Reciprocating motion, also called reciprocation, is a repetitive up-and-down or back-and-forth linear motion. It is found in a wide range of mechanisms, including

reciprocating engine A reciprocating engine, also often known as a piston engine, is typically a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common fea ...

s and

pump A pump is a device that moves fluids ( liquids or gases), or sometimes slurries, by mechanical action, typically converted from electrical energy into hydraulic energy. Pumps can be classified into three major groups according to the method the ...

s. The two opposite motions that comprise a single reciprocation cycle are called strokes. A

crank Crank may refer to: Mechanisms * Crank (mechanism), in mechanical engineering, a bent portion of an axle or shaft, or an arm keyed at right angles to the end of a shaft, by which motion is imparted to or received from it * Crankset, the compone ...

can be used to convert into reciprocating motion, or conversely turn reciprocating motion into circular motion. For example, inside an

internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal co ...

(a type of reciprocating engine), the expansion of burning fuel in the cylinders periodically pushes the

piston A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas- ...

down, which, through the connecting rod, turns the crankshaft. The continuing rotation of the crankshaft drives the piston back up, ready for the next cycle. The piston moves in a reciprocating motion, which is converted into circular motion of the crankshaft, which ultimately propels the vehicle or does other useful work. The reciprocating motion of a pump piston is close to, but different from, sinusoidal simple harmonic motion. Assuming the wheel is driven at a perfect constant rotational velocity, the point on the crankshaft which connects to the connecting rod rotates smoothly at a constant velocity in a circle. Thus, the displacement of that point, is indeed exactly sinusoidal by definition. However, during the cycle, the angle of the connecting rod changes continuously. So, the horizontal displacement of the "far" end of the connecting rod (i.e., connected to the piston) differs slightly from sinusoidal. Circumstances where the wheel is not spinning with perfect constant rotational velocity, such as a steam locomotive starting up from a stop, are very much not sinusoidal. So basically a up and down a repetitive up-and-down or back-and-forth linear motion.

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