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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 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.