General Mechanical

General Mechanical

Correct modal analysis conditions and results for shaft?

    • ENV159
      Subscriber

      Hello,


      I am doing a modal analysis (free vibration) for a rotating shaft. Are fixed supports in place of the bearings the correct boundary conditions? Do my results make sense in regards to the frequency (Hz) value? The first mode frequency when converted to RPM is 314400 RPM. Does this value seem too high? 




       


      Thank You

    • peteroznewman
      Subscriber

      Using Fixed Supports is wrong as it eliminates the correct torsional mode.  Does the shaft have gears or other large masses fastened to it?  The model should include a disk with the same mass moments of inertia as the actual part fastened to the shaft to get correct vibration modes of the shaft assembly.  The vibration of the shaft without the added parts is not that useful.


      Which coordinate direction is the axis of the shaft?  I will assume it is the Z axis for the description below.


      Create a Remote Displacement on one face. Set X, Y and Z displacement to zero, leaving the rotations free.
      Create a second Remote Displacement on the other face, set X and Y displacement to zero, leaving the others free.


      In the modal results, the first mode will have a zero (or very small) frequency. That is the rigid body rotation of the shaft. Just ignore Mode 1. All the other modes will have a non-zero frequency, including a torsional mode and various bending modes.


      Instead of two Remote Displacements, you could use two Bearing Constraints, one on each of the two faces. You have to also constrain the axial motion with a remote displacement and have only Z=0 leaving all others free. That would add the flexibility of the bearings (and housing depending on how you define K) to the modal analysis and the vibration modes will include radial displacement of the bearing faces on the shaft.


      Regards,
      Peter

    • ENV159
      Subscriber

      Thank you, this is what I was looking for. And the shaft is just simply bearings.

Viewing 2 reply threads
  • You must be logged in to reply to this topic.