wobble / wiggle on ballscrew double row angular contact bearing

[dsc]

I have an R1202 ballscrew mounted in a double row angular contact bearing (3205A2R), held by locknuts, the ballscrew has been machined to a tight slide fit and the runout on the ballscrew is +/-2um, the bearing is held in a housing which holds it tight, there's no play whatsoever. Yesterday I bolted the housing + bearing + ballscrew to a mount plate and here's what I got:



Any ideas why it wobbles / wiggles so much? the funny thing is that I had it mounted to a ballnut with some load on it and it rotates without any issues within the ballnut. Could it be that minimal load is needed for the bearing to work correctly?

Regards,
dsc.

[Clive S]

I think that I would be more than happy with that as you are not supporting it at the end with the dti on it. ..Clive

[Lee Roberts]

It looks to me like there is a small bend in the screw or, is there a nice flat face on the fixed end i.e the screw is sitting in the fixing square?

.Me

[dsc]

The thing is that the ballscrew is dead straight, it rolls on a surface plate with no whipping and I checked it on two V blocks, there's practically no runout. It was checked on a lathe after machining and the runout was 3um total, nothing even remotely close to what's visible on the video. The housing is bolted to a flat, machined surface, the bearing sticks out from the housing by around 0.1mm so when the housing is bolted down, the bearing is definitely pressed against the flat surface. I even tried it on the reverse side of the plate it bolts to (it's also machine) and it does the same wobble.

The strange thing is that it tilts in one direction, but if you go sideways it doesn't tilt by as much. If it was bearing clearance you'd expect to see that in any direction right? The annoying thing is that I have a similar problem on a large 25mm ID double row angular contact bearing, see here:



Same issue, tilts in one direction, but if I was to push on the end sideways, it wouldn't move as much. On that bigger bearing everything is tight, just like on the ballscrew, yet there's loads of play.

Regards,
dsc.

[mekanik]

dsc
Just had a quick look @ SKF
SKF double row angular contact ball bearings correspond in design to two single row angular contact ball bearings arranged back-to-back, but take up less axial space. They can accommodate radial loads as well as axial loads acting in both directions. They provide stiff bearing arrangements and are able to accommodate tilting moments.
So it appears there is some tolerance for angular alignment.
Fit & enjoy
Mike

[Robin Hewitt]

I don't think it has any reason to run true in this configuration. The ball circuits are usually less than 1 full turn so the nut can't hold the screw true if that screw is neither forced square nor held in tension.

[johnsattuk]

I had a similar problem while rebuilding my VTX, I spent some time trying to find the cause, In my case it turned out to be the locknut on the bearing, the face of the nut was not perpendicular to the thread, it had ~ 0.3mm out of square on 16 dia. I turned up a tight fitting thread in the lathe and ground the face of the nut square to the thread, cured it completely. I did all three nuts while I was at it, none of them were true which surprised me as they were INA nuts hardened and ground and had been on the m/c from new.

[dsc]

Thanks for the replies, I can certainly say the ballscrew works with a ballnut. As you said Robin, the wobble doesn't mean much as the ballnut is fixed to the axis plate and the plate is guided via linear rails. The locknuts used on this are SKFs, I haven't tested them for precision, but the wobble also happens on the big bearing which is held with a cap/bolt. This is why I don't think the wobble is caused by how the bearing is held down.

I guess this is normal as Mike said, I'll just use them and not worry.

Regards,
dsc.