Constant Force Spring

Constant force springs are precision-wound spiral springs that deliver a nearly uniform force throughout their deflection range. They are commonly used in applications requiring a steady force over long travel, such as counterbalancing heavy equipment, retracting cables, and in automotive seat belts. These springs are specified by force, travel, and dimensional parameters. Material grades include AISI 1095 high-carbon steel, AISI 301 stainless steel, 17-7 PH stainless steel, and Inconel X-750, each offering different fatigue life and corrosion resistance. Available force ranges from 0.5 to 500 N, with travel up to 2000 mm. Strip thickness ranges from 0.05 to 1.5 mm, and width from 1 to 100 mm. Drum diameter can be 10 to 200 mm. Force tolerance is typically ±5%, and fatigue life ranges from 10,000 to 100,000 cycles depending on material and stress. Operating temperature ranges from -40°C to 250°C for standard materials, with higher limits for Inconel. Surface finishes include bright or black oxide. End configurations can be hooked, looped, or plain. Relevant standards include ISO 10243, DIN 2095, and GB/T 1239.2. These springs are selected based on environmental conditions and fatigue requirements. It is essential to verify model-specific values and standards with the legal manufacturer or supplier before procurement.

The spring is wound into a tight spiral. When extended, it uncoils, and the force is generated by the elastic deformation of the strip material. The force remains nearly constant because the radius of curvature changes minimally over the working deflection, maintaining a consistent torque. This design allows for a steady force output over a long travel, making it suitable for applications requiring uniform tension or counterbalance.