On 2008-10-10, Christopher Tidy wrote:
REMOVE Tom wrote:

[ ... ]

Grease is a great insulator - the rollers will push it out of their
path forming a wall which prevents it from dissipating the heat.

Surely this is only going to be a problem in bearings which run at tens
of thousands of rpm?

That depends on the size of the bearing. let's say for a 1/4"
ID bearing, the balls run at approximately a 1/2" diameter, and get the
SFM for that at 10,000 RPM as a starting point:

1309 SFM

Now -- let's take the shaft diameter up to say 2", and say perhaps
2-1/2" for the diameter of the bearing ball's path:

5208 SFM

And a really serious electric motor with a 6" shaft (you can guess the
horsepower rating if you wish), and probably 8" diameter bearing ball path:

20,943 SFM

So -- at what speed does the grease migration start to become a problem?

Assuming the 1039 SFM as the top acceptable one with the 1/4"
shaft, we get 625 RPM for the maximum speed for the 6" shaft.

And how big *do* electric motor shafts get? (Of course, the
larger ones do run slower to keep from self-disassembly. :-)

Enjoy,
DoN.

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