Ex-stock constant force springs from Lee Spring offer a flat force curve over most of their extension range - it is this "table-top mountain" shape of force curve which ensures their suitability for certain applications such as counterbalances, door closers, cable retractors, hose retrievers, tool head returns, cabinet & furniture components and many more.
Motor springs might be used for cable retraction or power generation and power springs are used for retracting applications such as seat belt webbing, tape measures and dog leads, seat recliners and window regulator mechanisms. Where higher loads are encountered then multiple mounting can be used to increase the force to required levels. Constant force springs mounted back-to-back make the sum of their forces available at a single point - a method which also provides stable extension over long deflections. Tandem mounting makes the sum of two spring forces available at a single point, whilst pulley mounting doubles the force of a single constant force spring. Laminar mounting, inter-winding of two or more springs as an assembly, offers the sum of their forces in a minimum of space.
Lee Spring offers constant force springs in four Life Cycle ranges where each spring is a roll of high yield Type 301 Stainless Steel strip, exerting a nearly constant restraining force to resist uncoiling. When the strip is extended, the inherent stress resists the loading force at a near-constant rate. Considerable flexibility is possible with constant force springs because the load capacity can be varied by using different mounting configurations such as cavity mounts, multiple spring mounts, etc. Constant force springs are available in a wide variety of sizes and end configurations to suit the design situation. Many variations of constant force spring are possible over and above the simple increase in size, thickness, diameter etc. For example, doubling up by back-to-back mounting which not only doubles the available force but also increases stability; tandem mounting which doubles force and laminar mounting which doubles the force in a limited space.
Typically, stainless steel Type 301 is selected, but high-carbon steel, Inconel and other materials are also suitable for constant force springs. The fatigue life of these springs ranges from 2,500 cycles to over a million cycles, dependent upon the load and the size of the spring. Working deflections of 50 times the drum diameter can be achieved and these springs are often considered much easier to handle than a conventional wire spring can be.