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.
Another use is in pushback bars used for large aircraft. In this device, shear pins are frequently used to connect the "head" of the towbar – the portion that attaches to the aircraft – to the main shaft of the towbar. In this way, the failure of the shear pin will physically separate the aircraft and the tractor. The design may be such that the shear pin will have several different causes of failure – towbar rotation about its long axis, sudden braking or acceleration, excessive steering force, etc. – all of which could otherwise be extremely damaging to the aircraft.
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