The criteria used to determine the bearing friction of ball bearings are very complex and is still the subject of detailed investigations.
Research and experience have identified some of the key factors on which bearing friction depends:
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
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
In the majority of torque measuring instruments, the bearing to be measured is subjected to a pure axial load (which basically distributes the load on all balls within the bearing at the same time).
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
