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I purchased F688ZZ bearings from robotdigg and 8 mm shafts from Misumi. The shafts are 7.998 mm in diameter and the F688ZZ bearings are less than 7.98 mm inside diameter. Those seem to be too tight of a press fit for the application.
What have other people's experiences been?

I'm looking at buying bearings from Misumi, but don't see ID tolerances anywhere to know if they're going to be a better fit or not.

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Are your sure the end of the shaft doesn't have a small burr preventing assembly?  That shaft OD does seem a bit on the high side for normal bearing use. Normally though, quality bearings are on-size, so even a slightly undersized shaft should go in.  8mm ID's are very difficult to measure, and if your bearings are from a reputable source, your interference may be smaller than you think.  If the bearings are only going on the ends, try polishing the shafts locally with sand paper.  It won't take much polishing, and best if spun by a machine.  You might get them to go together just by freezing the shafts, but will be difficult to re-position or disassemble later. 

 

Never press the inner race onto an shaft by applying force to the outer race.

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Best that I can tell, it seems there are  a few different fits for this form factor of bearing. I'm guessing robotdigg has chosen a part with the wrong fit for this application.

I checked the rods that I ordered and they're supposed to be a g6 fit, which means they're supposed to be 7.986-7.995 mm in diameter.

 

I guess I could chuck the shafts in the lathe and polish them down just where the bearing will be fitted, like you suggest. I'd much rather find a better fit bearing and shaft combination, though. I don't think these should have much interference at all, just enough that there is no radial play in the shaft.

Edited by burdickjp

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