Reciprocating motion , also called reciprocation, is an up-and-down (or back-and-forth) motion which repeats over and over again. It is found in a wide range of mechanisms such as reciprocating engines and pumps. The two opposite motions that comprise a single reciprocation cycle are called strokes.
A crank can be used to convert circular motion into reciprocating motion, or conversely turn reciprocating motion into circular motion.
As an example, inside of an internal combustion engine an explosion within the cylinder periodically pushes the piston down, and through the connecting rod pushes the crankshaft around. 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 turns the car's wheels. The vibrations felt when the engine is running are a side effect of the reciprocating motion going on inside the cylinders.
In internal combustion engines, the reciprocating motion is not visible from outside. However, in early steam engines, particularly horizontal stationary engines, and outside-cylindered steam locomotives, the reciprocating action is clearly visible as the mechanism is not usually enclosed.
Mathematically, reciprocating motion is approximately sinusoidal simple harmonic motion. However, technically, the reciprocating motion produced by a rotating crank departs slightly from simple harmonic motion due to the changing angle of the connecting rod during the cycle.
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