Definition - What does Main Bearing mean?
Main Bearing in the piston engine is usually a journal or plain bearing on which a crankshaft rotates. It holds the crankshaft in a proper place and also prevents the forces which are created by the piston. These forces are transmitted to the crankshaft with the help of connecting rods and they force the crank to turn reciprocation movements into rotation. They have two parts – upper part and lower part. The upper part usually has oil groove on its inner surface. The Main Bearing also has a hole that allows oil to pass into the feed holes of the crankshaft.
Petropedia explains Main Bearing
Most of the engines have two Main Bearings that are placed one at each end of a crankshaft and have a number of crank pins. But some of the engines have only one main bearing which is designed to cope with the bending moment applied by the crank as an output of force from connecting rods. If the number of bearing is increased in the engine, it increases the size of the engine and also its cost. By increasing the number of bearings, stability can be provided to the crankshaft that would else have to encounter the greater bending moments.