Page 189 - Tribology in Machine Design

5 Sliding-element bearings

Sliding-element bearings, as distinguished from the rolling-element bear-
ings to be discussed in Chapter 7, are usually classified as plain journal or
sleeve, thrust, spherical, pivot or shoe-type thrust bearings. Another
method of classification is to designate the bearing according to the type of
lubrication used. A hydrodynamically-lubricated bearing is one that uses a
fluid lubricant (liquid or gas) to separate the moving surfaces. If the fluid
film gets thinner and is no longer able to separate the moving surfaces,
partial metal-metal contact can occur; this type of lubrication is referred to
as mixed lubrication. When the lubricating film gets even thinner and the
two contacting surfaces are separated by a film of a few angstroms thick the
bulk properties of the lubricant are not any longer important and its
physico-chemical characteristic comes into prominence. This type of
lubrication is usually called boundary lubrication. Boundary lubrication is
usually not planned by the designer. It depends on such factors as surface
finish, wear-in, and surface chemical reactions. Low-speed bearings,
heavily-loaded bearings, misaligned bearings and improperly lubricated
bearings are usually more prone to operate under mixed or boundary
lubrication. Boundary lubrication presents yet another problem to the
designer: it cannot be analysed by mathematical methods but must be dealt
with on the basis of experimental data. A completely separate class of
sliding element bearings constitute bearings operating without any external
lubrication. They are called self-lubricating or dry bearings.
In this chapter mainly hydrodynamically-lubricated bearings are
examined and discussed. The problem of bearing type selection for a
particular application is covered by ESDU-65007 and ESDU-67033.
Calculation methods for steadily loaded bearings are presented in
ESDU-84031 and ESDU-82029. The design and operation of self-
lubricating bearings are also briefly covered in this chapter. However, the
reader is referred to ESDU-87007 where there is more information on this
particular type of bearing.

5.1. Derivation of the It is well known from fluid mechanics that a necessary condition for
Reynolds equation pressure to develop in a thin film of fluid is that the gradient and slope of the
velocity profile must vary across the thickness of the film (see Chapter 2 for
details). Three methods for establishing a variable slope are commonly
used:
(i) fluid from a pump is directed to a space at the centre of the bearing,