Technical>Selection of bearing internal clearance
Selection of bearing internal clearance

The internal clearance of rolling contact bearings during operation (the operating clearance) is a factor that can affect bearing life, vibration, heat, sound, etc.. Therefore, to prevent clearance problems, unmounted bearing clearance should be selected so that operating clearance will be slightly positive.

Usually the bearing internal clearance refers to as Normal when the recommended and operating conditions are normal. Where interference fits are used for both bearing rings, where unusual temperatures prevail etc., bearings with greater or smaller internal clearance than Normal are required.

● Operating clearance
Operating clearance is defined as the clearance of a bearing operating in a machine at the operating temperature and load.

● Operating clearance & clearance influence factors

a ) Increase of clearance due to load (w)
When a bearing is subjected to a load, elastic deformation will occur and this deformation will cause an increase in internal clearance.

b) Temperature effects on radial internal clearance (t )
Under normal operation, the radial internal clearance of a bearing will decrease because of the temperature difference between the inner and outer rings. Typically, the temperatures of the inner ring and the rolling elements are higher than that of the outer ring. When the shaft is heated or the housing is cooled, the difference between the inner and outer rings is even larger.

Notes: Any increase in speed above the reference speed without taking any of these precautions could cause bearing temperatures to rise excessively. At the same time, the operational clearance in the bearing is reduced because of increased inner ring temperature, the final consequence would be bearing seizure. Usually, therefore, a bearing with a C3 internal clearance, which is greater than Normal, is required.

c ) Decrease in radial clearance due to fit (f,fi,fo)
The initial internal clearance (before mounting) is greater than the operational clearance because different degrees of interference in the fits and differences in thermal expansion of the bearing rings and the associated components cause the rings to be expanded or compressed. Therefore, when the inner or outer ring is tight-fitted to the shaft or the house, a decrease of radial internal clearance is caused by the expansion or contraction of the bearing rings.

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