Selecting a bearing

14. Torque

Torque of ball bearings depends on assembly, preload, and lubricants. These need to be selected based on the required specification.

Torque

There are two basic kinds of torque: Starting torque and running torque.
Starting torque is the torque required to rotate a bearing, starting in the static state.
In the static state, if loads like preload are applied to a shaft, the elastic contact deformation generated between the raceways and balls provide resistance to rotation. Force is required to overcome the elastic contact deformation, and to rotate the bearings. In order to overcome the lubricant forces between balls and raceways, another type of force is required. The total torque required to rotate the bearing from the static state is called "starting torque".
In addition to the force required to stir lubricants by balls, raceways, and retainer, running torque includes friction between the balls and retainer, and the balls and raceways.
The running torque has an impact on heat generation. In a motor application, bearing torque has an influence on the startup current, current rating, RPM rating, current fluctuation, and speed fluctuation. The followings are the factors and solutions.

Failure to reach speed

Some motors do not reach the designed nominal speed. Excessive amount of grease, tight fit, excessive preload, and grease churning can cause this problem.

Excessive startup current

Grease churning, grease fill amount, preload, and fits are the possible factors to be considered in order to solve this problem.

Speed fluctuation

Speed fluctuation is the phenomenon where the rotation speed fluctuates suddenly, and goes back to stable rotation after a while. This happens when grease caught between the balls and raceways loses its channel (wall), and running resistance changes momentarily. Torque is also affected. Decreasing the grease fill amount, changing the grease to higher channeling grease, or non churning type are the choices to make in order to prevent having this issue.

RPM and running torque

Generally, torque increases as RPM increases. Preload and grease are selected based on the RPM.

Grease fill amount and running torque

Generally, torque increases as the grease fill amount increases. Life could be adversely affected also if the grease fill amount is reduced only for the purpose of lowering the torque.

Temperature and running torque

Generally, running torque increases as the temperature goes lower. The reason for this is that the temperature reduction increases the viscosity of the base oil in the grease.

Grease fill position and running torque

Running torque varies with different grease fill positions.
During bearing rotation, whether or not the grease staying on the inner ring or outer ring or rotates with the retainer is critical because it affects the rotating torque of the bearing. The shear force of the grease affects torque.

Load and running torque

Load affects both the starting and running torque. The torque is higher if preload is applied.