There are two steps to motor bearing maintenance. The first step is to notice when a motor's bearing is deteriorating and replace the motor before it affects the performance of the equipment connected to it. The next step, which is equally important, is to carry out tests and inspections to find out why the failure occurred. Here's how you can conduct a proper inspection of the motor bearings to get to the root of the problem:
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Follow the Clues
1. Make a mark on the bearings to identify which part of the motor each came from, and to remember the orientation of the two bearings. Problems with misalignment or preloading can be more easily traced in this manner.
2. Take apart the bearings using a die grinder or chop saw to cut the outer race in half. Or you can use a grinder to remove the rivet heads holding the retainer together. Remember not to use a torch, which can destroy the evidence you are looking for.
3. Use a cloth to wipe away the grease and examine the ball wear paths so you can find out the cause of the problem.
Consider the Evidence
If the ball wear path follows a diagonal path, the problem with the bearing is that of misalignment.
If the inner race and the outer race are offset in opposite directions of the wear path, this means that the bearing has been preloaded during service. If both ends are preloaded outwards, it means there is not enough room in the motor for thermal expansion of the shaft. If just the drive-end bearing was preloaded, it's a result of external forces.
3. Shaft Current
If the wear path has a washboard appearance known as fluting, this indicates the existence of shaft currents, also known as bearing currents. This is a common problem with motors powered by variable frequency drives.
If the wear path seems highly frosted, it's because of either shaft currents or because of dirt which becomes embedded in the grease. If you can't see any dirt with your naked eye, use the help of a magnifier to examine the wear path. A rough cratering appearance indicates the presence of contaminants.
Keeping these points in mind will help ensure that the problem which leads to bearing failure is addressed, and new motors do not have to succumb to similar problems as well.