"We have an electric-motor bearing that is making noise. We have tried adding grease to the bearing, which stops the noise for a while, but then the noise returns. Is this cause for concern? Should the bearing be replaced? What else could be causing the noise?"
The first step is to isolate the problem. Since putting grease in the bearing quiets the noise and allows for a period of relief, more than likely the issue is with the bearing. It is quite common for bearings to begin making noises once a defect emerges.
After identifying the problem, you then must determine the underlying root cause. Some detective work will need to be performed. Among the various causes of a noisy bearing include manufacturing defects, lubrication, contamination, mounting conditions and application.
Manufacturing defects make up only a small portion of faults within bearings, as many manufacturers have provisions in place to eliminate these defects. Out-of-round housings, dirty surfaces and rotating shields or seals are often the result of poor installation or mounting conditions. Noises of this sort usually present themselves earlier in the bearing’s life. Shields and seals may also become damaged during lubrication of the bearing.
An inadequate grease volume, the wrong grease or the wrong viscosity are all lubrication-related problems. You can include contamination issues as well, since these regularly occur during relubrication. Foreign particles may become stuck to the end of a grease gun or Zerk and enter the machine during relubrication.
Because the noise stops when grease is applied to the bearing, several things may have occurred. A foreign particle may have entered the cavity and removed surface material from one of the internal surfaces, leaving behind a void. The grease volume could have dropped below a suitable level before relubrication took place. Overgreasing may have occurred at some point and caused the grease to fail, resulting in a lack of lubricant in the cavity. The wrong grease or an incompatible product could also have been used to relubricate the bearing.
Adding grease at this point would only mask the issue. This would be like putting a Band-Aid on a gaping wound that requires medical attention. To determine the severity of the situation, use the data that has already been collected for how long the relubrication interval can be before the bearing starts making noise. If you can employ other condition monitoring technologies like vibration analysis or thermography, these can also aid in predicting when the bearing needs to be replaced before a catastrophic failure occurs.