d - wire diameter
D - mean diameter, the diameter of the spring as measured at the wire centerline
ID - inside diameter, D-d
OD - outside diameter, D+d
Na - number of active coils
Nt - total coils, active coils plus any inactive coils. For a spring with closed ends, Nt=Na+2
FL - free length, the spring length with no load applied
P, F - load or force, the force exerted by the spring under a given deflection
l - instantaneous spring length, the spring length corresponding to a given applied load
x, s - instantaneous deflection, the amount the spring is compressed from free length to length l. x=FL-l
k - spring rate, the derivative of the load-deflection curve. k=P/x=(P2-P1)/(l1-l2)=(P2-P1)/(x2-x1)
C - spring index, the ratio of the mean diameter to the wire diameter. C=D/d