Bearing Pre-Load – Defined
The term 'pre-load' is used in many instances and differently between both people and companies. For this reason, below are a few generally accepted definitions of the term to help discussions one may have throughout the design process.
- Factory Pre-Load - The amount of force to axially align the bearing races
- Application Pre-Load - The optimal axial force to run a bearing within an application
- Spring Pre-Load or Spring Load – Installed spring force
In bearing catalogs there will be multiple pre-load values for each bearing as manufactured from the factory. These are usually defined as Light, Medium, and Heavy/Strong. But Extra-Light, specials, etc. also exist. For an example, GMN S6005 CTA A7 U_ can have L/70N, M/200N, and S/400 Newton’s of pre-load. These values have a defined axial offset from the inner race to the outer race. On smaller bearings, this offset can be hard to see even on a granite measuring table. On larger bearings it can sometimes be quite apparent. For the above mentioned GMN S6005… bearing, the axial offset difference from the L to the M is 5 microns per bearing. There is another 5 microns difference from the M to the S. What this means is that it will take another 130 Newton’s of axial force to push the extra 5 microns of offset of a medium vs. a light to align the races axially. In almost all rigidly pre-loaded applications, the races should be aligned axially at the completion of the install. With the races aligned axially, a proper pre-load will be established for the application. Please consult GMN USA Engineering for technical support when choosing a proper pre-load.
Side Note: Many high speed applications may reduce the factory pre-load utilizing spacers. This is done with unequal inner race to outer race axial spacer length. The bearing arrangement will determine which spacer is longer/shorter. This will affect the rigid pre-load of that particular rigid installation application. This can be done for a multitude of reasons; RPM, temperature, duty cycle, etc.
Side Note #2: Clamping force of a precision nut that secures a bearing is much higher than the factory pre-load required force mentioned above. This is for multiple reasons, but know that this clamping force is passed through to the abutment on the shaft and/or housing and will not damage the bearing itself when done properly. Suggested clamping forces and an example procedure.
A common second definition of pre-load has to do with the optimal axial load of a bearing for any application. This gets into a bit of vernacular of one company to another, but it is common for people within the bearing industry to call the optimal axial bearing load for a given application, the pre-load of the bearing. This is usually within the context of the static state of the application, before anything is rotating and before any application loads are applied.
This concept is usually arrived at with the support of a bearing engineer at GMN USA. However, the first step is to establish what the optimal dynamic axial load is for defined application parameters. Then the application effects/loads can be backed out, and the optimal installed static pre-load is known. This static value is commonly referred to as application pre-load.
Bearing Installation Example
- Picture of a spring pre-load installation example.
A spring can be utilized for bearing pre-load. A spring has multiple advantages for a bearing system strategy. An interesting advantage is that one can purchase any factory pre-load bearing. This is because the spring will establish the pre-load, not the L, M, or H values. Because of these purchasing options, the supply chain is sometimes greatly simplified. Secondarily, a spring allows for lower manufacturing costs of mating parts. Axial mating surfaces do not need to be ground to as tight of a tolerance because the spring will make up the few microns difference. Thirdly, a spring can allow some axial movement that is desired in certain applications. Conversely, a spring can be set up to act very rigidly, almost like a rigid installation (no spring). A spring pre-load strategy can have multiple interesting aspects to support a long successful application. Please consult GMN USA Engineering for pro’s and con’s on spring pre-loads.
A suggestion from a bearing engineer… be sure everyone within the conversation knows specifically what ‘pre-load’ is being talked about. Mixing these very specific topics can easily lead a successful application down a detrimental path. Please consult GMN Bearing USA engineering for technical support with any pre-load questions.