Torsional Vibrations of a shaft and a rotor
Torsional Vibrations of a shaft and a rotor
(OP)
Hi,
I want to find out the torsional frequency of a shaft attached to a rotor. the shaft is massless and with a torsional stiffness 5000Nmm/rad and the rotor is of a diameter of 200 mm and a thickness of 1.5 mm.. I am using the young's modulus of aluminum for the shaft and the rotor..Since I have considered the shaft as massless I have given very low density for the shaft material and the mass of the rotor is about 80 grams because I have considered the exact density of aluminum. The problem i am facing is that the the natural frequency should come about 17.79 hz if I use the torsional vibrations formula which is f =(sqrt(K/Izz))/2pi but in the FE analysis I am getting 0.000545 hz in the first mode.I am using 1 d cbar element for the shaft and 2d cquad element for the rotor
The Izz for rotor is 400kgmm^2. Please help
Thanks in advance
Bharat
I want to find out the torsional frequency of a shaft attached to a rotor. the shaft is massless and with a torsional stiffness 5000Nmm/rad and the rotor is of a diameter of 200 mm and a thickness of 1.5 mm.. I am using the young's modulus of aluminum for the shaft and the rotor..Since I have considered the shaft as massless I have given very low density for the shaft material and the mass of the rotor is about 80 grams because I have considered the exact density of aluminum. The problem i am facing is that the the natural frequency should come about 17.79 hz if I use the torsional vibrations formula which is f =(sqrt(K/Izz))/2pi but in the FE analysis I am getting 0.000545 hz in the first mode.I am using 1 d cbar element for the shaft and 2d cquad element for the rotor
The Izz for rotor is 400kgmm^2. Please help
Thanks in advance
Bharat





RE: Torsional Vibrations of a shaft and a rotor
Any SPC in your model?? it sounds like you have free body movement. If you have not any constraint that prevent the solid rigid movement in your model the first 6 modes that you will obtain are the six rigid body modes.These modes should have frequencies less than or equal to 1.0E-04 Hz. After these 6 modes you will obtain the vibration modes of your structure.
Hope this will help you.
Have a nice day!!
RE: Torsional Vibrations of a shaft and a rotor
Bharat
RE: Torsional Vibrations of a shaft and a rotor
RE: Torsional Vibrations of a shaft and a rotor
I know what is the problem, the problem are the CQUAD that you are using to modelate the rotor. For CQUAD element there is no stiffness associated to the rotation about the normal to the plate. The CQUAD can only represent in-plane, bending, and transverse shear behavior. That´s the problem!! you have a stiffness singularity in the connection of the cbar with the CQUAD, the rotation about the cbar axis is not constrained for the CQUAD.
Try to model your rotor with a mass element, CMASSi or CONMi, have a look to the "quick reference guide" to check what mass element fit better to your rotor, other solution will be to model your shaft with 2D elements...........another alternative could be to connect your cbar to the rotor by means of a RBE2, as independent node choose the bar extreme and as dependent nodes choose some symetric nodes on the CQUADS, I think that this can works.........
Hope that this will help!!
RE: Torsional Vibrations of a shaft and a rotor
RE: Torsional Vibrations of a shaft and a rotor
RE: Torsional Vibrations of a shaft and a rotor
Have a nice day
RE: Torsional Vibrations of a shaft and a rotor
I have attached the new model.
Have a nice day!!
RE: Torsional Vibrations of a shaft and a rotor
RE: Torsional Vibrations of a shaft and a rotor
RE: Torsional Vibrations of a shaft and a rotor
RE: Torsional Vibrations of a shaft and a rotor
I highly recommend you to read the MSC documentation for NASTRAN, you will learn a lot about FEM and the philosophy behind NASTRAN, you can find for free the docs in
http://simcompanion.mscsoftware.com/infocenter/ind...
You should start by the "Getting started User's Guide" and after reading it go to the "Linear Static User's Guide". Also you can find the "MSC Nastran Demonstration Problems" that will help teach you a lot.
Good luck with your way in the FEM and do not hesitate in contact me for further questions.
Have a nice day!!!
RE: Torsional Vibrations of a shaft and a rotor