A shear pin is a safety device designed to shear

Shearing (physics)

Shearing in continuum mechanics refers to the occurrence of a shear strain, which is a deformation of a material substance in which parallel internal surfaces slide past one another. It is induced by a shear stress in the material...

 in the case of a mechanical overload, preventing other, more-expensive parts from being damaged. As a mechanical sacrificial part

Sacrificial part

A sacrificial part is a part of a machine or product that is intentionally engineered to fail under excess mechanical stress, electrical stress, or other unexpected and dangerous situations...

, it is the analogue of an electric fuse.

The pin itself may be a plain metal

Metal

A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

 rod inserted through a hub and axle; the diameter of the rod is carefully chosen to allow the shear

Shearing (physics)

Shearing in continuum mechanics refers to the occurrence of a shear strain, which is a deformation of a material substance in which parallel internal surfaces slide past one another. It is induced by a shear stress in the material...

ing action when the desired breakaway force

Force

In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...

 or shock

Shock (mechanics)

A mechanical or physical shock is a sudden acceleration or deceleration caused, for example, by impact, drop, kick, earthquake, or explosion. Shock is a transient physical excitation....

 is reached. A cotter pin

Cotter pin

A split pin, also known in U.S. usage as a cotter pin or cotter key, is a metal fastener with two tines that are bent during installation, similar to a staple or rivet. Typically made of thick wire with a half-circular cross section, split pins come in multiple sizes and types.The British...

 may also be used as a low-tech shear pin.

They are most commonly used in drive trains

Powertrain

In a motor vehicle, the term powertrain or powerplant refers to the group of components that generate power and deliver it to the road surface, water, or air. This includes the engine, transmission, drive shafts, differentials, and the final drive...

, such as a snow blower

Snow blower

A snow blower or snow thrower is a machine for removing snow from an area where it is not wanted, such as a driveway, sidewalk, roadway, railroad track, rink, runway, or houses...

's auger

Auger

An auger is a drilling device, or drill bit, that usually includes a rotating helical screw blade called a "flighting" to act as a screw conveyor to remove the drilled out material...

 or the propeller

Propeller

A propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid is accelerated behind the blade. Propeller dynamics can be modeled by both Bernoulli's...

s attached to marine engines.

Another use is in pushback

Pushback

In aviation, pushback is an airport procedure during which an aircraft is pushed backwards away from an airport gate by external power. Pushbacks are carried out by special, low-profile vehicles called pushback tractors or tugs....

 bars used for large aircraft

Aircraft

An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

. 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 to swivel to the sides, which could be dangerous to both the pushback tractor and the personnel working nearby the plane.