## Flat Spring Design Analysis

## Flat Spring Design Analysis

(OP)

Hi all,

I have to design a spring and test it using Pro Mechanica Wildfire 3.

I don't know how should I approach the problem and what will be the boundary conditions. The spring sits between two curved areas. I will attach pictures. Thank you for your postings.

I have to design a spring and test it using Pro Mechanica Wildfire 3.

I don't know how should I approach the problem and what will be the boundary conditions. The spring sits between two curved areas. I will attach pictures. Thank you for your postings.

## RE: Flat Spring Design Analysis

Not precisely sure what your starting condition or input loads are, but from the following article, you can see that the flat spring problem is not a simple one & involves non-linearities, which Mechanica is not generally used to analyze.

www.iop.o

Maybe with some more information about your application, we could try to be more helpful with simplifications that could help you optimize your design.

PetieSmo

## RE: Flat Spring Design Analysis

The problem is this: This flat spring has to push in some lugs that sit on a bladder. This bladder when inflated has 80 PSI. The area of the lugs is 2.58in^2.The spring sits between the lugs and a steel tube with 2" ID. Practically the spring sits between two round surfaces. I attached pictures and the initial drawing of the spring. I also attached some hand calculations that make some idealizations: Uniform force and spring bended only on one direction. The space between the two round surfaces is smal: .16", so this will be the preload height of the spring. I calculated the force: F=P*A=80*2.58=206.4 LBF. We will use only 10% of that force: 20.64 LBF. This will be the load that I applied in Mechanica. I is a complicated problem for me, because I only used Mechanica for a very short time. I do not use it in a dayly bases, since we only have one licence at work. Any help will be appreciated. Thank you.

## RE: Flat Spring Design Analysis

## RE: Flat Spring Design Analysis

## RE: Flat Spring Design Analysis

I've uploaded a copy of the report. Sorry for the bad link.

In searching for some answers to assist you, I stumbled on your Eng-Tips thread from the Spring Engineering forum. In looking at your Mechanica constraints, I have a couple ideas:

1) First of all, I'm wondering if your units are consistent. In one post you stated: "21 LBS or force of 202.96 lbm*in/sec^2)". ProE's default unit system (lbm/in/sec) causes you to have to put in goofy numbers that are off by a factor of 386 inches/sec^2 (1g). I always convert the units system to IPS (inch/lbf/sec) - this way I can clearly input forces in LBF and get stresses out in PSI.

2) Secondly, I can not tell exactly what surface/edge you have selected at the four points that contact the bladder. However, you must be careful about constraining surfaces - if they are over-constrained, you end up preventing that surface from rotating freely, which introduces extra undesirable stiffness.

Let's start with those two things - Let us know how it goes

Pete

## RE: Flat Spring Design Analysis

About my units, I first used IPS and then I found some wired units for force, so I switch to metric N-mm-s and I applied -92N instead of 21 LB of force, which is the same thing. The animation does not look how it should, so I know my constraints are wrong. I am trying to apply the force from bellow on the two flat surfaces and fix the bow surface next, to see if I am getting the same result.

About edges selected...that's a problem.

## RE: Flat Spring Design Analysis

## RE: Flat Spring Design Analysis

Can you send a new image of your constraint set? The last image I saw (spring_fea_last.bmp), you had applied 92N to the top AND applied an enforced displacement to each foot (Dx: 1.828mm; Rz:0.8166 rad).

If these are still the constraints, I think you may be double dipping. Either the load will cause the feet to displace OR the enforced displacement of the feet will generate stresses in the spring. Applying both would be a problem.

Here is what I suggest for boundary conditions.

1) Prepare your spring for Quarter symmetry (cut away 3 of the feet, along the Front and Side datums, leaving one-quarter of your spring)

2) Apply 1/4 of your load to the top pad (92/4 = 23N)

3) Apply Symmetry constraints to the faces touching the Front and Side datums - these will take care of your X & Z displacements

4) Constrain the bottom edge of the foot in Y ONLY (note: you might get some localized stresses due to numerical error here, but they can be ignored)

5) Measure your Y displacements (Dy) at the center of the spring and use this to calculate your stiffness (92/Dy N/mm)

If you send the part file, I could give it a go as well.

Good luck!

Peter

## RE: Flat Spring Design Analysis

## RE: Flat Spring Design Analysis

Yes I did look at your report. Looking closer at the video and the details of the report, it appears that you have applied your displacement constraints to the entire upper surface - I think this is where your additional stiffness is coming from. As I mentioned in a previous post, you need to be very judicious when applying displacement constraints to surfaces, as they will generally add rotational stiffness to the model (sometimes it doesn't matter, but for flat springs this is a big deal). Each and every point on this surface is prevented from moving in any direction - this is unrealistic and drives all the deflections and stresses into the legs. This is why I suggested only applying a displacement constraint (NOT a fixed moment) to the bottom edge. Ideally, you would be using a contact interface on the top, which would restrain the surface with normal forces, but no moment; but this is another level of complexity.

You might also read the thread in this forum about Belleville Springs, originally posted by tazengr - the advice given there applies here as well.

One other thing I neglected to mention in my last post:

4b) In the definition of the static analysis, you need to enable the "Large Deflections" checkbox.

Peter

## RE: Flat Spring Design Analysis

Thank you much and I am looking forward for your foundings.

## RE: Flat Spring Design Analysis

I need stiffnes around 80-90lb/in at a preload height of .16".

## RE: Flat Spring Design Analysis

Your deflected shape looks much more believable. Based on your results, it appears you got:

92N / 6.27mm = 14.67 N/mm ~ 83.7 lbf/in

However, your vonMises stresses are ~275ksi

So, yes, I think you're getting closer to having a believable model. Once you get that, I think you can start doing the optimization for a final design.

Peter

PS I will try and run my symmetry version sometime today - just for comparison.

## RE: Flat Spring Design Analysis

Keep me posted about your foundings when you have time.

It's hard to believe that my flat spring has now 130 LB/in, but it is what Pro Mechanica tells me. Should I trust it?

What's your opinion on this?

Thank you much

## RE: Flat Spring Design Analysis

I did a couple runs in WF4 - sorry for the poor image quality in the report.

As you can see, the displacement constraints make a big difference!

Constraining the whole edge prevents rotation of the foot about the X axis giving spring constant ~65 lbf/in

Constraining only the corner against Y displacements allows that bottom edge to rotate and softens the result to about 33 lbf/in.

These two answers bound the approximate results computed by hand by desertfox. Also, the geometry has changed slightly since desertfox did his calcs. I think I've got the model in the ball park, but as you can see, the stresses are still way too high.

Good luck

Peter

## RE: Flat Spring Design Analysis

desertfox idealized the cross section of the spring (like I did in my hand calculations) and get almost the same solutions as I got, but in Pro Mechanica, the real part gave me different answers, so I knew that something was wrong with my boundary conditions, that's why I did this post. Thank you much.

## RE: Flat Spring Design Analysis

Q: "What have you done for rotational constraints in the first case (analysis 1)?:

A: I released the rotational constraints. My only constraint was Dy:Fixed (It is the same constraint type as shown for Analysis 2, I just selected an edge rather than a single point).

Q: "I never knew what is the difference between zero and fixed."

A: There is no difference between Dy=0 and Dy=Fixed.

Q: "My experience with FEA is at the beginer level so I do not know what should I expect either."

A: I would recommend running some simple analyses that you can easily check against hand calculations (e.g., flat plates, beams, bolted assembly, etc...). Try different boundary conditions and see how your errors change from theoretical. This will start to give you a sense of how to set up your problems. Analysis with FEA is much more than just running a software package - it's a skill that demands an understanding of the physics and when the software is giving you a bogus answer.

Best of luck in your endeavors,

Peter