Nonlinear Phenomena in Dynamic Response of Rotors in Anisotropic Mounting Systems

Abstract

In a previous paper (Ehrich, 1994), the author has cataloged a variety of unique rotordynamic responses which have been observed in a computer model of a simple Jeffcott rotor in a nonlinear anisotropic mounting system; that is, operating eccentrically within a clearance and in local intermittent contact with the stator. In addition to the critical synchronous resonant response similar to that found in linear systems, unique responses are found at subcritical operating speeds—superharmonic pseudo-resonances, and regions of chaotic and periodic response in transition zones between successive superharmonic orders. In the transcritical operating regime on both the subcritical and supercritical sides of the critical peak, spontaneous sidebanding is found when the system is very lightly damped. At supercritical operating speeds, subharmonic pseudoresonances, and regions of chaotic response and periodic response in transition zones between successive subharmonic orders are identified. These phenomena are characterized by their unique signature in the response curve, derived from a simple numerical model of a Jeffcott rotor with a bilinear stiffness in the direction normal to the plane of contact. Each phenomenon is further characterized by a typical example of the wave form and the wave form’s spectral analysis. All the waveforms display the tendency of the nonlinear system to have a significant asynchronous response component at its natural frequency irrespective of the rotational or stimulus frequency. In a more recent publication (Ehrich, 1995), recorded observations of rotordynamic response, in the format of “waterfall” or “cascade” charts, of operational high-speed turbomachinery in two typical instances have been compared with equivalent data from the same computer model used to illustrate the cataloged phenomena. The instances are representative of several of the cataloged phenomena. Excellent correspondence between the computed waterfall charts and the data from the actual operational machinery are achieved. The results may be of considerable interest to the community of vibrations engineers who deal with prevention of and remedial action for deleterious asynchronous vibration in operational high speed rotating machinery.