Re: ATM Mel's System

[andydtg@phoenix.net]

Re roller disk 'gear reducers':

I'm building/installing these in my version of alt-az-focal plane
polar tracking mounts, as they work much better than worm gears
& cost consideably less. Here's my comments on some of the
response to Chuck Shaw's and Al Kelly's postings on Mel's
and my own mount designs.

Stephen Tonkin wrote:

>Is there not a danger of periodic error if the 3" disk and 3/4" drive-
>roller are not perfectly circular in section and perfectly coaxial with
>the axis of rotation being perfectly central? 
>
>To what accuracy does this need to be made and is it realistically
>achievable by those of us who have minimal workshop kit and belong to
>the ambisinistrous brigade?

The rollers, disks, shafts, bearings, etc *do* have to be round and
concentric to achieve reduction of periodic errors to negligible
levels. Fortunately the commercial parts that can be easily obtained
(ground shafts, ball bearings, etc) meet the criteria easily.

Making the drive disk round and concentric is easy on a lathe, but
can also be done by mounting a good bearing in the center & rotating
the disk against a rotary sander.

Mel Bartels wrote:

>Two problems to overcome in pure roller drive systems:

>1. accuracy: for short term tracking, at least an accuracy of 2000:1 is
>needed (1 arcsecond error over 2 minutes tracking); this calls for
>machining to about 0.001" (also watch out for bearing slop) for the
>wheel and much better than that for the drive roller,

I agree, the disks, shafts, bearings, and rollers gave to be
round and concentric, easy to do with machine tools. I make my
drive rollers from commercial drill rod, which is typically round
to < .0001". It is not neccesary to have exact diameters as the
drive ratios have to be calibrated anyway.

>2. contact friction: getting enough of it without distorting the drive
>roller and/or wheel.

I've found that stainless steel rollers on plain steel disks work with
only moderate pressure if a fine grit lapping compound (300-500 grit)
is applied to both. This may seem a bit radical but it *works*, there
is very little wear because of the rolling (not sliding) contact
between the disk and roller. Aluminum disks with steel rollers work
for small (<12" reducers). 

I don't think the abrasive friction technique would work for high speed
high torque applications like slewing, because the acceleration and 
inertia loads would probably cause skidding at the contact point and
rapid wear. 


Andy Saulietis / DTG Alt-Az-Fp Drive Systems
HDPE Worm Gears, Custom designs & Machine work
12617 Harriet Ln
Santa Fe, TX 77510 USA
409-925-8854 Voice/Fax
e-mail: andydtg@phoenix.net