In a piston engine, the main bearings are the bearings on which the crankshaft rotates, usually plain or journal bearings.
All engines have a minimum of 2 main bearings, One at each end of the crank shaft. and they may have as many as one more than the number of crank pins. The number of main bearings is a compromise between the extra size, cost and stability of a larger number of bearings and the compactness and light weight of a smaller number. Both have advantages in terms of performance, as a shorter and more stable crankshaft will produce better engine balance.
Examples:
- All single- and V-twin-cylinder engines have at least two main bearings, one at each end.
- Parallel (inline) twin engines as used in motorcycles may have two, three, or four main bearings. Broadly speaking, older British twins had two main bearings. Japanese twins typically have four.
- Most modern four-cylinder petrol and some inline six engines have three main bearings, the third in the middle. However, four-cylinder inline diesel engines usually have five main bearings, due to the heavier loads imposed on the crankshaft.
- Almost all current production inline six-cylinder engines have seven main bearings. Older inline sixes often had either three or four main bearings.
- All modern V8 engines have five main bearings, with one crank pin between each pair of adjacent main bearings. Old designs, such as the Ford flathead V-8, produced from 1932 through 1953, often had three mains.
- Most straight-5 engines have six main bearings, to help counter the essential imbalance of this design.
When describing a crankshaft design, the number of main bearings is generally quoted, as the number of crank pins is determined by the engine configuration. For example, a crankshaft for an inline six engine will be described as three bearing or four bearing depending on its number of main bearings; The crank pins are not counted in this description. Similarly, when speaking of a crankshaft, the journals are the main bearing journals only. The crank pins are not normally called journals although they form the centre shafts of the big end bearings and are therefore journals in the more general sense.
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