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Posted by Desoto61 on Monday, April 13, 2009 at 08:40:13 :
In Reply to: Drive line noise posted by copey on Sunday, April 12, 2009 at 16:38:38 :
It's been a while since my engineering classes but if I remember correctly, roller type bearing normally fail by fatige. Essentially the compress/release cycle of the rollers as they rotate in their races work hardens the metal until they start developing surface damage. Just like when you break a piece of metal by twisting it back and forth repeatedly.
The grease/oil provides a barrier against direct metal-to-metal friction type wear, as well as for cooling purposes, but it doesn't change the weight loading on the bearing.
There are formulas for calculating the estimated number of rotations for a bearing under a given load before failure. The hard part in a vehicle is the loads are variable based on how the drivetrain operates (not a smooth, regular speed process like most machines), and (as with wheel bearings) are likely to experience frequent shock loading (going over a bump).
So pits in a bearing surface could be caused by corrosion if there are signs of moisture or neglect in the bearing, but otherwise they show inital signs of stress failure.
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