A shear pin is a safety device designed to shear in the case of a mechanical overload, preventing other, more-expensive parts from being damaged. As a mechanical sacrificial part, it is analogous to an electric fuse.
The pin itself may be a plain metal rod inserted through a hub and axle; the diameter of the rod is carefully chosen to allow the shearing action when the desired breakaway force or shock is reached. A cotter pin may also be used as a low-tech shear pin.
They are most commonly used in drive trains, such as a snow blower's auger or the propellers attached to marine engines.
Another use is in pushback bars used for large aircraft. In this operation, if the pilot accidentally commands the front wheel - in which the bar is engaged - to steer, the shear pin breaks the bar, preventing it from swivelling to the sides, which could be dangerous to both the pushback tractor and the personnel working near the plane.
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