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What is a Cotter?

Paul Scott
Paul Scott

A cotter is a term used to describe a selection of devices meant to lock or secure pins, nuts, bolts, or complete mechanisms. Depending on location, the cotter is also known as a split or cotter pin and may take the form of a simple metal pin or a machined wedge. Pin type cotters are generally passed through a hole in a shaft or into a machined groove to prevent locking mechanisms from working loose. In the United States, the term refers to the various pin types which include R-pins, bridge cotter pins, and spring cotter pins. In the UK, the term describes a cylindrical bar machined into a wedge at one end and commonly used to lock bicycle pedals and cranks.

Most fasteners subjected to vibration or repetitive motion have the annoying tendency of working loose after a while. One way of preventing this is the use of one of the many types of cotter pins designed to prevent fasteners from loosening unintentionally. These handy devices are available in a wide variety of designs to lock or secure an equally wide selection of fasteners. Most are simply spring wire pins with an application-specific shape. Some, however, may be solid steel rods with one wedge face used to friction-lock rotating mechanisms.


The simplest example of these devices is the well known split pin type which consists of a half round pin bent double with a loop at the closed end. The two tines or legs of the pin are inserted through a hole in a nut and bolt once the nut has been tensioned. The ends of the legs are then bent up to prevent the pin from working its way out of the hole. The pin passing through the nut and the threads it turns onto, in turn, prevents the nut from working loose. The same type of arrangement may be used on shafts where the pin prevents the shaft from pulling back through a bushing.

One particular type of shaft locking pin is known as a circle cotter and has a small belly or half loop formed in one leg. The one leg of the pin is passed through the hole in the shaft and the other passes over the outer surface until the belly or loop clips around the shaft. These pins are usually made of spring steel and are reusable. Another common spring steel example is the circle cotter. These look similar to a key ring and fit into a groove machined into a shaft to stop assemblies from working off the shaft. In the UK, the term cotter is usually applied to a round steel shaft which has a wedge face machined into one end used to lock bicycle pedals and cogs onto their shafts.

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