Constant-force springs are a special variety of extension springs. They consist of a spiral of strip material with built-in curvature so that each turn of the strip wraps tightly on its inner neighbor. The strip can then be extended, with the inherent stress resisting the loading force, but at a nearly constant rate. In a typical application, the spring is mounted on a drum, and the free end attached to a loading force, such as in a counterbalance application. Or, the relationship can be reversed. These springs can be used in tandem or back-to-back to multiply the load capacity.