The force in the compression spring is found from Hooke's Law,
The maximum shear stress t_{max} in the spring occurs on the inside surface of the coils. It is proportional to the spring force F, and is given by the formula,
where D is the mean diameter of the spring (measured from the centers of the wire crosssections), W is the Wahl correction factor to account for the spring curvature stress, and C is the spring index (essentially an aspect ratio of the spring crosssection),
Note that W approaches 1 in the limit as C gets large. In other words, as the spring's diameter increases or as its wire diameter decreases, the effect of curvature on the spring shear stress diminishes.
