Bifurcation Analysis of Ultrashort Self-Acting Gas Journal Bearings for MEMS

Abstract

The development of microrotational devices in microelectromechanical systems (MEMS) has introduced a kind of ultrashort self-acting gas journal bearing with low length-to-diameter ratios. The bifurcation of ultrashort self-acting gas journal bearing-rotor systems is studied in this paper. The system is modeled as a rigid rotor supported by bearing forces as a result of gas viscosity and rotational speed. The spectral collection method is employed to discretize the nonlinear Reynolds equation describing the pressure distribution of the working fluid of the bearing. A system of nonlinear partial differential equations, which couples the fluid equation and the equations of the rotor motion, is presented and solved using the Runge-Kutta method. The bifurcation diagram, rotor center orbits, phase portraits, frequency spectra, and Poincare maps are utilized to analyze the dynamic characteristics of the rotor-bearing system for different operating conditions. The effects of the rotational speed and length-to-diameter ratio on the system dynamic behaviors are investigated with both low and high initial eccentricity ratios. The analyses show that the system exhibits complicated behaviors at low eccentricity ratios as a result of the self-excited whirl motion. For high eccentricity ratios, the bearing behavior comprises synchronous and subharmonic motions. A further understanding of the nonlinear dynamics of gas journal bearing in MEMS is given by the analysis results.