Overhung rotors are widely used in industrial
turbo-machines. For certain gas turbines, gyroscopic effects of the disks may
almost double the critical speeds of the rotor systems, compared to normal
mechanical vibration systems. In addition, the asymmetry of the bearingstiffness will bring more complication to a rotor system. Even though there are
many established publications about how to theoretically model an overhung
rotor with anisotropic bearings , very few papers compared the theoretical
models with experimental results. Ishida et afl.derived a sophisticated
mathematical model to theoretically investigate the nonstationary vibration of
a flexible rotor with nonlinear spring parameters during acceleration.
They
employed an asymptotic method to compute the first approximate solution to
vibration response. Then, they calculated the amplitude variation curves of
each oscillation component using complex-FFT. Furthermore, they inspected how
each nonlinear component in polar format affected dynamic vibration. Theyproposed a unique signal processing method called complex-FFT where the rotorwhirling plane is coincident with the complex plane. The whirling direction of
the rotor can be judged by filtering the vibration signals at different
frequencies using complex-FFT method. However, the complex-FFT doesnot provide
instrumentation phase angle information and full spectrum cascaded plots.
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