Official Answer
Answer
The Work Height ($H_w$) is the axial space the spring occupies under a specific load. To avoid plastic deformation, the stress at $H_w$ must not exceed the yield strength ($S_y$) of the material. For nested springs, the stress is $S = \frac{3 \pi \cdot P \cdot D_m}{4 \cdot b \cdot t^2 imes N^2}$. The safety factor is $SF = S_y / S$. If $H_w$ is too close to the solid height ($H_s = n \cdot t$), the spring enters the non-linear range where the actual load exceeds the calculated linear load due to contact between turns. Designers must ensure that $H_w > H_s + (0.1 \times f_{total})$ to maintain linearity and prevent early fatigue failure caused by local stress concentrations at the solid height.
