Main bearing

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In a piston engine, the main bearings are the bearings on which the crankshaft rotates.

Nearly all engines have a minimum of two main bearings, one at each end of the crankshaft, 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 both a shorter crank and a more stable crank produce better engine balance.

Examples:

• Nearly all single and twin cylinder engines have two main bearings, one at each end.

• Most four cylinder petrol and some straight-6 engines have three main bearings, the third in the middle. However, four cylinder diesel engines usually have five main bearings, because of the higher compression pressures involved in diesels.

• The classic crossplane V8 engine has three main bearings.

• Some high-performance and luxury car straight six engines have four main bearings, with two crank pins in between each pair of adjacent main bearings.

• Some V8 engines have five main bearings, with one crank pin between each pair of adjacent main bearings.

• 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 a straight 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.