The spring constant (c) is the force that a spring exerts per mm compression (or extension for an extension spring); this is directly proportional in the case of cylindrical springs (see diagram).
The maximum force that the spring exerts is thus Fn. If the spring has a specified pretension this is usually called F1. Fn is decisive for the design of the spring.
Extension springs also have a defined preload. This is the force needed to pull the coils apart. The preload (Fo) is 5% to 15% of the Fn, depending upon the diameter of the spring.
τ w = torsional stress ( N/mm² )
Dm = diameter of spring centre-to-centre
F = spring force (N)
G = shear modulus (N/mm²)
= diameter of spring wire (mm)
nw = number of active coils
f = spring travel (mm)
c = spring constant (N/mm)