Torque required to put a bearing into motion from rest is usually higher than that required to keep it running once it starts. Starting friction, therefore, has an important influence on the power required in a drive system.
Externally pressurized bearings have very low starting torque. Roller bearings have a low starting torque and underpressurized sleeve (fluid-film) bearings have substantially higher starting torque. The coefficient of friction at start-up for self-lubricated bearings is highly variable. It may range from 0.04 to 0.16.
The fluid-film bearing has a high starting torque because it passes through boundary lubrication stages as it comes up to speed. Once running under a hydrodynamic film, the fluid-film bearing exhibits friction characteristics comparable to a rolling-element bearing.
At running speed, the externally pressurized bearing runs with low friction. Friction in a self-lubricating sleeve bearing is quite variable depending upon the application.
Running friction for a rolling-element bearing is lower than its starting friction. If torque characteristics are critical to a design, starting and running frictional characteristics should be measured experimentally.
When a bearing must be started repeatedly under heavy load, rolling-element bearings offer a better choice than sleeve bearings. When the increased complexity is acceptable, externally pressurized (hydrostatic) bearings are the best choice. When starting load is light and load increases gradually with speed, the conventional hydrodynamic sleeve bearing usually is preferred.
