Look behind you AEA.indd

Look behind you AEA.indd
LOOK BEHIND YOU!
A guide to trailed equipment braking
Contents
Introduction
Page 03
The effect of speed and vehicle weight
Page 04
Current UK agricultural vehicle
braking legislation
Page 06
Effect of gradient on vehicle braking performance
Page 08
Trailed equipment specifications
Page 10
Actuator specifications
Page 12
Getting trailed equipment braking systems
to “stop their weight”
Page 16
A fresh approach to tractor and trailed equipment purchase and
maintenance
Page 18
Acknowledgements
This document has been prepared through the Service Managers Group of the
Agricultural Engineers Association (AEA) and represents a common understanding
relating to the use of trailers and trailed implements with tractors.
The AEA represents the major tractor supply companies including: AGCO Ltd
(Challenger, Fendt, Massey Ferguson, Valtra) Case IH, Claas, Deutz, John Deere,
JCB, Kubota, Kioti, Landini, McCormick, New Holland, Same, Deutz-Fahr,
Lamborghini, Zetor, Yanmar plus suppliers of trailers, trailed equipment and
components.
Produced with assistance and advice from AGCO, BAGMA, the HSE and SCARLETT
RESEARCH LTD. The HSE do not endorse any commercial product or service.
SCARLETT RESEARCH LTD
02
Introduction
With the current trend for higher road speeds of 40-50 km/h in
the UK with the use of “eco transmissions”, it is imperative that
we have the correctly matched trailed equipment for the intended
speed and loadings seen in daily use.
Currently, we have an industrywide problem of “under
braked” trailed equipment
being used. This in turn can
lead to premature tractor brake
wear or even failure, causing
customer dissatisfaction and
conflict over who covers the
repair costs.
In this guide, we aim to
highlight the relevant legislation
and to guide you through some
of the available specs/speed
ratings of trailed equipment.
With this guide you will be
able to purchase the correct
specification of tractor or
trailed equipment for your
desired speed and application.
This will help to build a
pro-active approach to
investigating trailed equipment
and aid the prevention of
tractor brake failure.
03
The effect of speed and
vehicle weight
When travelling, a tractor and
trailer combination builds up
kinetic energy. To slow down or
stop, the kinetic energy must
be converted to heat through
friction in the tractor and trailed
equipment braking systems.
Both systems are designed to
withstand and dissipate the
heat generated at their rated
operating weights and top
road speeds.
This is fine if the braking
systems are designed to take
these speed induced loads, but
those fitted to many agricultural
trailers are not.
In these instances, braking
while “over speeding” will
subject the tractor and trailer
braking system to heat cycles
outside its design specification
and cause accelerated wear.
3
Travelling faster or operating
at greater weights increases
the kinetic energy levels the
braking systems have to cope
with, but there’s a sting in
the tail in the form of forward
speed.
If speed were increased by
50% (e.g. from 20 to 30 mph),
kinetic energy would increase
by almost 150%,
(see chart opposite).
What may be thought to be
an everyday speed increase
actually places nearly 2½ times
more load on the tractor and
trailer’s braking systems.
2.5
2
Kinetic energy (relative to 20mph)
Forward speed
If, for example, the total (gross)
weight of a tractor/trailer
combination were increased
by 50%, at any given speed its
kinetic energy and the resulting
braking system load would also
increase by 50%.
If not adjusted frequently, the
responsibility for braking is
progressively transferred to the
tractor. This then causes rapid
wear of the tractor’s system
and eventual long term damage
to braking components.
1.5
1
0.5
0
20
25
Vehicle Speed (mph)
Current (Legal GTW) tractor trailer combinations generate
nearly 150% more energy when travelling at 50Kph.
04
30
05
Current UK agricultural
vehicle braking
legislation
Prior to the introduction of harmonised European braking regulations for
agricultural trailers & trailed equipment, UK legal requirements are specified
by the Road Vehicles (Construction and Use) Regulations (1986) and
subsequent relevant amendments.
The C&U Regulations
effectively split agricultural
trailers and trailed equipment
braking requirements into
two speed related categories.
Up to 20 mph/32 km/h:
C&U Regulation 16
applies.
•
06
•
Trailer service braking
efficiency must be 45%
minimum.
•
Service braking system
must be dual-line and
failsafe, but not necessarily
pneumatic. The driver
must be able to apply the
trailer brakes should any
part of the towing vehicle’s
braking system fail. The
trailer brakes should also
apply (automatically) if the
tractor-trailer accidentally
separates.
Trailer service brake must
generate at least 25%
braking efficiency when
fully-laden.
•
Service braking system
must act on at least half
the wheels/axles.
•
Single-line hydraulic or
pneumatic brake actuation
is acceptable, but if trailer
gross weight exceeds
14230 kg, the service
brakes must be operated
directly from the tractor’s
service braking system.
•
Above 20 mph/32 km/h:
C&U Regulation 15 applies as
for commercial vehicles.
Parking brake must act on
at least half the wheels &
stop them from rotating
when applied!
•
ABS (antilock braking) has
been required since 2002.
•
Parking brake must hold
the laden, uncoupled trailer
stationary on a 16% slope.
So, if travelling faster than 20
mph, UK law requires nearly
twice as much trailer braking
performance plus additional
safety features to deal with
the unexpected. In either case
there is a legal requirement to
adjust and maintain braking
systems in good and efficient
working order.
Coincidentally, the higher
speed requirements (except
ABS) are very similar to those
proposed for future EU-wide
introduction.
Trailer braking force
Drawbar load max 3 tonnes
Total trailer axle load
Construction & Use
Regulations also specify
maximum operating weights
for both trailers and tractor/
trailer combinations
Conventional
‘unbalanced’
agricultural trailer:
•
Max. weight =
18,290 kg
Tractor/trailer combination:
•
Max train weight =
24,390 kg
So what is braking efficiency?
Braking efficiency is the overall measure of vehicle braking performance (the
terms braking rate or brake ratio are also sometimes used).
Braking force exerted by the wheels
Braking efficiency (%) =
x 100
Vehicle weight
So, a 14 tonne capacity twin axle trailer, loaded evenly to have a gross weight
of 18.29 tonnes, would typically carry 8 tonnes on each axle. To achieve 25%
braking efficiency will therefore require each axle to produce 2 tonnes of braking
force.
This would increase to 4 tonnes braking force to meet the approx. 50% braking
efficiency requirement for above 20 mph operation.
In practice, current UK
regulations prohibit the
use of trailers exceeding
approximately 14 tonnes
carrying capacity.
07
Effect of gradient
upon vehicle braking
performance
10
%
Gravity will accelerate the
combination down the gradient.
This effect has to be overcome
before deceleration can begin.
30% braking efficiency
available to decelerate
combined unit when on
level ground.
Only 20% braking efficiency
available to decelerate the
combination on a 10% gradient.
A tractor/trailer with a combined braking efficiency of 30% can apply all that braking effort into stopping on
level ground, but when travelling down a gradient it must overcome the weight acting down the gradient first.
So, on a 1 in 10 gradient (10 %) this leaves only 20% for stopping.
A vehicle travelling down a
gradient requires more braking
effort to stop than a vehicle
travelling at the same speed
on level ground.
This is because on a gradient,
gravity will try to accelerate the
vehicle downwards; the brakes
have to overcome this effect
before they can begin to stop
the vehicle.
08
The best way to consider the
effect of gradients is to think in
terms of percentage gradient.
A 1 in 10 gradient is a 10 %
slope. In this way the fraction
of a vehicle’s weight acting
down the gradient is equal to
the percentage slope of the
gradient.
So, to ensure that a vehicle can
travel safely down a gradient,
the total braking efficiency
must be equal to, or greater
than the gradient expressed as
a percentage, plus the amount
of braking efficiency necessary
to stop the vehicle.
09
Trailed equipment
specifications
With the trend for 40 and 50 km/h tractor speeds increasing, it is imperative
that the trailed equipment braking system is matched to the speed and load
requirements of the user and routinely maintained, if tractor brake failure is to
be avoided.
10
What is the ‘Foundation
Brake’?
All other things being equal,
trailer braking performance
depends upon brake size and
the force with which they are
applied. The brake drum and
shoes form the ‘foundation’ of
the braking system. They are
referred to in terms of drum
diameter and shoe width.
Higher capacity axles and/or
those used at higher speeds
tend to be fitted with larger
foundation brakes. Making the
wrong choice for your intended
use could prove to be a costly
mistake.
What is a ‘High-Speed/
Commercial Axle’?
Caution required here!!
There is a general opinion that
any trailer with 10-stud hubs
and super single wheels and
tyres is high-speed spec. This
is not necessarily the case, as
this wheel equipment is often
used on Ag. spec 20 mph
(32km/h) max. trailers.
What is an ‘Ag. spec axle’?
These are generally 6 or 8-stud
hubs with a typical brake drum
diameter and shoe width of
approx. Ø300x60 mm or
10-stud hubs with Ø400x
80 mm foundation brakes.
These axles are usually
intended to produce 25%
braking efficiency (This spec
can be used at up to 20 mph/
32 km/h). Ag. spec brakes also
use ‘flat’-type operating cams
which tend not to provide very
progressive brake control.
The sheer size of the drum
indicates how these brakes
are able to dissipate the heat
produced during higher-speed
braking. Depending on the air
chamber or brake ram size
fitted, these axles/brakes will
achieve 50% braking efficiency
when fully-laden.
A true “high-speed” spec axle
will usually have Ø406x120 mm
or Ø420x180 mm foundation
brakes operated by ‘S’-type
cams to give progressive
control.
If fitted with dual-line failsafe
and ABS systems, trailers
featuring these axles can be
towed above 20 mph (32km/h),
although UK C&U Regulations
prohibit such speeds if
used behind conventional
(unsuspended) agricultural
tractors).
So, you’ve selected decentsized axles and foundation
brakes for the operating loads
and maximum road speeds
your equipment will work at.
Problem solved?
Not necessarily!
Unless the brake actuator
(hydraulic ram or air chamber)
produces sufficient force,
overall braking performance
will be compromised. (See
graph on page 12).
Ø400 x 80mm Ag. spec: Bonded shoe ‘flat’ cam
actuation
Ø420 x 180mm high-speed brake: Riveted shoe
‘S’ cam actuation
Ø25mm actuating ram: Ag. spec
Ø35mm actuating ram - Commercial spec
11
Actuator specifications
Actuator specification
Trailer brake actuating rams
and air chambers come in a
range of diameters; bigger
rams produce more force.
Brake actuators push the brake
lever arms/slack adjusters to
operate the brakes and can
be attached in different holes
to achieve various degrees of
leverage.
Trailer manufacturers match
the brake actuator size, lever
arm length and foundation
brake size to suit the intended
operating speed and load
capability of the trailer. Operate
heavier or faster and the
braking system will be both
inadequate and overloaded.
Brake ram diameter typically
goes up in 5 mm steps; moving
up a ram size will increase
brake application force (but
not necessarily overall braking
efficiency) by 30-50%.
Most air-braked trailers
intended for higher-speed (50%
braking efficiency) operation
are fitted with Type 24 air
chambers.
A hydraulic braking system
would need Ø35 mm rams
to achieve comparable
performance, so don’t be
surprised if the Ø25 mm
hydraulic rams fitted to
dual-supply (air + hydraulic)
actuators don’t seem to deliver
the same braking performance
as the pneumatic system.
Where both air and oil are
fitted to a trailer, manufacturers
frequently size the hydraulic
system to deliver 25% braking
efficiency (for use up to
20 mph (32 km/h), specifying
the pneumatic system to
provide 50% efficiency for use
above 20 mph (32 km/h).
Unless both systems are fitted
with load sensing, the chances
are that the hydraulic system is
only rated for lower-speed use.
Brake actuator force (newtons) @ 100 bar hydraulic pressure/6.5 bar air pressure
14000
12000
10000
8000
6000
4000
2000
0
Ø20
Ø25 / T12
Ø30 / T16
Brake ram diameter
T20
Ø35 / T24
Air chamber size
The effect of brake actuator diameter/size upon on actuation force
12
Ø40 / T30
100%
90
o
75
o
60
o
BRAKING
POWER
97%
BRAKING
POWER
87%
BRAKING
POWER
90O angle (Brakes on) giving 100% braking force
45
o
71%
BRAKING
POWER
Regular actuator adjustment is important for two reasons: firstly, the reaction
speed of the foundation brake is directly affected by the distance the actuator
and lever arm has to travel. Secondly, the angle between the actuator and
the lever arm has a direct effect upon brake shoe application force and the
overall braking effort produced. Maximum force is applied when the angle is
90 degrees.
The effect of brake actuator/lever arm
angle upon efficiency of force transfer
Screw type brake lever arm aids easy
adjustment
Splined brake lever arm: free-play adjustment requires removal
from camshaft and replacement on alternative spline
13
Too much braking?
If the foundation brakes and
actuators are large enough,
50% trailer braking efficiency
can be achieved.
But remember, this requirement
is for a laden trailer. When
unladen, the brakes and rams
will generate the same braking
force, but on a vehicle weighing
70% less! A situation called
‘over-braking’ results, causing
the trailer wheels to lock,
rapid tyre wear and possibly
trailer instability.
Load sensing
Load sensing provides a
solution to the problem by
adjusting the braking effort
(actuator pressure) relative to
the load being carried. A simple
(hydraulic or pneumatic) valve
senses deflection of the trailer
suspension with load and
reduces brake line pressure
when partially loaded or
unladen. Load sensing should
always be specified for higher
performance (50% efficiency)
braking systems to avoid
wheel locking and provide a
controllable package.
Brake response time
An important factor in
preventing tractor brake
failure is to ensure the trailer
is contributing to overall
retardation as soon as possible.
For example; when braking to
decrease speed on a downhill
run, the tractor brakes will start
to retard the load immediately,
taking the entire combination’s
kinetic energy until the trailer
brakes respond. If trailer brake
response time is good, the
trailer system will contribute to
tractor/trailer deceleration as
quickly as possible, helping to
prevent tractor braking system
overload.
14
Hydraulic brake hose
diameter
Pneumatic trailer brakes
respond rapidly but hydraulic
systems are believed to be
slow. Hydraulic brakes have
to push a cold column of oil
along the trailer chassis right
back to the axles. The usual
ؼ” hose, fitted to keep costs
down, restricts oil flow and
increases brake response time.
If larger rams are fitted to get
more braking effort, greater oil
volume will be required,
further increasing brake
response time.
Replacing the trailer hydraulic
brake hose with a larger
(ؽ”) size has been shown
to dramatically improve
brake response time to levels
equivalent to air brakes!
Dual-line and failsafe
These features are required
for trailers which are operated
above 20 mph (32 km/h).
Dual-line pneumatic braking
systems already meet the
requirements, but existing
single-line hydraulic systems
need modification to be legal
(and safe!).
The law requires that the trailer
brakes must automatically
be applied if it accidentally
separates from the tractor.
Also, should the tractor
brakes fail (e.g. the engine
stops, a hose fractures), the
driver must be able to apply
the trailer brakes to bring the
combination to a halt.
These requirements are usually
met by placing a pressurised
(oil or air) reservoir on the
trailer. Should something go
wrong, the stored pressure is
triggered (automatically or by
the driver) to apply the trailer
brakes.
ABS
Antilock braking systems
are designed to prevent
wheel locking when braking
on slippery surfaces. They
are currently required on
agricultural trailers used
above 20 mph (32 km/h)
because of commercial vehicle
braking legislation. However,
it seems unlikely that future
EU Agricultural Braking
Regulations will require trailer
ABS at anything other than very
high speeds.
Some consider ABS to be an
acceptable substitute for load
sensing as a means to avoid
over-braking when unladen, but
this is not the case.
The ABS system ends up trying
to control an over-capacity
(when unladen) braking system
receiving full system pressure.
Rather like trying to use a
sledgehammer for cracking
nuts. Load sensing reduces the
size of the ‘hammer’ to more
manageable proportions.
Pneumatic load sensing (Note: balance beam between
axles and ‘screw-type’ slack adjusters)
Maintenance
Braking system adjustment,
to compensate for shoe lining
wear, is often overlooked and
is predominantly the root cause
of trailer braking problems. It
is important that system freetravel adjustment and lining
inspection are carried out on a
regular basis as this will have a
significant effect upon braking
performance.
As brake linings wear, system
free-travel increases and the
brake rams/actuators extend
further to compensate. This
increases trailer brake response
time because the rams have
to travel further to apply the
brakes.
Most brake rams have approx.
75 mm available stroke. If lining
wear increases free-travel to
the point where the rams are
fully-extended, no force will be
Hydraulic combined load sensing/emergency fail safe valve
(Note: balance beam between axles)
applied to the shoes and the
vehicle will have no brakes.
This then leaves the tractor to
stop the entire load.
Regular trailer brake free-travel
adjustment is vital if tractor
brake failure is to be avoided.
Use of under-sized foundation
brakes at higher speeds may
well increase shoe lining wear
rates beyond those previously
experienced. But regular
adjustment is only likely to
happen if it’s easy to do, and
often it isn’t on older trailers.
This is not a substitute for
brake drum removal and lining
inspection, but at least it
ensures the brake shoes will
actually be pushed onto the
drums during braking.
A simple and cheap solution is
to retro-fit manual ‘screw-type’
brake slack adjusters.
These replace the brake
lever arms and incorporate
an internal worm and wheel.
Turning an external nut rotates
the lever arm on the brake
camshaft and adjusts the
system free-travel.
15
Getting trailed
equipment braking
systems to
“stop their weight”
Remember, a fully-loaded trailer can account for up to 75% of a
tractor-trailer combination’s kinetic energy.
It is imperative that any
trailed equipment has a
braking system of the correct
specification for the top speed
travelled. It should also be
maintained to ensure that it
contributes appropriately to
the overall braking effort of the
combination, to prevent tractor
brake overload.
Air brakes
Currently, pneumatic brakes
give quicker brake actuator
reaction times, making them
more sensitive and easier to
control than conventional,
simple hydraulic brakes. They
are also cleaner and cheaper to
operate.
Air brakes are always used on
commercial lorry trailers and
may be the way
forward for agricultural
equipment.
16
However, upgraded
hydraulic brakes can deliver
comparable performance.
In summary
• Free-travel adjustment
and shoe lining inspection.
This will keep the brakes
working at maximum
efficiency and reduce
response time to a
minimum.
•
Increase brake actuator
ram size to give greater
braking force.
•
Fit screw type brake lever
arms: this encourages
and assists regular brake
adjustment.
•
Increase hydraulic
brake hose diameter to
improve system response
characteristics.
•
Load sensing – essential
when increasing braking
performance for higherspeed operation, if wheel
locking and excessive tyre
wear are to be avoided.
•
Air brakes give quicker
reaction times with greater
control than current
hydraulic systems, but
hydraulic brakes can be
comparable if specified
correctly.
17
A fresh approach to tractor and trailed
equipment purchase and maintenance
Think about...
•
The speeds that your
current tractor is capable of
travelling.
Is your current equipment
appropriate for the higher
speeds that could be
reached?
Remember a new trailer is likely
to outlast 2-3 replacement
tractors over its ‘frontline use’
lifetime. Higher-spec. trailer
brakes may cost more, but
they will cost less to run and be
much cheaper (and safer and
legal) in the long run.
18
Consider adding air brakes
to new tractor and trailed
equipment specifications,
or upgrade existing
hydraulic trailer brakes
and supplement them with
load sensing and failsafe
systems.
•
When Purchasing 50 km/h
tractors, have your current
trailed equipment fleet
inspected and tested at the
point of tractor installation.
•
Ask your supplier to advise
on any necessary repairs
or changes to tractor
and trailed equipment
specification required to
prevent possible problems
and comply with legislation.
•
Make sure, as an operator,
that you understand the
implications of “over
speeding” and its effects
on tractor and trailed
equipment braking systems
•
Ensure you have a thorough
maintenance plan for your
trailed equipment fleet as
this is often overlooked.
Components within trailed
equipment braking systems
may be wearing faster than
you think when used at
higher speeds with newer
tractors. Prevention is better
than cure!
19
The information in this document, which does not purport to be comprehensive,
has been provided by the AEA Service Managers Group. Whilst this information
has been prepared in good faith, no representation or warranty, express or
implied, is or will be made and no responsibility or liability is or will be accepted
by the AEA as to or in relation to the accuracy or completeness of this document.
No responsibility or liability is or will be accepted by the AEA in respect of any
changes made by third parties to the content of this document.
AEA
Samuelson House
Forder Way
Hampton
Peterborough
PE7 8JB
www.aea.uk.com
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