Cotter pins, also called split pins or cotter keys, are bent pins with a rounded head that are slid into a hole. Once inserted into the hole, the two ends are bent into different directions deforming the pin, so it cannot be easily removed from the hole. While cotter pins come in many shapes and sizes, the most common is the split pin variety.
Cotter pins are used with castle nuts to lock the nut onto a bolt and prevent it from spinning loose accidentally. They are usually made of a soft metal, so they can be bent by hand or with pliers to make them difficult to remove. It is common to see a cotter pin reused after being removed if it does not break during removal. This is not a good practice however; the pin should be replaced with a new pin as metal fatigue will prevent the pin from performing to standards once bent again.
*Due to the softness of materials, we do not advise using cotter pins where strong sheer force is a concern. In some rare cases, split pins may be used as low-tech shear pins.
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