Lubricant viscosity selection for bearings

According to the mechanism of lubrication, the viscosity of the lubricating base oil directly affects the formation of the lubricating oil film. Specifically, for the same speed and load, the high viscosity of the lubricating base oil is conducive to the formation of the oil film. Therefore, it is not difficult for many people to come to the conclusion that the higher the viscosity of the lubricating base oil, the better. In fact, as the viscosity of the lubricant increases, the movement resistance of the rolling elements on the raceway surface also increases, which leads to an increase in the resistance torque of the bearing running.

For some occasions that require the bearing resistance torque, it will cause the phenomenon that the bearing rotation is not flexible. For example, some air-conditioning manufacturers require the low-temperature starting torque of the motor to be within a certain range. When the bearing is at low temperature, the lubricant viscosity increases due to the temperature drop, which will cause the bearing starting torque to increase, which will cause starting problems.

On the other hand, the resistance torque caused by the high viscosity of the lubricant will eventually be dissipated as heat during the rotation of the bearing, so the high viscosity of the lubricant will also cause the temperature of the bearing to rise. In engineering practice, there have even been cases where the grease agglomerated due to the high viscosity of the grease, resulting in the inability of the grease to enter effectively between the rolling elements and the raceways to form an oil film.

The indicator for judging whether the viscosity of bearing grease is appropriate is the Kappa coefficient k. At the working temperature, when the Kappa coefficient k is greater than 4, it is obvious that the viscosity of the bearing grease is too high. Therefore, the Kappa coefficient should be checked against the working temperature when selecting lubrication. When the bearing generates heat related to lubrication, it is also necessary to check the kappa coefficient to ensure the correct selection of bearing grease.