Re-Meshing worm gear on a AP1600GTO

Craig Young

I have an AP1600GTO with CP4, auto adjusting worm box and absolute encoders.  Currently, I am installing a new OTA on the mount so I decided it was a good time to service the mount, including re-greasing the worm gears on both RA and DEC.

AP provided me with an excellent set of instructions on how to remove and then re-install the motors so I proceeded ahead.  Everything went to plan.  The old grease removed from the worm gear and new grease applied.  I then re-installed the motors and began initial testing.

The first test was to manually put the telescope into the Park 2 position with the OTA and CW bar removed.  The OTA saddle was attached to the DEC top plate.  I used APJog utility provided by AP to launch the AP V2 ASCOM driver after powering on the mount.

Using the V2 driver, I set the move speed to 24x and then clicked the N button.  Instead of a nice smooth movement of the gears I got a machine shop cutting lathe sound.  Not good.  So I clicked the S button and got a similar sound but at a lower frequency.  This sounded a lot like my iOptron CEM60 when I tightened the worm engagement bolt too much.

iOptron eventually provided a set of instructions on how to remove this noise.  You begin by powering off the mount and then tightening the engagement bolt for each axis.  Then you back that off by a half turn.  Power on the mount and then slew the telescope in each axis to check the engagement.  If the sound appears then back the bolt off a small amount each time until you have a smooth slew on both axis.  This worked quite well.

I was quite surprised to see the same problem on the AP mount given the auto adjusting worm mechanism should have guaranteed this would not happen.  After spending hours repeating the installation instructions and talking with George at AP I still had the same noise when slewing.  Time to give the iOptron trick a try.  This is the procedure I now use:

1. Power on the mount and use the APJog utility to connect to the mount.  The V2 ASCOM driver should now appear on the desktop and connect to the mount.  Expand the window (Expand button) so as to see the Park and Unpark buttons.  The mount should be unparked.  Toggle off the tracking in RA.

2. Set the Move rate (V2) to 24x.  Click the N button and check for a nice smooth sounding movement of each axis.  If you hear screaching noise then stop.  Repeat for S button.  If you do not hear any screaching, change the move speed to 200x and repeat the test, then 600x.  If you do not hear any screeching then the gears are meshed nicely and you are done.

Proceed with the next steps if you do hear screeching .. one axis at a time.

3. Park the mount (V2 button) using "Current Position" setting.  This will stop any small movement of the worm in the next steps.

4. With the auto mesh lever (lever) fully engaged, loosen all three motor attachment bolts.  Then slowly and evenly, tighten the bolts until they make first contact.  The key here is to NOT tighten the bolts, just have them make contact.

5. Loosen the lever bolts (2 of them) and following the AP instructions, adjust the backstop.  NOTE: do not tighten these two bolts, just make first contact.

6. Unpark the mount and repeat step 2.  If the slewing is nice and smooth at all speeds then you are done.  Otherwise, continue with next steps.

7. Repeat step 3.

8. Repeat step 4.

9. Repeat step 6. NOTE, do NOT repeat step 5 as it only needs to be done once.

10. If you still have noise then the next steps are executed.

11. With the mount unparked, but not slewing, loosen the 3 motor attachment screws just a tiny amount.  As you do this you will hear the worm motor making little sounds as it is adjusting.  Not sure if this is due to the auto engagement mechanism (springs, sensors) or the absolute encoders.

12. Continue to slowly loosen all three screws until you notice the motor box rotating in and out of mesh by itself.  You can do this manually if you grab the motor, but in this case you will see the box do it on its own.  The box will move back, make a few sounds and then a couple of seconds later move forward.  It repeats this back and forth movement.

13. Slowly tighten either the right or left motor attachment screw BUT ONLY do this when the motor box is in the forward position.  In other words, when the box moves out of mesh (springs releasing?) do not adjust the attachment bolts.  Wait for the box to move back into mesh (springs engaging?) and then slowly tighten the same attachment bolt (left or right, but always the same one).  Eventually the box will stay in position.

14. Now very carefully, tighten the center attachment bolt and the other (right or left) bolt.  Again, DO NOT tighten these bolts, only make first contact.

15. Repeat step 2.

16. Repeat steps 12 thru 15 until the mount slews quietly at 600x through the entire operating range for each axis.  For example, here in the Southern Hemisphere I slew DEC from +20 to -80 and RA from -6HA to 0HA.

17.  Once all of this is done then try a couple of meridian flips and random motions.  If you find any "High" spots where the noise comes back then repeat steps 11 thru 15.

This whole process can take an hour or more depending on your specific mount.  If you still have problems then I would recommend you contact AP for further assistance.


Emilio J. Robau, P.E.

Thanks Craig,

This sounds somewhat like the procedure I used to almost completely tweak out the noise.   Generally I would stop at the point of noise, loosen the bolts to where they just touched, giggle things a bit, and then tightened things again and tested for the sound.   I don't have the self adjusting mount, but I think there are some similarities in what you are doing to what I was doing to tweak for the proper sound.

Craig Young

Following Rolando's suggestion I rang David at AP to discuss the procedure.  He is going to read it over the next couple of days and possibly edit it to match more closely what they do at AP to tweak the motors off the assembly line.  The overall impression I got from the phone conversation with David is how critical the meshing is to obtain a nice smooth movement of the axis.  A tiny variation in alignment will likely result in a harmonic oscillation which makes for a terrible sounding noise during a slew.  This can also happen over the years of use or dramatic temperature changes, requiring a very slight tweak to the meshing of the gears.  It is encouraging to hear a similar result from your experience.  Means we are both on the right track and hopefully it is helpful for others.