Concentrated mass properties in Nastran/Patran

[mophie]

I use a concentrated mass (conm2) in my patran model. I use the lumped option. Now I have to fill in:
-mass (no problem)
-mass orientation ( is that the orientation of the gravity vector?)
- Inertia 1,1 1,2 etc. Is I(1,1)=Ixx, I(2,2)=Iyy, I(3,3)=Izz

Could someone please help me with de mass orientation and Inertia?!

Thanks

 

[pjhype]

mophie,

The mass orientation is the coordinate system in which the inertia matrix is defined. The most convenient method is to define the principal moments of inertia whereby the inertia matrix is diagonal and populated by I-xx, I-yy, & I-zz, with all other products of inertia = 0. However, you will need to define the coordinate system that the inertia matrix is aligned to. Alternately, you can define the inertia matrix in the global coordinate system (which is assumed by Nastran) and include the cross products.

If what you are trying to model is truely a concentrated mass, you may not have to worry about the inertia matrix. If you are uncertain how to calculate the inertia matrix, you might want to first try to include only the concentrated mass with no inertia terms. As you gain confidence in the solution, you might want to add the inertia matrix to see if there are any effects.

pj