The crankshaft turns the reciprocating motion of the pistons and connecting rods into the rotational motion that turns the transmission gears, axle shafts, and ultimately the wheels. The crankshaft turns on its axis in the cylinder block. The connecting rods are attached to offset crank throws, which enable the conversion of motion. Cast iron is commonly used to make low output engine crankshafts, while forged steel is needed for high horsepower motors.
Bearing journals are machined on the crankshaft so it can be supported in the cylinder block and to attach the big ends of the connecting rods to the crank throws. The crankshaft rotates in the block on plain bearings supported by a thin film of pressurized oil, and the rods are supported in the same manner on the crank throw rod journals. Most crankshafts have counterweights to offset the mass of the piston and rod assemblies and maintain engine balance.
