DYNAMIC RESPONSE OF A ROTOR-HYBRID GAS BEARING SYSTEM DUE TO

Proceedings of ASME Turbo Expo 2010: Power for Land, Sea and Air

GT2010

June 14-18, 2010, Glasgow, UK

GT2010-22277

DYNAMIC RESPONSE OF A ROTOR-HYBRID GAS BEARING SYSTEM DUE TO BASE INDUCED

PERIODIC MOTIONS

Luis San Andrés

Keun Ryu Yaying Niu

Mast-Childs Professor, Fellow ASME

Research Assistant Research Assistant

Mechanical Engineering Department, Texas A&M University

College Station, TX 77843

ABSTRACT

Rotating machinery in transportation systems experiences intermittent excitation from road conditions. Internal combustion (IC) engines exert (multiple) periodic load excitations into passenger vehicle turbochargers, for example. Too large base motions can produce severe rotor-bearing system damage, even failure. The paper shows the reliability of a rotor-hybrid gas bearing system to withstand intermittent base foundation motions induced by a shaker. The test rig consists of a rigid rotor, 190mm in length, 0.825 kg in mass, and 28.6 mm in diameter, supported on two hybrid, flexure pivot tilting pad type, gas bearings. The whole system, weighing 48 kg, is supported on two soft coil springs and its lowest natural frequency is just ~5 Hz. The rod connecting the shaker to the base plate is not affixed rigidly to the test rig base. The rod merely pushes on the base plate and hence the induced based motions are intermittent with multiple impacts and frequencies. The base induced motions are at a low main frequency (5-12 Hz) relative to the operating speed of the rotor-bearing system (max. 35 krpm). The recorded rotor responses, relative to the bearing housings, also contain the main excitation frequency and its super harmonics; and because of the intermittency of the base motions, it also excites the rotor-bearing system natural frequency, in particular when the gas bearings are supplied with a low feed pressure. Predicted rotor dynamic displacements induced by the base excitations show reasonable agreement with the test data.

INTRODUCTION

Gas bearings, offering lesser friction and heat generation than mineral oil lubricated bearings, are used in microturbomachinery1 (MTM) including turbo expanders, air-cycle turbines for airplanes, and auxiliary power units [1]. Gas bearings do not demand complex supply and evacuation systems and sealing. Besides, gas film bearings can operate at extremely high and low temperatures. However, gas bearings have low load carrying capacity and little damping due to the 1

IGTI defines MTM as units with power < 250 kW

inherently low viscosity of the gas. In addition, hydrodynamic gas bearings with rigid surfaces generate cross-coupled stiffnesses; and thus are prone to self-excited subsynchronous whirl motions leading to rotordynamic instability [2].

Tilting pad gas bearings permit rotor dynamically stable operation since the bearing pads are free to tilt and do not generate cross-coupled stiffnesses. However, complex mechanical structures and time-consuming installation, along with time-accumulated disadvantages including wear due to high contact stresses at pivot locations, limit their extensive applications in industry [3].

Hybrid flexure-pivot tilting-pad gas bearings (FPTPGBs) successfully overcome the drawbacks of conventional tilting-pad gas bearings, since the integral thin web supporting a pad acts as a pivot to allow free tilting motion, but without contact stress that often lead to pivot wear. In Ref. [4], supplied with pressurized air, a rotor supported on FPTPGBs achieved a speed of 99 krpm (motor maximum speed) without any instability.

The ability to withstanding external periodic or random loads, steady or transient as in shocks, is crucial for gas bearings used in transportation vehicles such as turbochargers and micro gas turbine engines. For example, air flow fluctuations, take off and landing, and sudden maneuvers can introduce random excitations or shocks to air cycle machines in aircrafts. Diesel engine induced vibrations and road conditions tend to introduce periodic and random and transient excitations to vehicle turbochargers. These load excitations could lead to serious failure due to direct impact or rubbing contact between the rotor and bearings. Therefore, it is necessary to evaluate rotor-gas bearing system reliability under operating conditions with external shocks or periodic load excitations introduced into the system.

Many researchers have developed predictive models and conducted experiments to determine the dynamic response of rotor-bearing systems subject to external random or periodic motions. Tessarzik et al. [5] study analytically and experimentally the rotor response of a turbo generator to random and single frequency load excitations. The rotor axial

response to the external random vibrations is modeled as a linear three-mass system. The calculated and measured root mean square (RMS) axial response amplitudes are in good agreement.

Duchemin et al. [6] conduct experiments on a flexible rotor-bearing system subject to single frequency base excitations. An electromagnetic shaker mounted under one end of the test rig excites the system. The vertical acceleration is controlled to within 0.75 g for safety considerations, since the test rig operates through the threshold speed of instability. Using the same test rig setup, Driot et al. [7] determine, analytically and experimentally, the dynamic behavior of a base-excited flexible rotor. The model includes two gyroscopic and parametric coupled equations. A shaker, as in [6], applies single frequency excitations to the test rig base. Measured rotor motion orbits exhibit excellent correlation with numerical results obtained from a time integration of the equations of motion.

Large shock loads can drive rotor-bearing systems into unexpected failure. Walton et al. [8] and Heshmat et al. [9] deliver shock load …… 此处隐藏：37968字，全部文档内容请下载后查看。喜欢就下载吧 ……