Page 20 - Tribology in Machine Design

Introduction to the concept of tribodesign 1

inducing a chemical type of wear. The continuous application and removal
of hydrodynamic pressure on the shaft may dislodge loosely held particles.
In many cases, however, it is the particles of foreign matter which are
responsible for most of the wear in practical situations. Most commonly,
the hard particles are trapped between the journal and the bearing.
Sometimes the particles are embedded in the surface of the softer material,
as in the case of white metal, thereby relieving the situation. However, it is
commonplace for the hard particles to be embedded in the bearing surface
thus constituting a lapping system, giving rise to rapid wear on the hard
shaft surface. Generally, however, the wear on hydrodynamically lubri-
cated bearings can be regarded as mild and caused by occasional abrasive
action. Chromium plating of crankshaft bearings is sometimes successful in
combating abrasive and corrosive wear.

1.2.2. Rolling contact bearings
Rolling contact bearings make up the widest class of machine elements
which embody Hertzian contact problems. From a practical point of view,
they are usually divided into two broad classes; ball bearings and roller-
bearings, although the nature of contact and the laws governing friction
and wear behaviour are common to both classes. Although contact is
basically a rolling one, in most cases an element of sliding is involved and
this is particularly the case with certain types of roller bearings, notably the
taper rolling bearings.
Any rolling contact bearing is characterized by two numbers, i.e. the
static load rating and L life. The static load-carrying capacity is the load
that can be applied to a bearing, which is either stationary or subject to a
slight swivelling motion, without impairing its running qualities for
subsequent rotation. In practice, this is taken as the maximum load for
which the combined deformation of the rolling element and raceways at any
point does not exceed 0.001 of the diameter of the rolling element. L 10 life
represents the basic dynamic capacity of the bearing, that is, the load at
which the life of a bearing is 1000000 revolutions and the failure rate is 10
per cent.
The practising designer will find the overwhelming number of specialized
research papers devoted to rolling contact problems somewhat bewilder-
ing. He typically wishes to decide his stand regarding the relative
importance of elastohydrodynamic (i.e. physical) and boundary (i.e.
physico-chemical) phenomena. He requires a frame of reference for the
evaluation of the broad array of available contact materials and lubricants,
and he will certainly appreciate information indicating what type of
application is feasible for rolling contact mechanisms, at what cost, and
what is beyond the current state of the art. As in most engineering
applications, lubrication of a rolling Hertz contact is undertaken for two
reasons: to control the friction forces and to minimize the probability of the
contact's failure. With sliding elements, these two purposes are at least co-
equal and friction control is often the predominant interest, but failure