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Precision springs
Custom extension springs
Custom extension springs (tension springs) from 0.2 to 2.0 mm wire, with machine loops, crossover loops, extended hooks or your own end form. We control initial tension and load at extended length, check hook orientation and gap, and can ship the springs with matching stamped parts. Orders start at 1,000 pieces.
Key specs at a glance
| Wire diameter | 0.2–2.0 mm |
|---|---|
| End forms | Machine loop, crossover loop, side loop, extended hook, custom |
| Key controls | Initial tension, load at length, hook orientation |
| Capacity | 2,000,000+ pcs per month (all spring types) |
| Minimum order | 1,000 pcs |
How is an extension spring different from a compression spring?
An extension spring is close-wound and resists being pulled apart. It usually has initial tension — a preload coiled into the body — so it does not start to extend until the applied force exceeds that value. Above it, force rises with extension at the spring rate.
The ends are what attach the spring to your product, and they are also where most extension springs fail. Hook geometry deserves as much attention as the body.
What to put on an extension spring drawing
| Parameter | Notes |
|---|---|
| Wire diameter and grade | 0.2–2.0 mm; for example SWP-B or SUS304-WPB |
| Outer diameter | Or inner diameter if the hook fits over a pin |
| Free length inside hooks | Measured from inside of one hook to inside of the other |
| Body coils | Close-wound coils between the ends |
| Initial tension | If it matters, give a value and tolerance |
| Load at extended length | One or two points: F1 at L1, F2 at L2 |
| Maximum extended length | The longest length in service, to check stress |
| Hook type and orientation | Relative angle of the two hooks and gap at the opening |
Common hook and loop types
| End type | Description | When to use |
|---|---|---|
| Machine loop (half loop) | Last coil bent up | Low cost, light loads |
| Crossover loop | Loop formed over the center of the coil | Centred load, general use |
| Full loop / side loop | Full turn at the side of the body | Easy assembly over a pin |
| Extended hook | Hook on a straight extension | Reaching a fixing point away from the body |
| Custom end | Your wire form | To drawing; confirmed per project |
Design rules that make hooks last
Hook failure is usually fatigue at the bend where the hook leaves the body. These rules of thumb help:
- Keep the bend radius at the hook root as large as the design allows; a tight bend concentrates stress.
- Avoid working the spring close to its maximum extension; leave margin between the longest service length and the elastic limit.
- Specify hook orientation only if assembly needs it; random orientation is cheaper.
- For high-cycle applications, tell us the cycle count; we may propose a crossover loop or a lower working stress.
- If the spring is installed by stretching over a pin, show the installation extension so we check that it does not overstress the hooks.
Typical wire grades for extension springs
| Wire (JIS) | Equivalent | Notes |
|---|---|---|
| SWP-A / SWP-B (G 3522) | ASTM A228 music wire | Highest strength for small diameters; plated or oiled |
| SUS304-WPB (G 4314) | ASTM A313 type 302/304 | Corrosion resistance, no plating needed |
How extension springs are inspected
First articles are checked for free length inside hooks, outer diameter, hook gap and orientation, and load at the specified extended lengths using force gauges. In production we sample each lot to AQL. A dimensional and load report can ship with each delivery.
Frequently asked questions
What is initial tension in an extension spring?
It is the internal preload coiled into the close-wound body. The spring will not extend until the applied force exceeds it. If your design depends on it, state the value and tolerance on the drawing.
Which hook type is strongest?
Crossover and full loops with generous bend radii generally last longer than machine half-loops. The best choice depends on load, cycles and how the spring is fixed.
Can you control hook orientation?
Yes. Give the angle between the two hooks on the drawing. If it does not matter, leave it random to reduce cost.
What is the minimum order?
1,000 pieces per part number.
What are the sample cost and lead time?
Both are confirmed per project in the quote.
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