dkshema
rrProfessor
Location: Cedar Rapids, IA
| A little surfing around the web led me to this site that deals with ball bearings, failure modes, wear-pattern recognition, and a whole host of technical information about preloading, not preloading....
http://www.gobearings.com/technical.htm
It's good reading, and may go a long way toward dispelling a lot of the guess, conjecture, and just plain BS that's been going on with regard to bearing failures in the OS 50.
FWIW, the bearings that I've removed from my OS 50's exhibit similar failure patterns, not random failure patterns. The failure mode shows the pattern shown in these pictures:
 
Note that the failure in the outer race is local -- it does NOT continue uniformly around the circumference. The damage to the outer race happens to be at the portion of the bearing that is found at the bottom of the crankcase, the center of the bad area would be at the 6 o'clock position , and extends between about the 5 to 7 o'clock positions.
The inner race shows uniform damage around its circumference.
The damage to the outer race indicates that the outer race, which is fixed in position and doesn't rotate, is the result of a repetitive radial load. The area of greatest damage is directly opposite the center of the cylinder head and indicates that the radial load is being presented to the bearing in a direction parallel to the vertical centerline of the cylinder. This would be consistent with the bearing seeing a heavier than expected load, most probably the result of the shock of the combustion process being transmitted from piston, through the con rod, to the crank, with the piston + or - a few degrees of top dead center. That would indicate that the heaviest radial load on the bearing is the result of absorbing the shock of combustion, the beginning of the power stroke of the motor.
The damage doesn't show up on the remainder of the outer race, the race appears undamaged from about the 7 o'clock position around to the 5 o'clock position.
Damage on the inner race is full circle and uniform. This is consistent with the fact that the inner race ist turning constantly and the balls in their cage are also moving around as the crank turns.
The actual failure mode is that the metal outer race is flaking off. Looking at the race under a stereo microscope looks like looking at a bowl of tiny cornflakes. The bits of recovered metal also resemble small cornflakes.
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It would appear that the failure mode is what this particular web site refers to as "fatigue flaking of the rolling sufaces".
| Quote |
| Nevertheless, all bearings have a finite life under use and will eventually fail to perform satisfactorily due to an increase in noise and vibration, loss of running accuracy, deterioration of lubricant, or fatigue flaking of the rolling surfaces. Such failure modes are considered "normal" and can be predicted using standardized techniques.
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* Making the World a Better Place -- One Helicopter at a time! *
Dave |
