Fix distance between polygon faces/nodes so that resultant displacement is equal

[St0RM33]

Hello. I am a little new to FEA, and i'm having an issue in finding a way to set up a constraint how i want to.

So here is this bracket

There are 2 pin constrains in the bottom holes and a force applied perpendicularly to the 2 holes above.
I can do a mesh and resolve the problem, no worries, but my constrains are not accurate.

My issue is that i want to couple the load faces to a fixed distance, so that any displacement occurring from the load equals for both faces. Essentially is like if i bolted an infinite stiffness member between the 2 faces.

How can i do this?

Kind regards

 

[JBlack68]

use an rigid element

 

[karachun]

Also, be careful with elements with large stiffness, compared to stiffness of whole model. High stiffness difference, like 10-15 orders of magnitude can produce ill-conditioned stiffness matrices Link. This can cause error 9137. Use RBE2 or RBE3 elements if it is possible. They do not add stiffness, instead they link DOF`s of nodes. If you have to use high stiffness elements then start with moderate stiffness difference, like 3 orders of magnitude bigger and increase stiffness until your link become acceptable stiff.

 

[St0RM33]

Thanks a lot i will do my research and find out how to achieve this now you told me the way

 

[St0RM33]

Update:
So i found this video (

https://www.youtube.com/watch?v=TWHoLZpz7RM

)that shows how to make a RBE connection and followed similar steps and ended up with this with NO LOADS:

My steps are:
1)Create new nodes at the center of the holes i want to attach
2)Create RBE2 connections between the new nodes and their related nodes
3)Create RBE2 connection between the 2 nodes

Also i haven't seen an option anywhere to set the stiffness of the connection. Only DOF's can be set.

What is going on here? (ps. I know this is a course mesh, i'm using it to test quickly each setup)

Kind regards

 

[St0RM33]

Update 2: Ok i've sorted it out
The elements are correct, the issue was that when i was unchecking the loads in NX it basically hides them, it doesn't disable them, and i had several test loads which made the results you see in my previous post.

Also i found out how to set elements with limited stiffness using CBEAM and CROD for example.
Then you go into Physical properties and set cross-section and stiffness (and i was wondering that you had to have cross-section to make any stiffness have an actual meaning in the simulation).

Now is there is an easy way to measure the deformed model results so i can confirm that the distance between the 2 holes didn't change? (since i do not see any measurement tools in the toolbar)

Kind regards

 

[karachun]

You can assume RBE2 element as absolutely stiff. Actually, they link DOF`s of selected nodes, they are not springs with large stiffness.
Search on web additional materials for better understanding of REB elements.

https://www.predictiveengineering.c...3_constraint_equations_and_cbush_elements.pdf

http://www2.me.rochester.edu/courses/ME204/nx_help/index.html#uid:id628196

https://www.quora.com/What-is-RBE2-...at-is-the-use-of-that-element-with-an-example

https://iberisa.wordpress.com/2015/10/13/rbe2-vs-rbe3-on-femap-with-nx-nastran/

(Article in Spanish, but you can translate whole page with google (it works in my Google Chrome).)