Extension spring testing and initial tension

Fixture the ends consistently

Hooks, loops and special ends can rotate or seat differently under load. Use fixtures that match the end geometry, avoid sharp local bending and allow consistent alignment with the load axis. Record the fixture configuration when it affects the result.

What initial tension means

Many close-wound extension springs contain a built-in force holding the coils together. The force must rise enough to overcome this initial tension before the body opens appreciably. On a force-extension graph, the useful linear region can therefore extrapolate to a positive force at zero extension.

Initial tension is not the same as spring rate. Rate is the slope of the linear region; initial tension relates to its force intercept and must be evaluated by the method required by the applicable specification.

Datum and direction

After the defined preload or seating condition, STM-X records a fixed reference. Extension test displacement is the nonnegative magnitude from that reference. The mode identifies the physical direction, while Machine Z remains an absolute coordinate.

Choose the analysis region

Avoid calculating rate from the transition where coils are only beginning to separate. Define a stable region on the open-coil part of the curve, remain within the approved load and travel envelopes, and use the same interval for each sample in the batch.

Review the return

The return trace helps reveal permanent set, hook movement, fixture slip and friction. If the spring does not return as expected, inspect both the component and the holding method before assigning the difference to the material.

Relevant standard family

ISO 22705-2:2023 covers measurement and test methods for general characteristics of cold-formed cylindrical helical extension springs made from round wire, excluding dynamic testing. Use the licensed document for normative details.