Initial Tension Extension Spring Calculator
Close-wound extension springs hold their coils shut with a built-in initial tension Fi, so force is F = Fi + k*x and nothing extends until the load beats Fi. Use the working extension spring calculator to include it properly.
Force with initial tension
An extension spring does not start at zero force. The winding process locks in an initial tension that must be exceeded before the coils separate:
F = Fi + k*x
Where Fi comes from
Initial tension is set by the winding tension and the spring index; it is commonly held between about 5% and 25% of the maximum working load. The working calculator accepts a measured or specified Fi directly and reports force at your extension.
Hooks govern the design
Most extension springs fail at the hook, not the body. After body force checks, run the hook and end-loop stress screen.
Frequently asked questions
How do I measure initial tension?
Pull the spring at two extensions and record both loads. The rate is k = (F2 - F1)/(x2 - x1), and the initial tension is Fi = F1 - k*x1. The spring constant calculator does this two-point math.
Can initial tension be zero?
Only if the spring is wound with space between coils (an open-wound extension spring), which is uncommon. Standard close-wound extension springs always carry some Fi.
Related pages
- Extension spring calculator - working rate, index and force calculator with initial tension.
- Hook stress calculator - first-pass hook and end-loop stress screen.
- Spring rate calculator - body-coil rate from wire, diameter and material.
- Spring constant calculator - measure k and Fi from two load points.