Loss of free length/force in a compression spring 1

[Jh0an1]

Hi.
I have this compression spring:
d = 7.50 mm
Nt = 5.5 REF.
ID = 84 mm
L0 = 130.60 mm REF.
Material SAE 1070; Tensile strength Sut = 1410 MPa (reported by the wire supplier).
L1 = 94 mm; F1 = 422 N +/- 10 %; corrected stress: 260 MPa (18.4 % Sut)
L2 = 58 mm; F2 = 837 N +/- 10 %; corrected stress: 517 MPa (36.6 % Sut) (the mating parts constraint the deflection, avoiding to obtain a height lesser than 58 mm).

The design seems safe. However, the first sample of the spring resulted in a loss of height/force. This issue affects the functionality of the spring.

Can I guess the actual tensile strength of the wire from the loss of length? I tried with the method of Bayliss (Springs Magazine, May 1990), getting a guessed value of 1290-1300 MPa for the tensile strength (I know this method is not intended to guess tensile strength of the wire). I tried with the Wahl methods (issued by Paredes&Jones,

https://www.researchgate.net/public...Compression_Springs_Part_1_theoretical_study,

https://www.researchgate.net/public...mpression_Springs_Part_2_experimental_study).

However, I miss the alpha parameter to apply method 2.

Have I forgotten another issue regarding this spring?

Notes:
I can't test the tensile strength of the wire.
I can't return the wire to the supplier.

 

[Jh0an1]

P.D.

Lost length = 3.60 mm
We can make a new spring 3.60 mm longer. However, I would like to know the root cause of the problem.

 

[Jh0an1]

For this wire, is the maximum allowable torsional stress 40 % or 45 %? (I see different values in different handbooks).

 

[desertfox]

Hi

I haven’t go through the calculations for the spring but what you describe is the spring is setting, I encountered this a few times in my design life and I used to get the springs made longer on the advice of the spring maker and pre stressed similar to the links you posted.
I had a spring that was losing height once and yet the stresses seemed okay at the designed compressions and found the answer as to why, after the spring was assembled it had to go through testing within the valve it was mounted in and during that test the spring got compressed beyond the designed compression and bingo it losses height. In that case I redesigned the spring so that it wasn’t over stressed at the test compressed length and end of problem.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[Jh0an1]

Thanks a lot for your valuable explanation, desertfox. It's very useful to know your experience with the spring for the valve.

 

[desertfox]

You’re very welcome jh0an1

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein