cotter

In mechanical engineering a cotter is a pin or wedge passing through a hole to fix parts tightly together.

Typical applications are in fixing a crank to its crankshaft, as in a bicycle, or a piston rod to a crosshead, as in a steam engine.

Nomenclature confusion: cotters vs. cotter pins

A cotter, particularly one with a circular cross-section, may be referred to as a cotter pin, but this is confusing. A cotter pin (or spilt-pin) is so named for its use in retaining a cotter. A cotter is an intended to take considerable shear loads, while a cotter pin is not, having to be soft enough to allow it to secured by bending it.

Bicycle Cotter

A wedge-shaped steel pin is used to mount cottered bicycle crank arms, though this is outdated technology that has generally been replaced by better interfaces such as the square taper and splined interfaces. (more about crank attachment technology). These cotters have a short threaded section at the narrower end of the taper, which is used to hold the cotter in place with a washer and nut.

A bicycle-specific tool, called a cotter press, is used to install and remove cotters. A cotter press is designed to apply extremely high pressure to either end of a cotter to alternately press it into place or remove it. Once a cotter is correctly pressed into place, a washer and nut should be installed on the protruding threaded stub to ensure that the cotter will not work loose over time. This threaded stub is not designed to pull the cotter into place, and using it for this will not sufficiently wedge the cotter into place and will often damage or break off the threaded stud.

Careful use of a hammer can be substituted for a cotter press, but this will usually damage cotters or even break them off beneath the surface of the crank. Positioning a small block of wood on top of the cotter will reduce the chance of damage to the cotter and bearings.

See also

• crankset
• bottom bracket

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