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Torque is the force needed to rotate an object around an axis. Starting torque is the force needed to initiate rotational motion, determined in part by static friction. It is important in threaded fasteners and cylinder engines, where too much or too little torque can cause issues.
In physics and mechanics, torque is the rotational force, or the force required to rotate an object around an axis. In most cases, more torque is required to initiate the rotational motion than is required to maintain it once it has begun. This initial force is called the starting torque.
The amount of starting 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 lug nut may have high starting 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 decreased.
Starting torque is almost always discussed in one of two contexts, the horsepower of a motor or the force required to rotate a threaded fastener such as a nut. If a mechanic wants to remove a nut from a bolt, he must apply torque to the nut using a wrench. As anyone who has ever done it knows, turning the key takes a lot of strength at first, but it usually gets 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 control. If the stripping force of a nut is too low, vibration from the equipment can cause the nut to come loose; if it’s too high, the threads may strip and the bolt may be impossible to remove. Part of fastener testing includes measuring torque as the fastener begins to move, after the torque has been exceeded. The audit can be performed using sensors to measure torque or by a trained operator applying torque manually.
Another area where starting torque is important is in cylinder engines. As with threaded fasteners, the starting torque of a motor is greater than the operating torque. In an engine, torque is used to turn a crankshaft. The crankshaft in turn moves the pistons up and down.
An engine designed with enough power to keep it running will fail. The motor also needs to have enough power to get the motion started in the first place. Yet it is a delicate balance. If the motor is allowed to run with enough torque to cause a breakaway, it may overheat. Once the breakaway has been obtained, the engine torque must be reduced to a normal operating speed.
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