Shaft slightly small for bearings

[leoms]

I have an outrunner which was making noise, and I think the reason is that the shaft doesn't fit tightly in the bearings. It's a 12mm shaft and the bearings are supposed to have 12mm bores, but it turns out that the fit is not tight. I'm going to try to fill the gap with retaining fluid. Anyone have tips for getting the shaft aligned, so the stator and rotor are perfectly parallel? Or, any other approaches which have worked for people? Seems like ideally I'd get a slightly bigger shaft and press-fit it, but I don't know how to obtain a shaft just a bit bigger than 12mm.

Thanks everyone.

 

[amberwolf]

Is there a possibility of changing the bearings for better ones that are the exact fit of the shaft? (or a few thousandths of an inch smaller; you can then heat the bearings and freeze the shaft, install, and have an interference fit).

Or is the shaft simply machined too small by some tiny amount?

 

[Papa]

the bearings are supposed to have 12mm bores, but it turns out that the fit is not tight.

Need more exact numbers - do you have a digital caliper to measure both inside and outside dimensions with?

6 in. Digital Caliper

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You should be able to buy thin, brass shim stock from automotive retailers. As thin as .001". Also, I've resorted to aluminum beverage containers (typically .003-.005" thickness). Also2, a 'feeler gauge' can be purchased and used for shims. But honestly... the correct fix will require knowing the exact dimensions of both.

 

[electric_nz]

The old mechanics trick is to use a punch and a hammer and put a handful of punch marks inside the bearing inner. This will make a shaft fit tightly but not so much that it can’t be removed later. You can also use a drop or 2 of Loctite or similar bearing retention glue.

 

[leoms]

Thanks all... My calipers are mechanical and not quite precise enough to capture this variation, even though they're ticked down to the thousandth of an inch. It appears that the bearings are not bad or worn out: they feel new, and the shaft play feels identical in both, suggesting that they're within a decent tolerance of each other, at least. So I think the shaft is machined too small (or smaller than the bearing manufacturers expected). I think the gap is a few thousandths of an inch, probably.

I like the idea of shims - I suppose I need the shim to go all the way around, for centeredness, right? And I'd somehow affix the shim inside the bearing and then use a hydraulic press or the heat-differential approach amberwolf suggested to put the shaft in?

electric_nz: Why do you suggest punching the bearing as opposed to the shaft? I worry I'd damage the bearing, and the shaft is easier to hold and hit. But point taken, I'll consider this approach.

 

[Chalo]

Loctite 680 "slip fit assist" will reconcile up to 0.015" of diametral clearance. If you let the bond cure with the shaft in a vertical orientation, it will float that bearing concentric with the shaft OD.

 

[leoms]

Oh that's great, sounds like the best option most likely. Thank you.

 

[Chalo]

Oh that's great, sounds like the best option most likely. Thank you.

Make sure the shaft and bearing are clean of particles, and there's nothing pushing on the bearing in any direction while the adhesive cures.

 

[leoms]

The shaft is attached to the outrunner's bell (rotor). If I leave it that way and glue the stator bearings, while sets, the stator will be surrounded by magnets, which will very possibly pull it in one direction or another... is this a problem? Think it's a better idea to un-set the shaft from the bell, do this gluing, and then re-attach the bell? I feel that the attachment in the bell is less robust and harder to get straight, which is why i wanted to do it first, but interested in whether you think that's right.

 

[Chalo]

The shaft is attached to the outrunner's bell (rotor). If I leave it that way and glue the stator bearings, while sets, the stator will be surrounded by magnets, which will very possibly pull it in one direction or another... is this a problem?

I can't really speculate on that. The surface tension that centers the bearing on the shaft is not a large force, so pretty much anything that pushes it off center will win.

 

[amberwolf]

It appears that the bearings are not bad or worn out: they feel new, and the shaft play feels identical in both, suggesting that they're within a decent tolerance of each other, at least. So I think the shaft is machined too small (or smaller than the bearing manufacturers expected). I think the gap is a few thousandths of an inch, probably.

While it's more likely the shaft is machined too small, there's nothing to indicate whether the bearings are made correctly or not, either. (presumably the shaft is also new, and identical at both ends, so the bearings appearing new and identical doens't mean they're not made wrong ).