Constant-force springs

Constant-force springs are mechanical components made from a pre-stressed, flat strip of spring steel coiled tightly around a drum or its own center, designed to provide a nearly constant force over a long range of linear motion. Unlike traditional helical springs, which obey Hooke's law (force increases with extension), constant-force springs maintain a uniform pull regardless of the extension length after an initial load is overcome.

Design and Function
The unique mechanism of a constant-force spring allows it to deliver a steady, smooth force while remaining compact.
Mechanism: The force primarily comes from the resistance of the material to being unrolled and bent into a straight line.
Energy Storage: When the flat metal strip is pulled, the material coming off the tight coil bends from its natural radius into a straight section, storing potential energy. This stored energy is released as a consistent pulling force when the load is removed.
Space Efficiency: The spring occupies minimal space in its coiled state but offers an extensive linear range of travel.
Mounting: They are typically mounted by tightly wrapping the coil on a drum and attaching the free end to the component that requires the force.
Materials
The material selection determines the spring's strength, durability, and resistance to environmental factors like corrosion and temperature.
301 Stainless Steel: The most common material, offering high tensile strength, good fatigue life, and corrosion resistance.
High Carbon Steel: A cost-effective option with high hardness and strength, though it may require protective coatings in corrosive environments.
Inconel: A nickel-chromium alloy used for applications in high-temperature or highly corrosive environments (e.g., saltwater exposure).
Common Applications
Due to their ability to provide consistent tension or counterbalancing, constant-force springs are utilized across various industries.
Counterbalances: Used in medical equipment (X-ray arm supports, hospital beds), window balancers, and industrial machinery to assist in lifting heavy moving parts.
Retractable Mechanisms: Found in tape measures, seatbelt retractors, vacuum cleaner cords, and point-of-purchase displays to ensure smooth and controlled extension and retraction.
Tensioning: Employed in automated machinery, cable management systems, and wire feeders to maintain uniform tension on materials.
Motors: Used as carbon brush springs in electric motors to maintain consistent pressure for optimal electrical contact.
Automotive: Integrated into components like sunroof systems, seat mechanisms, and trunk assists.