There are various end treatments that are possible with helical compression springs. The end treatments impact the number of active coils, N_a, that contribute to the stiffness of the spring, where N_t is the total number of coils in the spring. Squared and squared-ground ends are the most common with squared-ground being typically preferred unless the wire diameter, d, is less than 0.5 mm [1]. The following table allows the end treatment to be specified. The end treatment also impacts the shut length, L_s, of the spring which is the height where all of the coils are closed and the spring cannot be compressed any further. The shut length is also sometimes referred to as the solid length. The equations used in this sheet are from the Norton [1] and Shigley [2] machine design books.