Types of Friction  Functions of Engine Oil Requirements and Characteristics of  Reciprocating Engine Lubricants Friction may be defined as the rubbing of one object or  surface against another. One surface sliding over another  surface causes sliding friction, as found in the use of plain  bearings. The surfaces are not completely flat or smooth and  have microscopic defects that cause friction between the two  moving surfaces.  [Figure 6-1] Rolling friction is created  when a roller or sphere rolls over another surface, such as  with ball or roller bearings, also referred to as antifriction  bearings. The amount of friction created by rolling friction is  less than that created by sliding friction and this bearing uses  an outer race and an inner race with balls, or steel spheres,  rolling between the moving parts or races. Another type of  friction is wiping friction, which occurs between gear teeth.  With this type of friction, pressure can vary widely and loads  applied to the gears can be extreme, so the lubricant must be  able to withstand the loads. In addition to reducing friction, the oil film acts as a cushion  between metal parts.  [Figure 6-2]  This cushioning effect is  particularly important for such parts as reciprocating engine  crankshafts and connecting rods, which are subject to shock- loading. As the piston is pushed down on the power stroke,  it applies loads between the connecting rod bearing and  the crankshaft journal. The load-bearing qualities of the oil  must prevent the oil film from being squeezed out, causing  metal-to-metal contact in the bearing. Also, as oil circulates  through the engine, it absorbs heat from the pistons and  cylinder walls. In reciprocating engines, these components  are especially dependent on the oil for cooling. Oil cooling can account for up to 50 percent of the total  engine cooling and is an excellent medium to transfer the  heat from the engine to the oil cooler. The oil also aids in  forming a seal between the piston and the cylinder wall to  prevent leakage of the gases from the combustion chamber. Oils clean the engine by reducing abrasive wear by picking  up foreign particles and carrying them to a filter where they  are removed. The dispersant, an additive, in the oil holds the  particles in suspension and allows the filter to trap them as the  oil passes through the filter. The oil also prevents corrosion  on the interior of the engine by leaving a coating of oil on  parts when the engine is shut down. This is one of the reasons  why the engine should not be shut down for long periods of  time. The coating of oil preventing corrosion will not last on  the parts, allowing them to rust or corrode. The engine’s oil is the life blood of the engine and it is very  important for the engine to perform its function and to extend  the length between overhauls. While there are several important properties that satisfactory  reciprocating engine oil must possess, its viscosity is most  important in engine operation. The resistance of an oil to flow  is known as its viscosity. Oil that flows slowly is viscous or  has a high viscosity; if it flows freely, it has a low viscosity.  Unfortunately, the viscosity of oil is affected by temperature.  It was not uncommon for earlier grades of oil to become  practically solid in cold weather, increasing drag and making  circulation almost impossible. Other oils may become so thin at  high temperatures that the oil film is broken, causing a low load  carrying ability, resulting in rapid wear of the moving parts. The oil selected for aircraft engine lubrication must be light  enough to circulate freely at cold temperatures, yet heavy  enough to provide the proper oil film at engine operating  temperatures. Since lubricants vary in properties and since  no one oil is satisfactory for all engines and all operating  conditions, it is extremely important that only the approved  grade or Society of Automotive Engineers (SAE) rating be  used. Friction from metal-to-metal contact Oil film between parts preventing metal-  to-metal contact, reducing friction Figure 6-1.  Two moving surfaces in direct contact create excessive  friction.  Figure 6-2.  Oil film acts as a cushion between two moving surfaces. 6-2