How to calculate the number of active coils in a spring?

In any spring, some portion of the end coils will probably be inactive. The number of inactive coils varies depending on the spring end configuration and mating component geometry. The following equations give approximate active coil count, assuming that the spring are compressed between parallel plates.

For closed ends (ground or unground): Na ≈ Nt – 2 For open ground ends: Na ≈ Nt – 1 For open unground ends: Na ≈ Nt Na = Active coils Nt = Total coils

In practice, the number of inactive coils varies slightly as a spring is compressed. If the spring output at two operating heights is known, the number of active coils over the operating height range can be calculated using the following equation for any end configuration.

G = shear modulus of the spring material

d = wire diameter

OD = spring outside diameter

h1, h2 = spring operating heights

P1, P2 = spring force at heights h1 and h2, respectively.