Dome Rotation Motor Chassis Modification

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It started off as a well intended modification, but probably ended up unnecessary.

The two dome rotation motors are 180 degrees apart; east and west side of the observatory. The west motor wheels began to loose their grip on the underside of the dome support ring (
DSR). When I first inspected the trouble, it appeared that there was insufficient tension from the motor chassis tension spring, at the left side of the motor chassis as shown in image below.

So I went about replacing the tension spring because after checking with the vendor, Technical Innovations (TI), it was stated that the springs loose their tension over time. I wanted a tension system that could be easily adjusted, so I substituted the old 5/8" x 4-1/2" x 0.08" springs with 3/4" x 2" x 0.105" springs and 3/16" x 5-1/2" turnbuckles as shown in the image below. The new spring is more stiff, owing to its larger diameter and gauge.

To make the new springs and turnbuckles fit, I had to remove the small hook as seen above the spring in the top image. The new spring is attached directly to the bolt, which previously held the top hook, and is held in place with flat and split washers and a wing nut (for tool-less removal). The turnbuckles facilitate horizontal adjustment, if and when needed.

To estimate the proper compression of the motor wheels to the underside of the DSR, I measured the distance from the bottom arm of the chassis motor to the the underside of the dome base ring, and duplicated that measurement with the new spring hardware in place. Another good test is to ensure that the two dome rotation wheels on either side of the dome rotation motor do not move freely. if they do, then too much upward pressure is be applied by the dome rotation motor wheels.

Was this mod unnecessary? Probably. ​It turns out the my belts were worn too, although they did not look as such at first glance. The red herring was thinking that the tensioning spring was to blame. I arrived at that conclusion because the motor wheels began gripping the DSR as I applied a little upward pressure on the motor chassis.

After I installed the new springs and turnbukles, everything seemed fine, but then the wheels started slipping again. It then dawned on me that it could be worn belts. So I pulled out the belts and compared them to new back-up belts that I got from TI a few years ago in anticipation of replacement. The new belts are ~0.095" thick. The worn belts are ~0.08" thick. That 0.01" wear plus some stretching were the culprits. The peaks of the cogs on the old belts are rounded off due to wear, so the belt cogs were slipping out of the motor shaft cog grooves. You can see the slight differences between a new belt (top) and a worn belt (bottom) in the image below. The worn belt teeth are shorter and the tops rounded. Click on the image to enlarge.