Preload Strategies for Success
Preloading a precision bearing offers a lot of benefits, like higher speeds and long bearing life. A preload is required for angular contact bearings because these bearings need constant contact between the balls and raceway, which is achieved through a preload. This constant contact allows angular contact bearings to operate at higher speeds and have higher load ratings than a common radial ball bearing.
Sometimes the preload needs to be optimized or adjusted, sometimes it doesn’t need any adjusting. Regardless, you should probably be aware of three outside influences that can alter a bearing’s preload value, they are:
- Application Speed (RPM)
- Application Load (Type of Load)
- Assembly Materials
Understanding how these three factors affect bearing preload will give you the knowledge to counteract them and create a strong bearing preload strategy.
1. Application Speed (RPM)
Application speed is one of the most direct influences on bearing preload. The faster the rotational speeds (RPMs) the more centrifugal force is generated. Centrifugal force produces a powerful radial push on the balls as it rotates around the bearing.
Without a preload, an angular contact bearing with a 15°(C) contact angle could have that contact angle decreased to 5° or increased to 28°. Both situations would have a detrimental effect on the bearing and application.
Counter centrifugal force using bearing preload to keep the balls at their designated contact angle.
When creating your preload strategy, make sure to include the amount of force that will be generated when the application is operating at full speed. Then balance it out with preload to ensure the bearing raceways stay axially aligned.
2. Application Loads
Preload can be affected by static and dynamic application loads, as well as axial and radial application loads.
If there is a known static axial load, like a heavy vertical shaft, then this needs to be considered in a preload strategy (and with the bearing arrangement).
Why? A heavy vertical shaft that creates 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 lifespan but for opposite reasons – preload that’s too low vs. too high.
It’s critical to know and understand the type of application load the ball bearings will support during operation. Make sure to also incorporate an effective bearing arrangement plan into the preload strategy.
3. Assembly Materials
The type of mating material could increase or decrease a bearing press-fit, thereby canceling out bearing preload values.
For example, aluminum is considered a soft material, and the coefficient of thermal expansion is about double that of AISI 52100 bearing steel. If an aluminum housing is mated with our 15° bearing steel S6005 angular contact bearing, a 68°F (20°C) rise in application temperature from room temperature can create ~11 microns of growth between the steel bearing and aluminum housing. This growth differential could decrease the press-fit and cancel out the bearing preload!
To protect against bearing raceway creep during operation, an aluminum housing may require a heavier bearing press fit.
Make sure to account for the thermal expansion of mating materials when creating a preload strategy. Doing so could save you from a bearing failure and will prevent press-fit and preload changes from occurring during operation.
Preload Summary
Hopefully, this gave you insight into how these three factors, speed, loads, and materials could alter a bearing’s preload.
Knowing about these outside influences will give you the tools you need to counteract them and create a strong bearing preload strategy. If you’d like an engineer to help you with your preload strategy, reach out to us on our contact form or give us a call at 800.323.5725.
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