FORMULAS FOR DESIGN OF HELICAL EXTENSION SPRING UNDER STATIC LOADING

Formulas which are used to calculate extension spring parameters under the static loading are spring index formulas, Wahl factor formula, correction factor formulas and shear and tensile stress formulas . All these formulas are given in the following chart.

Note: For further information on extension spring design formulas, check Shigley's Mechanical Engineering Design.



Parameter Symbol Formula
Spring outer diameter OD Spring outer diameter
Spring inner diameter ID Spring inner diameter
Spring index C Spring Index
Spring index -1 C1 Spring index 1 in extension spring
Curvature correction factor for point A KA  Curvature correction factor at point A in extension spring
Tensile stress at point A σA Tensile stress at point A  in extension spring
Spring index -2 C2+ Spring index 2 in extension spring
Stress correction factor for point B KB   Stress correction factor at point B in extension spring
Shear stress at point B σB Shear stress at point B  in extension spring
Wahl factor Kw Wahl factor
Shear stress at spring body τsb Shear stress in spring body (wahl factor included)
Factor of safety at point A fosA Extension spring factor of safety at point A
Factor of safety at point B fosB Extension spring factor of safety at point B
Factor of safety at spring body fossb Extension spring factor of safety at spring body



Note 1 : + Recommended practice is to make C2 greater than 4.

List of Parameters
Symbol Definition
Na Number of active coils
Nb Number of body coils
d Wire diameter
D Spring mean diameter
Lf Spring free length
F Axial force on the spring

Reference: