Flat spring design FEA
Flat spring design FEA
(OP)
Hi all,
I am new to the forum and I have a problem to solve in FEA.
I am starting using it since last week and my hand calculation is not giving the same answer with FEA.
Here is the spring desing (I attached it) and this has a preload force of 16 LBS at a preload length of .16". What will be the constant k of the spring and the free length?
The main problem is to find out if my spring design works without failing. Thank you for your answer.
I am new to the forum and I have a problem to solve in FEA.
I am starting using it since last week and my hand calculation is not giving the same answer with FEA.
Here is the spring desing (I attached it) and this has a preload force of 16 LBS at a preload length of .16". What will be the constant k of the spring and the free length?
The main problem is to find out if my spring design works without failing. Thank you for your answer.





RE: Flat spring design FEA
Perhaps you can tell us how we can answer your problem without you giving us any dimensions of the spring or material and what does the preload length of 0.16" mean, does it mean 0.16" off the flattened state or 0.16" off some arc shape.
desertfox
RE: Flat spring design FEA
RE: Flat spring design FEA
RE: Flat spring design FEA
I considered B-B section as flat in my calc, just to get a close answer. In FEA, I considered it as an unsupported beam, I did not treat this as a spring. In fact I do not know how should I treat it in FEA, as I am a new user and I only use this ones in a while since we have just 1 license of it at work. I really think that I should rely on testing it, but I would first have to get close answers from both hand calc and FEA, which I did not get. Here is another picture of where is used. This sits on the curved base with two pins and all assembly sits inside a tube. There are three of these assemblies around inside the tube. The distance between the tube ID and the curved base is .16", so this will be the preload length of the spring. In this position spring will have to have 16 LBS of force, so this is the preload force of spring. I got a k=56LBS/in as a spring rate and a free length of .443" but I am not sure about my answers.
RE: Flat spring design FEA
RE: Flat spring design FEA
Thanks for the drawing.
Why is the beam curved across the the .825" width ie 1" rad on section B-B? if it was flat I have a method that would get you a stiffness figure assuming its not over stressed.
Alternatively if I can assume its flat then I'll have a look and see if I can get a ballpark figure.
Let me know if you want to go down this road.
Regards
desertfox
RE: Flat spring design FEA
RE: Flat spring design FEA
RE: Flat spring design FEA
See the picture
RE: Flat spring design FEA
I am looking at your spring now I'll post what I find later,
thanks for the information.
desertfox
RE: Flat spring design FEA
I need to check my figures again, but if they are correct your spring is far to weak, with a 16lb load according to my figures it will be flattened.
What is the f symbol in your formula for force is it allowable stress?
desertfox
RE: Flat spring design FEA
RE: Flat spring design FEA
RE: Flat spring design FEA
RE: Flat spring design FEA
I can't get the value of F that you get using your formula.
I started to model the spring with two ends on the ground and applying a load at the centre of the arch.
If you can post your workings out it may help, as soon as I re-check my figures I'll post again.
desertfox
RE: Flat spring design FEA
A=2.537in^2 (area of the curved spring)--from model
P=80PSI (pressure inside the tube)--> F (force)=202.96 lbf--> Load = 21 LB. That is the minium force or load that I need at the preload length (.16"). At this point the spring has to support a weight of 880LB (they are 96 springs, so for each one it will be another 9LB)
Preload length will be the entire space that I have between tubes: .16"
So the total load will be 21+9=30 LB for each spring. That means the spring will be compressed from the preload length another amount (I don't know how much, but I don't want this to fail or to be flattened). With this geometry, what will be the spring rate and the stroke? Will this going to work? FEA shows no movement at all. Is it too stiff? I am also in the green area.
RE: Flat spring design FEA
RE: Flat spring design FEA
Well its bad news, I checked my figures and your spring will go flat, its far to weak.
If you consider a simply supported flat beam of 2.25" long, 0.016" thick and 0.825" wide and apply a 1 lbf at centre and calculate the deflection it comes to 1.75".
I don't understand your comments:- It doesn't have to support any load. The force is 202.96lbf, so it will be 21 lb. I need to apply the force at the center of the arch.
Can you post your workings out so I can see exactly what you have done.
Regards
desertfox
RE: Flat spring design FEA
As soon I got a chance I can post pictures with FEA, but now I am at home.
Thank you.
RE: Flat spring design FEA
I found an error in my calculations to, however I have corrected this now and I can tell you that your spring stiffness is 37 lbf/inch which means that to get 0.16" deflection you need only 5.92 lbf which suggests your spring is to soft.
I checked my figures in mathcad using calculus and strain energy.
When I did the mathcad calculation, I didn't allow for the cutouts to go round the tubes so the spring came out marginally stiffer at 40 lbf/inch but in the graphical intergration I allowed for the cutouts, anyway the figures from both methods are within 10% when compared.
More importantly I used 65000lbs/in for the tensile stress of your steel and according to my calculations if the load at the centre of the arch exceeds 2 lbf your stress will be over the tensile strength.
I haven't allowed for the curvature across the spring width as we agreed to analsye it as a flat, neither have I allowed for any stress raisers which may occur at the bends or the cutouts.
Another piece of advice I used 0.016" thickness material for the calculation which was the average of the sizes you have given, however if you use 0,017" thick and compare it with 0.015" thick the stiffness goes up by a cubed law and not surprisingly a spring made from 0.017" is almost 1.5 times stiffer than one made from 0.015".
Your major problem with this spring is stress so its back to the drawing board.
regards
desertfox
RE: Flat spring design FEA
Then I plotted the graphic of displacements for several different heights. Idon't have the space here to explain what I did, but I think this time will be OK. I still don't know how you concluded that the spring is too soft, because I got it too stiff and I have to scoop it out and modify the geometry many times to get the rate down to 100lb/in which is close to the goal of 70 lb/in which i initially had. Thank you for yoour posting. You were the only one that took the time to think about my spring. It wasn't a simple analysis.
RE: Flat spring design FEA
I have uploaded my calculations assuming there is no curvature across the width of the spring,also I have ignored the cutouts in the spring where the tubes sit and that allowed for a constant "I" value across the spring.
The spring being symmetrical you only need to analyse one half of the spring which is what I have done.
The fact I have not allowed for the cutouts means the spring should be stiffer than with the cutouts but it still only comes to around 40 lbs/in for stiffness.
I have used metric units and converted the stiffness at the end,you will also find I have omitted the results of the intergration, but I have stated what needs to be intergrated and I checked them through Mathcad and you could probably do the same.
My graphical intergration method including the cutouts results in a stiffness of 37 lb/in stiffness which is in the same ball park, so unless I have made the same mistake in each method I am convinced my calculations are correct.
Regards
desertfox
RE: Flat spring design FEA
In addition the bending stress on this spring can be obtained using:-
stress = M*y/I
if you consider the stress to be the tensile strength of the material you will see if you put more than 2 lbf on the centre of the arch you will exceed your tensile strength.
I don't know what changes you have made to your spring since you posted the original drawing, however I hope you look at my posts before you go ahead with your design if only to serve as a reference.
regards
desertfox
RE: Flat spring design FEA
If you can please feed back if you could and let me now if you have solved your problem or if you have any questions about my calculations.
cheers
desertfox
RE: Flat spring design FEA
Thank you for your postings, very valuable. I concludes 36 lbs/in without cutouts myself by using some simple supported spring calculations and some beam idealizations. I think I have to start a new thread about what will be the bounderies conditions in Pro Mechanica for this application. Maybe I did not put them right.
Thank you again and I attached my conclusion apart from FEA
RE: Flat spring design FEA
Thanks for the feedback, I have looked at your conclusions which as you say closely reflect my findings.
So you believe your constraints within the original model may have given you the false results.
I don't know what your latest spring looks like or how different it is from the original posted, however if you post your latest one I'll look at it for you.
regards
desertfox
RE: Flat spring design FEA
RE: Flat spring design FEA
RE: Flat spring design FEA
I can see you scalloped out the spring centre but I can't see any dimensions for those centre scallops?
I agree there is no way that spring is 100lb/in on spring rate.
Now if you need a preload of say 20lbf at 0.16 in deflection, then you need a stiffness of 125lbf/in.
So my advice would be to go back to your original spring and increase the thickness and do the calculations again and increase thickness till we get to the rate you need.
desertfox
RE: Flat spring design FEA
There is also some technical information on the about power springs tab.