A constant force spring is a pre-stressed strip of spring steel or stainless steel, tightly coiled into a roll, that delivers nearly the same output force across its entire extension travel. Unlike a round-wire extension spring, whose force rises linearly with deflection under Hooke's Law, a constant force spring reaches its full rated load after unwinding roughly 1.25 times its coil diameter — then holds that force essentially flat for the rest of the stroke.
How It Works
The strip keeps the same radius of curvature as it unwinds, and that's really the whole trick — the force stays flat because the geometry stays flat, not because of some clever material property. It's one member of the broader Strip Springs family, distinguished from cousins like variable-force and disc springs by that flat force curve. See our guide to flat springs for how all these types compare.
Mount it on a drum and two rules keep it behaving: the drum diameter should run 10–20% larger than the spring's natural coil diameter, and at least 1.5 wraps need to stay on the drum at full extension. Skip either one and the strip starts kinking or losing its set well before it reaches its rated cycle life.
Advantages of Constant Force Springs
Compared with a round-wire extension spring sized for the same job, a constant force spring wins on three points: it holds output force flat over a long travel instead of climbing with Hooke's Law, it packages that travel into a tighter footprint since the strip nests on itself rather than stretching out in a line, and it avoids the free-length growth problem that makes long round-wire springs awkward to mount.
The tradeoff is that it needs guided travel — an unsupported strip buckles at long extensions — and it can't be back-bent against its natural curvature without permanent damage, so it's a poor fit for applications with reversing loads at the working end.
Constant Force vs. Standard Extension Springs
Round-Wire Extension Spring | Constant Force Spring | |
|---|---|---|
Force curve | Rises linearly with deflection | Flat after ~1.25× diameter of travel |
Cross-section | Round wire | Flat strip |
Best suited for | Short, defined travel | Long, uniform-force travel |
Mounting Configurations
Most constant force springs are drum-mounted, with the inner end fixed to a rotating hub — the standard choice for cable retractors and counterbalances. A Clip-On Constant-Force Spring swaps the pressed hub for a spring clip instead, so the spring can be replaced in the field without pulling the drum apart.
When You Need a Ramped Force Instead of a Flat One
Not every application wants a zero force gradient. A Variable-Force Spring uses a strip with a changing radius of curvature to produce a rising or falling force profile instead — the part to reach for when the load you're counterbalancing effectively changes weight through the stroke.
How It's Made
Constant force springs run on the same coiling equipment used for round-wire springs, retrofitted with tooling that grips and feeds flat strip instead of wire. From there, four inputs define the part: strip material, width and thickness, initial (natural) coil diameter, and the ID/OD of the finished roll.
Calculating Load and Sizing
The output force follows F = E·b·t³ / (26.4·Rn²) — modulus of elasticity times strip width times thickness cubed, divided by 26.4 times the natural radius squared. Published engineering references cite this as accurate to roughly ±10% for standard configurations. Notice the t³ term: doubling strip thickness raises the load eightfold, which is why thickness tolerance matters far more than width tolerance when a part is underperforming its rating.
Typical stock designs run a width-to-thickness ratio (b/t) around 100:1 and a mounted-to-natural diameter ratio (D0/Dn) around 1.2:1 — the same 10–20% oversizing rule mentioned above, just expressed as a ratio. Total developed strip length works out to roughly L = 1.56N(D0 + Dn), where N is the number of coil turns.
On cycle life, a commonly cited design target is keeping peak bending stress at or below 60% of the strip material's yield strength — parts designed to that margin typically clear well over a million cycles, while pushing stress higher trades life for a smaller, lighter spring.
Typical Tolerances
LILY's stock constant force line — JCJDKBBC included — ships with an OD tolerance of ±0.04 in., an ID tolerance of ±0.03 in., and a load tolerance of ±0.05 lbf. That's tight enough for most replacement and retrofit work; designs with tighter tolerance requirements typically call for a custom-specified part rather than off-the-shelf stock.
A Real Spec Example
[PRODUCT PHOTO: JCJDKBBC — insert here]
LILY carries a full line of Constant-Force Springs in stock, including JCJDKBBC — a 301 stainless steel constant force spring, 0.004 in. thick and 0.187 in. wide, delivering 0.49 lbf across a 15 in. extended length, rated for roughly 3,000 duty cycles. That combination — thin stainless strip, sub-1-lbf load, moderate cycle count — sits at the light-duty end of the category; heavier industrial units simply run thicker strip for more load, per the t³ relationship above.
Stainless vs. carbon steel is a separate decision, driven by corrosion exposure rather than load capacity. The two materials have similar elastic modulus, so switching between them doesn't meaningfully change how much force a given thickness delivers.
Common Applications
Counterbalance mechanisms — window sash counterbalances and medical/industrial table lifts use the flat force curve so the assist force doesn't change as the panel or platform moves through its range.
Cable and hose retractors — the spring rewinds the cable at a steady tension regardless of how much is paid out, without a ratchet or motor.
Tool-head return systems — pneumatic and production tooling uses a constant force spring to snap the head back to its home position after each cycle.
Cabinet and furniture closers — soft, consistent closing force across the full swing of a door or drawer.
Resistance mechanisms in fitness equipment — rowing machines and resistance trainers use the flat curve so effort feels the same at the start and end of the stroke.
Retail product dispensers — beverage cooler shelving uses constant force springs to push stock forward at a steady rate as items are removed.
End Configurations
The inner end (the one wound onto the drum or spool) is typically secured with a slot, clamp, or adhesive so it doesn't slip during rotation. The outer end — the one that actually connects to the load — is where more variation shows up:
Mounting tab — riveted or integral, for bolting to a bracket.
Hook or pull-loop — formed at the strip end, for attaching directly to a cable or lever.
Plain end — welded or fastened straight to the load with no hook or loop at all, which removes the stress concentration a hook or loop introduces — useful for parts running near their fatigue limit.
Custom end forms are available beyond these; the standard options above cover most off-the-shelf applications.
FAQ
Is a constant force spring the same as a power spring?
No. A power spring (what LILY lists as a Rotor Spring) stores torque over many turns and releases it in a rotary return stroke — think tape measures and seatbelt retractors. A constant force spring outputs linear force as the strip unwinds. Both start from a coiled flat strip, but the motion they produce is different.
Why would a constant force spring stop delivering constant force?
Usually one of three things: the drum is undersized — it should run 10–20% larger than the spring's natural coil diameter — so the strip binds instead of unrolling freely; an idler pulley smaller than the natural coil diameter is back-bending the strip against its set curvature; or the strip has simply been cycled past its fatigue life and lost its pre-stress. Watch for increasing friction, uneven pull, or visible kinking — those show up well before the part actually snaps.
Can the load and length be customized?
Yes — strip width, thickness, and coil diameter are the three variables a manufacturer adjusts to hit a specific load and travel length, since output force scales roughly with thickness cubed. That's why constant force springs are typically specified to the application rather than picked off a shelf beyond light-duty stock sizes.
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