All precision bearings should have a preload established to achieve a high RPM and long application life.
This is especially true for Angular Contact Bearings
An angular contact bearing requires a preload to ensure constant contact of the balls to the race ways.
This constant contact of the balls to the raceway is the main difference in why angular contact bearings perform differently than radial ball bearings.
Much higher RPM ratings and higher load ratings define an angular contact bearing vs. a common radial ball bearing. Having a preload applied to a radial ball bearing will greatly increase the running characteristics and life of most applications. Having established that a preload is almost always required, influences on the optimal preload 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
Application Speed (RPM)
The application RPM has one of the most direct influences on the required preload. Centrifugal forces acting on the balls as they rotate around the bearing push radially out with a lot of force.
This force needs to be counteracted axially — by the preload.
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.
Both static and dynamic, as well as both axial and radial application loads can affect the optimal installed preload.
If there is a known static axial load like a heavy vertical shaft, then this needs to be taken into account for the preload 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 preload 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 / preload.
Mating materials can also play a role in effecting optimal installed preload.
If you are using an Aluminum housing, make sure to take into account this material is considered soft and the coefficient of thermal expansion is around double that of AISI 52100 bearing steel.
Aluminum may require a heavier press fit than mating steel to ensure that there is no creep of the 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 preload. 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 a large amount of rigidity like a precision grinding spindle.
One way to achieve this is to use multiple bearings and to increase the preload 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 preload to achieve a greater level of stiffness.
Note: The term ‘preload’ can have multiple definitions. Please click here for an explanation of the different definitions of preload.