Lubrication between the moving surfaces is caused by a wedge-shaped oil film, which builds up between the surfaces (Fig. 11.2). The wedge-shaped oil film is referred to as boundary lubrication. The film is thicker at the front at leading edge than at the rear. When this film becomes thin so that the surface high spots touch, it is termed as boundary lubrication. With increase of the coefficient of friction more effort is necessary to move the surface. The least effort is required when the correct wedge-shaped oil film exists. Boundary lubrication is an intermediate state between dry friction and fluid hydrodynamic lubrication. It is basically a state where the lubricant fills the cavities, which exist between all friction surfaces. The friction occurs due to the shearing of the lubricant film itself and to the metal-to-metal contact of the high spots of the sliding surfaces.
The coefficient of friction increases during boundary lubrication or when the oil is too thick. The thickness of engine oil is extremely important in preventing boundary lubrication and providing a low coefficient of friction. The thickness of oil is referred to as its viscosity. The viscosity is defined as the tendency of oil to resist flowing. Figure 11.3 represents these
characteristic of lubricant, where Z is viscosity or thickness, N is surface speed, and P is pressure caused by the weight. The coefficient of friction is lowest for one value of ZN/P. If the load P is increased, the value of ZN/P is reduced and the force moves to the left on the graph towards boundary conditions. Any increase in speed increases ZN/P and moves the expression to the right on the graph. It takes more effort to increase the speed while using the same viscosity and load. For any constant speed and load, the oil film is dependent upon the oil viscosity.
Fig. 11.2. Hydrodynamic oil film.
Good boundary lubrication depends on chemical affinity between at least some of the molecules in the oil and the metal surface to be lubricated. Chemical affinity means chemical reactivity, and oil which has been refined to the maximum degree of chemical stability is therefore, unlikely to be able to provide very good boundary lubrication. On the other hand, crankcase oil must be sufficiently stable chemically to have a long life in service before breaking down due to oxidation. Special blending components are therefore used to achieve a compromise between the right degree of chemical ■ reactivity and a tendency towards rapid oxidation.
Boundary lubrication takes place between cylinder walls and piston assemblies, valve stems and their guides, and journal bearings
Fig. 11.3. ZNIP vs. coefficient of friction for lubricant.
when rotating from standstill. Figure 11.2 represents the condition for flat surface lubrication.
The hydrodynamic lubrication principles apply to the curved bearing surfaces in an engine such as main bearing, connecting rod, and camshaft bearings. A crankshaft main bearing lubrication is shown in Fig. 11.4.
Lubricating oil from the engine oil system is supplied through the hole in the upper half of the bearing shell. A groove in the bearing shell retains some oil in the bearing when the engine is stopped. The groove also assists in spreading a film of oil across the bearing surface when the engine is running. When the crankshaft is stationary, the load is straight down and the oil is squeezed out from between the shaft and the bearing (Fig. 11.4A). As the crankshaft rotates, hydrodynamic lubrication acts and a wedge-shaped hydrodynamic oil film is established around the bearing (Fig. 11.4B). This film supports the bearing and, when oil of the correct viscosity is
used, reduces the turning effort to a minimum. With the increase of the crankshaft speed the wedging action of the oil also increases, transferring the maximum pressure around the bearing to the left (Fig. 11.4C). Some oil leaks from the sides of the bearing, which flushes out contaminants and helps to cool the bearing. This requires continuous supply of fresh oil, which is provided by the oil pump to the bearing journal. The bearing wear mostly occurs during the initial start and continues until a hydrodynamic film is established.
Fig. 11.4. Main bearing lubrication.
The formation of hydrodynamic journal lubrication takes place in four stages :
(i) Stationary or Static Friction. When the shaft ^ stationary or revolving very slowly, there is intimate contact between the shaft and the bearing at the base.
(it) Boundary Lubrication. When the shaft starts rotating, it climbs up the bearing plane in a direction opposite to the direction of rotation until the limiting frictional force is reached.
(Hi) Semi-hydrodynamic Lubrication. As the journal’s speed increases, it drags with it a clinging layer of oil and another boundary layer of oil clings to the stationary bearing surface. Between these two layers, oil moves in the same direction as the journal surface movement. Oil is thus dragged into the thin end of the clearance space forming a converging wedge of oil film. This film generally becomes strong enough to support and separate the shaft from the bearing. Thicker or higher-viscosity provide stronger oil films and support heavier loads.
(iv) Hydrodynamic Lubrication. The thickness of the oil film formed between the two faces increases with rising speed and pushes the shaft axis in the direction of rotation, to the opposite side. In reality a stable mean position is established and the shaft axis ‘dances’ about this point with fluctuating loads.
Hydrodynamic lubrication takes place only because most lubricants have strong attachments to the metal surfaces, and relative motion is achieved due to the internal shearing of molecular layers within the oil-wedge film itself. In addition to crankshaft and camshaft journal lubrication, hydrodynamic lubrication often develops on the middle areas of cylinder walls, where piston speeds are highest.