The extension type of constant force spring represents the most basic, yet most versatile, type of constant force spring. It is a pre-stressed flat strip of spring material which is formed into virtually constant radius coils around itself or on a drum. When the strip is extended (deflected) the inherent stress resists the loading force, the same as a common extension spring, but at a nearly constant (zero) rate. A constant torque is obtained when the outer end of the spring is attached to another spool and caused to wind in either the reverse or same direction as it is originally wound.

The full rated load of the spring is reached after being deflected to a length equal to 1.25 times its diameter. Thereafter, it maintains a relatively constant force regardless of extension length. Load is basically determined by the thickness and width of the material and the diameter of the coil.

Fatigue life of springs ranges from 2,500 cycles to over a million cycles depending upon the load and size of the spring. Working deflections of 50 times the drum diameter can be achieved and these springs are much easier to handle than a conventional wire spring.

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.

Common Applications:

The constant force spring is an excellent device for applications where a constant load is required. Some of the many applications of constant force springs are in counterbalances, door closers, cable retractors, hose retrievers, tool head returns, cabinet & furniture components, gym equipment, hair dryers, toys, electric motors, appliances, space vehicles, and other long-motion functions.

Motor springs are used for cable retraction, power generation etc.  Power springs are used for retracting applications such as seat belt webbing, tape measures and dog leads, seat recliner, window regulator mechanisms etc.

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