Zhumell 12" Dobsonian Altitude Bearing Modification
By Andrew J. De Luca
I found the altitude & azimuth bearings to be very
loose on my Zhumell even when the knobs are turned down tight.
Upon removing the knob I found a roller bearing between two
washer. Behind that is a plastic clutch that tightens around the shaft
when pressure is applied by tightening the knob.
I tried to find the washer at Home Depot and my local hardware
store without luck. I opted to fabricate one out of a piece of 1" PVC
pipe. I cut 1/4 inch pieces and than used coarse sandpaper to get them
under 1/8" and finished them with emery cloth.
When rebuilding I left the outer washer off. The fabricated
washer is held in place by the shaft and the knob housing.
The tube still moves like butter but now the bearing clutch
grabs and holds. The tube stays erect when applying pressure or moving
the scope out of the garage. Total cost of project about $20.00
including tools completed in under 3 hours including planning &