Answer
The solid height $H_s$ of a wave spring is the axial length when the spring is compressed such that all waves are in contact. For a Crest-to-Crest spring, $H_s = N \cdot t$, where $N$ is the number of turns and $t$ is the material thickness. However, for springs with shim ends, the formula becomes $H_s = (N+2) \cdot t$. As the spring approaches $H_s$, the load-deflection curve becomes non-linear, exhibiting an exponential increase in force as the contact area moves from the wave peaks to the entire surface. Operating a spring near $H_s$ is discouraged because the 'solid stress' often exceeds the material's elastic limit, leading to permanent set. Designers use a safety factor $\eta = \sigma_{yield} / \sigma_{solid}$ and typically limit the maximum working deflection to $80\%$ of the available travel to $H_s$ to maintain predictable linear behavior.
