An extension spring, also known as a tension spring, is a helical length of tightly coiled metal, usually fitted with steel hooks or loops at either end for easy attachment to various fixtures, equipment and components. Extension springs are very often considered an integral part of general door hardware kits, but they’re also found playing any number of important roles in other applications, from industrial to automotive, agricultural to electronics, and many more.
The main job of extension springs is to absorb and store energy. As the spring stretches out (extends) under force loads, it absorbs and stores the energy transferred by those loads in its many coiled loops of metal. When the force is removed or relaxed, it redistributes the force back into motion that allows the attached object to return to its original position.
Extension springs thus provide the exact opposite type of support that a compression spring does. The latter is designed to do a similar job - storing energy, essentially, to help objects move between positions - but a compression spring does so by being squashed down, as opposed to stretched out. Either way, the process involves energy being stored and subsequently released by the spring. The extension spring provides resistance to a pulling force by simultaneously attempting to drag the moving object(s) back into the original position. Familiar examples would be the type of springs we often see around the edges of a trampoline, on the rear of a wooden garden gate, or along the inside hinges of a car door.
The main visual difference between an extension spring and a compression spring is in the spacing of the individual coils themselves. Unlike a compression spring, which features widely spaced coils to allow for squashing forces, an extension spring typically has coils that are tightly wound enough to rest alongside one another when not under load. As forces are transferred through the extension spring, the coils will be pulled apart, and will subsequently return to their ‘closed’ position once the force is removed.