In an engine, the rotating shaft driven by the pistons that converts their motion into rotation is the crankshaft.
Pistons move up and down, or reciprocate, inside the cylinders. Connecting rods link them to offset crankpins on the crankshaft. As combustion pushes the pistons downward, the connecting rods turn those crankpins, causing the crankshaft to rotate. This changes reciprocating motion into the rotary motion needed to drive the transmission and wheels.
The crankshaft sits in the engine block and rotates on main bearings. A flywheel is often attached to one end to smooth power delivery and reduce vibration, while the other end commonly drives accessories or connects to the transmission. Crankshafts are generally made from forged steel, cast iron or machined steel.
It is easy to confuse the crankshaft with the camshaft. The crankshaft converts piston movement into rotation; the camshaft uses rotation to open and close the engine’s valves. The driveshaft, by contrast, carries rotational power from the transmission to the driven wheels.