Official Answer
Answer
In high-precision assemblies like optical lens housings or EV motor bearings, the tolerance stack-up of the housing and mating components can exceed the desired load range of the wave spring. Shims are used to adjust the work height $L_w$. Since the load $P$ is a function of $(L_f - L_w)$, a small variation in $L_w$ can lead to a large variation in $P$ if the spring rate $k$ is high. Hardened steel shims should be used to provide a flat, parallel surface for the spring to bear against, preventing the wave crests from digging into softer aluminum or plastic housings, which would effectively change the free height and the resulting load.
