Ball Screw Issue

[please no smoke]

I need a bit of advise.

I am retrofitting a Hurco KMC-3P knee type CNC. I had 0.002" backlash in my x ball screw. It seemed to be visable at the nut interface with the screw. While taking things apart I accidentally pulled the ball screw out of the nut.

It is actually two nuts with 4 individual reciprocating ball paths each. There are ground shims in between the nuts for preload. I was going to reassemble it myself but a machine tech and the hurco tech told me that the balls are different sizes and are placed in a specific place and way. I also seem to not have an even amount of balls (some may have been lost?). As far as I can measure the balls are 0.1245" dia. If there are different sizes then the difference is in the 10 thou range. I can get 1/8" +-.0005 balls easily. Not sure what to do. Can I get new balls and reassemble myself?

A rebuilder I called wants 3 grand, and hurco has a new screw for $1500. A lot of money for an 1985 machine.

Thanks

Matt

[Perfect Circle]

My exp. with ballscrews has been the same "a big pain in the ass"
The smaller balls are to carry oil lube whaterever, and the bigger of the two carries the load. The ballscrew that I repaired is still working fine I just staggered the balls one big one small... try to find all the balls you can.
Good Luck with that job Ive been down that long road, its a lot of trial and cussing...

[underthetire]

We have re-balled screws in a pinch. Your re-builder is crazy, thats way to much. Just load it with new .125 balls and assemble it. If it's too tight, add shims to the nuts. Not a high speed machine by any means, so i wouldn't worry about it.

[please no smoke]

I measured the balls again tonight. They all mic out at 0.1245. Wouldn't .125 be too tight or would it just ride higher on the screw?

[Algirdas]

one micron doesn't matter at this case.

[diecutter]

When I replaced the balls in one of my ball screws I purchased the new balls from a company on Ebay. They had balls in increments of 0.0005 above and below the 0.1250 standard size

try this ToolSupply items - Get great deals on Chrome steel bearing balls, Manuals items on eBay Stores!

I have about 50 balls left over ( 0.1245 as far as I can measure) if you find you have a few missing. Also there are a couple videos on U-tube like this:
YouTube - Repacking the bearings into a ball nut.

[PixMan]

Originally Posted by Algirdas one micron doesn't matter at this case.

.0005" is not one micron. It's closer to 20.

[Algirdas]

sorry, that's 13 micron, my mistake. Not allowed for ball nut, sure.

[HuFlungDung]

A double nut does not really require finding balls of the exact diameter required to eliminate backlash. However, the correct ball diameter is required to ensure ball contact at the proper place in the raceway, which is probably somewhere in the 2 o'clock, 4, 8 or 10 o'clock positions, not at the 12, 3, 6 and 9 o'clock positions. But the average guy can probably assume the original ball size gives the correct contact angle.

A few missing balls is not going to be the end of the world. But you should definitely make sure that the new balls are the exact same diameter. A few big ones will make the nut feel gritty whereas a few small ones serve no purpose except to act as spacers between the larger balls that carry the load.

Rather, the backlash is eliminated through varying the thickness of the spacer. The spacer needs to be perfectly parallel sided or it will cock the two halves of the nut. You can play around with cutting spacer shims out of brass shimstock, but this might only get you close, since shimstock comes in limited thickness variations. Careful lapping of a new set of shims would be best.

The two halves of the double nut must be locked against rotation with the shim in place. There is typically a key screwed in place between the two halves of the double nut for this purpose.