— Type
Helical spring with close-wound coils that resists being stretched.
Extension
springs
Helical springs with close-wound coils that store force as they extend: they resist being stretched and exert a pulling force.
They start from an initial tension and, once it is overcome, respond linearly (Hooke's law).
Function
Pulling force
Behaviour
F = Fi + k · x
Ends
Machine · German · loop
Manufacturing
Custom
01
What an extension spring is and how it works
Helical springs with close-wound coils designed to store force as they stretch. Unlike a compression spring —which resists being shortened—, the extension spring resists being stretched and exerts a pulling force that tends to return it to its original length.
Their coils are made in contact with one another, with an initial tension built in during winding. That is why a minimum force is required to overcome this tension and start separating the coils; from there, the force grows linearly with the extension. A correctly sized spring offers a long service life with minimal maintenance.
After forming, extension springs usually undergo a stress-relief treatment to stabilize the initial tension and improve fatigue resistance.
— General specifications
— Wire section
Round (other sections custom).
— Behaviour
Linear from the initial tension onward: F = Fi + k · x.
— Initial tension
Fi · force that keeps the coils tight at rest.
— Ends
Machine · German · extended · loop · custom hook.
— Critical point
The hook — wire torsion plus bending at the transition.
— Materials
Music wire · stainless 302/304/316 · electrogalvanized.
— Manufacturing
Custom-made to drawing, with subsequent stress relief.
02
Initial tension and behaviour
The initial tension (Fi) is the force that keeps the coils pressed together at rest. It is the force that must be overcome before the spring begins to stretch.
Unlike a compression spring, whose curve starts at zero, the extension spring's curve starts at the initial tension value. The tension is "used up" as soon as a gap appears between the coils: it is not part of the spring rate, but adds to the load calculated with it.
The initial tension is tied to the spring index (C = Dm/d): high indices —large coils relative to the wire— give low tensions, and vice versa. That is why it must be specified when sizing the spring.
F = Fi + k · x
total forceC = Dm / d
spring indexSpring geometry
- Ø d Wire diameter
- Dm Mean coil diameter
- De External diameter
- Lc Body length (close-wound coils)
- LK Total length measured between the insides of the hooks
- Fi Initial tension — starting force
The basic dimensions of an extension spring: wire diameter (Ø d), outer (De) and mean (Dm) diameter, body length with the coils in contact (Lc) and total length between the insides of the hooks (LK).
The extension spring's line does not start at zero, but at the initial tension Fi. Once the coils have separated, the force grows linearly with the extension.
03
Hook types
The ends are formed into different geometries to suit the anchoring system of each application. The hook type determines both the mounting and the strength of the end.
Machine hook
machine hook · half hook
Economical and straightforward to manufacture; the first choice in most applications.
German hook
German hook
Closed loop, more resistant to unhooking; requires an extra forming operation.
Extended hook
extended hook
Greater reach for anchor points away from the spring body.
Loop / eye
loop / eye
For a pin or bolt; distributes the load better at anchor points with a shaft.
Custom
Beyond the four usual types, the end can be formed into special geometries —side hooks, double loops, threaded studs or connections to another part— depending on the anchoring of each application.
04
The hook is the critical point
The stresses in the hook are higher than in the body of the spring: a bending stress at the transition between the last coil and the hook adds to the wire's torsional stress. That is why an overloaded extension spring fails at the hook first.
The allowable stress in the hook is limited more than in the body, and it is advisable to design generous transition radii.
Design rule
Transition radius r > 2 · wire Ø to spread the bending and extend the fatigue life of the end.
At the hook, torsion and bending add together. A transition radius greater than twice the wire diameter spreads the stress better and improves fatigue life.
Shall we talk about your project?
Tell us the pulling force, travel, initial tension, available space, hook type and environment — our engineering team will advise you to size the optimal extension spring. Manufacturer since 1974.
05
Materials for the environment
The choice of material depends on the working environment: in dry conditions, mechanical strength comes first; with moisture or chemical agents, corrosion resistance.
Material by environment
| Environment | Recommended material | Note |
|---|---|---|
| Dry | Music wire | Maximum mechanical strength; best cost-performance ratio. |
| Moist | Stainless steel AISI 302 / 304 | Strength ~10% lower than music wire. |
| Aggressive chemical / salt water | Stainless steel AISI 316 | Maximum corrosion resistance. |
| Semi-moist · treated surface | Music wire electrogalvanized | Added surface protection on top of the music wire. |
— From dry to corrosive
In dry conditions, music wire for its higher strength. With moisture, move to stainless 302/304; with chemicals or salinity, to AISI 316. Electrogalvanizing covers the intermediate semi-moist case.
— Stress relief
After forming, the springs undergo a stress-relief treatment that stabilizes the initial tension and improves fatigue resistance.
06
Industrial applications
Extension springs are used wherever a pulling or stretch-return force is needed. They appear in return mechanisms, drive tensioning and high-volume consumer assemblies. To size the initial tension, the spring rate, the hook type and the material, Surisa's engineering team, a specialist manufacturer since 1974, offers free engineering support.
Industrial and agricultural machinery
Return · tensioning
Return and tensioning mechanisms in production lines, drives and agricultural equipment subjected to repetitive cycles.
Return mechanisms
Doors · flaps · pedals · levers
They return an element to its rest position: doors, flaps, pedals and levers that must come back after being actuated.
Drive tensioning
Belts · chains · pulleys
They keep belts, chains and drives under tension, compensating for clearances and wear throughout the system's life.
Home appliances
Washing machines · latching mechanisms
Suspension and return in washing machines, latching mechanisms and high-volume consumer assemblies that demand a repeatable pulling force.
Automotive
Controls · brakes · engine
Control returns, brake assemblies and engine components where a reliable pulling force and long fatigue life are required.
Custom pulling force
Engineering sizing
When no standard spring covers the tension–load–travel–space combination, a specific part is sized to drawing.
07
Frequently asked questions
01 What is an extension spring and how does it work?
An extension spring is a helical spring that stores force as it stretches: the more it elongates, the greater the force it stores and releases as it returns to its length. It resists being stretched and exerts a pulling force. Its coils are wound in contact with an initial tension, so a minimum force must be overcome before it begins to extend; from there, the force grows linearly with the extension.
02 What is initial tension?
It is the force that keeps the coils pressed together when the spring is at rest, built in during winding. It must be overcome for the coils to start separating, so the force-extension curve does not start at zero but at the initial tension value. It is not part of the spring rate: it is an additional force that adds to the one calculated with it. Its value depends on the spring index, so it must be specified when sizing.
03 How does it differ from a compression spring?
In the direction of the work and in where the curve starts. The compression spring resists being shortened and its force-deflection curve starts at zero; the extension spring resists being stretched and its curve starts at the initial tension. In addition, the extension spring has hooks at the ends (its weakest point), whereas the compression spring bears on its ends. If you need to push, compression; if you need to pull, extension.
04 What hook types exist and why do they matter?
The main ones are the machine hook (the most common and economical), the German hook, the extended hook and the loop or eye, plus custom geometries. The hook type is chosen according to the anchoring system. It matters a great deal because the hook is the critical point of the spring: it withstands a higher stress than the body (torsion plus bending) and is where an overloaded spring fails first. That is why generous transition radii are advisable.
05 Which material suits each environment?
In dry environments, music wire for its higher mechanical strength and better cost. In moist environments, AISI 302/304 stainless steel (with a strength roughly 10% lower). In the presence of aggressive chemical agents or salt water, AISI 316 stainless steel. For semi-moist environments requiring a treated surface, electrogalvanized music wire. The choice depends on the balance between mechanical strength and corrosion resistance.
Do you need an extension spring?
Tell us the pulling force, travel, initial tension, available space, hook type and environment — we select the material, wire diameter and end, and reply with the optimal solution. Free engineering support, manufacturer since 1974. Always custom-made.