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This page was last updated on 10/08/14.

Adjusting the RA Drive Worm

Worm Block Bearing Replacement Click the link if you actually need to replace your worm block bearings and want that information.

When my MI-250 RA axis started to show an increased periodic error, Larry Myers talked me through the process of adjusting the tension on the worm shaft bearing using the three Allen screws (shown below left).  While going through this process, I was introduced to the exacting parameters to which each mount is adjusted.  Under the worm assembly are a series of shims (below right),

RA Axis -- the left shim is 0.004" and the two stacked shims on the right are 0.003" and 0.0015" thick/thin, all required to square the worm with the worm gear!  WOW!

DEC Axis -- Adjuster side shims are 0.003" PLUS 0.0015", and on the motor side are 0.002" PLUS 0.0015" shims stacked.

(Below) Using a piece of .25" key stock, the worm assembly can be reinstalled very close to the needed position.  This is best done under a load, but can be accomplished without the load.  Final adjustments under load are more accurate than without the load.  Thus far I have adjusted the worm WITHOUT a load and have managed to attain just 5.7 arc. sec. of total P-T-P periodic error (MI-250 PEC Page).  When the weather clears and I again get motivated, I may go back and readjust the worm under a load and see just how small I can make the periodic error.

(below) RA shaft with large bearing still on shaft.  Also notice that Loktite was used to secure the large bearing to the RA housing.

(below) Notice how DEC bearing has been "pinged" to hold it in the DEC housing UNLIKE the RA housing above that was Loktited.