Clearance reduction caused by temperature difference between shaft, bearing rings and housing
The temperature behaviour of an application can create a difference in temperature between a bearing inner ring and outer ring, which changes the mounted bearing clearance/preload. For a steel shaft and steel or cast iron housing, the change can be estimated using
Δrtemp = ΔT x dm x 12 x 10-6
|Δrtemp||clearance reduction caused by temperature difference [mm]
||temperature difference between inner and outer ring [°C]|
|dm||the bearing mean diameter [mm]
= (d + D)/2
The operating temperature of a bearing reaches a steady state when there is thermal equilibrium – i.e. there is a balance between generated heat and dissipated heat. In the common case where the ambient temperature of the surroundings of the housing of a bearing arrangement is cooler than its shaft, a steady-state temperature gradient is developed that results in the inner ring of the bearing being hotter than the outer ring (→ ΔTsteady in diagram 1).
During start-up, the temperature gradient over the bearing is largely determined by the transient heat flow. Among the various components in contact with the bearing, the one that has the smallest thermal capacity will rise in temperature faster than the one that has the largest thermal capacity. Therefore, the start-up sequence can result in a larger temperature differential between bearing inner and outer ring than in the steady state condition. It results in a temperature peak during start-up (→ ΔTmax in diagram 1). This is especially pronounced in machines that either are working outdoors in a cold climate or have a heated shaft.
Whether during start-up or at steady state, higher speeds generate larger frictional losses. This typically results in a larger temperature differential between the bearing inner and outer ring and therefore a need for larger initial clearance.