High Thrust Bearing Temperatures Due to Varnish

Why is my thrust bearing running hot?

When you hear this question, it is natural to think that there is something wrong with the thrust bearing. This is not always the case. Bearing performance information can give you the symptoms of what is wrong with your machine or in this case the lubricant.

One example is a 7" LEG 6-shoe thrust bearing that over time, had pad temperatures increase from 100°C to as high as 140°C. Another example is a 6" LEG 6-shoe thrust bearing that after a few months in operation had a temperature difference of almost 20°C between two shoes. Note that rising pad temperatures and large differences in temperature readings over time are typical behaviors when varnish forms on thrust shoes.

Both bearings were installed in compressors and used ISO VG 32 viscosity oil. Each shoe shows signs of varnish deposits toward the trailing edge of rotation.

When reviewing the design conditions, the pad temperatures should have been much lower. A common thought is that the thrust load must be higher than expected. Some customers may even ask the bearing supplier to predict the actual thrust load based on the measured pad temperatures. This value can be misleading since the predicted thrust load with the higher pad temperatures does not take into account the effect of varnish deposits on the shoes.

These bearings were not overloaded. The shoe supports are in excellent condition and show no signs of indentation or hard contact.

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Contributed by: David M. Casper, Engineering Service Coordinator, Kingsbury, Inc