A flat spring is any spring made from a flat strip or sheet of metal rather than round wire, engineered to bend, flex, or coil while storing and releasing mechanical energy. Because the cross-section is rectangular instead of circular, you can get a flat, ramped, or stepped force curve out of the same basic strip — just by changing how it's coiled.

Flat Spring vs. Wire Spring

A wire spring stores energy mainly through torsion in the coiled wire. A flat strip stores it through bending across its width instead. It sounds like a small distinction, but it's why a flat strip can nest tightly on itself — exactly how every constant and variable force spring is built — and why flat stock is so easy to stamp or photo-etch into custom contact and switch springs.

Wire spring versus flat spring: how each stores energy Side by side comparison showing a round-wire coil storing energy through torsion in the wire, next to a flat strip coil storing energy through bending across its width. Round-wire spring Energy stored via torsion in the coiled wire Flat strip spring Energy stored via bending across the strip's width

Main Types of Flat Springs

All of the coiled types below live under LILY's Strip Springs category:

  • Constant-Force Springs — coiled flat strip, near-zero force gradient after initial travel. See our full guide to constant force springs for the mechanics.

  • Variable-Force Springs — engineered radius change for a ramped or degressive force curve.

    [PRODUCT PHOTO: Variable-Force Springs — insert here]

  • Clip-On Constant-Force Springs — same flat force output, quick-change clip mounting instead of a pressed hub.

    [PRODUCT PHOTO: Clip-On Constant-Force Springs — insert here]

  • Disc Springs — a conical flat-stock design (Belleville, wave, curved variants) built for high load in a very short axial space rather than long linear travel.

Materials

301/302 stainless steel is the default where corrosion resistance or washdown environments matter — it's what LILY's stock constant force line ships in. Per DIN 17224 (the spring-strip standard for stainless steel), the practical application temperature for spring-tempered stainless strip runs roughly 120–250°C (250–480°F) depending on load. The limiting factor is stress relaxation, not oxidation — the strip gradually loses its set well before the metal itself would be damaged by heat.

1074/1075 carbon spring steel costs less and covers most dry, indoor, non-corrosive applications, but loses its temper at noticeably lower temperatures than stainless. Beryllium copper shows up where electrical conductivity has to coexist with spring action, such as contact springs.

How to Choose the Right Flat Spring

Start with the force profile the application needs — flat, ramped, or high-load-short-travel — and that alone narrows the field to constant force, variable force, or disc spring territory. From there, environment decides the material, and layout decides the mounting method (drum-mounted, clip-on, or bolted stack).

On material, don't default to “stainless is just the tougher option” — 301/302 isn't necessarily more fatigue-resistant than a good carbon spring steel; it's chosen for corrosion resistance, not raw cycle life. The judgment call that actually matters: if the part will see frequent flexing and sits in a humid, washdown, or outdoor environment, specify stainless from the start.

A plated carbon steel strip can hold up fine on dry-cycle fatigue alone. But once repeated flexing wears through the plating, exposed corrosion pits become crack-initiation sites — the failure mode shifts from simple fatigue to corrosion-fatigue, and the part can fail well short of its rated dry-cycle life. Dry, low-corrosion-risk environments are where coated carbon steel earns its lower cost.

For a concrete reference point, LILY's stock constant force spring JCJDKBBC runs 0.004 in. 301 stainless rated to roughly 3,000 cycles — see our constant force spring guide for the full spec breakdown — which is a reasonable starting point if you're sizing something in the same load class.

Applications by Type

Each flat spring type tends to show up in a different corner of the same general problem — controlling force in a compact package:

  • Constant-force springs — cable and hose retractors, window sash counterbalances, tool-head returns.

  • Variable-force springs — ergonomic tensioners and exercise equipment where the resistance is meant to ramp up or down through the motion, not stay flat.

  • Clip-on constant-force springs — the same counterbalance and retractor uses as standard constant-force springs, in assemblies where the spring needs to be swapped without disassembling the drum.

  • Disc springs — preloading bolted joints, absorbing thermal expansion in flanged connections, and taking up axial play in bearing assemblies.

Flat Spring vs. Power Spring

[PRODUCT PHOTO: Rotor Spring — insert here]

A Rotor Spring (power/motor spring) is also wound from flat strip, but it's built to store rotational energy over many turns and release torque in a return stroke — not to produce a steady linear force. Need constant pull or push? Look at the flat spring types above. Need a spring-loaded rewind? That's a rotor spring, not a flat spring in the sense used here.

FAQ

Is a flat spring the same as an automotive leaf spring?

Both are flat-strip springs, but a leaf spring is a heavy-duty, low-cycle-count design built to absorb shock and support static load in vehicle suspensions. The flat springs covered here — constant force, variable force — are precision, high-cycle-count components built for linear force and motion control. Same starting material, different engineering target.

Can flat springs be custom-shaped for a specific assembly?

Yes. Flat stock can be stamped, photo-etched, or laser-cut, so custom cantilever and contact-spring shapes are common. That's a separate customization path from the coiled constant/variable force types, which are tuned by strip width, thickness, and coil diameter rather than outline shape.