One the most common problem we run into with extension springs is hook breakage. Hooks break for many reasons but the main cause is stress created from bending the wire to make the hook. Flipping up a hook causes the wire in that area to weaken. If high stresses are then applied to the extension spring, the hook may snap even though the rest of the spring can handle the load. But what’s the use of an extension spring with a broken hook?
If high stresses are expected, it might be a good idea to ask for swivel hooks. The idea around a swivel hook is to make the hook stronger by using larger wire and at a smaller OD than the spring body. To make a swivel hook, many times a nail is formed into a hook and the body of the spring is coned down to enclose the base of the nail. This permits the hook to rotate, or swivel, freely from the spring body itself.