A/B (anti-backlash) rack/pinion drive was Re: tensioned ballscrew

Doug,

Most routers using rack/pinion just use a spring to "load" the pinion
toward the rack.

At lower levels of preload, a split gear is used, with springs. This
works very similarly to the two nuts approach used with leadscrews;
the top Gear(half) presses against one rack flank, the bottom gear
(half) presses against the other rack flank, with the force of the
springs.

You could also "skew" the pinion axis in relation to the rack to
create anti-backlash. You'll create uneven wear, but it is a simple
approach. a spring could provide a constant (and adjustable) source
of "skew-ing" pressure.

Be sure to calc the tooth loads if you decide to use rack/pinion
drive, whether preloaded or not. And as I've mentioned before, you
will have less backlash due to center distance variations with the
older 14-1/2 degree Pressure angle, at the expense of some tooth
strength, compared to the modern 20 degree PA.

Just thinking here out loud: If you used a wider rack (to allow 1/2
tooth to carry the necessary load) you could fabricate a "torsion-
based" pinion of smaller diameter than the A/B "split" gears commonly
available.

Hope this helps.

Ballendo

--- In CAD_CAM_EDM_DRO@y..., "Doug Harrison" <prototype@c...> wrote:
> Hi Les;
>
>
> > Ok I will consider back to back AC or tapered roller bearing
tensioning a
> long screw using the screw's own elastic properties. This implies
that EI
> for its support is much higher.
>
> Yes, much higher.
>
>
> > First thing to note is the block-and tackle driven support blocks
pretty
> much eliminate buckling problems. A great idea, wot. ;^)
>
> Got it from our friend Slocum, with a little help from Art
Eckstein's DRO
> thingy.
>
>
> > OK now temperature.
> > Lets say it gets warm to the touch in use. Actually they often
get quite
> warm. But let's say 10c. So that gives 14 thousandths of expansion
of the
> screw.
>
> Bummer! I guess that's why all the big guys are going to coolant
through the
> screw. Rack and pinion looks better now when one considers bolting
the rack
> to a steel beam of 200lbs thermal mass.
>
> > Now let's consider the screw tensioning level.
> > Uh oh. There's a problem. A 500 lb preloaded screw will unload
completely
> with only a 1.7 degree c temperature
> > rise in the screw.
> >
> > But tensioning the ballscrew is a good thing for the reasons you
> mentioned. So do it with a preload spring. Something
> > that can maintain the force over a larger displacement. I use
stacked
> bellville washers.
>
> I used bellville washers on a machine we just finished. It worked
great.
> Your arguments, however, have me thinking again about rack and
pinion. Less
> friction, less resonance, minimal thermal growth and outrageous
rapid
> speeds. Got any suggestions on how to build a zero backlash pinion
drive?
>
> Doug