Bearing preload is a predetermined amount of axial force that’s applied to a bearing to align the bearing raceways.
Preloading a bearing helps prevent plastic deformation caused by overloaded bearings and optimizes bearing performance. Here are the two most common ways a bearing preload is applied:
- A Factory Preload is produced when the bearings are manufactured and becomes activated when the bearings are installed. This type of preload is usually labeled as Light, Medium, Heavy.
- A Spring Preload is when a separate external load, like springs or a wavy washer, is placed on the bearing
In this article, we’ll discuss these different types of bearing preload and the benefits of each.
Preload for Angular Contact Bearings
First, it’s important to know that a preload is required for angular contact bearings. This is because of the bearing design; it needs constant contact to happen between the balls and raceway. This constant contact is achieved through a bearing preload.
Radial ball bearings don’t need nonstop contact between the balls and raceway; therefore, a preload is usually not required.
However, sometimes a preload on a radial bearing is a better option than switching to a “preload required” angular contact bearing.
For example, a preloaded radial bearing would be beneficial in a centrifuge with tricky mounting requirements and where precision is required while spinning, but reverse load could occur during filling or emptying the centrifuge. Adding a preload using a wavy washer, Belleville spring, or something similar would be helpful in preventing a reverse load from occurring.
Benefits of Preloading a Bearing
Preloading a bearing helps optimize the performance of a bearing. Here are some specific benefits of bearing preload:
- Optimizes the ball spin to roll ratio
- Increases the rigidity of an application
- Protects from excessive ball skidding
- Decreases application vibration and sliding friction
- High running accuracy (even if load conditions keep changing)
- Increases bearing load capacity
The Two Main Types of Bearing Preload
1. Factory Preload
Most bearings, like our GMN angular contact bearings, have a factory preload already on the bearing when manufactured which activates when the bearings are installed. A factory preload is sometimes referred to as the offset value of a bearing.
A manufacturer calculates factory preload from the axial offset value (or clearance) between the inner and outer ring. It’s usually labeled as light, medium, or heavy (strong).
Factory Preload Example
Using our GMN S6005 C angular contact bearing as an example, it has an axial offset increase of 5 microns from a light to a medium preload. Going from a light to a heavy preload on this bearing increases the axial offset by 10 microns.
Why is this important?
Because 130 Newtons of axial force is needed to push those extra microns from light to medium and axially align the bearing raceways. Factory preload is that extra force that will align these raceways correctly.
Once installed, the bearing should be aligned axially and generate the proper preload.
Note: It’s a common practice to optimize the factory preload of a bearing for specific applications. Using spacers on the inner and outer ring of the bearing will make this possible. Our resources and downloads page has an entire section on calculations for changing factory preloads.
2. Spring Preload
A spring preload allows force to bump against it during operation and is a common way to change the factory preload on a bearing. It’s important to know that a spring preload will always override the factory preload values.
Adding a wavy washer or group of coil springs to the bearing arrangement is usually how a spring preload is achieved. Doing it this way will produce a constant axial force against the outer ring of the bearing.
A bearing with a spring preload has some added advantages:
- Most of the time, the mating parts are less expensive because they don’t require being ground to tight axial tolerances, the spring makes up the microns of difference.
- It’s very adaptable and allows a little flexibility in axial movement, which some applications like. But, it can also be very rigid, almost like a rigid installation with no spring.
Bearing Preload Summary
Application loads can sometimes get in the way with a bearing preload, so we recommend reading:
Hopefully, this article demystified ball-bearing preload and you have a good understanding of the differences between factory preload and spring preload and how they can both optimize bearing performance.
Check out our resources and downloads page, we have an entire section on calculations for changing preload values for angular contact bearings. If you have any questions reach out to our engineers they’d love to help.
Read our next article “3 Ways to Change Your Preload” for a deeper dive into ball bearing preload.