Inclusion of unbalance properties in modal analysis

[pure-machinery]

How can I include unbalance from the rotor itself and a coupling at the shaft end into the modal analysis?

Shouldn't they affect the natural frequencies of the system?

With run_unbalance_response i can only get how the rotor deflects for each mode, while run_modal takes no such parameters?

unbalance = rotor.run_unbalance_response(
    node=21,
    unbalance_magnitude=30e-6 * 0.5,
    unbalance_phase=0,
    frequency=frequency_range,
)

for mode in range(3):
   unbalance.plot_deflected_shape_3d(speed=frequency_range[mode]).show()

Are the units on the y-axis eg. 50p represented as 50 picometers or does it relate to something else?

[jguarato]

Hi, @pure-machinery !

I’ll try to clarify how these aspects work in ROSS, which might help explain why you’re not seeing the expected changes in natural frequencies:

• The modal analysis method .run_modal() in ROSS computes the natural frequencies and mode shapes of the system without any external or steady-state forces.
• The unbalance response method .run_unbalance_response() calculates how the rotor deflects under a given unbalance at specified speeds.

Unbalance represents a forcing condition (dynamic effect), not a change in the system’s stiffness or mass matrices. If you want the natural frequencies to change, you must modify the system matrices (mass, stiffness, damping).

Since a coupling introduces both mass and stiffness, including it in the model will indeed modify the system matrices. You can represent this in ROSS by using the CouplingElement, which will then influence the results of .run_modal() accordingly.

As for your question about the “50 p” label on the y-axis of the plot — it likely denotes picometers (10⁻¹² m).