Cotter (pin)

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A cotter is a pin or wedge with a flat face that goes through a hole to hold parts together. In Britain it’s called a cotter pin; in the United States it usually means a split pin. Common uses include fixing a bicycle crank to the crankshaft and a piston rod to a crosshead in a steam engine. The wedge angle controls how the parts are held, so on a bicycle the two pedal arms line up opposite each other only if the wedges on both pins are the same. In the past, bicycles often used cotters to mount cranks, but now people usually use easier options like square-taper or splined interfaces. These cotters have a short threaded end at the narrow end of the taper to secure them with a washer and nut. Cotters are also used to secure ceiling fans to prevent them from falling if the mounting nut loosens.