Fracture on torsion spring 1

[Mox-TT]

Hi
I have got problem that there was breakage on torsion spring. It occurred on end, but in previous REL test it occurred on squeezed part.
See attachment. I am a little confused about these two failures.
I want to improve design to distribute stress on end of spring. And considering to add two POM tube on end.
Can anyone provide some suggestions?

Thanks in advance!

 

[israelkk]

This is the worst configuration to use a torsion spring. In torsion spring the wire should work in bending. The two edges of the spring work in combined bending and "torsion" which makes the wire much weaker.

Can you provide exact detail of the spring? A blueprint contains dimensions and loading torque and deflection will be very helpful to see is this failure is an accident or could be foreseen. A detailed dimensions where the tangs are loaded with reference to the spring main body

Is this failure happened at first loading or after repeated loadings?
It is difficult from the pictures to decide if this is a fatigue failure or not.

https://sites.google.com/site/israelkk/

 

[btrueblood]

What Israelkk said. One of the more common torsion springs you see are roll-up type garage door springs. The spring ends are loaded by what is essentially a tapered plug with a "thread" groove to lay the spring wire in, see photo:

https://www.google.com/search?q=gar...hUN7WMKHXAXCnUQ_AUIBygC#imgrc=bgTHuYfb7yfAQM:

Very rarely do garage door springs fail at the connection, more commonly they fail near the center (due to sag?)

 

[EdStainless]

IF you need to use tab ends they should not go into holes, they should bear against a surface that matched the curvature.
That said this is a poor design.
You would be better off with a hook on the end that went over a pin.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[WKTaylor]

Mark Yao...

IsraelKK, Btrueblood and RB1957 Hit nail on head.

1. A few potential Issues that MAY/MAY NOT factor-in

Ensure spring material is high quality.
Spring surface finish is important: should be very fine [~polished and defect free] before/after forming.
Deep shot-peening after SR and before plating may be useful for fatigue life
Springs should be stress-relieved after coiling/forming. SR is based on spec requirements, alloy/temper, etc.
Cadmium, Zinc, zinc-Nickel, etc plating [and sometimes CRES passivation] requires hydrogen embrittlement relief.

2. May find following info useful in-the-long-run

SAE HS-795 Manual on Design and Application of Helical and Spiral Springs

SAE J1122 Helical Springs Specification Checklists

Spring Manufacturer's Institute
Handbook of Spring Design
Encyclopedia of Spring Design

http://www.smihq.org/public/pstorep.html#book2

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[Tmoose]

A closer look at the broken off piece of spring and the hole it goes into for damage or wear would be interesting. Deburring the edge of the hole or providing a thin tough plastic sleeve or lubricating the chafe points might help ease the situation.