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

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

(OP)
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

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

use an rigid element

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

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 DOFs 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.

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

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

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

(OP)
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

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

(OP)
Update 2: Ok i've sorted it out:p
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

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

You can assume RBE2 element as absolutely stiff. Actually, they link DOFs of selected nodes, they are not springs with large stiffness.
Search on web additional materials for better understanding of REB elements.
https://www.predictiveengineering.com/system/files...
http://www2.me.rochester.edu/courses/ME204/nx_help...
https://www.quora.com/What-is-RBE2-RBE3-in-a-finit...
https://iberisa.wordpress.com/2015/10/13/rbe2-vs-r... (Article in Spanish, but you can translate whole page with google (it works in my Google Chrome).)

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