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A component of all piston engines, a main bearing allows the crankshaft to rotate and is located, at a minimum, at each end of the crankshaft. Engines may have more than two main bearings, which improves overall engine balance but adds weight to the engine. Designers must weigh the benefit of the extra bearings against the additional weight and cost.
A main bearing is usually a plain bearing, also known as a journal bearing. This type of bearing is the simplest type of bearing and normally consists of two surfaces that slide against each other under lubrication and have no other moving parts or rolling elements. A shaft rotating within a sleeve is one simple type of journal bearing. The journal is the part that moves. The bearing is the part that holds the journal.
In a piston engine, the main bearing usually consists of two semicircular pieces of metal that enclose the crankshaft, forming a ring. A groove runs along the center of the ring, and one or more holes are cut into the groove. These design elements allow engine oil to enter the main bearing and lubricate the crankshaft as it rotates. Piston engines depend on precision of construction for efficiency and to run smoothly, so tolerances and clearances for a main bearing design are usually extremely small, and these parts are made to exacting specifications.
The number of cylinders in an engine and their configuration can influence the number of main bearings in the design. Longer crankshafts usually require extra main bearings, as they benefit from the increased stability and balance. Diesel engines will often have more main bearings than a similar gasoline engine, to compensate for the increased stress on the crankshaft. Most modern engines have multiple main bearings, with in-line configurations tending to have more than engines with the same number of cylinders arranged in a "V" configuration.
Engine and crankshaft designs are often described in terms of the number of main bearings present, and simply knowing the number of main bearings in a particular engine will allow an experienced mechanic to infer a great deal about the interior of an engine and its design. Bearing design and construction is an evolving field, as new materials and alloys are developed to optimize performance and durability. Lead, for example, once a common component in bearing alloys, is becoming less common as manufacturers attempt to find substitutes due to environmental concerns.