Constant Force Springs

Constant force springs are a special variety of extension spring.  They are tightly coiled wound bands of pre-hardened spring steel or stainless steel strip 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, the same as a common extension spring, but at a nearly constant (zero) rate.  The constant-force spring is well 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.  Constant force springs are available in a wide variety of sizes.

Common materials of construction for constant force springs include 301 stainless steel, high carbon steel, and Inconel®.  Type 301 is an austenitic chromium-nickel stainless steel capable of attaining high tensile strength and ductility by cold working. It is not hardenable by heat treatment. Type 301 is essentially nonmagnetic in the annealed condition and becomes magnetic with cold deformation.  High carbon steel is steel with more than 0.3% carbon. The more carbon that is dissolved in the iron, the less formable and the tougher the steel becomes. High-carbon steel's hardness makes it suitable for bedsprings, cutting edges, or other high-wear applications.  Inconel is an alloy of nickel-chromium.  It is a lightweight metal used for extreme durability, performance, and heat-resistance.  Inconel is a registered trademark of Special Metals Corporation.

Important performance parameters to consider include life cycle and load force.  Life cycle refers to the number of cycles the spring was designed to perform.  The load force is the force the spring exhibits.  Important dimensions parameters to consider when searching for compression springs include outside or drum diameter, bandwidth, and band thickness.  The outside or drum diameter refers to the outside diameter of the spring coil.  The bandwidth is the width of the spring band around the compression spring.  The band thickness is the thickness of the spring band around the compression spring.

• Mechanical Design Handbook 2nd Edition
  This Second Edition of the Mechanical Design Handbook has been completely reorganized from its previous edition and includes seven chapters from the Mechanical Design and Systems Handbook, the...
  (read more)
• White Paper Download: Springs, Constant Force
  When this spring is deflected by pulling out the outer end of the coil, see Figure 2, the following occurs: (1) a resisting force "P" results; (2) force P has a line of action through "Z";(3) force P...
  (read more)
• Springs Constant Force
  The process of selecting small hard-to-find rotating components for new prototypes or for production applications just became easier with the publication by Stock Drive Products and Sterling...
  (read more)

Pages: 1 - 3 of 103

Part Numbers for Constant Force Springs