Torsion springs are coil springs designed to apply or resist a twisting (torsional) force around a center point. They can also be manufactured with two sets of coils wound in opposite directions around the same center axis. These are referred to as "double torsion springs. The coils of a torsion spring are usually close wound with minimal tension between the coils, but they can also be made open wound to reduce friction or to create a spring that functions as both a torsion spring and a compression spring.
Torsion springs typically have two legs that can be formed into almost any configuration in order to be connected to other components. The simplest torsion springs have two straight tangential legs that simply push against the mating components.
Torsion springs can be either left-hand or right-hand wound, and they should be designed so that an applied force will wind up the spring (not unwind it). They function best when they are supported by a hub, shaft, or tube inserted through the body coils; however, the designer must keep in mind that the diameter of the coils gets smaller and the length of the body increases as the spring is wound up.