Extension spring and permanent damage Is there a way to calculate how far an extension spring can be extended before it suffers permanent damage? There are some online calculators, but how are they done? This calculator is the most popular one out there, but is it reliable?
I used it and entered the following:


*

*Diameter of spring wire, d:  0.500 mm   

*Outer diameter of spring, Douter :   20.000 mm    

*Length Inside Hook
(Free length) , Lfree    :   20.000 mm

*Material: A313


but it gives 1,292mm as the 'maximum safe travel'. That's over 60x the free length, which sounds a bit unreasonable. I can't quite imagine a steel spring could be extended 60x without suffering permanent damage. Is my intuition or the calculator correct?
Thanks
 A: Did you see section 4 of the calculator "warnings"? 

"Your index is too large" (index = ratio of diameter of spring to outer diameter. You have a ratio of 40 - a very thin wire wound on a very large diameter. And since length inside hook needs to be decreased by the diameter in order to get the length of the "actual spring", you basically have "almost no spring" and the calculations just won't work. That is the meaning of the second warning (in bold):

Hook Length 1 + Hook Length 2 is greater than or equal to your length inside hook.

Calculators have limits - assumptions. Your geometry violates the assumptions. The calculator ought not to give an answer at all - instead it gives a bad answer. That is not great design on their part, but it does at least give you warnings. Heed them. What is the geometry you are really trying to describe?
I actually got a negative answer... can't get the number you got. But either way - the problem is with your description of the geometry:

UPDATE
Following your clarifications (no hook spring, and using a thicker wire) you were still concerned about the result (89 mm extension). Let's see how reasonable that answer is.
A spring with 20 mm length and 1.5 mm diameter wire has 20/1.5 = 13.3 turns, so an "uncoiled length" of $2\pi D L / d = 1675\; mm$. The twist angle will be approximately 89/1675=0.05 rad. That is about 1 degree - not an unreasonable amount of twist for a 1.5 mm wire that's over one meter long.
I found a nice site with equations for coils . You could use that to confirm the results from your calculator. In general, it is always a good idea to be skeptical of information you get "from the internet". Including this answer...
