Page 77 - Tribology in Machine Design

3 Elements of contact mechanics
3.1. Introduction
There is a group of machine components whose functioning depends upon
rolling and sliding motion along surfaces while under load. Both surfaces
are usually convex, so that the area through which the load is transferred is
very small, even after some surface deformation, and the pressures and local
stresses are very high. Unless logically designed for the load and life
expected of it, the component may fail by early general wear or by local
fatigue failure. The magnitude of the damage is a function of the materials
and by the intensity of the applied load or pressure, as well as the surface
finish, lubrication and relative motion.
The intensity of the load can be determined from equations which are
functions of the geometry of the surfaces, essentially the radii of curvature,
and the elastic constants of the materials. Large radii and smaller moduli of
elasticity, give larger contact areas and lower pressures. Careful alignment,
smoother surfaces, and higher strength and oil viscosity minimize failures.
In this chapter, presentation and discussion of contact mechanics is
confined, for reasons of space, to the most technically important topics.
However, a far more comprehensive treatment of contact problems in a
form suitable for the practising engineer is given in the ESDU tribology
series. The following items are recommended:
ESDU-78035, Contact phenomena I; stresses, deflections and contact
dimensions for normally loaded unlubricated elastic
components;
ESDU 84017, Contact phenomena II; stress fields and failure criteria in
concentrated elastic contacts under combined normal and
tangential loading;
ESDU -85007, Contact phenomena III; calculation of individual stress
components in concentrated elastic contacts under com-
bined normal and tangential loading.
Although a fairly comprehensive treatment of thermal effects in surface
contacts is given here it is appropriate, however, to mention the ESDU
tribology series where thermal aspects of bearings, treated as a system are
presented, and network theory is employed in an easy to follow step-by-step
procedure. The following items are esentially recommended for the
practising designer:
ESDU-78026, Equilibrium temperatures in self-contained bearing
assemblies;
Part I - outline of method of estimation;