Downloaded from https://www.everyspec.com FM 1-506 lubrications certain to fail. Lubrication not only combats friction but also acts as a cooling agent. oil flow reduces operating temperatures of internal parts not directly cooled by the engine cooling system. Sealing Action WARNING Never mix reciprocating engine oils and gas turbine engine oils; they are not compatible. Mixing them causes engine failure!. Oil helps seal mating surfaces in the engine, and the film of oil on various surfaces is an effective pressure seal. In reciprocating engines the oil film between the cylinder wall and piston and piston rings is important in retaining the high gas pressure in the cylinder.
Cleaning Effect The primary purpose of a lubricant is to reduce friction between moving parts. Because liquid lubricants (oils) can be circulated readily, they are used universally in aircraft engines. In theory, fluid lubrication is based on actual separation of the surfaces so that no metal-to- metal contact occurs. As long as the oil film remains unbroken, metallic fiction is replaced by the internal fluid friction of the lubricant. Under ideal conditions friction and wear are held to a minimum. In addition to reducing friction, the oil film acts as a cushion between metal parts.
This cushioning effect is particularly import- ant for such parts as reciprocating engine crankshaft and connecting rods, which are subject to shock loading. As oil circulates through the engine, it absorbs heat from the parts. Pistons and cylinder walls in reciprocating engines especially depend on oil for cooling. Oil also aids in forming a seal between the piston and cylinder wall to prevent gas leaks from the combustion chamber. Oils also reduce abrasive wear by picking up foreign particles and carrying them to a filter to be removed Oil cleans the engine by picking up carbon and other foreign particles as it passes through and around engine parts.
It carries these particles through the system to a strainer where they are filtered from the oil. LUBRICATING OIL REQUIREMENTS The conditions which the engine operates under determine the requirements for lubricating oil. Condi- tions like temperature, contact pressure, and type and rate of motion vary so much that one lubricant cannot provide ideal lubrication for all components. Using a lubricating oil with all the desirable properties in* degrees will provide satisfactory results. TB 55-9150-200-24 specifies engine oils for use in Army aircraft. Some desirable lubrcating oil qualities are – Viscosity. Antifriction ability. Cooling ability. Chemical stability.
Friction Reduction Viscosity Lubricating oil decreases friction by preventing metal-to-metal contact at bearing points throughout the engine. Separating mating surfaces of moving parts by a thin film of oil changes dry or solid friction to fluid friction. The result is less heat generated in the moving parts and decreased wear on the parts. The degree of resistance of an oil flow at a specified temperature indicates its viscosity. An oil that flows slowly is described as a viscous oil or an oil of high viscosity. An oil that flows readily is said to possess a low viscosity. The viscosity of all oils is affected by temperature. AS the temperature increases, oil becomes thinner.
The rate at which an oil resists viscosity changes through a given temperature range is called its viscosity index The vis- cosity of aircraft engine oil is fairly high because of high operating temperatures high bearing pressures, and relatively large clearances inside an aircraft engine. Since aircraft engines are also subjected to a wide range of temperatures, an oil with a high viscosity index is re- quired. Cushioning Effect Lubricating oil cushions bearing surfaces by absorb- ing the shock between them. Cooling It has been noted that reducing friction results in less heat being generated.
Also, as oil is circulated through bearings and splashed on various engine parts, it absorbs a great amount of heat. Lubrication is particularly impor- tant in reciprocating engines to cool the piston and cylinder. An efficient lubrication system will absorb as much as 10 percent of the total heat content of fuel consumed by the engine. By carrying away this heat, the Antifriction The theory of fluid lubrication is based on the actual separation of metallic surfaces by an oil film. Lubricants should have high antifriction characteristics to reduce 1-5