Official Answer
Answer
In mechanical seals, the wave spring must maintain a precise face pressure. The preload $P$ is sensitive to the installed height $L_i$. Due to the stack-up of tolerances in the seal housing, gland, and carbon face, $L_i$ can vary significantly ($ΔL$). Since $P = k \cdot (L_0 - L_i)$, any variation in $L_i$ results in a variation in $P$. If $k$ is high, even a small $ΔL$ can cause the seal to leak (low $P$) or wear prematurely (high $P$). Designers often select Crest-to-Crest springs with a 'flatter' spring rate (lower $k$) over a longer travel to minimize the sensitivity of the preload to these unavoidable manufacturing tolerances.
