Friction: The Application of Statics to Machine Design

The physical cause of the resistance offered by friction to the motion of surfaces which slide on each other can be explained either by supposing an interlocking of the roughnesses of the surfaces, which cannot be separated unless they yield or break or are lifted over the tops of one another; or by supposing that the molecules of the surfaces of the two planes in contact are so close together that they develop a cohesion which must be overcome to produce the motion; experiment alone can enable us to determine the reality of these different causes. —Coulomb, Théorie des Machines Simples (1779).

For the present chapter we shall consider the application of the principles of statics to the solution of a number of problems taken from the field of machine design.

An analysis of the forces involved in the various members of a machine is one of the first steps in the design of any machine, and we shall find that the principles of statics, as we have developed them above, play a very important part in this force analysis.

In many of the problems which we have solved, we have stated that the forces between surfaces in contact are normal to the surfaces in contact, i.e., that the reactions were frictionless. We now wish to investigate more exactly the true nature of these reactions, and to include in our analysis the effects of frictional forces which in all physical problems must be present in some degree.

These frictional forces will sometimes represent undesirable elements of a physical situation, in that there will be associated with them an energy dissipation. At other times we may wish to take advantage of the frictional forces, as in the case of brakes, clutches, and some types of propulsion systems.