Super Precision Bearing Pre-Load Strategies
All precision bearings in general should have a pre-load to achieve a high RPM with a long system life; this is absolutely required for super precision angular contact bearings. An angular contact bearing requires a pre-load to ensure contact of the balls to the race ways. This constant contact is the crux of the design origin of angular contact bearings from radial ball bearings. Much higher RPM ratings and higher load ratings define an angular contact bearing vs. a common radial ball bearing. Having a pre-load applied to a radial ball bearing will also greatly increase the running characteristics and life of most applications. Having established that a pre-load is almost always required, influences on the optimal pre-load value include but are not limited to:
- Application RPM
- Application loads, static or dynamic and axial or radial
- Mating materials, i.e. shaft and housing
- Application operating temperature
- Required application rigidities/stiffness, axial and radial
- Simplified example of proper installation of a second bearing utilizing a tool that supports both inner and outer bearing races during installation.
- Simplified example of a 'Big O' arrangement with a rigid pre-load.
- Simplified example of a ‘Big O’ arrangement with a spring pre-load.
The application RPM has one of the most direct influences on the required pre-load. Centrifugal forces acting on the balls as they rotate around the bearing push radially out with much force. This force needs to be counteracted axially by the pre-load. This balance of loads will keep the balls riding at or near the designed bearing contact angle. A bearing designed for a 15°(C) contact angle, but running at either 5° (low) or 28° (high) contact angle will have a detrimental effect on bearing life. Some high speed applications with interesting loads may require a 25°(E) contact angle to provide optimal running conditions. Various options and configurations are available to create a successful application. i.e. 18° bearings have become fairly common. Please consult GMN Bearing Engineering for technical assistance with bearing choice, pre-load strategy, arrangement options, etc.
Both static and dynamic, and both axial and radial application loads can affect the optimal installed pre-load. If there is a known static axial load – maybe a heavy vertical shaft – then this needs to be taken into account for the pre-load strategy (and even the bearing arrangement). A vertical application with a heavy shaft creating a static axial load can increase the load on the lower bearings and reduce or maybe eliminate the pre-load on the upper bearings. In this scenario, both bearings may have an equally short life but for opposite reasons – too low vs. too high of axial load / pre-load. If the combination of application loads, pre-load strategy, and dynamic situations are a bit complicated, please consult GMN Bearing Engineering for technical assistance.
Mating materials can also play a role in effecting optimal installed pre-load. If one is utilizing an Aluminum housing for a multitude of reasons, it must be taken into account that this material is ‘soft’ and the coefficient of thermal expansion is around double that of AISI 52100 bearing steel. Softer aluminums may require a heavier press fit than mating steel to ensure that there is no creep of either bearing race while running. Also, if the application temperature is quite a bit higher than the installation temperature, thermal expansion needs to be looked at closely.
To show a thermal effects example, we are using the dimensions of GMN angular contact bearing S6005 CTA A7 UL. A 20°C rise in application temperature from ambient temperature can create a differential of ~11 microns growth between bearing steel and common aluminums. This has an opportunity to create an issue with one’s target running pre-load. This growth differential could either decrease or increase a press fit, depending on where the materials are located and other application parameters.
Rigidity / Stiffness
There are also applications that require an extreme amount of rigidity; i.e. a precision grinding spindle. One way to achieve this is to use multiple bearings. One may also choose to increase the pre-load of the bearings to create this rigidity/stiffness. There are many applications that will both increase the number of bearings at the front(nose) of the spindle and increase the pre-load to achieve a greater level of stiffness. Please contact GMN USA Engineering with any application questions.
Note: The term ‘pre-load’ can have multiple definitions. Please click here for an explanation of the different definitions of pre-load.