Basics of theory of coil springs
d – rod diameter, mm.
D1 – outer diameter of spring, mm.
D – average diameter of spring, mm.
n – number of working spring coils.
n1 – full number of spring coils.
l – length of spring in released condition, mm.
t – spring pitch in released condition, mm.
с – spring stiffness, N/mm.
G – modulus of shearing (for spring steel 78500 MPa).
c – spring stiffness is the energy required for spring deformation.
c = Gd4/8D3n, N/mm
The wound cylindrical spring with constant pitch has linear stiffness, if the spring has a cylindrical shape. For instance, if it has a conical shape then a load-deformation curve is made by the formula:
R – internal radius of projection of the largest coil.
r – internal radius of projection of the smallest coil.