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15 years ago
Electric motor lubrication
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15 years ago
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15 years ago
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15 years ago
Over lubrication is the primary cause of premature bearing failure. Check out lube recommendations on the bearing or grease manufacturers web site. Depending on the capacity of your grease gun usually a couple of shots periodically will suffice.
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15 years ago
.. soap being of course animal fat boiled up with lye
Grant
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15 years ago
Interesting. I never heard that before. I had always assumed that the first grease was derived from animal fat.
Best wishes,
Chris
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15 years ago
Is that true? What damage does it do to the bearings? The worst I've seen is for the excess grease to squirt out and make a mess.
Best wishes,
Chris
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15 years ago
Grease is a great insulator - the rollers will push it out of their path forming a wall which prevents it from dissipating the heat.
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15 years ago
Surely this is only going to be a problem in bearings which run at tens of thousands of rpm?
Best wishes,
Chris
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15 years ago
Good point, Grant. I'd overlooked that.
Does anyone know of a book which describes the process of making grease from oil and soap flakes? I'd be interested to read about it.
Best wishes,
Chris
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15 years ago
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 SFMNow -- 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 SFMAnd 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 SFMSo -- 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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15 years ago
I don't think thermal conduction is the culprit. I am pretty sure that the friction from churning the higher viscosity lubricant raises the temperature. It is an effect not commonly thought of but you can get an idea by running a power paint stirrer in thick paint. Essentially all of the power it takes to spin the stirrer is converted to heat. I had a "bush-hog" gearbox overheat and ruin the seals because I had put in oil which was too thick.
Don Young
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15 years ago
Over greased bearings start heating up at < 1500 rpm
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15 years ago
I'm thinking a few thousand horsepower for a 6" diameter shaft at 625 rpm, but that's just a guess.
I thought the issue Tom was talking about was the thermal insulating effect of a thick grease barrier?
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
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15 years ago
I've sometimes seen two maximum speeds quoted on bearing data sheets: a lower speed for grease lubrication and a higher speed for oil lubrication. I guess the advantage of oil is that it can circulate and carry the heat away, so it doesn't have to rely heavily on thermal conduction.
Best wishes,
Chris
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15 years ago
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15 years ago
Right, Don. Thanks. Got it now!
Best wishes,
Chris