A sailor checks the diameter of a breaking pen aboard the USS John C. Stennis
A breaker or breaker bolt is a safety device designed to break in case of mechanical overload, for example to prevent costly components from being damaged. The breaking bolt is thus sacrificed, and can be seen as a mechanical equivalent of an electrical fuse.
In the consumer sphere, the crank pin is inserted through the shaft of a screw with a simple outboard engine. If the ship sails across a beam or the screw hits a stone under water, the breaker pin breaks down and does not turn off a blade of the screw or damages the engine internally.
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