Extension spring: how it works and how to calculate it

An extension (tension) spring works the opposite way to a compression spring: it resists when you try to stretch it. The coils sit closed at rest and separate under load, pulling the two anchor points back together.

Initial tension

The big difference of an extension spring is initial tension: the force that holds the coils closed before any displacement. You must overcome this initial tension before the spring starts to extend. It is set during winding.

Hook types

• Machine hook (half loop): the most common and economical.
• Crossover hook: centred on the spring axis.
• Extended hook: to reach distant points.
• Swivel loop: lets the anchor rotate to any angle.

Where the hook usually fails

The most stressed point of an extension spring is the hook bend, not the body. Hooks with a very tight radius concentrate stress and reduce fatigue life. In cyclic applications it pays to ease the radius or use swivel loops.

Example and tool

A stainless spring with d = 2 mm, OD = 18 mm, 10 coils and 8 N initial tension needs 8 N just to start opening; after that each millimetre of opening adds k newtons. In the designer you set the hook-to-hook length, hook type and initial tension and get the force and quote instantly.