Constant Force Springs

Constant force springs represent a specialized category of extension springs, precision-engineered from high-strength, prestressed metal ribbons (typically stainless steel). Unlike conventional helical springs—where the force increases linearly with deflection ( $F = k x$ )—a constant force spring exerts a uniform, nearly zero-gradient load throughout its entire extension. This unique characteristic makes them the ideal solution for applications requiring long linear travel and perfectly balanced motion.

Critical Design Parameters

To ensure optimal mechanical performance and fatigue life, the following engineering specifications must be defined:

Material Thickness : The gauge of the metal strip. This parameter has a cubic relationship with the output force and is the most critical factor in load calculation.
Material Width : The width of the ribbon. Output force increases proportionally (linearly) with the strip width.
Wound Diameter : The natural diameter of the spring in its relaxed, coiled state. The spring is typically mounted on a storage spool or arbor sized to match the natural ID.
Rated Load : The specific, constant pulling force provided. Unlike standard springs, this load remains consistent regardless of how far the spring is pulled.
Total Strip Length : The full uncoiled length. For maximum service life and safety, a “dead coil” reserve of 1.5 to 2 turns must remain on the spool at full extension.

Typical Applications

Constant force springs excel in mechanisms requiring smooth, effortless counterbalancing or retraction over long distances:

Ergonomic Workspace Solutions:
- Height-Adjustable Monitor Stands: Enables displays to “hover” effortlessly at any position, providing a weightless user experience without drifting.
Advanced Interior Design:
- Cordless Blinds & Window Shades: Allows for precise positioning; the blinds remain stationary at any height by perfectly counterbalancing the weight of the fabric.
Retail & Industrial Equipment:
- Automated Shelf Pushers: Ensures products are smoothly glided to the front of the display as items are removed, maintaining a “fronted” appearance with uniform pressure.
Automotive & Safety:
- Seatbelt Retractors & Recoil Starters: Provides consistent retraction tension for safety belts and small engine pull-starts.
- Cable & Hose Reels: Ensures uniform retraction speed, whether the hose is fully extended or nearly reeled in.

As the spring is unrolled, the material coming off the roll bends from the radius of the roll into a straight line between the reel and the load. Because the material tension-stiffness of the straight section is orders of magnitude greater than the bending stiffness of the ribbon, the straight section does not stretch significantly, the restoring force comes primarily from the deformation of the portion of the ribbon near the roll.