Common Angular Contact Bearing Preload Definitions
- Factory Preload: The amount of force used to axially align the bearing races
- Application Preload: The optimal axial force to run a bearing in an application
- Spring Preload: A bearing that “springs” or allows force to bump against it while in use but still has controlled movement.
Bearing catalogs usually list preload values as light, medium, and heavy/strong (extra-light and specials are also seen).
These ‘factory preload’ terms have a defined axial offset value that’s measured from the inner race to the outer race.
For example our GMN S6005 bearing has an axial offset difference of 5 microns from the Light to the Medium preload. If you are going from Medium to Strong (Heavy) preload, then another 5 microns of difference is offset.
Why is this important? Because 130 Newtons of axial force is needed to push those extra 5 microns to align the races axially.
For rigidly preloaded applications, the races should be aligned axially once installation is complete. Once this happens, the proper preload is established.
Reduced Factory Preload
Many high speed applications reduce factory preload using spacers with unequal inner race to outer race axial spacer length. The arrangement of the bearings on the application will determine which spacer is longer or shorter.
This will affect the rigid preload of that particular install. Usually this is done because of RPM, temperature, duty cycle, etc.
Clamping force from a precision nut that secures a bearing is much higher than the factory preload required. The clamping force is passed through to the abutment on the shaft or housing and will not damage the bearing itself when done properly. Suggested clamping forces and an example procedure.
This type of preload is about finding the right axial load of a bearing for any application, usually when the application is in it’s static state. A static state means when the application is still or not rotating and before any loads are applied to the application.
Here are the steps you would take to find the answer:
- First you will need to establish the optimal dynamic axial load for the application.
- Once you have the above, you can back out the applications efffects/loads to reach the optimal static preload.
This static value is commonly referred to as application preload.
A spring preload has multiple advantages.
- You can purchase any factory preload bearing & use a spring to change the preload. This is because the spring will create the preload, not the factory Light, Medium, or Heavy values.
- A spring preload has lower manufacturing costs of mating parts. Axial mating surfaces don’t need to be ground to as tight of a tolerance because the spring will make up the few microns difference.
- A spring allows some flexibility in axial movement. This is sometimes desired in certain applications. But on the flip side, a spring can be set to act very rigid, almost like a rigid installation with no spring.
A spring preload strategy has many benefits that support a long successful running application.