A bearing is any surface that supports or is supported by another surface. A quality bearing is made of material strong enough to withstand the pressure it is exposed to while permitting the other surface to move with minimal friction and wear. The bearing must be held tightly in position with close tolerance to provide efficient and quiet operation, yet still be allowed to move freely. To do this while reducing friction and power loss, aircraft engines employ many different types of bearings.
Bearings experience radial loads, thrust loads, or a combination of the two. An example of a radial load is a rotating shaft that is being held or contained in a radial plane. An example of a thrust load would be a rotating shaft being contained from moving axially along the axis shafts. There are two different ways in which bearing surfaces move relative to one another. One way, sliding friction, is by the sliding movement of one metal against another. The second, rolling friction, occurs when one surface rolls over the other. There are three types of bearings associated with general use: plain, roller, and ball. Let’s take a look at each.
Plain bearings are used in engine components such as the crankshaft, cam ring, connecting rods, and accessory drive shaft bearings. Plain bearings are usually only used to withstand radial loads, but some types are able to take thrust loads as well. Plain bearings are typically made from nonferrous (lacking iron) materials including silver, bronze, and aluminum, as well as alloys of copper, tin, or lead. In some engines, master rod or crank pin bearings are thin shells of steel plated with silver both internally and externally, in addition to a lead-tin plate over the internal surface. Smaller plain bearings are called bushings. These bearings, such as porous oilite bushings, are used in instances where a shaft in the accessory section of an engine needs support. Bushings of this type are filled with oil such that the heat of friction brings the oil to the bearing’s surface to lubricate during engine operation.
The second type of bearing, ball bearings, consist of grooved inner and outer races, one or more sets of balls, and a bearing retainer. They are used for shaft bearings and rocker arm bearings in certain types of reciprocating engines. Specially-made deep groove ball bearings are used to transmit propeller thrust and radial loads to the engine nose section of radial engines. Because ball bearings can withstand both radial and thrust loads, they are used in gas turbine engines to support one end of a shaft (radial) and keep the shaft from moving axially (thrust).
The final type of general use bearings are roller bearings. While these come in many types, the two most commonly used in aircraft are straight and tapered roller bearings. Straight roller bearings are used for radial loads, while tapered roller bearings, which are cone shaped, can support both radial and thrust loads. Straight roller bearings are ideal for use in the crankshaft of high-powered reciprocating aircraft engines, but are also frequently used in gas turbines where radial loads are high. Most commonly, the rotating shaft in a gas turbine engine is supported by a deep-groove ball bearing one one end and a straight roller bearing on the other end.
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