Friction in a bearing can be described as the total resistance to rotation. Contributing factors include, but are not limited to:
- elastic deformation of the rolling elements and raceways under load
- lubricant and lubrication method
- sliding friction between the rolling elements and cage, flanges and guide rings, and between the seals and their counterfaces
Each of these contributes to the frictional heat generated by the bearing. The bearing operating temperature is attained when frictional heat and heat dissipated by the application are in balance.
For detailed information about friction in super-precision bearings, contact the SKF application engineering service.
Effects of clearance and preload on friction
High operating temperatures or high speeds can reduce the internal clearance or increase the preload in a bearing. Either of these changes can increase friction. This is particularly important for super-precision bearing arrangements because they are typically preloaded and are extremely sensitive to changes in preload.
For applications that are sensitive to changes in clearance or preload, contact the SKF engineering application service.
Effects of grease fill on friction
During initial start-up, or after relubrication, the frictional moment of a grease lubricated bearing can be exceptionally high during the first few hours or days of operation. This high initial frictional moment, which can be seen as a temperature spike, is caused by the uneven distribution of grease within the bearing free space.
After a running-in period, the frictional moment and bearing operating temperature are typically similar to the values for oil lubricated bearings. Bearings filled with an excessive amount of grease may have higher frictional values.
Frictional behaviour of hybrid bearings
The lower density of silicon nitride rolling elements, compared with steel, reduces internal centrifugal forces. This, combined with their low coefficient of friction, significantly reduces bearing temperatures at high speeds. Cooler running extends the service life of both the bearing and the lubricant.