The main bearing secure the crankshaft in its place. The largest number of main bearings a crankshaft can have is one more than the number of cylinders, but it can have one less bearing than the number of cylinders. Not only do the bearings support the crankshaft, but one bearing must control the forward-backward movement of the crankshaft. This bearing rubs against a ground surface of the main journal, and is called the “thrust bearing.”
More detail :
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.
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.