If a gear reducer is running hot, don't automatically think that it needs a higher viscosity oil. Take a sample of oil for a metals analysis. If the sample shows high wear metals, then the high temperatures could be a result of metal-to-metal contact, in which case, a higher viscosity oil would be warranted.

If the wear metals in the sample are low, then the heat could be the result of having too high a viscosity oil, and fluid friction is the source of heat. In this case, a lower viscosity oil may solve the high temperature problem. If these solutions don't work, check the temperature of the drive train (the electric motor, coupling and the reducer). If the electric motor is hotter than the reducer, then maybe the electric motor is undersized and the reducer is acting as a heat sink. In this case, have the electricians install the proper size motor for the load. The oil viscosity may have been correct. (Arnie Russell, Senior Tech Services Advisor, Petro-Canada)

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Book Bits: Synthetics for Extending Oil Drains

From "Basic Handbook of Lubrication"

The use of a synthetic oil may allow oil drain intervals to be extended. In a clean, dry system, a synthetic oil may allow for extended drain periods (double or triple the drain interval of a mineral oil is possible) if the mineral oil was being changed due to oxidation. If the oil drain interval for the system (with mineral oil in use) was set due to external contamination entering the oil, then the use of a synthetic oil will not allow the drain period to be extended. Oil monitoring by laboratory analysis is the key to understanding the situation.

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"We recently switched electric motor rebuild shops. Since switching, a number of bearings have failed, typically just a few months after putting the motor back into service. For the most part, these failures have been attributed to inadequate lubrication. On closer inspection, the grease appeared to have thinned-out to almost a liquid consistency. We suspected that the rebuild shop was using a grease inferior to our electric motor grease, but they assure us they are using a premium-quality synthetic grease. What is your opinion?"

Without more details, it's hard to attribute an exact root cause. However with greases, one of the most commonly encountered problems is incompatibility between different types of grease made from different thickeners.

For electric motors, the most commonly used greases are made either from a lithium complex soap thickener or polyurea material. While both polyurea and Li complex-based greases can be used in this application, the two are usually considered to be incompatible with each other and should not be mixed unless proper compatibility testing has been performed.

To avoid these types of problems, it is advisable to request that the rebuild shop use the same grease that you plan to use to regrease the bearings, or at the very least, indicate the exact type and brand of grease being used, so that you can determine if there are any serious compatibility issues between the two greases.

It is often advisable to provide a tube of grease to the rebuild shop whenever a motor is sent to rebuild to avoid these issues.

Mark Barnes, Noria Corporation

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