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.

G11RA-Slack-Location-IMG_0653On the G11 the Right-Ascension worm gear is covered by a milled aluminum cover. This comes off by loosening two tiny set screws.

G11RA-Slack-Measure-IMG_0655OVision 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.)

G11RA-Slack-Loosen-IMG_0656Two 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, end RA shaft, to adjust the slack between the gears.

G11RA-Slack-Gap-IMG_0654To make the adjustment repeatable when dealing with the tiny motions needed, use a set of engine feeler gauges between the worm block and the RA housing. Pressing tightly against a given feeler gives a defined amount of gear slack; then a different feeler can be used to increase or decrease the slack.

1 thought on “Adjusting G11 Worm Gear Backlash

  1. Francesco Megli

    Hello!
    I am going to give a try to all this at my G11 Losmandy dating more than 20 years ago. In fact something may be going wrong at the RA worm, since I get jumpy stars: double, triple or more in a row in RA.
    I had the mount inspected, checked up and lubricated by a good italian firm, but good results lasted short.
    By now I get 25-30% ruined frames even working with 15-s exposures: the worm must be crowded with dirt speckles, or the new grease turned to be sticky. I diluted it a bit by putting some droplets of Marine CRC, a nautical libricant, in the worm thread and bearings and things seem to go better, but still deserve optimization.
    Do you think multiple stars are the symptom of bad worm-gear slack? Should I take a step more dismounting and cleaning the worm thread and bearings? Should I go even further and change the worm and bearing set ( I found good quality both at Teleskope Express.De.
    I would greatly appreciate your reply. I would also be grateful to you for detailed directions how to dismount the worm and bearings, if required.
    Thank you.
    Best regards
    Francesco Megli

    Reply

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