1100GTO Reduction Spur Gear-set Play Adjustment

Oct 31, 2017



#59277 Oct 31, 2017

I have experienced greater declination backlash from day one in my 1100GTO purchased new in December of 2015 than I would have anticipated.�� I have played around with adjusting the worm mesh using both the slewing and park 3 push on the gearbox methods, but have not seen any significant change in the behavior.�� I do not feel any backlash when I try to wiggle the declination axis, so I believe the preload on the worm bearing is adequately tight.

The leaves the reduction spur gear-set as the next possible source of backlash.�� I took the cover off of the gear box and indeed I found that there was about 3 teeth worth of play at the small gear on the motor shaft.�� I removed the intermediate gear with the three holes spaced 120 degrees apart and discovered what the holes are for.�� The position of the pivot point of this gear is adjustable and the hex head screw that locks the pivot position in place is accessed through these holes.�� With the gear in place I rotated the gear to allow the hex wrench to loosen the screw.�� Using rotation of the motor shaft gear in one direction or the other tightens or loosens the lash of the gear-set.�� I was able to take out virtually all the lash without the gears binding.�� I then readjusted the worm mesh and ran the axis through 360 degrees to verify there was no binding anywhere in the full range of motion.

For the heck of it I opened up the RA gearbox and found that it too had significant backlash, though less at about 2.5 teeth of motion.�� I went ahead and adjusted the gearbox and then the worm mesh and verified full 360 operation.�� Of course, in theory, this should have no effect on guiding as long as the guide sped is 1 or below.

The proof is in the pudding, but of course the weather here is going to be bad for at least the next week, so it will be a while before I will know if this has any positive effect on the declination issue.�� I have not found anything about making this adjustment on the 1100GTO on this site or the AP site.�� But I did find a link on this site to AP instructions on various adjustments on the 1200GTO, but not the reduction spur gear-set play.�� Here is the link: ��www.astro-physics.com/tech_support/mounts/1200gto/Gear%20Mesh%201200.pdf.�� At least it defines the proper terminology to use for various aspects and does show some other possible sources of lash that could be seen in other models as well.

I will post with results when I finally to get suitable conditions.�� I am interested if anyone out there has played with this adjustment.�� It is not clear that this adjustment is possible on other models, and may be a refinement first incorporated on the 1100GTO.�� I had been looking at retrofitting with the self-adjusting worm gear boxes, but if this results in the kind of improvement I think is possible, I may not need that.�� It would be interesting to compare the performance in this regard to the original like mine and the latest version.


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