CBUSH MODELLING 1

[SoftcraftIT]

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.

 

[Onda]

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

 

[SoftcraftIT]

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.

 

[SoftcraftIT]

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.

 

[Onda]

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

 

[40818]

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.

 

[SoftcraftIT]

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