Re: [ATM] How to Reduce Bearing Friction EQ version

[Kevin MIchael Zabbo <chaosopher23@yahoo.com>]

My EQ platform will have, as its north pole axis, a
bicycle rim devoid of its innards and tire, contacting
on two bearings at about 100 degrees apart.  One of
these contact points is a driven wheel and the other
is an idler.  The driven wheel is smallish, about a
quarter of an inch in diameter and is coupled to a
stepper motor via small gears.  This will take the
load from the little motor.  The idler is going to be
taken from a skateboard or roller skate that's still
in good shape, or maybe the front hub of the bicycle
rim that is now the north pole of an EQ mount.

Oh, yeah, the south pole is going to be a rather heavy
3/4" Dodge pillow-block bearing bolted firmly to a
frame big enough to support it.  These are rather
expensive but they are very smooth and won't collect
dirt in the bearing, and have a grease zerx where it
can be reached so that some super slippery stuff can
be squirted in.

The diametric difference will produce a strong and
lightweight cone that will reduce that dreaded polar
axis flex found in cheap forks and German mounts.  I
can even put a setting circle on this old rim between
the fork and the rim itself.

It will probably weigh more than I want to carry, but
when I do get it to a place to observe, 1. I'd have
lost 2 pounds, 2. It won't move unless it's told to
move.  It will carry my 8" Newt or the 13.25" Cass
without complaints, and possibly something larger,
like a refrigerator-sized RFT.

The DEC bearings aren't as important as the RA
bearings.  These are simple front hubs from a bicycle
and a locking bar.  I may add a rack and pinion
fine-adjust to the DEC axis if I get rid of my general
laziness.

The fork is open framework of #2 cheap pine 1x2,
though I may change this a little to white cedar, but
that's just the boat builder in me crying for nicer
wood.

Kevin of Eastern Iowa
Seeker of the Darkness

ps. Boat builder was a previous incarnation.  I built
some nice boats for the Vikings back around the turn
of the millenium.  The British didn't like me very
much. :-P

--- Nils Olof Carlin <nilsolof.carlin@telia.com>
wrote:

> 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 
> 
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 


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