Extension Spring Calculator

Inputs

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Diameter of the spring wire.
mm
Center-to-center across the coil (outside diameter − wire diameter).
mm
Coils that actually deflect (body coils that carry the extension).
Sets the shear modulus G.
Built-in force holding the coils closed; the spring won't extend until load exceeds it.
N
Working extension beyond the closed-coil length — used for the force at that point.
mm

Results

Default result
Edit inputs
Spring rate(k)
2.904N/mm

Also computed

Spring index(C)Pass8

Force at extension(F)34.04N

7.653 lbf

= initial tension + rate × extension

Initial tension5N

The spring stays closed until the load exceeds this.

Method notes 3 notes
  • Static load; ignores fatigue, set and stress relaxation.
  • The coils stay closed until the load exceeds the initial tension Fᵢ; below that the spring does not extend.
  • Hook/loop ends are the highest-stressed region and are not stress-checked here.

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