Method of Testing Initial Tension

Most extension springs are wound with initial tension. The measure of the initial tension 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. This is graphically illustrated in figure below. This built-in load, called initial tension, can be varied within limits, decreasing as the spring index increases. If the designer needs an extension spring with no initial tension, the spring should be designed with space between the coils.
An accurate method for testing initial tension is as follows:

1. Extend the spring any convenient amount (such as 1”) and measure the load.
2. Extend the spring exactly twice that amount (in this case, 2”) and again measure the load.
3. Subtract the first load from the second.
4. Subtract this difference from the first load and the remainder obtained is the exact amount of initial tension.