Custom Extension Springs

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Initial Tension

Most extension springs are wound with initial tension.This is an internal force that holds the coils tightly together. 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.


Note that there is a range of stress (and, therefore, force) for any spring index that can be held without problems. If the designer needs an extension spring with no initial tension, the spring should be designed with space between the coils.


Often stresses are higher in the spring ends than in the spring body. Unless special design precautions are taken, the allowable body stress must be reduced.  The hook stress in torsion should not exceed 40-45 percent of tensile strength, while hook stress in bending should not exceed 75 percent of tensile strength.

The maximum bending stress in a machine loop (see spring image below) occurs at location B, while the maximum torsion stress occurs at T. Stresses at these locations are estimated as follows:

Equations Spring-Stresses-Diagram


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