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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 that they are using a premium-quality synthetic grease. What is your opinion?"
Without more details, it is difficult 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 lithium-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.
Grease compatibility is often confusing to grease users, even though most grease manufacturers produce compatibility charts. This is because the charts from the various manufacturers often disagree with one another on certain thickener-type combinations.
In bygone days, when simple soaps and clay were the primary thickener types, compatibility was relatively straightforward. Lithium and calcium soaps were compatible with one another, and neither was particularly good when mixed with a clay-based grease.
Today, with not only the aforementioned thickeners but also complex soaps, polyurea, calcium sulfonate and even more exotic thickeners used in many greases, the issue of compatibility has become much more complicated.
To add to the confusion, there are some grease specifications that are based solely on grease performance without regard to grease composition. If greases of different thickener types (both of which meet the performance requirements of the specification) get mixed in service, dire consequences can result.