FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesMedium
A technician replacing main bearings on a reciprocating engine crankshaft installs plain (sleeve-type) bearings rather than ball or roller bearings. What is the primary advantage of plain bearings in this high-load, high-speed application?
- AThey provide a larger bearing surface area that better distributes and absorbs shock loads
- BThey eliminate the need for crankshaft counterweights
- CThey require no lubrication once installed
- DThey allow the crankshaft to be made from a lighter alloy
Show answer & explanationAnswer & explanation
Correct answer: A. They provide a larger bearing surface area that better distributes and absorbs shock loads
Plain bearings spread the load over a larger contact area than the point or line contact of ball or roller bearings, allowing them to better absorb shock loads and vibration typical of reciprocating engine operation, provided a continuous supply of pressurized lubricating oil is maintained to prevent metal-to-metal contact.
Why the other options are wrong
- B. Counterweights balance reciprocating and rotating mass forces and are unrelated to bearing type.
- C. Plain bearings absolutely require continuous pressurized oil flow to prevent wear and seizure.
- D. Bearing type does not determine crankshaft alloy selection, which is based on strength and fatigue requirements.
Plain (Sleeve) Bearings
Bearings that support a rotating shaft through a full-surface sliding contact rather than rolling elements, offering greater load distribution and shock absorption when supplied with pressurized oil.
- Larger contact area than ball/roller bearings
- Require continuous pressurized oil film to prevent metal-to-metal contact
- Common for main and connecting rod bearings in reciprocating engines
Memory trick: Plain bearings SPREAD the load like a wide hand catching a ball, cushioning the shock.