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"How hot is too hot for a grease-lubricated bearing?"
In the final analysis, if the bearing is hot enough to evaporate the oil off the grease between relubrication cycles, then it is too hot for grease lubrication.
The answer to this question, though, depends on several factors. Let's address a few of them.
When dealing with elevated temperatures, we should first ask, "What is the source of the heat?" Is it ambient? Is it from the process (transmitted down the shaft)? Is it wear-related?
Is it lubrication-related? These questions have a lot of impact on our selection of the grease type and in determining the appropriate answer to the question.
If the temperature is from the process or atmosphere and we know that we have to deal with that condition perpetually, then we should adjust the grease composition to meet the requirements. Perhaps we should select a synthetic oil with a non-soap thickening system that gives us a low evaporation rate. If the temperature is not process or atmosphere-related, then we should make sure that we are not over-filling the bearing with grease.
Too much grease in a cavity causes churning and excessive temperatures. Too little grease, or too infrequent relubrication, may cause friction-generated heat.
If we are using precisely the right volume and we are sure that the heat is internally generated rather than atmospheric or process-related, then perhaps we should examine whether the grease has sufficient strength to support the dynamic load. In this case we might look at selecting a grease with a heavier viscosity base oil.