Extension Spring End Types: Hooks, Loops and Configurations
Short answer: The main extension spring end types are machine hooks (full loop, half loop, short hook), side loops, cross loops, extended hooks, and threaded or swaged ends. Machine hooks are the default and cheapest, formed in-line with the body at no extra tooling cost. Side and cross loops are used when the spring must pull along a different axis, and extended hooks or threaded ends appear when the assembly needs a long reach or a bolted connection. End type changes the free length, the hook stress concentration, and how the spring seats on its anchor — so it should be specified before the wire is ordered, not after. BQUQ forms all standard end types in-house on the same four production lines in Dongguan.
Why the end type matters more than most buyers expect
On a compression spring, the ends only need to sit flat. On an extension spring, the ends carry the entire working load. Every kilogram of force the spring applies passes through two small bends of wire that are often only 60–70% of the body diameter in cross-section and have a stress concentration factor of roughly 1.3 to 1.6 at the inside of the bend.
That is why extension springs rarely fail in the middle of the body. In pull-test data from our own production floor, the great majority of extension spring failures we see in returned samples initiate at the hook radius, not at the coils. If you specify the wrong loop, you can lose 20–40% of the spring's rated load capacity before the body ever reaches its design stress.
The end type also sets the spring's free length, its installed length, and the geometry of whatever bracket it attaches to. Changing from a machine hook to an extended hook after tooling is cut usually means a new fixture on the mating part. Get it right at RFQ stage.
What are the standard extension spring end types?
There are six families you will see in real drawings. The table below summarises them, their typical applications, and their relative cost.
| End type | Description | Typical use | Tooling impact |
|---|---|---|---|
| Full loop (machine hook) | Wire bent 180° back to the body, full circle | General purpose, high cycle counts | None — formed in-line |
| Half loop | Wire bent 180° but open, no return to body | Short installed length, low cost | None — formed in-line |
| Short hook | Wire bent less than 180°, roughly 90–120° | Tight spaces, low load | None — formed in-line |
| Side loop | Loop formed in the plane of the body, offset to one side | Pull direction off-axis | Secondary operation |
| Cross loop (double loop) | Two loops formed perpendicular to the body axis | Balanced two-point anchoring | Secondary operation |
| Extended hook | Straight or shaped leg several coil diameters long | Long reach, custom anchor geometry | Secondary operation, often CNC-formed |
Machine hooks — full loop, half loop and short hook — are the default because they are formed on the coiler in the same pass as the body. There is no separate setup, no extra labour, and no additional fixture. If your application can accept one of these three, you get the lowest unit price and the shortest lead time.
Side loops, cross loops and extended hooks require a secondary forming step. On our lines that is usually a dedicated wire-forming station or a small CNC-bent leg. It adds a setup, but the per-piece cost is still modest at volume.
Full loop vs half loop: the practical difference
A full loop returns the wire to the body of the spring, closing the loop. This distributes the load around the full circumference of the hook and gives the best fatigue life. It also means the hook occupies more axial space — typically 1.5 to 2 wire diameters of extra free length per end.
A half loop stops before returning to the body. It is shorter, cheaper in material, and fits into tighter pockets, but the load path is less balanced and the hook tends to open under high extension. Use half loops for static or low-cycle applications, and full loops whenever the spring sees more than roughly 100,000 cycles.
Short hooks and when they are acceptable
A short hook is essentially a partial bend — often 90° to 120° — rather than a closed loop. It is common on very small springs where the wire diameter is under about 0.4 mm and bending a full loop would work-harden the wire excessively.
Short hooks are also used where the spring hooks over a pin rather than through a hole. If the pin is captive and the load is light, a short hook is perfectly adequate. If the spring can slip off the pin, it is not.
Side loops and cross loops: when the pull is not axial
Sometimes the spring cannot pull along its own centreline. The anchor points are offset, or the assembly needs two attachment points, or the spring has to clear an obstruction.
| Configuration | Load path | Best for | Watch out for |
|---|---|---|---|
| Side loop (single) | Off-axis, single point | Offset anchors, low-to-medium load | Bending moment at the loop root |
| Cross loop (double) | Two symmetric points | Balanced pull, reduced twist | Requires two matched anchors |
| Extended hook, straight | Axial but long reach | Reaching through a housing wall | Buckling if the leg is too long |
| Extended hook, shaped | Custom | Fitting an existing bracket | Must be specified as a formed feature |
A side loop is formed in the plane of the body but offset to one side. It is a good solution when the spring has to reach around a boss or connect to a lever arm. The trade-off is a bending moment at the root of the loop, which reduces fatigue life compared with a centred machine hook.
A cross loop — sometimes called a double loop or cross-over loop — puts two loops perpendicular to the body axis. It gives a symmetric two-point attachment, which is useful when you want to avoid the spring twisting under load. It costs more because both loops must be formed and aligned to each other.
Extended hooks and threaded ends
An extended hook is simply a hook with a longer leg. The leg can be straight, or it can be shaped — bent, offset, or formed into a specific profile to match a mating feature. Extended hooks are common in automotive interior mechanisms, appliance latches, and medical device assemblies where the spring has to reach across a gap.
Threaded ends and swaged ends are a different category. Here the end of the spring is not a hook at all but a machined or formed termination: an internal thread, an external thread, or a swaged fitting. These are used when the spring must be bolted or screwed into place rather than hooked. They are almost always a secondary machining operation, and they change the spring from a commodity part into a small assembly.
If your design needs a threaded end, mention it at RFQ stage — it affects material selection, because threading a hardened spring wire is not practical and you may need to specify the end fitting as a separate component.
How do end types affect spring performance?
Three things change when you change the end type.
Free length. A full loop adds roughly 1.5–2 wire diameters per end. A half loop adds about 1 wire diameter. An extended hook can add 10 wire diameters or more. If your installed cavity is fixed, the end type directly determines how much body length you have left for coils.
Stress concentration. The inside of a hook bend sees a stress concentration factor typically between 1.3 and 1.6. That means the hook can be the limiting feature even when the body is comfortably within its design stress. This is the single most common reason a spring that "calculates fine" fails in testing.
Initial tension. Extension springs are usually wound with initial tension — the force required to separate the coils. The end type does not change the initial tension value, but it does change how repeatably you can measure it, because a poorly formed hook can shift during the first extension. For more on this, see our guide to initial tension in extension springs.
A note on hook stress and index
Spring index — the ratio of mean coil diameter to wire diameter — interacts strongly with hook design. Low-index springs (index below about 5) are hard to form with a good hook because the bend radius is tight relative to the wire. High-index springs (index above about 12) form easy hooks but are more prone to buckling and tangling. The usable band is roughly index 5 to 12 for most machine-hook designs. Our article on spring index and stress covers the calculation in detail.
How to specify extension spring ends on a drawing
A good extension spring drawing specifies the end type unambiguously. Vague notes like "hook both ends" cause rework.
Include these details:
- End type name — full loop, half loop, short hook, side loop, cross loop, extended hook.
- Loop orientation — in-line with the body, or rotated by a stated angle. If the two ends are at 90° to each other, say so.
- Loop inner diameter — the hole or pin the loop must clear, with tolerance.
- Hook length — measured from the end of the body to the outside of the loop.
- Material and finish — the hook is often the first place corrosion shows, so plating or coating matters. See our notes on spring corrosion protection.
- Load at installed length and at maximum extension — with the hook included in the rating.
If you can supply a sample or a 3D model of the mating bracket, we can usually confirm the end geometry before cutting tooling. That is the cheapest possible place to catch a mistake.
Typical tolerances for formed ends
| Feature | Typical tolerance | Notes |
|---|---|---|
| Loop inner diameter | ±0.15 mm | Tighter on request |
| Hook length | ±0.3 mm | ±0.1 mm on CNC-formed legs |
| Loop orientation angle | ±5° | ±2° on request |
| Body free length | ±1% or ±0.5 mm | Whichever is greater |
| Wire diameter | ±0.02 mm | Per wire supplier spec |
These are indicative figures for typical production. Exact capability depends on wire diameter, material and end geometry — we confirm on the quote.
Materials and finishes for hooked ends
The hook is a cold-formed bend, so it work-hardens the wire. That is usually fine, but it matters for two material families.
Austenitic stainless steels such as 302 and 304 work-harden readily. A tight hook bend can make the wire locally brittle, which is why very small stainless springs often use short hooks rather than full loops. If your spring is stainless and the hook is tight, ask for a stress-relief pass after forming.
Carbon steels such as music wire and oil-tempered wire take a hook well and hold it. They are the default for high-cycle machine-hook springs. They do need a finish — zinc plating, powder coat, or a phosphate and oil — because bare carbon steel will rust at the hook first.
Nickel alloys and beryllium copper are used where the spring sees high temperature or needs non-magnetic behaviour. Hooks on these materials are usually formed with a larger radius to avoid cracking.
For a fuller comparison of spring materials and how they behave at the hook, see our custom spring RFQ guide, which walks through what information to send.
Manufacturing the ends: what actually happens on the line
At BQUQ, extension springs are wound on CNC coilers across our four production lines in one Dongguan factory. Machine hooks are formed in-line: the coiler winds the body, then a cam-driven finger bends the loop at each end before the spring is cut. That is why machine hooks carry no tooling charge.
Side loops, cross loops and extended hooks come off the coiler with straight legs and go to a secondary wire-forming station. There, a dedicated tool bends the leg to the drawing. For low-volume or prototype work we can form the leg on a CNC wire bender instead, which avoids cutting a hard tool at all.
After forming, springs are stress-relieved where the material and application call for it, then finished and inspected. Hook geometry is checked against the drawing on an optical comparator or vision system, and load is verified at the specified test lengths on a calibrated tester.
If your project also includes compression or torsion springs, we run those on the same floor — see custom compression springs and torsion springs — which means one RFQ, one quality system, one shipment.
Frequently Asked Questions
Q: What is the most common extension spring end type?
A: The full loop machine hook is by far the most common. It is formed in-line with the body, adds no tooling cost, distributes load around the full circumference of the hook, and gives the best fatigue life. If your application has no special geometric constraint, start with a full loop on both ends and only change it if testing shows a problem.
Q: Can I mix end types on one spring?
A: Yes, and it is common. A spring might have a full loop at one end and an extended hook at the other, or a machine hook at one end and a side loop at the other. Mixed ends are formed in two operations, so there is a modest cost increase. Specify each end separately on the drawing, including the relative orientation between them.
Q: How much load capacity do I lose with a half loop?
A: A half loop typically reduces hook strength by roughly 15–30% compared with a full loop of the same wire diameter, because the load path is less balanced and the hook can open under extension. The exact figure depends on wire diameter and loop geometry. If your spring operates near its rated load, use a full loop.
Q: Do extension spring ends need a separate tolerance callout?
A: Yes. Loop inner diameter, hook length and loop orientation angle should each carry their own tolerance, because they interface with different features on the mating part. A single general tolerance note on the drawing usually leads to arguments at inspection. State the critical one — normally loop inner diameter — and let the rest follow a general block tolerance.
Q: What lead time should I expect for custom end configurations?
A: BQUQ issues quotes within 12 working hours for extension springs with standard or custom ends. Prototype quantities with CNC-formed legs typically ship in about one to two weeks depending on material availability; production volumes depend on quantity and finishing. Flexible MOQ applies, so prototype and pilot builds are welcome alongside production orders.
Related Resources
- About BQUQ and our Dongguan factory: /about/
- Custom extension springs with any end configuration: /extension-custom-springs/
- Compression and torsion springs from the same floor: /compression-springs/, /torsion-springs/
- Industry trends in spring and metal component sourcing: /industry-dynamics/
- Technical articles on spring design and manufacturing: /bquq-blog/
- Frequently asked questions about quoting and tolerances: /faq/
- Case studies from appliance, automotive and industrial programs: /case/
- Send drawings for a 12-hour quote: /contact/
Authored by the BQUQ Engineering Team. BQUQ (Dongguan) runs CNC machining (±0.005 mm), metal stamping, custom springs, and heat sink production in one ISO9001 factory. Source-direct from Dongguan, China — quote in 12 hours: sc@bquq.com | WhatsApp +86 13713157787 | www.bquq.com
