spring connectors, how to use

[LARA-Engineering]

Hello,

Here you see a pneumatic cylinder with spring support.
In the middle you see a centering ring to prevent the springs to bend to much.
But in fact, these 4 springs may considered as 1 spring, without that centring ring in the middle.

Now, I need to substitute the geometry of the 4 springs by 1 springcontact with certain stiffness (mm/N) between piston bottom and cylinder front.
but I notice, Nastran does not have these springcontacts like Autodesk Stress Analysis do.
Now I wonder, can I use spring-connectors for these solution?
If yes, can I fill in the spring rate (mm/N) somewhere in the table?

How does it work?
I just miss the basic layout in the help for using these connectors.
Can’t find any good thorough explanation how to use these springconnectors, except this URL.
http://help.autodesk.com/view/NINCAD/2018//ENU/?guid=GUID-11D9471C-D976-4B2A-A980-199723DAEBF3
But that is not good enough to learn how to work with.
So I tried some, but whatever I click to select parts or faces, I can’t get these spring-connectors into the analysis.

[John_Holtz · Accepted solution]

Hi Freddy,

The spring connector goes between two vertices (points) in the model. In your case where you want the springs to act over a face, you may want to create a rigid connected on each face (or edge), where ever you want the load of the spring to be transmitted into the model. Then you would define the spring connector between the center points of the two rigid connectors.

The stiffness of the spring connectors is explained in this article: How to enter spring stiffness in Autodesk Nastran In-CAD

Let us know if you have any other questions.

[LARA-Engineering]

Thanks John,
I'll try to do as you explained.
I'll let you know soon.

[LARA-Engineering]

Hi John,

I followed ur advice and got some results.

I understand, when using spring-connectors, always first make rigids.

And release the Translation of the rigid in the springcompression/extension direction, in my case that is X so K2 . . . is that correct?

As I said, i got some results. only 1,166 mm translation of the piston in the pulling direction against the spring.

But the piston should move at least 200 mm, for I have a pulling force=10kN on the rod and spring stiffness K2=50N/mm

The constraint between piston and cylinderwall and between rod and cylinder fronthole are all separation type, so sliding is possible.

I got max 632 MPa, so there is something happening, but I dont see logical movements.

Or may it be that the spring compression distance and movements not really be displayed.

[John_Holtz · Accepted solution]

Hi Freddy,

Just to make sure we understand what you are doing, I have created a schematic of the arrangement as I envision it. I do not know what loads you are applying, but I think they are not directly related to the questions, so you can ignore them. As far as the spring, the points to discuss are as follows:

1. One rigid link is applied to the piston. This creates "spokes" or lines to a common point. (This common point can be anywhere, but the software has an option to create it at the center if you have not already created a work point to use.)
2. Another rigid link is applied to the cap.
3. A spring is created between the rigid connectors.
4. Since the rigid connectors need to transmit the compression force of the spring, the translation direction on the rigid elements need to be checked. I think you do not want to release (uncheck, free-up) the axial translation on the rigid element.
5. The spring only needs stiffness in the axial direction. Based on the orientation, that would be the global X direction. Correct? The spring is assigned to the coordinate system "analysis model.....". So what you need to determine is which direction in the coordinate system "analysis model...." corresponds to the global X direction. Normally, the coordinates are the same, so I would think that you want a stiffness for the K1 direction (X), not K2 (Y).
6. You may want to enter a small stiffness for the other directions of the spring just to provide some stability.

[LARA-Engineering]

Wow John . .  its working. Really it does !!
And exactly the 200 mm as I already calculated by hand. Really great
I already filled in all the things you mentioned as the rigids and so on.
The only stupid thing was the K2.
This was indeed wrong and had to be K1 as you said, don’t know how I figured out to fill in K2

You know, this aircylinder is only a small part of a preliminary study for a quite large assignment for train brakesystems for the German public rail company Deutsche Bahn. They asked me to analyse their systems on several criteria as stress, and lifecycle. So it's all about putting this to a good end, as you’ll understand. I think that's going to succeed now. I'm happy, my customer happy, everybody happy.
Thanks a lot, this is a great step forwards in the project. Here you read a really happy engineer 

btw, do you know why the colorbar and the min/max markers disappears when I make a halfsectionview?

[LARA-Engineering]

I forgot this view, you can see the rigids and spring

[John_Holtz · Accepted solution]

Hi Freddy,

Glad the analysis is working now.

"btw, do you know why the colorbar and the min/max markers disappears when I make a halfsectionview?"

If you are using the Inventor command to slice the model, the issue with the legend/color bar is a known issue. Use the "Section View" command on the Plot dialog instead. (See Contour legend is not visible in Autodesk Nastran In-CAD.)

[LARA-Engineering]

awesome

[Anonymous]

Hello and sorry for bothering. I am trying to perform a similar analysis for a drone building team at Universidad Politecnica de Valencia. While I have followed the steps shown in this thread, i have not been able to obtain proper results. I was wondering if there is something wrong with analysis i have been performing that has not been detected. If you could find some time to check it I would really appreciate it. Thank you in advance.