Extension springs along with compression springs are the most widely used group of springs. Extension springs are characterized by the fact that during operation they are only stretched. Another unique feature of this type of springs is that extension springs are the only spring with the so called pretension feature, which can be defined as the cumulative force in the spring and that should be applied to stretch the spring from rest position L0. All the coils of this type of springs are open and are located at the ends of the hooks for mounting the spring. There are several basic types of hooks in extension springs: A- German hook, B - English hook, C - catch hook, D - rotary hook and also hooks specifically shaped for cooperation in a unit where springs are applied. Hooks in springs may be positioned as follows: 1) - central (in the spring axis), 2) - side (adjacent to the cylindrical surface of the spring), or 3) - external (facing the outside of the spring).