How does the flatness of mating surfaces impact the load distribution of a wave spring?

Wave springs require flat, parallel mating surfaces to distribute the load evenly across all wave crests. If the mating surface is concave, the inner diameter of the spring will be loaded more than the outer diameter, creating a moment that can cause the spring to twist. The parallelism should be within $0.02$ mm per $25$ mm. In many automotive clutches, hardened and ground shims are placed between the wave spring and cast-iron housings to provide a suitable mating surface and prevent the spring from 'digging' into the softer material.