ABSTRACT
Friction occurs between two surfaces that are pressed together by a force F1 acting normal to the surfaces and prevents them from sliding the one over the other. When a force F2 is applied parallel with the surfaces great enough to just cause them to slide over each other, the limiting frictional force developed can be defined by a coefficient of friction μ, such that F2 = μF1 or F2/F1= tanϕ, where ϕ is called the angle of friction. This angle or the coefficient remains constant over quite a range of normal loads. It seemed strange that two smooth surfaces should respond to increasing contact stress by this proportional increase in sliding force: a very detrimental behaviour for bearings. But it was known that a layer of oil between bearing surfaces reduced the sliding force and made it a function of oil viscosity, independent of normal load, unless it increased sufficiently to squeeze out the oil and permit surface to surface contact, which could cause the bearing to seize. Bowden and Tabor (1945) threw light on this problem by microscopic study of surfaces. They found that apparently smooth metal surfaces are really very irregular with asperities several microns high. When two such surfaces are put together, contact is made only by the asperities and these carry the normal load F1. As F1 is increased, the asperities are compressed and additional asperities make contact. If a 32is the actual area of contact, and p is the pressure required to cause the plastic flow of the asperities, then F1 = ap. Similarly, if s is the shear strength at the asperities, then F2 = as. F2 increases with F1 because, although the apparent area of contact remains constant the actual microscopic area of contact increases in proportion to F1.
