The force in the compression spring is found from Hooke's Law,
The maximum shear stress tmax 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 cross-sections), 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 cross-section),
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.