Dead Coils

Dead coils are the inactive coils at each end of a torsion spring that are gripped and immobilized by the spring's end hardware - the winding cone on the winding end and the stationary cone on the anchor end. These coils are clamped in place by set screws driven into the cone body. Because they cannot flex or rotate relative to the shaft, they store no energy and generate no torque. All mechanical work is done by the spring's active coils, which are the turns between the two dead-coil sections.

A standard residential torsion spring has roughly 2 dead coils per end, so 4 dead coils total on a typical spring. This can vary by manufacturer. The dead coil count matters because when measuring the physical length of a spring for replacement, the total coil count includes dead coils, but when calculating the spring's torque output (IPPT, or inch-pounds per turn), only the active coils are used.

Why dead coils matter for sizing:

If an installer measures a spring and counts the total coils without subtracting the dead coils, the active coil count used in the IPPT calculation will be wrong, leading to a spring that is slightly over- or under-powered for the door. On a standard spring with 30 total coils and 4 dead coils, the active coil count is 26. Using 30 in the formula overstates the IPPT by more than 10 percent.

Dead coils also matter when diagnosing spring failure. A broken torsion spring almost always fractures in the active coil section rather than at a dead coil end, because active coils flex through the full winding and unwinding cycle. A fracture at or near a dead coil end suggests a manufacturing defect or a cone set-screw that was over-tightened and created a stress concentration.

The dead coil concept applies to both residential torsion springs and commercial torsion springs. Extension springs have a similar concept at their hooks, but the terminology is less commonly applied there.

Want to put numbers to this? Use the interactive torsion spring winding and ippt estimator below, or open the full torsion spring winding and ippt estimator with examples and notes.