How Extension Springs Work

An Extension Spring is activated when a linear force is applied along the length of the coil, which will stretch it open. The spring’s initial tension is created by resisting the “stretch”. As the spring tries to maintain or return to its original shorter length, it utilizes the force created by the tension.

How Extension Springs are Applied

Extension springs are used to provide tensioning in a wide range of industries and applications, like small medical devices, automotive (interior and exterior components), push/pull levers, screen doors, trampolines, garage door assemblies, farm machinery, balance scales, off-road equipment, brake springs, toys, carburetors.

Different Types of Custom Extension Springs manufactured by Spiros Industries

Types of Extension Springs

Extension Springs are manufactured in any of the following configurations:

  • Straight Coil Springs
  • Constant Force Springs
  • Drawbars
  • Garter Springs

Straight Coil Springs – are the same diameter along its entire length.

Constant Force Springs – are a spiral made from winding a flat strip of metal, with the long dimension of the strip coiled tightly.The resulting “form” looks similar to a tape measure. One end of the strip is inaccessible (at the center of the spiral). The outside end will resist being pulled out and, when released, will snap back into its tightly coiled shape.

Drawbar Springs – (or Drawbars) are a specialized extension spring that has hooks or arms extending beyond the spring on each end. When these arms are pulled outward under load, the spring itself compresses. As the spring gets shorter, the total length of part gets longer. The Drawbar extension spring is able to withstand a greater load than an ordinary compression spring.

Garter Springs – can have functions similar to an extension or compression spring. Its ends are connected so the spring coils form a continuous circle, which exerts radial forces. A garter spring is loaded when stretched outwards; when released, it snaps back – returning to its smaller size.

Extension Spring Designed by modifying the Wire Diameter, Free Length, Stretch Length, Number of Coils and Ends (using Pro/PROGRAM).

Coil Ends for Extension Springs

Extension Springs normally have Hooks or Loops at each end that attach to other components. Besides the common Hook and Loop options, there are unlimited possibilities for the end designs based on the specific engineering application. Unless a custom coil end design is required, the shape may follow one of the following:

  • Teardrop or Rectangular Shapes
  • Expanded or Reduced Eyes
  • Threaded Inserts
  • Extended Twist Loops
  • Crossover Center Loops

Critical Design Specifications for Extension Springs

  • Length (Maximum and minimum)
  • Diameters (Inside and outside limits)
  • Number of Coils
  • Winding (Right hand or Left hand)
  • Wire Size
  • Wire Material
  • Length of Coil
  • Length Inside Hooks
  • Load Required
  • Maximum Extended Length
  • Deflection, or Distance of Travel
  • Frequency of Extension
  • End Positioning (How important is end positioning? Making the ends of springs bear a definite spatial relationship to one another will usually add to the manufacturing costs.)
  • End Style
  • Finish

Stress relieving or heat treating affects extension springs as follows:

  1. Reduction in initial tension
  2. Change in hook position
  3. Change in spring diameter