LO 10-2 Describe the purpose, properties, function, composition, types, and applications of diesel engine lubricating oils

Friction and Lubrication

Understanding how lubricant works is a starting point for understanding the purpose and function of a lubricating system. A typical lubricant molecule consists of a long chain of carbon and hydrogen atoms. Because even the most highly polished metal surfaces are rough and irregular at a microscopic level, moving surfaces need to be separated to avoid wear and friction. Oil works by forming a separating film between parts to minimize direct contact between moving parts. Because oil is slippery, it reduces friction and heat buildup caused by friction. Oil is capable of separating moving parts because its molecules behave like billions of tiny ball bearings that physically separate moving metal surfaces Figure 6. At a molecular level, the adhesive forces attracting oil molecules to metal surfaces are greater than the cohesive forces causing oil molecules to stick to one another. This permits lubricants to adhere to the irregular surface of metal yet easily slide over other lubricant molecules. When a supply of oil is available between a rotating shaft and a plain bearing surface, shaft rotation generates oil pressure to form what is called a hydrodynamic wedge . A hydrodynamic wedge actually lifts a rotating shaft journal from direct contact with a plain bearing surface, much like a water ski is pushed up by the water pressure beneath the ski. Like a water ski sliding over a water surface, a shaft glides over a bearing surface with enough lubricant between the two parts Figure 7. When the shaft’s rotational speed increases, more oil is forced into the wedge and the pressure separating the shaft and bearing journal increases Figure 8. The leading rotational edge of the shaft drags sticky oil into the clearance between the shaft and the bearing surface to increase the amount of oil supplied to the hydrodynamic wedge.

FIGURE 10-6: Oil lubrication behaves like ball bearings, enabling metal surfaces to slide over one another without metal-to-metal contact.

FIGURE 10-7: A hydrodynamic wedge is formed between a rotating shaft and a bearing surface. The leading edge of the rotating shaft drags sticky oil between the surfaces.

FIGURE 10-8: The faster a shaft rotates, the greater the pressure formed by a hydrodynamic wedge.