Introduction

When two engineering surfaces are loaded together there will always be some distortion of each of them. These deformations may be purely elastic or may involve some additional plastic, and so permanent, changes in shape. Such deflections and modifications in the surface profiles of the components can be viewed at two different scales. For example, consider the contact between a heavily loaded roller and the inner and outer races in a rolling-element bearing. In examining the degree of flattening of the roller we could choose to express the deflections as a proportion of their radii, that is, to view the distortions on a relatively macroscopic scale. On the other hand, as we have seen in Chapter 2, at the microscale no real surface, such as those of the roller or the race, can be truly smooth, and so it follows that when these two solid bodies are pushed into contact they will touch initially at a discrete number of points or asperities. The sum of the areas of all these contact spots, the ‘true’ area of contact, will be a relatively small proportion of the ‘nominal’ or geometric contact area–perhaps as little as only a few per cent of it. Some deformation of the material occurs on a very small scale at, or very close to, these areas of true contact. It is within these regions that the stresses are generated whose total effect is just to balance the applied load.