One of the properties of a liquid is that it can-not be forced into a smaller space than it already occupies. A liquid is, for all practical purposes, incompressible. It is this incompressible property of a liquid that allows for moving metal surfaces to be kept separated from each other by fluid lubrication. Because of the importance of incompressibility, most materials that are used for lubrication purposes are in liquid form. As long as the lubricant film remains unbroken, fluid friction is able to replace sliding friction and rolling friction.

In any process involving friction, some power is always consumed and some heat is always produced. Overcoming sliding friction consumes the greatest amount of power and produces the greatest amount of heat. Overcoming fluid friction consumes the least amount of power and produces the least amount of heat.

LANGMUIR THEORY

A presently accepted theory of lubrication is based on the Langmuir theory regarding the action of the fluid films or layers of oil between two surfaces, one or both of which are in motion. According to this theory, at least three layers of oil exist between two lubricated bearing surfaces. These layers of oil are shown in figure 8-1. Two of the oil films shown are boundary films and are indicated by roman numerals I and V. A BOUNDARY film is a condition in which the oil film is neither so thin as to cause seizure nor so thick as to create a full film of oil between the shaft and the bearing. As shown in view A, one of the boundary films (I) clings to the surface of the rotating journal. The other boundary film (V) clings to the stationary lining of the bearing. However, boundary film lubrication alone is not sufficient to protect metal surfaces from friction

Figure 8-1.-Theory of oil film lubrication showing boundary and fluid oil films.

and wear. Between the two boundary films are one or more FLUID films that slide layer upon layer. These fluid films are indicated in views A and B by roman numerals II, III, and IV. Refer to view B of figure 8-1. When the rotating journal is set in motion, a wedge of oil (W) is formed. Contact between the two metal surfaces is prevented when oil films II, III, and IV slide between boundary films I and V.

This theory is again illustrated in figure 8-2. Views A, B, and C of figure 8-2 represent a journal or shaft rotating in a solid bearing. The clearances are enlarged in each view to show the formation of the oil film. The shaded portion in

Figure 8-2.-Journal rotation in a solid bearing showing the distribution of the oil film.

each view represents the clearance filled with oil.

The position of the oil wedge (W) is shown with respect to the position of the journal as the journal starts and continues in motion. In view A, the oil film is in the process of being squeezed out while the journal is at rest (stationary). As the journal begins to turn (view B), the oil adhering to the surface of the journal is carried into a space between the journal and the bearing. As shown in view B (starting), the oil film increases in thickness and tends to lift the journal away from the bearing. (Remember, a liquid is incompressible.) As the shaft speed increases, the journal takes a position similar to that in view C (running). It is estimated that in a diesel engine the pressure in the oil wedge can build up to several hundred psi. This pressure, along with the correct viscosity of the oil, is necessary so that the fluid films can slide into place and prevent metal-to-metal contact. With-out this protection, boundary line lubrication alone cannot prevent the damage to metal surfaces that would result from sliding and rolling friction.