The torque in ball bearings depends on assembly, preload, seal type and lubricants. These need to be selected based on the required specification.
There are two types of torque in a ball bearing:
Starting Torque and Running Torque
Starting torque is the initial torque required to rotate a bearing in the static state. 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. The total torque required to overcome these is called “starting torque.”
In addition, running torque includes friction between balls and retainer as well as friction between 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, speed rating, current fluctuation, and speed fluctuation. The following are some of the factors and solutions.
Fail to Reach Speed
Some motors fail to reach the designed nominal speed. This could be caused by an excessive amount of grease, excessive interference fit, excessive preload, and use of churning type grease.
Excessive Startup Current
The possible factors to be considered for excessive startup current are: grease fill amount, high viscosity greases, preload, and fit conditions.
Speed fluctuation is the phenomenon where the rotation speed fluctuates unexpectedly and goes back to stable rotation speed after a while. This can be seen when grease loses its channel (wall) and is caught between the balls and raceways changing running resistance momentarily. Decreasing the grease fill amount, changing the grease to higher channeling grease, or non churning type are options to prevent having this issue.
Speed and Running Torque
Generally, torque increases as speed increases. Preload and grease are selected based on the speed.
Grease Fill Amount & Running Torque
Generally, torque increases as the grease fill amount increases. Life could be adversely affected if the grease fill amount is reduced for the purpose of lowering the torque.
Temperature & 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 & Running Torque
Running torque may vary based on the grease fill position. Especially with viscous greases, the shear force of the grease can affect torque.
Load and Running Torque
Load affects both the starting and running torque. The torque is higher if preload is applied.
Testing Reference Standard
GB/T 32562-2016 Rolling bearings – Measuring methods for friction torque
Note: GB/T means Chinese national standard