Answer
In a gap-type wave spring, the mean diameter $D_m$ is the cubic denominator in the load formula $P = \frac{E b t^3 N f}{K D_m^3}$. This means that load is extremely sensitive to $D_m$. However, the bending stress $\sigma$ is proportional to $D_m / (b t^2)$. If $D_m$ increases while maintaining the same load $P$, the stress decreases significantly because the lever arm of the wave increases, but the thickness/width provides the resistance. In precision medical devices where space is constrained, engineers often have to minimize $D_m$, which drastically increases the stress for a given load. This often forces a transition from a gap-type to a multi-turn crest-to-crest design to distribute the total deflection across more waves and reduce the per-wave stress.
