Nonstationary Vibrations of a Rotating Machinery Through Critical Speeds and Active Damping

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Rotating machinery has nonlinear spring characteristics for various reasons. In this research, nonstationary oscillations through critical speeds caused by nonlinear spring characteristics are investigated.1. In an experimental apparatus in which a rotor was mounted on an elastic shaft, various kinds of subharmonic and summed-and-differential harmonic oscillations are observed.2. In the theoretical and experimental analyses, we originated Complex-FFT method which utilizes a digital signal processing technique. By using Complex-FFT method, phase plane analyses, and the asymptotic methods, the following results are obtained : (1) In the case of the major critical speed, the maximum amplitude gamma max in nonstationary oscillation has only one value for a given angular acceleration. The rotor can pass the critical speed when the acceleration is higher than a certain value. The bigger the acceleration, the smaller the maximum amplitudes. (2) In the case of subharmonic oscillation of order 1/2, the maximum amplitude gamma max varies between 0 and (gamma max)_1 in randam even if the angular acceleration is the same. (3) In the case of summed-and-differential harmonic oscillation caused by unsymmetrical nonlinearity, the maximum amplitude gamma max varies between two values (gamma max)_1 and (gamma max)_2 in random for a given angular acceleration. (4) In the case of subharmonic oscillation of order 1/3, the rotor can pass the critical speed if the value of accelaration is very small or very large. Otherwise, the nonstationary amplitude is drawn toward stationary amplytude and the rotor cannot pass the critical speed.3. The active damping technique is now under development and we could not obtain a completed results.