Allowable stresses of an extension spring 4

[Jh0an1]

Hi, I'm a mechanical engineer from Venezuela. I'm helping to my friend with his seats factory. He need to improve an extension spring.

The current spring has the following characteristics:
- Material: EN 10270-1 Grade SM
- Tensile Strength, Rm: 1690 N/mm² minimum.
- Wire diameter, d: 2,50 mm
- Coils number, Nt: 27 ½
- Free length, L0: 114,50 mm
- Outside diameter, OD: 22,50 mm
- Initial tension, F0: 27 N
- Assembly Length, L1: 147 mm
- Maximum Operation Length, L2: 200 mm

The springs are manufactured in an automatic coiler machine, with secondary forming of one hook.

This spring has permanent deformation (excesive stresses). Besides, the spring can not to move totally the seat from the maximum operation length.

Then, he called me and he explained me the trouble. We could to find an offering of wire 3,20 mm EN 10270-1 SH, indicated for static applications with high stresses. I am planning to suggest him the following design:

- Material: EN 10270-1 Grade SH
- Tensile Strength, Rm: 1820 N/mm² minimum.
- Wire diameter, d: 3,20 mm
- Coils number, Nt: 21
- Free length, L0: 120 mm
- Outside diameter, OD: 25,45 mm (maximum value allowable by assembly restrictions)
- Initial tension, F0: 83,2 N
- Assembly Length, L1: 147 mm
- Maximum Operation Length, L2: 200 mm

But I see that the coil torsion stress (body) is so high: 929 N/mm² (49% of Rm).

Here is my question: SMI Springs Design Handbook suggests a maximum coil torsion stress of 819 N/mm². On the other hand, the european standard EN 13906-2 specifies the same value for maximum coil torsion stress (819 N/mm²). But both documents have different rules for to calculate the real stress when the application is static: SMI method increases the torsion stress with Wahl factor (to 929 N/mm², exceeding the torsion allowable stress), while EN 13906-2 uses only the value 8*D*F/(pi*d³) (i.e., without curvature correction). With the european rule, the real stress is 765 MPa, within the allowable range.

With EN 13906-2 the design would be accepted, but according SMI method, it would be rejected. (?)

We have coiled a first sample of the spring and we have to extended it to 200 mm, by 15 hours, without permanent deformation, and without force loss.

Do you think that the SMI method is overdesigned for static springs?

Best regards, and thank you very much for your help.

P.D.: I'm suggesting increasing the bending radii and reducing the last coil to avoid hooks failure.

 

[TVP]

The Wahl factor is typically used for dynamic applications, not static.

 

[MintJulep]

I can't see any spring application in an automotive seat being truly static.

 

[Jh0an1]

TVP, thank you by the help. I will use your information.

 

[Jh0an1]

MintJulep, you're OK. The analyzed spring it's not truly static. Thank you for your comment.

 

[desertfox]

Hi

The Wahl factor is a geometrical factor used to correct the shear stress due to spring curvature.

see link below

http://www.efunda.com/designstandards/springs/spring_design.cfm

I'll check EN13906 if I can get my hands on a copy.

 

[desertfox]

see clause 10.2 En13906-2 the max stress is limited to 0.45 X Rm of material

 

[israelkk]

Jh0an1

What do you mean by the term "Initial tension"? Is it the force to start deflection or the force at 147 mm?
What is the desired force at loaded length of 147 mm?
What is the desired force at loaded length of 200 mm?
It seams that the two springs doesn't give the same forces at 147 and 200 mm length therefore, how the second spring design can replace the first spring design?

 

[desertfox]

Hi jhOan1

I have looked at the above specification EN13906-2 and agree that it for static springs it ignores the stress due to spring curvature, its many years since I did any spring design though I did quite a lot and always included the Wahl factor.
I looked on the internet and found this paper which says the stress due to curvature can be ignored if the spring is static because the stress will be relieved by yielding on the first operation. (see clause 10.2 of link below)

http://faculty.ksu.edu.sa/essam/Documents/ME301chapter10.pdf

Now moving on I ran your spring and that of your friends through EN13906-2 and according to my calculations both springs for static application should be okay.
The spring your colleague specified is actually not as highly stressed as that you proposed so I wonder what is wrong, it could be my calculations (very possible) or the information on the spring operating parameters are wrong perhaps you could check your colleague's spring to the European specification.
Two things I wasn't certain about was how you obtained the initial tension force ie the 83.2N and what the number of active coils was for both springs, in the latter case I just used the total number of coils, so maybe you can enlighten us to how you obtained those figures.

desertfox

 

[Jh0an1]

Hi, israelkk,

Initial tension: force to begin the clearance between the coils.

Force to 147 mm: 150 N minimum

Force to 200 mm: 450 N around(according to equations of forces equilibrium applied by me)

The two springs doesn't give the same forces at 147 and 200 mm length because the first spring can not to move totally the seat from the maximum operation length. We need more force to 200 mm

Thank you very much for your help.

Jh0an1

 

[Jh0an1]

Hi, desertfox,

The initial tension of 83,2 N was obtained after the first sample, by experimental measurement. I had to estimated initially this characteristic in 32.2 N (the value suggested by SMI 1991 divided by 2 to consider the stress relieving). The springs of this automatic coiler machine have a high initial tension!

I have run the first spring with this web tool (EN 13906-2):

http://www.graniteng.com/tool_eng.php?subcontext=extension_spring

The result: the spring can to deflect to 81,7 mm (static) (the design is rejected according to this method, because maximum deflection = 85,5 mm). I will check this result by hand tomorrow.

With SMI method, the spring can to deflect to 70,7 mm (the design is rejected according to this method too).

The first spring (of my friend) had two problems: the force @ 200 mm is not enough and the spring has permanent deformation.

In my calculations: active coils = total coils. I think this is OK for extension springs. What do you think about this issue?

Thank you very much.

Jh0an1

 

[desertfox]

Hi JOah1

I couldn't access the calculator you posted but I ran the numbers for your friends spring by hand and still can't get the spring over stressed.
Using all the information above and the intial tension of 27N as you posted I get spring stress of 591.87Mpa, the allowable being 760Mpa
(0.45 * 1690Mpa) without correction.
If I take account of curvature in the spring the stress rises to 693.9Mpa and still within the allowable.
Can't scan my calculation in at present but will try if required over the weekend.

desertfox

 

[israelkk]

Jh0an1

Using the following requirements:

Force to 147 mm: 150 N minimum

Force to 200 mm: 450 N

Max. outside diameter 25.4 mm

Only static use springs may be designed. For cyclic operation use you will need to lower loads and/or allow larger OD.