Discovering the Secrets of Proper Bearing Lubrication

Dave Garner, Dow Corning

At the heart of almost every industrial facility are electric motors, which can render equipment inoperative should their rolling element bearings fail -an occurrence more common than necessary due to strain resulting from improper lubrication.

Typical causes of failure include the following:

  • Loss of Lubricant. If the bearing is not regreased at the appropriate interval with the proper amount of grease, or if the oil separates from the base thickener of the grease by overheating, loss of lubricant and lubrication can occur and contribute to equipment failure.

  • Grease Incompatibility. Not all greases are compatible with each other. It is important to stay with the same grease or a compatible substitute for the life of a bearing.

  • Incorrect Grease. Be sure to use the correct grease for your application. Some bearing designs and applications need only non-EP or general purpose (GP) grease while others require extreme pressure (EP) grease.

  • Grease Degradation. Grease hardening, chemical breakdown caused by excessive heat, and oil separation from grease base are common types of grease degradation.

  • Excess Lubrication. This occurs mainly with open face bearings, when excess grease can cause an excessive temperature increase in the bearings due to churning and also be pushed back into the windings of the motor and can also cause overheating and deterioration of the electrical insulation on the windings.

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Proper regreasing procedures also play a large role in preventing bearing failure. Following strict regreasing steps in the correct sequence helps to minimize failures:

  1. Regrease a bearing while the motor is running and hot, or after the motor is removed from service and the grease is still hot. Under operational conditions, the grease is less viscous.

  2. Ensure the grease gun contains the right lubricant for the bearings to be regreased.

  3. Clean the areas around the fill and drain fittings to ensure contaminants are not introduced into the bearing cavity.

  4. Remove the drain fitting to allow any excess grease to escape from the bearing.

  5. Leave the drain plug out for the duration of the regreasing process.

  6. With the motor running at operating temperature, add the recommended quantity of grease, or add slowly until it begins to move into the relief tube.

  7. After excessive grease has been purged, reinstall the drain plug and clean excessive grease from the drain area.

Upon installation, monitor motor noise and vibration to establish a baseline. At regular intervals, check these measurements; any sudden or significant change should result in inspection of the bearing lubrication. Motor bearing temperature is also a valuable analysis tool and should be monitored and trended over time. An increasing trend could indicate the need to renew the grease or that the bearings are being overgreased or that a bearing needs to be replaced. Finally, pay attention to any high-frequency (ultrasonic) noise output or vibration level because it may be a sign of bearing lubrication trouble.

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