Re: [CAD_CAM_EDM_DRO] Ultimate backlash free reduction

Graham Stabler wrote:

>What is the best way of acheiving approximately 5:1 gear ratio with
>as little backlash as humanly possible?
>
>

For CNC motion control, a toothed-belt drive is very good. I am quite
positive
I have NO backlash in any of my belt drive systems. I used the old
belts that
were on one system when I got it, and you could hear the belts creaking
as they
slipped a few thousandths over the teeth. When I replaced the belts,
the creaking
was gone.

>The application is the increase of optical rotary encoder resolution
>so there are no large forces involved. You can also assume the
>encoder has ball bearings or is at least coupled to a shaft with
>them. The encoders are already 2500CPR/10000PPR so we are talking a
>requirement of 0.007 degrees of backlash.
>
>

Now, if you need not only no backlash but also very precise positioning
without
any cyclical error introduced by the belts and the sprockets, I don't
think you can
get that with belts. I have the encoders on my system running directly
from the
leadscrews by helically-slitted couplers, only the motors drive through
the belts.
I wanted to avoid any cyclical errors that the belts and sprockets would
introduce.
Even still, I had to rebore all my sprockets, as the factory bores were
way off
center!

There are precision gearboxes made for this purpose, such as running
fine and coarse
resolvers on radar dish mounts. They use gears in pairs, with springs
between the
two gears, to prevent backlash. You can even buy these gears from the
likes of
W. M. Berg. The gears wouldn't set you back more than a few hundred $.
But, I have no idea what short term and long term cyclical errors you'd
get from
them. Berg can, of course, not only provide gears to the accuracy spec
you require,
but provide test certificates for them. That will drive up the price
quite a bit.

Jon