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.