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A 'shear pin' is the mechanical analogue of an electric fuse. Installed in a drive train, it is designed to break in the case of a mechanical overload, preventing other, more-expensive parts of the drive train from being damaged.
A shear pin may be 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 load is reached. A cotter pin may also be used as a low-tech shear pin.
A common use of shear pins is in the drive train of a snow blower's auger or the propellers attached to marine engines.