Extension springs are springs which absorb and store energy by offering resistance to a pulling force. Various types of ends are used to attach the extension spring to the source of the force.
Initial Tension of Extension Springs Most extension springs are wound with initial tension. This is an internal force that holds the coils of an extension spring tightly together. The measure of the initial tension of an extension spring is the load necessary to overcome the internal force and just start coil separation, unlike a compression spring, which has zero load at zero deflection, an extension spring can have a preload at zero deflection. When manufacturing extension springs, the operator applies what is termed back pitch to the wire using the pitch tool. Typically, the pitch tool is recognized as a means to create the distance between coils on compression springs during coiling.
Extension Spring Ends The variety of ends that can be used on extension springs is unlimited and may include threaded inserts, reduced and expanded eyes on the side or in the center of the extension spring, extended loops, hooks or eyes at varying positions or distances from the body of the extension spring and even rectangular or teardrop shaped ends. (The end is a loop when the opening is less than one wire size; the end is a hook when the opening is greater than one wire size.) The most common ends for extension springs are the machine loop and crossover loop. The extension spring ends are made with standard tools in one operation and should be specified whenever possible to minimize costs. It should be remembered that as the space occupied by the machine loop is shortened, the transition radius is reduced creating significant stress. Most extension spring failures occur in the area of the end. To maximize the life of the extension spring, the path of the wire should be smooth and gradual as it flows into the end. A minimum bend radius of 1-1/2 times the wire diameter is recommended for extension springs.
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The variety of ends that can be used on extension springs is unlimited and may include threaded inserts, reduced and expanded eyes on the side or in the center of the extension spring, extended loops, hooks or eyes at varying positions or distances from the body of the extension spring and even rectangular or teardrop shaped ends. (The end is a loop when the opening is less than one wire size; the end is a hook when the opening is greater than one wire size.) The most common ends for extension springs are the machine loop and crossover loop. The extension spring ends are made with standard tools in one operation and should be specified whenever possible to minimize costs. It should be remembered that as the space occupied by the machine loop is shortened, the transition radius is reduced creating significant stress. Most extension spring failures occur in the area of the end. To maximize the life of the extension spring, the path of the wire should be smooth and gradual as it flows into the end. A minimum bend radius of 1-1/2 times the wire diameter is recommended for extension springs. 