Re: Tuning backlash on iExos 100

Wes Mcdonald


Backlash on the Exos2 (and 100) comes from two sources:  The preload on the worm bearings (which is what you adjusted) and the mesh between the worm and the wheel gear.  Once the preload is just enough to ensure the worm does not move left and right (along its axis) then it is set correctly.  Most have found that if you overtighten it, the worm picks up a cogging action from the ball bearings and while we have not quantified what effect this might have, most remove it by having just enough preload.

The second issue is how tightly the worm meshes into the wheel.  This is one of those things that is a compromise.  If you get it too tight, then the mount will bind and stall.  If it is too loose, then you have backlash.  You can find a paper by Bob Hoskins that explains how to set the backlash (and other good to know stuff) in the Files section of the MAIN group.  

The backlash in the RA axis is less critical, because the mount is always being driven in the same direction in RA.  Even when guiding, the drive never reverses, it is just slowed down a bit or sped up a bit.  Dec is another story as guiding will cause dec to reverse directions at which time the gears will disengage.  So you want to really work on the DEC mesh to get it just as tight as you can without causing a bind. 

The tricky part is that the wheels are not round and the shafts may not be exactly eccentric.  This means there will be areas of loose mesh and areas  of tight mesh.  You have to find that mesh adjustment that does not bind but minimizes the loose part -- and for that you have to check the mesh 360 degrees around the wheel.

Hope this helps

On Wed, Sep 9, 2020 at 6:33 PM Vijay <vijaysam@...> wrote:
I think I figured out where to tune for RA.
At the end of the worm gear, there is a large screw.Tightening/loosening seems to reduce the shift but not all of it though. There is some shift still, though it has reduced reasonably.

Need to try the mount out once the sky is clear and see how well it performs in PHD2.

Wes, Southport NC
EXos2-GT PMC-8, iExos 100
ES ED 127, 10" LX200GPS+wedge, Astro-Tech 8" Newt, ETX-90, 60mm no-name guide scope ~ 260mm FL
Polemaster, Orion ST-80 and SAG, ZWO 290MM, D5300 astro modified
Nina, Bootcamped Mac Mini control computer, RDP to iMAC
110 amp hour lead acid deep discharge battery for field power
Electrical Engineer, Retired

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