How do you design a helical spring for fatigue loading?

For fatigue loading, use Soderberg or Goodman criteria. Calculate mean stress tau_m and alternating stress tau_a from operating load range. The Wahl factor K = (4C-1)/(4C-4) + 0.615/C accounts for curvature and direct shear (C = D/d). Apply fatigue strength reduction for surface finish, size, and reliability. Ensure tau_a/Se + tau_m/Ssy < 1/FoS. Shot peening improves fatigue life by inducing compressive residual stresses. Design for at least 10^6 cycles minimum.