Counting the coils in cylindrical helical compression springs: A clarification note

Despite an early paper by Vogt addressing the difference between free and active coils and between end and dead coils, current design recommendations for the coil count in cylindrical helical compression springs do not enter in such details. This results in underestimating the flexibility of the spring and overestimating its solid length. This technical note clarifies the meaning of the relevant coil numbers and provides formal relationships that help the designer improve the prediction of the spring rate and of the solid length. Though the analysis can be applied to any type of spring ends, it is particularly effective for the typical squared and ground end setup, which is most frequently encountered in practice.