Tension springs are used in a variety of industrial, recreational, and household applications to attach objects of an assembly while allowing movement during operation. A trampoline or hammock are two common recreational examples where tension springs are used. When at rest, tension springs are in a near coil to coil state, or solid condition in which each coil of the spring may touch the next. For each application, a user may specify a desired force at length characteristics among other tolerances for the spring's intended use.
A particularly notable characteristic of tension springs is their
initial tension or Fi as it is represented by the industry. This value represents the force required to manipulate the spring from a coil to coil state to one in which the coils begin to separate. In other words, the force at which the spring begins to lengthen. When testing a tension spring, (see image on right) the Fi, or initial tension, can easily be determined from two very distinct slopes that occur - one before and one during the point at which the coils begin to separate. As the force vs. position curve in the graph demonstrates the Y-intercept of the two distinct slopes marks the initial tension value. In addition to Fi, other resultant information such as multiple force at length, and spring rate values can easily, and quickly be determined.
Note: Unlike many other materials and components, widely agreed upon specifications for testing springs do not exist