FEM analysis of a spring

[gfraulini]

Hi all,

I've designed a spring and I want to see if the value of tensions are, more or less, the same I've calculated with the formulas.

The matetial is "springflex" with

E=206000 MPa

G=81500 MPa

Su=2270 MPa

d=0.9mm

In my calculation, the spring is verified.

Anyway, the results that Mechanica gives me (I've used WF5 for the FEM simulation) provides tensions much much high...

I've done two tipe of analysis:

1) I've imposed the displacement (35 mm)

2) I've imposed the force (25 N)

but, for me, the results are incorrect but I don't know why.

I've tryed "linear" and "non-linear" analisys for with large deformations.

Greetings

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[SylvainA.]

I have found a stiffness of 2N/mm with the force imposed analysis (displacement of 5,94mm for a load of 11,77N), it doesn't seem wrong...

Note that in your analysis the load isn't 25N, it's 11,77N.

[gfraulini]

You're right.
I should have put 25.938 N as force.
But, anyway, what can you tell me about the tension over the wire?

I attach results of Von Mises Stress in MPa.

[SylvainA.]

At first sight, it looks OK.

Why did you think it was incorrect ?

[gfraulini]

Because about 2000 Mpa internally, and about 1700 MPa externally on the wire, are too high.

I attach a photo of a capture of my MathCad sheet where I've done the calculus.

II was expecting tensions near 700 MPa.

[SylvainA.]

"tensione tangenziale" means shear stress and the above results represent VM stress, right ?

Did you try to look at maximum shear stress in Mechanica ?

[gfraulini]

Ok ok...calm...

I don't know why but I tried to do the analysis at home and it gave me 2000 MPa of tension (VM)
Now I've done the same analysis at work and it gives me about the same results I've found by the calculus...

I did not see the "maximum shear stress", but only the VM because, on the wire, the shear due to torsion is the major one and mediate with the VM method should give the same results of "maximum shear stress"

But as you can see in the picture, there is again a discrepancy of about 200 MPa

[SylvainA. · Accepted solution]

Don't worry, I'm calm

I just want to help you investigate your issue, I'm sorry if my message seemed to be unfriendly.

On your results comparison one can clearly see that the maximum shear stress is between 460 and 690MPa when the VM stress is comprised between 800 and 1200MPa. That's why, regarding the shear stress you were expecting, I was proposing you to look at the shear stress.

[346gnu]

The VM stress in pure shear is :

sigma_vm = sqrt(3) x Tau

lower end of your numbers : sigma_vm = sqrt(3) x 460 = 797MPa

upper end of your numbers : sigma_vm = sqrt(3) x 690 = 1195MPa

Seems about right

[gfraulini]

You are right!
There is a factor of "sqrt(3) " between those two results...

Thanks at all

[gfraulini]

I'm sorry but "calm calm" was told ironically because was ironic the situation that on different pc I've had two different results with the same simulation...

It was like "calm calm guys I've found the issue..."