When should you replace a bearing?
The amount of time from the first (initial) damage until the bearing becomes unserviceable can vary considerably. At higher speeds, it can take a few seconds. In large, slow rotating machines, it can take months. The question, "When should I replace the bearing?", is best answered by monitoring the condition of the bearing.
A hard contaminant was over-rolled and made an indentation in the inner ring raceway of a cylindrical roller bearing. The surface initiated fatigue resulting in a spall started just behind the indentation. Over a period of time, spalling became more and more pronounced. If the machine was not stopped in time, secondary damage to machine components could have occurred. The initial indentation is no longer recognizable.
If a damaged bearing goes undiagnosed, and is not replaced before it fails catastrophically, secondary damage to the machine and its components can result. Also, when a bearing fails catastrophically, it can be difficult, even impossible, to determine the root cause of the failure.