Bridgeport Ballscrew adjustment

Good Day,

I have a .001 backlash on ground ballscrews. The ball screw assembly is good and has no backlash. It is the bearing on the end. The bearing has a shoulder washer ( in hand) keeping it in place that uses shims to adjust the pressure on the bearing. I've adjusted out all the slop but .001. It seems the bearing is defective. I don't understand exactly how the bearing works and want to take it out to inspect it.

How do I remove the bearing for inspection? The bearing race fits loosely (it slides in and out) but the nut looks to be locktite in place. It appears that I should use an impact wrench and some special tool to remove the nut. Do I make the tool or is that a standard tool?

What else do I need to know?

This is a ballscrew from a Millpwr installation on a Bridgeport mill.

http://i230.photobucket.com/albums/e...2/DSCN0249.jpg

Not quite sure why the bearing retaining washer would have shims. The nut tightens the inner race (s) [I think there are two bearings back to back on the thrust side] to a shoulder on the ball screw. The retaining washer tightens the outer races to the housing. All of that should be solid so if there are shims in there that might be the problem. Neither the washer nor the nut put any preload on the bearing.

You said you have adjusted out all but .001 of slope; What were you adjusting?

You can buy a spanner wrench to the nut or just make one.

Quote:

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Originally Posted by LeftCoastCNC

Not quite sure why the bearing retaining washer would have shims. The nut tightens the inner race (s) [I think there are two bearings back to back on the thrust side] to a shoulder on the ball screw. The retaining washer tightens the outer races to the housing. All of that should be solid so if there are shims in there that might be the problem. Neither the washer nor the nut put any preload on the bearing.

You said you have adjusted out all but .001 of slope; What were you adjusting?

You can buy a spanner wrench to the nut or just make one.

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Hi Leftcoast,
The shims are there so You can seat the outside of the beaing into the bore it is in without crushing it. I believe he was changing the amount of shime to set the bearing position and endplay.

Pofo,
What you are looking for is called a hook spanner. You could also make a socket with the 4 square pins to work with a 3/8 ratchet.

If that is high temp red loctitie, you will mos likely have to apply heat to soften the loctite but don't go crazy with the heat or you will just screw up the ball screw.
Are you sure that there is only one bearing on this end? There are normally 2 angular contact bearing in oopositon so that one will take the load in one direction and the other takes the load in the other direction.


Merry Christmas

Mike

Thanks guys.

I made a wrench from a socket. Indeed, there are two bearings as you described.

Yes, the shims determine how firm the washer presses down on the outer race. I removed a shim and reduced the backlash from .005 to .001. That was before I took the nut off and had a look at the bearings.

Somehow I made things worse. I'm back to .005 backlash. I did learn that the bearings need to be put together in a specific orientation (sorry, I don't know correct terminology), such that the inner races face each other such that they touch each other and the nut presses them tightly against the ballscrew. Then the outer races are pressed towards each other with the shoulder washer to take out the slop. At least that is the way I have it figured out. Maybe that is backwards because it is worse now, not better. I'll take another look at it and sort it out. I think I am close to understanding how it works and will be able to make the proper adjustments to get it right.

All part of the learning curve. But that's why I started this project.

Quote:

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Originally Posted by pofo

Thanks guys.

I made a wrench from a socket. Indeed, there are two bearings as you described.

Yes, the shims determine how firm the washer presses down on the outer race. I removed a shim and reduced the backlash from .005 to .001. That was before I took the nut off and had a look at the bearings.

Somehow I made things worse. I'm back to .005 backlash. I did learn that the bearings need to be put together in a specific orientation (sorry, I don't know correct terminology), such that the inner races face each other such that they touch each other and the nut presses them tightly against the ballscrew. Then the outer races are pressed towards each other with the shoulder washer to take out the slop. At least that is the way I have it figured out. Maybe that is backwards because it is worse now, not better. I'll take another look at it and sort it out. I think I am close to understanding how it works and will be able to make the proper adjustments to get it right.

All part of the learning curve. But that's why I started this project.

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Remove the bearings, and look carefullly at the inner races. You will see each inner race is wider on one side than the other. The wider sides must face outwards, away from each other. I would then get trid of that "shim" ( which looks more like a gasket....). The inner races should be locked tightly together by that nut. The outer races also should be locked tightly together, by the retainer. There is nothing there that can be over-tightened in a way that will harm the bearings. Both the locknut and the retainer MUST be tight for the bearings to work properly. They are *meant* to be tightly held together, and if they're not, you will have backlash. They are a ground pair of angualr contact bearings, which will be properly pre-loaded and backlash-free *only* when the inner races are locked tightly together, and the outer races are locked tightly together.

Pack them with clean, good-quality grease before reassembling them as well.

Regards,
Ray L.

Thanks Ray. That was a nice clear explanation. That fixed it.

Two questions:

- You say I can't overtighten. But might I not put too much force on the bearings without the shim?

- Similarly, without the shim, might I put more force on one bolt over another and put an uneven force on the bearings?

Thanks.

Also make sure the bearings do not have alignment marks. Usually precision bearings do need alignment to keep them from fighting each other and wearing out prematurely.
I usually put a dial indicator on the end of the ball screw to make sure it is not moving in and out. That would indicate a bearing issue. If the ball screw has no play but the table does, the problem may be in the fit of the ball nut to the ball screw or the mounting of the ball nut.

George

Quote:

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Originally Posted by pofo

Thanks Ray. That was a nice clear explanation. That fixed it.

Two questions:

- You say I can't overtighten. But might I not put too much force on the bearings without the shim?

- Similarly, without the shim, might I put more force on one bolt over another and put an uneven force on the bearings?

Thanks.

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Over-tightening would require distorting the bearing races - not easy to do. The way the bearings are made, if you install them in the correct orientation, you *can't* over-pre-load them, unless you get really carried away in tightening the fasteners. Just torque the cover bolts equally, and you'll be fine.

Regards,
Ray L.

Hi pofo

!!! Similarly, without the shim, might I put more force on one bolt over another and put an uneven force on the bearings???

YES you will bend the flange on the cover plate & put uneven force on the bearing

YOU need to have the shimes, to stop this happening.

You will need about .004/.005 MAX push on the front cover to get the right preload on the bearing face ajust the shim thickness to get this

These photo may help some as well

Thanks once again guys. That's very helpful.

Mac, those diagrams really cleared it up for me. Now I understand preload. I also now understand what Ray was saying about not being able to overtighten them.

I'm going to shim it anyway to keep the washer flat.