×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

CBUSH MODELLING

CBUSH MODELLING

CBUSH MODELLING

(OP)
Hi, I'm a quite expertise fem user and i've a problem that not meet before today.
I modelled some bolts and to simplify the model I have used CBUSH NASTRAN ELEMENTS.

the .f06 indicate that there are a mechanisms in X direction.
all the degree of freedom are locked because the cbush connect two RBE2 (123456) and the PBUSH has K=10e10 stifnes in 123456 direction.
What's happen?
Thanks to advance to everybody give me any suggest.


 

RE: CBUSH MODELLING

You should give us some more information.
You could post your bdf or some pictures to let us understand.
we can't say anything from the indication you already give.
Were are your constrains? on a RBE2 on the independent grid points?
Onda

RE: CBUSH MODELLING

(OP)
thank you Onda for attention.
you are right, but I posted the previous message too hurriedly.
The CBUSH in question is shown in the attached file.
The question is:
why the same modelling technique applied for others items, not cause any problems? and for 5 nodes like that in the figure gives the mechanism error?

ever in my models I have modelled by this method.
Nb: the problem occurs not for all the simulation and not appears running free-free modal analisys.

RE: CBUSH MODELLING

(OP)
thank you Onda for attention.
you are right, but i posted the previous message too hurriedly.
The CBUSH in questions is shown in the attached file.
The question is:
Why the same modelling technique applied for others items, not cause any problems? and for 5 nodes like in the figure gives the mechanism error?

Ever in my models I have modelled by this method.
Nb: the problem occurs not for all the simulation and not appears running free-free modal analisys.

RE: CBUSH MODELLING

I really don't know why this happen.
but I've never used Cbush in this way. I don't like to use CBUSH with non coincident grid point.
So, just try to move the two grid point to be coincident, in the middle of the bar. And define the Cbush with a CID.
Also, you should post which Units are you using.
Look strange to me that translational DOF, and rotational DOF have the same stiffness value.
Onda

RE: CBUSH MODELLING

The stiffnesses are veeerrry large to properly represent a fastener position. I would definately see about using more representative values for the translational and axial stiffnesses, and lower the rotational stiffnesses and remove the drilling degree of freedom.
CBush elements dont need to be coincident, but its always best to have them such.
Without more info, my guess would be a massive stiffness variation between the shell elements dof and the RBE2/CBUSH dof, with the result of fatals.
With regard to the free free modal analysis, although it is a good indicator for some modeling accuracies, static analysis is always better at finding flaws.

RE: CBUSH MODELLING

(OP)
Thank you 40818 the solution of the problem it is correct. but alone I have resolved it two days ago.
thaks the same one.  

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources