Torsion Spring Leg Design: Angles, Directions and Torque
Short answer: Torsion spring leg design starts with three decisions made together, never separately: the leg angle (the free position of each arm relative to the coil axis), the winding direction (which way the coil closes under load), and the required torque at the working rotation. Get the direction wrong and the spring opens instead of closing, torque collapses, and the legs bend at the bend radius. For a typical 1.2 mm music-wire torsion spring with a 10 mm mean coil diameter, each 90° of deflection produces roughly 0.35–0.45 N·m depending on active coils — indicative only, since body length, leg geometry and end conditions all shift the number. BQUQ machines and winds custom torsion springs in one ISO9001 Dongguan factory, with quotes returned in 12 working hours.
Why leg geometry decides torsion spring performance
A torsion spring is not a compression spring bent into a circle. It stores energy by twisting wire around the coil axis, and every bit of that energy has to pass through two points: the bend where the wire leaves the helix, and the leg itself. That is why leg design is not a cosmetic detail added after the torque calculation — it is part of the load path.
Three failure modes dominate real-world torsion spring complaints:
1. Wrong winding direction. The spring is installed so that deflection opens the coil rather than closing it. The legs splay outward, torque drops sharply, and the spring may slip off its mandrel.
2. Leg bend radius too tight. The wire cracks or necks down at the bend, and the spring breaks after a few thousand cycles even though the coil body is fine.
3. Leg angle mismatch. The arms do not sit where the assembly needs them at free state, so the spring is pre-loaded during installation. Pre-load eats into the available deflection range before the spring has done any work.
All three are geometry problems, and all three are cheap to fix on paper and expensive to fix in tooling.
What is the leg angle on a torsion spring?
The leg angle is the angular position of each arm in the free (unloaded) state, measured relative to a reference — usually the coil axis, a line through the coil centre, or the mounting feature in the assembly. Two numbers matter:
- Angle between the two legs in free state. This defines where the arms will sit before any load is applied.
- Angle of each leg relative to the coil axis (the axial tilt). A leg that leaves the coil at 90° to the axis behaves very differently from one that leaves parallel to the axis.
In practice, buyers specify the free-state leg angle and the working angle. The difference between them is the deflection the spring must deliver, and that deflection drives torque.
Free angle vs working angle
| Parameter | What it defines | Why it matters |
|---|---|---|
| Free leg angle | Arm position with zero load | Sets installation fit and pre-load |
| Working leg angle | Arm position at the loaded state | Determines required torque |
| Deflection (Δθ) | Working angle − free angle | The rotation the spring actually delivers |
| Pre-load angle | Installation angle − free angle | Consumes usable travel; raises stress at rest |
| Over-travel allowance | Extra rotation beyond working angle | Protects against hard stops and shock |
A common mistake is specifying the free angle to match the assembly's rest position exactly. In a real assembly, you almost always want a small pre-load — typically 5° to 15° of the total travel — so the arm stays seated against its stop and does not rattle. Beyond about 20% pre-load, you are paying for deflection you never use.
Leg angle tolerances you can actually hold
Wound torsion springs are not machined parts. Angle tolerances of ±5° are routine; ±3° is achievable with dedicated tooling and inspection on a comparator or optical system. If your assembly needs better than ±3° on the free angle, design the mounting feature to absorb the variation rather than tightening the spring tolerance — it is cheaper and more reliable.
How do you choose torsion spring winding direction?
Winding direction describes how the coils are formed: right-hand (clockwise) or left-hand (counter-clockwise) when viewed along the axis. The rule that matters:
A torsion spring should be loaded in the direction that winds the coil tighter, not the direction that unwinds it.
When you load the spring in the winding direction, the coil diameter shrinks slightly, the spring grips its mandrel, and the wire is in clean torsion. When you load it against the winding direction, the coil grows, the spring can walk off the mandrel, and the stress distribution across the wire becomes less predictable.
Direction selection table
| Winding | Load direction that tightens coil | Effect if loaded the wrong way |
|---|---|---|
| Right-hand | Clockwise (viewed from the specified end) | Coil expands, arm may slip, torque falls off |
| Left-hand | Counter-clockwise (viewed from the specified end) | Same, mirrored |
| Double torsion (two coils, centre leg) | Both legs load toward the centre | Uneven loading if legs are asymmetric |
Two practical notes. First, always state the viewing direction when you specify handedness — "right-hand viewed from the leg end" is unambiguous; "clockwise" alone is not. Second, if your assembly can be rotated either way during service, a double torsion spring with a central leg is usually the better answer than a single spring that will be abused in the unwind direction.
How do you calculate torsion spring torque?
The working formula for a round-wire torsion spring is:
T = (E · d⁴ · θ) / (3667 · D · N)
Where T is torque (N·mm), E is the modulus of elasticity (MPa, typically 203,000 MPa for music wire), d is wire diameter (mm), θ is deflection in degrees, D is mean coil diameter (mm), and N is the number of active coils. The constant 3667 packages the unit conversions for degrees and millimetres.
The important structural point: torque scales with the fourth power of wire diameter and inversely with mean coil diameter. A 10% increase in wire diameter raises torque by roughly 46%. That is why wire diameter selection deserves its own analysis — see our guide to spring wire diameter selection for the trade-offs between stress, torque and envelope.
Indicative torque reference
The table below shows typical torque per 90° of deflection for music wire (E ≈ 203,000 MPa), 4 active coils, legs at 90° to the axis. Values are indicative and will shift with body length, leg form and end conditions.
| Wire dia. (mm) | Mean coil dia. (mm) | Torque per 90° (N·mm) | Torque per 90° (N·m) |
|---|---|---|---|
| 0.8 | 6 | 62 | 0.062 |
| 1.0 | 8 | 88 | 0.088 |
| 1.2 | 10 | 132 | 0.132 |
| 1.5 | 12 | 275 | 0.275 |
| 2.0 | 16 | 651 | 0.651 |
| 2.5 | 20 | 1,271 | 1.271 |
Two things to read from this table. First, the spread between a 0.8 mm and a 2.5 mm spring is more than twenty-fold — torsion springs are extremely sensitive to wire size. Second, a 1.2 mm spring at 90° gives about 0.13 N·m, not the 0.35–0.45 N·m cited in the short answer, because that figure assumed a shorter body and fewer active coils. Always calculate from your own geometry rather than borrowing a number.
Stress check, not just torque
Torque alone does not tell you whether the spring survives. The bending stress in the wire is:
σ = (32 · T) / (π · d³) × K
where K is a stress correction factor that accounts for the curvature of the coil and typically runs from about 1.1 to 1.3 depending on the spring index (D/d). Keep the calculated stress below roughly 60–70% of the material's tensile strength for static applications, and lower — often 40–50% — for cyclic service. If you are running near the limit, spring stress relief after winding can recover a meaningful margin.
What are the common torsion spring leg types?
Leg form is where design intent meets manufacturability. The main families:
- Straight legs. Simplest and strongest. The wire leaves the coil and runs straight. Best choice unless the assembly forces something else.
- Bent or hooked legs. The end is bent at an angle to engage a hole, slot or pin. Each additional bend is a stress riser and a tooling cost.
- Inward / outward radial legs. The leg points toward or away from the coil centre. Common in clamps and clips.
- Axial legs. The leg runs parallel to the coil axis, useful when the spring sits in a deep bore.
- Double torsion with centre leg. Two coil bodies sharing a central straight leg, giving balanced torque in both directions.
- Shortened or trimmed legs. Legs cut to length after winding. Cheap, but the cut end has no radius and should not bear on a sharp edge.
Leg design rules that prevent field failures
1. Bend radius ≥ 1× wire diameter, ideally 1.5×. Tighter bends concentrate stress and are the single most common cause of leg fracture.
2. Keep the first bend at least 1.5× wire diameter from the coil body. Bending right at the coil junction damages the helix and creates a weak point.
3. Give the leg a defined bearing surface. A leg pressing on a sharp edge will wear a groove and eventually fail. Design a rounded stop or a pin.
4. Avoid legs that must flex. If the leg is expected to bend during assembly, it will fatigue. Let the coil do the work.
5. Account for leg deflection in your torque budget. A long, thin leg absorbs some rotation before the coil sees it, so the effective spring rate at the arm tip is lower than the coil calculation suggests.
If your application involves repeated cycling, leg geometry and surface condition matter as much as coil stress — our notes on spring rotational fatigue cover the cycle-life side in more detail.
Materials and finishes for torsion spring legs
Music wire (ASTM A228) is the default for small torsion springs: high tensile strength, good fatigue resistance, low cost. Stainless steel — typically 302 or 304 — is the choice when corrosion resistance matters, at the cost of roughly 10–15% lower modulus and reduced fatigue strength. Chrome-silicon and oil-tempered wire suit larger, higher-load springs.
For legs specifically, finish matters more than it does for the coil body, because legs are exposed and often rub against mating parts. Zinc plating, black oxide, and passivation are all common. Note that plating adds thickness — typically 5–15 µm — which can change the fit of a leg in a tight slot. If your leg sits in a precision pocket, specify the finish before you finalise the slot dimension.
Design checklist before you request a quote
| Item | What to specify |
|---|---|
| Wire diameter | Nominal plus tolerance |
| Coil OD / ID | Which one is critical to the assembly |
| Body length | Free-state, with tolerance |
| Active coils | Or total coils and end condition |
| Winding direction | Right or left, with viewing direction stated |
| Free leg angle | Between legs and relative to axis |
| Working angle | Deflection required in service |
| Torque at working angle | With tolerance band |
| Leg form | Straight, bent, hooked, axial; bend radii |
| Material and finish | Grade and coating |
| Cycle life | Expected cycles and environment |
Send this and BQUQ can quote in 12 working hours. Flexible MOQ applies, so a prototype run and a production run can use the same drawing.
Frequently Asked Questions
Q: Can a torsion spring be used in both directions?
A: A single torsion spring is designed to load in one direction — the direction that winds the coil tighter. Loading it the other way expands the coil, reduces torque predictability and risks the spring walking off its mandrel. If your assembly genuinely needs bidirectional resistance, specify a double torsion spring with a central leg, which gives balanced torque in both directions. Otherwise, use two opposed springs.
Q: How tight can the leg bend radius be?
A: Keep the inside bend radius at a minimum of one wire diameter, and preferably 1.5× wire diameter. Tighter bends concentrate stress at the outside of the bend and are the most common cause of leg fracture in service. Also keep the first bend at least 1.5× wire diameter away from where the wire leaves the coil body, so the helix is not distorted during forming.
Q: Why does my torsion spring lose torque after installation?
A: The usual cause is installation pre-load. If the free leg angle does not match the assembly's rest position, the spring is already deflected before it does any work, which consumes usable travel and raises resting stress. A second cause is loading against the winding direction, which opens the coil and drops the effective rate. Check both before changing the spring specification.
Q: What leg angle tolerance is realistic for a wound spring?
A: ±5° on the free-state leg angle is routine for production wound torsion springs. ±3° is achievable with dedicated tooling and optical or comparator inspection. If your assembly needs tighter than ±3°, redesign the mounting feature to absorb the variation — that is almost always cheaper and more reliable than tightening the spring tolerance further.
Q: Does plating affect torsion spring leg fit?
A: Yes, and it is often overlooked. Zinc plating, black oxide and passivation all add or remove a small amount of material — plating typically adds 5–15 µm. On a leg that seats in a precision slot or hole, that can turn a comfortable fit into an interference fit. Specify the finish before finalising mating dimensions, and tell your supplier which surfaces are critical.
Related Resources
- About BQUQ and our Dongguan ISO9001 factory: /about/
- Custom torsion springs and other spring products: /torsion-springs/
- Industry trends in spring and metal component sourcing: /industry-dynamics/
- Technical articles on spring design and manufacturing: /bquq-blog/
- Frequently asked questions: /faq/
- Case studies from production: /case/
- Contact the engineering team: /contact/
Authored by the BQUQ Engineering Team. BQUQ (Dongguan) runs CNC machining (±0.005 mm), metal stamping, custom springs, and heat sink production in one ISO9001 factory. Source-direct from Dongguan, China — quote in 12 hours: sc@bquq.com | WhatsApp +86 13713157787 | www.bquq.com
