A customer recently called to inform me that a bearing they inspected rattled when shaken. My first thought was to ask, "Why are you shaking the bearings during inspection?" but knowing that many parts experience intense vibration in various applications I decided to ask a question a little more specific to their concern. It turned out that shaking the bearings was a new inspection procedure. While I don't advocate shaking bearings as part of an inspection, this experience did bring to mind a common misunderstood feature of bearing design.

Nearly all bearings with an inner and outer ring have some radial play (also known as internal clearance, or fit). Ranging from five millionths of an inch in very small bearings, to several inches in very large bearings, this play, or space, leaves room among components so a bearing system operates properly.

Most bearings have radial play for two main reasons. The first reason is that radial play leaves room for imperfections in a bearing system. Inside a bearing, individual components are held to very tight tolerances, in the case of balls, as close as one millionth of an inch. Still, no two components are identical. In a bearing with 11 balls, 2 rings, and a retainer, the stack up of tolerances can lead to relatively high variation in a system. Outside a bearing, tolerances in a housing or on a shaft, as well as out-of-roundness, can lead to interference fits among mating components. An excessive interference fit can cause brinelling (denting) of bearing race surfaces or even crushing of components.
An interference fit will also cause excess friction, the second main reason for radial play. Lack of radial play leads to excessive friction throughout a bearing system. In a bearing with appropriate radial play, internal components are not resisting each other or carrying loads in more than one direction. Fewer points of resistance and less loading means less friction, longer life, and lower heat throughout a bearing system.

So What Gives? Radial Play!

There are many choices among bearing radial plays to allow for a wide variety of specific application based solutions. One thing is for sure, an improper fit will lead to premature bearing failure. If you have questions about an application and its radial play requirements, or housing and shaft tolerances, please contact your NPB field sales representative, or a inside technical sales specialist through the contact link on the left side of this page.

A new product will be available through NPB for the dental aftermarket on May 8th 2009. The distribution of ceramic ball dental bearings has been restricted by a patent in the US since 1990 but coinciding with the expiration of that patent, NPB will be stocking a range of ceramic ball bearings for the dental aftermarket. While standard dental bearings remain a high quality and low cost solution to many replacement requirements, ceramic ball bearings will offer an advantage where longer life and quieter operation are the highest values. Hybrid ceramic ball bearings have been used in a wide variety of applications for more than 20 years and have proven their value as solutions to some of the toughest bearing applications. Please contact your NPB technical sales representative with questions about dental bearing availability or ceramic hybrid bearings for your application.

NPB eNews is now monthly. We are eager to bring you industry and product information as a service. Last month's article on the importance of DFARS requirements in doing business with the US Government was of critical importance to some of our customers. See it on the NPB website as a link from the Government page.