Spring types

Extension Springs

Springs that produce a counter force when pulled apart, with closed coils and a connection form at each end. We manufacture them with hook, loop and threaded end options.

0.20–40 mmproduction wire range

4machine groups

4–12healthy spring index

1978production began

How extension springs work

Unlike a compression spring, an extension spring is coiled so that its turns touch one another. Those closed coils are not merely a shape preference: they allow initial tension to be set into the wire during coiling. A spring with initial tension does not extend at all until a certain force is reached, and only opens beyond that threshold. Part of what is called "the spring has gone soft" in the field is not breakage but the loss of this initial tension.

Once the body does begin to open, the spring rate follows exactly the same relation as a compression spring: k = G·d⁴ / (8·Dm³·n). Body behaviour is identical in both types; the difference lies at the ends.

The weakest point of an extension spring is not its body but its hook. The entire load passes through the bend radius of the hook and stress concentrates there, which is why the great majority of breakages happen at the root of the hook. Increasing the bend radius reduces that concentration; where space allows, a generous hook radius gives longer life.

The end type also determines assembly. A hook end fits quickly and is practical in field conditions, while a threaded end allows initial tension to be adjusted in place. Confirming the existing end type on the machine before ordering prevents surprises during fitting, because changing the end type also changes the free length measured over the hooks.

Extension springs in our catalogue run from forage harvester ball springs to mower drum suspension springs, and from sprayer arm springs to baler groups. When giving dimensions it matters whether the free length includes the hooks; if only the body length is quoted, two springs can look identical and behave differently.

Dimensions we need for a quotation

On an extension spring, stating whether the length is measured over the hooks or not is the single detail that prevents the most mistakes.

Dimension	What it means
Wire diameter · d	The diameter of the wire the spring is coiled from.
Outside diameter · Dd	Outer measurement of the body; mean diameter Dm = Dd − d.
Free length · L₀	End-to-end length including hooks, unloaded. Please state which measurement you are giving.
Body length · Lk	Coil length excluding hooks; approximately (n+1)·d.
Coil count · n	The number of closed coils.
End type	Hook, loop or threaded end. A threaded end is chosen where initial tension has to be adjusted.

Extension Springs calculator

Enter your dimensions and we will return the spring rate, solid length and spring index instantly — with a warning if the index leaves the healthy band.

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Photographs from production

**Extension springs**S-hook and loop-hook extension springs in three lengths.

**Extension spring range**Extension springs with different hook types.

**Drum suspension spring**Long-bodied extension spring with loop hooks at both ends.

Which machine groups use it

Springs of this type appear in the following groups in our catalogue, each listed in detail on its own page.

3 itemsForage Harvester Springs 1 itemsMower Springs 2 itemsSprayer Springs 54 itemsBaler Springs

How it is made

Every step after coiling happens in our own plant; nothing is outsourced. This shortens lead times and keeps quality responsibility in one pair of hands.

CNC coiling

Formed across the 0.20–40 mm wire range.

Tempering

Stress is relieved; elasticity becomes permanent.

End grinding

Seating faces are flattened on compression springs.

Galvanising

Surface protection against corrosion.

Packing

Domestic and export logistics included.

Frequently asked questions

What is initial tension in an extension spring?

It is the force set into the wire during coiling that presses the turns against each other. The spring does not begin to extend until that force is exceeded. Initial tension is a production parameter and can be set to the value you need.

Why do springs always break at the root of the hook?

The entire load passes through the bend radius of the hook and stress concentrates there. Increasing the bend radius reduces that concentration; where space allows, a generous hook radius gives longer life.

Should free length be measured over the hooks?

It is enough to state which measurement you took. Length over the hooks is the common convention; if only the body length is given, two springs can look identical and behave differently.

What is the difference between hook and threaded ends?

A hook end fits quickly and is practical in the field. A threaded end allows initial tension to be adjusted in place. The choice follows the existing connection type on the machine.

Does an extension spring have a solid length?

No — solid length does not apply to extension springs; the limit is on the extension side. A spring pulled too far takes on permanent length once the gap between coils has opened.

Can you reproduce a spring that has broken?

Yes. With the broken part in hand, wire diameter, outside diameter and coil count are measured to recover the original geometry; a photograph is enough for the hook form.

Other spring types

Compression SpringsCylindrical coil springs that work under axial compressive load.Torsion and Wire Form SpringsTorsion springs that transmit a moment through their legs, together with wire form parts.Conical SpringsCompression springs whose diameter changes along their length so the coils can nest inside one another.

Cannot find the spring you need?

We produce to your dimensions in the 0.20–40 mm wire range, with no tooling dependency. A sample, a technical drawing, or simply a photograph of the old spring is enough.

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Or call us directly: 0850 840 54 17