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Partially supported inner bearing surface

Partially supported inner bearing surface

Partially supported inner bearing surface

Hi all,
I am looking into an application of an "unfortunate" square-peg-in-a-round-hole. I need to fit a 40-mm square extrusion into a 50mm ID bearing.
The application is low speed, <500RPM with low loads (robotic arm).

I am looking into turning (lathe) the square 40mm extrusion so that I end up with 4 rounded corners, with an approximate arc coverage of 7mm per corner (so 7mm supported times 4). I would re-anodize to restore the surface hardness after.

I would thus have around 20% of the nominal bearing surface covered, equally distributed.

Does anybody have any pointers or applications where they've seen this before? Should I be worried about inner race cracking?


RE: Partially supported inner bearing surface

Bearing races are meant to be fully supported.
Even before it cracks, yours will suffer from reduced stiffness in certain directions, and will make funky noises.

Better to bush the bearing ID to fully support it, and terminate the bushing to the extrusion separately somehow.

Mike Halloran
Pembroke Pines, FL, USA

RE: Partially supported inner bearing surface

Do you need the low starting torque of a ball/roller bearing?
Are you concerned with a few 'thou radial ( diametral) clearance?
Maybe use a 1-1/2" square bore agricultural bearing and reduce your 40 mm shaft ~ 0.074 "?

Tolerances on the square shaft were not easily Googled for some reason

RE: Partially supported inner bearing surface

Agree. Don't do that.

We once needed to make a test bench with two 12 mm thick steel supports where we put two 6205 bearings. It was necessary to change the bearings easily and we made a two mm slot in each support so we should be able to pry open the bearing seat and replace the bearings.

It never worked. There was practically no load at all but we could hear each ball bumping across that slot at low speed and we destroyed the bearings very quickly when running at 1500 RPM. Or, rather, we didn't let it run that far. The noise wasn't bearable.

Your 20% coverage is a lot less than our 98 - 99% Don't do it.

Gunnar Englund
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Partially supported inner bearing surface

Great, concise answers.

I'll think of something else,


RE: Partially supported inner bearing surface

Turn your 40 mm square down to a 40 mm round and then press on a 40 mm ID x 50 mm OD bushing to fit the bearing.

RE: Partially supported inner bearing surface

Unfortunately I can't as it is not solid aluminium but an extrusion with a specific shape (similar to T-slot).

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