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very stupid mistake

snowman

Diamond
Joined
Jul 31, 2004
Location
Southeast Michigan
Ok, so I overtraveled my ball screw and lost some balls. It's a thomson.

Can I reload them myself, or should I send it in to Thomson and have them do it.

It's got a screw to remove the little tube that recirculates them.

If I can reload it, any particular lubrication that I should use on the screw?

-Jacob
 
Hello Snowman.

Installing the balls is not difficult with a little care. We do it often, even replace all the balls when the assembly is starting to loose pre-load. If possible, remove the screw to a bench and set it up on vee-blocks.

Loading the screw is just a matter of inserting the balls into the reticulation holes, then rotating the screw slightly so that the balls rotate down and around the screw until the train of them appears at the outlet to the other hole. Often 2 or 3 turns or circuits of the screw.

It’s hard to tell sometimes if there are gaps between the balls. I normally get a pin punch just slightly smaller than the ball return tube, and place it down the escape hole on the shoulder that the return tube sits on. Holding the nut with the holes always facing up. Rotate the screw until all the balls run away down the opposite hole, and keep filling it until the balls come up to the level of that shoulder the return tube sits on.

Then load the reticulation tube with balls, leaving one or two balls short of a complete length, hold them in with some grease and carefully insert the tube. The pick up fingers on the return tube are often hand fitted. Before I dismantle them, I normally mark them with a needle file. “Nicks” 1 – 2 -3-4 etc, so that you get them back in the same place and orientation. Generally you will find they have been marked by the manufacturer under the saddle clamp. Be careful when fitting the return tubes back, not to tap them to hard, bruises on the tubes are no good.

The only problem, and the one you have now, is that you have to ensure that every ball is either in the correct circuit on the screw or contained in the ball return tube. Loose balls that get trapped on the wrong side of the pick up and locked between the wiper will do damage.

If you have over-travelled the screw, I’d strip the whole thing, to make sure you know where every ball is. It’s not difficult to start from scratch. I’m assuming this is a single nut? With a double nut pre-load setup, remove the key that locks the 2 nuts together, and rotate them apart, until they are loaded.

I have several screws here in the shop for re-balling. If you have any problem, email me, I can take Pics and mail them to you.

You might also consider installing some Poly-urethane bumpers to the bearing housings to prevent this in the future.

Best regards.
Phil
[email protected]
 
snowman,

I'm not 100% positive, but I think the balls in ballscrews are not all identical: there are several sizes, all within a few thou of each other, so you're gonna hafta mic each one and reinstall in the correct sequence. I would check with Thompson on this.

Or is this an old mechanic's wives tale?
 
Ferrous, I believe that's one of those tales. They're generally sized in the millionths for uniformity because any oversize ball will be carrying a disproportionate share of the load. A real knowledgable fellow on another board answered this same question a while back, and explained that this myth got started when some less than reputable "rebuilders" found they could drop a few oversize balls into a worn out screw and remove most of the backlash. In this particular instance another poster was recommending a few oversize balls as an easy way to convert a standard ballnut into a near zero backlash model. That can in fact be done, but it also has to use a full complement of balls with the same oversize, and can't be done if there's more wear in some areas of the screw than other areas, so it's pretty much confined to use with new screws.
 
When my internist was in his residency some of his colleagues treated a man in the ER who arrived after having gotten a good way into giving himself an appendectomy. The surgeons made the shrinks see him first. :eek:
 
My experiance with reloading ballscrews, at least with NSK ballscrews is that every other ball in the circuit is about 0.0005 smaller. They are spacer balls and rotate in the oposite direction. They are only under tension from being pinched by the larger balls, hence they rotate the oposite direction. If all the balls are the same size, they will constantly be rubbing each other. A ballscrew is basically a ball bearing running in a helical path but without the ball seperators. That said, I do beleive that there are some cheap ballscrews produced that have the same size balls. A good bearing house can supply oversized balls to do a quick and dirty job of reloading a worn unit. Balls are avalable in increments of 0.0001 Hope this helps.
Bill
 








 
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