Helical extension springs are similar to helical compression springs, but are loaded in tension. Hooks or loops are provided to allow a pull force to be applied. Usually, extension springs are attached at both ends to other components. When these components move apart, the spring tries to bring them together again. Extension springs absorb and store energy as well as create a resistance to a pulling force. It is initial tension that determines how tightly together an extension spring is coiled. This initial tension can be manipulated to achieve the load requirements of a particular application. Extension Springs are wound to oppose extension. They are often tightly wound in the no-load position and have hooks, eyes, or other interface geometry at the ends to attach to the components they connect. They are frequently used to provide return force to components that extend in the actuated position.

Common Applications

Applications for extension springs include automotive interiors and exteriors, garage door assemblies, vise-grip pliers, carburetors, trampolines, washing devices, farm machinery, toys as well as thousands of other uses. Extension springs come in a wide array of sizes, from small medical devices to off-road machinery brake springs.

A standard loop is shown in the figure below, but many variations are possible.

Typical Extension Spring Dimensions

Free Length - is the overall dimension of an extension spring in the unloaded position. The free length is measured from the inside of one end loop (or hook) to the other and can be varied by changing the end configuration without changing the number of coils.

Body Length - is the measure of the spring length, excluding the loops.

Extended Length - is the length at full rated extension.

Maximum Load is the load at full extension.

Loop Length - measures the length of a loop from the end of a body to the inside diameter of the loop.

Hook Length - measures the length of a hook from the end of a body to the inside diameter of the hook.

Gap - is the dimension that controls the loop or hook opening.

Initial Tension - is the force that keeps the coils of an extension spring closed and which must be overcome before coils start to open.

Active Coils - All coils in the body are considered active coils, but one coil is typically added to the number of active coils to obtain the body length.