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Miniature & small ball bearings


The torque of rolling ball bearings varies depending on the assembly method, the amount of preload, and the type and amount of lubricant. These need to be selected based on the required specification.


There are two basic kinds of torque: Starting torque and running torque.

(1) Starting torque
The torque required to rotate a non-rotating ball bearing. A ball bearing in the static state has the elastic contact deformation generated between the raceways and balls if the loads, like preload, are applied to the shaft. A force to overcome the elastic contact deformation is required to rotate the bearings. Also, a force to overcome the lubricant fill between the balls and raceways is required. These forces (resistance) are the starting torque.

(2) Running 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, grease churning, tight fit, and excessive preload can cause this problem.

Excessive startup current

Grease fill amount, grease churning, 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.

Rotation speed 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 decreases. 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.
For example, the torque fluctuates depending on whether or not the grease on the retainer is connected to the inner and outer raceways.
The shear force of the grease also affects torque.

Load and running torque

Once a load is applied to a ball bearing, both the starting torque and running torque change. For example, torque is higher when preload is applied as compared to when preload is not applied.

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