Ball screw backlash options

[Stevie]

Hi Guys

Well i got a long ballscrew off eBay (53" 0.500" dia 0.500" lead 2 start) with a single nut (2 ball tracks)
However there is a fair bit of play in it; Ive not measured it but must be 3-4 thou
So as I got it for a good price i have 3 options (as I can't find who made it for a replacement nut; and i already re-loaded the nut with new balls)
option 1
Toss it
option 2
build it into a third lathe as the Z axis (I'll have to cut it down) and add some form of backlash compensator; this could be a weight on a thin wire rope hanging from a pulley
option 3
I cast a plastic nut with bronze dust mixed into the resin; I've already tried this and other than the shrinkage it will work (I had no dust in it for the test)
I can allow for the diameter shrinkage by wrapping the ball screw with teflon tape before the pour; but it's the longitudinal shrinkage that i can't dream up a way of allowing for
This nut will be following the ball nut with a spring to push the ball nut against 1 side of the threads; the ball nut will be positioned to thrust in the direction of cut; so the backlash is always compensated for (unless your backcutting)

Any ideas guys

[HuFlungDung]

You could try here for some oversize balls:

http://www.precisionballs.com/

Its difficult to estimate what size you'll need, but you might be able to come a lot closer than what you have. That's not saying that the internal ball contact angle will be correct if its a crude screw and nut, but, it may get you by.

[High Seas]

Option 4 - How about using a short piece, make it into a tap - and drill/tap deltin/acetal rather than make a cast plastic nut? I and many other here have taken this approach with very good results. Jim

[Stevie]

Thanks guys

I was told oversize ball did not exist

[ger21]

Moglice. I saw a website before, where they bored out the nut, and injected moglice into the nut while on the screw, so you end up with a ballscrew without balls, and no backlash. But I can no longer find the site.

[Stevie]

you mean this

[ger21]

Yes. Give tham a call and ask about that application. I also heard on one of the Yahoo groups that they have packages a limited number of smaller quantity packages, as the price is a little high. I think it was around $50 for the smaller package, but don't quote me on that.

[HuFlungDung]

Originally Posted by ger21 Yes. Give tham a call and ask about that application. I also heard on one of the Yahoo groups that they have packages a limited number of smaller quantity packages, as the price is a little high. I think it was around $50 for the smaller package, but don't quote me on that.

Oops, my bad (I quoted him )

[D Walck]

The website for Moglice is www.moglice.com. Hope this helps. d

[Swede]

If the ballscrew is rolled (it probably is) then the backlash that is present is normal. Rolled screws have natural lead variation, and if a ball-nut is loaded with oversized balls, as the nut travels in areas with tighter pitch, then the nut will begin to bind. If you look at Thomson-type rolled ballscrews, they eliminate backlash with spring-loaded double nuts back to back. If it was a simple matter to load oversized balls, then I suspect they'd do it.

Although thinking more while typing ( ) perhaps the hand labor associated with oversized balls makes it cheaper for them to do the double-nut trick, so perhaps oversized balls will do the job for you! It certainly won't be an expensive test, just labor-intensive.

[HuFlungDung]

Yes, the trick is to guess what size balls to get. 1/8" balls are about 20 cents each, but as Swede says, it would be a lot of trial to load the nut and try it the full length of the screw to find the size that won't bind up anywhere.

It is good to know that every other ball can be smaller diameter. This lets you get away with purchasing only half as many "working balls" as the nut really requires. The small balls act as spacers between the large ones, to help prevent skidding and jamming of the balls against one another (think of what happens when you let your front bicycle tire rub against your buddy's back bicycle tire )

[Stevie]

The screw looks ground
I'm still waiting to hear from the guys about oversized balls