Material & Concept thoughts on this simple spring Mechanism

[see3p0]

Heres my problem.

I need to design a small spring type device that has minimal moving parts for a cleanroom environment.

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A sketch of the spring lever mechanism

http://files.engineering.com/getfil...a6ed-46c1-b2fa-0a7dc9ffa946&file=untitled.bmp

Part one is the lever arm
Part two is the spring material that is fixed to the lever arm
-The general shape of the stainless steel spring

http://files.engineering.com/getfile.aspx?folder=a3251a41-b37b-4c5e-9c30-2f7a5f4ec572&file=B_1.jpg

parts three is a cover piece
part four is a main body that the latch will contact with
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The latch releases when a small downward force is applied to the top of the lever (part 1) as shown in my fairly rough sketch.

A force of 15 - 50 grams would suffice. I dont need a lot of force in the spring to keep the clip in the closed position

I would like to use a spring material (part 2) that will transfer this downward force and release the latch, part one will be of a ridgid shape and would ideally not flex when the downward force is applied.

The spring material (part two) would do all the flexing.

I was looking at a stainless spring steel something like, Stainless Type 302, ASTM A313.

Im not getting great results with the force to deflection of the spring (part 2).

If I use a small sized spring part it fails long before i am getting my desired results.

So my two questions are..

Is there a more suitable spring material that would deflect under small loads and not fail like the spring steel.

or maybe the shape of the mechanism i am trying to design is totally wrong.

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note: this mechanism must be made from metal and kept as simple as possible. I have ruled out hinges and small individual springs as they are not ideal for the clean environment that this mechanism will be required to operate in.

Any input is greatly appreciated

 

[see3p0]

Hi folks,
Thanks again for the input.
I am using the FEA program in Unigraphics to run an analysis of the spring. I hadnt set it up correctly in my previous post. I seem to be getting ok results now (but I would really rather use hand calculations). The analysis setup allows one to choose from a list of materials and does not allow the input of specific material properties.
I chose AISI 303 - cold drawn Stainless Steel.

I essentially need a piece of spring steel as shown in my initial post,

http://files.engineering.com/getfile.aspx?folder=a3251a41-b37b-4c5e-9c30-2f7a5f4ec572&file=B_1.jpg

I ideally require a radius between 5 and 6 mms, with overall dimensions as shown previously. A thickness and width that will suit the deflection of 5mm's (horizontal) or approximately 20 degrees as Zekeman worked out earlier.

I ran a simple analysis with 50 grams force
I used a 6mm radius, 14mm horizontal beam and 14mm in the vertical. I used a thickness of 1.25mm's and width of 6mm.
The model was set up as Desertfoxes sketch (PDF)

I got a displacement of 4.82mm and a stress of 434 Mpa. The only info this simple fea gives is that it wont fail (safety factor 1.7)
I just dont know how good these results are and would prefet the hand calcs to compare to, and obviously some advice of fatigue and safe stress levels.

Hi Desertfox,
Thank you again for the valuable advice and input. Regarding Materials books, I have 'Mechanics of Materials -Beer Johnson, De-Wolfe. It discusses deflection of a beam using strain energy. But does not mention a circular/ or quarter section unfortunately. What book did you reference to get the formulae would you mind me asking?
I got results that matched yours from you uploaded PDF, when I set a simple analysis up as illustrated in your sketch.
stress was 251.7 Mpa and deflection was 4.42mm.
Your results were also 251 and about 5mm deflection.

Hi Zekeman,
Once again thank you too for your great input. Yes I meant 5mm width previously, apologies.
So the spring was failing because of my flawed FEA setup and I dont think its the best program to complete these calculations.
I was running through your calculation method, could you direct me to anywhere I could find the formulae online or method. I was having a bit of trouble following it, my apologies.
What did you mean by
''@=Pi/2 this is the spring working length angle'' ??
in your first post.

''Stainless 302 data I have is 130,000 psi working stress for flat spring material''

130,000 psi = 896.3 N/mm^2
Would this mean that If I am calculating a working stress of around 66,000 or 450 N/mm^2 then the design should be within safe limits and will not fail. Presuming I am using a 302 spring steel?

I am getting values

0.2% Offset Yield Strength, of 30,000 PSI

http://www.upmet.com/302-mechanical.shtml

and 40,000 PSI Yield Strength

http://www.espimetals.com/tech/stainlesssteel.pdf

But I presume the difference is because they are not Stainless Spring Steel, or just bad numbers?

Thank you for the input Unclesyd and GerhardL, I will definitely try alter the shape if i cant get good results from this 1/4 round shape.

 

[desertfox]

Hi see3p0

Try O'Roarks formula's for stress and strain it might have curved beams in there.
I am slightly confused why your saying 130000psi for a working stress when the steel as a yield stress of 30000psi, to me you should be below 30000 if you don't want your spring to yield.
I'll be home tomorrow so I'll try to find some other references for you.

desertfox

 

[see3p0]

Hi Desert fox,

In my last post I was quoting what Zekeman said about the working stress of stainless steel 302.

''Stainless 302 data I have is 130,000 psi working stress for flat spring material''

Maybe It was a typo, the statement just confused me.

I have Roarks so will give that a look.

Thank you,

 

[zekeman]

No, not a typo.

It comes in sheets work hardened to those and higher values, typically for spring applications.

By googling I found this link.

http://www.aksteel.com/pdf/markets_products/stainless/austenitic/301_Data_Sheet.pdf

 

[see3p0]

Thank you Zekeman,

Your are wealth of Knowledge.

So a steel is hardened to have a 130,000 psi working stress, would I be correct in presuming this steel would therefore have a greater resistance to an applied bending force?
For example the L shaped spring we discussed earlier, Its shaped, and once hardened It will no longer deflect the 20 degrees with the same 0.1 lb force?

And Zekeman would you possibly have a reference to where you completed your calculations from, some online source or in a book?

Your assistance is greatly appreciated

 

[zekeman]

"For example the L shaped spring we discussed earlier, Its shaped, and once hardened It will no longer deflect the 20 degrees with the same 0.1 lb force?"


No, the elasticity is essentially constant and unaffected by the hardness.I,E. the modulus of elasticity remains

E=30,000,000 psi.

Also
"And Zekeman would you possibly have a reference to where you completed your calculations from, some online source or in a book?"

I am trying to upload the analysis I did in more detail. The fundamental equation from beam theory is used and is available in any strength of materials book. The solution is classical. I didn't use the energy approach which is genrally more powerful but not necessary here.

So here goes

 

[see3p0]

Once again Zekeman thank you, the info is much appreciated.

 

[desertfox]

Hi see3p0

Well I had a look round the site you posted and found the hardened stock suitable with a suitable stress level for your spring however I couldn't find any section below 0.009" thickness.

Here is a link to the Strain energy methods for deflection of curved or shaped beams, start at page 8.

http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT Kharagpur/Structural Analysis/pdf/m1l3.pdf

desertfox

 

[see3p0]

Hi Desertfox,

Once again thank you for the info and that nice link. You have been a great help in assisting with this design.

 

[SnTMan]

Sounds like the material aspect is well covered, about the concept:

What keeps it from going sideways when pressed?

Regards,

Mike

 

[chicopee]

To keep the latch closed, you could have a spring loaded ball at the bottom of the hook fitting in the recess. The ball would be under slight spring load and would travel downward against spring load when pressing the top of the lever.