Power springs and spring motors are rotational-drive springs that are wound tightly and mounted on an arbor. Typically, power springs and spring motors are used in applications such as retractable reels, tape measures, and retracting seat belts. A spring is a flexible elastic object used to store mechanical energy. Often, springs are made of hardened steel. Most power springs and spring motors are produced by tightly winding tempered spring material on an arbor and attaching the free end to a metal or plastic case. The spring’s natural tendency to expand imparts a moment to either the arbor or case, producing usable torque. The torque generated is cumulative, and increases the force as the spring is wound tighter and decreases as it unwinds. Power springs and spring motors work inside a housing and exert torsional force against either a central shaft or the housing itself.