Packing A Ball Screw Nut

[Baithog]

I have a Thompson 8105-448-014 ball screw and nut. It feels loose and Thompson rates its repeatability (back lash) as .005. I would like to tighten it up with new balls. No problem getting the balls out or back in, but what size should I get for a start? It is supposed to have 1/8" balls. The ones that are in it are actually .1234. Funny number, but that's what they are.

 

[Wreck™Wreck]

I have a Thompson 8105-448-014 ball screw and nut. It feels loose and Thompson rates its repeatability (back lash) as .005. I would like to tighten it up with new balls. No problem getting the balls out or back in, but what size should I get for a start? It is supposed to have 1/8" balls. The ones that are in it are actually .1234. Funny number, but that's what they are.

Not a funny number at all as there is always a variation during a production run, if one were to search the ABEC standards this is well within one of the tolerance classes.
Often the balls are seperrated into the different classes after finishing and matched with races that finish with a similar accuracy and assembled to the more precise standards, the looser ones are assembled into the less precise standard. A grade 5 ball will be closer to what you want, you may not like the prices however.
Look here http://www.abbottball.com/products/precision-balls.php

 

[rdean]

I would start with 0.125 +- 0.00005 balls from McMaster-Car or maybe a lower priced source. The one time I was connected with this same problem we installed what the manufacture stated the ball size was and it worked well.

Ray

 

[Baithog]

Thanks Ray. I thought about you yesterday, but we had to go to Sam's, Bed Bath, Pinched Penny, S-bucks, Beals, .... So by the time I got the shopping done I was more than willing to just head home. I'll get over some day. I'd really like to see your CNC stuff.

About the balls - I had forgotten about McMaster selling the balls. I looked at Bal-Tec and they have a better selection and better pricing. The only gotcha is the $20 minimum order. That works out to 133 grade 5 .1250 balls. Them grade 3 balls are really pricey, and I can't imagine a mini mill stiff enough to need them. The grade 5's should do just fine.

 

[jumps4]

the balls are also available on ebay
Steve

 

[4GSR]

You may find out that the .125" od ball could make the screw/nut too tight. In fact, you may not get them to work or even in the grooves and channels. Don't hurt to try. One thing to remember on high end ball screws, you don't load the same size ball in the assembly for all of the balls. Every other ball should be about .0005-.0010" smaller in diameter than the other. This keeps the balls from skewing against each other as well as in the grooves. Ken

 

[Baithog]

I took Ray's suggestion and ordered .1250 balls. They came from an ebay vendor that just happens to be down in Tampa. I loaded them last night and it runs smooth and with a lot less floppy. I won't know just how much backlash I have until they are mounted.

I am now contemplating what and how to lube the screws. Oil or grease?

 

[rdean]

I have seen them both ways and don't know what is the best. In my case I ran copper lines out to places where you could get to them and mounted grease jerks.
I am probably the worlds worst when it comes to remembering to oil and grease things as I have only greased them 4 times in two years. They are fairly well protected from dust and metal chips being down under everything.

Ray

 

[chevydyl]

Oil allows higher rpm, I think you would have to oil more than you'd have to grease. Grease isn't as much fun as oil to clean up when maintenance periods arise. I use oil on my ball screws, and I would pick oil almost everytime, when I repacked my Y screw with larger balls I used run of the mill lithium grease on the balls for assembly