The English name of this component sends engineers in the wrong direction, and the German and Polish names do the same. A torsion spring does not work in torsion. The moment rotates the coils around the axis, and the wire itself is loaded in bending – which changes the modulus that governs the calculation, and with it every material substitution decision made later in the project.
Bending, not torsion, and what follows from it
In compression and extension springs the wire is twisted and the rate depends on the shear modulus G. In a torsion spring the governing property is Young’s modulus E.
The practical consequence shows up in every grade change. Converting a carbon steel torsion spring to 1.4310 does not mean comparing 81 500 with 70 000 N/mm²; it means comparing 206 with 185 kN/mm². The loss of stiffness is then about 10 %, not the 14 % that applies to a compression spring. Calculation and design are covered by EN 13906-3, which deals with cold and hot coiled helical torsion springs with a linear characteristic.
Moment per degree: the equation worth keeping at hand
For a torsion spring made of round wire the angular rate follows:
R = (E · d⁴) / (3667 · D · n), with R expressed in N·mm per degree
where E is Young’s modulus, d the wire diameter, D the mean coil diameter and n the number of active coils. The constant 3667 comes from converting radians to degrees in the classical bending equation.
For 2 mm wire to EN 10270-1, a mean diameter of 16 mm and eight coils, the result is roughly 7 N·mm per degree. A 90° rotation therefore produces a moment in the region of 630 N·mm. The equation ignores the contribution of the legs – with long legs their own bending noticeably reduces the stiffness of the assembly, and EN 13906-3 treats that separately.
Wire diameter appears to the fourth power again. In torsion springs that hurts more than in compression springs, because the geometric room for manoeuvre is usually smaller: the arbor and the housing are already designed, and the only free variables left are the wire and the coil count.
Wind direction: a binary parameter that can scrap a whole batch
A torsion spring is loaded in the direction that closes the coils. The body diameter then decreases, the spring tightens onto the arbor and works in a stable position. Loaded the other way it unwinds, the diameter grows, the stress distribution turns unfavourable and service life drops.
Wind direction, right hand or left hand, is therefore not supplementary information. It is a functional dimension. A left hand and a right hand spring with otherwise identical parameters are two separate part numbers and two separate tool set-ups. The mistake surfaces at assembly, usually after an entire batch has been received.
Put it on the drawing twice: in words in the parameter table, and graphically, by marking the direction of the working moment.
The arbor: diameter, clearance and body elongation
As the spring rotates in the closing direction, its inside diameter shrinks and the body grows longer. Both effects have to be allowed for in the housing.
The guide arbor is sized so that the spring does not clamp onto it at the maximum working angle. In practice that means a diameter clearly below the minimum inside diameter of the spring in its end position, typically around 90 % of that value. An arbor that is too tight produces friction, accelerated coating wear and moment hysteresis – invisible in the calculation, highly visible in warranty claims.
Body elongation calls for axial clearance. A spring seated between two rigid walls will start generating an axial force in service and will rub against them instead of rotating freely.
Legs: length, shape and angle tolerance
The legs transfer the moment from the body to the mating parts, and they are usually what drives the price. A straight tangential leg is the cheapest option. A leg bent in three dimensions, with a hook or an eye, normally requires additional operations and dedicated tooling.
The angle between the legs in the unloaded state has to be given together with a tolerance. On springs with a low coil count, that angular tolerance is hard to tighten in practice, because a small variation in wire length translates into a large spread of the free angle. Where the application needs a precise rest position, designing adjustment into the housing is often cheaper than tightening the tolerance on the spring.
Long legs also act as a lever on the point where they leave the body. The bend radius at that transition should be as generous as the design allows.
Pitch between the coils
Torsion springs are usually wound with a small pitch so that the coils do not touch during operation. Friction between adjacent coils introduces hysteresis and abrades the corrosion protection exactly where nobody can inspect it afterwards.
In high cycle applications – flaps, lids, latch mechanisms in domestic appliances – this detail can decide the difference between 200 thousand and one million cycles.
Material and operating temperature
The grade range matches other spring types, but torsion springs meet elevated temperatures more often, in oven hinges, lighting fixtures and mechanisms sitting close to a heat source.
| Material | Standard | E [kN/mm²] | Temperature range | Notes |
|---|---|---|---|---|
| Patented wire SH / DH | EN 10270-1 | 206 | -200 to +80 °C | default choice for dry interiors |
| 1.4310 | EN 10270-3 | 185 | -200 to +160 °C | humidity, steam, washing |
| 1.4568 | EN 10270-3 | 195 | -200 to +200 °C | higher strength after precipitation hardening |
| 1.4401 | EN 10270-3 | 180 | -200 to +300 °C | chemical environments, higher temperatures |
Exceeding the upper limit does not produce an immediate failure. It produces relaxation: after a few hundred hours in service the spring gives back part of its moment and the flap stops closing. That is one of the harder failure modes to diagnose, because the part still looks perfectly serviceable.
Double torsion springs, when one body is not enough
When the required moment exceeds what a single body can deliver within the available diameter, a double torsion spring is used: two bodies wound in opposite directions and joined by a common length of wire. The moments add up, while the axial forces of the two halves cancel each other, so the part pulls less to one side.
The solution costs noticeably more to manufacture and needs accurate guidance in the housing, yet in compact hinges it is often the only way to reach the required moment without increasing wire diameter.
Checking the moment in series production
Moment is measured on a test rig at a defined angle, with a defined way of supporting the legs. The result depends on that support more strongly than intuition suggests – a leg supported at a point and a leg supported along its length give different readings for the same part.
The measuring method therefore has to be agreed before the series starts and written into the inspection documentation. Without it, any discussion about batch conformity turns into a comparison of two different measurements of the same component, taken on two different instruments.
Tolerances to DIN 2194
Requirements and testing for cold coiled helical torsion springs are covered by DIN 2194, which applies to wire up to 17 mm diameter. The standard tolerates the coil diameter in the unloaded state, the spring moment, the relative angle of the legs, the body length when wound with a pitch, leg lengths, bend radii and bend angles on the legs.
This matters because torsion springs have no equivalent of EN 15800. Copying „accuracy grade 2” across from a compression spring drawing carries no technical meaning here.
For repeat production it also pays to agree a reference moment for the first part of each batch, and a threshold beyond which the machine settings are corrected. A torsion spring reacts to tool wear faster than a compression spring, because the free angle depends on the length of wire that is cut off.
Three things to check before releasing the drawing
- Wind direction stated unambiguously, ideally with an arrow showing the working moment
- Angle between the legs in the free state with a realistic tolerance, plus the moment at the specified working angle
- Arbor diameter and axial clearance that allow for the change of geometry under load
Where these springs earn their volume
The largest quantities go into opening and closing mechanisms: flaps, lids, hinges, handles and latches. We produce them within our technical spring manufacturing, with legs bent to size. A considerable part of that output serves the household appliance sector and the automotive industry, where moment repeatability across runs of hundreds of thousands of pieces, and resistance to moisture and cleaning agents, decide acceptance.
Designing a mechanism around a torsion spring? Send us your enquiry – we will help select the parameters and the material for your duty cycle.
