Extension springs are used in many applications such as push and pull levers, screen doors, and anywhere else extended spring force is necessary. Continuous extension springs, such as garter springs, can be used in applications that require metal reinforcement and can be connected with spring-threaded ends. A garter spring is also a tension spring that is tightly wound and can be connected by making one end tapered and screwed into the other end.
Extension springs made are wound with initial tension. Extension spring initial tension is the amount of force required to break the coils apart. When designing extension springs, certain criteria should be considered such as maximum load, which is determined by spring material type, spring wire diameter, and the application of the extension spring. An extension spring should be designed within torsion limits of the extension spring material or about forty percent of the tensile strength of the spring material.
In designing extension springs, other design specifications that should be included are free length; overall, or inside the extension spring hooks. Outside diameter of the extension spring body and/or hooks. The extension spring wire size is preferred or can be referenced and determined by the spring manufacturer if there is an extension spring load or rate. The number of coils or maximum extension spring body length should be referenced if the spring load is known, extension spring style of ends, right or left-hand wound. What the extension springs maximum extended length is¦inside the hooks. How often is the extension spring deflected over the spring’s lifetime? The position of the extension springs hooks is important to the installation position in relation to the extension springs usage.
The extension springs ends can come in many varieties including hooks, loops, plain, threaded, or extended loops. The most common are the machine loop and crossover loop and should be specified whenever possible to minimize cost. Special care should be taken in designing the radius ends as this area can be an area of great stress and shorten the life of the extension spring. Tool marks and other stress concentrations should be held to a minimum. A minimum bend radius of one and a half times the wire diameter is recommended.
To summarize, the general steps of designing an extension spring are as follows. Determine if the extension spring has to fit over a shaft. Determine if the spring is going to fit in a hole. Does the extension spring need to fit into a fixed space? This would indicate that the extension spring length inside the hooks and the body length need to be at least a certain length or can be no longer than a certain length. How is the extension spring being used¦what is its general function? This will allow the spring manufacturing engineer to know how much initial tension is needed and how much force the extension spring will need to have in order to perform its desired function. What types of hooks are required of the extension spring? Do the extension spring hooks need to have a certain gap between the end of the hook and the body of the spring for a rod of the shaft to slide into or are the closed or threaded loops? Determine what type of spring material will be necessary for this extension spring to function correctly. For example; stainless spring steel or music spring wire, is the working environment corrosive or non-corrosive? Whenever possible consult a spring manufacturing engineer to help with the final details of your extension spring design.