In reciprocating aircraft engines, bearings support rotating components while minimizing friction, wear, and power loss. They maintain precise alignment of shafts and moving parts under radial and thrust loads, ensuring smooth and reliable engine operation. Proper bearing design and lubrication are critical for durability, efficiency, and overall powerplant performance.
A bearing is any surface which supports, or is supported by, another surface. A good bearing must be composed of material that is strong enough to withstand the pressure imposed on it and should permit the other surface to move with a minimum of friction and wear. The parts must be held in position within very close tolerances to provide efficient and quiet operation, and yet allow freedom of motion. To accomplish this, and at the same time reduce friction of moving parts so that power loss is not excessive, lubricated bearings of many types are used. Bearings are required to take radial loads, thrust loads, or a combination of the two. An example of a radial load would be a rotating shaft being held or contained in one position on a radial plane. A thrust load prevents the rotating shaft from moving axially along its axis. These radial and thrust loads are illustrated in Figure 1.
 |
| Figure 1. Radial and thrust loads |
There are two ways in which bearing surfaces move in relation to each other. One involves sliding contact between two surfaces (sliding friction), and the second is for one surface to roll over the other (rolling friction).
The three primary types of bearings used in aircraft engines are plain, ball, and roller bearings. [Figure 2]
 |
| Figure 2. Bearings |
| Bearing Type | Load Capability | Typical Application |
|---|---|---|
| Plain Bearing | Radial | Crankshaft mains, connecting rods, camshafts. |
| Ball Bearing | Radial & Thrust | Propeller thrust bearings, rocker arms, turbine shafts. |
| Straight Roller | Radial Only | High-power crankshaft mains, gas turbine supports. |
| Tapered Roller | Radial & Thrust | Wheel bearings (tapered shape handles side loads). |
Plain Bearings
Plain bearings are generally used for the crankshaft, cam ring, camshaft, connecting rods, and the accessory drive shaft bearings. Such bearings are usually subjected to radial loads only, although some have been designed to take thrust loads. Plain bearings are usually made of nonferrous (having no iron) metals, such as silver, bronze, aluminum, and various alloys of copper, tin, or lead. Master rod or crankpin bearings in some engines are thin shells of steel, plated with silver on both the inside and the outside surfaces and with lead-tin plated over the silver on the inside surface only. Smaller bearings, such as those used to support various shafts in the accessory section, are called bushings. Porous Oilite bushings are widely used in this instance. They are impregnated with oil so that the heat of friction brings the oil to the bearing surface during engine operation.
Ball Bearings
A ball bearing assembly consists of grooved inner and outer races, one or more sets of balls, in bearings designed for disassembly, and a bearing retainer. They are used for shaft bearings and rocker arm bearings in some reciprocating engines. Special deep-groove ball bearings are used to transmit propeller thrust and radial loads to the engine nose section of radial engines. Since this type of bearing can accept both radial and thrust loads, it is used in gas turbine engines to support one end of a shaft (radial loads) and to keep the shaft from moving axially (thrust loads).
Roller Bearings
Roller bearings are made in many types and shapes, but the two types generally used in the aircraft engine are the straight roller and the tapered roller bearings. Straight roller bearings are used where the bearing is subjected to radial loads only. In tapered roller bearings, the inner- and outer-race bearing surfaces are cone-shaped. Such bearings withstand both radial and thrust loads. Straight roller bearings are used in high power reciprocating aircraft engines for the crankshaft main bearings. They are also used in gas turbine applications where radial loads are high. Generally, a rotating shaft in a gas turbine engine is supported by a deep-groove ball bearing (radial and thrust loads) on one end and a straight roller bearing (radial loads only) on the other end.