Cotter pin
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- For split pins as an item of stationery see split pin.
A cotter pin (also known as a cotter key or a split pin) is a metal fastener with two tines that are bent during installation, similar to a staple or rivet. Typically made of wire with a half-circular cross section, a new cotter pin (see figure, A) will have its flat inner surfaces touching one another for most of its length, so that it appears to be a split cylinder (figure, D). Once inserted, the two ends of the pin are bent apart, locking it in place (figure, B). In order to facilitate the initial separation of the tines, one tine of the cotter pin is often noticeably longer than the other.
Cotter pins are frequently used to secure other fasteners, i.e. clevis pins, as well as being used in combination with hardboard discs as a traditional jointing technique for teddy bears. A castellated nut is castellated so that a cotter key can be used to keep it from coming loose.
Cotter pins are typically made of very soft metal, making them easy to install and remove, but also making it inadvisable to use them to resist strong shear forces, and it is advisable to always replace them rather than to reuse them, lest metal fatigue cause them to fail in use.
Spring-type cotter pins are also available, which are not designed to be permanently bent. In this design, only one section of the pin passes through the shaft to be secured, the other section being curved to wrap around the outside of the shaft (figure, C). This type of pin is usually made of round wire of a harder heat treatment than is appropriate for traditional cotter pins.
Cotter pins may be used in some applications as low-tech shear pins.
A cotter pin can also refer to a slightly conical pin with one flattened face. This pin passes through a hole in a hub aligned perpendicular to the shaft passing through. There is normally a flattened portion on the shaft against which the face of the cotter pin engages. There may also be a screw thread formed at the narrow end of the cotter pin to enable a nut to secure the pin against vibration. This form of cotter pin is commonly found on older bicycle bottom brackets and are used to secure the chainwheel and pedal crank.
