Crankshaft

A rotating shaft with offset crankpins that converts reciprocating piston motion into torque, or torque into reciprocating motion. It is a central component of piston engines and reciprocating machines.

Crankpin: An offset journal on a crankshaft where a connecting rod attaches. The connecting rod applies piston force to this offset surface, creating turning moment.
Reciprocating motion: Motion that repeatedly travels back and forth along a path. Pistons supply the back-and-forth motion the crankshaft converts.
Internal combustion engine: An engine that produces power by burning fuel inside a working chamber. Crankshafts turn piston forces into output rotation in piston-based examples of this engine.
Camshaft: A shaft with shaped lobes that operates engine valves in timed relation to crankshaft rotation. It is driven in synchronization with the crankshaft but controls valves rather than converting piston force.
Hand crank: A lever attached to a rotating shaft, turned by hand to produce rotary motion. It is a simple crank form that shares the shaft's conversion between reciprocating effort and rotation.
Connecting rod: A link that connects a piston to a crankshaft and transmits force between them. It carries force between the piston and crankpin as the piston reciprocates.
Torque: The turning effect of a force about an axis. Torque is the rotational output produced by force acting through crank geometry.
Inline engine: A piston engine whose cylinders are arranged in a single row. Its shared crankshaft serves cylinders positioned along one bank.
Eccentric shaft: A rotating shaft with an offset section used to create or transmit eccentric motion. It can produce reciprocating motion, but uses an eccentric rather than crankpins and connecting rods.
Roman watermill: A water-powered mill used in ancient Rome to grind grain or perform other mechanical work. Crank mechanisms appeared in ancient machinery before piston engines existed.

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