Hi I’m trying to reload a ball screw I have 0.002 backlash on the screw , I took some of the balls out and measured them some are 0.1223 and others are 0.1242. What’s the reason for the two different size balls?
Should I reload it with all the same size or reload the 0.1242 and then load some 0.126 or 0.125 balls in it?
Thanks For the help

I'm not sure why they would have different sized balls, however I know they can wear and appear smaller. I have some small thk ways that I screwed up and dropped once. They weren't caged balls. I did recover all the balls though and I could see that some were worn. They were larger in one dimension than the other.
Someone else will have to fill you on on the appropriate oversized balls to use.

The reason for the alternate size balls was explained in a prior backlash elimination thread for ball screws on the CNC Zone website. To save you the time and trouble of searching for it via the us of the handy, dandy search engine:

The balls were alternately loaded big, little, big, little INTENTIONALLY by the OEM. The small balls act as idlers as they'll tend to try to rotate in the opposite direction as you move the system.

By using all the same size balls, more friction can be encountered and/or the rolilng action can wipe the lube of the balls as they try to spin "against" each other. Premature wear can result.

If you're reloading to reduce backlash, you should only go up in size a few 0.0001" at a time. If you go too big too fast, you can jamb the balls while loading and even brinnell the raceways - NOT GOOD.

NOTE and Caveat: When the system was originally built, the OEM probably had balls in size increments of 0.001mm or so to choose from. You'll be hard pressed to find that level of ball increment in the aftermarket. THey then selectively fit the balls to custom tailor the stiffness and/or backlash.

Reload with care and caution. Your well intended efforts could create more harm than good if you get carried away with preloading the system.

I found those articles on reloading ball screws; I didn’t realizes how small of an increment change affects the overall out come of the backlash. My question is, do I replace the small balls with the same size balls or just change the bigger balls to adjust the backlash

Some common sense goes a long way on this project.

The smaller balls are essentialy spacers/idlers. They probably did not/could not contact the raceway. Thus, changing them to the same size as your existing large balls will not do much if anything to affect backlash/clearance in the device.

Now, if you find some balls that are 0.0001" or 0.0002" larger than your current "big" balls, you could use the current "big" balls as the idler and the new ones as the load carrying/clearance compensating members.

Keep in mind that a 0.0001" ball size change is TWICE that effectively - you have a ball on either side of the screw/nut so the size change is magnified. Moreover, due to the raceway geometry, it could even be more than that - some racedways are "gothic arches" and the ball geometery change affects these in a disproportionate amount.

You'd really want to maintain the same/large ball relationship, especially if you're tryiing to tighten things up....