The increasing complexity of aircraft engine systems has facilitated more requirements of proper lubrication. Aircraft engines require lubrication to prevent friction from reducing the engines’ efficiency; oil is the lifeblood of the engine. If the oil flow to the bearings stops, the lubricating films break down and cause degradation, wear and tear, and burning between moving parts. Fortunately, the engine fuel pump and oil system are very reliable. Like the circulatory system of the human body, they quietly perform their functions.
The central purpose of lubricant is to reduce friction caused by metal foil resistors. Lubricating oils provide a film that permits surfaces to glide over one another with less friction. Therefore, lubrication is essential to prevent wear in mechanical devices where surfaces rub together repeatedly. The selection of the proper lubricant depends on the design of the equipment and the operating conditions. Maintenance instruction manuals or maintenance requirements cards list the type of lubricant required for specific aircraft. With an understanding of the different types of lubricants, their characteristics, and purposes, you will know why we must utilize the proper lubricant. Using the wrong type of lubricant, mixing different types, or lubricating improperly can cause extra unnecessary maintenance and part failures.
Lubricants are classified according to their source—petroleum, mineral, or synthetic. Mineral-base lubricants are usually divided into three groups: solids, semisolids, and liquids. Petroleum-based oils were used in early aircraft engines. This oil was distributed in two grades—1010 for normal use and 1005 for extremely low temperatures. A MIL-PRF 6081 grade 1010 is still used as reserve oil in fuel systems. As the power output of jet engines increased, aircraft were able to fly higher. The operation of jet engines at these higher, colder altitudes and higher engine temperatures created greater demands on lubricating oils. This, in turn, required the development of synthetic lubricants that could withstand these higher bearing temperatures.
Engine lubrication is one of the primary functions oil provide. Oils should have the following characteristics to lubricate properly:
Lubricating oil must cool moving parts by carrying heat away from gears and bearings. This is a vital process considering the numerous parts located next to burners or turbine wheels, where temperatures can reach over 1700°F. Liquid lubricants cool by pumping or spraying oil on/around bearings or gears. The oil absorbs the heat and dissipates it through oil coolers.
Another function of lubricating oil is cleaning. Oil carries dirt, small carbon and metal particles, gum, and varnish to filters. This has become increasingly important with the higher compression ratios, engine speeds, operating temperatures, and closer tolerances between parts in newer engines.