Constant Force Springs
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. The load is basically determined by the thickness and width of the material and the diameter of the coil.
Guide to Mounting Constant Force Springs
A Constant Force Spring is usually mounted by first tightly wrapping it on a drum, then attaching the free end to the loading force such as in a counterbalance application. This relationship can also be reversed.
1. The drum diameter should be 10% to 20% larger than its natural inside diameter.
2. One and one-half wraps should remain on the drum at maximum extension.
3. The strip becomes unstable at long extensions and should be guided to prevent twisting or kinking on recoil.
4. Idler pulleys must be larger in diameter than the natural diameter and should never be used to cause back-bending against the natural radius of curvature.
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.
