Parallel combination of Extension Springs

[laviadahc]

Hi all,
I have tested a set of Extension Helical Springs in parallel configuration obtaining that the equivalent stiffness equation is quite similar to F0+nKx where:
F0 Initial tension
n number of springs (all with the same F0 and K)
K stiffness of a single spring
x elongation

while I was waiting the equation n(F0+Kx)
Could anybody explain me the cause of this phenomena?

 

[Biggadike]

Unless I'm misunderstanding your description, it sounds like you are simply measuring the situation you are creating. To explain:

I have one extension spring whos rate is 1Nmm. I load it with 10N by extending it 10mm. I then place another spring beside it and, pulling on both, I achieve the same extension with 20N ( 2 x 10 x 1Nmm) As I add springs in parallel, the apparent spring rate will be total combined rate of all the springs, pre-load force with be multiplied accordingly.
I'd be very supprised if anything else than this occurred unless your springs differ significantly from eachother.
You might find it easier to measure the force and extension at equal extension intervals and plot these, ignoring the pre-load. That should give a straight line but for the very beginning and the point of overextension.