Page 26 - Tribology in Machine Design

2 Basic principles of tribology

Years of research in tribology justifies the statement that friction and wear
properties of a given material are not its intrinsic properties, but depend on
many factors related to a specific application. Quantitative values for
friction and wear in the forms of friction coefficient and wear rate, quoted in
many engineering textbooks, depend on the following basic groups of
parameters:
(i) the structure of the system, i.e. its components and their relevant
properties;
(ii) the operating variables, i.e. load (stress), kinematics, temperature and
time;
(iii) mutual interaction of the system's components.
The main aim of this chapter is a brief review of the basic principles of
tribology. Wherever it is possible, these principles are presented in forms of
analytical models, equations or formulae rather than in a descriptive,
qualitative way. It is felt that this approach is very important for a designer
who, by the nature of the design process, is interested in the prediction of
performance rather than in testing the performance of an artefact.

2.1. Origins of sliding Whenever there is contact between two bodies under a normal load, W, a
friction force is required to initiate and maintain relative motion. This force is called
frictional force, F. Three basic facts have been experimentally established:
(i) the frictional force, F, always acts in a direction opposite to that of the
relative displacement between the two contacting bodies;
(ii) the frictional force, F, is a function of the normal load on the contact,
W,

where/is the coefficient of friction;
(iii) the frictional force is independent of a nominal area of contact.
These three statements constitute what is known as the laws of sliding
friction under dry conditions.
Studies of sliding friction have a long history, going back to the time of
Leonardo da Vinci. Luminaries of science such as Amontons, Coulomb and
Euler were involved in friction studies, but there is still no simple model
which could be used by a designer to calculate the frictional force for a given
pair of materials in contact. It is now widely accepted that friction results