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This calculator will determine the spring rate constant related to a compression spring from basic geometry and material data input. Default units are shownin inches, etc however SI (metric) can be used.
This equation will determine (estimate) the
Spring Rate (k) of a compression spring. Spring rate is best
determined by test and measurement techniques.
Open Compression Spring Constant (k) Calculator
(This calculator requires a java  enabled browser)
Equation:
k = Gd^{4}/[8nD^{3}]
Where:
k = constant, pounds of load per inch of deflection
G = modulus of rigidity
of spring material, pounds per square inch
d = wire diameter, inches
n = number of active coils, which is the number of coils subjected to flexure (always less than the total number of coils)
D = mean coil diameter, inches = Outer Diameter  Wire Diameter
G is calculated from the spring materials Elastic moduls and Poisson Ratio n
G = E/(2(1 + v)
The pitch of the spring coil can be found by
C_{p} = L_{f} / n_{c}
Where:
L_{f }= Uncompressed length of spring
n_{c} = Number of coils
The angle of the spring coil realtive to the mounting base is found:
f = atan [
C_{p} / p D ]
Compression Spring Design Suggestions
