Official Answer
Answer
A 'Shim End' wave spring features a flat 360-degree surface at each extremity, whereas a 'Plain End' spring terminates at the peak of a wave. The Shim End configuration provides a uniform $360^{\circ}$ distribution of the load $P$ to the mating component, effectively eliminating the point-loading associated with plain ends. Mathematically, this reduces the localized contact stress $\sigma_{contact}$ and prevents the spring from 'digging' into soft housing materials (like aluminum). From a rate perspective, the shim turns act as rigid foundations, ensuring that the number of active turns $Z$ remains constant throughout the deflection range, whereas plain ends can 'roll' and slightly alter the effective $N$ as they flatten against the seat.
