DoN. Nichols wrote:
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

I'm thinking a few thousand horsepower for a 6" diameter shaft at 625
rpm, but that's just a guess.

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

I thought the issue Tom was talking about was the thermal insulating
effect of a thick grease barrier?

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. :-)

Well, I've seen alternators with 12" diameter shafts at a place where I
had a summer job once. But they had plain bearings. Hydrostatic when
starting, hydrodynamic when up to speed. Rumour has it than when the oil
pump is switched on, you can rotate a 90-ton rotor in those bearings by
hand. That's what I was told anyway.

Perhaps your reasoning explains why they don't use roller bearings in
those huge alternators, as they do 3,000 rpm. I just remember being told
that rolling element bearings didn't survive under those conditions.

I've also seen a ship with a half metre diameter propellor shaft
(apparently solid too).

Best wishes,

Chris