Torsion or double torsion are helical springs used to apply torque ( a force that produces rotation ) or store rotational energy. A torsion spring exerts a force (torque) in a circular arc and the arms rotate about the central axis. The stress is in bending and not in torsion. It is customary to specify torque with deflection or with the arms at a definite position. Formulas for torque are in pound-inches or inches-pounds. If ounce-inches are specified the value should be divided by 16 to use the formulas in the inch-pound system.