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Torque, torsion and tension: These seem like simple terms that many of us should know; however, there are just as many users who may either confuse the terms, or more importantly, fail to understand the implications of improper usage and the many variables which affect the results that are expected.
Both actions of torque and tension are measures of force; one is used to accomplish the other. Units of force are measured as an amount of applied energy times a unit of distance. Tension is the result of the application of torque and torsion.
When torque is applied to a fastener, we are rotating the nut onto the threads of a bolt. Therefore, torque is a twisting force causing rotation about an axis: the bolt. This force is measured in Newton-meters (N-m) or pound-feet (lb.-ft.). As the nut contacts the joint surface and begins to compress the joint, the nut encounters resistance. Now the torque becomes measurable as the amount of exerted force required to continue to rotate the nut against the joint at a distance of 1 meter or 1 foot.
As the nut has additional torque applied to it, the shank of the bolt may now experience a certain amount of twisting caused from the contact pressure between the thread flanks of the nut and the bolt. This contact pressure is related to friction. The torsion is a force that is now measurable as Newtons-per-meter squared (Pa, which is a Pascal) or as pounds-per-square inch (psi). However, we don't really measure torsion, we measure its resultant force: clamp load. As soon as the nut stops rotating, the torsional forces immediately dissipate, and we have pure tension on the bolt.
Because the nut can still advance on the threads of the bolt, the entire bolt does not experience as much torsion as does turning the bolt head, just the portion around the nut. However, when tightening the head of the bolt, the torsional forces create twist from the top of the bolt head to the nut or bottom threads of a tapped hole. This is the reason why...