Extension springs are helical springs that are subjected to axial tension, and extension springs are generally made of materials with circular cross-sections. When not under load, the circles of extension springs are generally tightened with no gaps between them.
A number of different end devices or "hooks" are used to ensure the source of tension in extension springs. Extension springs work on the opposite principle to compression springs. Compression springs work in reverse when they are compressed, while extension springs work in reverse when they are extended or pulled apart. When the ends of an extension spring are pulled apart, the spring tries to pull them back together. Like compression springs, extension springs absorb and store energy. Unlike compression springs, however, most extension springs are usually under a certain level of tension, even without any load. This initial tension determines how tightly the extension spring coils without any load.
