Constant Force Springs

Stock sizes in stainless steel

Constant-force springs are a special variety of extension spring. 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. When the strip is extended (deflected) the inherent stress resists the loading force, just as in a common extension spring, but at a nearly constant (zero) rate. The accompanying load/deflection curves illustrate this.

The constant-force spring is will suited to long extensions with no load build-up. In use, the spring is usually mounted with the ID tightly wrapped on a drum and the free end attached to the loading force, such as in a counterbalance application. This relationship can be reversed, however, with the free end mounted stationary and the spring itself providing the working force, as with carbon brushes in electrical apparatus.

Considerable flexibility is possible with constant-force springs because the load capacity can be multiplied by using two or more strips in tandem, or back-to-back, as illustrated.

Material

• Type 301 stainless steel.

Note: Be sure to allow at least 1 1/2 coils of material on the drum at full extension. The spring ID will wrap tightly on the drum so that in most applications no fastening method on the drum is required.

How to multiply constant-force spring load

Tolerances

Width and dimensions A and B: ± 0.005" (0.13 mm), R: ± 0.06" (1.53 mm)