Extension springs
Extension springs, or coil extension springs, are used wherever a tensile force must be applied. They are commonly employed as return springs in vehicle construction, as well as in garage doors, locks, bed bases, and relays in plant and equipment engineering. Various types of eyelets allow for connection to adjacent components. The selection depends on the load and application.
For the production of extension springs, we offer the optimal manufacturing process for all requirements. We produce individual units in our factory. For small series, we use semi-automatic production, combining the advantages of manual and large-scale manufacturing. For large series, we generally rely on fully automated production.
Our machinery includes a wide range of automatic spring coilers for pre-assembled extension springs with standard eyelets, as well as CNC-controlled production centers with up to 28 individually programmable axes for producing special eyelets and complex geometries.
Our greatest strength lies in the wire thickness range from 0.20 mm to 5.00 mm. Depending on the geometry, design, and quantity, we can manufacture springs with a maximum wire thickness of up to 16.00 mm.
Materials
Surface Treatment
Useful information
Formula symbols
Contact Person
Steel wire and strip steel used for the production of technical springs are applied across all areas of technology — particularly in electrical engineering, communications, medical technology, and the automotive industry.
The choice of material grade depends on the mechanical stress and the operating environment of the spring. The following list of materials provides a representative cross-section of the types we primarily process.
Thanks to our extensive stock of raw materials, we are usually able to meet customer requirements at short notice.
| Standard / Material | Material Description | Load, Properties, Applications | Operating Temperature |
|---|---|---|---|
| DIN EN 10270-1 SM | Spring steel | Medium static or rarely dynamic load Compression, torsion or tension springs, bent parts | max. 80° |
| DIN EN 10270-1 SH | Spring steel | High static or low dynamic load Compression, torsion or tension springs, bent parts | max. 80° |
| DIN EN 10270-1 DH | Spring steel | High static or medium dynamic load Compression, torsion, shaped or tension springs, bent parts | max. 80° |
| DIN EN 10270-2 FDSiCr | Tempered spring steel | High static load, compression and leg springs | max. 130° |
| DIN EN 10270-2 TDSiCr | Tempered spring steel | High static load, moderate fatigue strength, compression and leg springs | max. 130° |
| DIN EN 10270-2 VDSiCr | Tempered spring steel | High static and dynamic load, high fatigue strength, compression and leg springs | max. 130° |
| DIN EN 10270-3 1.4310 | (X10CrNi18-8) | Stainless steel for use at higher temperatures | max. 250° |
| DIN EN 10270-3 1.4401 | (X5CrNiMo17-12-2) | Non-magnetic, more corrosion-resistant than 1.4310 | max. 250° |
| DIN EN 10270-3 1.4568 | (X7CrNiAl17-7) | Less corrosion-resistant than 1.4310, high load-bearing capacity | max. 300° |
| DIN EN 10270-3 1.4571 | (X6CrNiMoTi17-12-2) | Seawater-resistant | max. 300° |
| DIN EN 1654 CuSn | Bronze | Non-magnetic, electrically conductive | |
| DIN EN 1654 CuZn | Brass | Non-magnetic | |
| DIN EN 1654 CuNiZn | Nickel silver | Corrosion-resistant and electrically conductive | |
| 2.4610 Hastelloy C-4 | (NiMo16Cr16Ti) | Extremely corrosion-resistant, high service temperature, seawater-resistant | -200° to max. +400° |
| 2.4669 Inconel X-750 | (NiCr15Fe7Ti2Al) | Non-magnetic, highly corrosion-resistant, high operating temperature | max. 370° |
| DIN EN 10016-2 C9D | Wire | Bent parts | |
| DIN EN 10016-2 C10D | Wire | Bent parts |
Springs may experience relaxation at elevated temperatures. Alternative materials for special applications are available upon request.
The information provided above is for general guidance only and does not claim to be exhaustive.
Surface coatings can be applied to provide additional corrosion protection for springs. This is especially important when a non-corrosion-resistant material is used in spring production due to technical or economic considerations. When stainless spring wire is used, additional corrosion protection is generally not required. In addition, various surface coatings can be applied to enhance the functional or visual properties of the springs (e.g., anti-friction coatings).
Below is a selection of surface treatments we offer. If the coating you require is not listed, please do not hesitate to contact us.
| Surface Treatment | Note |
|---|---|
| Zinc Plating | Electrolytic zinc plating is the most commonly used and cost-effective method of corrosion protection for technical springs. |
| Zinc Plating with Passivation | Zinc plating followed by passivation is a cost-efficient method for achieving good corrosion protection for technical springs. |
| Nickel Plating | Nickel plating provides an attractive finish and good sliding properties along with high corrosion resistance. |
| Chrome Plating | Chrome-plated parts are decorative and offer good heat and corrosion resistance. |
| Pickling | Pickling chemically removes impurities bound to the surface. |
| Copper Plating | Copper plating is often used as a base for further coatings but can also be applied for color-coding springs. |
| Bluing (Black Oxide) | Creates an iron oxide layer resulting in a deep black surface. When oiled, it provides some corrosion protection. |
| Phosphating | Used as corrosion protection and as an adhesion base for paint or KTL coating. |
| Zinc Flake Coating | An environmentally friendly coating system that meets growing corrosion protection demands. Chrome- and heavy metal-free, with high temperature resistance and low hydrogen embrittlement. |
| KTL Coating | A dip-painting process that meets high standards in corrosion protection and resistance to under-rusting. |
| Powder Coating | Ideal when a scratch- and impact-resistant surface with high corrosion protection is required. |
| Gleitmo | Air-drying lubricant coating based on specially processed PTFE. Operating temperature: -180°C to +250°C. Provides low friction, is clean and food-safe. |
| Tin Plating | Offers excellent solderability. |
| Silver and Gold | High-gloss and premium finishes for decorative and technical purposes. |
| Shot Peening | Increases the dynamic lifespan of technical springs. Offers little to no corrosion protection (limited applicability). |
| Tumbling / Vibratory Finishing | Removes burrs and sharp edges from stamped parts. Offers limited or no corrosion protection. |
Tension springs are typically wound right-handed. Left-handed wound springs can be produced, but this generally involves more effort and higher manufacturing costs.
Coil Direction
Compression coil springs are typically wound right-handed. Spring sets often incorporate both right-hand and left-hand wound springs, with the outer spring usually being right-hand wound.
Left
Right
In addition to standardized eyelets, a wide variety of shapes are possible. The geometry should then be specified using a dimensioned drawing. In a tension spring, the eyelets are usually the weakest points. As a rule of thumb, it can be assumed that the eyelets can withstand approximately 70% of the maximum load of the spring body.
Dynamic applications reduce these values by approximately 30%. For more precise assessments and calculations, all influencing factors such as installation length, stroke, load change frequency, load variation curve, temperature, ambient media, and connection conditions must be taken into account. For tension springs, the wire thickness and material selection—depending on the coil and eyelet diameter—determine the maximum achievable load. A larger wire diameter corresponds to a higher load capacity.
The wire length (dDn) of the spring body determines the maximum spring deflection. More wire length corresponds to greater spring deflection. With a constant coil ratio, the ratio of wire length to spring deflection is directly proportional. However, larger coil ratios negatively affect the load on the eyelet root.
Types of Eyelets
| Visual Representation: | Designation: | Designation according to EN 13906 (currently valid) | Designation according to DIN 2097 (obsolete) |
|---|---|---|---|
 | Half German eyelet Lh=0,55 to 0,8 * Di | Image A.1 | Image 2 |
 | Full German eyelet Lh=0,8 to 1,1 * Di | Image A.2 | Image 3 |
 | Double German eyelet Lh=0,8 to 1,1 * Di | Image A.3 | Image 4 |
 | Full German eyelet raised on the side Lh = ca. Di | Image A.4 | Image 5 |
 | Double German eyelet raised on the side Lh = ca. Di | Image A.5 | Image 6 |
 | Hook eye positioned centrally | Image A.6 | Image 7 |
 | Hook eye positioned sideways | Image A.7 | Image 8 |
 | English eye Lh= ca. 1,1 * Di Lh = ca. Di | Image A.8 | Image 9 |
 | Hook rolled in | Image A.9 | Image 10 |
 | Threaded bolt rolled in | Image A.10 | Image 11 |
 | Threaded plug screwed in Number of screwed-in turns 2 to 4 | Image A.11 | Image 12 |
 | Schraublasche Screw tab screwed in Number of screwed-in turns 2 to 4 | Image A.12 | Image 13 |
 | Entire German eye tilted up | Image A.13 | Image 14 |
Spring Testing
Quality is our top priority. Springs are tested on our production equipment according to sampling plans and customer specifications to ensure compliance with tolerances. We perform and document all testing according to the customer’s requirements, including individual spring tests. Spring forces and geometries are measured using state-of-the-art equipment, employing tactile or optical measuring methods. Corresponding test reports are provided as standard.
Production Compensation
Production compensation is necessary in spring manufacturing to ensure compliance with the specified load conditions.
| Prescribed sizes: | Production compensation: |
|---|---|
| One spring force, the corresponding compressed spring length, and the free length L0 | F0 and D |
| One spring force, the corresponding compressed spring length, and the initial force F0 | L0, n and d or L0 and De (Di ; D) |
Two spring forces and their corresponding compressed spring lengths | L0, n and d or F0 and De (Di ; D) |
It should be noted that changing d to the nearest standardized wire gauge usually results in a significant jump in the values. The number of turns (n) can also only be specified in “whole” turns, with a proportional effect relative to the total number of turns. Both inevitably lead to a change in L0, unless it can be corrected using Lh.
Calculation
The spring is calculated according to customer requirements using modern software and in compliance with EN 13906-2:2013.
Tolerances
Unless otherwise specified, DIN 2097 Grade 2 tolerances are applied.
Types of Load
Static loads are defined as loads that remain constant over time. Quasi-static loads are loads that vary over time, with higher stresses but with load cycles fewer than 10,000.
Dynamic loads are loads that vary over time, with load cycles exceeding 10,000 and stresses greater than 0.1 times the fatigue limit.
Geometric dimensioning
| Sign | Description | Unit |
|---|---|---|
| d | Wire diameter | mm |
| Di | Inner diameter of spring body | mm |
| De | Outer diameter of spring body | mm |
| D | Mean diameter of spring body (theoretical calculated value, not suitable for measurement) | mm |
| L0 | Free length between eyelets in the delivered state | mm |
| Lk | Length of spring body with coils touching, without eyelets | mm |
| Lh1, Lh2 | Eyelet height measured from inside | mm |
| m1, m2 | Eyelet opening | mm |
| n | Active coils | |
| nt | Total coils (for rolled or screwed eyelet types, if+ non-resilient coils) |
Information on spring forces
| L0 | Free length in delivered state | mm |
| F0 | Initial preload force | N |
| s1 | Deflection from delivered state to installed position | mm |
| L1 | Installed length | mm |
| F1 | 1. force at installed position | N |
| s2 | Deflection from delivered state to end position | mm |
| L2 | Length at end position | mm |
| F2 | 2. force at end position | N |
| sn | Maximum permissible deflection | mm |
| Ln | Maximum permissible installed length | mm |
| Fn | Maximum permissible spring force | mm |
| sh | Stroke of the spring (working travel) | mm |
Information on the calculation and testing of forces
| Dh | Working sleeve diameter | mm2 |
| w=D/d | Coil ratio | |
| F0 | Built-in preload, cannot be measured directly, only calculated | N |
| R | Spring rate (slope of the characteristic curve) | N/mm |
| τ, τ1,τ2,… | Shear stresses in the spring body assigned to spring forces | N/mm² |
| τ, τκ1,τκ2 | Corrected shear stresses considering stress factor q in the spring body | N/mm² |
| σ, σ1, σ2,… | Bending stresses at the eyelets assigned to spring forces | N/mm² |
| σk, σk1, σk2,… | Corrected bending stresses considering stress factor q at the eyelets | N/mm² |
| σzul | Permissible bending stress of the eyelets | N/mm² |
| σkh | Corrected stroke bending stress of the eyelets | N/mm² |
| σhzul | Permissible stroke bending stress of the eyelets | N/mm² |
| τzul | Permissible shear stress in the spring body | N/mm² |
| τkh | Corrected stroke shear stress in the spring body | N/mm² |
| τhzul | Permissible stroke shear stress in the spring body | N/mm² |
Types of Eyelets
| Visual Representation: | Designation: | Designation according to EN 13906 (currently valid) | Designation according to DIN 2097 (obsolete) |
|---|---|---|---|
| Half German eyelet Lh=0,55 to 0,8 * Di | Image A.1 | Image 2 |
| Full German eyelet Lh=0,8 to 1,1 * Di | Image A.2 | Image 3 |
| Double German eyelet Lh=0,8 to 1,1 * Di | Image A.3 | Image 4 |
| Full German eyelet raised on the side Lh = ca. Di | Image A.4 | Image 5 |
| Double German eyelet raised on the side Lh = ca. Di | Image A.5 | Image 6 |
| Hook eye positioned centrally | Image A.6 | Image 7 |
| Hook eye positioned sideways | Image A.7 | Image 8 |
| English eye Lh= ca. 1,1 * Di Lh = ca. Di | Image A.8 | Image 9 |
| Hook rolled in | Image A.9 | Image 10 |
| Threaded bolt rolled in | Image A.10 | Image 11 |
| Threaded plug screwed in Number of screwed-in turns 2 to 4 | Image A.11 | Image 12 |
| Schraublasche Screw tab screwed in Number of screwed-in turns 2 to 4 | Image A.12 | Image 13 |
| Entire German eye tilted up | Image A.13 | Image 14 |
Contact Person
REINHARD HIMMER
Technical Consulting | Development
DOMINIC HIMMER
Operations management
24-Hour Service
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*Available upon request