Extension Springs Information
Helically wound to oppose resistant forces, extension springs have consistent mechanical energy to return to its no-load, compressed position. The ends of the spring are attached to components intended to move apart, with the extension spring providing a reliable return force.
This type of coil spring is closely wound when at rest; the internal force pulling the coils together is known as initial tension. Initial tension is the measure of load required to begin coil separation and extension springs can have a pre-load that does not begin to deflect the spring. This standard value of force is required with each use, and must be discounted when an operating limit is determined for an extension spring.
Initial tension is directly correlated to spring index, which is measured by dividing the spring's mean diameter by wire diameter. High spring indexes result in lower initial tension requirements, with fewer load required to begin spring extension. As the spring lengthens, a heavier load force is required to continue the expansion. Spring composition also determines the linear elasticity of the spring, with more rigid materials providing stiffer resistance and a higher return force.
Extension springs are typically metal because of the stiffness of the material. Hard drawn steel, music wire, spring steel, stainless steel and brass are most common, but other materials may be incorporated in custom applications. Some extensions springs may have pitch between coils, but that can make initial torsion harder to identify. Manufacturers will often provide design units for extension springs in both imperial and metric standards.
Extension spring manufacturing Video credit: Master Springs & Wire Form Co. via YouTube
While extension springs are straight-forward in their identification, there are two notable variances.
Drawbar extension springs are a type of pitched extension springs with the loads attached to the further ends of the spring, connected through the center of the spring with a metal loop. When a static load is applied, the spring will compress accordingly and provide a set stop. These types need to be oiled to reduce friction.
Constant force extension springs are unique type of spring, formed of prestressed, flat metal stripping. Formed into a tight coil, when deflected the spring straightens and lengthens in a linear fashion. The return force is provided by the spring's attempt to return to its coiled radius.
Video credit: Vulcan Spring via YouTube
Extension springs are wound in either a clockwise or counter-clockwise direction. Since the load is applied laterally, wind direction is usually unimportant.
Extensions springs offer a variety of ending inserts, loops, hooks, and eyes to aid in their attachment. Common loop types are illustrated below.
Extension Spring Specifications
Free length is the length of the extension spring at rest measured between the inner surface of the ends.
Wire diameter is the width of the wire used to make the coil.
Mean coil diameter is the total width of the spring at rest, minus the wire diameter.
Spring rate is the amount of force, in pounds, needed to extend the spring one inch. Affected by wire diameter and material. Planetspring.com offers an automated extension spring rate calculator.
Maximum extended length is the furthest a spring can be stretched before the spring warps or fails.
Maximum load is the stress weight at the spring's full extension.
Spring index is the mean coil diameter divided by the wire diameter and correlates with the initial tension needed to begin extending the spring.
Spring pitch is the distance between wire centers in adjacent coils. For closely wound coils, it is the same as the wire diameter.
Number of extension coils refers to the number coils that deflect under load. For standard hook-end configurations, it is typically equal to the number of coils. For springs with threaded ends or swivel hooks, dead coils, or coils that never separate, may be incorporated on the end of the coil body to reinforce the attachment component.
Initial tension is the force required to overcome the standard pitch of the extension spring at rest.
Extension spring failure can be more catastrophic than failure in other springs. Compared to a compression spring of similar size and material, safe working stresses are limited to ¾ of the load weight for extension springs. Also, while the extension of a spring may be within its safe working load, the spring's ending attachment (hook, loop, eye, etc.) is considerably weaker than the spring coil and the stress point is concentrated on a small surface area. This may warp or snap under high stress.
Steel is the most common material used for torsion springs, though custom fabrication options exist.
- Hard drawn steel is inexpensive, but has a low working load.
- Music wire is a common and relatively inexpensive high-carbon steel alloy used for spring manufacture. It is cold drawn and offers uniform tensile strength.
- Spring steel is a standard industrial grade of steel specifically used for spring making. It exhibits good elastic and return properties.
- Stainless steel exhibits good corrosion resistance for specialty applications.
- Brass is inexpensive, easy to mold, and corrosion resistant, but may not be strong enough for the application.
Specialty metals and alloys can be customized per application. This may include beryllium copper, beryllium nickel, niobium, tantalum, and titanium.
Coating methods can offer additional properties to untreated springs
- Black oxide coating is an inexpensive way to prevent corrosion with a durable finish.
- Gold iridite is a chemical coating offering durability and conductivity.
- Passivation adds a thin layer of oxide or nitride onto the metal, enhancing corrosion resistance.
- Zinc offers a galvanized layer of protection to the coils.
Other finishes may be offered by individual manufacturers.
Extension springs are common in trampolines, screen and storm doors, garage doors, toys, farm machinery and thousands of other uses. A common--albeit mechanically unconventional--use for extension springs is in the use of a plumbing snake. A long, plain-end extension spring is fed through pipes to dislodge pipe congestion.