TABLE OF CONTENTS When the surfaces of two solid bodies are in contact a certain amount of force must be applied to one of them if relative motion is to occur. Taking a simple example, if a dry steel block is resting on a dry steel surface, relative sliding motion will not start until a force approximately equal to one fifth the weight of the steel block is applied. In general, the static friction between any two surfaces of similar materials is of this magnitude, and is expressed as a coefficient of friction of 0.2. As soon as the initial resistance is overcome, a very much smaller force will keep the slider moving at uniform velocity. This second frictional condition is called dynamic friction. In every bearing or sliding surface, in every type of machine, these two coefficients are of vital importance. Static friction sets the force required to start the machine and dynamic friction absorbs power which must be paid for in terms of fuel consumed. Also, friction resistance of unlubricated surfaces causes heating, rapid wear and even, under severe conditions, actual welding together of the two surfaces.
Lubrication, in the generally accepted sense of the word, means keeping moving surfaces completely separated by means of a layer of some liquid. When this is satisfactorily achieved the frictional resistance no longer depends on the solid surfaces but solely on the internal friction of the liquid which, in turn, is directly related to its viscosity. The more viscous the...
