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ATM [Fwd: Re: Making worm gears]

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Alan Shinn wrote:
> 
> Louis Boyd <boyd@pegasus.la.asu.edu> wrote:
> 
> >Ken Carter wrote:
> >>
> >> Anyone know of a reference for making worm gears? What kind of equipment is
> >> need and procedures?  I build telescopes and would like to make my own worm
> >> gears for the guidance systems.
> >> Thanks,
> 
> >Not to throw cold water on your project but why would you want to
> >use worm gears to drive a telescope?  By using disk and roller drives
> >you can simplify the machining and greatly reduce the periodic error
> >and backlash.  Many modern small research telescopes use this
> >technology.
> >A single microstepped stepper motor on each axis can give excellent
> >tracking and slew rates with no gear shifting or clutches.
> 
> >Lou Boyd
> >Fairborn Observatory
> 

> What's a disk and rollor drive? A brief description would be enjoyed.

It's simply a round metal disc, typically one to two times the diameter
of
the telescope mirror.  There's one for each axis.  The telescope design
can be equatorial or alt-azimuth.  The disk is usually made of a
moderately
hard stainless but it can be aluminum. The roller is considerably
smaller
in diameter and gives the first reduction.  The rollers may also be used
to support the polar axis of an equatorial mount.   The rollers are
driven
either by a second disk and roller reduction or by a belt drive.  This
is
directly driven by a microstepped motor. 

The key is that the pressure between the disk and roller must be high 
enough to prevent any slipping but low enough to prevent damage to
the surfaces.  The coefficient of friction of dry steel on steel is
about
0.1 so with 100 lbs of loading it takes a 10 lb tangential force to
cause
any slipage.  Our .5 meter telescopes run about that.  This provides a 
safety factor in case something bumps the telescope. It will slip before
any damage is done.   The edge of the disk and the diameter of the
roller
must be large enough to keep the surface deformations low enough to not
damage the metal.  For a light weight 30" telescope (made entirely of
steel)
a 48" disk with a 1" wide face and a 1" diameter roller works fine. 
This
gives a 48:1 initial reduction.  We use a belt drive 7.2:1 secondary
reduction,
then a x256 microstepped 400 step/revolution stepper motor.  For larger
telescopes ( 1 metere & up)  DC servo motor are typically used with 
both velocity and position encoders.  On larger telescopes the disk
diamter
increases proportionately with mirror diameter while the face width and
roller diameter increase at a faster rate to accomidate the weight which
goes up roughly with the cube of the mirror diameter.  

The things critical on a disk and roller drive are that the axis
of the disk and roller bearings be parallel to each other and the
edge of the disk be cylindrical. No crown is used.  The roller
must be mounted on low friction bearings.  Normal shielded ball bearings 
are sufficient.  The material used for the disk and the roller should
have
similar coefficients of expansion or the drive rate will change slightly 
with temperature.  The roller should be harder than the disk for best 
wear characteristics.  While stainless materials are desirable, plain 
steel works ok in a reasably dry environment.  The disk stays clean on
the rolling surfaces like a railroad track.  With this type of drive
subarcsecond corrections may be made and and promptly seen at the focal 
plane without backlash.  That doesn't mean the telescope will point with
arcsecond accuracy.  There is still mount flexure.  These driver are 
repeatable in pointing to a few arcseconds and thus flexure models may
be 
made in software and pointing error can be corrected.  

For some examples check http://24.1.225.36/fairborn.html. 
All but the first of these scopes use disk and roller drives.  They
are simple, but are doing very high quality photometric research.

Lou Boyd
Fairborn Observatory

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