Spring types

Extension springs and hooks

An extension helix produces force as it lengthens. The body and its ends behave differently and need separate evaluation.

Helical extension spring with close-wound coils and hooks at both ends. — Educational illustration generated for this guide.

Initial tension

Coils are often close wound and require an initial force to separate. In a linear model, load combines that initial tension with body rate and subsequent extension; actual behavior can differ from the idealization.

Ends matter

Hooks, loops, and transitions concentrate stress and may govern strength or service life. The calculator approximates the body and does not verify local hook geometry, opening, orientation, or end load capacity.

Body relationship

For the idealized helix, k = G·d⁴/(8·D³·n) and F = F₀ + k·s. With N/mm² and mm inputs, k is in N/mm; F₀ is initial tension in N and s is extension after coil separation.

Worked example

With d=2 mm, D=16 mm, n=8, and G=79,300 N/mm², k≈4.8401 N/mm. If F₀=15 N and s=10 mm, F≈63.4009 N. This estimates the body; it does not verify the hook.