Preloading a ball bearing is required for angular contact bearings because they need constant contact between the balls and raceway, which is achieved through bearing preload.
The constant contact between the balls and raceway allows angular contact bearings to operate at higher speeds and have higher load ratings than a common radial ball bearing.
In our previous article, we discussed the two common types of ball-bearing preload, factory preload, and spring preload. Sometimes those types of preload need to be further optimized or adjusted, other times the factory preload is good as-is and doesn’t need any adjusting.
The Outside Forces That Could Change Your Ball Bearing Preload
Regardless if you want to change the preload or you’re happy where it’s at, you need to be aware of these three outside influences that could change a ball bearing’s preload:
- Application Speed (RPM)
- Application Load (Type of Load)
- Shaft & Housing Materials
Understanding how these three factors affect bearing preload will give you the knowledge to counteract it 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 outward force on the balls as it rotates around the bearing.
Without enough preload, a high-speed application with a 15°(C) angular contact bearing could have the contact angle (on the outer ring) decrease to 5° or less, and that contact angle (on the inner ring) increase to 28° or more. This would have dire consequences and could lead to the bearing locking up due to pinching or the balls would go from rolling to skidding on the bearing raceway and cause excessive heat and damage.
Preload counters centrifugal force and forces the balls back down in the right spot to keep the balls at their designated contact angle.
When creating your preload strategy, make sure to think about the amount of force that will be generated when operating at full speed. Once you know this, then you can balance it out with the right amount of bearing preload.
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. Shaft & Housing 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, so if we compare it to 52100 bearing steel, the thermal expansion for aluminum is about double that of steel. This means the aluminum expands twice as much as 52100 bearing steel.
Why is this important? Well, if an aluminum housing is mated with our 52100 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 could decrease the press-fit and cancel out the bearing preload.
To protect and prevent this from happening, an aluminum housing may benefit from a heavier press fit on the bearing.
Remember Thermal Expansion When Determining Bearing Preload
To summarize, make sure to know the shaft and housing material and the thermal expansion that may result during operation.
Knowing this could save you from experiencing a bearing failure and will prevent press-fit and preload changes from occurring during operation.
If you want to make sure that your shaft and housing units are machined within the right tolerance range, check out our tolerance calculator. Our thermal range calculator will show you how the material of the bearing, housing, and shaft will grow or shrink when in operation.
Preload Summary
Hopefully, this gave you insight into how these three factors: application speeds, application loads, and shaft & housing materials can alter a bearing’s preload.
Understanding these outside influences and how it affects the bearing’s preload will allow you to prepare a strong preload strategy that will counteract those effects.
Our onsite engineers geek out enjoy working with other engineers to help them put together a strong preload strategy. And they’d enjoy working with you too! Reach out to us on our contact form or give us a call at 800.323.5725 with any questions you may have.
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