New Dome Motor

In January 2020 we suffered a failure of both the telescope and dome, with neither one being able to move. Investigating, we discovered that the dome motor was tripping the circuit breaker (located in the larger electrical box in the dome, to the left of the DC MaxDome box). We removed the motor, gearbox and wheel assembly from the dome and brought them down to investigate the issue.

We discovered that the motor was burned out, and needed replacing. We had to use a hydraulic jack to remove the wheel from the gearbox shaft, as it was impossible to remove by hand. Over time the wheel had slipped down the gearbox shaft, to the point that the edge of the gearbox was cutting in to the rubber wheel. After removing the wheel I was able to open the gearbox, and found it was in remarkably good condition, with the only issue being an oil leak.

To bring the dome back in to service we ordered a new motor, a ½ hp Marathon AC motor (model no 5KC42GN0018X) from [grainger] We also ordered oil for the gearbox (Mobil 1 synthetic gear lube 75w-90) and gasket material to replace the gaskets. Finally, I ordered a replacement wheel from It’s a 10”x3” rubber wheel, with a 1” keyed bore.

The gearbox/wheel is held against the dome by a spring. To mount the gearbox to the dome, undo the nut that’s constraining the spring and remove the nut, washer and spring. The spring is not under any force when the nut is at the end of the bolt, so you don’t have to worry about it flying off. You can then swing the mounting plate for the gearbox forwards and secure the gearbox to it. We used 5 washers to help space the wheel off of the gearbox, and prevent the gearbox cutting in to the rubber. Install the wheel on to the gearbox shaft (you can do this before mounting the gearbox, but it adds considerable weight to the gearbox), and tap in the key. Swing the mounting plate back in to position against the dome, reinstall the spring and washer, and tighten down the nut.

The motor mounts to the gearbox. Install the gear towards the end of the motor shaft, insert it into the gearbox and then attach the 4 bolts to hold the motor on to the gearbox. Note that if the motor is attached to the gearbox, you won’t be able to swing out the gearbox as noted above to mount/dismount it. The motor is just long enough to hit the dome wall when you try.

We had to get electricians from facilities to install the motor for us as it’s not straightforward to get it to run in both forwards and backwards directions. It required installing an extra relay to properly reverse the motor wiring. Marco Rodriguez performed the work.

Telescope Balancing/Greasing

We discovered that the telescope was not moving because it was shaking/juddering while moving, and this was shaking loose the set-screws on the RA motor. We found that physically pulling against the direction of motion stopped the shaking, but pushing in the direction of motion had no effect. We disconnected and removed the torque motor from the telescope, and verified that this was still working. The telescope showed no difference in behaviour with the torque motor removed. We rebalanced the telescope in the RA direction, but this had no effect on the issues. Finally, we re-greased the RA gear and worm gear. The telescope manual calls for annual regreasing, but Jon H. didn’t think this had been done in recent years. This immediately improved things. For future reference, the recommended grease is a molybdenum disulfide (moly) grease. I ordered some off of grainger.

Last modified 16 months ago Last modified on Jul 30, 2021, 7:41:27 AM
Note: See TracWiki for help on using the wiki.