What is Extension Spring?- Working, and Application

What is an Extension Springs?

Extension springs absorb and store energy as well as create a resistance to a pulling force. These springs are usually attached to other components at both ends and when those components come apart the spring tries to bring them back together again.

It is the initial tension that determines how tightly together the spring is coiled. This initial tension can be manipulated to meet the load requirements of a particular application. Designs usually have hooks, eyes, or other interface geometry on the ends that attach to the opposing components.

They are often used to provide a restoring force to components that extend in an actuated position.

How do Extension Springs work?

Extension springs generally have a hook or eye at each end to facilitate attachment. 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. Since extension springs are under tension, they do not need any support along their length. This often results in a simpler mechanism than if a compression spring were used.

The ends of an extension spring are attached between two mechanisms. The hooks and loops of the tension spring store and absorb energy. An extension spring provides the connected mechanisms with a restoring force through hooks or loops.

Tightly coiled extension springs typically sit in the idle position. More stress in the end hook in contrast to the spring body limits the performance of extension springs.

Extension springs

Key Design Parameters


  • Outside Diameter, Inside Diameter, Wire Diameter, Free Length and Extended Length
  • The Free Length is the length of a spring in the unloaded position (measured from inside the end loops).
  • The Extended Length is the length at full rated extension.

Spring Rate and Maximum Load

  • The Spring Rate is the force per unit spring deflection, such as lbs per inch travel.
  • The Maximum Load is the load at full rated extension.
  • Unit of Measure


Extension springs use a variety of hook or loop end configurations to serve specific functions. Tension spring ends include threaded inserts, extended twist loops, crossed center loops, hooks, expanded eyelets, reduced eyelets, square ends, and teardrop-shaped ends.

Change the length of the hooks and spring body spacing for custom tension spring fits and functions.

Find extension springs in a wide variety of everyday items from garage doors to tools to washing machines and toys. The variety of sizes makes extension springs versatile as they are used in small medical devices and off-road machines.

Common applications of extension springs include:

  • Car interiors and exteriors
  • Garage door assemblies
  • Vise-grip pliers
  • Carburetors
  • Trampolines
  • Washing devices
  • Farm machinery
  • Toys