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.
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