The "crank" is a mechanical device employed for converting the parallel or reciprocating motion of the piston into a rotary motion. It is connected by a key to the shaft, which carries the fly-wheel. The power transmitted to the crank exactly represents that exerted by the steam in the cylinder against the piston, minus the friction.

Between the piston and crank, connection is made by means of a cross-head and connecting rod; the cross-head runs to and fro between guides. This motion of the cross-head is necessary to prevent the piston rod from being broken or bent by the oblique positions of the connecting rod when the crank is at mid-travel. The distance from the center of the shaft to the center of the crank pin is called the crank's throw, and is half the piston stroke.