Key takeaways
- A flat spring loads in bending across its thickness, so it needs width and a short travel rather than a large diameter.
- Rate is very sensitive to thickness on a flat spring and to wire diameter on a coil spring, so both need a tolerance call.
- A sheared edge is the usual fatigue start on a flat spring; a wire spring starts at a bend, a hook or a coil contact.
- Flat springs win on lateral stiffness and shallow height; wire springs win on travel, cost at low volume and axial symmetry.
The space question is not one number
A tight space is usually described by one dimension: the gap between a cover and a housing, or the height under a PCB. That number alone does not choose the spring. A flat spring can be very thin in the load direction, but it needs width and length to store energy. A wire spring can be compact in diameter, but it needs length along its axis to give travel.
The useful question is which dimensions are scarce. If the height is fixed at 3 mm and the footprint is open, a flat cantilever or a wave washer can fit where a coil cannot. If the footprint is a 6 mm hole and the length is open, a small compression spring or a torsion spring is the natural choice.
The load direction matters as much as the envelope. A flat spring pushes perpendicular to its face and is stiff sideways. A wire spring pushes along its axis and is soft sideways unless it is guided.
How each one stores energy
A flat spring stores energy in bending. A cantilever strip deflects at the tip, a simply supported strip deflects in the middle, and a disc or wave washer deflects through a shaped section. The spring rate depends on the modulus, the width, the cube of the thickness and the cube of the length.
A wire spring stores energy in torsion or in bending of the wire. A compression or extension spring twists the wire as the coil deflects. A torsion spring bends the wire around the coil axis. The rate depends on the wire diameter to the fourth power, the mean coil diameter and the number of active coils.
The difference in the exponents explains the behaviour. A flat spring is dominated by thickness, and a wire spring is dominated by wire diameter.
Load in a tight envelope
Take an illustrative flat cantilever in stainless strip, 0.3 mm thick, 6 mm wide and 20 mm long. The rate works out at about 1 N/mm, so a 3 mm deflection gives about 3 N. The bending stress at that deflection is roughly 650 MPa, which is high for a static part and marginal for a cycling one unless the material and the edge condition are controlled.
To match that load with a wire compression spring in the same 3 mm of height, the spring would need a coil diameter that fits the footprint and a wire size that gives the rate. If the footprint allows a 10 mm diameter, the coil can be made to work. If the footprint is also 6 mm, the wire becomes very thin and the spring becomes fragile and hard to coil.
That is the practical trade. A flat spring converts a scarce height into a generous width. A wire spring converts a scarce footprint into a generous length. Neither is universally smaller; each uses the dimension that the design can spare.
Rate tolerance and consistency
Rate on a flat spring moves with the cube of the thickness. A strip at the top of its thickness tolerance can be several per cent stiffer than one at the bottom, and the load at a given deflection moves with it. The strip standard and the thickness tolerance class belong on the drawing, along with the grain direction if the part is formed across the strip.
Rate on a compression spring moves with the fourth power of the wire diameter and inversely with the cube of the mean coil diameter. Wire diameter tolerance classes exist for this reason, and a drawing that gives a nominal wire size without a class is asking for a rate spread.
In practice a stamped flat spring often carries a wider rate tolerance than a coiled spring, because the stamping process adds burr, edge taper and residual stress to the strip variation. A wire spring can be adjusted at the coiler by changing the pitch or the coil count.
Fatigue and edges
A flat spring starts its fatigue life at an edge. Stamping leaves a shear zone, a burr and sometimes a micro crack on the cut face, and the tension side of the bend is where a crack opens. Tumbling, edge radiusing and shot peening all improve the life, and the drawing should say which one is required.
A wire spring starts its fatigue life at a bend, a hook or a coil contact. The drawn surface of the wire is relatively clean, so the failure is usually at a geometric feature rather than at a cut edge. That is an advantage for a part with a high cycle count, provided the bends are generous and the hooks are designed for the stress.
Both families suffer from corrosion fatigue. A plated flat spring with a poor coating at the edge, or a wire spring with a crack in a plating layer, will fail earlier in a damp environment. The finish call matters as much as the material call.
Cost and tooling
A flat spring is a stamped part. At volume it needs a die, and the die cost is recovered over the run. At prototype or low volume it can be laser cut or waterjet cut from strip, which avoids the die but leaves an edge that may need finishing for fatigue.
A wire spring is a formed part. A CNC coiler or former can make a new design from a program and a small set of tools, so the entry cost is low and the change cost is low. At high volume a dedicated coiler setup or a four-slide can bring the piece price down, but the tooling is still lighter than a stamping die.
The cost crossover depends on the part. A simple flat spring with a loose tolerance can be laser cut cheaply at low volume. A complex profile with tight tolerances needs a die, and then the volume has to justify it. A wire spring rarely needs a heavy tool, so it is usually the lower risk route for an uncertain programme.
The comparison
The table sets the two families against the requirements that decide a spring in a tight space. It assumes a similar load and a similar deflection, so the differences are about geometry and process rather than about size.
| Requirement | Flat spring | Wire spring |
|---|---|---|
| Dimension used | Height and width, with a short length | Diameter and axial length |
| Load direction | Perpendicular to the face | Along the axis, or torque about it |
| Lateral stiffness | High across the width | Low, may need a rod or bore |
| Rate sensitivity | Cube of the thickness | Fourth power of the wire diameter |
| Fatigue start | Sheared edge or burr | Bend, hook or coil contact |
| Tooling | Die at volume, laser for prototypes | Program and pins, light tooling |
| Best volume | Thousands upward with a die | Prototype to millions |
| Typical failure | Edge crack or permanent set | Buckling, coil clash or hook break |
A comparison at similar load and deflection. The right answer depends on which dimension the envelope can spare.
How to choose
Start with the scarce dimension and the load direction. If the height is the limit and the load is perpendicular to a flat face, a flat spring is the natural candidate. If the footprint is the limit and the load is axial, a wire spring is the natural candidate.
Then check the travel. A flat cantilever gives a short travel before the stress becomes high or the strip takes a set. A coil spring can give a long travel in the same footprint, provided the length is available and the spring is guided.
Finally, check the volume and the change risk. A flat spring with a die is efficient at volume but expensive to change. A wire spring is cheap to change and suits a programme that is still moving. For a part that needs both a shallow height and a long travel, a flat wire coil spring can be the compromise: a coiled spring made from rectangular wire packs more material into a small radial space.
- Identify the dimension that cannot grow
- Check the load direction and the lateral support
- Estimate the travel before the stress limit
- Compare the tooling charge at your volume
- Consider flat wire coils when height and travel both matter
Frequently asked questions
Can a flat spring fit in a 3 mm gap where a coil spring cannot?
Often yes, if the footprint has room for width and length. A flat cantilever can be 0.3 mm thick and still carry a useful load. A coil spring in the same 3 mm height would need a small diameter and a thin wire, which limits the load and the travel.
Which type holds a tighter rate tolerance?
A coiled wire spring usually holds a tighter rate because the coiler can adjust the pitch and the coil count, and the wire diameter is tightly controlled. A stamped flat spring carries the strip thickness variation plus burr and edge effects. Test at the installed height and set the tolerance from the function.
Can I use flat wire in a coil spring?
Yes. A flat wire compression spring winds rectangular wire on edge, so it packs more material into a small radial envelope than a round wire spring. It is a useful compromise when the height is limited but the spring still needs travel. The forming and coiling route is different, so ask for a supplier who has made the type before.
Which is cheaper at 5,000 pieces?
Usually the wire spring, because it needs a program and pins rather than a stamping die. A simple flat spring can be laser cut at that volume and may compete, but the edge finish and the rate tolerance have to be checked. Compare the total cost, including any finishing step.
Sources and further reading
- EN 10132-4, Cold rolled narrow steel strip for heat treatment, technical delivery conditions, spring steels
- EN 10270-1, Steel wire for mechanical springs, patented cold drawn unalloyed spring steel wire
- Qumek spring selection and DFM notes for flat and wire springs
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
Not sure which spring fits the envelope?
Send the space, the load and the travel. The engineering desk will compare a flat and a wire option on the dimensions you can spare.
Send the envelope