Compression spring open end

The end shapes of compression springs are "closed end" and "open end."

The typical type is a closed end, where the tip of the coil at the end is in contact with the next coil. This single turn at the end is called a "seat coil" (zamaki). The seat coil serves to improve the stability of the spring when it is upright, but it does not have any functional role as a spring. The turns that function as a spring are those excluding the seat coil, and the number of turns that effectively function as a spring is referred to as the effective number of turns. The spring in the photo is an atypical "open end." The tip of the coil at the end is not in contact with the next coil, creating a gap (indicated by the arrow). The seat coils at both ends are less than one turn, resulting in fewer seat coils compared to a closed end. The reason for deliberately adopting a non-typical open end is mainly in the following cases: 1) When you do not want to change the spring constant (want to keep the effective number of turns fixed), and 2) When you want to keep the height at which the spring is in contact low (want to reduce the total number of turns). While reducing the seat coils may worsen the stability of the spring, it allows you to maintain the spring constant while keeping the contact height low.

Spring