Everett,
Yes, its true! Excessive lubrication is a definite failure mode for
anti-friction bearings. This is the failure mechanism … too much grease
within the bearing housing causes an increase in bearing temperature due to
the friction of the bearing moving through the grease pack. This excess heat
can cause failure of the seals (if equipped) and/or deformation of the
rolling element cage. If the seals fail … they can’t keep grease in or
foreign material out. If the cage fails, the rolling elements lose their
operating geometry and rub against each other (creating even more heat) or
carry the load unequally (cyclic loading). Ultimately, the bearing will fail!
The best analogy I can give you for why too much lubrication (typically
grease) is a detrimental to bearing life is as follows:
If you are walking in the surf at the beach, its far easier to walk with
water at your ankles than water at your knees. Much more effort involved
from walking through deep water. Its the same for a bearing … much easier
(less heat generated) to move through the correct amount of lubricant than
attempting to push through a heavy grease pack.
What is the correct amount of grease for a front wheel bearing?
Each bearing should have its cage full of grease with the races and rollers
well lubed. The bearing housing should be filled approximately half way (50%
full) with grease to act as a reservoir. Do not pack the housing completely
full!
Excessive bearing lubrication is NOT the same thing as frequent
re-lubrication intervals. One can pull the bearings out as often as you like
… if you simply inspect and re-lube the bearings, nothing theoretically
wrong with this practice. However, if you understand that the majority of
failures are caused by incorrect installation practices, then you start to
question the need for frequent inspection. Everytime you take the front
bearings apart, you have the potential for incorrect installation. Sometimes
from an engineering perspective, there’s a lot of wisdom in the old proverb
of “If its not broken, don’t fix it!”
Or as my departed grandmother was fond of saying, " The road to hell is paved
with good intentions." Things can get screwed up from the best of intentions
… as in, you didn’t quite get the bearings seated correctly, you punctured
the seal, or you forgot to apply the correct pre-load. Lots of things can
happen!
Hope this helps …
John Pring