Written by Jody Muelaner
Extension springs, also known as tension springs, are springs that can be stretched to increase their length. When extended, these springs are under tension. Typically, each end of the extension spring is attached to a different component, when the components are pulled apart the spring tried to pull them back together. Most extension springs are coiled springs manufactured from sprung steel spring wire.
Generally, extension springs have a hook or an eye at each end to facilitate fastening. Variations on the ends include open hooks, extended hooks, side hooks, and double full loops.
Extension springs are often used to create a restoring force in mechanisms such as latches and counterbalances. Because extension springs are under tension, they require no support along their length. This often leads to a simpler mechanism than if a compression spring were used.
Hooke’s law is often used as an approximation for elastic objects. It states that the force (F) required to extend or compress a spring by some distance x is proportional to that distance. The rate that the force increases will depend on the stiffness of the spring k.
This can be expressed as: F = kx
Real extension springs do not follow Hooke’s Law since they have some initial tension (F1) that must be overcome before any extension takes place.
A better approximation for the force in an extension spring is given by: F = F1 + kx
The initial tension (F1) can be difficult to control and may vary significantly between springs of the same part number.
The dimensions of extension springs are normally given in their relaxed state. They may be specified in terms of outside diameter and inside diameter or wire diameter. The length of the body coils may be given in addition to the unloaded length inside the hooks as well as the maximum extended length inside the hooks.
Typically, a sprung steel spring wire is coated with zinc but stainless steel springs are also available. Rubber bands, elastic bands, or bungee cord can provide low-cost alternatives to steel extension springs.