The reason of Bearing temperature rising is mainly caused by  friction of the bearing that turns into  heat inside the bearing, which causes the temperature of the bearing to rise.

 The heat generated by friction torque can be expressed by the following formula:

If the heat generation and the heat rejection are balanced, the bearing temperature is stable. In normal operation, the temperature rises sharply at the beginning, but it is basically stable when it reaches the normal state. The time until it reaches a stable state, and the temperature varies depending on the amount of heat generated, the heat capacity of the bearing housing, the cooling area, the amount of lubricating oil, and the surrounding temperature. If it is always unstable and cannot reach a stable state, it can only be judged as some kind of abnormality.

The reasons for the abnormal temperature rise are: bearing torsion (torque load); too small clearance, too large preload, too much or insufficient lubricant; foreign matter mixing and heating of the sealing device, etc.