LUBRICATION

When two surfaces are in relative motion with a lubricant between them, the phenomenon will fall into one of three rough categories or types of lubrication. First, it is essential to realize that the surface of a smooth piece of metal is not smooth. Smooth is a relative term, and is entirely qualitative. Metal is given a finish which is specified in terms of the permissible roughness. A cylinder wall, for example, might have a 2 µm finish, meaning that the rms roughness is 2 × 10-6 m high. Bearing materials have similar finishes. Boundary lubrication (see Figure 4.1), the first type, involves metal-to-metal contact between the tops of the roughness elements (or asperities) of the two surfaces. This involves deformation and fracture of the surfaces and removal of bits of the surface. The second type is called mixed-film lubrication. The surfaces are separated by a slightly thicker film of lubricant now, and the metal-to-metal contact is only occasional. As the surfaces move farther apart and the lubricant film is thicker, the third type arrives, hydrodynamic lubrication. Now the surfaces never touch, and no wear takes place.

In an automobile engine, the main and connecting rod bearings are intended to operate in the hydrodynamic regime. However, when the engine is first started, if it has not run for several hours, say overnight, the oil film has probably been squeezed out of the bearing, and when first started it is operating in boundary lubrication. As the oil pressure rises, the oil film is replenished, and the lubrication becomes hydrodynamic.