Sources and scope for this page: Industry convention — no single governing standard (conventional usage, definitions vary between sources), covering the definition of spring index. No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
Definition
Spring index governs whether a spring is manufacturable and how it behaves. Below about 4 the coils are too tight to wind and stress concentrates severely on the inside of the coil. Above about 12 the spring is floppy and prone to buckling. Six to ten is the comfortable range.
It appears in the Wahl correction factor, which accounts for direct shear and for the stress concentration at the inside of the coil — the point where a spring actually fails.
Common misuse
Designing a spring with a very low index for a high rate. It cannot be wound reliably and will fail at the inside of the coil.