Xiao-Li Wang

Effects of Gas Rarefaction on Dynamic Characteristics of Micro Spiral-Grooved Thrust Bearing

The effects of gas-rarefaction on dynamic characteristics of micro spiral-grooved-thrust-bearing are studied. The Reynolds equation is modified by the first order slip model, and the corresponding perturbation equations are then obtained on the basis of the linear small perturbation method. In the converted spiral-curve-coordinates system, the finite-volume-method (FVM) is employed to discrete the surface domain of micro bearing. The results show, compared with the continuum-flow model, that under the slip-flow regime, the decrease in the pressure and stiffness become obvious with the increasing of the compressibility number. Moreover, with the decrease of the relative gas-film-thickness, the deviations of dynamic coefficients between slip-flow-model and continuum-flow-model are increasing.

Dynamic Analysis of a High-Speed Rotor-Ball Bearing System Under Elastohydrodynamic Lubrication

The nonlinear dynamic behaviors of a high-speed rotor-ball bearing system under elastohydrodynamic lubrication (EHL) are investigated. First, the numerical curve fittings for stiffness and damping coefficients of lubricated contacts between rolling elements and races are undertaken, and then the fitted formulae are introduced to the equations of motion of the rotor-ball bearing system to investigate its nonlinear characteristics. Furthermore, the time responses, power spectra, phase trajectories, orbit plots, and bifurcation diagrams for cases of ignoring and considering the lubrication condition in bearings are inspected and compared. The results indicate that, when lubrication is taken into account, the amplitudes of vibration displacements and velocities of the rotor system increase, and the appearance of different regions of periodic, quasi-periodic, and chaotic behavior is strongly dependent on the speed and load.

Modeling and Analysis of Micro Hybrid Gas Spiral-Grooved Thrust Bearing for Microengine

The micro hybrid spiral-grooved thrust bearing is a promising candidate to support the rotating elements in power MEMS devices such as micro gas turbine engines. However, the realization of hybrid thrust bearings has encountered a number of technical challenges due to the very high rotating speed and DN number (the product of the inner diameter and the rotational speed of the bearing, mm · rpm) to achieve high power density, the super thin gas film between rotors and thrust pad, and the relative large fabrication uncertainties according to the imperfection of the fabrication technology. In this paper, the configuration of a micro hybrid spiral-grooved thrust bearing for power MEMS is designed, and the steady and dynamic characteristics of this kind of bearing are then analyzed comprehensively, with the consideration of both the rarefaction effects and the influence of potential microfabrication defects. The nonlinear equations of molecular gas-film lubrication describing the gas rarefaction effects in a micro hybrid bearing are discretized by the finite volume method and solved by the Newton–Raphson techniques. The small perturbation technique is employed to study the dynamic behavior of a micro hybrid bearing. The results show that the micro hybrid thrust bearing exhibits better steady-state and dynamic performance than the existing micro hydrodynamic and hydrostatic bearings and that the hybrid bearings are likelier to be stable than their hydrodynamic counterparts, especially when the frequency number is high. The load capacity of the micro hybrid bearing increases slightly with the number of orifices and gradually with the diameter ratio of the orifice. The microfabrication defects of clogged orifices could lessen the load capacity and the dynamic coefficients of the hybrid thrust bearing. The model developed in this paper can serve as a useful tool to provide insight into micro hybrid gas thrust bearing-rotor systems.

A numerical study of the rolling friction between a microsphere and a substrate considering the adhesive effect

A numerical model of the rolling friction between a microsphere and a substrate is established by introducing the adhesion hysteresis between the front and rear sides of the contact region into Zhangs adhesive contact model. Effects of the size ratio which is defined as the sphere radius divided by the equilibrium separation, relative amount of adhesion hysteresis and Tabor parameter on the dimensionless maximum rolling friction torque in the case of zero normal force are inspected, and the quantitative relationship between the maximum rolling friction torque and the normal force is achieved. Results indicate that due to adhesion hysteresis at microscale, the dimensionless maximum rolling friction torque at zero normal force is not zero, which not only increases with decreasing size ratio, showing clear size effects, but also increases with increasing relative amount of adhesion hysteresis and Tabor parameter. In addition, the maximum rolling friction torque at microscale presents a sublinear relationship with the normal force, and the exponent of the normal force is influenced by the size ratio, relative amount of adhesion hysteresis and Tabor parameter, which are remarkably different from the superlinear relationship at macroscale.