Handbook of Turbomachinery Second Edition Revised - Ventech!

eplacement of a failed bearing with a like part, with high confidence of a
proper fit. Several classes of fit (shaft-to-bore) are used, depending on the
loads involved and the wishes of the designer. These vary from a light press
fit to interference fits that require a shaft-to-bearing temperature difference
of several hundred degrees for bearing installation. The outside diameter of
the inner race is sometimes termed the inner raceway. It contains a
circumferential groove of a specific depth and radius of curvature. The balls
or ‘‘rolling elements’’ ride in this groove. The groove radius is normally 50–
55% of the ball diameter.
The outer race fits inside a turbomachine housing, or in some cases fits
into a bearing retainer or cartridge, which is itself retained in the machine
housing. The outside diameter of the outer race constitutes the outer
diameter of the bearing. On the inside diameter of the outer race is the outer
raceway, which contains a groove similar to that in the inner raceway.
Rolling in the inner and outer raceway grooves is the bearing’s
complement of rolling elements, which in this example are balls. The balls
maintain the proper spacing between the rotating shaft and stationary
housing. In this example, the ball bearing is said to be of the Conrad variety.
This term makes reference to both the bearing assembly technique and the
fact that the number of balls is such that balls are not immediately adjacent
to one another. In order to maintain proper spacing of the rolling elements
under these conditions another component, the cage, is used. The cage,
which is a relatively thin webbed piece, contains a ‘‘pocket’’ for each rolling
element and many times rides against either the inner or outer raceway.
Depending on the nature of the application, cages have been made of many
different materials, from various plastics to moderately hard alloy steels, as
well as brass, bronze, and fiberglass. Some materials, particularly plastics,
may be unsatisfactory by themselves and have been augmented where
necessary with metallic sidebands to meet strength requirements. It should
be noted here that while it may appear that the cage is not a load-reacting
member, this is definitely not the case. While a detailed force analysis is
beyond the scope of this chapter, it should be made clear that in many
instances the cage is a highly loaded component that requires as much
attention to detail as any other bearing component. In those cases where
cage structural failure has occurred during operation, catastrophic
turbopump failure has resulted.
The current state of the art in rolling element bearings bears witness to
the excellence in engineering, manufacturing, and quality control that is
prevalent in American industry. A bearing sized appropriately for a piece of
machinery, and likely the most highly loaded component in the machine,
can be purchased routinely ‘‘over the counter’’ for a price that is normally
insignificant compared to the cost of the other machine components. In the
Copyright © 2003 Marcel Dekker, Inc.