Crankshaft is the first part in the power transmission system onto which the reciprocating motion of the piston is converted into rotating motion with the help of connecting rod. A crankshaft consists of crank pin, crank arm, balancing weight and main journals. Big end of the connecting rod is connected to the crank pin of the crankshaft. Center to center distance between the crank pin and crankshaft is half of the piston displacement during a stroke. Thus one complete revolution of the crankshaft makes two stroke of the piston. The parts of the crankshaft inside the main bearings are called the main journals. The crankshaft is supported by the main bearings on the main journals. Balancing weights are provided on the opposite side of the crank arms for balancing. 

The crankshaft is made of casting or forging of heat treated alloy steel and is machined and ground to provide suitable journals for the connecting rods and main bearings. It must be strong enough to take the thrust of the pistons during the power stroke without excessive distortions. Crankshaft have drilled oil passage through which oil can flow from the main bearing to the connecting rod bearing. The front end of the crankshaft carries three devices- The gear or sprocket that drives the camshaft, the vibration damper to control torsional vibration and the fan belt pulley. This pulley drives the engine fan, water pump and generator with a V-belt. The rear end of crankshaft carries flywheel. The inertia of the flywheel tends to keep the crankshaft turning at constant speed. The number of main bearings varies with design of the engine and the number of cylinders. The more main bearings, the less the possibility of vibration and distortion of the crankshaft of a given size. To reduce vibration in the engine to a minimum, the crankshaft and flywheel are balanced separately and then are often casted for balance when mounted together. Balancing is necessary to prevent severe damage to the engine, specifically to the bearings.

The crankshaft should be inspected for: