A bearing is a surface that supports or is supported by another surface. Bearings must be comprised of materials that are strong enough to withstand pressure they are exposed to and should also allow the other surface to move with minimal friction and wear. The parts within the system must be held in close proximity to each other so as to provide efficient and quiet operation while still providing space for freedom of motion. Bearings are also required to withstand radial loads, thrust loads, and combinations of the two. There are two ways that bearing surfaces move in relation to each other and experience friction. The first is the friction that occurs when two metals slide against one another. The second, rolling friction, occurs when surfaces roll over each other. To withstand these conditions, there are four main types of bearings: plain, roller, thrust, and ball bearings.
Plain bearings are typically used for the crankshaft, cam ring, camshaft, connecting rods, and accessory drive shaft bearings. These bearings are typically only subject to radial loads, although there are configurations of plain bearings that can withstand thrust loads. Plain bearings are machined from metals that have no iron, such as silver, bronze, aluminum, and various alloys of copper, tin, or lead. Smaller types of plain bearings, particularly those used to support various shafts in the accessory section of an engine, are called bushings. Bushings operate with sliding motion between moving surfaces. The type of bushing depends on rotational speeds and applied load. Generally speaking, bushings do not require additional lubrication to operate.
Roller bearings come in many types, shapes, and sizes, but the two most commonly used in aircraft engines are the straight and tapered roller bearings. Straight roller bearings are used in areas where they will be subjected only to radial loads. Tapered bearings are able to withstand both radial and thrust loads, due to their inner and outer raceways being cone-shaped. Straight roller bearings are commonly used in high power aircraft engines, serving as the main bearings for the crankshaft. They are also used in gas turbine engines to withstand the high radial loads.
The thrust bearing is a type of rotary bearing. Like other types of bearings, they allow movement between surfaces, but thrust bearings are designed to support an axial load. Thrust bearings have many uses in the aerospace sector, and also have many marine and automotive applications. Thrust bearings are used in aircraft and automobiles to deal with the axial forces caused by the helical gears of the transmission gearbox. These gears are important in the smooth, quiet operation of an engine, but create a significant amount of force. High speed applications of thrust bearings require oil lubrication to function properly.
The ball bearing features a grooved inner and outer raceway, one or more sets of spherical balls, and a bearing retainer. These are used in reciprocating engines as shaft bearings and rocker arm bearings. specially machined deep-groove ball bearings are used to transmit axial and radial propeller loads to the nose of radial engines. Because ball bearings are able to withstand radial and axial loads, they are used in turbine engines to support the shaft and prevent it from moving axially.
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