Most enquiries that reach a European spring plant from abroad carry the same four numbers: outside diameter, free length, wire diameter, annual quantity. A quotation can be built on that. What cannot be built on it is confidence that the part will work, because none of those four numbers describes what the spring is supposed to do inside the assembly.
Specify the duty, then the geometry
A compression spring is a functional element. The designer rarely knows the geometry first; what is known is the boundary condition. There is an installation envelope of a given length and diameter, the element has to deliver force X at deflection Y, and it has to return to its starting position for a defined number of cycles.
Everything else can be calculated from there. Working the other way round, starting from a wire diameter picked off a catalogue page, turns selection into a search through tables.
The minimum data set for a serious quotation is short: working force F1 at length L1, force F2 at length L2 (or the rate R in N/mm), the maximum permissible envelope, the nature of the load (static or dynamic), operating temperature and corrosive environment. Annual volume belongs in the same set, because volume decides the manufacturing route rather than the spring itself.
Rate, and why wire diameter dominates everything
For a helical compression spring made from round wire the rate follows:
R = (G · d⁴) / (8 · D³ · n)
where G is the shear modulus, d the wire diameter, D the mean coil diameter and n the number of active coils.
The practical consequence sits in the exponent. Wire diameter enters to the fourth power. A spring wound from 2.0 mm patented wire to EN 10270-1, with a 16 mm mean diameter and eight active coils, has a rate of roughly 4.97 N/mm. Change nothing but the wire, moving to 2.1 mm, and the rate rises by more than 21 %. That is not a deviation. That is a different spring.
This is why an enquiry should state which parameter governs. If force is critical, tolerance the force and let free length float. If the envelope is critical, do the opposite. Holding both to tight limits raises the price and, in a share of cases, simply cannot be manufactured.
Spring index: the window a coiler can actually work in
The ratio of mean coil diameter to wire diameter, the spring index w = D/d, decides whether a part can be coiled repeatably at all. The sensible window runs from about 4 to 12.
Below 4 the inner fibre of each coil carries markedly higher stress than the outer one, cracking risk grows and coiling calls for special tooling. Above 12 the spring becomes geometrically soft: it tangles in transport, it is harder to feed automatically on an assembly line and it loses stability under load much sooner.
When the calculation pushes the index outside that range, the answer usually lies in the installation space, not in the material.
Material: carbon steel, stainless, or something in between
Grade selection is settled by three things at once: stress level, operating temperature and how aggressive the environment is. The values below cover the wire types used in most European industrial applications.
| Material | Standard | E [kN/mm²] | G [N/mm²] | Temperature range |
|---|---|---|---|---|
| Patented wire SH / DH | EN 10270-1 | 206 | 81 500 | -200 to +80 °C |
| Oil hardened VDSiCr | EN 10270-2 | 206 | 79 500 | up to +120 °C |
| Stainless 1.4310 | EN 10270-3 | 185 | 70 000 | -200 to +160 °C |
| Stainless 1.4401 | EN 10270-3 | 180 | 68 000 | -200 to +300 °C |
| Stainless 1.4568 | EN 10270-3 | 195 | 73 000 | -200 to +200 °C |
The gap between 81 500 and 70 000 N/mm² is not cosmetic. The same geometry coiled from 1.4310 instead of patented wire comes out roughly 14 % softer. Swapping the grade on a drawing without recalculating the spring is one of the most reliable ways to generate a force complaint on the first production batch.
Patented wire to EN 10270-1 remains the default for dry indoor duty. Stainless grades take over wherever there is moisture, steam or chemical washing, which in practice means domestic appliances, fittings and sanitary equipment. Precipitation hardened 1.4568 buys back size where 1.4310 no longer carries the stress.
Buckling, the condition most often missed
A slender spring compressed between two flat plates stops compressing at some point and starts escaping sideways. As a rough guide, the problem appears once the ratio of free length to mean diameter, L0/D, passes about 4 with both ends free to rotate. With one end guided the limit is lower still.
Guiding the spring on an arbor or inside a sleeve raises that limit to something in the region of 8 to 10, at the cost of friction. Clearance on the guide has to allow for deflection: the outside diameter grows as the spring is compressed, and the pocket has to accept that growth.
Tolerances to EN 15800
EN 15800 sets the quality requirements for cold coiled compression springs made of round wire. Its scope covers wire from 0.07 to 16 mm, mean coil diameters from 0.63 to 200 mm and free lengths up to 630 mm. Tolerated features include coil diameters, deviation from straightness e1, parallelism of the ground ends e2, free length L0 and the working forces.
The standard defines three accuracy grades. Grade 1 gives the narrowest tolerance band and the highest price, grade 3 the widest. Grade 2 is the default and covers most industrial applications comfortably. Grade 1 earns its cost when the spring sits in a measuring system or controls a valve, not when the drawing simply asks for more precision.
End coils and seating
End coils can be closed and ground or closed without grinding. Grinding improves the parallelism of the bearing surfaces, reduces the tilting moment and stabilises how the spring sits in its pocket, but it adds an operation to the process. Below 0.5 mm wire diameter grinding is normally not performed: the bearing surface is too small for the operation to produce a measurable effect, while the extra cost still lands in the unit price.
Stress relieving, coatings and the hydrogen embrittlement trap
After coiling, the spring retains residual stresses from bending and torsion of the wire. Stress relieving temperature depends on the grade: for patented wire it sits around 230 to 250 °C, austenitic 1.4310 needs considerably more, in the region of 340 to 400 °C, and 1.4568 requires full precipitation hardening close to 480 °C.
Corrosion protection is a separate subject with an expensive failure mode. Springs are high tensile parts, frequently above 1500 MPa, which places them in the highest risk group for hydrogen embrittlement after electroplating. Where an electrolytic coating is unavoidable, de-embrittlement baking has to follow plating quickly, at around 200 to 230 °C, in line with ISO 9588. For heavily loaded parts, zinc flake systems applied without an electrolytic process are the safer route.
Corrosion requirements belong in the specification as hours to first corrosion in neutral salt spray testing to EN ISO 9227, not as the name of a coating. A line reading „zinc plated” with no figure behind it is not a requirement.
Dynamic duty: shot peening and presetting
A spring working statically and a spring running millions of cycles are two different engineering tasks behind one identical drawing. Under cyclic load it is the stress amplitude that decides service life, not the peak value.
Two operations extend it. Shot peening introduces compressive stress into the surface layer, delaying fatigue crack initiation. Presetting, compressing the spring to solid length before delivery, produces controlled plastic deformation and limits later force relaxation. Both belong in the enquiry if the application is dynamic, since adding them after production has started always costs more.
What the drawing has to carry
- Wire diameter and material grade with the standard quoted, for example EN 10270-1 SH
- Spring diameter, clearly identified as mean, outside or inside
- Free length L0 and working forces F1/F2 at their corresponding lengths
- Total and active coil count, plus coiling direction
- End coil type and whether the ends are ground
- Accuracy grade to EN 15800 and a statement of which parameter governs, force or length
- Surface finish, corrosion requirement and operating temperature
Why the conversation is worth having before the drawing is released
A spring is one of the few components where changing a single dimension rebuilds the whole characteristic. A review before the documentation is frozen rarely changes the function of the assembly, yet it often takes a double digit percentage out of the unit price, by moving the wire to a standard size, simplifying the ends or dropping an accuracy grade the application never asked for.
In our technical spring manufacturing we work either from customer drawings or from the boundary conditions alone: force, stroke, installation space. A significant share of that output goes to the household appliance industry, where force repeatability across long production runs and resistance to washing and humidity decide whether a component is accepted.
Need compression springs manufactured to a specific duty rather than a catalogue number? Send us your enquiry – we will run the calculation and propose a material suited to the working conditions.
