Key takeaways
- Initial tension is the load that holds the coils closed, and it has to be specified as well as the rate.
- Hooks are the usual failure point because the wire bends sharply and the load path is offset.
- A full loop hook carries more load than a half hook at the same wire size, and it costs more to make.
- Test load at two lengths so that both the rate and the initial tension can be separated.
What is different about an extension spring
A compression spring sits in a seat and pushes. An extension spring hangs between two points and pulls. The coils are wound tight in the free state, and the ends are formed into hooks that transfer the load into the body. There is no seat to square the spring up, so the hooks and the alignment of the ends carry the whole load path.
The tight coils create initial tension. The wire is twisted as it is coiled so that the coils press against each other, and a force is needed to separate them before the spring starts to extend. That force is useful in an assembly: it keeps a latch closed or a belt tensioned without rattle, and it lets a designer use a softer rate without the spring going slack.
Initial tension and the load curve
Plot load against extension and an extension spring gives a straight line that does not pass through zero. The intercept on the load axis is the initial tension. The slope is the rate. The spring stays closed until the load reaches the initial tension, then extends at the rate.
The worked example makes this concrete. Take a spring with an illustrative rate of 0.8 N/mm and an initial tension of 2.5 N. At 5 mm of extension the load is 2.5 plus 4.0, which is 6.5 N. At 15 mm the load is 2.5 plus 12.0, which is 14.5 N. From those two readings the rate is 8 N over 10 mm, or 0.8 N/mm, and the intercept is 6.5 minus 4.0, which is 2.5 N.
That is why the drawing should give two loads at two lengths, or a load at one length plus a rate and an initial tension. A single load at a single length can be met by a spring with the wrong combination, and the assembly will behave differently at the other end of its travel.
Initial tension is limited by the coil geometry and the material. A spring with a low index and a lot of initial tension can be difficult to coil and may open unevenly.
Hook types and where each fits
A full loop hook turns the wire through a complete circle, so the load is carried around a generous bend and the hook can rotate on a pin. It is the strongest of the common hooks and the most expensive to form.
A half hook turns through about half a circle and hooks over a wire or a pin. It is cheaper and shorter, but the load path is less symmetrical and the hook can open under a high load. It suits a light duty spring or a spring that is retained by a feature that supports the hook.
A side hook points to the side of the coil axis. It is used where the spring has to pull at an angle, and it introduces a bending moment at the hook root that the drawing should acknowledge. An extended hook is a straight length of wire beyond the coil, often with a bend at the end. It moves the attachment point away from the body, but the longer lever raises the stress at the transition.
- Full loop: high load, articulation on a pin, best fatigue resistance
- Half hook: light load, short installation, lower cost
- Side hook: pull at an angle, check the bending moment at the root
- Extended hook: remote attachment, watch the stress at the transition
- Threaded insert or swivel: adjustable tension, higher cost
Hook stress and design rules
The hook is a curved beam with a stress concentration at the inside of the bend. The load is offset from the wire centreline, so the hook sees bending as well as tension. That combination is why a hook fails before the body does, even when the body stress is comfortably inside the material limit.
Keep the hook bend radius generous and the transition from the coil smooth. A sharp change of section, a tool mark or a flattening at the hook root raises the stress. If the hook is formed after coiling, a stress relief can recover some ductility, but it will also relax the initial tension, so the two effects have to be balanced.
Align the hooks with the load. A hook that is twisted relative to the pull direction puts a torque into the body and can make the spring rotate in service. The drawing should show the angular relationship between the two hooks, not just their type.
Support the hook where possible. A pin through a full loop shares the load around the bend. A hook that hangs on a sharp edge concentrates the load at one point and can open or crack. The mating part is part of the spring design, even when it is not on the spring drawing.
Body stress and fatigue
The body of an extension spring sees a mean stress from the initial tension and the installed extension, plus a range from the working travel. Fatigue life depends on the range more than on the mean, but the mean still matters because it sets where the cycle sits on the material curve.
Keep the stress range within the material and the cycle count. A spring that cycles a few thousand times can run at a higher stress than one that cycles millions. Shot peening the coils puts the surface into compression and raises the fatigue limit, and stress relief after coiling reduces the residual stress from forming.
Corrosion is the other multiplier. A plated spring that cracks in service, or one with a coating that is damaged at the hook, will fail earlier in a damp environment. For high strength wire, the post plating bake is part of the fatigue story, not just a corrosion control.
Failure modes and checks
The table lists the ways an extension spring usually fails and the check that catches each one before release. The first two account for most of the field failures in this family.
| Failure mode | Where it starts | Design check | Prevention |
|---|---|---|---|
| Hook break | Inside of the hook bend | Hook stress at the maximum load | Full loop, larger radius, stress relief |
| Body fatigue | First or last active coil | Stress range over the working travel | Shot peen, stress relief, lower range |
| Initial tension loss | Coils separate early | Load at two lengths | Control coiling tension, check after relief |
| Hook opening | Hook root or the contact point | Hook bending and the mating part | Support the hook, reduce the load or thicken the wire |
| Corrosion fatigue | Plated surface at the hook | Finish and bake requirement | Correct finish, bake, avoid damage in assembly |
Checks are a starting point for a design review. Confirm the stress and the cycle life against the material data for the actual wire.
Tolerances and testing
Test load at two lengths. The two readings give the rate and the initial tension, and they catch a spring that meets one number by luck. If the assembly only cares about one position, test at that position and add a second check at the maximum extension.
Hook alignment needs its own check. Put the spring on two pins at the installed length and measure the angle between the hooks and the squareness of the coils. A spring that is straight in the free state can lean under load if the hooks are not aligned.
What to put on the drawing
Give the wire diameter with its tolerance class, the outside diameter, the free length inside the hooks, and the number of coils. Name the hook type at each end, give the hook opening and the angle between the hooks, and say which end is which if the spring is handed.
Add the load at the installed length and at the maximum working length, plus the rate and initial tension if the assembly needs them. State the material, the temper, the finish and any stress relief or peening. A drawing with those calls gives a supplier enough to make the spring and enough to prove it.
- Wire diameter and tolerance class, outside diameter, free length
- Hook type at each end, opening and angular alignment
- Load at the installed length and at the maximum length
- Rate and initial tension where the assembly needs them
- Material, temper, finish, stress relief and peening
- Maximum extension and any limit on the hook load
Frequently asked questions
What is initial tension in an extension spring?
It is the force that holds the coils closed before the spring extends. It comes from the way the wire is twisted during coiling. On a load curve it is the intercept, and it is separate from the rate. Specify it because a spring with the right rate and the wrong initial tension will not behave as the assembly expects.
Can I specify an extension spring by rate alone?
No. Rate alone does not define the load at the installed length, and it says nothing about the initial tension. Give the load at two lengths, or give the load at one length plus the rate and the initial tension. That is also how the spring should be tested.
Why do extension springs fail at the hook?
The hook is a curved beam with a stress concentration, and the load is offset from the wire centreline, so it sees bending as well as tension. A sharp bend, a tool mark or a hook that is loaded on a sharp edge will start a crack. A full loop, a generous radius and a stress relief all help.
Does a longer hook reduce the stress?
Not necessarily. A longer hook moves the attachment point away from the body but increases the bending moment at the hook root. What helps is a generous bend radius, a smooth transition and a load path that is aligned with the hook. Test the hook at the maximum load rather than assuming a longer hook is stronger.
Sources and further reading
- EN 13906-2, Cylindrical helical springs made from round wire and bar, extension springs
- EN 10270-1, Steel wire for mechanical springs, patented cold drawn unalloyed spring steel wire
- Qumek spring design and DFM notes for extension springs
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
Check an extension spring before release
Send the hook drawing, the installed length and the working travel. The engineering desk will check the hook stress, the initial tension and the load at both lengths.
Send the spring details