Extension springs are closed-coiled helical springs that extend under a pulling force. Many design considerations must be made with extension springs including; initial tension, stress and deflection of hooks, hook end types.
There are a variety of extension spring end types. Machine loop and hook over center, side loop, full loop over center, no hook, extended hook, hook end with swivel, v-hook over center, double twisted full loop over center, and many more.
A hook is open to fit over a projection; a loop is a closed hook. A regular machine hook or loop is made on an automatic looper and is the least expensive and often is the most satisfactory type. Hook stresses can be reduced by reducing hook diameter, using swivel hooks (which can be expensive) and minimizing sharp radius bends.
Initial tension is the force which presses the coils of a closed-wound extension coils together. Initial tension is the force needed to open these coils. Initial tension can be wound into cold-coiled extension springs made from hard-drawn or oil-tempered wires only, such as music wire, 300 series stainless steel, phosphorous-bronze, Monel, Inconel, and their pre-hardened materials.