U Clip Stress Analysis

[dteich]

We designed a U clip 0.87" long, 0.62" wide and 0.62 high, made of 0.060" spring steel. The force the clip needs to work under (and yield to) is up to 1,000 lbs. at its tips (the two tips get compressed together half way with a 0.25" metal in the middle of the U preventing it from overtravel).

We need help simulating this so we know how to change the material to prevent it from going plastic (which it does now). We clearly exceed the allowable strain.

Thanks from Dori at: eng-tips@rmtai.com

 

[israelkk]

The force is way beyond the capability of this U clip. The stress in the clip is 10 times this spring can hold. I would say 100lbf is more likely but it depends on the type of use of this clip (dynamic, static etc.)

 

[JStephen]

The geometry you have together with the deflection will fix the strain that you must be able to develope. Assuming the clip is steel, that fixes your modulus of elasticity. You can calculate a working stress from that, and if no steel is available to meet that stress, then you need to rework your geometry. Maybe you need a spring shaped more like a clothespin spring or something.

I would think you could get a reasonable (not perfect) analysis of the spring just by treating it as 3 beams attached together.

I'm assuming that if it is spring steel, it is properly hardened. In that case, a lot of springs will break, rather than bend plastically. If the load is 10 times what it will take, that won't matter, tho.

 

[corus]

I'd use curved beam theory as a guide, but you can always use finite elements with free software

corus

 

[israelkk]

corus

Which free software you recommend?

 

[desertfox]

Hi dteich


split the u clip in half at the base and assume clip at the split line is built in support. Then use strain energy in bending to find deflection of one half the clip.I asumed a 2lb load at the top of the clip and using strain energy theory came out with a deflection of 0.00048" for the 2lb load. I ignored any strain energy due to direct tensile loading on the base leg of the 'u' clip.
Now and providing I have interpreted your dimensions correctly each leg would need to deflect by 0.185" which is
excessive compared to the dimensions of the clip. However I also calculate using strain energy theory that the deflection of the clip would be 0.0603" with a load of 500lb
on one half the clip and therefore a 1000lb load ie 500lb on each leg would only deflect the u clip 0.12" in total.
I don't know what grade of steel your using but lets say your using a moderate strengh spring steel with a rockwell hardness of say 40 and an approximate tensile strength of 170000psi if we take say 80% of this as a safe limit for the spring, then the spring stress should not exceed 136000psi unfortunatly just using the normal bending formula
with 500lb on each leg of your spring the stress at the base of the u clip is 1187739.7318psi a factor of almost ten above the safe limit. Working back the other way using 136000psi your load on the u clip should not exceed about 57lbs force on each leg and this would equate (using strain energy theory) to a deflection of 0.0137" per leg or 0.0275"
total for the u clip. I may be able to help further with your problem if you can give details of the material and also if you can load the spring in steps and try to measure at what deflection the u clip leg will not return to its original position ie:- if you deflect one leg by 0.0137" has it gone plastic.

regards desertfox

 

[dteich]

Hi desertfox,

Thanks for the excellent analysis. It is clear to me, at long last, that the clip just can not offer the forces that I need.

I am looking at a torsion solution. Can you recommend any reasonable (free is very reasonable) software/calculator that provide the angular rotation of a beam under offset radial forces?

 

[israelkk]

desertfox

I believe you have an error assuming that on each arm of the U-clip the force is 500lb. It is 1000lb as you can see in the following scematic sketch

V 1000lb
V
V
----------
I
I
I
----------
^
^
^ 1000lb

 

[desertfox]

Hi israelkk

I read the post as the clip withstands a force of upto 1000lb which would be 500lb on each leg, if it was a 1000lb
on each leg then the post would read the clip withstands a force of upto 2000lb. However as the clip cannot stand anything like a 1000lb or even 500lb it seems irrelevant.

dteich

I know of no free software for your application have you tried a search.
If your going to design a flat beam like a leaf spring then there is a good graphical method for doing so in a book titled:- "spring design" author W.R.Berry.

regards desertfox

 

[GBor]

dteich,

Many FEA packages offer limited time "trial" versions or limited node packages. NENastran has a 300 node limited version which would probably be plenty for this application. COSMOS and SolidWorks will give you limited time demo...30 day or 90 day...

Garland E. Borowski, PE

 

[corus]

For free FE pacakges for simple jobs I've used JLAnalyser, FEAPPv with geompack++, and Beam!2D. The latter will probably do to model the curved section of the clip, though that won't calculate the stress intensity at the radius. With Beam!2D It's a simple matter to change the beam properties to get the size of clip you need to satisfy the stress limits you'll impose (about 2/3 yield).

corus

 

[dteich]

Thanks to all so much. Your help and support are greatly appreciated. Dori

 

[Cockroach]

This is a definite Curved Beam application, and one that is very well documented.

For an exact example of this U clip, see Advanced Mechanics of Materials, Boresi & SideBottom. The discussion and development of stress analysis will be more than sufficient for your application.

Good luck with it.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada