Extension Springs

Extension springs (tension springs) with hooks in more than 2,600 standard sizes, in spring steel and stainless steel. Every spring is listed with full technical data: wire diameter d, outer diameter Do, free length L0, maximum extended length, spring rate R and initial tension F0. Filter the table, use the extension spring calculator or ask Springy to find the right spring, or contact us for custom springs.

Extension spring calculator →
Wire diameter d (mm)
Outer diameter Do (mm)
Free length L0 (mm)
Test length L1 (mm)
Force F1 (N)
Material
Spring rate R (N/mm)
Initial tension F0 (N)
Extension spring dimensions
DoOuter diameter of the spring body. Determines the radial space needed.
dWire diameter. Affects spring force, spring rate and load capacity.
L0Free length of the unloaded spring, measured between the inner contact points of the loops.
F0Initial tension: the force that must be exceeded before the coils start to separate.
L1Test length: the defined extended length at which the force F1 is specified.
F1Spring force at the test length L1.
RSpring rate: force increase per millimetre of extension (N/mm).
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Part No. Do (mm)i d (mm)i L0 (mm)i L1 (mm)i F1 (N)i R (N/mm)i Material In stock Price / pc Ex. VAT
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How to select an extension spring

1
Installation space

Check the maximum outer diameter Do and measure the distance between the attachment points, both unloaded and in the working position.

2
Spring travel and forces

Work out the travel from the free length L0 to the working length. The force is the initial tension F0 plus the spring rate R times the extension. Keep the working length below the test length L1.

3
Loops and loading

Make sure the loops fit the mounting points and that the pull is as straight as possible. The transition between body and loops is the most stressed area, especially under frequent cycling.

4
Material

Spring steel 1.1200 suits dry indoor applications. Stainless steel 1.4310 is recommended for humid or corrosive environments.

Material

Spring steel 1.1200
High-strength spring steel wire for dry indoor environments. Not corrosion resistant without additional surface protection.

Stainless steel 1.4310
Stainless spring steel (X10CrNi18-8) with good corrosion resistance for most industrial applications and humid environments. Limited suitability for permanent contact with seawater or media with a high chloride content. May be slightly magnetic due to cold forming.

Calculating the spring force

In the unloaded state the coils lie tightly together with a built-in initial tension F0. The spring only starts to extend once the applied force exceeds F0. Within the linear working range, the force is:

F = F0 + R × s

where s = L - L0 is the extension beyond the free length in mm. At the test length L1, F1 is therefore approximately F0 + R × (L1 - L0). Due to manufacturing tolerances, the calculated value may differ slightly from the stated F1.

Test length L1

The test length L1 is the defined length at which F1 is specified, not a recommended installation length. Only extend the spring beyond L1 after technical verification, as this can cause permanent deformation, loss of initial tension or damage to the loops.