The criteria used to determine the friction of ball bearings are very complex and are still the subject of detailed investigation. Research and experience have identified some of the key factors on which bearing friction depends:
Key Factors Affecting Bearing Friction
- Dimensional precision, design and surface quality of the raceways
- Dimensional precision of the balls
- Material of the balls and rings
- Design, material and guidance of the cages
- Properties, quantity, quality and distribution of the lubricant
- Precision of the housing and shaft in or on which the bearings are installed
- The fit tolerances used to set the clearance when installing the bearings
- Magnitude and direction of external loads
- Position of the ball bearing axis
Determining Bearing Sensitivity
The sensitivity of ball bearings is determined by the relative value of one or more of the following forces:
- Starting frictional torque
- Running frictional torque
- Friction peaks
Torque Measurement
In the majority of torque measuring instruments, the bearing to be measured is subjected to a pure axial load (which essentially distributes the load onto all of the balls within the bearing at the same time).
Axial Test Load
- 0.75 N: For ball bearings with an outside diameter of up to and including 10 mm, or up to .375" for bearings in inch dimensions
- 4 N: For ball bearings with an outside diameter of more than 10 mm, or an outside diameter of more than .375" for bearings in inch dimensions