A constant-force spring is a tightly rolled strip of prestressed spring steel that delivers nearly the same force across its entire extension. Unlike a helical spring, where force rises linearly with displacement (F = kx), a constant-force spring's load stays flat once the strip unrolls past the first inch or so. This makes it the go-to solution when you need steady tension regardless of position.

The physics: the strip is manufactured with a natural "curl" radius. Pulling the strip off the drum forces it to straighten, and the internal bending stress that resists straightening produces the output force. Since only the small transition zone at the drum is being deformed at any moment, the force doesn't depend on how much strip you've pulled out — extend it 3 inches or 30 inches, the load is the same.

Where you see them:

• Retractable tape measures — the coiled strip inside the case pulls the tape back with steady tension whether you've extended 1 foot or 25 feet.
• Cable retractors on tool balancers hanging over assembly lines — the operator feels the same pull-down force at every arm position.
• Counterbalances in laptop hinges, monitor arms, and hospital bed lift mechanisms.
• Carbon brush holders in electric motors — constant force keeps the brush pressed against the commutator as it wears down over thousands of hours.
• Window sash balances that hold a double-hung window at any height without a counterweight.

Sizing rule of thumb: Force output is approximated by:

F ≈ (E × b × t³) / (26.4 × R²)

Where E is Young's modulus (~30×10⁶ psi for spring steel), b is strip width, t is thickness, and R is the natural coil radius. The cube on thickness matters — doubling thickness gives 8× the force. Manufacturers publish force curves; typical off-the-shelf units deliver anywhere from 0.5 lb to 30+ lb.

Design gotchas:

• Life vs. extension ratio: Cycle life plummets if you extend the strip past ~20× the drum diameter. Design for the shortest travel that meets the requirement.
• Direction matters: The strip must wind onto a smaller-diameter output drum (not free-hanging) for true constant force. A loose strip will spring back into a tangled mess.
• Fatigue is bending-driven, not tension-driven. The transition zone gets fully stress-reversed on every cycle, so surface finish and edge quality dominate life.
• They can't push. Constant-force springs only pull. For pushing applications, use constant-torque springs (motor-style spool arrangement).