Removing Declination Axis backlash - 400QMD

Note: Backlash is a gap between the worm and worm wheel gears. This can be felt by locking the dec axis clutch knobs and moving the cradle plate and/or telescope back and forth in declination.

Step 1

Remove the dec axis drive assembly cover plate. Unscrew all four 4-40 socket button head screw with a 5/64 hex key. Note: On some mounts one or two of the inner screws (closest to the mount axis) have been modified to improve clearance. Please take note of the screw and location before removal.

Step 2

With plate removed, place finger tips on the exposed worm and motor spur gears. Holding the worm spur firm, note the slight movement or play of the motor spur. This is proper and necessary to prevent the drive from stalling.

Step 3

Remove the two 8-32x1/2 socket flat head screws to release the motor assembly.
( 3/32 inch hex key )

Step 4

Get a 7/64 inch hex key to turn the center 6-32x1/2 socket head cap screw. This screw provides "push" to draw the gears closer.

Use a 5/64 inch hex key to turn the pair of 8-32x1/4" socket set screws. These screws provide the "pull" to draw the gears futher apart. In practice only one of the two screws is activated.

Remove the black, thimble like, cap to reveal a silver cylinder with a cap screw. (Arrowed)

Step 5

Mount your cradle plate and lock dec axis clutch to allow you to feel the backlash. Unscrew one of the two set screws, one 1/4 turn at a time, while tightening the center socket head cap screw. Feel for the reduction in backlash as you proceed by moving the cradle plate after each adjustment. Stop immediatly when backlash is no longer felt.

Verify that the gear mesh is now not too tight by rotating the worm. Rotate the worm using a hex key on the cap screw of the silver cylinder exposed earlier by removing the black cap.

Step 6

Return the motor box to the dec drive. Once screws are tight, verify that play still exists between the two spur gears. If not, readjust the motor placement by loosening the three 8-32 socket head cap screws. Please note that turning the inner screw will require shortening the length of a 9/63" hex key.

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This page was last modified: January 28, 2004

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