Hi,
I want to find out the torsional frequency of a component with 2 shafts,each shaft fixed to a frame on either side.The shafts are completely constrained at one of its ends and the other end is attached to the frame.Torsional stiffness of each shaft is 5000 N-mm. The frame is inturn attached to a circular plate of thickness 1.5 mm, located above the frame, using rigid elements The model is a bit difficult to explain so I have attached the .bdf file. The problem I am facing is as I increase the thickness of the frame , the nastran values and calculation values match but with less thickness of the frame the values in nastran is decreasing whereas it has to increase because the mass of the model is reduced. The formula I am using is f =(sqrt(K/Iyy))/2pi. I got Iyy of the model using the preproc itself.If I take thickness of the frame as about 10mm in Nastran,the values are matching(I am getting 1st natural frequency as 6 Hz). But practically I have to build a frame of 0.4mm thickness for which the frequency in Nastran is coming about 3Hz where as if I use the torsional vibration formula it should come about 19Hz.
I am not able to understand the discrepancies in the values. Can anybody please help me.
Bharat
I want to find out the torsional frequency of a component with 2 shafts,each shaft fixed to a frame on either side.The shafts are completely constrained at one of its ends and the other end is attached to the frame.Torsional stiffness of each shaft is 5000 N-mm. The frame is inturn attached to a circular plate of thickness 1.5 mm, located above the frame, using rigid elements The model is a bit difficult to explain so I have attached the .bdf file. The problem I am facing is as I increase the thickness of the frame , the nastran values and calculation values match but with less thickness of the frame the values in nastran is decreasing whereas it has to increase because the mass of the model is reduced. The formula I am using is f =(sqrt(K/Iyy))/2pi. I got Iyy of the model using the preproc itself.If I take thickness of the frame as about 10mm in Nastran,the values are matching(I am getting 1st natural frequency as 6 Hz). But practically I have to build a frame of 0.4mm thickness for which the frequency in Nastran is coming about 3Hz where as if I use the torsional vibration formula it should come about 19Hz.
I am not able to understand the discrepancies in the values. Can anybody please help me.
Bharat
