Custom Torsion Springs: An OEM Buyer's Guide

What a torsion spring actually does

A torsion spring stores energy through rotation, not linear travel. Torque is applied through its legs, the body winds up, and the spring pushes back. That single difference drives everything else in this guide — because when a torsion spring winds up, its geometry changes: the coils get tighter, the inside diameter shrinks, and the body gets longer.

Design around the free state alone and the spring will bind on its shaft the first time it is loaded.

The five decisions that determine whether it works

1. Wind direction — the mistake we see most often

A torsion spring must be loaded in the direction that winds the coils tighter, not the direction that unwinds them.

Load it the wrong way and three things happen: the coil diameter grows instead of shrinking, the spring loses the favourable residual stress left by the coiling process, and its usable torque and fatigue life drop sharply.

So the wind direction — left-hand or right-hand — is not a preference. It is dictated by your assembly. State it explicitly on the drawing, and state the direction of the applied load. "Torsion spring, 5 coils" without a wind direction is not a manufacturable specification.

2. The shaft — and the ID that shrinks around it

Under load, the mean coil diameter decreases. The wire length is fixed, so as coils are added by the deflection, the diameter must give way:

D₂ = D₁ × N / (N + θ)

where D₁ is the free mean diameter, N is the number of active coils, and θ is the deflection in revolutions.

This is why torsion springs seize. The shaft was sized to the free ID, the spring wound up, the ID closed onto the shaft, and now the torque output is unpredictable and the spring is wearing itself out.

Rule of thumb: the shaft should be no more than 90% of the spring's minimum ID at maximum deflection. Not the free ID — the loaded one.

The body also lengthens as coils are added. Leave axial clearance for it.

3. Leg configuration — your moment arm is part of the spring

Straight, bent, hooked, offset, extended — the legs are not just mounting features. They carry the load, they set the moment arm, and they deflect too, which means they contribute to the spring's effective rate.

Long legs give you mechanical advantage but also flex, which softens the effective rate and makes torque less predictable. Short legs are stiffer and more repeatable but can be harder to locate in the assembly.

Specify leg length, leg angle, and any bend radii. If the legs are the interface to your part, they need tolerances the same way the coil body does.

4. Spring index — the manufacturability check nobody runs

Spring index C = D / d (mean diameter ÷ wire diameter).

• C below about 4: severe forming stress, tooling wear, cracking risk, wide diameter scatter
• C between 5 and 10: the comfortable zone — repeatable, stable, cost-effective
• C above about 14: loose tolerance control, springs that vary part to part

If your design lands at C = 3 or C = 18, it can often still be made — but expect higher cost, wider tolerances, and more scrap. Checking the index takes ten seconds and prevents a lot of arguments later.

5. Stress and material — and one counterintuitive point about heat treatment

Torsion springs work in bending, not torsion of the wire itself (despite the name). The stress that matters is at the inner surface of the coil, and it needs a correction factor:

σ = K_i × 32M / (π d³)

with the inner-fibre correction factor:

K_i = (4C² − C − 1) / (4C(C − 1))

Here is the counterintuitive part: coiling leaves residual stress in the favourable direction for a torsion spring — the same direction the spring is loaded. Aggressive high-temperature stress relief removes that benefit and can actually reduce the spring's capacity.

This is the opposite of compression springs, where full stress relief is routine. For torsion springs, heat treatment should be deliberate and low-temperature — enough to stabilise the part, not so much that it undoes the coiling. It is one of those details that separates a spring that holds its free angle from one that drifts.

A worked example

Say you need roughly 0.12 N·m at 90° of deflection, in music wire.

Starting point:

• Wire: ASTM A228 music wire, d = 1.0 mm
• Mean coil diameter: D = 8.0 mm
• Active coils: N = 5
• Deflection: θ = 90° (0.25 revolutions)
• E ≈ 207,000 MPa

Step 1 — Check the index.
C = D/d = 8.0 / 1.0 = 8 — comfortably in the 5–10 zone.

Step 2 — Rate per degree.
k = E·d⁴ / (3667 · D · N)
k = (207,000 × 1.0⁴) / (3667 × 8.0 × 5) = 207,000 / 146,680 = 1.41 N·mm per degree

Step 3 — Torque at 90°.
M = 1.41 × 90 = 127 N·mm ≈ 0.127 N·m — meets the target.

Step 4 — Check the stress.
K_i = (4×8² − 8 − 1) / (4×8×7) = 247 / 224 = 1.10
σ = 1.10 × (32 × 127) / (π × 1.0³) = ≈ 1,427 MPa

A228 at 1.0 mm has a tensile strength of roughly 2,250 MPa (it is diameter-dependent — always check the spec for your exact wire size). So we are at about 64% of UTS. For a static or low-cycle application that is acceptable — the usual static allowance for torsion springs runs to roughly 70–80% of UTS. For high-cycle fatigue, this is too high; you would drop the stress by increasing the wire diameter or the coil count.

Step 5 — Size the shaft. This is the step that gets skipped.

• Free ID = D − d = 8.0 − 1.0 = 7.0 mm
• Mean diameter at 90°: D₂ = 8.0 × 5 / (5 + 0.25) = 7.62 mm
• ID at 90° = 7.62 − 1.0 = 6.62 mm

The ID closed by 0.38 mm. Applying the 90% rule: maximum shaft = 0.9 × 6.62 = 5.96 mm → specify a 5.9 mm shaft.

If you had sized the shaft off the free ID of 7.0 mm, the spring would bind.

Step 6 — Body length.
Free: L = d(N+1) = 6.0 mm. At 90°: L = d(N + 1 + 0.25) = 6.25 mm. Leave axial clearance.

One honest note on the math: these formulas assume no friction. In reality the coils rub against each other and the shaft, so the measured rate usually comes out slightly higher than calculated. This is exactly why the sample stage exists — the calculation gets you close, the sample tells you the truth.

Tolerancing: what to specify, and what to leave alone

A common way to make a torsion spring expensive is to over-constrain it.

Tolerance what you need:

• Torque at a stated deflection angle — this is almost always the functional requirement
• Wind direction
• Leg positions and lengths
• ID (with the loaded condition in mind)

Be careful with:

• Free angle. The free angle of a torsion spring naturally scatters — it is one of the widest tolerances in spring manufacturing. Holding it tight adds cost and sorting.
• Free angle AND torque at deflection, both tight. These are linked. Constraining both over-determines the spring, and the manufacturer is left with an impossible trade-off.

The best specification we receive reads: "X N·mm ± Y% at Z degrees, right-hand wound." It tells us what the spring has to do and lets us hit it.

What to send with your RFQ

The difference between a quote in a day and three rounds of emails:

• Drawing or sketch — even hand-drawn with dimensions
• Wire diameter (or the torque you need, and let us pick it)
• Mean diameter or ID — and say which
• Number of coils
• Wind direction — left-hand, right-hand, or double
• Leg length, leg angle, bend details
• Torque at a stated deflection angle — the most important line
• Shaft diameter, if the spring runs on one
• Material, or the environment (temperature, chemicals, corrosion) so we can propose one
• Finish / plating requirements
• Cycle life — static, or how many cycles?
• Annual volume and prototype quantity
• Certification requirements — material certs, PPAP, dimensional reports

No drawing? Send 3–5 physical samples. We analyse the material and performance and produce matched prototypes.

Five mistakes worth avoiding

1. No wind direction on the drawing. The single most common omission. Half the springs made from an incomplete drawing are backwards.
2. Shaft sized to the free ID. Covered above. It binds.
3. Free angle toleranced to ±1°. Expensive, and rarely what the part actually needs.
4. Ignoring the spring index. C = 3 designs get quoted at three times the price for a reason.
5. Specifying material by habit rather than environment. 1.4310 (301) stainless is an excellent general spring steel — but in an aggressive chemical environment it is the wrong choice, and finding that out in the field is expensive. Tell your supplier the environment, not just the grade.

Working with us

We are a custom spring manufacturer in Shanghai — IATF 16949 certified, 20+ years, producing single, double and shaped torsion springs to customer drawings. Not a catalogue supplier: every part is made to specification.

Where a specification calls for something unusual, we are willing to solve the whole problem rather than decline it — including sourcing difficult materials and absorbing mill minimums that would otherwise stop a project at the prototype stage.

Send us your drawing, your samples, or just the torque and the angle. We will tell you honestly what we can do.

Request a quote →

FAQ

1.What is the difference between a single and a double torsion spring?

A double torsion spring is two coil bodies — one right-hand wound, one left-hand — connected by a bridge, working in parallel. It roughly doubles the torque in the same space and balances the side load. Use it when you need more torque without more diameter.

2.Which way should my torsion spring be wound?

Whichever direction the load winds it tighter. Determine it from your assembly and state it on the drawing.

3.Can you make torsion springs from a sample without a drawing?

Yes. Send 3–5 pieces and we will analyse the material and performance and produce matched prototypes.

4.What is your MOQ for custom torsion springs?

Production starts at 1,000 pcs. Prototypes and samples are supported below that.

5.How tight can you hold torque tolerance?

It depends on the wire, the index and the geometry. ±10% is routine for most designs; tighter is possible but should be discussed against your actual functional need — over-tolerancing is one of the biggest cost drivers in custom springs.

6.What material should I use?

Tell us the environment — temperature, media, corrosion, cycle life — rather than picking a grade by habit. For most industrial applications a carbon or stainless spring steel is right; for aggressive chemistry or high temperature, we can evaluate specialty grades.