Bearing Preload

What Is Bearing Preload?

Bearing Preload is an axial or thrust load applied to a bearing that removes excess play. There are many ways to apply preload, as well as many benefits and potential problems with preload. This article will provide an overview of bearing preloading in common radial and angular contact ball bearing applications.

Why Preload?

The purpose of preload in a bearing system is to eliminate clearance that is designed into standard ball bearings. Almost all radial ball bearings are made with a clearance or space between their components to allow for free movement. This space, if not taken up by a preload can allow the rolling elements to slide rather than roll, or even allow for races to misalign.

How Much Preload?

The best preload for a system should be individually determined based on a bearings size and the system's required stiffness, starting torque, running torque, life, and loading parameters. Excessive preload can cause increased heat, fatigue, and torque. Insufficient preload can allow resonant vibration causing fretting of the raceways.

Left image: A bearing with no play, or an interference fit has all rolling elements loaded, wears and heats up excessively.

Center image: A bearing with standard play in the free state has low rigidity, and rolling elements can slide or skid instead of rotating.

Right image: A standard bearing with proper preload applied will provide system rigidity, reduced vibration, and optimal bearing life.

What is the Best Way to Preload a System?

Preload can be applied using two standard methods, solid preload and spring preload. A solid preload is achieved by holding the inner and outer rings of the bearing in place with spacers or locking mechanisms while applying an axial load. Figure two illustrates a solid preload applied mechanically to two radial ball bearings. A spring preload is created using a coil spring or a spring wave washer applying a constant axial load to the inner or outer rings.

Spring preload is the most commonly used method because they are often simpler to assemble. Spring preloads are also more forgiving of slight misalignments or thermal expansion and contraction of components. The advantages of a solid preload is that system stiffness is high, and the design of a mechanical preload system straightforward. The disadvantage is that solid components expand and contract with thermal variation, leading to changes in preload depending on system temperature.

If you have a question about preload in your bearing system, or would like to get engineering advice on your application, contact us today.