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Applying mass to an RBE3

Pioneer
Pioneer

I recently asked a question on how to get the rotation in a body during a random response analysis. You can see it here. The solution mentioned that the output would be near the correct mean rotation but different by the weight. What is meant by this? The solid that I connected with the RBE3 has some density that gives it mass. Does a mass need to be applied to the new central node of the RBE3 to get an accurate output? Would it be applied with a MASS element?

 

I made a separate topic for this because it seems like a separate discussion from the other topic.

2 REPLIES 2

Re: Applying mass to an RBE3

Solution Partner Phenom Solution Partner Phenom
Solution Partner Phenom

Hello!,

To help you better always post the FEMAP model (you always can create a pilot study) with pictures describing the problem, if not if will be difficult to understand your problem. Let's see if I understand you:

  • Create an RBE3 element and Select "at least" three nodes of your meshed body to enter as INDEPENDENT. Make sure to activate only TX, TY & TZ.
  • For the DEPENDENT node you can choose the option NEW NODE AT CENTER, this is the geometric center of all the INDEPENDENT nodes previously selected, and FEMAP will create a new DEPENDENT node at the center of the INDEPENDENT nodes. Take note of the label of this node because you need it to track the resultant displacements & rotations after running the corresponding analysis. Make sure to activate all DOF.

rbe3-dof.png

The typical use of the spider rigid RBE3 element is to allow application of remote loads applied at the DEPENDENT node and transmitted to the INDEPENDENT nodes. Also another typical use is to transmit to flange the effect of concentrated mass (CONM2 elements) located at the CoG of a body, when you only are interested in the mass of the body, not its stiffness, very typical in a NORMAL MODES/EIGENVALUE analysis using NX NASTRAN (SOL103).

rbe3.png

To learn more about RBE2 & RBE3 rigid elements please visit my blog in the following address:
https://iberisa.wordpress.com/2015/10/13/rbe2-vs-rbe3-on-femap-with-nx-nastran/

bracket-forced-response-fe-model

Alternatively, if you don't want to 3-D mesh your body (in the previous image, for instance, the electric motor) because don't contribute to the stiffness of the structure simply compute the CoG of the body, put there a node (or use command TOOL > MASS PROPERTIES > SOLID PROPERTIES that make the job for you) and joint that node with the flange using a rigid spider RBE3 element. If the mass of the electric motor is important for you because you are running a dynamic analysis, then create a concentrated mass using the node at the CoG of the electric motor, simply, OK?.

Best regards,
Blas.

Blas Molero Hidalgo, Ingeniero Industrial, Director
IBERISA • 48004 BILBAO (SPAIN)
WEB: http://www.iberisa.com
Blog Femap-NX Nastran: http://iberisa.wordpress.com/

Re: Applying mass to an RBE3

Phenom
Phenom

Yes, the question asked in your previous post is a little different to the question asked in this post.  In the last post you were asking about finding overall rotation in a body (of solid elements).  The RBE3 element provided a method where the dependent (central) node of the element would have rotation results calculated using the translations from those solid nodes which were selected as the RBE3 independent nodes.  The "weight" refers to the influence on the averaging, it is not related to mass at all.  RBE3 independent nodes can have a "weighting" or influence factor on the averaging.  Default weighting is 1.0 for every independent node, but Femap can automatically create distance based weighting, which is probably more physically realistic.  A small test model where you know the expected outcomes would help you understand the consequences.

 

A mass element is simply a way to add mass without creating a specific FE mesh of the part/assembly (eg. an engine in a car for handling response, or the seats/passengers in a train model for dead load analysis).   You do not need to add a mass element just to find the rotation result at the "central" node of an RBE element.

 

Cheers,