900 Mount Gear adjustMents - Astro

900 Mount Gear adjustMents - Astro
900 Mount Gear Adjustments
Procedures to Remove Worm Gear Backlash and to Minimize Gear Play
A telescope mount is a precision piece of equipment that allows a telescope to be pointed and to track with sub arc-second
accuracy. To be able to do so requires that the gear mesh be periodically checked and adjustments occasionally performed.
This becomes more important if the mount is frequently jostled around in a vehicle while transported to observing sites, than if
the mount remains fixed in an observatory.
Accurate meshing of the worm gear to the worm wheel is important for peak performance of your mount. A gear mesh that
is too loose will result in backlash, which can be problematic, especially where accurate declination reversal is required for
excellent guiding. Gear mesh that is too tight can add unnecessary stress to the motors and can result in R.A. tracking that is
not smooth, or Dec. guider movements that are not precise.
Definitions:
Backlash (as defined by Astro-Physics):
Backlash is the result of a gap between the worm gear teeth and the worm wheel teeth. For clarity of communication, we
restrict the term backlash to ONLY apply to this worm gear to worm wheel mating. If a looseness can not be felt when
making the tests that follow, then the adjustments described here will not be needed. Please note that other issues can
give the false appearance of backlash when guiding or making fine adjustments with the direction buttons. However, only
the tests below will truly identify backlash as we define it.
Play (as defined by Astro-Physics):
The movement in an axis where there was none before. It can be felt as a back and forth movement of any gearing setup
or seen as an image shift in the eyepiece where there should be none. All backlash is evidenced by play, but not all play is
necessarily backlash in the way that we define it. By necessity, there will always be some small amount of play in any spur
gear system. This play is required in order for the gears to be able to move. There are five principal sources of play that
occur in gear systems:
1. Backlash - (true worm wheel / worm gear backlash): The result of a gap between the worm screw gear and the worm
wheel. This can be felt as a slight play in the axis when moved back and forth as if to rotate it.
2. Worm gear / worm spur gear: The play caused by a loose set screw or similar mating issue between the worm gear’s
shaft and the spur gear to which it is mated. This is easily remedied and should be zero in normal operation.
3. Reduction spur gear-set play: The cumulative play that occurs between the motor drive shaft’s first spur gear and the
spur gear that is on the worm shaft.
4. Spur gear cluster separation: Although extremely rare, this can occur when one member of a two-gear cluster breaks
free from its paired partner. This has only been seen in the Delrin / brass cluster that is the next set after the motor in
some of our mounts. When this happens, the smaller brass spur gear is able to turn without the larger Delrin gear also
turning.
5. Worm gear tangential end play: A motion of the worm gear that is tangential to the worm wheel. It generally occurs
because of inadequate pressure or preload on the bearings that hold the worm screw gear in place.
Part 1 - Backlash: Gear Mesh Issues
Factors contributing to gear mesh problems from greater to lesser:
1. Transporting mounts. Carrying or shipping mounts to local and distant observing sites causes mounts to experience
vibration and jostling which can put pressure on the gear boxes and change meshing.
2. Seasonal temperature changes. Mounts located in geographical areas that experience extreme temperature differences between summers and winters will change gear mesh. A mount with proper gear mesh in the summer may show
a loose meshing in the winter and vice-versa.
3. Time and wear. Over time, gear wear will cause a small change in the gear mesh.
1
Test for Right Ascension (R.A.) and Declination (Dec.) Backlash
Proper gear mesh is when no looseness is felt in the worm gear / worm wheel mesh and the spur gears turn freely without
binding. This two-step process is discussed below.
The 900 mount should be firmly attached to its pier, powered off and with clutch knobs engaged. The mount / telescope
should be put into a Park 3 position so that proper centering and meshing of the gears will take place without undue stress
or pressure.
R.A. Axis:
The R.A. test is done with the counterweight shaft pointing down in the Park 3
position. Place your hand near the end of the counterweight shaft and move
the shaft back and forth as if to rotate the R.A. axis. This test can be done with
the mount alone on its pier or with the telescope and counterweights attached.
The play you are looking for is rather subtle. You will have a better sense of
feel if you hold the counterweight shaft with your fingertips. Be sure that the
counterweight shaft is not loose and wiggling.
If no play is felt, then you do not have an R.A. backlash problem.
If play is felt when attempting to rotate the shaft back and forth, proceed with the
adjustment as described below.
Dec. Axis:
The Dec. test is done with the telescope pointing towards the Pole in the Park 3
position. Place your hand near the end of the telescope focuser, or at the end of the
cradle plate, if the telescope is off the mount. Be sure that whatever you are using
is itself solid and flex-free. Move the telescope or plate back and forth as if to rotate
the Dec. axis. This test must be done with the assembled mount on its pier. Again,
the play you are looking for is rather subtle. You may want to grab the scope or plate
at each end and use two hands. Again, fingertips will tend to give the most accurate
sense of feel.
If no play is felt, then you do not have a Dec. backlash problem.
If play is felt, proceed with the adjustment as described below.
Adjustments - Apply the Following Steps to Both the Dec. and R.A. Axes
Tools needed:
●●
●●
●●
●●
5/32” hex key
5/64” hex key
3/32” hex key (see sidebar)
Paper towel for greasy fingers
Procedure to Tighten Gear Mesh
1. Put the mount into Park 3 position. This is very important to ensure that there is not uneven pressure on the gears
due to an out of balance load when gear meshing.
Important Sidebar Instructions (applies to mounts produced prior
to the end of 2005): Early production runs of the 900GTO mounts are
identified by two set screws located on the top of both the R.A. and
Dec. motor / gearbox housings (not present in more recent mount
runs). These additional instructions (lettered “a” through “d” below) apply only to these mounts and are to be disregarded if the your mount
does not have these additional screws.
a) Using a 3/32” hex key, remove the set screws shown in the photo
at right. Retain these screws for re-installing.
b) Next, insert a 5/64” hex key into the hole opened by the set screw
2
removal. Pass it through the entire housing and into the
set screw that is in the painted bracket on other side. (See
photo with insert at right) Completely remove the set
screws (they will be discarded, as it has been found that
their use may increase worm gear pressure and are no
longer recommended).
c) You will need to remove the rear cover (with servo cable
connector) in order to retrieve the set screws which will have
dropped inside the housing. Simply use the 5/64” hex key
to remove the four socket head screws attaching the cover
and retrieve the two screws for disposal. Be careful that the
connector’s wiring is handled gently.
Rear
Cover
d) Re-attach this cover with the four socket head screws and
then restore the two set screws (referred to in “a” above) to their positions in the housing. They serve the purpose
of hole plugs for dust and moisture protection.
(End of special sidebar instructions.)
2. Loosen the gearbox lock-down screws. Using a 5/32” hex key, loosen both socket head cap screws shown in
the photo below left. Note that these screws are stainless steel (not painted). You do not want to loosen the painted
screws, as those screws attach the bracket to the mount. Loosen the indicated screws sufficiently to be able to rock
the motor / gearbox, but not so much that it floats around.
ck
Ro
3. Properly seat the worm gear. Gently rock the motor / gearbox from side to side while applying a gentle pressure to
be sure that the worm is fully seated in the gears of the worm wheel. The
trick is to feel the center-point where the teeth of the gears are fully engaged. This action moves the worm gear closer to the worm wheel, which
removes the gap that you felt earlier. Removing the space between the
gears is a delicate adjustment. Use only a gentle pressure, such as the
weight of your hand resting on a table.
L
Lock
Down
Screws
R
4.Re-tighten the lock-down screws. Snug the LEFT screw first. It is
critical for proper worm mesh to snug the LEFT screw first. As you snug,
rock the box slightly so that it finds its center as the screw is gradually
tightened (remember the gentle hand pressure). It is best to hold the hex
key by the short end when doing the first tightening. Next, snug the right
screw. Repeat the “left bolt first, right bolt second” tightening procedure,
this time using the longer end of the hex key.
5. Re-check for looseness. Repeat your earlier test to see if the backlash has been removed. If you still feel backlash,
then repeat the re-meshing procedure described in Steps 2-5. Repeat until correct mesh is achieved.
NOTE: These screws are not the lug nuts that hold the wheel onto your car. If you are unsure how tight to make the
attachment screws, I would suggest that you err on the side of caution and don’t risk over tightening. It is easier to do this
whole process over making everything a bit tighter the second time around than it is to undue the damage from too heavy
a hand on the hex key. We have found that a good practice is to have the long end of the hex key in the hole, so that you
only have the short end for leverage. Make it as tight as you can with this short lever, and then reverse the hex key and
tweak the tightness by no more than 5-10 additional degrees.
Check for Excess Tightness in the Dec. Gearbox:
1. Put the mount into a Park 3 position. This is very important to ensure
that there is not uneven pressure on the gears due to an out of balance
load when gear meshing. Be sure that the mount is powered off.
2. Remove the Gearbox Cover. Please see the photo at right to locate the
screws to be removed. Use a 5/64” hex key.
3. Rotate the Foremost Spur Gear. The gear should turn freely with your fingers a full turn in both directions. If it is not easy to turn, proceed to Steps
4 and 5. Otherwise, restore the cover and you are done with Dec. (See
following photo at left)
3
5.Re-tighten the lock-down screws. Snug the screws from left to right. Once all are snug,
return to the left screw and finish tightening. Again rotate the spur gear in both directions to
ensure that the spur gears still turn freely and also check for looseness in the mesh.
ot
ate
4.If tight, loosen the gearbox lock-down screws. (Refer to the photo on previous page)
If the worm gear is too tight, and it is difficult to rotate the foremost spur gear, try the following solution. Holding the Gearbox in place with finger pressure only, slightly loosen the
lock-down screws again. With no hand pressure applied to the housing, attempt to rotate the
aluminum spur gear again. The rotation of the spur gear should release the excess pressure
between the worm and worm wheel. If it does, re-tighten the screws and check for backlash
play. If the aluminum spur gear cannot be rotated with this action, remesh the gears as
described previously. Repeat until correct mesh is achieved.
R
6.Replace the Gearbox Cover. Using the 5/64” hex key, replace the six gearbox cover
screws and you are finished.
Check for Excess Tightness in the R.A. Gearbox:
Important: The instructions to check for excessive tightness in the R.A. axis are similar to those for the Dec. axis
with one very important exception. The spur gears need to return to the original gear angle following rotation
so that the stored PE Curve is not lost. It will be necessary to mark both gears shown since the lower gear turns
more rapidly and will make several complete turns while rotating the top gear a full turn in each direction. Use a
pencil so that the marks can be removed for a future mesh check. If you are a visual observer or an imager who
does not use the PE correction, then the PE Curve is not important and you can check both gearboxes identically
without concern for gear angle. However, if the R.A. gear angle is changed, it will be necessary to turn off PE
correction or, alternatively, install a new PE Curve using PEMPro™.
1. Put the mount into a Park 3 position. This is very important to ensure that there is not uneven pressure on the gears
due to an out of balance load when gear meshing. Be sure that
the mount is powered off.
Make Two Pencil
4. Rotate the Foremost Spur Gear. The gear should turn freely
with your fingers a full turn in both directions. If it is not easy to
turn, proceed to numbers 5 through 7 in these instructions. If the
gear turns freely, re-establish the gear angle via the pencil marks,
restore the cover, and you are finished.
tate
3. Make pencil marks as shown in the photo at the right. Place
marks on both spur gears so that they can be returned to the
same gear angle following the test.
Marks & Note
Position
Ro
2. Remove the Gearbox Cover. Please see the photo on the previous page to locate the screws to be removed. Use a 5/64” hex
key.
5. If tight, loosen the gearbox lock-down screws. Refer to the
photo on the previous page. If the worm gear is too tight, and it is difficult to rotate the foremost spur gear, try the
following solution. Holding the Gearbox in place with finger pressure only, slightly loosen the screws again. With no
hand pressure applied to the housing, attempt to rotate the aluminum spur gear again. The rotation of the spur gear
should release the excess pressure between the worm and worm wheel. If it does, re-tighten the screws and check for
backlash play. If the aluminum spur gear cannot be rotated with this action, remesh the gears as described previously.
Repeat until correct mesh is achieved. Re-establish the gear angle so that the stored PE curve is not lost.
6. Re-tighten the lock-down screws. Snug the screws from left to right. Once all are snug, return to the left screw and
finish tightening. Again rotate the spur gear in both directions to ensure that the spur gears still turn freely. Re-establish the gear angle via the pencil marks. Make a quick check for gear mesh looseness, as described on the previous
page.
7. Replace the Gearbox Cover. Using the 5/64” hex key, replace the six gearbox cover screws and you are finished.
Remember: If the R.A. gear angle is changed, it will be necessary to turn off PE correction or, alternatively, install
a new PE Curve using PEMPro™.
4
Part 2 - Loose Spur Gear Set Screw
Symptoms
If you are experiencing long delays in direction reversal, but
you are confident that your worm gear to worm wheel mesh is
good, then the problem is most likely a loose Spur Gear on the
worm gear shaft. First be sure that your gear mesh is correct
for your mount. The Spur Gear becomes suspect when you
cannot feel backlash in the system, but you experience delays
in direction reversal when moving with the direction buttons
at slow speed like 1x or in autoguiding. A loose Spur Gear
will not cause a perpendicular motion, but since that problem
may require the removal of the Spur Gear, it is suggested that
you read on and not skip ahead. The illustration shows what
happens when the set screw is loose or is not centered on the
flat surface on the worm shaft. This is a relatively easy thing to
both diagnose and repair. See illustration at right.
Worm
S haft
Worm
Shaft
Worm
Shaf
t
Correct - Tight Set Screw Causes Worm to Turn with Spur Gear
Worm
Shaft
Worm
Shaft
Worm
Shaft
Loose Spur Gear Set Screw Causing Slop in Gear Train
Important Note: Removing the top spur gear, as required by this procedure, will result in the loss of your
stored PEM curve. You will need to use PEMPro™ to create a new curve.
Tools needed:
●●
●●
●●
●●
●●
1/16” hex key (older 900/1200)
5/64” hex key (all mounts)
Flat head screwdriver (for 900/1200)
3/8” nutdriver (for Mach1GTO)
Paper towel for greasy fingers
Applying the Fix (900/1200/Mach1GTO mounts - appearances may differ)
1. Start by removing the gearbox cover from the motor / gearbox on the axis in
question. To remove the cover, simply remove the socket head cap screws
indicated by the arrows (number of screws may differ by mount). Be careful
not to lose the screws! (See photo at
right)
Worm Shaft
Worm
Spur Gear
2.Remove the cover to expose the clustered spur gears beneath. The worm’s
spur is hidden underneath the large
spur gear that is on top of the cluster.
This top cluster gear must be removed.
3.Use a standard flat head screwdriver (900GTO & 1200GTO) or 3/8” socket
(Mach1GTO) to undo the spur gear bolt that holds the gear in place and then
remove the gear. (See bottom right photo
on following page)
4.Take hold of the worm’s spur gear and
turn it back and forth. If the set screw is
loose, you will feel some slop between the spur and the worm shaft. The spur
gear should feel as if it were welded to the worm. If the spur can turn without
subsequently turning the worm, you have found the source of the problem.
See photo above left
5. The set screw is on the collar on the side of the spur gear. You will probably
have to turn the spur and worm to get clear access to the screw. On older
900/1200 mounts, the set screw probably takes a 1/16” hex key. Newer
mounts including all Mach1s will have larger set screws requiring a 5/64” hex
key. See photo at right
Worm Spur Gear
Set Screw
6. Be careful to fully seat the hex key into the socket of the set screw so that you don’t wallow out the socket hole. The
Bondhus™ hex key set, included with your mount, has a ball end on the long leg of each hex key that will be helpful.
5
7.Before tightening the set screw, you need to properly position the spur gear on the worm
shaft. There are two parts to this. The spur gear must be positioned laterally along the shaft
to properly engage the next gear, and it must have the set screw centered radially over the
flat spot on the worm shaft.
8.For the lateral positioning on the 900/1200 mounts, start by making sure that the spur is
roughly flush with the end of the worm shaft. With some mounts, you may need to push the
spur gear on a tiny bit further to prevent contact with the gearbox housing. For the Mach1GTO, fully seat the spur gear until it stops. (See photo at left)
Spur Gear Flush
with Worm Shaft
9.To align the set screw with the worm shaft’s flat spot, turn the spur back and forth and
position it in the center of its free play. Turn the set screw just until it makes contact, and
then test the centering of the spur over the worm shaft’s flat spot by turning it back and forth
again. It is very important that the set screw is dead-centered over the worm shaft’s flat
spot.
10.If you are unsure where the center of the flat spot is, remove the spur gear completely and make a small mark on the
end of the worm shaft with a Sharpie marker to mark the spot so that you can see it when the spur gear is re-installed.
11.Tighten the set screw, and give the spur gear one final test by turning it back and forth. The spur gear and worm should
feel like they are made from one piece of material, and should have NO play or slop whatsoever between them. (Of
course, you must distinguish any play between the spur and worm from any play caused by the mesh between the
worm gear and worm wheel.)
12.If you wish to re-grease the spur gears while the gearbox is open, clean them with a soft cloth and then use a good
quality lithium grease rated for the temperatures that the mount is likely to encounter. It will not generate significant
additional heat in operation, so a high temp grease is unnecessary. You should probably be more concerned about the
cold end of the grease’s temperature rating. Astro-Physics uses “AeroShell 33”, but this is certainly not a requirement.
13.As important as lubricating the gears themselves, is to place some
lubricant onto the spur gear bolt shafts that serve as the axles for each
reduction spur gear. In ALL applications of lubricant, only a very light
greasing is recommended. Do not pack in extra grease!
14.Replace the top spur gear that connects the worm spur to the motor
side of the reduction gear train and tighten the spur gear bolt. DO NOT
OVERTIGHTEN! This is especially true with the MACH1GTO where
you will have the leverage of a 3/8” wrench on the bolt (it is better to
use a nutdriver instead of a wrench to minimize risk of damage). (See
photo at right)
900 Dec. Gearbox
15.Replace the gearbox cover and you are finished.
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