What is Torque?
Torque is a "turning" or "twisting" force and differs from tension, which is created by a straight pull. However, we use torque to create a tension.
(Diagram A) As the nut and bolt are tightened, the two plates are clamped together. The thread angle in the bolt converts the force applied into tension (or stretch) in the bolt shank. The amount of the tension created in the bolt is critical.
A bolt tensioned properly works at its optimum efficiency and will resist coming undone. However, if the tension is too low, the nut could vibrate or work loose. If the tension is too high (overstretched), the bolt could break. Every bolt has a correct optimum torque/tension figure for each fastening application. It is important to have these figures available so that the end product will be safe, efficient and economical.
HOW DO WE MEASURE TORQUE?
(Diagram B) Torque is the result of multiplying the value of Force applied by the Distance from the point of application.
Comparing the two examples, please note that the same Torque result can be achieved with a lower Force if the Distance from the nut/bolt is increased.
Another factor that affects applied torque when using torque wrenches is if it is "length dependent," which means that the actual torque applied to the fastener varies if the hand position on the wrench is varied (even if the wrench is preset). This occurs if the pivot point of the wrench mechanism is not coincidental with the point of application of torque.
THE IMPORTANCE OF TORQUE CONTROL?
The reliability of machine parts subjected to fluctuating loads and stress depends on the fatigue strength of the materials. A threaded fastener, however, relies upon an elastic interaction between the mating components. Its objective is to clamp parts together with a tension greater than any external force trying to separate them. The bolt then remains under almost constant stress and is immune to fatigue. If the initial bolt tension is too low, the fluctuating load in the shank in much greater and it will quickly fail. Reliability, therefore, depends on correct initial tension and is ensured by specifying and controlling the tightening torque.
Torque = force x lever length of wrench:
Force of 20 lbs. x 1 ft. = 20 lbf.ft
Force of 10 lbs. x 2 ft. = 20 lbf.ft