Answer
Solid height is the point where all waves are compressed flat. Compressing a wave spring to solid height often exceeds the material's elastic limit, especially in high-load designs. This results in 'set', where the free height $H_{free}$ decreases. The stress at solid height $\sigma_s = \frac{3 π E t (H_{free}-t_{solid})}{4 N^2 D_m^2}$ should be compared against the yield strength. If $\sigma_s > \sigma_y$, the spring will not return to its original height. In some manufacturing processes, 'presetting' is performed: the spring is intentionally compressed to solid height to 'remove' the initial set and stabilize the load. If a spring in the field is found to have a reduced free height, it is a clear indicator that the assembly reached a 'bottom-out' condition during operation.
