Re: [ATM] How to Reduce Bearing Friction

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

Arjan,

> There is another force component called static friction, or stiction. This
> stiction exposes itself as a threshold you have to overcome when changing
> from steady to moving state. The user experiences this as a jerkyness in
> movement: the force to set the scope in motion is higher than the force
> required to keep it moving.
>
> What you ideally want is to have zero threshold, i.e. equal static and
> dynamic friction. This prevents the jerkyness and increases pointing
> precision. Teflon based bearings have a static to dynamic friction ratio 
> of
> almost one, the trick is to get rid of that final bit.

Actually, the teflon-laminate friction can vary with velocity of motion - 
with my OTA unbalanced just enough to overcome stiction, it will move ever 
so slowly, at essentially constant speed. This is of course nice when 
tracking. On the other hand, I've tried teflon-laminate bearings in an eq 
platform with little success (so far) - the vibration of the stepping motor 
in "normal" mode seems to break up stiction (as well as image sharpness 
:-( ) but going to continuous, vibration-free drive would make the platform 
move in little jerks - an arcminute a jerk or so. Further experimenting 
might have found a working solution, but using ball bearings instead solved 
this problem for me. But it demonstrates the complexity of the matter - the 
real-life friction of Teflon against laminate just isn't well described by 
the common, simplified model of a constant friction coefficient.
>
> Ebony star apparently fits the bill. Getting the ratio down to 1 does not
> require a  different placement, however it will still change the friction 
> as
> such.
One feature of Ebony Star or ther wavy surfaces is of course that the actual 
area of touch is only a small fraction of the total area, and I believe that 
for smooth friction the pressure at the contact "points" must be high enough 
for the Teflon to deform some. So, to get smoother, "buttery" friction, 
smaller Teflon pads *might* work better than larger. So might increasing the 
bearing diameter.

Nils Olof 


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