Decoupling DOFs

Altair Forum User

I have a model which contains a linkage similar to a one sided universal joint. One half of the linkage is fixed while the other is coupled in all but one rotational degree of freedom. I am attempting to construct this particular part of the model using only beam elements but I am unfamiliar with the process of setting up the contstraints/geometry such that one DOF is not shared at the node of intersection. Can someone please run me through the steps?

Thanks!

Altair Forum User

Hi Shawman,

with Beam elements you can use the PIN option of a CBEAM element (Nastran, Radioss Bulk).

Go to 1D->bars (element type CBEAM) and look at pins a= and Pins b= (for node1 and node2 of beam)

Here you can 'release' DOF's, up to 5 per side.

Help file:

PA, PB: Pin flags for beam ends A and B respectively. Used to remove connections between the grid point and selected degrees-of-freedom of the beam. The degrees-of-freedom are defined in the element’s coordinate system and the pin flags are applied at the offset ends of the beam. The beam must have stiffness associated with the PA and PB degrees-of-freedom to be released by the pin flags. For example, if PA=4, the PBEAM entry must have a non-zero value for J, the torsion stiffness.

No default (Integer > 0; up to 5 of the unique digits 1-6 with no embedded blanks)

Remark:

Any five of the six forces at either end of the element may be set equal to zero by using the pin flags on the CBEAM entry. The integers 1 to 6 represent the axial force, shearing force in Plane 1, shearing force in Plane 2, axial torque, moment in Plane 2 and moment in Plane 1, respectively.

Example: pins a=123 ... release dof x,y,z at end a of the beam

Regards,

Mario

Altair Forum User

Unfortunately, I failed to mention that I am working under the LS Dyna profile. That solution sounds otherwise perfect. To do this in Dyna, I thought maybe if I had two unequivalenced nodes at the same point, I could somehow constrain all but the z-rotational degree of freedom within the local coordinate system of my 'un-fixed' beam. Many of the *Constrained_... cards in Dyna seem promising but I'm having trouble implementing them so a phone call to support may be in order.

Altair Forum User

Hi Shawman,

with LSDyna i propose to use one of the following cards:

*CONSTRAINED_JOINT_SPHERICAL

*CONSTRAINED_JOINT_REVOLUTE

*CONSTRAINED_JOINT_CYLINDRICAL

*CONSTRAINED_JOINT_PLANAR

... and so on

in your case the second one, defined by four nodes (the lines drawn between nodes (1,3) and (2,4) must be perpendicular).

The joints work only with rigid bodys, so you have to create a rigid body on your parts (if they are not rigid bodys).

Regards,

Mario

Altair Forum User

Not sure if there is an equivalent in LSDYNA but CELAS, linear spring, elements can be used between equivalent nodes. Each spring has one DOF, so just use what you need to transmit load. However, I prefer the pin flags approach.

Altair Forum User

On the *ELEMENT_BEAM card in LS-DYNA, the RT1, RT2, RR1, and RR2 options can release specific freedoms at either end of hte beam element. This is comparable to the pin flags for NASTRAN.

Altair Forum User

There exists something similar to the pin flags for RADIOSS ???