What Is a Constant Force Spring? Mechanics, Mounting, and Applications
A constant force spring is a coiled flat strip that keeps the same radius of curvature as it unwinds, producing a flat force curve over long travel unlike a round-wire extension spring governed by Hooke’s Law. Proper drum mounting is critical: the drum diameter should be 10–20% larger than the natural coil diameter, with at least 1.5 wraps remaining at full extension to prevent kinking, set loss, and reduced cycle life. Load is governed by strip material, width, thickness, and natural radius, with force scaling roughly with thickness cubed, while guided travel and correct end configurations help avoid buckling and fatigue issues. Common uses include counterbalances, cable and hose retractors, tool-head return systems, cabinet closers, fitness resistance mechanisms, and retail product dispensers where steady linear force is required.
