Specific features of helical compression springs
Mechanical springs. Axial load

In addition to the N useful coils used to calculate flexibility (or its inverse, stiffness), the end coils of compression springs, called bases, are shaped to adapt to the system that receives the spring, so that the external force is as close as possible to an axial force, and the spring operates under good conditions.

In a conventional design, the spring is supported on flat cups. In this case, the most common practice is to add a coil with a pitch equal to the wire diameter to each end of the compression spring, known as the close end. Preferred ends are close-coupled and ground, to ensure the correct application of forces. For small springs, grinding is sometimes not possible, so it's best to use close ends (not ground). The...