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Fatigue is an issue in springs subjected to cyclic loading, where the force on the spring varies between a maximum and minimum value. For example, the springs in a car's shock absorption system would be subject to fatigue loading. Over time, excessive cyclic stresses from fatigue can cause the spring to fail.

Use this calculator to predict if a known spring subjected to a known range of loading forces will be in danger of failing from fatigue. The calculator outputs a GO / NO-GO assessment after checking the maximum shear stresses in the spring against Soderberg's criterion.

Inputs
  Spring free length, Lfree:    
  Spring constant, k:    
  Maximum force on spring, Fmax:    
  Minimum force on spring, Fmin:    
  Wire diameter, d:    
  Outer diameter of spring, Douter:    
  Material's limiting fatigue stress:    
  Material's yield stress:    
  Factor of safety:    
Answers
  Minimum length of spring (under Fmax):  3.85  in
  Mean component of shear, tmean:  13.4  ksi
  Alternating component of shear, talt:  2.68  ksi
  Soderberg limit for talt:  3.91  ksi
 
Fatigue Check:  Your spring is safe from fatigue.
Equations Behind the Calculator
See the companion page to view the equations for this calculator.