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Q & A History:

Question #: 1

Subject: Bearing Oil

Date: June 6, 2024

Question: How do you know when to change oils, replace or re-grease bearings?


Good question

Surprising how many seem to get it wrong. For example, a study by the Electric Power Research Institute found that more motor bearings failed because of overgreasing rather than undergreasing. Others report that most rolling element bearings never reach their design fatigue life with the leading cause being improper lubrication.

The answer depends on the life cycle of the component. For example, if under warranty then follow the equipment manufacturers recommends. After that you can do more to optimize the maintenance. The reason being that the product you are using might be better or worse than the standard, and/or the environment, duty cycle, crafts available might be different.

Other factors to consider include importance of that equipment to safety, production, and/or the environment.   Plus, the nature of any consequential damage, the repair costs, and the risk that the repair will not be as good or better than the original.  Some equipment will justify the best efforts while others might be allowed to fail. An example of the latter might be a bathroom fan bearing. It will start to make noise and possibly seize with the motor tripping out on high amps. But even so it does not mean doing nothing, but that most such bearings cannot be relubed and bearing monitoring not likely justified. Bearings are generally not very expensive but if the shaft or bearing fit is damaged the repair costs can mount.

There are many ASTM and other tests for testing lubricants but which ones you want to monitor would be ones that best indicate adverse deterioration and/or which might be a required characteristic. This will vary depending on the equipment and the lubricant. When to change can be best determined by trending results or have a limit based on past experiences. As a rule, it is also less costly to prevent contamination then to remove it. Similarly, considering filtering or dehydrating the oil, rather than just changing it. The reason being that just putting in new oil can be many times the cost of only the oil when all the factors including purchasing, stocking, transferring, and disposal are considered.

In addition, oils are usually easier to monitor than greases but it can be done and ultrasonic have been shown to be a good tool determine when to regrease bearings and with how much grease.

In summary, it depends. There is no one answer but there can be the right answer for each application.