To many manufacturers, the concept of torque is a mystery. Ask them to describe the physics of torque and you'll likely meet a blank stare. Cracking open a dictionary won't help much either. To the layman the definition reads like a foreign language that owes more to mysticism than science. Despite the confusion, proper control of this mysterious force remains a critical component for quality manufacturing and has a direct impact on the bottom line issues like product quality, reliability and safety.
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.
SOURCE: Mountz Learning Centre White Pages