Shimming up the CG-5 (EQ-4) Drive Motor
Why shim in the first place? If you are just using the scope for visual observing, you probably don't need to. However,
if you're trying to do imaging or autoguiding, you'll need to get all the slop and backlash out of the drive possible. One source
of backlash is the adjustment of the worm gear. See Jeff Detray's site on the CG-5
on how to adjust worm gear.
Another source of backlash is the slop between the output shaft and its bushing. You can see how much slop there is by wiggling the spur gear back and forth. This is what we're going to try to rectify here. Why the factory didn't use tighter tolerances is a mystery to me. This is a case where a small outlay in quality control would have made a big difference. No matter, though. It's simple to fix. BTW, all the small pictures on this page are linked to larger ones. |
Obviously, the first step is to disassemble the works and remove the brass spur gear. Not so obvious: I'd advise
using a permanent pen to mark the location of gear box within the housing to the motor. There
are about six different
ways everything can line up. Although, it will only work one way, you'll save yourself a lot of time if mark the parts first.
Also, if you never had the thing apart before, you might want consider having some lithium grease handy. The grease on
the gear reduction assembly looked okay, but Celestron China isn't known for using high-quality lubricants and
I chose to replace it with a lithum/teflon grease as a matter of course. You can use spray carburetor cleaning to rinse out the old grease but careful; it will
remove your permanent pen alignment marks as well.
Click on the picture to see a larger version. |
Cut a small piece of .002 brass shim stock. You can find this at your local machine tool supply. I used a piece about
3/16ths of an inch wide and about an inch long. Trim one end at an angle and curve the shim by pressing it against
the shaft. This will make it easier to insert.
Click on the picture to see a larger version. |
Put a dab of grease on the shaft and carefully work the shim in around the shaft. It will only go in about 1/8 of
an inch but that should be enough.
Click on the picture to see a larger version. |
Clip off most of the excess shim and bend it over out of the way.
Click on the picture to see a larger version. |
This is the "it won't hurt" category. I folded several layers of the shim stock over for a thickness of
about .010 and wedged it between the housing and the outer hub. The idea here is to make certain the spur gear on
the motor is firmly engaged on gear housing. Don't over do it.
Click on the picture to see a larger version. |