A crankshaft is a component in an engine that converts reciprocating motion of the pistons into rotational motion. It is also responsible for transferring power from the crankshaft to other engine components. It should be replaced if it becomes damaged or worn out, which can result in engine vibration, loss of power, and even engine failure. Signs of a bad crankshaft include metallic grinding noises or bearings knocking sound coming from the engine.