Handbook of Turbomachinery Second Edition Revised - Ventech!

of the bearing. Although rocket turbomachinery and groundbased
machinery are similar in the criteria used for bearing selection, unique
problem areas exist regarding the operation of rolling element bearings in
typical rocket propulsion system applications. We attempt to account for,
analyze, and otherwise discuss some of these unique problems.
In our discussions, we focus on bearings of the rolling element type, that
is, those bearings that allow relative motion between a shaft and housing by
allowing rolling motion between a set of rotating elements (either sphericalor
cylindrical-shaped) and a pair of cylindrical rings, or races. Although a
class of fluid-film bearings (referred to as hydrostatic bearings) has undergone
a recent resurgence of interest, rolling element bearings continue to constitute
the vast majority of bearings in use in rocket propulsion systems. There are
several reasons for this. Rolling element bearings are capable of high radial
stiffness, with values of 2,000,000 lb/in being reported. Also, to ‘‘tune’’ the
critical speeds of a rotor, control of bearing radial stiffness may be required.
This is possible with rolling element bearings by providing an axial bearing
preload to impart the desired radial stiffness.
The radial stiffness of a rolling element bearing is relatively constant
under most reasonable regimes of turbopump operating speeds. The bearing
needs no auxiliary equipment to maintain stiffness at low speeds, in contrast
to hydrostatic bearings, which require an adequate source of pressurized
fluid for operation. If this source is the turbopump itself, a second fluid
source may be needed if a hydrostatic bearing is required to operate before
adequate pump discharge pressure is available. Also, with few exceptions, a
rolling element bearing package of a given load reaction capability tends to
occupy less axial space than other bearing types. This can be important in
machinery where weight is a strong consideration.
For readers who may be unfamiliar with rolling element bearings and
their terminology, a short review might prove constructive. Perhaps the
most commonly encountered type is the general purpose Conrad ball
bearing. This type of bearing consists of four main components. The inner
race fits over a shaft, which, in rocket engine turbomachinery applications,
is usually in rotation relative to a stationary housing of some kind. The
portion of the shaft over which the inner race fits is known as a journal and
is normally of at least moderate hardness and machined/ground to close
tolerances. The inner race inside diameter (known as the bore), measured in
millimeters, is used to partially identify the bearing. For example, a ball
bearing with an inner race diameter of 45 mm is known as a ‘‘45-mm ball
bearing.’’ For a given bore diameter, several bearing ‘‘series’’ exist, with
varying load-carrying capacities and varying outside diameters.
The bearing bore is controlled by bearing manufacturers to very
exacting standards of size, uniformity, and surface finish. This allows ready
Copyright © 2003 Marcel Dekker, Inc.