cotter pin

Cotter pins: A: New B: Installed C: Spring type D: Cross-section of traditional design

A car hub showing a castellated nut cover and cotter pin (near center).

In the United States, cotter pin (also known as a cotter key^[1] 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, cotter pins come in multiple sizes and types.

In the UK, the term split pin has traditionally been used to describe the same device.^[2] The term "cotter pin" is reserved for the tapered round cotter that was used to fix bicycle pedal cranks to their shaft, being installed offset from the diameter of the driven crank, flat on one side to prevent rotation and tapered so as to tighten the entire union. The different usage between the two countries is often a cause of confusion when companies of both countries work together.

Use

A spring-type cotter pin

A new cotter pin (see figure A) has its flat inner surfaces touching 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; and in order to ease insertion into a hole, the longer tine is often slightly curved or beveled to overlap the tip of the shorter tine.

Cotter pins are frequently used to secure other fasteners, e.g. clevis pins, as well as being used in combination with hardboard discs as a traditional joining technique for teddy bears.^[3] A castellated nut is castellated so that a cotter pin can be used to keep it from coming loose.

Cotter pins are typically made of soft metal, making them easy to install and remove, but also making it inadvisable to use them to resist strong shear forces. It is advisable to always replace the cotter pin rather than to reuse it, lest metal fatigue cause it to fail in use.

Spring-type cotter pins, also called hitch pins, sometimes known as R-pins from their shape, 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 metal than is appropriate for traditional cotter pins.

Applications

Cotter pins may be used in some applications as low-tech shear pins. Most stock car race cars use several spring-type cotter pins called hood pins to hold down the car's hood and trunk.

Round cotter pins

In the UK, the term cotter pin has traditionally referred only to the round cotter, a short rod with a tapered flat on one side and a thread on one end.^[4] This was commonly used to hold bicycle cranks on tightly. The device described on this page was universally referred to as a split pin, although there are signs that manufacturers and stockists are increasingly listing both names together to avoid confusion. The term 'split cotter' may also be found.

Types

• Standard
• Humped
• Clinch
• Hitch, R-clip

See also

• Circlips, which have replaced cotter pins in some applications.
• R-clip, sometimes used where easier removal and re-use are desirable.
• Linchpin, sometimes used where easier removal and re-use are desirable.

References