Engineering and Tolerance

Learn About: Extension Springs

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 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.

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.

Lee Spring's Stock Extension Springs are supplied with full diameter loops (either machine or crossover center) at a random position with the exception of Metric Extension Springs specified to comply with DIN Standards. Loop openings are approximately one wire diameter and the direction of wind is factory optional. Lee Stock Extension Springs are available in both Inch and Metric designs. If exact direction of helix is required, Custom Extension Springs can be made to specification. To fit assembly functions, a multitude of hook or loop configurations may be specified. Close winding of the body provides initial tension in the spring to help manipulate the load and rate.

Extension spring ends include threaded inserts, extended twist loops, crossover center loops, hooks, expanded eyes, reduced eyes, rectangular ends and teardrop-shaped ends, which can all be produced to vary in distance from the spring body. At the design stage of Custom Extension Springs, the length of the hooks at each end of the spring can be adjusted in order to precisely obtain the required spring load at any extended position.

Continuous Length Extension Springs are designed to be cut to desired length by user to meet particular load requirement. Various loops or hooks can be formed on the ends using looping pliers or other appropriate instruments; as wire size increases, use different tools including a vise to hold the coil body and a pry bar to bend up the last coil.

Another common type of extension spring is the drawbar spring. In a drawbar, the load is applied at the ends of long steel loops which pass through the spring's center and are hooked around the opposite end, thus compressing the spring upon loading. Drawbar springs are excellent for use in potential overload situations and offer a built-in definite stop that will continue to carry a static load after reaching the maximum extended length. A common application of the drawbar spring is to support a porch swing.

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
  • Lee Spring's Stock Springs are specified in both Imperial (inch and pound) and Metric units. Be sure to select the preferred unit of measure when looking for a Stock Spring using Lee Spring's search engines.

Types of Ends:

Lee Spring Part PDF