In physics and mechanics, torque is rotational force, or the force necessary to cause an object to rotate around an axis. In most cases, more torque is needed to start the rotational motion than is needed to keep it going once it has begun. This initial force is called breakaway torque.
The amount of breakaway torque needed to move something is determined in part by static friction. Static friction is the force that exists between two physical bodies to keep them from moving. For example, a wing-nut may have high breakaway torque because there is a lot of static friction between the nut and the bolt. If, however, the bolt is greased, the torque will be lower because the static friction has been decreased.
Breakaway torque is almost always discussed in one of two contexts, either the power of an engine or the force needed to turn a threaded fastener such as a nut. If a mechanic wants to remove a nut from a bolt, he or she must apply torque to the nut using a wrench. As anyone who has ever done this knows, turning the wrench takes a lot of force at first, but usually becomes easier after a few turns.
Mechanics and engineers often measure the breakaway torque of threaded fasteners in part to ensure product integrity as part of a process called torque auditing. If the breakaway force of a nut is too low, the vibration of the equipment may cause the nut to loosen; if it's too high, the threads may strip and the bolt may be impossible to remove. Part of testing the fastener includes measuring the torque at the moment the fastener starts moving, after the torque has been overcome. Auditing can either be done using sensors to measure the torque or by a trained operator applying torque by hand.
Another area where breakaway torque is important is in cylinder engines. As with threaded fasteners, the breakaway torque of an engine is greater than the running torque. In an engine, torque is used to spin a crankshaft. The crankshaft in turn causes the pistons to move up and down.
An engine that is designed with only enough power to keep the engine running will fail. The engine must also have enough power to get the movement started in the first place. Yet it is a delicate balance. If the engine is allowed to continue running with enough torque to cause breakaway, it may overheat. Once the breakaway has been achieved, the engine torque should be reduced to a normal operating speed.