Main bearing
Encyclopedia
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 two main bearings, one at each end of the crankshaft, and they may have as many as one more than the number of crank pin
s. 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:
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.
Bearing (mechanical)
A bearing is a device to allow constrained relative motion between two or more parts, typically rotation or linear movement. Bearings may be classified broadly according to the motions they allow and according to their principle of operation as well as by the directions of applied loads they can...
on which the crankshaft
Crankshaft
The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation...
rotates, usually plain
Plain bearing
A plain bearing, also known as a plane bearing or a friction bearing is the simplest type of bearing, comprising just a bearing surface and no rolling elements. Therefore the journal slides over the bearing surface. The simplest example of a plain bearing is a shaft rotating in a hole...
or journal bearings.
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 pin
Crank pin
In a reciprocating engine, the crankpins, also known as crank journals are the journals of the big end bearings, at the ends of the connecting rods opposite to the pistons....
s. 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
Engine balance
Engine balance is the design, construction and tuning of an engine to run smoothly. Improving engine balance reduces vibration and other stresses and can improve the overall performance, efficiency, cost of ownership and reliability of the engine, as well as reducing the stress on other machinery...
.
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 four-cylinder petrol and some inline sixStraight-6The straight-six engine or inline-six engine is a six-cylinder internal combustion engine with all six cylinders mounted in a straight line along the crankcase...
engines have three main bearings, the third in the middle. However, four-cylinder inline diesel engineDiesel engineA diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...
s 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-8Ford Flathead engineThe Ford flathead V8 was a V8 engine of the flathead type, designed by the Ford Motor Company and built by Ford and various licensees...
, produced from 1932 through 1953, often had three mains.
- Most straight-5Straight-5The straight-five engine or inline-five engine is an internal combustion engine with five cylinders aligned in one row or plane, sharing a single engine block and crankcase...
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
Engine configuration
Engine configuration is an engineering term for the layout of the major components of a reciprocating piston internal combustion engine. These components are the cylinders and crankshafts in particular but also, sometimes, the camshaft....
. 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.