On: Sun, 15 Aug 1999 00:56:06 EDT
Smokerbros@aol.com wrote:
> I wrote:
>
> > I can't seem to find anyone local with a spring rate tester that I
can use,
> > and neither of my Carroll Smith books seems to have the formula
for spring
> > rate. Anyone have it handy?
>
> And I forgot a couple of things... Is a spring with a pigtail
(coil diameter
> gets smaller at one end) figured any differently?
>
> What do you do with the portion that's ground flat on one end? Start
> counting coils where it ends?
>
> CHD
Charlie,
The formula is K = (G * d^4) / (8 * D^3 * N), where K is the spring rate
in lbf/in, G is the shear modulus of the spring material (11.5 million psi
for spring steel), d is the wire diameter (inches), D is the coil diameter
(to the center of the wire, also in inches) and N is the number of _active_
coils. For springs that are flattened and ground, you need to start
counting coils where they stop touching each other. If the coils get
smaller at one end, it gets complicated in a hurry. Also if the spring is
painted or coated, you need to know the thickness of the coating pretty
accurately. If your wire diameter measures 0.400" and it has 0.005" thick
paint on it, failing to account for the thickness of the paint will results
in a 10% error in the calculated spring rate, because d is raised to the
fourth power.
Don
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