A shear pin is a mechanical safety device that fails at a preset stress load to protect the rest of a system, as well as operators and technicians who may be in the vicinity. One of the most common uses for such devices is in drive trains, where uncontrolled pressure could damage the system if it wasn't stopped with the aid of a device like a shear pin. Such devices are intended to be part of a larger safety network with countersafes that can activate when other safety measures fail, to reduce the risk of damages, injuries, or loss of life.
The shear pin consists of a bolt or other fastener like a cotter pin, carefully engineered to break at the right stress level. Designers do not want the shear pin failing under normal operating conditions, but need it to snap in the event of a high stress situation. For example, such devices are commonly used in snow blowers to provide protection in the event that something gets caught in the blades, which would put stress on the drivetrain of the device.
It is important to use a shear pin rated for a given application. Manuals typically provide specifications, and manufacturers may stock pins for the benefit of customers. The pin will need to be replaced whenever it fails, and some devices are designed in such a way that they will not work without the pin in place. This is a safety measure to prevent accidents if an operator decides to keep using a machine even after the pin breaks off.
Like other sacrificial parts, the main function of a shear pin is to prevent expensive damage to a system and to protect human health and safety. These parts are typically very inexpensive to make them easy to replace, and they can be replaced by operators, since the point of insertion is usually easy to spot. If a shear pin fails repeatedly, it may be a sign that there is a problem with the equipment that is causing undue strain, and a mechanic may need to look at it to explore possible causes of the problem.
Hardware and home supply stores may carry shear pins. The packaging should note the stress rating and will also provide information about the size and method of attachment. There are several ways to fit a shear pin into a system, and it may help to bring in the broken component to compare with replacements. This can increase the chances of getting the right part on the first try if a specific model number is not available.