Principles of Tribology

What does the word mean? It is derived from the Greek word TRtsos meaning rubbing, so that a literal translation would be ‘the science of rubbing’. The word is so new as to appear in only the latest editions of dictionaries where it is there defined as ‘the science and technology of interacting surfaces in relative motion and of related subjects and practices’. This latter definition, although embracing the literal translation, is of even wider significance and was created to bring together the interest in friction and wear of chemists, engineers, metallurgists, physicists and the like. This wide-ranging concern with tribology immediately illustrates the interdisciplinary nature of the subject. In a sense it is the name alone which is new because man’s interest in the constituent parts of tribology is older than recorded history. Clearly, the invention of the wheel illustrates man’s concern with reducing friction in translationary motion, and this invention certainly predates recorded history. That man should have been so concerned with the tribological problems of friction and wear is not surprising because our involvement with such phenomena affects almost every aspect of our lives.

In the field of tribology it is usually necessary to widen the simple interpretation of a surface as being a geometric plane separating two media. A surface must be recognised as a layer that grows organically out of the solid and has physical properties of considerable functional significance. The surface layer of metals is known to consist of several zones having physico-chemical characteristics peculiar to the bulk material itself.

It is clear that any study of tribology must incorporate a detailed understanding of the mechanics of contact of solid bodies. This involves an understanding of the nature of the associated deformations and the stresses induced by any applied loading to bodies of a wide variety of geometric shapes. In particular we are concerned not only with the deformation and stresses at the surfaces of solids but also throughout the depth of the surface layers. Any load inducing a deformation of solids may readily be resolved into a normal and a tangential component, and it is generally convenient to consider these two influences separately with respect to the stresses and deformation which they induce, and then by superimposing the two obtain the total effect. In such cases the principle of superposition is acceptable since the systems are essentially statically determinate.

Friction is the resistance to motion which is experienced whenever one solid body slides over another. The resistive force, which is parallel to the direction of motion is called the ‘friction force’. If the solid bodies are loaded together and a tangential force is applied, then the value of the tangential force which is required to initiate sliding is the ‘static friction force’. The tangential force required to maintain sliding is the ‘kinetic (or dynamic) friction force’. Kinetic friction is generally lower than static friction.

Wear occurs as a natural consequence when two surfaces with a relative motion interact with each other. Although our understanding of the various mechanisms of wear is now improving, no reliable and simple quantitative law comparable to that for friction has been evolved. This is not surprising since the wear process involves many diverse phenomena, interacting in a largely unpredictable manner. Furthermore, whereas the coefficients of friction of most materials lie between 0.1 and 1.0, corresponding wear rates can vary over many orders of magnitude.

In previous chapters on friction and wear theories we have described the basic interactions between moving surfaces, and the effects of these inter­actions on friction coefficients and wear rates. In the present chapter the aim is to explain the effects of material properties and environment on the observed behaviour and, in particular, to describe the behaviour of materials which have good tribological properties.

Unwanted vibrations which may arise during the operation of machines are costly in terms of reduction of performance and service life, sometimes endangering equipment and personnel. This chapter is concerned with those vibrations occurring through the agency of friction forces at the sliding parts. It introduces methods of study, examines the characteristics, and considers the prevention or alleviation of the vibrations.

It is not without significance to society that the principle of rolling as a method of translation dates from antiquity. None the less, this method of translation may be properly claimed to be the product of man’s ingenuity, and indeed it is known that such an advanced society as the Incas did not discover this principle.

The term lubricant generally suggests oil or grease simply because they are the most common lubricants in use; but they are not exclusive and, in fact, any fluid can be used as a lubricant in the right circumstances. In modern applications a very wide range of fluids are used as lubricants. Air or gas bearings are now quite common, there are numerous examples of the use of water as a lubricant and there is an increasing use of process fluids, one rather exotic example being the use of liquid sodium as a lubricant in nuclear reactors. In some situations solid lubricants are used but the main emphasis in this chapter is on fluid lubricants.

The introduction of a film of fluid between components with relative motion forms the solution of a vast number of triboiogical problems in engineering. In chapter 9 we have seen how lubricants may be supplied to the contact at high pressure. However, in many cases the viscosity of the fluid and the geometry and relative motion of the surfaces, may be used to generate sufficient pressure to prevent solid contact without any external pumping agency. If the bearing is of a convergent shape in the direction of motion, the fluid adhering to the moving surface will be dragged into the narrowing clearance space, thus building-up a pressure sufficient to carry the load. This is the principle of hydrodynamic lubrication, a mechanism which is essential to the efficient functioning of the whole of modern industry. Motor vehicles, locomotives, machine tools, engines of all types, domestic appliances, aircraft, surface and underwater vessels, gearboxes, pumps and spacecraft are only a small part of an almost endless list of equipment and machines which rely heavily on hydrodynamic films for their operation. Although usually so beneficial, hydrodynamic films sometimes occur in situations where they are undesirable or even dangerous. For example, care has to be taken to prevent the formation of such a film of water between the pantograph of an electric locomotive and the conductor in wet weather, and the tread pattern of a motor tyre is an attempt to prevent ‘aquaplaning’, which is the build-up of a hydrodynamic film between the tyre and the road, resulting in a loss of grip.

In the previous chapter we discussed the formation of a hydrodynamic film of lubricant to support a normal load without examining the effects of the size of this load or, more usefully, the value of the load per unit area. We now look more closely at ‘highly loaded’ contacts, where loads act over relatively small contact areas. Such contacts are to be found in the so-called ‘line contacts’ of gear teeth and roller bearings and the ‘point contact’ of ball-bearings. As the contact areas in the latter cases are typically only about one-thousandth of those occurring in such situations as journal bearings, the mean pressures will be about one thousand time greater. We may appreciate that such high pressures will affect the behaviour so that the hydrodynamic solutions which were used to study journal and pad bearings will have to be modified. Indeed we shall find that these high pressures can lead both to changes in the viscosity of the lubricant and elastic deformation of the bodies in contact, with consequent changes in the geometry of the bodies bounding the lubricant film.

We have seen in chapter 10 that hydrodynamic or self-acting bearings only operate effectively if the two following features exist in their construction:

Before considering tribological solutions a designer must first consider the possibility of eliminating the tribological difficulty by using. an alternative design. Designs with fewer moving parts not only eliminate some of the tribological headaches, but invariably provide a cheaper and more elegant solution to the overall problem. In chapter 1 this principle was illustrated by a rather graphic example, but many other less glamorous examples can be quoted. The use of elastometers and flexible linkages to provide a range of oscillatory motions without rubbing interfaces is an established practice, while the recently developed Wankel engine clearly illustrates the same basic principle.