Lazy Susan Bearing for a 200mm Dob Base
Submitted: Monday, 5th March 2007 by Geoff White
My pre-loved Bintel Dob has only had one bothersome problem, the azimuth motion, which was prone to sticking and jumping. This was evidently due to the fact that base mods by the previous owner, to fit a Tech2000 DDR2 Dobdriver, had introduced rough spots into the slide path of the Teflon pads fitted to this style of telescope. These were at places where some type of cup or roller bearing had been fitted.
I was well aware of this condition at sale as the previous owner had generously allowed me an extensive trial beforehand, and I was quite happy to take on the interesting project of bringing the azimuth motion into a smooth condition.
I set out to smooth the rough patches by filling them with epoxy and smoothing them out to the same level as the surrounding surface, which seems to be ebony star laminate or similar. I then applied dry wax lubricant to the surface. This was an improvement, but it still called for more. I then tried milk bottle washers at the centre, but they produced no worthwhile difference (I could not expect much really).
So then it was a choice between relaminating the surface with Ebony Star, which comes in 2 different textures, and which I had previously seen when looking at kitchen bench tops, or a lazy susan bearing.
Although it is quite easy to relaminate the surface, and I had experience doing this with our kitchen, the engineer in me could not resist the thought of a steel bearing. It’s so Isambard Brunel. I found a 12” Howard Silver bearing at Bunnings for $27. A bit expensive for what it is, but minor in the total scheme of things.
These low-load low-speed bearings have pressed-steel races, which are swaged or clenched together to form an assembly which traps the balls in place. They can be secured to the top and bottom halves of the dob base, but it is really only necessary to screw the bearing to one surface.
The critical step is to centre the bearing around the securing bolt which holds the dob base together. Since there is about 1mm of lateral play in the bearing when it is unloaded, tape-measure accuracy is OK for this, but it is worthwhile being as careful as possible within those limits.
I carefully marked the screw positions for the flange of the bearing, and screwed it to the upper base only. A longer centre bolt was necessary, but the original steel sleeve in the centre hole was still long enough to bear in both upper and lower bases.
On reassembly, with a drop of Loctite on the centre bolt threads to retain the nut, I found that the law of conservation of angular momentum was well in evidence. The azimuth motion was now quite smooth, and very light, but the telescope just kept right on slewing to point in the opposite direction! With my Newtonian fitted with a RA finder, Telrad, 10:1 Crayford focuser, and an Orion Stratus EP at one end, the necessary counterweight is 2 speaker magnets at the other. All that weight now needed a brake of some sort.
This was supplied by 2 pieces of closed cell foam stuck to one surface and dragging against the other, and now the azimuth motion is ideal. This is certainly not the only way to go as I have acknowledged, but it worked for me.