. The strength of the spring determines how aggressive the LSD becomes.
four wheel drive (4wd) systems
There are many different varieties of four wheel drive vehicle, so it's important
to understand how to make the most of each system before embarking on an
off-road journey. The three main varieties are:
•
Permanent / full time four wheel drive
•
Manually selectable four wheel drive
•
Automatically selected four wheel drive
This article explains how each of these three systems work, and how to use them
in the most effective way when driving off-road or in challenging conditions.
permanent / full time four wheel drive
Permanent four wheel drive systems all follow the same principles, although the
mechanical layouts can vary. Firstly, the drive (which exits the engine via a drive
shaft) needs to be split in order to power the front and back axles
simultaneously. This is done via a mechanical gadget called a differential. Full
time four wheel drive systems have a centre differential (a), as well as a rear (b)
and front diff (c) shown in Figure 1 below. The front and rear diffs split the drive
to the wheels, and this is how all four wheels can be powered by a single engine.
Figure 1: Permanent four wheel drive
introduction to differentials
Differentials allow the drive to be split, but they also have another function,
which is to allocate drive to each wheel in a flexible manner. When turning
corners, each wheel takes a different path (see Figure 2). The inside wheels take
a tighter arc than the outside, and in addition the front wheels take a slightly
different path than the back. To allow this to happen, the same differentials
which split the drive must also be able to distribute the drive at different speeds
while maintaining forward propulsion - a tough job for one component.
Figure 2: The different arc of the inside and outside front wheels
•
The side effect of differentials
This is all well and good, however this flexible approach to distributing drive has
a side effect - it allows drive to 'escape' via the easiest route. Let's say we're
driving a front wheel drive vehicle on a tarmac road, but one of the driven
wheels is positioned on frictionless ice. When the driver tries to pull away they
would find the wheel on ice spinning wildly, but the wheel on tarmac would not
rotate forwards to propel the vehicle. This same principle applies to permanent
4WD cars - the flexible division of drive provided through the three separate
differentials can prevent forward motion from occurring if just one wheel is on a
slippery surface (a, Figure 3).
Figure 3: Drive loss through one wheel (a) on a slippery surface
This situation is far from ideal, especially as four wheel drive is meant to provide
better performance in slippery conditions. But there is a solution - locking
differentials and traction control. Most full time 4WD vehicles are fitted with a
locking centre differential which, when engaged prevents the flexible division of
drive between the front and the rear axles (b, Figure 4). So, as long as either
the front or the back axles have decent grip, forward propulsion can be
maintained. Suddenly that patch of ice isn't so much of a big deal.
Figure 4: A locked centre differential (b) prevents drive loss between the front and rear
axles, and maintains forward motion
However, if you're really unlucky, one of the wheels on both the front and the
rear axles could be on a slippery patch, and we're stuck yet again through the
action of a spinning rear wheel (c, Figure 5).
Figure 5: Even though the centre diff is locked (b) - drive is escaping through two wheels
on slippery ground (a and c)
To get over this problem, modern vehicles are usually fitted with electronic
traction control system. Traction control systems prevent drive from escaping
from a spinning wheel by applying the brakes to that wheel. If the brakes are on,
the easiest route for the drive to travel is to the wheel on the high grip surface,
thus forcing the car to continue moving. Alternatively, some hard-core off-road
machines have front and rear locking differentials. With both these solutions we
now have a robust four wheel drive powered car which can cope with a variety
slippery conditions without getting stuck - phew!
manually and automatically selected four wheel
drive systems
The mechanics of a vehicle with selectable 4WD are quite similar to that of a
permanent four wheel drive system, this includes a method of splitting the drive
to the front and rear (a), and some differentials at each axle (b and c) to split
the drive again for the wheels.
Both manually and automatically selected for 4WD systems operate using the
same principle - drive is permanent at one axle (usually the rear), and the other
axle can be connected to the engine when required.
The ethos of selectable four wheel drive is to engage four wheel drive only when
conditions get tricky. This can be done manually via a switch or button, or
automatically via some clever technology. The mechanism of connecting an
additional axle to the engine can be done in a variety of different ways such as
via clutch plates, viscous couplings, or other clever means. Depending on the
vehicle you're driving, this can be done on the move or in some cases the car
Note: Most selectable four wheel drive systems do not have a centre differential,
which means they shouldn't be used in high traction conditions with 4WD
engaged. Doing so may put unnecessary strain on the drive line when turning
corners.
differentials & limited slip differentials (lsds)
Firstly, some definitions...
A differential is a mechanical device which allows a flexible division of drive
between wheels to allow cornering
A limited slip differential (LSD) is a device which automatically reduces the loss
of drive which can result from spinning wheels on one side of an axle.
Spinning wheels are most likely to result from cornering while on the gas, pulling
away from a stand still or accelerating in a car with lots of power
Now a little more depth...
differentials - an introduction
Before understanding why a limited slip or locking differentials are important,
first we'll briefly need to touch on why we need a differential in the first place. In
simple terms, a differential is a device which allows for the differences in wheel
speed which naturally occurs when a car turns a corner.
As you can see in Diagram 1, the inside and outside wheels of a car turn in
different radius corners, and thus need to rotate at different speeds (with the
outside wheels travelling faster). However at least two of the wheels will also
need to be linked to allow the car to be two wheel drive. Consider a front wheel
drive car with the two front wheels linked together with no flexibility, such as
with a solid axle between them.
Diagram 1: A car driving in a circle
In this case a certain amount of tension would build up when cornering as the
outside wheel tries to rotate quicker that the inside wheel (due to the bigger arc
it must go through). Eventually this tension would relive itself with a wheel
skipping over the surface, or with a drive shaft snapping. This situation is
obviously not a good one, so differentials where invented (see Diagram 2).
Diagram 2: A basic differential positioned between two driven wheels
As you can see, a diff is essentially a combination of cogs which work together to
turn the wheels. It looks complicated, but it uses simple mechanics to allow the
two wheels to rotate at different rates.
Diagram 3: A close up of a differential
how a differential works:
The drive from the engine rotates the large yellow crown wheel (1), which is
attached to the smaller blue cogs (2). These planetary gears can rotate freely,
but work together to turn the green side gears, which are connected to the half
shafts (3). If one wheel needs to rotate faster than the other, the green cogs
permit this to happen. Simple really!
limited slip differentials
Differentials work by allowing a flexible distribution of drive between the wheels
on an axle, which allows for the different rates of rotation while cornering.
However this flexibility is also the differential's weakness, as it will always allow
drive to 'escape' via the easiest route. So if you are turning a corner while hard
on the gas in a powerful car, you can find that the inside wheel starts spinning
(due to the weight transfer leading to less grip), and you lose the ability to put
power down on the road via the outside wheel. This isn't good, especially if
you're trying to put in a good time on the track, and this is why the limited slip
differential (LSD) was invented. The differentials shown in the diagrams above
are known as 'open' diffs which means they have no mechanism to prevent this
drive loss. The first LSDs connected the two half shafts together with a clutch
pack allowing a limited amount of clutch slip between each side of the axle. This
allowed for the relatively small differences in rotation while cornering, but
prevented violent wheelspin from just one of the wheels which could lead to loss
of drive.
types of limited slip differential
Today there are a variety of differentials which can reduce unwanted wheelspin
on one side of an axle, which is prevented using either viscous, mechanical,
hydraulic and electronic systems. A simplified example of a clutch type LSD is
illustrated in Diagram 4 below. Many race bred cars have LSDs fitted as
standard, especially powerful front wheel drive cars which are more prone to
wheelspin while pulling out of a corner.
Diagram 4: A clutch type LSD
In Diagram 4 above, the simple open differential has been fitted with a clutch
(1). This clutch prevents the two blue side gears from freely rotating
independently which can help in the occasions when drive loss would be an
issue, however there is enough flexibility in the system to allow small differences
such as when cornering. Clutch packs such as these as usually held together by
a spring, which automatically keeps the clutch tight even when it has worn
down. The strength of the spring determines how aggressive the LSD becomes.
A trailing-arm suspension is an automobile suspensiondesign in which one or more arms (or
"links") are connected between (and perpendicular to and forward of) the axle and the chassis. It
is usually used on rear axles. A "leading arm", as used on a Citroën 2CV, has an arm connected
between (and perpendicular to, and to the rear of) the axle and the chassis. It is used on the front
axle.
Semi-trailing arm suspension of a 1969Volkswagen Beetle
Trailing-arm designs in live axle setups often use just two or three links and a Panhard rod to
locate the wheel laterally. A trailing arm design can also be used in an independent
suspension arrangement. Each wheel hub is located only by a large, roughly triangular arm
that pivots at one point, ahead of the wheel. Seen from the side, this arm is roughly parallel to the
ground, with the angle changing based on road irregularities. A twist-beam rear suspension is
very similar except that the arms are connected by a beam, used to locate the wheels and which
twists and has an anti-roll effect.
A semi-trailing arm suspension is a supple independent rear suspension system
for automobiles where each wheelhub is located only by a large, roughly triangular arm that
pivots at two points. Viewed from the top, the line formed by the two pivots is somewhere
between parallel andperpendicular to the car's longitudinal axis; it is generally parallel to the
ground. Trailing-arm and multilink suspensiondesigns are much more commonly used for the rear
wheels of a vehicle where they can allow for a flatter floor and more cargo room. Many small,
front-wheel drive vehicles feature aMacPherson strut front suspension and trailing-arm rear axle.
External
Wikimedia Commons has
media related to: Trailing-arm
suspension
S
emi-trailing arm
Suspension
Transaxle
Unsourced material may be challenged and removed. (May 2009)
In the automotive field, a transaxle is a major mechanical component that combines the functionality
of the transmission, the differential, and associated components of the driven axle into one integrated
assembly.
Transaxles are near universal in all automobile configurations that have the engine placed at the same
end of the car as the driven wheels: the front-engine, front-wheel drive layout, rear-engine, rear-wheel
drive layout and rear mid-engine, rear-wheel drive layout arrangements.
Many mid- and rear-engined vehicles use a transverse engine and transaxle, similar to a front wheel
drive unit. Others use a longitudinal engine and transaxle like Ferrari's 1989 Mondial t which used a "t"
arrangement with a longitudinal engine connected to a transverse transaxle, a design the company
continues to this day. Front-wheel drive versions of modern Audis, from the A4 upwards, along with
their related marques from the Volkswagen Group (which share the same automobile layout) also use
a similar layout, but with the transaxle also mounted longitudinally.
Contents
[hide]
1 Front-engine, rear-wheel drive transaxles
2 Rear-engine, rear-wheel drive transaxles
3 Four-wheel drive
5 References
Front-engine,
rear-wheel drive transaxles
Csonka transaxle from 1908.
Front-engine, rear-wheel drive vehicles tend to have the transmission up front just after the engine, but
sometimes a front engine drives a rear-mounted transaxle. This is generally done for reasons of
weight distribution, and is therefore common on sports cars. Another advantage is that as
the driveshaft spins at engine speed it only has to endure the torqueof the engine, instead of that
torque multiplied by the 1st gear ratio. This design was pioneered in the 1934 Škoda Popular, and then
in the 1950 Lancia Aurelia, designed by the legendary Vittorio Jano.
Since this placement of the gearbox is unsuitable for a live axle (due to excessiveunsprung weight),
the rear suspension is either independent, or uses a de Dion tube (notably in theAlfa Romeos). Rare
exceptions to this rule were the Bugatti T46 and T50 which had a three speed gearbox on a live axle.
The Nissan GT-R is unique in that it uses a rear transaxle with an AWD layout, the transaxle in this
case also contains the differential sending power back to the front wheels via a separate driveshaft.
Drive shaft
(Redirected from Driveshaft)
Drive shaft with universal joints at each end and a splinein the centre
citations toreliable sources. Unsourced
material may
be challengedand removed. (June 2010)
A drive shaft, driveshaft, driving shaft,propeller shaft (prop shaft), or Cardanshaft is a
mechanical component for transmitting torque and rotation, usually used to connect other components
of a drive train that cannot be connected directly because of distance or the need to allow for relative
movement between them.
Drive shafts are carriers of torque: they are subject to torsion and shear stress, equivalent to the
difference between the input torque and the load. They must therefore be strong enough to bear the
stress, whilst avoiding too much additional weight as that would in turn increase their inertia.
To allow for variations in the alignment and distance between the driving and driven components, drive
shafts frequently incorporate one or more universal joints, jaw couplings, or rag joints, and sometimes
a splined joint or prismatic joint.
Contents
[hide]
1 History
2 Automotive drive shafts
o
o
2.1 Vehicles

2.1.1 Front-engine, rear-wheel drive

2.1.2 Front-wheel drive

2.1.3 Four wheel and all-wheel drive
2.2 Drive shaft for Research and Development (R&D)
3 Motorcycle drive shafts
4 Marine drive shafts
5 Locomotive drive shafts
6 Drive shafts in bicycles
o
o
8 References
History
The term drive shaft first appeared during the mid 19th century. In Storer's 1861 patent reissue for
aplaning and matching machine, the term is used to refer to the belt-driven shaft by which the machine
is driven.[1] The term is not used in his original patent.[2] Another early use of the term occurs in the
1861 patent reissue for the Watkins and Bryson horse-drawn mowing machine.[3] Here, the term refers
to the shaft transmitting power from the machine's wheels to the gear train that works the cutting
mechanism.
In the 1890s, the term began to be used in a manner closer to the modern sense. In 1891, for
example, Battles referred to the shaft between the transmission and driving trucks of his Climax
locomotive as the drive shaft,[4] and Stillman referred to the shaft linking the crankshaft to the rear axle
of his shaft-driven bicycle as a drive shaft.[5] In 1899, Bukey used the term to describe the shaft
transmitting power from the wheel to the driven machinery by a universal joint in his Horse-Power.[6] In
the same year, Clark described his Marine Velocipede using the term to refer to the gear-driven shaft
transmitting power through a universal joint to the propeller shaft.[7] Crompton used the term to refer to
the shaft between the transmission of his steam-powered Motor Vehicle of 1903 and the driven axle.[8]
Automotive
drive shafts
Vehicles
An automobile may use a longitudinal shaft to deliver power from an engine/transmission to the other
end of the vehicle before it goes to the wheels. A pair of short drive shafts is commonly used to send
power from a central differential, transmission, or transaxle to the wheels.
A truck double propeller shaft
Front-engine, rear-wheel drive
Main article: Front-engine, rear-wheel drive layout
In front-engined, rear-drive vehicles, a longer drive shaft is also required to send power the length of
the vehicle. Two forms dominate: The torque tube with a single universal joint and the more
common Hotchkiss drive with two or more joints. This system became known as Système
Panhard after the automobile company Panhard et Levassor patented it.
Most of these vehicles have a clutch and gearbox (or transmission) mounted directly on the engine
with a drive shaft leading to a final drive in the rear axle. When the vehicle is stationary, the drive shaft
does not rotate. A few, mostly sports, cars seeking improved weight balance between front and rear,
and most commonly Alfa Romeos or Porsche 924s, have instead used a rear-mounted transaxle. This
places the clutch and transmission at the rear of the car and the drive shaft between them and the
engine. In this case the drive shaft rotates continuously as long as the engine does, even when the car
is stationary and out of gear.
Early automobiles often used chain drive or belt drive mechanisms rather than a drive shaft. Some
used electrical generators and motors to transmit power to the wheels.
Front-wheel drive
In British English, the term "drive shaft" is restricted to a transverse shaft that transmits power to the
wheels, especially the front wheels. A drive shaft connecting the gearbox to a rear differential is called
a propeller shaft, or prop-shaft. A prop-shaft assembly consists of a propeller shaft, a slip joint and
one or more universal joints. Where the engine and axles are separated from each other, as on fourwheel drive and rear-wheel drive vehicles, it is the propeller shaft that serves to transmit the drive force
generated by the engine to the axles.
A drive shaft connecting a rear differential to a rear wheel may be called a half shaft. The name
derives from the fact that two such shafts are required to form one rear axle.
Several different types of drive shaft are used in the automotive industry:

One-piece drive shaft

Two-piece drive shaft

Slip-in-tube drive shaft
The slip-in-tube drive shaft is a new type that also helps in crash energy management. It can be
compressed in the event of a crash, so is also known as a collapsible drive shaft.
Four wheel and all-wheel drive
These evolved from the front-engine rear-wheel drive layout. A new form of transmission called the
transfer case was placed between transmission and final drives in both axles. This split the drive to the
two axles and may also have included reduction gears, a dog clutch or differential. At least two drive
shafts were used, one from the transfer case to each axle. In some larger vehicles, the transfer box
was centrally mounted and was itself driven by a short drive shaft. In vehicles the size of a Land
Rover, the drive shaft to the front axle is noticeably shorter and more steeply articulated than the rear
shaft, making it a more difficult engineering problem to build a reliable drive shaft, and which may
involve a more sophisticated form of universal joint.
Modern light cars with all-wheel drive (notably Audi or the Fiat Panda) may use a system that more
closely resembles a front-wheel drive layout. The transmission and final drive for the front axle are
combined into one housing alongside the engine, and a single drive shaft runs the length of the car to
the rear axle. This is a favoured design where the torque is biased to the front wheels to give car-like
handling, or where the maker wishes to produce both four-wheel drive and front-wheel drive cars with
many shared components.
Drive
shaft for Research and Development (R&D)
The automotive industry also uses drive shafts at testing plants. At an engine test stand a drive shaft is
used to transfer a certain speed / torque from the Internal combustion engine to a dynamometer. A
"shaft guard" is used at a shaft connection to protect against contact with the drive shaft and for
detection of a shaft failure. At a transmission test stand a drive shaft connects the prime mover with
the transmission.
Motorcycle
drive shafts
The exposed drive shaft on BMW's first motorcycle, the R32
Drive shafts have been used on motorcycles almost as long as there have been motorcycles. As an
alternative to chainand belt drives, drive shafts offer relatively maintenance-free operation and long
life. A disadvantage of shaft drive on a motorcycle is that gearing or a Hobson's joint or similar is
needed to turn the power 90° from the shaft to the rear wheel, losing some power in the process. On
the other hand, it is easier to protect the shaft linkages and drive gears from dust, sand and mud.
The best known motorcycle manufacturer to use shaft drive for a long time—since 1923—is BMW.
Among contemporary manufacturers, Moto Guzzi is also well known for its shaft drive motorcycles.
The British company,Triumph and all four Japanese brands, Honda, Suzuki,Kawasaki and Yamaha,
have produced shaft drive motorcycles. All geared models of the Vespa scooter produced to date have
been shaft-driven. The automatic models, however, use a belt.
Motorcycle engines positioned such that the crankshaft is longitudinal and parallel to the frame are
often used for shaft driven motorcycles. This requires only one 90° turn in power transmission, rather
than two. Bikes from Moto Guzzi and BMW, plus the Triumph Rocket III and Honda ST series all use
this engine layout.
Motorcycles with shaft drive are subject to shaft effect where the chassis climbs when power is
applied. This is counteracted with systems such as BMW's Paralever, Moto Guzzi's CARC and
Kawasaki's Tetra Lever.
Marine
drive shafts
On a power-driven ship, the drive shaft, or propeller shaft, usually connects the transmission inside the
vessel directly to the propeller, passing through a stuffing box or other seal at the point it exits the hull.
There is also a thrust block, a bearing to resist the axial force of the propeller. As the rotating propeller
pushes the vessel forward, any length of drive shaft between propeller and thrust block is subject
tocompression, and when going astern to tension. Except for the very smallest of boats, this force isn't
taken on the gearbox or engine directly.
Cardan shafts are also often used in marine applications between the transmission and either a
propeller gearbox or waterjet.
Locomotive
drive shafts
The rear drive shaft, crankshaft and front drive shaft of a Shay locomotive.
The Shay, Climax and Heisler locomotives, all introduced in the late 19th century, used quill drivesto
couple power from a centrally mounted multi-cylinder engine to each of the trucks supporting the
engine. On each of these geared steam locomotives, one end of each drive shaft was coupled to the
driven truck through a universal joint while the other end was powered by
the crankshaft, transmission or another truck through a second universal joint. A quill drive also has
the ability to slide lengthways, effectively varying its length. This is required to allow the bogies to
rotate when passing a curve.
Cardan shafts are used in some diesel locomotives(mainly diesel-hydraulics, such as British Rail Class
52) and some electric locomotives (e.g. British Rail Class 91). They are also widely used in diesel
multiple units.
Drive
shafts in bicycles
A shaft-driven bicycle.
The drive shaft has served as an alternative to a chain-drive in bicycles for the past century, never
becoming very popular. A shaft-driven bicycle (or "Acatane", from an early maker) has several

Drive system is less likely to become jammed, a common problem with chain-driven bicycles

The rider cannot become dirtied from chain grease or injured by "Chain bite" when clothing or
a body part catches between an unguarded chain and a sprocket

Lower maintenance than a chain system when the drive shaft is enclosed in a tube

More consistent performance. Dynamic Bicycles claims that a drive shaft bicycle can deliver
94% efficiency, whereas a chain-driven bike can deliver anywhere from 75-97% efficiency based
on condition

Greater ground clearance: lacking a derailleur or other low-hanging machinery, the bicycle
has nearly twice the ground clearance

A drive shaft system weighs more than a chain system, usually 1-2 pounds heavier

Many of the advantages claimed by drive shaft's proponents can be achieved on a chaindriven bicycle, such as covering the chain and gears

Use of lightweight derailleur gears with a high number of ratios is impossible, although hub
gearscan be used

Wheel removal can be complicated in some designs (as it is for some chain-driven bicycles
with hub gears).
See
also
Sway bar
An anti-sway bar (in black) in the rear axle of a Porsche, which traverses the underside of the car from left to right.
A flexible sway bar bushing attaches it to the chassis. Also visible is one of the sway bar end links that connects
the bar vertically to an axle. The sway bar end link is the structural member that twists the anti-sway bar when the
vehicle is cornering.
A sway bar or anti-roll bar or stabilizer bar is a part of an automobile suspension that helps reduce
the body roll of a vehicle during fast cornering or over road irregularities. It connects opposite
(left/right) wheels together through shortlever arms linked by a torsion spring. A sway bar increases
the suspension's roll stiffness—its resistance to roll in turns, independent of its spring rate in the
vertical direction. The first stabilizer bar patent was awarded to the Canadian S. L. C. Coleman
of Fredericton, New Brunswick on April 22, 1919.[1]
Contents
[hide]
1 Purpose and operation
2 Principles
o
2.1 Main functions
o
2.2 Drawbacks
o
3 Active systems
5 References
Purpose
and operation
An SUV, with the sway bars removed, shows how one wheel can be much lower than the opposite side, as the
body tilts, or rolls, further without the bar.
Photo of 2 front-wheel springs, with the tires removed. Each suspension spring is connected to the central sway
bar assembly.
An anti-sway or anti-roll bar is intended to force each side of the vehicle to lower, or rise, to similar
heights, to reduce the sideways tilting (roll) of the vehicle on curves, sharp corners, or large bumps.
With the bar removed, a vehicle's wheels can tilt away by much larger distances (as shown by
the SUV image at right). Although there are many variations in design, a common function is to force
the opposite wheel's shock absorber, spring or suspension rod to lower, or rise, to a similar level as
the other wheel. In a fast turn, a vehicle tends to drop closer onto the outer wheels, and the sway bar
will soon force the opposite wheel to also get closer to the vehicle. As a result, the vehicle tends to
"hug" the road, closer in a fast turn, where all wheels are closer to the body. After the fast turn, then
the downward pressure is reduced, and the paired wheels can return to their normal height against the
vehicle, kept at similar levels by the connecting sway bar.
Because each pair of wheels is cross-connected by a bar, then the combined operation causes all
wheels to generally offset the separate tilting of the others, and the vehicle tends to remain level
against the general slope of the terrain. A negative side-effect, of connecting pairs of wheels, is that a
jarring or bump to one wheel tends to also jar the opposite wheel, causing a larger impact applied
across the whole width of the vehicle. If there are several potholes scattered in the road, then a vehicle
will tend to rock, side-to-side, or waddle, due to the action of the bar at each pair of wheels. Other
suspension techniques can be used to delay, or dampen, the effect of the connecting bar, as when
hitting small holes which momentarily jolt just a single wheel, whereas larger holes or longer tilting
would then tug the bar with the opposite wheel.
Principles
A sway bar is usually a torsion spring that resists body roll motions. It is usually constructed out of a
wide, U-shaped steel bar that connects to the body at two points, and at the left and right sides of the
suspension. If the left and right wheels move together, the bar rotates about its mounting points. If the
wheels move relative to each other, the bar is subjected to torsion and forced to twist. Each end of the
bar is connected to an end link through a flexible joint. The sway bar end link in turn connects to a spot
near a wheel or axle, permitting forces to be transferred from a heavily-loaded axle to the opposite
side.
Forces are therefore transferred:
2. to the connected end link via a bushing
3. to the anti-sway (torsion) bar via a flexible joint
4. to the connected end link on the opposite side of the vehicle
5. to the opposite axle.
The bar resists the torsion through its stiffness. The stiffness of an anti-roll bar is proportional to the
stiffness of the material, the fourth power of its radius, and the inverse of the length of the lever arms
(i.e., the shorter the lever arm, the stiffer the bar). Stiffness is also related to the geometry of the
mounting points and the rigidity of the bar's mounting points. The stiffer the bar, the more force
required to move the left and right wheels relative to each other. This increases the amount of force
required to make the body roll.
In a turn the sprung mass of the vehicle's body produces a lateral force at the centre of gravity (CG),
proportional to lateral acceleration. Because the CG is usually not on the roll axis, the lateral force
creates a moment about the roll axis that tends to roll the body. (The roll axis is a line that joins the
front and rear roll centers (SAEJ670e)). The moment is called the roll couple.
Roll couple is resisted by the suspension roll stiffness, which is a function of the spring rate of the
vehicle's springs and of the anti-roll bars, if any. The use of anti-roll bars allows designers to reduce
roll without making the suspension's springs stiffer in the vertical plane, which allows improved body
control with less compromise of ride quality.
One effect of body (frame) lean, for typical suspension geometry, is positive camber of the wheels on
the outside of the turn and negative on the inside, which reduces their cornering grip (especially with
cross ply tires).
Main
functions
Anti-roll bars provide two main functions. The first function is the reduction of body lean. The reduction
of body lean is dependent on the total roll stiffness of the vehicle. Increasing the total roll stiffness of a
vehicle does not change the steady state total load (weight) transfer from the inside wheels to the
outside wheels, it only reduces body lean. The total lateral load transfer is determined by the CG
height and track width.
The other function of anti-roll bars is to tune the handling balance of a
car. Understeer or oversteerbehavior can be tuned out by changing the proportion of the total roll
stiffness that comes from the front and rear axles. Increasing the proportion of roll stiffness at the front
will increase the proportion of the total load transfer that the front axle reacts and decrease the
proportion that the rear axle reacts. In general this will cause the outer front wheel to run at a
comparatively higher slip angle, and the outer rear wheel to run at a comparatively lower slip angle,
which is an understeer effect. Increasing the proportion of roll stiffness at the rear axle will have the
opposite effect and decrease understeer.
Drawbacks
Because an anti-roll bar connects wheels on the opposite sides of the vehicle together, the bar will
transmit the force of one-wheel bumps to the opposite wheel. On rough or broken pavement, anti-roll
bars can produce jarring, side-to-side body motions (a "waddling" sensation), which increase in
severity with the diameter and stiffness of the sway bars. Excessive roll stiffness, typically achieved by
configuring an anti-roll bar too aggressively, will cause the inside wheels to lift off the ground during
very hard cornering. This can be used to advantage: many front wheel drive production cars will lift a
wheel when cornering hard, in order to overload the other wheel on the axle, limiting understeer.
bars
Some anti-roll bars, particularly those intended for use in auto racing, are externally adjustable while
the car is in the pit whereas some systems can be adjusted in real time by the driver from inside the
car, such as in Super GT. This allows the stiffness to be altered by increasing or reducing the length of
the lever arms. This permits the roll stiffness to be tuned for different situations without replacing the
entire bar.
Active
systems
Some high-priced cars, such as the Range Rover Sport and BMW 7-series, have begun to use "active"
anti-roll bars that can be proportionally controlled automatically by a suspension-control computer,
reducing body lean in turns while improving rough-road ride quality. The first [2] to use this was
theCitroen Xantia Activa, a medium sized sedan sold in Europe. The Activa system featured an antiroll bar that could be stiffened under the command of the suspension ECU during hard cornering. The
car rolled at any time at most 2 degrees. Mercedes S-class ABC system uses another approach: the
computer uses sensors to detect lateral load, lateral force, and height difference in the suspension
strut, then uses hydraulic pressure to raise or lower the spring to counter roll. This system removes the
anti-roll bar. Most active roll control systems allow a small degree of roll to give a more natural feel.
See
also
A starter is an electric motor, pneumatic motor, hydraulic motor, or other device for rotating
an internal-com
bustion engine so as to initiate the engine's
operation under its own power.
Bendix drive
A Bendix drive is a type of engagement mechanism used in starter motors of internal combustion
engines. The device allows the pinion gear of the starter motor to engage or disengage the flywheel of
the engine automatically when the starter is powered or when the engine fires, respectively. It is
named after its inventor, Vincent Hugo Bendix.
Operation
The Bendix system places the starter drive pinion on a helical drive spring. When the starter motor
begins turning, the inertia of the drive pinion assembly causes it to wind the spring forcing the length of
the spring to change and engage with the ring gear. When the engine starts, backdrive from the ring
gear causes the drive pinion to exceed the rotative speed of the starter, at which point the drive pinion
is forced back and out of mesh with the ring gear.
The main drawback to the Bendix drive is that it relies on a certain amount of "clash" between the teeth
of the pinion and the ring gears before they slip into place and mate completely; the teeth of the pinion
are already spinning when they come into contact with the static ring gear, and unless they happen to
align perfectly at the moment they engage, the pinion teeth will strike the teeth of the ring gear side-toside rather than face-to-face, and continue to rotate until both align. This increases wear on both sets
of teeth.
References
Automotive battery
(Redirected from Car battery)
A typical, 12 V, 40 Ah Lead-acid car battery
An automotive battery is a type of rechargeable batterythat supplies electric energy to an automobile.
[1]
Usually this refers to an SLI battery (starting, lighting, ignition) to power the starter motor, the lights,
and the ignition systemof a vehicle’s engine.
Automotive SLI batteries are usually lead-acid type, and are made of six galvanic cells in series to
provide a 12 voltsystem. Each cell provides 2.1 volts for a total of 12.6 volt at full charge. Heavy
vehicles such as highway trucks or tractors, often equipped with diesel engines, may have two
batteries in series for a 24 volt system, or may have parallel strings of batteries.
submerged into an electrolyte solution of about 35% sulfuric acid and 65% water.[2] This causes
a chemical reactionthat releases electrons, allowing them to flow through conductors to
produce electricity. As the batterydischarges, the acid of the electrolyte reacts with the materials of the
plates, changing their surface tolead sulfate. When the battery is recharged, the chemical reaction is
reversed: the lead sulfate reforms into lead dioxide and lead. With the plates restored to their original
condition, the process may now be repeated.
Battery recycling of automotive batteries reduces the need for resources required for manufacture of
new batteries, diverts toxic lead from landfills, and prevents risk of improper disposal.
Contents
[hide]
1 Types
2 Use and maintenance
o
2.1 Fluid level
o
2.2 Charge and discharge
o
2.3 Storage
o
2.4 Changing a battery
o
2.5 Freshness
3 Failure
4 Exploding batteries
5 Terms and ratings
6 Terminal voltage
8 References
Types
Lead-acid batteries for automotive use are made with slightly different construction techniques,
depending on the application of the battery. The "flooded cell" type, indicating liquid electrolyte, is
typically inexpensive and long-lasting, but requires more maintenance and can spill or leak. Flooded
batteries are distinguished by the removable caps that allow for the electrolyte to be tested and
maintained.
More costly alternatives to flooded batteries are "valve regulated lead acid" (VRLA) batteries, also
called "sealed" batteries. The absorbed glass mat (AGM) type uses a glass mat separator, and a "gel
cell" uses fine powder to absorb and immobilize the sulfuric acid electrolyte. These batteries are not
serviceable: the cells are sealed so the degree of charge cannot be measured by hydrometer and the
electrolyte cannot be replenished. They are typically termed "maintenance-free" by proponents, or
"unable to be maintained" by skeptics.[3][4][5] Both types of sealed batteries may be used in vehicular
applications where leakage or ventilation for vented gasses is a concern. However, this article deals
with the classic, flooded-type of car battery.
The starting (cranking) or shallow cycle type is designed to deliver large bursts of power for a short
time, as is needed to start an engine. Once the engine is started, the battery is recharged by the
engine-driven charging system. Starting batteries are intended to have a low depth of discharge on
each use. They are constructed of many thin plates with thin separators between the plates, and may
have a higher specific gravity electrolyte to reduce internal resistance. [1]
The deep cycle (or motive) type is designed to continuously provide power for long periods of time (for
example in a trolling motor for a small boat, auxiliary power for a recreational vehicle, or traction power
for a golf cart or other battery electric vehicle). They can also be used to store energy from
aphotovoltaic array or a small wind turbine. Deep-cycle batteries have fewer, thicker plates and are
intended to have a greater depth of discharge on each cycle, but will not provide as high a current on
heavy loads. The thicker plates survive a higher number of charge/discharge cycles. The specific
energy is in the range of 30-40 watt-hours per kilogram.[2]
Some cars use more exotic starter batteries–the 2010 Porsche 911 GT3 RS offers a lithium-ion
batteryas an option to save weight over a conventional lead-acid battery. [6]
Use
and maintenance
Fluid
level
Filling a (flooded lead-acid type) car battery with distilled water
Car batteries using lead-antimony plates would require regular watering to replace water lost due
to electrolysis on each charging cycle. By changing the alloying element to calcium, more recent
designs have lower water loss, unless overcharged. Modern car batteries have reduced maintenance
requirements, and may not provide caps for addition of water to the cells. Such batteries include extra
electrolyte above the plates to allow for losses during the battery life. If the battery has easily
detachable caps then a top-up with distilled water may be required from time to time. Prolonged
overcharging or charging at excessively high voltage causes some of the water in the electrolyte to be
broken up into hydrogen and oxygen gases, which escape from the cells; this is called gassing. If the
electrolyte liquid level drops too low, the plates are exposed to air, lose capacity, and are damaged.
The sulfuric acid in the battery normally does not require replacement since it is not consumed even
on overcharging. Impurities or additives in the water will reduce the life and performance of the battery.
Manufacturers usually recommend use of demineralized or distilled water, since even potable tap
water can contain high levels of minerals.
Charge
and discharge
In normal automotive service the vehicle's charging system powers the vehicle's electrical systems and
restores charge used from the battery during engine cranking. When installing a new battery or
recharging a battery that has been accidentally discharged completely, one of several different
methods can be used to charge it. The most gentle of these is called trickle charging. Other methods
include slow-charging and quick-charging, the latter being the harshest.
The voltage regulator of the charge system does not measure the relative currents charging the
battery and for powering the car's loads. The charge system essentially provides a fixed voltage of
typically 13.8 to 14.4 V (Volt), adjusted to ambient temperature, unless the alternator is at its current
limit. A discharged battery draws a high charge current of typically 20 to 40 A (Ampere). As the battery
becomes charged the charge current typically decreases to 2—5 amperes. A high load is when
multiple high-power systems such as ignition, radiator fan, heater blowers, lights and entertainment
system are running at the same time. In older vehicles (read: 80's and earlier) the battery voltage may
decrease unless the engine is running at a higher than idle rpm and the alternator/generator is
delivering enough current to power the load. This is not an issue for modern vehicles where alternators
provide enough current for all loads and a regulator keeps charging voltage in check. In such cars rpm
has little influence on the battery voltage - tests show near normal voltage regardless of the AC /
headlights / music / fan / defrosting / other electrical loads, even at idle.
Some manufacturers include a built-in hydrometer to show the state of charge of the battery, a
transparent tube with a float immersed in the electrolyte visible through a window. When the battery is
charged, the specific gravity of the electrolyte increases (since all the sulfate ions are in the electrolyte,
not combined with the plates), and the colored top of the float is visible in the window. When the
battery is discharged, or the electrolyte level is too low, the float sinks and the window appears yellow
(or black). The built-in hydrometer only checks the state of charge of one cell and will not show faults
in the other cells. In a non-sealed battery each of the cells can be checked with a portable or handheld hydrometer.
A positive (red) jumper cable connected to battery post. An optional hydrometer window is visible by the single
jumper clamp. (The black negative jumper clamp is not shown)
In emergencies a vehicle can be jump started by the battery of another vehicle or by a portable battery
booster.
Whenever the car's charge system is inadequate to fully charge the battery, a battery charger can be
used. Simple chargers do not regulate the charge current, and the user needs to stop the process or
lower the charge current to prevent excessive gassing of the battery. More elaborate chargers, in
particular those implementing the 3-step charge profile, also referred to as IUoU, charge the battery
fully and safely in a short time without requiring user intervention. Desulfating chargers are also
commercially available for charging all types of lead-acid batteries.
Storage
Batteries last longer when stored in a charged state. Leaving an automotive battery discharged will
shorten its life, or make it unusable if left for a long time (usually several years); sulfation eventually
becomes irreversible with normal charging. Batteries in storage may be monitored and periodically
charged, or attached to a "float" charger to retain their capacity. Batteries are prepared for storage by
charging and cleaning deposits from the posts. Batteries are stored in a cool, dry environment for best
results since high temperatures increase the self discharge rate and plate corrosion.
Changing
a battery
When changing a battery, battery manufacturers recommend disconnecting the negative ground
connection first to prevent accidental short-circuits between the battery terminal and the vehicle frame.
Conversely the positive cable is connected first. A study by the National Highway Traffic Safety
Association estimated that in 1994 more than 2000 people were injured in the United States while
working with automobile batteries.
The majority of automotive lead-acid batteries are filled with the appropriate electrolyte solution at the
manufacturing plant, and shipped to the retailers ready to sell. Decades ago, this was not the case.
The retailer filled the battery, usually at the time of purchase, and charged the battery. This was a
time-consuming and potentially dangerous process. Care had to be taken when filling the battery
withacid, as acids are highly corrosive and can damage eyes, skin and mucous membranes.
Fortunately, this is less of a problem these days, and the need to fill a battery with acid usually only
arises when purchasing a motorcycle or ATV battery.
Freshness
Because of "sulfation", lead-acid batteries stored with electrolyte slowly deteriorate. Car batteries are
date coded to ensure installation within one year of manufacture. In the United States, the
manufacturing date is printed on a sticker. The date can be written in plain text or using an
alphanumerical code. The first character is a letter that specifies the month (A for January, B for
February and so on).[7] The letter "I" is skipped due to its potential to be mistaken for the number 1.
The second character is a single digit that indicates the year of manufacturing (for example, 6 for
2006). When first installing a newly purchased battery a "top up" charge at a low rate with an external
battery charger (available at auto parts stores) may maximize battery life and minimize the load on the
vehicle charging system.
Failure
Common battery faults include:

Shorted cell due to failure of the separator between the positive and negative plates

Shorted cell or cells due to build up of shed plate material below the plates of the cell

Broken internal connections due to corrosion

Broken plates due to vibration and corrosion

Low electrolyte level

Cracked or broken case

Broken terminals

Sulfation after prolonged disuse in a low or zero charged state
Corrosion at the battery terminals can prevent a car from starting due to electrical resistance. The
white powder sometimes found around the battery terminals is usually lead sulfate which is toxic by
inhalation, ingestion and skin contact. The corrosion is caused by an imperfect seal between the
plastic battery case and lead battery post allowing sulfuric acid to react with the lead battery posts. The
corrosion process is also expedited by over charging. Corrosion can also be caused by factors such as
salt water, dirt, heat, humidity, cracks in the battery casing or loose battery terminals. Inspection,
cleaning and protection with a light coating of dielectric grease are measures used to prevent
corrosion of battery terminals.
Sulfation occurs when a battery is not fully charged. The longer it remains in a discharged state the
harder it is to overcome sulfation. This may be overcome with slow, low-current (trickle) charging.
Sulfation is the formation of large, non-conductive lead sulfate crystals on the plates; lead sulfate
formation is part of each cycle, but in the discharged condition the crystals become large and block
passage of current through the electrolyte.
The primary wear-out mechanism is the shedding of active material from the battery plates, which
accumulates at the bottom of the cells and which may eventually short-circuit the plates.
Early automotive batteries could sometimes be repaired by dismantling and replacing damaged
separators, plates, intercell connectors and other repairs. Modern battery cases do not facilitate such
repairs; an internal fault generally requires replacement of the entire unit. [1]
Exploding
batteries
Car battery after explosion
Any lead-acid battery system when overcharged (>14.34 V) will produce hydrogen gas (gassing
voltage) by electrolysis of water. If the rate of overcharge is small, the vents of each cell allow the
dissipation of the gas. However, on severe overcharge or if ventilation is inadequate, or the battery is
faulty, a flammable concentration of hydrogen may remain in the cell or in the battery enclosure. An
internal spark can cause ahydrogen and oxygenexplosion, which will damage the battery and its
surroundings and which will disperse acid into the surroundings. Anyone close to the battery may be
injured.
Sometimes the ends of a battery will be severely swollen, and when accompanied by the case being
too hot to touch, this usually indicates a malfunction in the charging system of the car. Reversing the
positive and negative leads will damage the battery. When severely overcharged, a lead-acid battery
produces high levels of hydrogen and the venting system built into the battery cannot handle the high
level of gas, so the pressure builds inside the battery, resulting in the swollen ends. An unregulated
alternator can quickly ruin a battery by excessive voltage. A swollen, hot battery is dangerous.
Another potential cause of explosion is when the battery terminals are short-circuited via a very low
resistance path (like a wrench or other tool dropped or lying across the terminals). Apart from the
sparks which usually occur in a short circuit, heating due to the internal resistance of the battery can
cause the electrolyte to boil, also leading to explosion due to buildup of water vapor pressure
(unrelated to electrolysis).
Persons handling car batteries should wear protective equipment (goggles, overalls, gloves) to avoid
injury by acid spills. Any open flame or electric sparks, including lit tobacco products like cigarettes,
cigars or pipes, in the area also present a danger of ignition of any hydrogen gas emanating from a
battery (this is the reason, when recharging the battery in place in the vehicle or jump starting, that the
negative cable of the recharger or attached to the other vehicle's jumping battery negative post is
always attached away from the battery to ground on the engine or frame, and is always attached to
complete the circuit only after the positive cable has been attached to the battery's positive terminal
(and is removed in the reverse order, i.e., negative cable first from the frame or engine, breaking the
circuit, then positive cable from the battery) - in this fashion, any sparks which may occur will occur at
the more distant location of the negative cable attachment point, away from the battery and potentially
explosive gases, and no sparks will occur, as the circuit is no longer complete, when the positive cable
is attached or detached from the battery).
Terms

and ratings
Ampere-hours (A·h) is a measure of electrical charge that a battery can deliver. This quantity
is one indicator of the total amount of charge that a battery is able to store and deliver at its rated
voltage. Its value is the product of the discharge-current (in amperes), multiplied by the duration
(in hours) for which this discharge-current can be sustained by the battery. Generally, this value
(or rating) varies widely with the duration of the discharge period (see: Peukert's Law), therefore
the value is typically only meaningful when the duration is specified. This rating is rarely stated for
automotive batteries, except in Europe where it is required by law.
Cranking amperes (CA), also sometimes referred to as marine cranking amperes (MCA), is

the amount of current a battery can provide at 32 °F (0 °C). The rating is defined as the number of
amperes a lead-acid battery at that temperature can deliver for 30 seconds and maintain at least
1.2 volts per cell (7.2 volts for a 12 volt battery).
Cold cranking amperes (CCA) is the amount of current a battery can provide at 0 °F (−18 °C).

The rating is defined as the current a lead-acid battery at that temperature can deliver for 30
seconds and maintain at least 1.2 volts per cell (7.2 volts for a 12-volt battery). It is a more
demanding test than those at higher temperatures.
Hot cranking amperes (HCA) is the amount of current a battery can provide at 80 °F (26.7 °C).

The rating is defined as the current a lead-acid battery at that temperature can deliver for 30
seconds and maintain at least 1.2 volts per cell (7.2 volts for a 12-volt battery).
Reserve capacity minutes (RCM), also referred to as reserve capacity (RC), is a battery's

ability to sustain a minimum stated electrical load; it is defined as the time (in minutes) that a leadacid battery at 80 °F (27 °C) will continuously deliver 25 amperes before its voltage drops below
10.5 volts.
Battery Council International group size (BCI) specifies a battery's physical dimensions, such

as length, width, and height. These groups are determined by the Battery Council
Internationalorganization.[8]
Peukert's Law states that the capacity available from a battery varies according to how rapidly

it is discharged. A battery discharged at high rate will give fewer ampere hours than one
discharged more slowly.
The hydrometer measures the density, and therefore indirectly the amount of sulfuric acid in

the electrolyte. A low reading means that sulfate is bound to the battery plates and that the battery
is discharged. Upon recharge of the battery, the sulfate returns to the electrolyte.
Terminal
voltage
The open circuit voltage, is measured when the engine is off and no loads are connected. It can be
approximately related to the charge of the battery by:
Open circuit voltage
Approximate Relative
charge
acid density
12 V
6V
12.60 V
6.32 V
100%
1.265 g/cm3
12.35 V
6.22 V
75%
1.225 g/cm3
12.10 V
6.12 V
50%
1.190 g/cm3
11.95 V
6.03 V
25%
1.155 g/cm3
11.70 V
6.00 V
0%
1.120 g/cm3
Open circuit voltage is also affected by temperature, and the specific gravity of the electrolyte at full
charge.
The following is common for a six-cell automotive lead-acid battery at room temperature:

Quiescent (open-circuit) voltage at full charge: 12.6 V

Fully discharged: 11.8 V

Charge with 13.2–14.4 V

Gassing voltage: 14.4 V

Continuous-preservation charge with max. 13.2 V

After full charge the terminal voltage will drop quickly to 13.2 V and then slowly to 12.6 V

Open circuit voltage is measured 12 hours after charging to allow surface charge to dissipate
and enable a more accurate reading.

All voltages are at 20 °C, and must be adjusted -0.022V/°C for temperature changes (negative
temperature coefficient - lower voltage at higher temperature).
See
also


Battery


42-volt electrical system



Vehicle-to-grid
specific energy
30–40 Wh/kg
energy density
60–75 Wh/l
specific power
180 W/kg
Charge/discharge efficiency
50%–92% [3]
Energy/consumer-price
7(sld)-18(fld) Wh/US\$ [4]
Self-discharge rate
3–20%/month [5]
Cycle durability
500–800 cycles
Nominal cell voltage
2.105 V
Lead–acid batteries, invented in 1859 by French physicist Gaston Planté, are the oldest type
of rechargeable battery. Despite having a very low energy-to-weight ratio and a low energy-to-volume
ratio, their ability to supply high surge currents means that the cells maintain a relatively large powerto-weight ratio. These features, along with their low cost, make them attractive for use in motor
vehicles to provide the high current required by automobile starter motors.
Lead–acid batteries (under 5 kg) account for 1.5% of all portable secondary battery sales in Japan by
number of units sold (25% by price).[1] Sealed lead–acid batteries accounted for 10% by weight of all
portable battery sales in the EU in 2000.
[2]
Contents
[hide]
1 History
2 Electrochemistry
o
2.1 Discharge
o
2.2 Charging
3 Voltages for common usages
4 Measuring the charge level
5 Construction
o
5.1 Plates
o
5.2 Separators
6 Applications
7 Cycles
o
7.1 Starting batteries
o
7.2 Deep cycle batteries
o
7.3 Fast and slow charge and discharge
8 Valve regulated
9 Sulfation and desulfation
10 Stratification
11 Risk of explosion
12 Environment
o
12.1 Environmental concerns
o
12.2 Recycling
14 Corrosion problems
15 Maintenance precautions
17 References
History
Main article: History of the battery
In 1859, Gaston Planté's lead-acid battery was the first battery that could be recharged by passing a
reverse current through it. Planté's first model consisted of two lead sheets separated by rubber strips
and rolled into a spiral.[3] His batteries were first used to power the lights in train carriages while
stopped at a station. In 1881, Camille Alphonse Faure invented an improved version that consisted of
a lead grid lattice into which a lead oxide paste was pressed, forming a plate. This design was easier
to mass-produce.
The lead-acid battery is still used today in automobiles and other applications where weight is not a big
factor. In the 1970s the valve regulated lead acid battery (often called "sealed") was developed that
used a gel electrolyte instead of a liquid, allowing the battery to be used in different positions without
leakage.
Electrochemistry
Discharge
Fully Discharged: Two identical lead sulfate plates
In the discharged state both the positive and negative plates become lead(II) sulfate (PbSO4) and
the electrolyteloses much of its dissolved sulfuric acid and becomes primarily water. The discharge
process is driven by the conduction of electrons from the positive plate back into the cell at the
negative plate.
Negative plate reaction: Pb(s) + HSO−
4(aq) →PbSO4(s) + H+(aq) + 2e
Positive plate reaction: PbO2(s) + HSO−
4(aq) + 3H+(aq) + 2e → PbSO4(s) + 2H2O(l)
The total reaction can be written:
Pb(s) + PbO2(s) + 2H2SO4(aq) → 2PbSO4(s) + 2H2O(l)
The sum of the molecular weights of the reactants is 642.6, so theoretically a cell can
produce twofaradays of charge from 642.6 g of reactants, or 83.4 amp-hours per kg (or
13.9 amp-hours per kg for a 12-volt battery). At 2 volts per cell, this comes to 167 watthours per kg, but lead-acid batteries in fact give only 30 to 40 watt-hours per kg due to
the weight of the water and other factors.
Charging
In the charged state, each cell contains negative plates of elemental lead (Pb) and
positive plates of lead(IV) oxide(PbO2) in an electrolyte of approximately 33.5% v/v (4.2
Molar) sulfuric acid (H2SO4). The charging process is driven by the forcible removal of
electrons from the negative plate and the forcible introduction of them to the positive
plate.
Negative plate reaction: PbSO4(s) + H+(aq) + 2e →Pb(s) + HSO−
4(aq)
Positive plate reaction: PbSO4(s) + 2H2O(l) →PbO2(s) + HSO−
4(aq) + 3H+(aq) + 2e
Overcharging with high charging voltages generates oxygen and hydrogen gas
by electrolysis of water, which is lost to the cell. Periodic maintenance of lead
acid batteries requires inspection of the electrolyte level and replacement of any
water that has been lost.
Due to the freezing-point depression of water, as the battery discharges and the
concentration of sulfuric acid decreases, the electrolyte is more likely to freeze
during winter weather.
Voltages
for common usages
This section does
not cite anyreferences or
sources.(February 2012)
These are general voltage ranges for six-cell lead-acid batteries:

Open-circuit (quiescent) at full charge: 12.6 V (2.1V per cell)

Open-circuit at full discharge: 11.7 V

Loaded at full discharge: 10.5 V.

Continuous-preservation (float) charging: 13.4 V for gelled electrolyte; 13.5
V for AGM (absorbed glass mat) and 13.9 V for flooded cells
1.
All voltages are at 20 °C (68 °F), and must be adjusted −0.0235V/°C
for temperature changes.
2.
Float voltage recommendations vary, according to the manufacturer's
recommendation.
3.
Precise float voltage (±0.05 V) is critical to longevity; insufficient
voltage (causes sulfation) is almost as detrimental as excessive
voltage (causing corrosion and electrolyte loss)

Typical (daily) charging: 14.2 V to 14.4 V (depending on temperature and
manufacturer's recommendation)

Equalization charging (for flooded lead acids): 15 V for no more than 2.205
hours. Battery temperature must be absolutely monitored.

Gassing threshold: 14.4 V
Portable batteries, such as for miners' cap lamps headlamps typically have two
or three cells. [4]
Measuring
the charge level
A hydrometer can be used to test the specific gravity of each cell as a measure of its
state of charge.
A battery's open-circuit voltage can be used to estimate the state of charge, in this case
for a 12-volt battery.
Because the electrolyte takes part in the charge-discharge reaction, this battery
has one major advantage over other chemistries. It is relatively simple to
determine the state of charge by merely measuring the specific gravity (S.G.) of
the electrolyte, the S.G. falling as the battery discharges. Some battery designs
include a simple hydrometer using colored floating balls of differing density.
When used in diesel-electric submarines, the S.G. was regularly measured and
written on a blackboard in the control room to indicate how much longer the
boat could remain submerged.[5]
The battery's open circuit voltage can also be used to gauge the state of
charge.[6] If the connections to the individual cells are accessible, then the state
of charge of the each cell can be determined which can provide a guide as to
the state of health of the battery as a whole.
Construction
Plates
The lead–acid cell can be demonstrated using sheet lead plates for the two
electrodes. However such a construction produces only around one ampere for
roughly postcard sized plates, and for only a few minutes.
Gaston Planté found a way to provide a much larger effective surface area. In
Planté's design, the positive and negative plates were formed of two spirals of
lead foil, separated with a sheet of cloth and coiled up. The cells initially had low
capacity, so a slow process of "forming" was required to corrode the lead foils,
creating lead dioxide on the plates and roughening them to increase surface
area. Initially this process used electricity from primary batteries; when
generators became available after 1870, the cost of production of batteries
greatly declined.[7] Planté plates are still used in some stationary applications,
where the plates are mechanically grooved to increase their surface area.
Faure pasted-plate construction is typical of automotive batteries. Each plate
consists of a rectangular lead grid alloyed with antimony or calcium to improve
the mechanical characteristics. The holes of the grid are filled with a paste
of red lead and 33% dilute sulfuric acid. (Different manufacturers vary the
mixture). The paste is pressed into the holes in the grid which are slightly
tapered on both sides to better retain the paste. This porous paste allows the
acid to react with the lead inside the plate, increasing the surface area many
fold. Once dry, the plates are stacked with suitable separators and inserted in
the battery container. An odd number of plates is usually used, with one more
negative plate than positive. Each alternate plate is connected.
The positive plates are the chocolate brown color of lead dioxide, and the
negative are the slate gray of "spongy" lead at the time of manufacture. In this
charged state the plates are called 'formed'.
One of the problems with the plates is that the plates increase in size as
the active material absorbssulfate from the acid during discharge, and decrease
as they give up the sulfate during charging. This causes the plates to gradually
shed the paste. It is important that there is room underneath the plates to catch
this shed material. If it reaches the plates, the cell short-circuits.
The paste contains carbon black, blanc fixe (barium sulfate) and lignosulfonate.
The blanc fixe acts as a seed crystal for the lead–to–lead sulfate reaction. The
blanc fixe must be fully dispersed in the paste in order for it to be effective. The
lignosulfonate prevents the negative plate from forming a solid mass during the
discharge cycle, instead enabling the formation of long needle–like crystals.
The long crystals have more surface area and are easily converted back to the
original state on charging. Carbon black counteracts the effect of inhibiting
formation caused by the lignosulfonates. Sulfonatednaphthalene condensate
dispersant is a more effective expander than lignosulfonate and speeds up
formation. This dispersant improves dispersion of barium sulfate in the paste,
reduces hydroset time, produces a more breakage-resistant plate, reduces fine
lead particles and thereby improves handling and pasting characteristics. It
extends battery life by increasing end–of–charge voltage. Sulfonated
naphthalene requires about one-third to one-half the amount of lignosulfonate
and is stable to higher temperatures.[8]
Practical cells are usually not made with pure lead but have small amounts
of antimony, tin, calciumor selenium alloyed in the plate material to add strength
and simplify manufacture. The alloying element has a great effect on the life of
the batteries, with calcium-alloyed plates preferred over antimony for longer life
and less water consumption on each charge/discharge cycle.
About 60% of the weight of an automotive-type lead–acid battery rated around
60 Ah (8.7 kg of a 14.5 kg battery) is lead or internal parts made of lead; the
balance is electrolyte, separators, and the case. [7]
Separators
Separators between the positive and negative plates prevent short-circuit
through physical contact, mostly through dendrites (‘treeing’), but also through
shedding of the active material. Separators obstruct the flow of ions between
the plates and increase the internal resistance of the cell. Wood, rubber, glass
fiber mat, cellulose, and PVC or polyethylene plastic have been used to make
separators. Wood was the original choice, but deteriorated in the acid
electrolyte. Rubber separators were stable in the battery acid.
An effective separator must possess a number of mechanical properties; such
as permeability, porosity, pore size distribution, specific surface area,
mechanical design and strength, electrical resistance, ionic conductivity, and
chemical compatibility with the electrolyte. In service, the separator must have
good resistance to acid and oxidation. The area of the separator must be a little
larger than the area of the plates to prevent material shorting between the
plates. The separators must remain stable over the battery's operating
temperature range.
Applications
Most of the world's lead–acid batteries are automobile starting, lighting and
ignition (SLI) batteries, with an estimated 320 million units shipped in 1999. [7] In
1992 about 3 million tons of lead were used in the manufacture of batteries.
Wet cell stand-by (stationary) batteries designed for deep discharge are
commonly used in large backup power supplies for telephone and computer
centers, grid energy storage, and off-grid household electric power systems.
[9]
Lead–acid batteries are used in emergency lighting and to power sump
pumps in case of power failure.
Traction (propulsion) batteries are used for in golf carts and other battery
electric vehicles. Large lead–acid batteries are also used to power the electric
motors in diesel-electric (conventional) submarinesand are used on nuclear
submarines as well. Valve-regulated lead acid batteries cannot spill their
electrolyte. They are used in back-up power supplies for alarm and smaller
computer systems (particularly in uninterruptible power supplies ("UPS")) and
for electric scooters, electric wheelchairs,electrified bicycles, marine
applications, battery electric vehicles or micro hybrid vehicles, and motorcycles.
Lead–acid batteries were used to supply the filament (heater) voltage, with 2 V
Cycles
Starting
batteries
Main article: Car battery
Lead acid batteries designed for starting automotive engines are not designed
for deep discharge. They have a large number of thin plates designed for
maximum surface area, and therefore maximum current output, but which can
easily be damaged by deep discharge. Repeated deep discharges will result in
capacity loss and ultimately in premature failure, as the electrodes disintegrate
due to mechanical stresses that arise from cycling. Starting batteries kept on
continuous float charge will have corrosion in the electrodes which will result in
premature failure. Starting batteries should be kept open circuitbut charged
regularly (at least once every two weeks) to prevent sulfation.
Starting batteries are lighter weight than deep cycle batteries of the same
battery dimensions, because the cell plates do not extend all the way to the
bottom of the battery case. This allows loose disintegrated lead to fall off the
plates and collect under the cells, to prolong the service life of the battery. If this
loose debris rises high enough it can touch the plates and lead to failure of a
cell, resulting in loss of battery voltage and capacity.
Deep
cycle batteries
Main article: Deep cycle battery
Specially designed deep-cycle cells are much less susceptible to degradation
due to cycling, and are required for applications where the batteries are
regularly discharged, such as photovoltaic systems,electric vehicles (forklift, golf
cart, electric cars and other) and uninterruptible power supplies. These batteries
have thicker plates that can deliver less peak current, but can withstand
frequent discharging.[10]
Some batteries are designed as a compromise between starter (high-current)
and deep cycle batteries. They are able to be discharged to a greater degree
than automotive batteries, but less so than deep cycle batteries. They may be
referred to as "Marine/Motorhome" batteries, or "leisure batteries".
Fast
and slow charge and discharge
Charge current needs to match the ability of the battery to absorb the energy. Using too
large a charge current on a small battery can lead to boiling and venting of the
electrolyte. In this image a VRLA battery case has ballooned due to the high gas
pressure developed during overcharge.
The capacity of a lead–acid battery is not a fixed quantity but varies according
to how quickly it is discharged. An empirical relationship exists between
discharge rate and capacity, known as Peukert's law.
When a battery is charged or discharged, this initially affects only the reacting
chemicals, which are at the interface between the electrodes and the
electrolyte. With time, the charge stored in the chemicals at the interface, often
called "interface charge", spreads by diffusion of these chemicals throughout
the volume of the active material.
If a battery has been completely discharged (such as by leaving the car lights
on overnight) and then is given a fast charge for only a few minutes, the battery
plates charge only near the interface between plate and electrolyte. The battery
voltage may rise to be close to the charger voltage so that the charging current
decreases significantly. After a few hours this interface charge will spread to the
volume of the electrode and electrolyte, leading to an interface charge so low
that it may be insufficient to start the car.[11]
On the other hand, if the battery is given a slow charge, which takes longer,
then the battery will become more fully charged. During a slow charge the
interface charge has time to redistribute to the volume of the electrodes and
electrolyte, while being replenished by the charger. The battery voltage remains
below the charger voltage throughout this process allowing charge to flow into
the battery.
Similarly, if a battery is subject to a fast discharge (such as starting a car, a
current draw of more than 100 amps) for a few minutes, it will appear to go
dead, exhibiting reduced voltage and power. However, it may have only lost its
interface charge. If the discharge is halted for a few minutes the battery may
resume normal operation at the appropriate voltage and power for its state of
discharge. On the other hand, if a battery is subject to a slow, deep discharge
(such as leaving the car lights on, a current draw of less than 7 amps) for hours,
then any observed reduction in battery performance is likely permanent.
Valve
regulated
In a valve regulated lead acid (VRLA) battery the hydrogen and oxygen
produced in the cells largely recombine into water. Leakage is minimal,
although some electrolyte still escapes if the recombination cannot keep up with
gas evolution. Since VRLA batteries do not require (and make impossible)
regular checking of the electrolyte level, they have been called maintenance
free batteries. However, this is somewhat of a misnomer. VRLA cells do require
maintenance. As electrolyte is lost, VRLA cells "dry-out" and lose capacity. This
can be detected by taking regular
internal resistance, conductance orimpedance measurements. Regular testing
reveals whether more involved testing and maintenance is required. Recent
maintenance procedures have been developed allowing "rehydration", often
restoring significant amounts of lost capacity.
VRLA types became popular on motorcycles around 1983,[12] because the acid
electrolyte is absorbed into the separator, so it cannot spill. [13] The separator
also helps them better withstand vibration. They are also popular in stationary
applications such as telecommunications sites, due to their small footprint and
installation flexibility.[14]
The electrical characteristics of VRLA batteries differ somewhat from wet-cell
lead–acid batteries, requiring caution in charging and discharging.
Sulfation
and desulfation
Lead–acid batteries lose the ability to accept a charge when discharged for too
long due to sulfation, the crystallization of lead sulfate. They generate electricity
through a double sulfate chemical reaction. Lead and lead dioxide, the active
materials on the battery's plates, react with sulfuric acid in the electrolyte to
divided, amorphous state, and easily reverts to lead, lead dioxide and sulfuric
acid when the battery recharges. As batteries cycle through numerous
discharges and charges, some lead sulfate is not recombined into electrolyte
and slowly converts to a stable crystalline form that no longer dissolves on
recharging. Thus, not all the lead is returned to the battery plates, and the
amount of usable active material necessary for electricity generation declines
over time.
Sulfation occurs in all lead–acid batteries during normal operation. It impedes
recharging; sulfate deposits ultimately expand, cracking the plates and
destroying the battery. Eventually so much of the battery plate area is unable to
supply current that the battery capacity is greatly reduced. In addition, the
sulfate portion (of the lead sulfate) is not returned to the electrolyte as sulfuric
acid. The large crystals physically block the electrolyte from entering the pores
of the plates. Sulfation can be avoided if the battery is fully recharged
immediately after a discharge cycle.[15] A white coating on the plates may be
visible (in batteries with clear cases, or after dismantling the battery). Batteries
that are sulfated show a high internal resistance and can deliver only a small
fraction of normal discharge current.
Sulfation also affects the charging cycle, resulting in longer charging times, less
efficient and incomplete charging, and higher battery temperatures.
The process can often be at least partially reversed by a desulfation technique
called pulse conditioning, in which short but powerful current surges are
repeatedly sent through the damaged battery. Over time, this procedure tends
to break down and dissolve the sulfate crystals, restoring some capacity. [16]
Desulfation is the process of reversing the sulfation of a lead-acid battery.
Desulfation is achieved by high current pulses produced between the terminals
of the battery. This technique, also called pulse conditioning, breaks down the
sulfate crystals that are formed on the battery plates. Short high current pulses
tend to work best. Electronic circuits are used to regulate the pulses of different
widths and frequency of high current pulses. These can also be used to
automate the process since it takes a long period of time to desulfate a battery
fully. Battery chargers designed for desulfating lead-acid batteries are
commercially available. A battery will be unrecoverable if the active material has
been lost from the plates, or if the plates are bent due to over temperature or
over charging.
Batteries which have sat unused for long periods of time can be prime
candidates for desulfation. A long period of self-discharge allows the sulfate
crystals to form and become very large. Some typical cases where lead acid
batteries are not used frequently enough are planes, boats (esp sail boats), old
cars, and home power systems with battery banks that are under utilized.
Some charging techniques can aid in prevention such as equalization charging
and cycles through discharging and charging regularly. It is recommended to
follow battery manufacturer instructions for proper charging.
SLI batteries (starting, lighting, ignition; i.e. car batteries) have less deterioration
because they are used more frequently vs deep cycle batteries. Deep cycle
batteries tend to require more desulfation, can suffer from overcharging, and
can be in a very large bank which leads to unequal charging and discharging.
Stratification
A typical lead–acid battery contains a mixture with varying concentrations of
water and acid. There is a slight difference in density between water and acid,
and if the battery is allowed to sit idle for long periods of time, the mixture can
separate into distinct layers with the water rising to the top and the acid sinking
to the bottom. This results in a difference of acid concentration across the
surface of the plates, and can lead to greater corrosion of the bottom half of the
plates.[7]
Frequent charging and discharging tends to stir up the mixture, since
the electrolysis of water during charging forms hydrogen and oxygen bubbles
that rise and displace the liquid as the bubbles move upward. Batteries in
moving vehicles are also subject to sloshing and splashing in the cells, as the
vehicle accelerates, brakes, and turns.
Risk
of explosion
Car battery after explosion
Excessive charging electrolyzessome of the water, emitting hydrogen and
oxygen. This process is known as "gassing". Wet cells have open vents to
release any gas produced, and VRLA batteries rely on valves fitted to each cell.
Wet cells come with catalytic caps to recombine any emitted hydrogen. A VRLA
cell normally recombines any hydrogenand oxygen produced inside the cell, but
malfunction or overheating may cause gas to build up. If this happens (for
example, on overcharging) the valve vents the gas and normalizes the
pressure, producing a characteristic acid smell. Valves can sometimes fail
however, if dirt and debris accumulate, allowing pressure to build up.
If the accumulated hydrogen and oxygen within either a VRLA or wet cell is
ignited, an explosionresults. The force can burst the plastic casing or blow the
top off the battery, spraying acid and casing shrapnel. An explosion in one cell
may ignite the combustible gas mixture in remaining cells.
The cell walls of VRLA batteries typically swell when the internal pressure rises.
The deformation varies from cell to cell, and is greater at the ends where the
walls are unsupported by other cells. Such over-pressurized batteries should be
carefully isolated and discarded. Personnel working near batteries at risk for
explosion should protect their eyes and exposed skin from burns due to
spraying acid and fire by wearing a face shield, overalls, and gloves.
Using goggles instead of a face shield sacrifices safety by leaving one's face
exposed to acid and heat from a potential explosion.
Environment
Environmental
concerns
According to a 2003 report entitled, "Getting the Lead Out," by Environmental
Defense and the Ecology Center of Ann Arbor, Mich., the batteries of vehicles
on the road contained an estimated 2,600,000 metric tons (2,600,000 long tons;
2,900,000 short tons) of lead. Some lead compounds are extremely toxic. Longterm exposure to even tiny amounts of these compounds can cause brain and
kidney damage, hearing impairment, and learning problems in children. [17] The
auto industry uses over 1,000,000 metric tons (980,000 long tons; 1,100,000
short tons) every year, with 90% going to conventional lead-acid vehicle
batteries. While lead recycling is a well-established industry, more than 40,000
metric tons (39,000 long tons; 44,000 short tons) ends up in landfills every year.
According to the federal Toxic Release Inventory, another 70,000 metric tons
(69,000 long tons; 77,000 short tons) are released in the lead mining and
manufacturing process.[18]
Attempts are being made to develop alternatives (particularly for automotive
use) because of concerns about the environmental consequences of improper
disposal and of lead smelting operations, among other reasons. Alternatives are
unlikely to displace them for applications such as engine starting or backup
power systems, since the batteries are low-cost although heavy.
Recycling
Lead–acid battery recycling is one of the most successful recycling programs in
the world. In the United States 97% of all battery lead was recycled between
1997 and 2001.[19] An effective pollution control system is a necessity to prevent
lead emission. Continuous improvement in battery recyclingplants and furnace
designs is required to keep pace with emission standards for lead smelters.
Since the 1950s chemical additives have been used to reduce lead sulfate build
up on plates and improve battery condition when added to the electrolyte of a
vented lead–acid battery. Such treatments are rarely, if ever, effective. [20]
Two compounds used for such purposes are Epsom salts and EDTA. Epsom
salts reduces the internal resistance in a weak or damaged battery and may
allow a small amount of extended life. EDTA can be used to dissolve
the sulfate deposits of heavily discharged plates. However, the dissolved
material is then no longer available to participate in the normal
charge/discharge cycle, so a battery temporarily revived with EDTA will have a
reduced life expectancy. Residual EDTA in the lead–acid cell forms organic
acids which will accelerate corrosion of the lead plates and internal connectors.
The active materials change physical form during charge/discharge, resulting in
growth and distortion of the electrodes, and shedding of electrode into the
electrolyte. Once the active material has fallen out of the plates, it cannot be
restored into position by any chemical treatment. Similarly, internal physical
problems such as cracked plates, corroded connectors, or damaged separators
cannot be restored chemically.
Corrosion
problems
Corrosion of the external metal parts of the lead–acid battery results from a
chemical reaction of the battery terminals, lugs and connectors.
Corrosion on the positive terminal is caused by electrolysis, due to a mismatch
of metal alloys used in the manufacture of the battery terminal and cable
connector. White corrosion is usually lead or zinc sulfate crystals. Aluminum
connectors corrode to aluminum sulfate. Copper connectors produce blue and
white corrosion crystals. Corrosion of a battery's terminals can be reduced by
coating the terminals with petroleum jelly or a commercially available product
If the battery is over-filled with water and electrolyte, thermal expansion can
force some of the liquid out of the battery vents onto the top of the battery. This
solution can then react with the lead and other metals in the battery connector
and cause corrosion.
The electrolyte can weep from the plastic-to-lead seal where the battery
terminals penetrate the plastic case.
Acid fumes that vaporize through the vent caps, often caused by overcharging,
and insufficient battery box ventilation can allow the sulfuric acid fumes to build
up and react with the exposed metals.
Maintenance
precautions
Ammonia can neutralize spilled battery acid. Surplus ammonia and water
evaporate, leaving anammonium sulfate residue. Sodium bicarbonate (baking
soda) is also commonly used for this purpose.
See
also
Contact breaker
Unsourced material may be challenged and removed. (February 2008)
Breaker arm with contact points at the left. The pivot is on the right and the cam follower is in the middle of the
breaker arm.
A contact breaker (or "points") is a type of electricalswitch, and the term typically refers to the
switching device found in the distributor of the ignition systemsof spark-ignition internal combustion
engines.
Contents
[hide]
1 Purpose
2 Operation
Purpose
The purpose of the contact breaker is to interrupt thecurrent flowing in the primary circuit of the ignition
coil. When this occurs, the collapsing current induces a high voltage in the secondary winding of the
coil, which has many more windings. This causes a very high voltage to appear at the coil output for a
short period - enough to arc across the electrodes of a spark plug.
Operation
The contact breaker is operated by an engine-driven cam, and the position of the contact breaker is
set so that they open (and hence generate a spark) at the exactly correct moment needed to ignite the
fuel at the top of the piston's compression stroke. The contact breaker is usually mounted on a plate
that is able to rotate relative to the camshaft operating it. The plate is rotated by a centrifugal
mechanism, thus advancing the ignition timing (making the spark occur earlier) at higher revolutions.
This gives the fuel time to burn so that the resulting gases reach their maximum pressure at the same
time as the piston reaches the top of the cylinder. The plate's position can also be moved a small
distance using a small manifold vacuum-operated servomechanism, providing advanced timing when
the engine is required to speed up on demand. This helps to prevent pre-ignition (or pinging).
of contact breakers
Since they open and close several times every turn of the engine, contact breaker points and cam
follower suffer from wear - both mechanical and pitting caused by arcing across the contacts. This
latter effect is largely prevented by placing a capacitor parallel across the contact breaker - this is
usually referred to by the more old fashioned term condenser by mechanics. As well as suppressing
arcing, it helps boost the coil output by creating a resonant LC circuit with the coil windings.
A drawback of using a mechanical switch as part of the ignition timing is that it is not very precise,
needs regular adjustment of the dwell (contact) angle, and at higher revolutions, its mass becomes
significant, leading to poor operation at higher engine speeds. These effects can largely be overcome
using electronic ignition systems, where the contact breakers are retrofitted by a magnetic (Hall effect)
or optical sensor device. However, because of their simplicity, and since contact breaker points
See
also
Distributor
Unsourced material may be challenged and removed. (September 2007)
Typical distributor with distributor cap
Also visible are mounting/drive shaft (bottom), vacuum advance unit (right) and capacitor (centre)
A distributor is a device in the ignition system of aninternal combustion engine that routes high
voltage from theignition coil to the spark plugs in the correct firing order. The first reliable battery
operated ignition was developed byDayton Engineering Laboratories Co. (Delco) and introduced in the
1910 Cadillac. This ignition was developed by Charles Kettering and was considered a wonder in its
day.
Contents
[hide]
1 Description
2 Distributor cap
3 Direct & distributorless ignition
5 References
Description
A distributor consists of a rotating arm or rotor inside the distributor cap, on top of the distributor shaft,
but insulated from it and the body of the vehicle (ground). The distributor shaft is driven by a gear on
the camshaft on most overhead valve engines, and attached directly to a camshaft on most overhead
cam engines. (The distributor shaft may also drive the oil pump.) The metal part of the rotor contacts
the high voltage cable from the ignition coil via a spring-loaded carbonbrush on the underside of the
distributor cap. The metal part of the rotor arm passes close to (but does not touch) the output
contacts which connect via high tension leads to the spark plug of each cylinder. As the rotor spins
within the distributor, electrical current is able to jump the small gaps created between the rotor arm
and the contacts due to the high voltage created by the ignition coil.
The distributor shaft has a cam that operates the contact breaker. Opening the points causes a
highinduction voltage in the system's ignition coil.
The distributor also houses the centrifugal advance unit: a set of hinged weights attached to the
distributor shaft, that cause the breaker points mounting plate to slightly rotate and advance the spark
the timing even further as a function of the vacuum in the inlet manifold. Usually there is also
a capacitorattached to the distributor. The capacitor is connected parallel to the breaker points, to
suppresssparking to prevent excessive wear of the points.
Around the 1970s[citation needed] the primary breaker points were largely replaced with a Hall effect
sensor or optical sensor. As this is a non-contacting device and the ignition coil is controlled by solid
state electronics, a great amount of maintenance in point adjustment and replacement was eliminated.
This also eliminates any problem with breaker follower or cam wear, and by eliminating a side load it
extends distributor shaft bearing life. The remaining secondary (high voltage) circuit stayed essentially
the same, using an ignition coil and a rotary distributor.
Most distributors used on electronically fuel injected engines lack vacuum and centrifugal advance
units. On such distributors, the timing advance is controlled electronically by the engine computer. This
allows more accurate control of ignition timing, as well as the ability to alter timing based on factors
other than engine speed and manifold vacuum (such as engine temperature). Additionally, eliminating
vacuum and centrifugal advance results in a simpler and more reliable distributor.
Distributor
cap
A distributor cap is used in an automobile's engine to cover the distributor and its internal rotor.
The distributor cap has one post for each cylinder, and in points ignition systems there is a central post
for the current from the ignition coil coming into the distributor. There are some exceptions however,
as some engines (many Alfa Romeo cars, some 1980's Nissans) have two spark plugs per cylinder, so
there are two leads coming out of the distributor per cylinder. Another implementation is the wasted
spark system, where a single contact serves two leads, but in that case each lead connects one
cylinder. In General Motors high energy ignition (HEI) systems there is no central post and the ignition
coil sits on top of the distributor. Some Toyota and Honda engines also have their coil within the
distributor cap. On the inside of the cap there is a terminal that corresponds to each post, and the plug
terminals are arranged around the circumference of the cap according to the firing order in order to
send the secondary voltage to the proper spark plug at the right time.
The rotor is attached to the top of the distributor shaft which is driven by the engine's camshaft and
thus synchronized to it. Synchronization to the camshaft is required as the rotor must turn at exactly
half the speed of the main crankshaft in the 4-stroke cycle. Often, the rotor and distributor are attached
directly to the end of the one of (or the only) camshaft, at the opposite end to the timing drive belt. This
rotor is pressed against a carbon brush on the center terminal of the distributor cap which connects to
the ignition coil. The rotor is constructed such that the center tab is electrically connected to its outer
edge so the current coming in to the center post travels through the carbon point to the outer edge of
the rotor. As the camshaft rotates, the rotor spins and its outer edge passes each of the internal plug
terminals to fire each spark plug in sequence.
Engines that use a mechanical distributor may fail if they run into deep puddles because any water
that leaks into the distributor can short out the electric current that should go through the spark plug,
rerouting it directly to the body of the vehicle. This in turn causes the engine to stop as the fuel is not
ignited in the cylinders. This problem can be fixed by removing the distributor's cap and drying the cap,
cam, rotor and the contacts by: wiping with tissue paper or a clean rag, by blowing hot air on them, or
using a moisture displacement spray i.e. WD-40 or similar. Oil, dirt or other contaminants can cause
similar problems, so the distributor should be kept clean inside and outside to ensure reliable
operation. Some engines include a rubber o-ring or gasket between the distributor base and cap to
help prevent this problem. This gasket should not be discarded when replacing the cap. Most
distributor caps have the position of the number 1 cylinder's terminal molded into the plastic. By
referencing a firing order diagram and knowing the direction the rotor turns, (which can be seen by
cranking the engine with the cap off) the spark plug wires can be correctly routed. Most distributor
caps are designed so that they cannot be installed in the wrong position. Some older engine designs
allow the cap to be installed in the wrong position by 180 degrees, however. The number 1 cylinder
position on the cap should be noted before a cap is replaced.
The distributor cap is a prime example of a component that eventually succumbs to heat and vibration.
It is a relatively easy and inexpensive part to replace if its bakelite housing does not break or crack
first. Carbon deposit accumulation or erosion of its metal terminals may also cause distributor-cap
failure.
As it is generally easy to remove and carry off, the distributor cap can be taken off as a means of theft
deterrence. Although not practical for everyday use, because it is essential for the starting and running
of the engine, its removal prevents any attempt at hot-wiring the vehicle.
Breaker arm with contact points at the
left. The pivot is on the right and the cam
follower is in the middle of the breaker
arm.
Distributor cap. At the center is a springloaded carbon button that bears upon the
rotor. The number of distribution points
(in this case 4) is determined by the
number of cylinders in the engine
Direct
& distributorless ignition
Modern engine designs have abandoned the high-voltage distributor and coil, instead performing the
distribution function in the primary circuit electronically and applying the primary (low-voltage) pulse to
individual coils for each spark plug, or one coil for each pair of companion cylinders in an engine (two
coils for a four-cylinder, three coils for a six-cylinder, four coils for an eight-cylinder, and so on).
In traditional remote distributorless systems, the coils are mounted together in a transformer oil filled
'coil pack', or separate coils for each cylinder, which are secured in a specified place in the engine
compartment with wires to the spark plugs, similar to a distributor setup. General
Motors, Ford,Chrysler, Hyundai, Subaru and Toyota are among the automobile manufacturers known
to have used coil packs. Coil packs by Delco for use with General Motors engines allow removal of the
individual coils in case one should fail, but in most other remote distributorless coil pack setups, if a
coil were to fail, replacement of the whole pack would be required to fix the problem.
More recent layouts utilize a coil located very near to or directly on top of each spark plug (Direct
Ignition, 'DI' or coil-on-plug). This design avoids the need to transmit very high voltages, which is often
a source of trouble, especially in damp conditions.
Both direct and remote distributorless systems also allow finer levels of ignition control by the engine
computer, which helps to increase power output, decrease fuel consumption and emissions, and
implement features such as Active Fuel Management. Spark plug wires, which need routine
replacement due to wear[citation needed], are also eliminated when the individual coils are mounted directly
on top of each plug, since the power is transported a very short distance from the coil to the plug.
Four-stroke 2-cylinder engines can be built without a distributor, as in the Citroen 2CV of 1948 and
BMW boxer twin motorcycles. Both spark plugs of the boxer twin are fired simultaneously, resulting in
a wasted spark on the cylinder currently on its exhaust stroke.
Four-stroke 4-cylinder engines can be built without a distributor, as in the Citroen ID19. Two coils are
used with one coil firing two of the spark plugs simultaneously, resulting in a wasted spark on the
cylinder currently on its exhaust stroke, and the other coil used for the other two cylinders.
Four-stroke one-cylinder engines can be built without a distributor, as in many lawn mowers. The spark
plug is fired on every stroke, resulting in a wasted spark in the cylinder when on its exhaust stroke.
See
also
Ignition system
these issues on the talk page.
2007)
standards. (October 2011)
For other uses, see Ignition system (disambiguation).
An ignition system is a system for igniting a fuel-air mixture. Ignition systems are well known in the
field of internal combustion engines such as those used in petrol (gasoline) engines used to power the
majority of motor vehicles, but they are also used in many other applications such as in oil-fired and
gas-fired boilers, rocket engines, etc.
The first ignition system to use an electric spark was probably Alessandro Volta's toy electric pistolfrom
the 1780s. Virtually all petrol engines today use an electric spark for ignition.
Diesel engines rely on fuel compression for ignition, but usually also have glowplugs that preheat
thecombustion chamber to allow starting of the engine in cold weather. Other engines may use a
flame, or a heated tube, for ignition.
Contents
[hide]
1 History
o
1.1 Magneto systems
o
1.2 Switchable systems
o
1.3 Battery-operated ignition
2 Modern ignition systems
o
2.1 Mechanically timed ignition
o
2.2 Electronic ignition
o
2.3 Digital electronic ignitions
3 Engine management
4 Turbine, jet, and rocket engines
6 References
History
Magneto
systems
Magneto ignition coil
The simplest form of spark ignition is that using a magnet. The engine spins a magnet inside a coil, or,
in the earlier designs, a coil inside a fixed magnet, and also operates acontact breaker, interrupting the
current and causing the voltage to be increased sufficiently to jump a small gap. The spark plugs are
connected directly from the magnetooutput. Early magnetos had one coil, with the contact breaker
(sparking plug) inside the combustion chamber. In about 1902, Bosch introduced a double-coil
magneto, with a fixed sparking plug, and the contact breaker outside the cylinder. Magnetos are not
used in modern cars, but because they generate their own electricity they are often found on pistonengined aircraft engines and small engines such as those found
in mopeds, lawnmowers, snowblowers, chainsaws, etc. where a battery-based electrical system is not
present for any combination of necessity, weight, cost, and reliability reasons.
Magnetos were used on the small engine's ancestor, the stationary "hit and miss" engine which was
used in the early twentieth century, on older gasoline or distillate farm tractors before battery starting
and lighting became common, and on aircraft piston engines. Magnetos were used in these engines
because their simplicity and self-contained operation was more reliable, and because magnetos
weighed less than having a battery and dynamo or alternator.
Aircraft engines usually have multiple magnetos to provide redundancy in the event of a failure. Some
older automobiles had both a magneto system and a battery actuated system (see below) running
simultaneously to ensure proper ignition under all conditions with the limited performance each system
provided at the time.This gave the benefits of easy starting (from the battery system) with reliable
sparking at speed (from the magneto).
Switchable
systems
Ford Model T ignition circuit
The output of a magneto depends on the speed of the engine, and therefore starting can be
problematic. Some magnetos include an impulse system, which spins the magnet quickly at the proper
moment, making easier starting at slow cranking speeds. Some engines, such as aircraft but also the
Ford Model T, used a system which relied on non rechargeable dry cells, (similar to a large flashlight
battery, and which was not maintained by acharging system as on modern automobiles) to start the
engine or for starting and running at low speed. The operator would manually switch the ignition over
to magneto operation for high speed operation.
To provide high voltage for the spark from the low voltage batteries, a 'tickler' was used, which was
essentially a larger version of the once widespread electric buzzer. With this apparatus, the direct
current passes through an electromagnetic coil which pulls open a pair of contact points, interrupting
the current; the magnetic field collapses, the spring-loaded points close again, the circuit is
reestablished, and the cycle repeats rapidly. The rapidly collapsing magnetic field, however, induces a
high voltage across the coil which can only relieve itself by arcing across the contact points; while in
the case of the buzzer this is a problem as it causes the points to oxidize and/or weld together, in the
case of the ignition system this becomes the source of the high voltage to operate the spark plugs.
In this mode of operation, the coil would "buzz" continuously, producing a constant train of sparks. The
entire apparatus was known as the 'Model T spark coil' (in contrast to the modern ignition coil which
isonly the actual coil component of the system). Long after the demise of the Model T as transportation
they remained a popular self-contained source of high voltage for electrical home experimenters,
appearing in articles in magazines such as Popular Mechanics and projects for school science fairs as
late as the early 1960s. In the UK these devices were commonly known as trembler coils and were
popular in cars pre-1910, and also in commercial vehicles with large engines until around 1925 to ease
starting.
The Model T (built into the flywheel) differed from modern implementations by not providing high
voltage directly at the output; the maximum voltage produced was about 30 volts, and therefore also
had to be run through the spark coil to provide high enough voltage for ignition, as described above,
although the coil would not "buzz" continuously in this case, only going through one cycle per spark. In
either case, the low voltage was switched to the appropriate spark plug by the 'timer' mounted on the
front of the engine. This performed the equivalent function to the modern distributor, although by
directing the low voltage, not the high voltage as for the distributor. The timing of the spark was
adjustable by rotating this mechanism through a lever mounted on the steering column. As the precise
timing of the spark depends on both the 'timer' and the trembler contacts within the coil, this is less
consistent than the breaker points of the later distributor. However for the low speed and the low
compression of such early engines, this imprecise timing was acceptable.
Battery-operated
ignition
With the universal adaptation of electrical starting for automobiles, and the concomitant availability of a
large battery to provide a constant source of electricity, magneto systems were abandoned for
systems which interrupted current at battery voltage, used an ignition coil (a transformer) to step the
voltage up to the needs of the ignition, and a distributor to route the ensuing pulse to the correct spark
plug at the correct time.
The first reliable battery operated ignition was developed by the Dayton Engineering Laboratories Co.
(Delco) and introduced in the 1910 Cadillac. This ignition was developed by Charles Kettering and was
a wonder in its day. It consisted of a single coil, points (the switch), a capacitor and a distributor set up
to allocate the spark from the ignition coil timed to the correct cylinder. The coil was basically a
transformer set up to step up the low (6 or 12 V) voltage supply to the high ignition voltage required to
jump a spark plug gap.
The points allow the coil to charge magnetically and then, when they are opened by
a camarrangement, the magnetic field collapses and a large (20 kV or greater) voltage is produced.
The capacitor is used to absorb the back EMF from the magnetic field in the coil to minimize point
contact burning and maximize point life. The Kettering system became the primary ignition system for
many years in the automotive industry due to its lower cost, higher reliability and relative simplicity. [1]
Modern
ignition systems
The ignition system is typically controlled by a key operated Ignition switch.
Mechanically
timed ignition
Distributor cap
Most four-stroke engines have used a mechanically timed electrical ignition system. The heart of the
system is the distributor. The distributor contains a rotating cam driven by the engine's drive, a set
of breaker points, a condenser, a rotor and a distributor cap. External to the distributor is theignition
coil, the spark plugs and wires linking the distributor to the spark plugs and ignition coil. (see diagram
Below)
The system is powered by a lead-acid battery, which is charged by the car's electrical system using
a dynamo oralternator. The engine operates contact breaker points, which interrupt the current to
an induction coil (known as the ignition coil).
The ignition coil consists of two transformer windings sharing a common magnetic core—the primary
and secondary windings. An alternating current in the primary induces alternating magnetic field in the
coil's core. Because the ignition coil's secondary has far more windings than the primary, the coil is a
step-up transformer which induces a much higher voltage across the secondary windings. For an
ignition coil, one end of windings of both the primary and secondary are connected together. This
common point is connected to the battery (usually through a current-limiting ballast resistor). The other
end of the primary is connected to the points within the distributor. The other end of the secondary is
connected, via the distributor cap and rotor, to the spark plugs.
Ignition Circuit Diagram - Mechanically Timed Ignition
The ignition firing sequence begins with the points (orcontact breaker) closed. A steady charge flows
from the battery, through the current-limiting resistor, through the coil primary, across the closed
breaker points and finally back to the battery. This steady current produces a magnetic field within the
coil's core. This magnetic field forms the energy reservoir that will be used to drive the ignition spark.
As the engine turns, so does the cam inside the distributor. The points ride on the cam so that as the
engine turns and reaches the top of the engine's compression cycle, a high point in the cam causes
the breaker points to open. This breaks the primary winding's circuit and abruptly stops the current
through the breaker points. Without the steady current through the points, the magnetic field generated
in the coil immediately and rapidly collapses. This change in the magnetic field induces a high voltage
in the coil's secondary windings.
At the same time, current exits the coil's primary winding and begins to charge up
the capacitor("condenser") that lies across the now-open breaker points. This capacitor and the coil’s
primary windings form an oscillating LC circuit. This LC circuit produces a damped, oscillating current
which bounces energy between the capacitor’s electric field and the ignition coil’s magnetic field. The
oscillating current in the coil’s primary, which produces an oscillating magnetic field in the coil, extends
the high voltage pulse at the output of the secondary windings. This high voltage thus continues
beyond the time of the initial field collapse pulse. The oscillation continues until the circuit’s energy is
consumed.
The ignition coil's secondary windings are connected to the distributor cap. A turning rotor, located on
top of the breaker cam within the distributor cap, sequentially connects the coil's secondary windings
to one of the several wires leading to each cylinder's spark plug. The extremely high voltage from the
coil's secondary -– often higher than 1000 volts—causes a spark to form across the gap of the spark
plug. This, in turn, ignites the compressed air-fuel mixture within the engine. It is the creation of this
spark which consumes the energy that was stored in the ignition coil’s magnetic field.
The flat twin cylinder 1948 Citroën 2CV used one double ended coil without a distributor, and just
contact breakers, in a wasted spark system.
High performance engines with eight or more cylinders that operate at high r.p.m. (such as those used
in motor racing) demand both a higher rate of spark and a higher spark energy than the simple ignition
circuit can provide. This problem is overcome by using either of these adaptations:

Two complete sets of coils, breakers and condensers can be provided - one set for each
half of the engine, which is typically arranged in V-8 or V-12 configuration. Although the two
ignition system halves are electrically independent, they typically share a single distributor which
in this case contains two breakers driven by the rotating cam, and a rotor with two isolated
conducting planes for the two high voltage inputs.

A single breaker driven by a cam and a return spring is limited in spark rate by the onset of
contact bounce or float at high rpm. This limit can be overcome by substituting for the breaker
a pair of breakers that are connected electrically in series but spaced on opposite sides of the
cam so they are driven out of phase. Each breaker then switches at half the rate of a single
breaker and the "dwell" time for current buildup in the coil is maximized since it is shared between
the breakers. The Lamborghini V-12 engine has both these adaptations and therefore uses two
ignition coils and a single distributor that contains 4 contact breakers.
A distributor-based system is not greatly different from a magneto system except that more separate
elements are involved. There are also advantages to this arrangement. For example, the position of
the contact breaker points relative to the engine angle can be changed a small amount dynamically,
allowing the ignition timing to be automatically advanced with increasing revolutions per minute (RPM)
or increased manifold vacuum, giving better efficiency and performance.
However it is necessary to check periodically the maximum opening gap of the breaker(s), using a
feeler gauge, since this mechanical adjustment affects the "dwell" time during which the coil charges,
and breakers should be re-dressed or replaced when they have become pitted by electric arcing. This
system was used almost universally until the late 1970s, when electronic ignition systems started to
appear.
Electronic
ignition
The disadvantage of the mechanical system is the use of breaker points to interrupt the low-voltage
high-current through the primary winding of the coil; the points are subject to mechanical wear where
they ride the cam to open and shut, as well as oxidation and burning at the contact surfaces from the
constant sparking. They require regular adjustment to compensate for wear, and the opening of the
contact breakers, which is responsible for spark timing, is subject to mechanical variations.
In addition, the spark voltage is also dependent on contact effectiveness, and poor sparking can lead
to lower engine efficiency. A mechanical contact breaker system cannot control an average ignition
current of more than about 3 A while still giving a reasonable service life, and this may limit the power
of the spark and ultimate engine speed.
Example of a basic electronic ignition system
Electronic ignition (EI) solves these problems. In the initial systems, points were still used but they
handled only a low current which was used to control the high primary current through a solid state
switching system. Soon, however, even these contact breaker points were replaced by
an angularsensor of some kind - either optical, where a vaned rotor breaks a light beam, or more
commonly using a Hall effect sensor, which responds to a rotating magnet mounted on the distributor
shaft. The sensor output is shaped and processed by suitable circuitry, then used to trigger a switching
device such as athyristor, which switches a large current through the coil.
The first electronic ignition (a cold cathode type) was tested in 1948 by Delco-Remy,
[2]
while Lucasintroduced a transistorized ignition in 1955, which was used on BRM and Coventry
Climax Formula One engines in 1962.[2] The aftermarket began offering EI that year, with both
the AutoLite Electric Transistor 201 and Tung-Sol EI-4 being available.[3] Pontiac became the first
automaker to offer an optional EI, the breakerless magnetic pulse-triggered Delcotronic, on some 1963
models; it was also available on some Corvettes.[3] Ford fitted a Lucas system on the Lotus
25s entered at Indianapolisthe next year, ran a fleet test in 1964, and began offering optional EI on
some models in 1965.[3]Beginning in 1958, Earl W. Meyer at Chrysler worked on EI, continuing until
1961 and resulting in use of EI on the company's NASCAR hemis in 1963 and 1964.[3]
Prest-O-Lite's CD-65, which relied on capacitance discharge (CD), appeared in 1965, and had "an
unprecedented 50,000 mile warranty."[3] (This differs from the non-CD Prest-O-Lite system introduced
on AMC products in 1972, and made standard equipment for the 1975 model year.) [3] A similar CD unit
was available from Delco in 1966,[4] which was optional on Oldsmobile, Pontiac, and GMC vehicles in
the 1967 model year.[3] Also in 1967, Motorola debuted their breakerless CD system.[3]
FIAT became the first company to offer standard EI, in 1968, followed by Chrysler (after a 1971 trial) in
1973 and by Ford and GM in 1975.[3]
In 1967, Prest-O-Lite made a "Black Box" ignition amplifier, intended to take the load off of the
distributor's breaker points during high r.p.m. runs, which was used by Dodge and Plymouth on their
factory Super Stock Coronet and Belvedere and drag racers.[3] This amplifier was installed on the
interior side of the cars' firewall, and had a duct which provided outside air to cool the unit. [citation
needed]
The rest of the system (distributor and spark plugs) remains as for the mechanical system. The
lack of moving parts compared with the mechanical system leads to greater reliability and longer
service intervals.
Chrysler introduced breakerless ignition in mid-1971 as an option for its 340 V8 and the 426 Street
Hemi. For the 1972 model year, the system became standard on its high-performance engines (the
340 cu in (5.6 l) and the four-barrel carburetor-equipped 400 hp (298 kW) 400 cu in (7 l)) and was an
option on its 318 cu in (5.2 l), 360 cu in (5.9 l), two-barrel 400 cu in (6.6 l), and low-performance
440 cu in (7.2 l) . Breakerless Ignition was standardised across the model range for 1973.
For older cars, it is usually possible to retrofit an EI system in place of the mechanical one. In some
cases, a modern distributor will fit into the older engine with no other modifications, like
the H.E.I.distributor made by General Motors, the Hot-Spark electronic ignition conversion kit and the
aforementioned Chrysler-built electronic ignition system.
Other innovations are currently available on various cars. In some models, rather than one central coil,
there are individual coils on each spark plug, sometimes known as direct ignition or coil on plug (COP).
This allows the coil a longer time to accumulate a charge between sparks, and therefore a higher
energy spark. A variation on this has each coil handle two plugs, on cylinders which are 360 degrees
out of phase (and therefore reach TDC at the same time); in the four-cycle engine this means that one
plug will be sparking during the end of the exhaust stroke while the other fires at the usual time, a socalled "wasted spark" arrangement which has no drawbacks apart from faster spark plug erosion; the
paired cylinders are 1/4 and 2/3. Other systems do away with the distributor as a timing apparatus and
use a magnetic crank angle sensor mounted on the crankshaft to trigger the ignition at the proper time.
Digital
electronic ignitions
At the turn of the 21st century digital electronic ignition modules became available for small engines on
such applications as chainsaws, string trimmers, leaf blowers, and lawn mowers. This was made
possible by low cost, high speed, and small footprint microcontrollers. Digital electronic ignition
modules can be designed as either capacitor discharge ignition (CDI) or inductive discharge
ignition(IDI) systems. Capacitive discharge digital ignitions store charged energy for the spark in a
capacitor within the module that can be released to the spark plug at virtually any time throughout the
engine cycle via a control signal from the microprocessor. This allows for greater timing flexibility, and
engine performance; especially when designed hand-in-hand with the engine carburetor.
Engine
management
In an Engine Management System (EMS), electronics control fuel delivery and ignition timing. Primary
sensors on the system are crankshaft angle (crankshaft or Top Dead Center (TDC) position), airflow
into the engine and throttle position. The circuitry determines which cylinder needs fuel and how much,
opens the requisite injector to deliver it, then causes a spark at the right moment to burn it. Early EMS
systems used an analogue computer to accomplish this, but as embedded systems dropped in price
and became fast enough to keep up with the changing inputs at high revolutions, digital systems
started to appear.
Some designs using an EMS retain the original ignition coil, distributor and high-tension leads found
on cars throughout history. Other systems dispense with the distributor altogether and have individual
coils mounted directly atop each spark plug. This removes the need for both distributor and hightension leads, which reduces maintenance and increases long-term reliability.
Modern EMSs read in data from various sensors about the crankshaft position, intake manifold
temperature, intake manifold pressure (or intake air volume), throttle position, fuel mixture via the
oxygen sensor, detonation via a knock sensor, and exhaust gas temperature sensors. The EMS then
uses the collected data to precisely determine how much fuel to deliver and when and how far to
advance the ignition timing. With electronic ignition systems, individual cylinders [citation needed] can have
their own individual timing so that timing can be as aggressive as possible per cylinder without fuel
detonation. As a result, sophisticated electronic ignition systems can be both more fuel efficient, and
produce better performance over their counterparts.
Turbine,
jet, and rocket engines
Gas turbine engines, including jet engines, have a capacitor discharge ignition system using one or
more ignitor plugs, which are only used at startup or in case the combustor(s) flame goes out.
Rocket engine ignition systems are especially critical. If prompt ignition does not occur, thecombustion
chamber can fill with excess fuel and oxidiser and significant overpressure can occur (a "hard start") or
even an explosion. Rockets often employ pyrotechnic devices that place flames across the face of
the injector plate, or, alternatively, hypergolic propellants that ignite spontaneously on contact. Such
engines do away with ignition systems entirely and cannot experience hard starts, but the propellants
are highly toxic and corrosive.
See
also
Wikibooks has a book on the
plugs for racing

Saab Direct Ignition

Spark-ignition

Charles Kettering Inventor of battery ignition system

Electromagnetism


Ignition coil

Inductor

Induction coil

Magnetic field
Ignition coil
Bosch ignition coil.
Dual ignition coils (blue cylinders, top of picture) on a Saab 92.
Citroën 2CV wasted spark ignition system
An ignition coil (also called a spark coil) is an induction coilin an automobile's ignition
system which transforms thebattery's low voltage to the thousands of volts needed to create an electric
spark in the spark plugs to ignite the fuel. Some coils have an internal resistor while others rely on a
resistor wire or an external resistor to limit the current flowing into the coil from the car's 12 volt supply.
The wire which goes from the ignition coil to the distributor and the wires which go from the distributor
to each of the spark plugs are called spark plug wires or high tension leads.
Originally, every ignition coil system required mechanicalcontact breaker points, and
a capacitor (condensor). More recent electronic ignition systems use a power transistor to provide
pulses to the ignition coil. A modern passenger automobile may use one ignition coil for each engine
cylinder (or pair of cylinders), eliminating a distributor to route the high voltage pulses.
Ignition systems are not required for Diesel engines which rely on compression to ignite the fuel/air
mixture.
Contents
[hide]
1 Basic principles
2 Construction
3 Use in cars
o
3.1 Modern ignition systems
4 Related coils
6 Patents
7 References
Basic
principles
An ignition coil consists of a laminated iron core surrounded by two coils of copper wire. Unlike a
powertransformer, an ignition coil has an open magnetic circuit - the iron core does not form a closed
loop around the windings. The energy that is stored in the magnetic field of the core is the energy that
is transferred to the spark plug.
The primary winding has relatively few turns of heavy wire. The secondary winding consists of
thousands of turns of smaller wire, insulated for the high voltage by enamel on the wires and layers of
oiled paper insulation. The coil is usually inserted into a metal can or plastic case with insulated
terminals for the high voltage and low voltage connections. When the contact breaker closes, it allows
a current from the battery to build up in the primary winding of the ignition coil. The current does not
flow instantly because of the inductance of the coil. Current flowing in the coil produces a magnetic
field in the core and in the air surrounding the core. The current must flow long enough to store
enough energy in the field for the spark. Once the current has built up to its full level, the contact
breaker opens. Since it has a capacitor connected across it, the primary winding and the capacitor
form a tuned circuit, and as the stored energy oscillates between the inductor formed by the coil and
the capacitor, the changing magnetic field in the core of the coil induces a much larger voltage in the
secondary of the coil. More modern electronic ignition systems operate on exactly the same principle,
but some rely on charging the capacitor to around 400 volts rather than charging the inductance of the
coil. The timing of the opening of the contacts (or switching of the transistor) must be matched to the
position of the piston in the cylinder. The spark must occur after the air/fuel mixture is compressed.
The contacts are driven off a shaft that is driven by the engine crankshaft, or, if electronic ignition is
used, a sensor on the engine shaft controls the timing of the pulses.
The amount of energy in the spark required to ignite the air-fuel mixture varies depending on the
pressure and composition of the mixture, and on the speed of the engine. Under laboratory conditions
as little as 1 millijoule is required in each spark, but practical coils must deliver much more energy than
this to allow for higher pressure, rich or lean mixtures, losses in ignition wiring, and plug fouling and
leakage. When gas velocity is high in the spark gap, the arc between the terminals is blown away from
the terminals, making the arc longer and requiring more energy in each spark. Between 30 and 70
millijoules are delivered in each spark.
Construction
Formerly, ignition coils were made with varnish and paper insulated high-voltage windings, inserted
into a drawn-steel can and filled with oil or asphalt for insulation and moisture protection. Coils on
modern automobiles are cast in filled epoxy resins which penetrate any voids within the winding.
A single-spark system has one coil per spark plug. To prevent premature sparking at the start of the
primary pulse, a diode or secondary spark gap is installed in the coil to block the reverse pulse that
would otherwise form.
In a coil meant for a dual-spark system, the secondary winding has two terminals isolated from the
primary, and each terminal connects to a spark plug. With this system, no extra diode is needed since
there would be no fuel/air mixture present at the inactive spark plug.
[1]
In a low-inductance coil, fewer primary turns are used, so primary current is higher. This is not
compatible with the capacity of mechanical breaker points, so solid-state switching is used.
Use
in cars
Very early gasoline (petrol) internal combustion engines used a magneto ignition system, since no
battery was fitted to the vehicle; magnetoes are still used in piston-engine aircraft. The voltage
produced by a magneto is dependent on the speed of the engine, making starting difficult. A batteryoperated coil can provide a high-voltage spark even at low speeds, making starting easier.
[2]
When
batteries became common in automobiles for cranking and lighting, the ignition coil system displaced
magneto ignition.
In older vehicles a single (large) coil would serve all the spark plugs via the ignition distributor. Notable
exceptions are the Saab 92, Some Volkswagens, and the Wartburg 353 which have one ignition coil
per cylinder. The flat twin cylinder 1948 Citroën 2CV used one double ended coil without a distributor,
and just contact breakers, in a wasted spark system.
Modern
ignition systems
Transmission (mechanics)
(Redirected from Gearbox)
"Gearbox" redirects here. For the video game developer, see Gearbox Software.
5-speed gearbox + reverse, the 1600 Volkswagen Golf (2009).
A machine consists of a power source and a power transmission system, which provides controlled
application of the power. Merriam-Webster defines transmission as an assembly of parts including the
speed-changing gears and the propeller shaft by which the power is transmitted from an engine to a
live axle.[1] Often transmissionrefers simply to the gearbox that uses gearsand gear trains to
provide speed and torqueconversions from a rotating power source to another device. [2][3]
In British English, the term transmission refers to the whole drive train, including gearbox, clutch, prop
shaft (for rear-wheel drive), differential and final drive shafts. In American English, however, the
distinction is made that a gearbox is any device which converts speed and torque, whereas a
transmission is a type of gearbox that can be "shifted" to dynamically change the speed-torque ratio
such as in a vehicle.
The most common use is in motor vehicles, where the transmission adapts the output of the internal
combustion engine to the drive wheels. Such engines need to operate at a relatively high rotational
speed, which is inappropriate for starting, stopping, and slower travel. The transmission reduces the
higher engine speed to the slower wheel speed, increasing torque in the process. Transmissions are
also used on pedal bicycles, fixed machines, and anywhere else where rotational speed and torque
Often, a transmission will have multiple gear ratios (or simply "gears"), with the ability to switch
between them as speed varies. This switching may be done manually (by the operator), or
automatically. Directional (forward and reverse) control may also be provided. Single-ratio
transmissions also exist, which simply change the speed and torque (and sometimes direction) of
motor output.
In motor vehicles, the transmission will generally be connected to the crankshaft of the engine. The
output of the transmission is transmitted via driveshaft to one or more differentials, which in turn, drive
the wheels. While a differential may also provide gear reduction, its primary purpose is to permit the
wheels at either end of an axle to rotate at different speeds (essential to avoid wheel slippage on
turns) as it changes the direction of rotation.
Conventional gear/belt transmissions are not the only mechanism for speed/torque adaptation.
Alternative mechanisms include torque converters and power transformation (for example, dieselelectric transmission and hydraulic drive system). Hybrid configurations also exist.
Contents
[hide]
1 Explanation
2 Uses
3 Simple
4 Multi-ratio systems
o
4.1 Automotive basics
o
4.2 Manual
o
4.3 Non-synchronous
o
4.4 Automatic
o
4.5 Semi-automatic
o
4.6 Bicycle gearing
5 Uncommon types
o
5.1 Dual clutch transmission
o
5.2 Continuously variable
o
5.3 Infinitely variable
o
5.4 Electric variable
6 Non-direct
o
6.1 Electric
o
6.2 Hydrostatic
o
6.3 Hydrodynamic
8 References
Explanation
Transmission types
Manual

Sequential manual

Non-synchronous

Preselector
Automatic
Manumatic

Semi-automatic

Electrohydraulic

Dual clutch

Saxomat
Continuously variable
Bicycle gearing

Derailleur gears

Hub gears

V

T

E
Interior view of Pantigo Windmill, looking up into cap from floor -- cap rack, brake wheel, brake and wallower.
Pantigo Windmill is located on James Lane, East Hampton, Suffolk County, Long Island, New York.
Early transmissions included the right-angle drives and other gearing in windmills, horse-powered
devices, and steam engines, in support of pumping,milling, and hoisting.
Most modern gearboxes are used to increase torque while reducing the speed of a prime mover output
shaft (e.g. a motor crankshaft). This means that the output shaft of a gearbox will rotate at a slower
rate than the input shaft, and this reduction in speed will produce amechanical advantage, causing an
increase in torque. A gearbox can be set up to do the opposite and provide an increase in shaft speed
with a reduction of torque. Some of the simplest gearboxes merely change the physical direction in
which power is transmitted.
Many typical automobile transmissions include the ability to select one of several different gear ratios.
In this case, most of the gear ratios (often simply called "gears") are used to slow down the output
speed of the engine and increase torque. However, the highest gears may be "overdrive" types that
increase the output speed.
Uses
Gearboxes have found use in a wide variety of different—often stationary—applications, such as wind
turbines.
Transmissions are also used in agricultural, industrial, construction, mining and automotive equipment.
In addition to ordinary transmission equipped with gears, such equipment makes extensive use of the
hydrostatic drive and electrical adjustable-speed drives.
Simple
The main gearbox and rotor of a Bristol Sycamore helicopter
The simplest transmissions, often called gearboxes to reflect their simplicity (although complex
systems are also called gearboxes in the vernacular), provide gear reduction (or, more rarely, an
increase in speed), sometimes in conjunction with a right-angle change in direction of the shaft
(typically in helicopters, see picture). These are often used on PTO-powered agricultural equipment,
since the axial PTO shaft is at odds with the usual need for the driven shaft, which is either vertical (as
with rotary mowers), or horizontally extending from one side of the implement to another (as
with manure spreaders, flail mowers, and forage wagons). More complex equipment, such
as silagechoppers and snowblowers, have drives with outputs in more than one direction.
The gearbox in a wind turbine converts the slow, high-torque rotation of the turbine into much faster
rotation of the electrical generator. These are much larger and more complicated than the PTO
gearboxes in farm equipment. They weigh several tons and typically contain three stages to achieve
an overall gear ratio from 40:1 to over 100:1, depending on the size of the turbine.
(For aerodynamic and structural reasons, larger turbines have to turn more slowly, but the generators
all have to rotate at similar speeds of several thousand rpm.) The first stage of the gearbox is usually a
planetary gear, for compactness, and to distribute the enormous torque of the turbine over more teeth
of the low-speed shaft.[4] Durability of these gearboxes has been a serious problem for a long time. [5]
Regardless of where they are used, these simple transmissions all share an important feature: thegear
ratio cannot be changed during use. It is fixed at the time the transmission is constructed.
For transmission types that overcome this issue, see Continuously Variable Transmission, also known
as CVT.
Multi-ratio
systems
Tractor transmission with 16 forward and 8 backward gears
Amphicar gearbox cutaway w/optional shift for water going propellers
Many applications require the availability of multiple gear ratios. Often, this is to ease the starting and
stopping of a mechanical system, though another important need is that of maintaining good fuel
efficiency.
Automotive
basics
The need for a transmission in an automobile is a consequence of the characteristics of the internal
combustion engine. Engines typically operate over a range of 600 to about 7000 revolutions per
minute (though this varies, and is typically less for diesel engines), while the car's wheels rotate
between 0 rpm and around 1800 rpm.
Furthermore, the engine provides its highest torque and power outputs unevenly across the rev range
resulting in atorque band and a power band. Often the greatest torque is required when the vehicle is
moving from rest or traveling slowly, while maximum power is needed at high speed. Therefore, a
system that transforms the engine's output so that it can supply high torque at low speeds, but also
operate at highway speeds with the motor still operating within its limits, is required. Transmissions
perform this transformation.
A diagram comparing the power and torque bands of a "torquey" engine versus a "peaky" one
The dynamics of a car vary with speed: at low speeds, acceleration is limited by the inertia of vehicular
gross mass; while at cruising or maximum speeds wind resistance is the dominant barrier.
Many transmissions and gears used inautomotive and truck applications are contained in a cast
iron case, though more frequently aluminium is used for lower weight especially in cars. There are
usually three shafts: a mainshaft, a countershaft, and an idler shaft.
The mainshaft extends outside the case in both directions: the input shaft towards the engine, and the
output shaft towards the rear axle (on rear wheel drive cars- front wheel drives generally have the
engine and transmission mounted transversely, the differential being part of the transmission
assembly.) The shaft is suspended by the main bearings, and is split towards the input end. At the
point of the split, a pilot bearing holds the shafts together. The gears and clutches ride on the
mainshaft, the gears being free to turn relative to the mainshaft except when engaged by the clutches.
Types of automobile transmissions include manual, automatic or semi-automatic transmission.
Manual
Main article: Manual transmission
Manual transmission come in two basic types:

a simple but rugged sliding-mesh or unsynchronized / non-synchronous system, where
straight-cut spur gear sets are spinning freely, and must be synchronized by the operator
matching engine revs to road speed, to avoid noisy and damaging "gear clash",

and the now common constant-mesh gearboxes which can include non-synchronised,
orsynchronized / synchromesh systems, where typically diagonal cut helical (or sometimes either
straight-cut, or double-helical) gear sets are constantly "meshed" together, and a dog clutch is
used for changing gears. On synchromesh boxes, friction cones or "synchro-rings" are used in
addition to the dog clutch to closely match the rotational speeds of the two sides of the
(declutched) transmission before making a full mechanical engagement.
The former type was standard in many vintage cars (alongside e.g. epicyclic and multi-clutch systems)
before the development of constant-mesh manuals and hydraulic-epicyclic automatics, older heavyduty trucks, and can still be found in use in some agricultural equipment. The latter is the modern
standard for on- and off-road transport manual and semi-automatic transmission, although it may be
found in many forms; e.g., non-synchronised straight-cut in racetrack or super-heavy-duty applications,
non-synchro helical in the majority of heavy trucks and motorcycles and in certain classic cars (e.g. the
Fiat 500), and partly or fully synchronised helical in almost all modern manual-shift passenger cars and
light trucks.
Manual transmissions are the most common type outside North America and Australia. They are
cheaper, lighter, usually give better performance, and fuel efficiency (although automatic transmissions
with torque converter lockup and advanced electronic controls can provide similar results). It is
customary for new drivers to learn, and be tested, on a car with a manual gear change.
In Malaysiaand Denmark all cars used for testing (and because of that, virtually all those used for
instruction as well) have a manual transmission. In Japan, the
Philippines, Germany, Poland, Italy, Israel, theNetherlands, Belgium, New Zealand, Austria, Bulgaria,
the UK,[6][7] Ireland,[7] Sweden, Norway,Estonia, France, Spain, Switzerland, the Australian states
of Victoria, Western Australia and Queensland, Finland and Lithuania, a test pass using an automatic
car does not entitle the driver to use a manual car on the public road; a test with a manual car is
required.[citation needed] Manual transmissions are much more common than automatic transmissions
in Asia, Africa, South Americaand Europe.
Manual transmissions can include both synchronized and unsynchronized gearing. For example,
reverse gear is usually unsynchronised, as the drive is only expected to engage it when the vehicle is
at a standstill. Many older (up to 1970s) cars also lacked syncro on first gear (for various reasons cost, typically "shorter" overall gearing, engines typically having more low-end torque, the extreme
wear which would be placed on a frequently used 1st gear synchroniser...), meaning it also could only
be used for moving away from a stop unless the driver became adept at double-declutching and had a
particular need to regularly downshift into the lowest gear.
Some manual transmissions have an extremely low ratio for first gear, which is referred to as a
"creeper gear" or "granny gear". Such gears are usually not synchronized. This feature is common on
pickup trucks tailored to trailer-towing, farming, or construction-site work. During normal on-road use,
the truck is usually driven without using the creeper gear at all, and second gear is used from a
standing start. Some off-road vehicles, most particularly the Willys Jeep and its descendents, also had
transmissions with "granny first"s either as standard or an option, but this function is now more often
provided for by a low-range transfer gearbox attached to a normal fully synchronised transmission.
Non-synchronous
Main article: Non-synchronous transmissions
There are commercial applications engineered with designs taking into account that the gear shifting
will be done by an experienced operator. They are a manual transmission, but are known as nonsynchronized transmissions. Dependent on country of operation, many local, regional, and national
laws govern the operation of these types of vehicles (see Commercial Driver's License). This class
may include commercial, military, agricultural, or engineering vehicles. Some of these may use
combinations of types for multi-purpose functions. An example would be a power take-off (PTO) gear.
The non-synchronous transmission type requires an understanding of gear range, torque, engine
power, and multi-functional clutch and shifter functions. Also see Double-clutching, and Clutchbrakesections of the main article.
Automatic
Main article: Automatic transmission
Epicyclic gearing or planetary gearing as used in an automatic transmission.
Most modern North American and Australian and some European and Japanese cars have
an automatic transmission that will select an appropriate gear ratio without any operator intervention.
They primarily usehydraulics to select gears, depending on pressure exerted by fluid within the
transmission assembly. Rather than using a clutch to engage the transmission, a fluid flywheel,
or torque converter is placed in between the engine and transmission. It is possible for the driver to
control the number of gears in use or select reverse, though precise control of which gear is in use
may or may not be possible.
Automatic transmissions are easy to use. However, in the past, automatic transmissions of this type
have had a number of problems; they were complex and expensive, sometimes had reliability
problems (which sometimes caused more expenses in repair), have often been less fuel-efficient than
their manual counterparts (due to "slippage" in the torque converter), and their shift time was slower
than a manual making them uncompetitive for racing. With the advancement of modern automatic
transmissions this has changed.[citation needed]
Attempts to improve the fuel efficiency of automatic transmissions include the use of torque
converterswhich lock up beyond a certain speed, or in the higher gear ratios, eliminating power loss,
and overdrive gears which automatically actuate above certain speeds; in older transmissions both
technologies could sometimes become intrusive, when conditions are such that they repeatedly cut in
and out as speed and such load factors as grade or wind vary slightly. Current computerized
transmissions possess very complex programming to both maximize fuel efficiency and eliminate any
intrusiveness, and we are at a point in technological advancement where automatics are beginning to
outperform manuals in both performance and efficiency. [citation needed]. This is due mainly to electronic
advances rather than mechanical ones although improvements in CVT technology and the use of
automatic clutches have also helped. The 2012 model of the Honda Jazz sold in the UK actually
claims marginally better fuel consumption for the CVT version than the manual version.
For certain applications, the slippage inherent in automatic transmissions can be advantageous; for
instance, in drag racing, the automatic transmission allows the car to be stopped with the engine at a
high rpm (the "stall speed") to allow for a very quick launch when the brakes are released; in fact, a
common modification is to increase the stall speed of the transmission. This is even more
advantageous for turbocharged engines, where the turbocharger needs to be kept spinning at high
rpm by a large flow of exhaust in order to keep the boost pressure up and eliminate the turbo lag that
occurs when the engine is idling and the throttle is suddenly opened.
Semi-automatic
Main article: Semi-automatic transmission
A hybrid form of transmission where the an integrated control system handles manipulation of
theclutch automatically, but the driver can still - and may be required to - take manual control of gear
selection. This is sometimes called a "clutchless manual," or "automated manual" transmission. Many
of these transmissions allow the driver to fully delegate gear shifting choice to the control system,
which then effectively acts as if it was a regular automatic transmission. They are generally designed
using manual transmission "internals", and when used in passenger cars, have synchromesh operated
helical constant mesh gear sets.
Early semi-automatic systems used a variety of mechanical and hydraulic systems - including
centrifugal clutches, torque converters, electro-mechanical (and even electrostatic) and servo/solenoid
controlled clutches - and control schemes - automatic declutching when moving the gearstick, preselector controls, centrifugal clutches with drum-sequential shift requiring the driver to lift the throttle for
a successful shift, etc. - and some were little more than regular lock-up torque converter automatics
with manual gear selection.
Most modern implementations, however, tend to be standard or slightly modified manual transmissions
(and very occasionally modified automatics, even including a few cases of CVTs with "fake" fixed gear
ratios), with servo-controlled clutching and shifting under command of the central engine computer.
These are intended to be a combined replacement option both for more expensive and less efficient
"normal" automatic systems, and for drivers who prefer manual shift but are no longer able to operate
a clutch, and users are encouraged to leave the shift lever in fully automatic "Drive" most of the time,
only engaging manual-sequential mode for sporty driving or when otherwise strictly necessary.
Specific types of this transmission include: Easytronic, Tiptronic and Geartronic, as well as the
systems used as standard in all ICE-powered Smart-MCC vehicles, and on geared step-through
scooters such as the Honda Cub or Suzuki Address.
A dual-clutch transmission uses two sets of internals which are alternately used, each with its own
clutch, so that a "gearchange" actually only consists of one clutch engaging as the other disengages,
making for a supposedly "seamless" shift with no break in (or jarring reuptake of) power transmission.
Each clutch's attached shaft carries half of the total input gear complement (with a shared output
shaft), including synchronised dog clutch systems that pre-select which of its set of ratios is most likely
to be needed at the next shift, under command of a computerised control system.
Specific types of this transmission include: Direct-Shift Gearbox.
There are also sequential transmissions which use the rotation of a drum to switch gears, much like
those of a typical fully manual motorcycle.[8] These can be designed with a manual or automatic clutch
system, and may be found both in automobiles (particularly track and rally racing cars), motorcycles
(typically light "step-thru" type city utility bikes, e.g. the Honda Cub) and quadbikes (often with a
separately engaged reversing gear), the latter two normally using a scooter-style centrifugal clutch.
Bicycle
gearing
Shimano XT rear derailleur on amountain bike
Main articles: Bicycle gearing, Derailleur gears, and Hub gear
Bicycles usually have a system for selecting different gear ratios. There are two main types: derailleur
gears and hub gears. The derailleur type is the most common, and the most visible,
usingsprocket gears. Typically there are several gears available on the rear sprocket assembly,
attached to the rear wheel. A few more sprockets are usually added to the front assembly as well.
Multiplying the number of sprocket gears in front by the number to the rear gives the number of gear
ratios, often called "speeds".
Hub gears use epicyclic gearing and are enclosed within the axle of the rear wheel. Because of the
small space, they typically offer fewer different speeds, although at least one has reached 14 gear
ratios and Fallbrook Technologies manufactures a transmission with technically infinite ratios.[9]
Causes for failure of bicycle gearing include: worn teeth, damage caused by a faulty chain, damage
due to thermal expansion, broken teeth due to excessive pedaling force, interference by foreign
objects, and loss of lubrication due to negligence.
Uncommon
Dual
types
clutch transmission
Main article: Dual clutch transmission
This arrangement is also sometimes known as a direct shift gearbox or powershift gearbox. It seeks to
combine the advantages of a conventional manual shift with the qualities of a modern automatic
transmission by providing different clutches for odd and even speed selector gears. When changing
gear, the engine torque is transferred from one gear to the other continuously, so providing gentle,
smooth gear changes without either losing power or jerking the vehicle. Gear selection may be
manual, automatic (depending on throttle/speed sensors), or a 'sports' version combining both options.
Continuously
variable
Main article: Continuously variable transmission
The Continuously Variable Transmission (CVT) is a transmission in which the ratio of the rotational
speeds of two shafts, as the input shaft and output shaft of a vehicle or other machine, can be varied
continuously within a given range, providing an infinite number of possible ratios. The CVT allows the
relationship between the speed of the engine and the speed of the wheels to be selected within a
continuous range. This can provide even better fuel economy if the engine is constantly running at a
single speed. The transmission is in theory capable of a better user experience, without the rise and
fall in speed of an engine, and the jerk felt when poorly changing gears.
CVTs are increasingly found on small cars, and especially high-gas-milage or hybrids vehicles. On
these platforms the torque is limited because the electric motor can provide torque without changing
the speed of the engine. By leaving the engine running at the rate that generates the best gas milage
for the given operating conditions, overall milage can be improved over a system with a smaller
number of fixed gears, where the system may be operating at peak efficiency only for a small range of
speeds. CVTs are rare on other platforms, especially high-torque applications, as they are generally
constructed using rubber belts or similar devices that are subject to slippage at high torque.
Infinitely
variable
The IVT is a specific type of CVT that includes not only an infinite number of gear ratios, but an
infiniterange as well. This is a turn of phrase, it actually refers to CVTs that are able to include a "zero
ratio", where the input shaft can turn without any motion of the output shaft while remaining in gear.
Zero output implies infinite ratios, as any "high gear" ratio is an infinite number of times higher than the
zero "low gear".
Most (if not all) IVTs result from the combination of a CVT with an epicyclic gear system with a fixed
ratio. The combination of the fixed ratio of the epicyclic gear with a specific matching ratio in the CVT
side results in zero output. For instance, consider a transmission with an epicyclic gear set to 1:-1 gear
ratio; a 1:1 reverse gear. When the CVT side is set to 1:1 the two ratios add up to zero output. The IVT
is always engaged, even during its zero output. When the CVT is set to higher values it operates
conventionally, with increasing forward ratios.
In practice, the epicyclic gear may be set to the lowest possible ratio of the CVT, if reversing is not
needed or is handled through other means. Reversing can be incorporated by setting the epicyclic
gear ratio somewhat higher than the lowest ratio of the CVT, providing a range of reverse ratios.
Electric
variable
The Electric Variable Transmission (EVT) combines a transmission with an electric motor to provide
the illusion of a single CVT. In the common implementation, a gasoline engine is connected to a
traditional transmission, which is in turn connected to an epicyclic gear system's planet carrier. An
electric motor/generator is connected to the central "sun" gear, which is normally un-driven in typical
epicyclic systems. Both sources of power can be fed into the transmission's output at the same time,
splitting power between them. In common examples, between ¼ and ½ of the engine's power can be
fed into the sun gear. Depending on the implementation, the transmission in front of the epicyclic
system may be greatly simplified, or eliminated completely. EVTs are capable of continuously
modulating output/input speed ratios like mechanical CVTs, but offer the distinct benefit of being able
to also apply power from two different sources to one output, as well as potentially reducing overall
complexity dramatically.
In typical implementations, the gear ratio of the transmission and epicyclic system are set to the ratio
of the common driving conditions, say highway speed for a car, or city speeds for a bus. When the
drivers presses on the gas, the associated electronics interprets the pedal position and immediately
sets the gasoline engine to the RPM that provides the best gas milage for that setting. As the gear
ratio is normally set far from the maximum torque point, this set-up would normally result in very poor
acceleration. Unlike gasoline engines, electric motors offer efficient torque across a wide selection of
RPM, and are especially effective at low settings where the gasoline engine is inefficient. By varying
the electrical load or supply on the motor attached to the sun gear, additional torque can be provided
to make up for the low torque output from the engine. As the vehicle accelerates, the power to the
motor is reduced and eventually ended, providing the illusion of a CVT.
The canonical example of the EVT is Toyota's Hybrid Synergy Drive. This implementation has no
conventional transmission, and the sun gear always receives 28% of the torque from the engine. This
power can be used to operate any electrical loads in the vehicle, recharging the batteries, powering
the entertainment system, or running the air conditioning. Any residual power is then fed back into a
second motor that powers the output of the drivetrain directly. At highway speeds this additional
generator/motor pathway is less efficient than simply powering the wheels directly. However, during
acceleration, the electrical path is much more efficient than engine operating so far from its torque
point.[10] GM uses a similar system in the Allison Bus hybrid powertrains and the Tahoe and Yukon
pick-up trucks, but these use a two-speed transmission in front of the epicyclic system, and the sun
gear receives close to half the total power.
Non-direct
Electric
Electric transmissions convert the mechanical power of the engine(s) to electricity with electric
generators and convert it back to mechanical power with electric motors. Electrical or
electronicadjustable-speed drive control systems are used to control the speed and torque of the
motors. If the generators are driven by turbines, such arrangements are called turbo-electric. Likewise
Diesel-electric arrangements are used on many railway locomotives, ships, large mining trucks, and
some bulldozers. In these cases, each driven wheel is equipped with its own electric motor, which can
be fed varying electrical power to provide any required torque or power output for each wheel
independently. This produces a much simpler solution for multiple driven wheels in very large vehicles,
where drive shafts would be much larger or heavier than the electrical cable that can provide the same
amount of power. It also improves the ability to allow different wheels to run at different speeds, which
is useful for steered wheels in large construction vehicles.
Hydrostatic
Hydrostatic transmissions transmit all power hydraulically, using the components of hydraulic
machinery. They are similar to electrical transmissions, but hydraulic fluid as the power
distribution system rather than electricity.
The transmission input drive is a central hydraulic pump and final drive unit(s) is/are a hydraulic
motor, or hydraulic cylinder (see: swashplate). Both components can be placed physically far
apart on the machine, being connected only by flexible hoses. Hydrostatic drive systems are used
on excavators, lawn tractors, forklifts, winch drive systems, heavy lift equipment, agricultural
machinery, earth-moving equipment, etc. An arrangement for motor-vehicle transmission was
probably used on the FergusonF-1 P99 racing car in about 1961.
The Human Friendly Transmission of the Honda DN-01 is hydrostatic.
Hydrodynamic
If the hydraulic pump and/or hydraulic motor make use of the hydrodynamic effects of the fluid
flow, i.e. pressure due to a change in the fluid's momentum as it flows through vanes in a turbine.
The pump and motor usually consist of rotating vanes without seals and are typically placed in
close proximity. The transmission ratio can be made to vary by means of additional rotating
vanes, an effect similar to varying the pitch of an airplane propeller.
The torque converter in most automotive automatic transmissions is, in itself, a hydrodynamic
transmission. Hydrodynamic transmissions are used in many passenger rail vehicles, those that
are not using electrical transmissions. In this application the advantage of smooth power delivery
may outweigh the reduced efficiency caused by turbulence energy losses in the fluid.
See
also
Transmission (mechanics)
(Redirected from Gearbox)
"Gearbox" redirects here. For the video game developer, see Gearbox Software.
5-speed gearbox + reverse, the 1600 Volkswagen Golf (2009).
A machine consists of a power source and a power transmission system, which provides controlled
application of the power. Merriam-Webster defines transmission as an assembly of parts including the
speed-changing gears and the propeller shaft by which the power is transmitted from an engine to a
live axle.[1] Often transmissionrefers simply to the gearbox that uses gearsand gear trains to
provide speed and torqueconversions from a rotating power source to another device. [2][3]
In British English, the term transmission refers to the whole drive train, including gearbox, clutch, prop
shaft (for rear-wheel drive), differential and final drive shafts. In American English, however, the
distinction is made that a gearbox is any device which converts speed and torque, whereas a
transmission is a type of gearbox that can be "shifted" to dynamically change the speed-torque ratio
such as in a vehicle.
The most common use is in motor vehicles, where the transmission adapts the output of the internal
combustion engine to the drive wheels. Such engines need to operate at a relatively high rotational
speed, which is inappropriate for starting, stopping, and slower travel. The transmission reduces the
higher engine speed to the slower wheel speed, increasing torque in the process. Transmissions are
also used on pedal bicycles, fixed machines, and anywhere else where rotational speed and torque
Often, a transmission will have multiple gear ratios (or simply "gears"), with the ability to switch
between them as speed varies. This switching may be done manually (by the operator), or
automatically. Directional (forward and reverse) control may also be provided. Single-ratio
transmissions also exist, which simply change the speed and torque (and sometimes direction) of
motor output.
In motor vehicles, the transmission will generally be connected to the crankshaft of the engine. The
output of the transmission is transmitted via driveshaft to one or more differentials, which in turn, drive
the wheels. While a differential may also provide gear reduction, its primary purpose is to permit the
wheels at either end of an axle to rotate at different speeds (essential to avoid wheel slippage on
turns) as it changes the direction of rotation.
Conventional gear/belt transmissions are not the only mechanism for speed/torque adaptation.
Alternative mechanisms include torque converters and power transformation (for example, dieselelectric transmission and hydraulic drive system). Hybrid configurations also exist.
Contents
[hide]
1 Explanation
2 Uses
3 Simple
4 Multi-ratio systems
o
4.1 Automotive basics
o
4.2 Manual
o
4.3 Non-synchronous
o
4.4 Automatic
o
4.5 Semi-automatic
o
4.6 Bicycle gearing
5 Uncommon types
o
5.1 Dual clutch transmission
o
5.2 Continuously variable
o
5.3 Infinitely variable
o
5.4 Electric variable
6 Non-direct
o
6.1 Electric
o
6.2 Hydrostatic
o
6.3 Hydrodynamic
8 References
Explanation
Transmission types
Manual

Sequential manual

Non-synchronous

Preselector
Automatic
Manumatic

Semi-automatic

Electrohydraulic

Dual clutch

Saxomat
Continuously variable
Bicycle gearing

Derailleur gears

Hub gears

V

T

E
Interior view of Pantigo Windmill, looking up into cap from floor -- cap rack, brake wheel, brake and wallower.
Pantigo Windmill is located on James Lane, East Hampton, Suffolk County, Long Island, New York.
Early transmissions included the right-angle drives and other gearing in windmills, horse-powered
devices, and steam engines, in support of pumping,milling, and hoisting.
Most modern gearboxes are used to increase torque while reducing the speed of a prime mover output
shaft (e.g. a motor crankshaft). This means that the output shaft of a gearbox will rotate at a slower
rate than the input shaft, and this reduction in speed will produce amechanical advantage, causing an
increase in torque. A gearbox can be set up to do the opposite and provide an increase in shaft speed
with a reduction of torque. Some of the simplest gearboxes merely change the physical direction in
which power is transmitted.
Many typical automobile transmissions include the ability to select one of several different gear ratios.
In this case, most of the gear ratios (often simply called "gears") are used to slow down the output
speed of the engine and increase torque. However, the highest gears may be "overdrive" types that
increase the output speed.
Uses
Gearboxes have found use in a wide variety of different—often stationary—applications, such as wind
turbines.
Transmissions are also used in agricultural, industrial, construction, mining and automotive equipment.
In addition to ordinary transmission equipped with gears, such equipment makes extensive use of the
hydrostatic drive and electrical adjustable-speed drives.
Simple
The main gearbox and rotor of a Bristol Sycamore helicopter
The simplest transmissions, often called gearboxes to reflect their simplicity (although complex
systems are also called gearboxes in the vernacular), provide gear reduction (or, more rarely, an
increase in speed), sometimes in conjunction with a right-angle change in direction of the shaft
(typically in helicopters, see picture). These are often used on PTO-powered agricultural equipment,
since the axial PTO shaft is at odds with the usual need for the driven shaft, which is either vertical (as
with rotary mowers), or horizontally extending from one side of the implement to another (as
with manure spreaders, flail mowers, and forage wagons). More complex equipment, such
as silagechoppers and snowblowers, have drives with outputs in more than one direction.
The gearbox in a wind turbine converts the slow, high-torque rotation of the turbine into much faster
rotation of the electrical generator. These are much larger and more complicated than the PTO
gearboxes in farm equipment. They weigh several tons and typically contain three stages to achieve
an overall gear ratio from 40:1 to over 100:1, depending on the size of the turbine.
(For aerodynamic and structural reasons, larger turbines have to turn more slowly, but the generators
all have to rotate at similar speeds of several thousand rpm.) The first stage of the gearbox is usually a
planetary gear, for compactness, and to distribute the enormous torque of the turbine over more teeth
of the low-speed shaft.[4] Durability of these gearboxes has been a serious problem for a long time. [5]
Regardless of where they are used, these simple transmissions all share an important feature: thegear
ratio cannot be changed during use. It is fixed at the time the transmission is constructed.
For transmission types that overcome this issue, see Continuously Variable Transmission, also known
as CVT.
Multi-ratio
systems
Tractor transmission with 16 forward and 8 backward gears
Amphicar gearbox cutaway w/optional shift for water going propellers
Many applications require the availability of multiple gear ratios. Often, this is to ease the starting and
stopping of a mechanical system, though another important need is that of maintaining good fuel
efficiency.
Automotive
basics
The need for a transmission in an automobile is a consequence of the characteristics of the internal
combustion engine. Engines typically operate over a range of 600 to about 7000 revolutions per
minute (though this varies, and is typically less for diesel engines), while the car's wheels rotate
between 0 rpm and around 1800 rpm.
Furthermore, the engine provides its highest torque and power outputs unevenly across the rev range
resulting in atorque band and a power band. Often the greatest torque is required when the vehicle is
moving from rest or traveling slowly, while maximum power is needed at high speed. Therefore, a
system that transforms the engine's output so that it can supply high torque at low speeds, but also
operate at highway speeds with the motor still operating within its limits, is required. Transmissions
perform this transformation.
A diagram comparing the power and torque bands of a "torquey" engine versus a "peaky" one
The dynamics of a car vary with speed: at low speeds, acceleration is limited by the inertia of vehicular
gross mass; while at cruising or maximum speeds wind resistance is the dominant barrier.
Many transmissions and gears used inautomotive and truck applications are contained in a cast
iron case, though more frequently aluminium is used for lower weight especially in cars. There are
usually three shafts: a mainshaft, a countershaft, and an idler shaft.
The mainshaft extends outside the case in both directions: the input shaft towards the engine, and the
output shaft towards the rear axle (on rear wheel drive cars- front wheel drives generally have the
engine and transmission mounted transversely, the differential being part of the transmission
assembly.) The shaft is suspended by the main bearings, and is split towards the input end. At the
point of the split, a pilot bearing holds the shafts together. The gears and clutches ride on the
mainshaft, the gears being free to turn relative to the mainshaft except when engaged by the clutches.
Types of automobile transmissions include manual, automatic or semi-automatic transmission.
Manual
Main article: Manual transmission
Manual transmission come in two basic types:

a simple but rugged sliding-mesh or unsynchronized / non-synchronous system, where
straight-cut spur gear sets are spinning freely, and must be synchronized by the operator
matching engine revs to road speed, to avoid noisy and damaging "gear clash",

and the now common constant-mesh gearboxes which can include non-synchronised,
orsynchronized / synchromesh systems, where typically diagonal cut helical (or sometimes either
straight-cut, or double-helical) gear sets are constantly "meshed" together, and a dog clutch is
used for changing gears. On synchromesh boxes, friction cones or "synchro-rings" are used in
addition to the dog clutch to closely match the rotational speeds of the two sides of the
(declutched) transmission before making a full mechanical engagement.
The former type was standard in many vintage cars (alongside e.g. epicyclic and multi-clutch systems)
before the development of constant-mesh manuals and hydraulic-epicyclic automatics, older heavyduty trucks, and can still be found in use in some agricultural equipment. The latter is the modern
standard for on- and off-road transport manual and semi-automatic transmission, although it may be
found in many forms; e.g., non-synchronised straight-cut in racetrack or super-heavy-duty applications,
non-synchro helical in the majority of heavy trucks and motorcycles and in certain classic cars (e.g. the
Fiat 500), and partly or fully synchronised helical in almost all modern manual-shift passenger cars and
light trucks.
Manual transmissions are the most common type outside North America and Australia. They are
cheaper, lighter, usually give better performance, and fuel efficiency (although automatic transmissions
with torque converter lockup and advanced electronic controls can provide similar results). It is
customary for new drivers to learn, and be tested, on a car with a manual gear change.
In Malaysiaand Denmark all cars used for testing (and because of that, virtually all those used for
instruction as well) have a manual transmission. In Japan, the
Philippines, Germany, Poland, Italy, Israel, theNetherlands, Belgium, New Zealand, Austria, Bulgaria,
the UK,[6][7] Ireland,[7] Sweden, Norway,Estonia, France, Spain, Switzerland, the Australian states
of Victoria, Western Australia and Queensland, Finland and Lithuania, a test pass using an automatic
car does not entitle the driver to use a manual car on the public road; a test with a manual car is
required.[citation needed] Manual transmissions are much more common than automatic transmissions
in Asia, Africa, South Americaand Europe.
Manual transmissions can include both synchronized and unsynchronized gearing. For example,
reverse gear is usually unsynchronised, as the drive is only expected to engage it when the vehicle is
at a standstill. Many older (up to 1970s) cars also lacked syncro on first gear (for various reasons cost, typically "shorter" overall gearing, engines typically having more low-end torque, the extreme
wear which would be placed on a frequently used 1st gear synchroniser...), meaning it also could only
be used for moving away from a stop unless the driver became adept at double-declutching and had a
particular need to regularly downshift into the lowest gear.
Some manual transmissions have an extremely low ratio for first gear, which is referred to as a
"creeper gear" or "granny gear". Such gears are usually not synchronized. This feature is common on
pickup trucks tailored to trailer-towing, farming, or construction-site work. During normal on-road use,
the truck is usually driven without using the creeper gear at all, and second gear is used from a
standing start. Some off-road vehicles, most particularly the Willys Jeep and its descendents, also had
transmissions with "granny first"s either as standard or an option, but this function is now more often
provided for by a low-range transfer gearbox attached to a normal fully synchronised transmission.
Non-synchronous
Main article: Non-synchronous transmissions
There are commercial applications engineered with designs taking into account that the gear shifting
will be done by an experienced operator. They are a manual transmission, but are known as nonsynchronized transmissions. Dependent on country of operation, many local, regional, and national
laws govern the operation of these types of vehicles (see Commercial Driver's License). This class
may include commercial, military, agricultural, or engineering vehicles. Some of these may use
combinations of types for multi-purpose functions. An example would be a power take-off (PTO) gear.
The non-synchronous transmission type requires an understanding of gear range, torque, engine
power, and multi-functional clutch and shifter functions. Also see Double-clutching, and Clutchbrakesections of the main article.
Automatic
Main article: Automatic transmission
Epicyclic gearing or planetary gearing as used in an automatic transmission.
Most modern North American and Australian and some European and Japanese cars have
an automatic transmission that will select an appropriate gear ratio without any operator intervention.
They primarily usehydraulics to select gears, depending on pressure exerted by fluid within the
transmission assembly. Rather than using a clutch to engage the transmission, a fluid flywheel,
or torque converter is placed in between the engine and transmission. It is possible for the driver to
control the number of gears in use or select reverse, though precise control of which gear is in use
may or may not be possible.
Automatic transmissions are easy to use. However, in the past, automatic transmissions of this type
have had a number of problems; they were complex and expensive, sometimes had reliability
problems (which sometimes caused more expenses in repair), have often been less fuel-efficient than
their manual counterparts (due to "slippage" in the torque converter), and their shift time was slower
than a manual making them uncompetitive for racing. With the advancement of modern automatic
transmissions this has changed.[citation needed]
Attempts to improve the fuel efficiency of automatic transmissions include the use of torque
converterswhich lock up beyond a certain speed, or in the higher gear ratios, eliminating power loss,
and overdrive gears which automatically actuate above certain speeds; in older transmissions both
technologies could sometimes become intrusive, when conditions are such that they repeatedly cut in
and out as speed and such load factors as grade or wind vary slightly. Current computerized
transmissions possess very complex programming to both maximize fuel efficiency and eliminate any
intrusiveness, and we are at a point in technological advancement where automatics are beginning to
outperform manuals in both performance and efficiency. [citation needed]. This is due mainly to electronic
advances rather than mechanical ones although improvements in CVT technology and the use of
automatic clutches have also helped. The 2012 model of the Honda Jazz sold in the UK actually
claims marginally better fuel consumption for the CVT version than the manual version.
For certain applications, the slippage inherent in automatic transmissions can be advantageous; for
instance, in drag racing, the automatic transmission allows the car to be stopped with the engine at a
high rpm (the "stall speed") to allow for a very quick launch when the brakes are released; in fact, a
common modification is to increase the stall speed of the transmission. This is even more
advantageous for turbocharged engines, where the turbocharger needs to be kept spinning at high
rpm by a large flow of exhaust in order to keep the boost pressure up and eliminate the turbo lag that
occurs when the engine is idling and the throttle is suddenly opened.
Semi-automatic
Main article: Semi-automatic transmission
A hybrid form of transmission where the an integrated control system handles manipulation of
theclutch automatically, but the driver can still - and may be required to - take manual control of gear
selection. This is sometimes called a "clutchless manual," or "automated manual" transmission. Many
of these transmissions allow the driver to fully delegate gear shifting choice to the control system,
which then effectively acts as if it was a regular automatic transmission. They are generally designed
using manual transmission "internals", and when used in passenger cars, have synchromesh operated
helical constant mesh gear sets.
Early semi-automatic systems used a variety of mechanical and hydraulic systems - including
centrifugal clutches, torque converters, electro-mechanical (and even electrostatic) and servo/solenoid
controlled clutches - and control schemes - automatic declutching when moving the gearstick, preselector controls, centrifugal clutches with drum-sequential shift requiring the driver to lift the throttle for
a successful shift, etc. - and some were little more than regular lock-up torque converter automatics
with manual gear selection.
Most modern implementations, however, tend to be standard or slightly modified manual transmissions
(and very occasionally modified automatics, even including a few cases of CVTs with "fake" fixed gear
ratios), with servo-controlled clutching and shifting under command of the central engine computer.
These are intended to be a combined replacement option both for more expensive and less efficient
"normal" automatic systems, and for drivers who prefer manual shift but are no longer able to operate
a clutch, and users are encouraged to leave the shift lever in fully automatic "Drive" most of the time,
only engaging manual-sequential mode for sporty driving or when otherwise strictly necessary.
Specific types of this transmission include: Easytronic, Tiptronic and Geartronic, as well as the
systems used as standard in all ICE-powered Smart-MCC vehicles, and on geared step-through
scooters such as the Honda Cub or Suzuki Address.
A dual-clutch transmission uses two sets of internals which are alternately used, each with its own
clutch, so that a "gearchange" actually only consists of one clutch engaging as the other disengages,
making for a supposedly "seamless" shift with no break in (or jarring reuptake of) power transmission.
Each clutch's attached shaft carries half of the total input gear complement (with a shared output
shaft), including synchronised dog clutch systems that pre-select which of its set of ratios is most likely
to be needed at the next shift, under command of a computerised control system.
Specific types of this transmission include: Direct-Shift Gearbox.
There are also sequential transmissions which use the rotation of a drum to switch gears, much like
those of a typical fully manual motorcycle.[8] These can be designed with a manual or automatic clutch
system, and may be found both in automobiles (particularly track and rally racing cars), motorcycles
(typically light "step-thru" type city utility bikes, e.g. the Honda Cub) and quadbikes (often with a
separately engaged reversing gear), the latter two normally using a scooter-style centrifugal clutch.
Bicycle
gearing
Shimano XT rear derailleur on amountain bike
Main articles: Bicycle gearing, Derailleur gears, and Hub gear
Bicycles usually have a system for selecting different gear ratios. There are two main types: derailleur
gears and hub gears. The derailleur type is the most common, and the most visible,
usingsprocket gears. Typically there are several gears available on the rear sprocket assembly,
attached to the rear wheel. A few more sprockets are usually added to the front assembly as well.
Multiplying the number of sprocket gears in front by the number to the rear gives the number of gear
ratios, often called "speeds".
Hub gears use epicyclic gearing and are enclosed within the axle of the rear wheel. Because of the
small space, they typically offer fewer different speeds, although at least one has reached 14 gear
ratios and Fallbrook Technologies manufactures a transmission with technically infinite ratios.[9]
Causes for failure of bicycle gearing include: worn teeth, damage caused by a faulty chain, damage
due to thermal expansion, broken teeth due to excessive pedaling force, interference by foreign
objects, and loss of lubrication due to negligence.
Uncommon
Dual
types
clutch transmission
Main article: Dual clutch transmission
This arrangement is also sometimes known as a direct shift gearbox or powershift gearbox. It seeks to
combine the advantages of a conventional manual shift with the qualities of a modern automatic
transmission by providing different clutches for odd and even speed selector gears. When changing
gear, the engine torque is transferred from one gear to the other continuously, so providing gentle,
smooth gear changes without either losing power or jerking the vehicle. Gear selection may be
manual, automatic (depending on throttle/speed sensors), or a 'sports' version combining both options.
Continuously
variable
Main article: Continuously variable transmission
The Continuously Variable Transmission (CVT) is a transmission in which the ratio of the rotational
speeds of two shafts, as the input shaft and output shaft of a vehicle or other machine, can be varied
continuously within a given range, providing an infinite number of possible ratios. The CVT allows the
relationship between the speed of the engine and the speed of the wheels to be selected within a
continuous range. This can provide even better fuel economy if the engine is constantly running at a
single speed. The transmission is in theory capable of a better user experience, without the rise and
fall in speed of an engine, and the jerk felt when poorly changing gears.
CVTs are increasingly found on small cars, and especially high-gas-milage or hybrids vehicles. On
these platforms the torque is limited because the electric motor can provide torque without changing
the speed of the engine. By leaving the engine running at the rate that generates the best gas milage
for the given operating conditions, overall milage can be improved over a system with a smaller
number of fixed gears, where the system may be operating at peak efficiency only for a small range of
speeds. CVTs are rare on other platforms, especially high-torque applications, as they are generally
constructed using rubber belts or similar devices that are subject to slippage at high torque.
Infinitely
variable
The IVT is a specific type of CVT that includes not only an infinite number of gear ratios, but an
infiniterange as well. This is a turn of phrase, it actually refers to CVTs that are able to include a "zero
ratio", where the input shaft can turn without any motion of the output shaft while remaining in gear.
Zero output implies infinite ratios, as any "high gear" ratio is an infinite number of times higher than the
zero "low gear".
Most (if not all) IVTs result from the combination of a CVT with an epicyclic gear system with a fixed
ratio. The combination of the fixed ratio of the epicyclic gear with a specific matching ratio in the CVT
side results in zero output. For instance, consider a transmission with an epicyclic gear set to 1:-1 gear
ratio; a 1:1 reverse gear. When the CVT side is set to 1:1 the two ratios add up to zero output. The IVT
is always engaged, even during its zero output. When the CVT is set to higher values it operates
conventionally, with increasing forward ratios.
In practice, the epicyclic gear may be set to the lowest possible ratio of the CVT, if reversing is not
needed or is handled through other means. Reversing can be incorporated by setting the epicyclic
gear ratio somewhat higher than the lowest ratio of the CVT, providing a range of reverse ratios.
Electric
variable
The Electric Variable Transmission (EVT) combines a transmission with an electric motor to provide
the illusion of a single CVT. In the common implementation, a gasoline engine is connected to a
traditional transmission, which is in turn connected to an epicyclic gear system's planet carrier. An
electric motor/generator is connected to the central "sun" gear, which is normally un-driven in typical
epicyclic systems. Both sources of power can be fed into the transmission's output at the same time,
splitting power between them. In common examples, between ¼ and ½ of the engine's power can be
fed into the sun gear. Depending on the implementation, the transmission in front of the epicyclic
system may be greatly simplified, or eliminated completely. EVTs are capable of continuously
modulating output/input speed ratios like mechanical CVTs, but offer the distinct benefit of being able
to also apply power from two different sources to one output, as well as potentially reducing overall
complexity dramatically.
In typical implementations, the gear ratio of the transmission and epicyclic system are set to the ratio
of the common driving conditions, say highway speed for a car, or city speeds for a bus. When the
drivers presses on the gas, the associated electronics interprets the pedal position and immediately
sets the gasoline engine to the RPM that provides the best gas milage for that setting. As the gear
ratio is normally set far from the maximum torque point, this set-up would normally result in very poor
acceleration. Unlike gasoline engines, electric motors offer efficient torque across a wide selection of
RPM, and are especially effective at low settings where the gasoline engine is inefficient. By varying
the electrical load or supply on the motor attached to the sun gear, additional torque can be provided
to make up for the low torque output from the engine. As the vehicle accelerates, the power to the
motor is reduced and eventually ended, providing the illusion of a CVT.
The canonical example of the EVT is Toyota's Hybrid Synergy Drive. This implementation has no
conventional transmission, and the sun gear always receives 28% of the torque from the engine. This
power can be used to operate any electrical loads in the vehicle, recharging the batteries, powering
the entertainment system, or running the air conditioning. Any residual power is then fed back into a
second motor that powers the output of the drivetrain directly. At highway speeds this additional
generator/motor pathway is less efficient than simply powering the wheels directly. However, during
acceleration, the electrical path is much more efficient than engine operating so far from its torque
point.[10] GM uses a similar system in the Allison Bus hybrid powertrains and the Tahoe and Yukon
pick-up trucks, but these use a two-speed transmission in front of the epicyclic system, and the sun
gear receives close to half the total power.
Non-direct
Electric
Electric transmissions convert the mechanical power of the engine(s) to electricity with electric
generators and convert it back to mechanical power with electric motors. Electrical or
electronicadjustable-speed drive control systems are used to control the speed and torque of the
motors. If the generators are driven by turbines, such arrangements are called turbo-electric. Likewise
Diesel-electric arrangements are used on many railway locomotives, ships, large mining trucks, and
some bulldozers. In these cases, each driven wheel is equipped with its own electric motor, which can
be fed varying electrical power to provide any required torque or power output for each wheel
independently. This produces a much simpler solution for multiple driven wheels in very large vehicles,
where drive shafts would be much larger or heavier than the electrical cable that can provide the same
amount of power. It also improves the ability to allow different wheels to run at different speeds, which
is useful for steered wheels in large construction vehicles.
Hydrostatic
Hydrostatic transmissions transmit all power hydraulically, using the components of hydraulic
machinery. They are similar to electrical transmissions, but hydraulic fluid as the power
distribution system rather than electricity.
The transmission input drive is a central hydraulic pump and final drive unit(s) is/are a hydraulic
motor, or hydraulic cylinder (see: swashplate). Both components can be placed physically far
apart on the machine, being connected only by flexible hoses. Hydrostatic drive systems are used
on excavators, lawn tractors, forklifts, winch drive systems, heavy lift equipment, agricultural
machinery, earth-moving equipment, etc. An arrangement for motor-vehicle transmission was
probably used on the FergusonF-1 P99 racing car in about 1961.
The Human Friendly Transmission of the Honda DN-01 is hydrostatic.
Hydrodynamic
If the hydraulic pump and/or hydraulic motor make use of the hydrodynamic effects of the fluid
flow, i.e. pressure due to a change in the fluid's momentum as it flows through vanes in a turbine.
The pump and motor usually consist of rotating vanes without seals and are typically placed in
close proximity. The transmission ratio can be made to vary by means of additional rotating
vanes, an effect similar to varying the pitch of an airplane propeller.
The torque converter in most automotive automatic transmissions is, in itself, a hydrodynamic
transmission. Hydrodynamic transmissions are used in many passenger rail vehicles, those that
are not using electrical transmissions. In this application the advantage of smooth power delivery
may outweigh the reduced efficiency caused by turbulence energy losses in the fluid.
See
also
Double wishbone suspension
Unsourced material may be challenged and removed. (May 2012)
Wishbones and upright painted yellow
In automobiles, a double wishbone (orupper and lower A-arm) suspension is anindependent
suspension design using two (occasionally parallel) wishbone-shaped arms to locate the wheel. Each
wishbone or arm has two mounting points to the chassis and one joint at the knuckle. The shock
absorberand coil spring mount to the wishbones to control vertical movement. Double wishbone
designs allow the engineer to carefully control the motion of the wheel throughout suspension travel,
controlling such parameters as camber angle, caster angle,toe pattern, roll center height, scrub radius,
scuff and more.
Contents
[hide]
1 Implementation
3 Uses
5 References
Implementation
Double wishbone suspension
The double-wishbone suspension can also be referred to as "double A-arms," though the arms
themselves can be A-shaped, L-shaped, or even a single bar linkage. A single wishbone or A-arm can
also be used in various other suspension types, such asMacPherson strut and Chapman strut. The
upper arm is usually shorter to induce negative camber as the suspension jounces (rises), and often
this arrangement is titled an "SLA" or "short long arms" suspension. When the vehicle is in a turn, body
roll results in positive camber gain on the lightly loaded inside wheel, while the heavily loaded outer
wheel gains negative camber.
Between the outboard end of the arms is a knuckle with a spindle (the kingpin), hub, or upright which
carries the wheel bearing and wheel.
To resist fore-aft loads such as acceleration and braking, the arms require two bushings or ball joints
at the body.
At the knuckle end, single ball joints are typically used, in which case the steering loads have to be
taken via a steering arm, and the wishbones look A- or L-shaped. An L-shaped arm is generally
preferred on passenger vehicles because it allows a better compromise of handling and comfort to be
tuned in. The bushing inline with the wheel can be kept relatively stiff to effectively handle cornering
loads while the off-line joint can be softer to allow the wheel to recess under fore-aft impact loads. For
a rear suspension, a pair of joints can be used at both ends of the arm, making them more H-shaped
in plan view. Alternatively, a fixed-length driveshaft can perform the function of a wishbone as long as
the shape of the other wishbone provides control of the upright. This arrangement has been
successfully used in the Jaguar IRS. In elevation view, the suspension is a 4-bar link, and it is easy to
work out the camber gain (see camber angle) and other parameters for a given set of bushing or balljoint locations. The various bushings or ball joints do not have to be on horizontal axes, parallel to the
vehicle centre line. If they are set at an angle, then antidive and antisquat geometry can be dialed in.
In many racing cars, the springs and dampers are relocated inside the bodywork. The suspension
uses a bellcrank to transfer the forces at the knuckle end of the suspension to the internal spring and
damper. This is then known as a "push rod" if bump travel "pushes" on the rod (and subsequently the
rod must be joined to the bottom of the upright and angled upward). As the wheel rises, the push rod
compresses the internal spring via a pivot or pivoting system. The opposite arrangement, a "pull rod,"
will pull on the rod during bump travel, and the rod must be attached to the top of the upright, angled
downward. Locating the spring and damper inboard increases the total mass of the suspension, but
reduces the unsprung mass, and also allows the designer to make the suspension more aerodynamic.
The advantage of a double wishbone suspension is that it is fairly easy to work out the effect of moving
each joint, so the kinematics of the suspension can be tuned easily and wheel motion can be
optimized. It is also easy to work out the loads that different parts will be subjected to which allows
more optimized lightweight parts to be designed. They also provide increasing negative camber gain
all the way to full jounce travel, unlike the MacPherson strut, which provides negative camber gain only
at the beginning of jounce travel and then reverses into positive camber gain at high jounce amounts.
The disadvantage is that it is slightly more complex than other systems like a MacPherson strut. Due
to the increased number of components within the suspension setup it takes much longer to service
and is heavier than an equivalent MacPherson design.
Uses
The double wishbone suspension was introduced in the 1930s. French carmaker Citroën used it since
1934 in their Rosalie and Traction Avant models. Packard Motor Car Company of Detroit,
Michiganused it on the Packard One-Twenty from 1935.[citation needed], and advertised it as a safety
feature. Prior to the dominance of front wheel drive in the 1980s, many everyday cars used double
wishbone front-suspension systems, or a variation on it. Since that time, the MacPherson strut has
become almost ubiquitous, as it is simpler and cheaper to manufacture. In most cases, a MacPherson
strut requires less space to engineer into a chassis design, and in front-wheel-drive layouts, can allow
for more room in the engine bay. A good example of this is observed in the Honda Civic, which
changed its front-suspension design from a double wishbone to a MacPherson strut after the year
2000 model.
Double wishbones are usually considered to have superior dynamic characteristics as well as loadhandling capabilities, and are still found on higher performance vehicles. Examples of makes in which
double wishbones can be found include Alfa Romeo, Honda and Mercedes-Benz. Short long arms
suspension, a type of double wishbone suspension, is very common on front suspensions for mediumto-large cars such as the Honda Accord, Peugeot 407, or Mazda 6/Atenza, and is very common on
sports cars and racing cars.It also provide least camber change at
Short long arms suspension
Unsourced material may be challenged and removed. (January 2007)
A short long arms suspension (SLA) is also known as an unequal length double wishbone
suspension. The upper arm is typically an A-arm, and is shorter than the lower link, which is an A-arm
or an L-arm, or sometimes a pair of tension/compression arms. In the latter case the suspension can
be called a multi-link, or dual ball joint suspension.
The four-bar link mechanism formed by the unequal arm lengths causes a change in the camber of the
vehicle as it rolls, which helps to keep the contact patch square on the ground, increasing the ultimate
cornering capacity of the vehicle. It also reduces the wear of the outer edge of the tire.
SLAs can be classified as short spindle, in which the upper ball joint on the spindle is inside the wheel,
or long spindle, in which the spindle tucks around the tire and the upper ball joint sits above the tire.
Short spindle SLAs tend to require stiffer bushings at the body, as the braking and cornering forces are
higher. Also they tend to have poorer kingpin geometry, due to the difficulty of packaging the upper
ball joint and the brakes inside the wheel.
Long spindle SLAs tend to have better kingpin geometry, but the proximity of the spindle to the tire
restricts fitting oversized tires, or snowchains. The location of the upper balljoint may have styling
implications in the design of the sheetmetal above it.
SLAs require some care when setting up their Bump Steer characteristic, as it is easy to end up with
excessive, or curved, bump steer curves.
Epicyclic gearing
2011)
Epicyclic gearing is used here for increasing output speed. The planet gear carrier (green) is driven by an input
torque. The sun gear, in the center, but almost hidden, has a prominent yellow shaft which provides the output
torque, while the ring gear (pink) is fixed. Note the red marks both before and after the green input drive is rotated
45° clockwise.
Epicyclic gearing or planetary gearing is a gear system consisting of one or more outer gears,
or planet gears, revolving about a central, or sun gear. Typically, the planet gears are mounted on a
movable arm orcarrier which itself may rotate relative to the sun gear. Epicyclic gearing systems also
incorporate the use of an outer ring gear orannulus, which meshes with the planet gears. Planetary
gears (or epicyclic gears) are typically classified as simple and compound planetary gears. Simple
planetary gears have one sun, one ring, one carrier, and one planet set. Compound planetary gears
involve one or more of the following three types of structures: meshed-planet (there are at least two
more planets in mesh with each other in each planet train), stepped-planet (there exists a shaft
connection between two planets in each planet train), and multi-stage structures (the system contains
two or more planet sets). Compared to simple planetary gears, compound planetary gears have the
advantages of larger reduction ratio, higher torque-to-weight ratio, and more ﬂexible conﬁgurations.
The axes of all gears are usually parallel, but for special cases like pencil sharpeners they can be
placed at an angle, introducing elements of bevel gear (see below). Further, the sun, planet carrier and
annulus axes are usually coaxial.
Contents
[hide]
1 History
2 Gear ratio
o
2.1 Compound planetary gears
4 Gallery
6 References
History
Bookwheel, from Agostino Ramelli's Le diverse et artifiose machine, 1588
Epicyclic differential gearing, used for calendrical computation, has been identified in the
Greek Antikythera mechanism dating to around 87 BC.[1]
Richard of Wallingford, an English abbot of St Albans monastery is credited for reinventing epicyclic
gearing for an astronomical clock in the 14th century.[1]
In 1588, Italian military engineer Agostino Ramelli invented thebookwheel, a vertically-revolving
bookstand containing epicyclic gearing with two levels of planetary gears to maintain proper orientation
of the books.[2]
The Antikythera mechanism (main fragment)
Gear
ratio
In this example, the carrier (green) is held stationary while the sun gear (yellow) is used as input. The planet gears
(blue) turn in a ratio determined by the number of teeth in each gear. Here, the ratio is -24/16, or -3/2; each planet
gear turns at 3/2 the rate of the sun gear, in the opposite direction.
The gear ratio in an epicyclic gearing system is somewhat non-intuitive, particularly because there are
several ways in which an input rotation can be converted into an output rotation. The three basic
components of the epicyclic gear are:

Sun: The central gear

Planet carrier: Holds one or more peripheral planetgears, all of the same size, meshed with
the sun gear

Annulus: An outer ring with inward-facing teeth that mesh with the planet gear or gears
In many epicyclic gearing systems, one of these three basic components is held stationary; one of the
two remaining components is an input, providing power to the system, while the last component is
an output, receiving power from the system. The ratio of input rotation to output rotation is dependent
upon the number of teeth in each gear, and upon which component is held stationary.
In other systems, such as hybrid vehicle transmissions, two of the components are used as inputs with
the third providing output relative to the two inputs.[3]
In one arrangement, the planetary carrier (green) is held stationary, and the sun gear (yellow) is used
as input. In this case, the planetary gears simply rotate about their own axes (i.e., spin) at a rate
determined by the number of teeth in each gear. If the sun gear has Ns teeth, and each planet gear
has Np teeth, then the ratio is equal to -Ns/Np. For instance, if the sun gear has 24 teeth, and each
planet has 16 teeth, then the ratio is -24/16, or -3/2; this means that one clockwise turn of the sun gear
produces 1.5 counterclockwise turns of each of the planet gear(s) about its axis.
This rotation of the planet gears can in turn drive the annulus (not depicted in diagram), in a
corresponding ratio. If the annulus has Na teeth, then the annulus will rotate by Np/Na turns for each
turn of the planet gears. For instance, if the annulus has 64 teeth, and the planets 16, one clockwise
turn of a planet gear results in 16/64, or 1/4 clockwise turns of the annulus. Extending this case from
the one above:

One turn of the sun gear results in

One turn of a planet gear results in
turns of the planets
turns of the annulus
So, with the planetary carrier locked, one turn of the sun gear results in
turns of the
annulus.
The annulus may also be held fixed, with input provided to the planetary gear carrier; output rotation is
then produced from the sun gear. This configuration will produce an increase in gear ratio, equal to
1+Na/Ns.
These are all described by the equation:
where n is the form factor of the planetary gear, defined by:
If the annulus is held stationary and the sun gear is used as the input, the planet carrier will
be the output. The gear ratio in this case will be 1/(1+Na/Ns). This is the lowest gear ratio
attainable with an epicyclic gear train. This type of gearing is sometimes used in tractors and
construction equipment to provide high torque to the drive wheels.
In bicycle hub gears, the sun is usually stationary, being keyed to the axle or even machined
directly onto it. The planetary gear carrier is used as input. In this case the gear ratio is simply
given by (Ns+Na)/Na. The number of teeth in the planet gear is irrelevant.
Compound planets of a Sturmey-ArcherAM bicycle hub (gear ring removed)
Compound
planetary gears
Stepped planet series of the Rohloff Speedhub internally geared bicycle hub with the smaller planet
series meshing with the sun wheel and the larger planet series meshing with the annulus.
"Compound planetary gear" is a general concept and it refers to any planetary gears involving
one or more of the following three types of structures: meshed-planet (there are at least two
more planets in mesh with each other in each planet train), stepped-planet (there exists a
shaft connection between two planets in each planet train), and multi-stage structures (the
system contains two or more planet sets).
Some designs use "stepped-planet" which have two differently-sized gears on either end of a
common casting. The large end engages the sun, while the small end engages the annulus.
This may be necessary to achieve smaller step changes in gear ratio when the overall
package size is limited. Compound planets have "timing marks" (or "relative gear mesh
phase" in technical term). The assembly conditions of compound planetary gears are more
restrictive then simple planetary gears,[4] and they must be assembled in the correct initial
orientation relative to each other, or their teeth will not simultaneously engage the sun and
annulus at opposite ends of the planet, leading to very rough running and short life.
Compound planetary gears can easily achieve larger transmission ratio with equal or smaller
volume. For example, compound planets with teeth in a 2:1 ratio with a 50T annulus would
give the same effect as a 100T annulus, but with half the actual diameter.
More planet and sun gear units can be placed in series in the same annulus housing (where
the output shaft of the first stage becomes the input shaft of the next stage) providing a larger
(or smaller) gear ratio. This is the way some automatic transmissions work.
During World War II, a special variation of epicyclic gearing was developed for
portable radar gear, where a very high reduction ratio in a small package was needed. This
had two outer annular gears, each half the thickness of the other gears. One of these two
annular gears was held fixed and had one tooth fewer than did the other. Therefore, several
turns of the "sun" gear made the "planet" gears complete a single revolution, which in turn
made the rotating annular gear rotate by a single tooth. [citation needed]
The mechanism of a pencil sharpenerwith stationary annulus and rotating planet carrier as input. Planet
gears are extended into cylindric cutters, rotating around the pencil that is placed on the sun axis. The
axes of planetary gears join at the pencil sharpening angle.
Advantages of planetary gears over parallel axis gears include high power density, large
reduction in a small volume, multiple kinematic combinations, pure torsional reactions, and
complexity.[5][6]The planetary gearbox arrangement is an engineering design that offers many
of both compactness and outstanding power transmission efficiencies. A typical efficiency
loss in a planetary gearbox arrangement is only 3% per stage. This type of efficiency ensures
that a high proportion of the energy being input is transmitted through the gearbox, rather
than being wasted on mechanical losses inside the gearbox.
Another advantage of the planetary gearbox arrangement is load distribution. Because the
load being transmitted is shared between multiple planets, torque capability is greatly
increased. The more planets in the system, the greater load ability and the higher the torque
density.
The planetary gearbox arrangement also creates greater stability due to the even distribution
of mass and increased rotational stiffness.
Gallery
Continuously variable transmission
these issues on the talk page.
Transmission types
Manual

Sequential manual

Non-synchronous

Preselector
Automatic

Manumatic
Semi-automatic

Electrohydraulic

Dual clutch

Saxomat
Continuously variable
Bicycle gearing

Derailleur gears
Hub gears


V

T

E
A continuously variable transmission (CVT) is a transmission that can change steplessly through
an infinite number of effective gear ratios between maximum and minimum values.
This contrasts with other mechanical transmissions that offer a fixed number of gear ratios. The
flexibility of a CVT allows the driving shaft to maintain a constant angular velocity over a range of
output velocities. This can provide better fuel economy than other transmissions by enabling
the engine to run at its most efficient revolutions per minute (RPM) for a range of vehicle speeds.
Alternatively it can be used to maximize the performance of a vehicle by allowing the engine to turn at
the RPM at which it produces peak power. This is typically higher than the RPM that achieves peak
efficiency. Finally, a CVT does not strictly require the presence of a clutch, allowing the dismissal
thereof. In some vehicles though (e.g. motorcycles), a centrifugal clutch is nevertheless added,
[1]
however this is only to provide a "neutral" stance on a motorcycle (useful when idling, or manually
reversing into a parking space).
Contents
[hide]
1 Uses
2 Types
o
2.1 Variable-diameter pulley (VDP) or Reeves drive
o
2.2 Toroidal or roller-based CVT (Extroid CVT )
o
2.3 Magnetic CVT or mCVT
o
2.4 Infinitely Variable Transmission (IVT)
o
2.5 Ratcheting CVT
o
2.6 Hydrostatic CVTs
o
2.7 Naudic Incremental CVT (iCVT)

2.7.1 High frictional losses

2.7.2 Shock and durability

2.7.3 Torque transfer ability and reliability
o
2.8 Cone CVTs
o
o
2.10 Planetary CVT
3 History
5 Notes
6 References
Uses
A Chain-driven CVT
Principle of Variator
Many small tractors for home and garden use have simple rubber belt CVTs. For example, the John
Deere Gator line of small utility vehicles use a belt with a conical pulley system. They can deliver an
abundance of power and can reach speeds of 10–15 mph (16–24 km/h), all without need for a clutch
or shifting gears. Nearly all snowmobiles, old and new, and motorscooters use CVTs, typically the
rubber belt/variable pulley variety.
Some combine harvesters have CVTs. The CVT allows the forward speed of the combine to be
adjusted independently of the engine speed. This allows the operator to slow or accelerate as needed
to accommodate variations in thickness of the crop.
CVTs have been used in aircraft electrical power generating systems since the 1950s and in Sports
Car Club of America (SCCA) Formula 500race cars since the early 1970s. CVTs were banned from
Formula 1 in 1994 due to concerns that the best-funded teams would dominate if they managed to
create a viable F1 CVT transmission.[2] More recently, CVT systems have been developed for gokarts and have proven to increase performance and engine life expectancy. The Tomcar range of offroad vehicles also utilizes the CVT system.
Some drill presses and milling machines contain a pulley-based CVT where the output shaft has a pair
of manually adjustable conical pulley halves through which a wide drive belt from the motor loops. The
pulley on the motor, however, is usually fixed in diameter, or may have a series of given-diameter
steps to allow a selection of speed ranges. A handwheel on the drill press, marked with a scale
corresponding to the desired machine speed, is mounted to a reduction gearing system for the
operator to precisely control the width of the gap between the pulley halves. This gap width thus
adjusts the gearing ratio between the motor's fixed pulley and the output shaft's variable pulley,
changing speed of the chuck. A tensioner pulley is implemented in the belt transmission to take up or
release the slack in the belt as the speed is altered. In most cases the speed must be changed with
the motor running.
CVTs should be distinguished from Power Sharing Transmissions (PSTs), as used in newer hybrid
cars, such as the Toyota Prius, Highlander and Camry, the Nissan Altima, and newer-model Ford
Escape Hybrid SUVs. CVT technology uses only one input from a prime mover, and delivers variable
output speeds and torque; whereas PST technology uses two prime mover inputs, and varies the ratio
of their contributions to output speed and power. These transmissions are fundamentally different.
However the Mitsubishi Lancer, Proton Inspira, Honda Insight, Honda Fit, and Honda CR-Z hybrids,
theNissan Tiida/Versa (only the SL model), Nissan
Cube, Juke, Sentra, Altima, Maxima, Rogue, Murano,Micra, Honda Capa, Honda Civic HX, Jeep
Patriot and Compass, and Subaru Impreza, Legacy andOutback offer CVT.
Types
Toyota Super CVT - i
Variable-diameter
pulley (VDP) or Reeves drive
In this most common CVT system,[3] there are two V-belt pulleys that are split perpendicular to their
axes of rotation, with a V-belt running between them. The gear ratio is changed by moving the two
sheaves of one pulley closer together and the two sheaves of the other pulley farther apart. Due to the
V-shaped cross section of the belt, this causes the belt to ride higher on one pulley and lower on the
other. Doing this changes the effective diameters of the pulleys, which in turn changes the overall gear
ratio. The distance between the pulleys does not change, and neither does the length of the belt, so
changing the gear ratio means both pulleys must be adjusted (one bigger, the other smaller)
simultaneously in order to maintain the proper amount of tension on the belt.
The V-belt needs to be very stiff in the pulley's axial direction in order to make only short radial
movements while sliding in and out of the pulleys. This can be achieved by a chain and not by
homogeneous rubber. To dive out of the pulleys one side of the belt must push. This again can be
done only with a chain. Each element of the chain has conical sides, which perfectly fit to the pulley if
the belt is running on the outermost radius. As the belt moves into the pulleys the contact area gets
smaller. The contact area is proportional to the number of elements, thus the chain has lots of very
small elements. The shape of the elements is governed by the static of a column. The pulley-radial
thickness of the belt is a compromise between maximum gear ratio and torque. For the same reason
the axis between the pulleys is as thin as possible. A film of lubricant is applied to the pulleys. It needs
to be thick enough so that the pulley and the belt never touch and it must be thin in order not to waste
power when each element dives into the lubrication film. Additionally, the chain elements stabilize
about 12 steel bands. Each band is thin enough so that it bends easily. If bending, it has a perfect
conical surface on its side. In the stack of bands each band corresponds to a slightly different gear
ratio, and thus they slide over each other and need oil between them. Also the outer bands slide
through the stabilizing chain, while the center band can be used as the chain linkage. [note 1]
Nissan Motors Extroid CVT
Toroidal
or roller-based CVT (Extroid CVT )
Toroidal CVTs are made up of discs and rollers that transmit power between the discs. The discs can
be pictured as two almost conical parts, point to point, with the sides dished such that the two parts
could fill the central hole of a torus. One disc is the input, and the other is the output. Between the
discs are rollers which vary the ratio and which transfer power from one side to the other. When the
roller's axis is perpendicular to the axis of the near-conical parts, it contacts the near-conical parts at
same-diameter locations and thus gives a 1:1 gear ratio. The roller can be moved along the axis of the
near-conical parts, changing angle as needed to maintain contact. This will cause the roller to contact
the near-conical parts at varying and distinct diameters, giving a gear ratio of something other than
1:1. Systems may be partial or full toroidal. Full toroidal systems are the most efficient design while
partial toroidals may still require a torque converter, and hence lose efficiency.
Some toroidal systems are also infinitely variable, and the direction of thrust can be reversed within the
CVT[4].
Diagrams:

Animated image of a toroidal CVT on HowStuffWorks
Magnetic
CVT or mCVT
A magnetic continuous variable transmission system was developed at the University of Sheffield in
2006 and later commercialized.[5] mCVT is a variable magnetic transmission which gives an electrically
controllable gear ratio. It can act as a power split device and can match a fixed input speed from a
prime-mover to a variable load by importing/exporting electrical power through a variator path.
The mCVT is of particular interest as a highly efficient power-split device for blended parallel hybrid
vehicles, but also has potential applications in renewable energy, marine propulsion and industrial
drive sectors.
Infinitely
Variable Transmission (IVT)
A specific type of CVT is the infinitely variable transmission (IVT), in which the range of ratios of output
shaft speed to input shaft speed includes a zero ratio that can be continuously approached from a
defined "higher" ratio. A zero output speed (low gear) with a finite input speed implies an infinite inputto-output speed ratio, which can be continuously approached from a given finite input value with an
IVT.Low gears are a reference to low ratios of output speed to input speed. This low ratio is taken to
the extreme with IVTs, resulting in a "neutral", or non-driving "low" gear limit, in which the output speed
is zero. Unlike neutral in a normal automotive transmission, IVT output rotation may be prevented
because the backdriving (reverse IVT operation) ratio may be infinite, resulting in impossibly high
backdriving torque; ratcheting IVT output may freely rotate forward, though.
The IVT dates back to before the 1930s; the original design converts rotary motion to oscillating
motion and back to rotary motion using roller clutches.[6] The stroke of the intermediate oscillations is
adjustable, varying the output speed of the shaft. This original design is still manufactured today, and
an example and animation of this IVT can be found here. [7] Paul B. Pires created a more compact
(radially symmetric) variation that employs a ratchet mechanism instead of roller clutches, so it doesn't
have to rely on friction to drive the output. An article and sketch of this variation can be found here
[8]
Most IVTs result from the combination of a CVT with a planetary gear system (which is also known as
an epicyclic gear system) which enforces an IVT output shaft rotation speed which is equal to the
difference between two other speeds within the IVT. This IVT configuration uses its CVT as a
continuously variable regulator (CVR) of the rotation speed of any one of the three rotators of the
planetary gear system (PGS). If two of the PGS rotator speeds are the input and output of the CVR,
there is a setting of the CVR that results in the IVT output speed of zero. The maximum output/input
ratio can be chosen from infinite practical possibilities through selection of additional input or output
gear, pulley or sprocket sizes without affecting the zero output or the continuity of the whole system.
The IVT is always engaged, even during its zero output adjustment.
IVTs can in some implementations offer better efficiency when compared to other CVTs as in the
preferred range of operation because most of the power flows through the planetary gear system and
not the controlling CVR. Torque transmission capability can also be increased. There's also possibility
to stage power splits for further increase in efficiency, torque transmission capability and better
maintenance of efficiency over a wide gear ratio range.
An example of a true IVT is the Hydristor because the front unit connected to the engine can displace
from zero to 27 cubic inches per revolution forward and zero to -10 cubic inches per revolution
reverse. The rear unit is capable of zero to 75 cubic inches per revolution. However, whether this
design enters production remains to be seen. Another example of a true IVT that has been put into
recent production[9] and which continues under commercial development [10] is that of Torotrak.
Ratcheting
CVT
The ratcheting CVT is a transmission that relies on static friction and is based on a set of elements that
successively become engaged and then disengaged between the driving system and the driven
system, often using oscillating or indexing motion in conjunction with one-way clutches or ratchets that
rectify and sum only "forward" motion. The transmission ratio is adjusted by changing linkage
geometry within the oscillating elements, so that the summed maximum linkage speed is adjusted,
even when the average linkage speed remains constant. Power is transferred from input to output only
when the clutch or ratchet is engaged, and therefore when it is locked into a static friction mode where
the driving & driven rotating surfaces momentarily rotate together without slippage.
These CVTs can transfer substantial torque, because their static friction actually increases relative to
torque throughput, so slippage is impossible in properly designed systems. Efficiency is generally high,
because most of the dynamic friction is caused by very slight transitional clutch speed changes. The
drawback to ratcheting CVTs is vibration caused by the successive transition in speed required to
accelerate the element, which must supplant the previously operating and decelerating, power
transmitting element.
Ratcheting CVTs are distinguished from VDPs and roller-based CVTs by being static friction-based
devices, as opposed to being dynamic friction-based devices that waste significant energy through
slippage of twisting surfaces. An example of a ratcheting CVT is one prototyped as a bicycle
transmission protected under U.S. Patent 5,516,132 in which strong pedalling torque causes this
mechanism to react against the spring, moving the ring gear/chainwheel assembly toward a
concentric, lower gear position. When the pedaling torque relaxes to lower levels, the transmission
self-adjusts toward higher gears, accompanied by an increase in transmission vibration.
Hydrostatic
CVTs
Honda DN-01 motorcycle: Swashplate animation.
The Japanese Type 10 tank uses Hydraulic Mechanical Transmission (HMT).
Hydrostatic transmissions use a variable displacement pump and a hydraulic motor. All power is
transmitted by hydraulic fluid. These types can generally transmit more torque, but can be sensitive to
contamination. Some designs are also very expensive. However, they have the advantage that the
hydraulic motor can be mounted directly to the wheel hub, allowing a more flexible suspension system
and eliminating efficiency losses from friction in the drive shaft and differential components. This type
of transmission is relatively easy to use because all forward and reverse speeds can be accessed
using a single lever.
An integrated hydrostatic transaxle (IHT) uses a single housing for both hydraulic elements and gearreducing elements. This type of transmission has been effectively applied to a variety of inexpensive
and expensive versions of ridden lawn mowers and garden tractors.
One class of riding lawn mower that has recently gained in popularity with consumers is zero turning
radius mowers. These mowers have traditionally been powered with wheel hub mounted hydraulic
motors driven by continuously variable pumps, but this design is relatively expensive.
Some heavy equipment may also be propelled by a hydrostatic transmission; e.g. agricultural
machinery including foragers, combines, and some tractors. A variety of heavy earth-moving
equipment manufactured by Caterpillar Inc., e.g. compact and small wheel loaders, track type loaders
and tractors, skid-steered loaders and asphalt compactors use hydrostatic transmission. Hydrostatic
CVTs are usually not used for extended duration high torque applications due to the heat that is
generated by the flowing oil.
The Honda DN-01 motorcycle is the first road-going consumer vehicle with hydrostatic drive that
employs a variable displacement axial piston pump with a variable-angle swashplate.
Naudic
Incremental CVT (iCVT)
The neutrality of this section is disputed. Please do not remove this
message until the dispute is resolved. (April 2012)
This is a chain-driven system which is advertised at *[2] Although an iCVT works, it has the following
weakness:
High frictional losses
The variator pulley of an iCVT is choked using two small choking pulleys. Here one choking pulley is
positioned on the tense side of the chain of the iCVT. Hence there is a considerable load on that
choking pulley, the magnitude of which is proportional to the tension in its chain. Each choking pulley
is pulled up by two chain segments, one chain segment to the left and one to the right of the choking
pulley; here if the two chain segments are parallel to each other, then the load on the choking pulley is
twice the tension in the chain. But since the two chain segments are most likely not parallel to each
other during operations of an iCVT, it is estimated that the load on a choking pulley is between 1 to 1.8
times of the tension of its chain.
Also, a choking pulley is very small so that its moment arm is very small. A larger moment arm reduces
the force needed to rotate a pulley. For example, using a long wrench, which has a large moment arm,
to open a nut requires less force than using a short wrench, which has a small moment arm. Assuming
that the diameter of a choking pulley is twice the diameter of its shaft, which is a generous estimate,
then the frictional resistance force at the outer diameter of a choking pulley is half the frictional
resistance force at the shaft of a choking pulley.
Shock and durability
The transmission ratio of an iCVT has to be changed one increment within less than one full rotation of
its variator pulley. Has to be changed one increment means that the transmission diameter of the
variator pulley, made generally from rubber, has to be changed from a diameter that has a
circumferential length that is equal to an integer number of teeth to another diameter that has a
circumferential length that is equal to an integer number of teeth; such as changing the transmission
diameter of the variator pulley from a diameter that has a circumferential length of 7 teeth to a
diameter that has a circumferential length of 8 teeth for example. This is because if the transmission
diameter of the variator pulley does not have a circumferential length that is equal to an integer
number of teeth, such as a circumferential length of 7½ teeth for example, improper engagement
between the teeth of the variator pulley and its chain will occur. For example, imagine having a bicycle
pulley with 7½ teeth; here improper engagement between the bicycle pulley and its chain will occur
when the tooth behind the ½ tooth space is about to engage with its chain, since it is positioned a
distance of ½ tooth too late relative to its chain.
Regarding the previous paragraph, the chain of an iCVT forms an open loop on its variator pulley that
partially covers its variator pulley such that an open section, which is not covered by the chain, exist.
This is similar to a sprocket of a bicycle where there is a section of the sprocket that is covered by its
chain, and a section of the sprocket that is not covered by its chain. During one complete rotation, the
toothed section of the variator pulley of an iCVT passes by the open section and re-engages with the
chain. Here if the transmission diameter of the variator pulley does not represent an integer number of
teeth, improper re-engagement between the teeth of the variator pulley and its chain will occur. Also,
the transmission diameter of the variator pulley cannot be changed while the toothed section of the
variator pulley is covering the entire open section of its chain loop. Since this is similar to where a plate
is glued across the open section of a chain loop, which does not allow expansion or contraction of the
chain loop as required for transmission diameter change of the variator pulley. Therefore the
transmission diameter of the variator pulley has to be changed one increment during an interval where
the variator pulley rotates from an initial position where a portion of the toothed section of the variator
pulley is positioned at the open section of the chain loop but not covering the entire open section, to
the final position where the toothed section of the variator pulley passes by the open section of the
chain loop and is about to re-engage with the chain. Since it takes less than one full rotation to rotate
the variator pulley from its initial position to its final position mentioned in the previous sentence, the
transmission diameter of the variator pulley has to be changed one increment within less than one full
rotation.
In addition, as the transmission diameter is increased, the chain has to be pushed up the inclined
surfaces of the pulley halves of the variator pulley, while the tension in the chain tends to pull the chain
towards the opposite direction. Hence a large force, which is larger than the tension in the chain, is
required to change the transmission diameter. Since the transmission ratio has to be changed within
less than one full rotation of the variator pulley, a large force has to be applied on the pulley halves
within a very short duration. If for example the variator pulley rotates at 3600 rpm, which is equivalent
to 60 revolutions per second, then the force required to change the transmission ratio has to be
applied within 1/60 seconds. This would be similar to hitting something with a hammer. Therefore, here
significant shock loads are applied to the variator pulley during transmission ratio change that
increases the transmission diameter. These shock loads my cause comfort problem for the driver of
the vehicle using an iCVT. Also an iCVT has to be designed as to be able to resist these shock loads
which would most likely increases the cost and weight of an iCVT.
Torque transfer ability and reliability
The teeth of the variator pulley of an iCVT are formed by pins that extend from one pulley half to the
other pulley half and slide in the grooves of the pulley halves of the variator pulley. Here torque from
the chain is transferred to the pins and then from the pins to the pulley halves. Since the pins are
round and the grooves are curved, line contact between the pins and the grooves are used to transfer
force from the pins to the grooves. The amount of force that can be transmitted between two parts
depend on the contact area of the two parts. Since the contact areas between the pins and their
grooves are very small, the amount of force that can be transmitted between them, and hence also the
torque capacity of an iCVT, is limited.
Another possible problem with an iCVT is that the pins of the variator pulley can fall-out when they are
not engaged with their chain, and wear of the pins and the grooves of the pulley halves can cause
some serious performance and reliability problems.
Cone
CVTs
The Evans friction cone, a type of cone CVT
A cone CVT varies the effective gear ratio using one or more conical rollers. The simplest type of cone
CVT, the single-cone version, uses a wheel that moves along the slope of the cone, creating the
variation between the narrow and wide diameters of the cone.
The more-sophisticated twin cone mesh system is also a type of cone CVT. [11][12]
In a CVT with oscillating cones, the torque is transmitted via friction from a variable number of cones
(according to the torque to be transmitted) to a central, barrel-shaped hub. The side surface of the hub
is convex with a specific radius of curvature which is smaller than the concavity radius of the cones. In
this way, there will be only one (theoretical) contact point between each cone and the hub at any time.
A new CVT using this technology, the Warko, was presented in Berlin during the 6th International CTI
Symposium of Innovative Automotive Transmissions, on 3–7 December 2007.
A particular characteristic of the Warko is the absence of a clutch: the engine is always connected to
the wheels, and the rear drive is obtained by means of an epicyclic system in output.[13] This system,
named “power split”,[14] allows the engine to have a "neutral gear":[15] when the engine turns (connected
to the sun gear of the epicyclic system), the variator (i.e., the planetary gears) will compensate for the
engine rotation, so the outer ring gear (which provides output) remains stationary.
roller CVT
The working principle of this CVT is similar to that of conventional oil compression engines, but,
instead of compressing oil, common steel rollers are compressed.[16]
The motion transmission between rollers and rotors is assisted by an adapted traction fluid, which
ensures the proper friction between the surfaces and slows down wearing thereof. Unlike other
systems, the radial rollers do not show a tangential speed variation (delta) along the contact lines on
the rotors. From this, a greater mechanical efficiency and working life are claimed.
Planetary
CVT
In a planetary CVT, the gear ratio is shifted by tilting the axes of spheres in a continuous fashion, to
provide different contact radii, which in turn drive input and output discs. The system can have multiple
"planets" to transfer torque through multiple fluid patches. One commercial implementation is
theNuVinci Continuously Variable Transmission.
History
Split annulus, compound planet, epicyclic gears of a
car rear-view mirror positioner
Reduction gears on Pratt & Whitney Canada
PT6 gas turbine engine.
See
also
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