An extension spring — also called a tension spring — stores energy when stretched and returns a load to its starting position. Garage doors, trampolines and countless industrial assemblies rely on them daily. Unlike a compression spring that pushes apart, an extension spring resists a pulling force — and its hooks, not the body, are the most common failure point. This guide covers extension spring hook types, design basics and material choices.
What Is an Extension Spring?
An extension spring is a close-wound helical spring that absorbs and stores energy under axial tension. In its free state the coils usually touch, and the spring carries an initial tension — a built-in force that must be overcome before the coils separate.
The spring rate uses the same formula as a compression spring:
k = G × d⁴ / (8 × D³ × n)
where G is the shear modulus of the wire (about 79,300 MPa for steel), d the wire diameter (mm), D the mean coil diameter (mm) and n the number of active coils. Rate is the force per millimetre of extension, constant for a given geometry regardless of the hooks.
The hooks complicate the picture: a hook adds bending stress on top of the torsional stress in the body. A well-designed extension spring balances three numbers: body rate, initial tension and hook geometry. If you are still choosing a spring type, our article on compression vs. extension vs. torsion springs walks through each family.
Extension Spring Hook Types
Hooks transfer the pulling load into the body, and their shape determines how the spring mounts. The most common types are:
| Hook type | Description | Typical use |
|---|---|---|
| Regular hook (open) | Half loop in the plane of the last coil | General purpose |
| Full loop | Complete ring in the plane of the last coil | Pivot mounting |
| Side hook | Loop at a right angle to the coil axis | Angled mounting |
| Cross-over hook | Loop crossing the spring body | Compact assemblies |
| Extended hook | Straight leg ending in a loop | Distant anchor points |
| Threaded end | Machined stud or thread instead of a loop | Heavy loads |
Every bend concentrates stress — the tighter the hook radius, the higher the concentration. A common rule: keep the hook inside radius at least one wire diameter; smaller radii risk cracking at the inner fibre. Our extension springs (tension springs) product page shows the hook styles we manufacture, and the spring glossary defines drawing terminology.
Design Basics: Initial Tension and Hook Stress
1. Initial tension. Initial tension comes from winding the coils tightly so they press on each other. It typically ranges from about 5% to 15% of the maximum working load — above 15% the spring is hard to coil consistently, below 5% the coils may gap in the free state. Specify the value on the drawing rather than leaving it to the manufacturer.
2. Hook stress. The hook is the weak link. Approximate bending stress at the hook is:
σ = 32 × F × r / (π × d³)
where F is the applied load and r the hook inside radius. The hook works in bending while the body works in torsion, so it often reaches its stress limit first — a spring that looks fine on body stress can still fail at the hook. If the design cannot fit a larger bend radius, choose a full loop, a different hook style or a stronger material.
3. Free length. The free length of a close-wound extension spring is roughly the number of coils times wire diameter, plus the hook lengths. State the load at two or three extended positions — installed, working and maximum — so the manufacturer can verify rate and travel. Our spring engineering notes cover manufacturing limits for hooks, coil count and body length.
Materials and Applications
Because the hook concentrates stress, material choice matters more on extension springs than on any other type. Music wire (ASTM A228) is the standard for small springs and gives the highest tensile strength; oil-tempered chrome silicon (ASTM A401) suits larger, dynamically loaded springs; stainless grades such as 302 and 316 add corrosion resistance for outdoor and food-contact service. The spring steel materials guide compares these grades in detail.
Typical applications include garage door counterbalance systems (see our garage door spring buying guide), trampolines and fitness equipment, agricultural machinery and general tensioning devices. For demanding duty, specify shot peening or preset treatment to improve fatigue life.
FAQ
What is the difference between an extension spring and a tension spring?
None — the terms are interchangeable. “Extension spring” describes the working mode (it extends under load); “tension spring” describes the load itself.
How much initial tension should my extension spring have?
Typical close-wound springs carry 5–15% of the maximum working load as initial tension. State the value on your drawing; if the mechanism needs zero initial tension, specify open-wound coils.
Why do extension springs most often fail at the hook?
The hook works in bending while the body works in torsion, and the tight bend radius concentrates stress at the inner fibre. Increasing the hook radius, changing hook style or upgrading the wire material are the standard fixes.
Need an extension spring engineered for your load, space and environment? Send us the working space, the installed and maximum lengths, the required load at each position and your preferred hook style — our engineers will check rate, initial tension and hook stress, then quote the right material and finish. Request a quote from the Chenji team, or email liu@chenjisprings.com / call +86 158 5311 1612. We have manufactured custom extension springs since 2003 and inspect every batch.