Written by Jody Muelaner
Cotter pins are typically used to prevent axial movement along a shaft. They’re either a wedge or tapered pin that’s driven into a hole.
The tapered nature of this pin means that it’s compressed as it is driven into the hole, causing friction, which prevents it from working loose. However, the term “cotter pin” is also sometimes referred to as a split pin, an R-clip, or a circle cotter.
Here are a few of the different types of fasteners that are also considered cotter pins:
Split pins: Often called cotter pins, split pins are bent in half so that both ends may be inserted through the same hole. The pins are usually manufactured from a half-circular profile so that the two ends together form a circle, which fits into the hole. The bent end is formed into an enlarged end and the double end can be bent outwards to prevent the pin being removed.
R-clips or R-pins: These pins are sometimes referred to as hairpin cotters. They’re sprung pins with a straight section that bends to form a ring at the head, with a curved section offset from the straight pin. The straight section may be pushed through a hole in a shaft, while the curved section clips around the outside of the shaft, retaining the pin in place. A finger can be inserted through the ring at the head to pull the pin out of the hole.
Split ring: These components are also called a circle cotter or cotter ring. Essentially, it’s a wire loop that extends beyond 360 degrees allowing it to be threaded through a hole. Key rings are a common application for this type of fastener although they may also be used to retain pins and shafts.
This brings us to the traditional cotter pin, which is a wedge that’s driven or drawn into a hole. Conventionally, it’s been used to attach cranks onto shafts (say, for example, on bicycles and steam engines). Cotters might have a threaded shaft protruding from the narrower end of the taper. This is used to draw the cotter into the hole and retain it in position. This type of cotter has been largely replaced by splines and square shafts.