Click on the images below to view a larger (1600x1066) version.

Ball Bearings

440 SS, Double Shield, 0.25" ID, 0.625" OD, 0.196" width. Number of dots denote ABEC rating (ABEC7 from SDP-SI part# S9912Y-G2562PS2M, ABEC5 from McMaster-Carr part # 57155K377 and ABEC3? stock from Losmandy)

Worm Bearing Blocks

Worm shaft side up. Note the ball bearing pocket has been reamed out with a Dremel tool so the ball bearing does not get pinched when the block is tightened down to the mounting plate. Any curves or bumps between the mounting plate and the bottom of the bearing block will translate into a warpage of the round bearing pocket, and hence pinch the bearing, since the material is thinnest at the bottom of the block.

Worm Bearing Blocks

Worm shaft side down. I had to drill a hole through the far block (left block in picture) in order to punch out the original bearing since it was so tightly pressed in. The nub diameter was reduced on the block nearest the gearbox (right block in picture) to allow for better coupler to worm shaft alignment and simultaneously set the proper gear backlash spacing. As designed the block nearest the motor can only rotate, the far block can rotate and translate.This is not always enough freedom of movement for ideal alignment.

DEC Worm Assembly

This is the retrofitted DEC worm assembly with the original high precision worm, the unmodified bearing blocks, new ABEC7 ball bearings, new helical coupler and gear box spacers (two 8-32 nuts).

DEC Worm Assembly

Retrofitted DEC worm assembly.


Various couplers I have used on my G11 mount (3/16" x 1/4" Bore). From left to right:
McMaster-Carr part# 6259K12, Type 303 SS Helical Coupling, clamp hub, 1" Length, 5/8" OD, 288 oz-in peak torque
SPD-SI part# S5904Y-G409-17, Type 303 SS Helical Coupling, pin hub, 0.75" Length, 0.469" OD, 80 oz-in peak torque, 1.5'/oz-in rigidity.
Stock Losmandy from Renbrandt, Inc. Flexible Couplings, Tinymite Oldham coupler, clamp hub, 21/32" length, 0.5" OD, 80 oz-in peak torque.

The McMaster-Carr coupler does not fit on the G11 without modification. I had to shave down the heads of M3 button head screws (replacing the cap head screws) in order for the coupler to clear the mounting plate while rotating. The performance it gave was worse than the stock Oldham coupler, so I switched back. Eventually after a lot of fiddling I was able to get very good performance from the Oldham. Recently I have tried a different helical coupler from SDP-SI, it has given me good performance albeit the PE is higher (AFAIK 12" p-p), the tracking error is smooth allowing the autoguider to do its job. And this was with very little tweaking of the worm/gear shaft alignment. I have only a few objects imaged with this setup so the jury is still out on whether I will keep this configuration.

DEC Backlash Setup

These two pictures show the amount of backlash I have set for the DEC axis. I can turn the coupler about 3mm (at the outer diameter) between the teeth of the worm wheel.