A simple rule for troubleshooting bearing temperatures: No more than 180 degrees Fahrenheit (82 degrees Celsius) on the housing. The bearing outer ring can be up to 20°F (11°C) hotter than the housing. The lubricant originally specified for the application was likely selected to run at lower temperatures. A temperature increase of 50°F (28°C) may cause oil viscosity to drop by 50% or more. This could be the "tipping point" for the application, causing it to go from a hot (but stable) condition, to a hotter, unstable condition where thermal runaway occurs. Eventually, it doesn't matter how much lubricant you add to the application: the oil film is too thin inside the bearing to prevent metal-to-metal contact. Friction and heat build up, which could lead to catastrophic machinery seizure.
A better procedure for "hot" bearings would be:
Check the housing surface temperature. It might be OK, even though most people find anything over 130°F (55°C) too hot to touch.
Slowly add the proper amount of fresh grease to the application (use SKF Dialset). If it's a motor or other machine with a grease relief, open the relief and allow excess grease to escape.
Monitor the temperature and make sure it doesn't run away. (You'll often get a temperature spike after adding grease as the bearing ejects the excess into the housing. This might take 30 minutes to several hours to stabilize, depending on the size of the machine.)
If the temperature keeps going up, call for help - you will probably have to shut down and investigate.
Catastrophic failures can have untold consequences. Contact SKF to apply our Knowledge Engineering to your application - we can help you with temperature issues in bearings. A simple call could save you time, money, and perhaps someone's life.
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