Re: [ATM] need help with an altitude bearing problem

["Arjan te Marvelde" <arjan.te.marvelde@hetnet.nl>]

Dale,

> discovered that is not the case.  What you want is
> something with high stiction and then low friction as
> you start it moving.  Ebony star and Teflon were found

Not sure if I understand what you mean. To my knowledge the static friction
should be equal to the dynamic friction, in order to prevent the relaxation
effects that cause a jerky movement. Teflon, used with many other materials,
has this characteristic which makes it so useful. I believe that the
combination with polished stainless is in fact a good one. The CoF is about
0.05 -0.1 or so, which value you can use to calculate what kind of force on
the pads you need in order to produce enough friction for your set of
eyepieces.
Note that the contact area in fact hardly affects the friction, other than
that the pressure should be higher than a certain value. Ebony star in
effect decreases the contact area due to the bumpy surface, only the bumps
make contact.

Concluding, the solution to the original question would be to either
increase the force on the bearing, for example by larger separation.
Otherwise you would need to go for some weight compensation method.

Bob Lombardi has written an ebook about ATM, which contains a good treatise
on teflon bearings for dobs.

Cheers,
Arjan

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