Re: [CAD_CAM_EDM_DRO] tensioned ballscrew

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