Adjusting G11 Worm Gear Backlash

The Losmandy G11 mount in the observatory is fitted with the OVision upgraded worm gear for higher-precision tracking. This page illustrates the adjustment of the slack for this gear – the amount of “looseness” between the worm and the gear mounted on the RA shaft. An equivalent adjustment is available on all mounts, and the right balance of looseness (for smooth tracking) and tightness (for minimal backlash) must be found.

OVision gears are run quite loose. (Generally, one need not worry about backlash too much, on the right ascension gear, because the drive is being constantly driven in the same direction. Backlash only matter on a gear when you change directions.)

On the G11 the Right-Ascension worm gear is covered by a milled aluminum cover. This comes off by loosening two tiny set screws.

OVision says to adjust the gear to produce enough slack so that the end of the counterweight can be deflected 1 to 2mm.

To measure this, I used a couple of clamps to hold a small ruler at an appropriate angle near the end of the shaft. (One clamp is clamped to a table, and a second clamps the ruler to the first clamp.)

Two hex screws secure the worm block, one at each end. The one near the motor needs to be loosened slightly, and the one away from the motor a lot. This allows the end of the worm gear that is far from the motor to be pressed closer to, or pulled further from, the RA shaft, to adjust the slack between the gears.

To make the adjustment repeatable when dealing with the tiny motions needed, I use a set of engine feeler gauges (used to adjust spark plug or valve gaps) between the worm block and the RA housing. Such a set of feeler gauges is a very useful tool, and is inexpensive at your local mass-market hardware store. Pressing tightly against a given feeler gives a well-defined and repeatable amount of gear slack; then a different feeler can be used to increase or decrease the slack.