Extension Springs

Custom Extension Springs · Engineering Review · Drawing Fast-Track

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End Loop Types — How Your Spring Attaches

The hook is where extension springs carry the load — and where they most often fail. Choosing the right type matters as much as the spring itself.

Machine Hook

Extended Hook

Side Hook

Crossover Hook

Threaded Hook

Coiling Methods — Material & Hook Considerations

The choice between cold and hot coiling affects not only the spring body, but also how the hooks are formed — and how long they last under load.

Cold Coiling

Hot Coiling

Each method affects hook strength and fatigue life. We evaluate your drawing against these process boundaries — not to reject, but to align method with expectation.

Spring Geometry

Extension springs share many of the same body shapes as compression springs,
but with one difference: the hooks are added after coiling.

Cylindrical — constant diameter, most common
Conical — decreasing diameter, progressive rate
Barrel — larger diameter at center, side load stable
Swivel Hook — Hook can rotate at spring end
Nested / multi-spring — multiple concentric springs, compact high load
Custom shapes — as per your drawing

We manufacture to the geometry specified in your drawing — hooks included.

After Coiling — What Makes an Extension Spring Reliable

The hooks are formed after coiling. What happens next determines whether they hold — or fail.

Stress Relieving

Pre‑stretching-optional

Shot Peening-optional

These steps are engineering decisions — applied only when your application requires them. We recommend during engineering review, not before.

Wire Cross‑Section

Note: Extension springs are almost exclusively manufactured from round wire. Square, rectangular, or flat wire is generally not used due to hook forming difficulties and stress concentrations at corners — available only for special applications upon engineering review.