不同压力腔的气体静压轴承静特性的数值模拟

气体静压轴承以其低摩擦、高精度成功应用于高精密回转运动台,其定位精度已达到纳米级水平,达到了物理极限,因此压力腔形状对气体静压轴承的影响无法忽略。通过CFD数值模拟,研究了不同压力腔形状对气体静压轴承静态性能的影响。针对矩形压力腔、圆形压力腔和锥形压力腔,研究分析了气膜内压力分布、气体质量流量和承载力等静态性能。结果表明:相同的条件下,锥形压力腔承载力较大,且压力腔内部气旋强度较弱,比较适合应用于高压重载的场合。     

Characteristics of a hybrid journal bearing with one recess: Part 1: Dynamic considerations

A fluid film hybrid journal bearing with one recess, as used in tandem cold rolling mills, is studied theoretically in two ways. The dynamic response of the hybrid bearing, under isothermal conditions due to the decrease in hydrostatic pressure, is considered and presented in Part 1 of the study, while the thermal effects on the load capacity, temperature distribution, etc of the bearing will be dealt with and described in Part 2.In this paper, the dynamic behaviour of the journal due to the decrease in hydrostatic pressure is presented in the form of transient orbits and squeezed-film speeds, which are shown to be dependent on the initial equilibrium conditions. In the analysis, when the recess pressure is dropped below the hydrodynamic pressure generated by the fluid film, it is found to be difficult to obtain a convergent solution. The dynamic response of the bearing, due to the shut-off of external pressure is, therefore, simulated by the dynamic behaviour of the journal due to a series of pressure drops in arbitrary time intervals. The results show that the journal is quite stable in such conditions.     

CFD investigation of pressure depressions in aerostatic circular thrust bearings

Under certain operating conditions, the pressure distributions of aerostatic thrust bearings experience an undesirable pressure depression which decreases their load carrying capacity. Many investigators reasoned this phenomenon to the occurrence of shock waves in the bearing clearance. Recently, some investigators reasoned this phenomenon to the transition from laminar to turbulent flow and claimed that no shock wave is generated at the boundary between supersonic and subsonic flows. As such, there is a contradiction between these two opinions. In this paper, the rationale of the pressure depression phenomenon in aerostatic thrust bearings is investigated using computational fluid dynamics (CFD) simulations. The turbulent full Navier-Stokes equations for steady, three-dimensional, compressible flows are numerically solved in this study. Two circular bearing configurations are analyzed. The obtained results showed that the predicted pressure distributions along the fluid film compare well with the corresponding experimental data of other investigators. The present computational methodology allowed a clear capturing of the coherent structures of the flowfield in the bearing inlet region which include the coalescing of compression waves into shock waves and the region of shock/boundary layer interaction (pseudo-shock). The thorough understanding of this phenomenon is the first step towards the development of its appropriate control methods.     

气穴现象对动静压浮环径向轴承压力场的影响研究

由Navier-Stokes方程得出计入惯性项的变密度、变粘度的深浅腔动静压浮环径向轴承的内、外油膜无量纲非定常Reynolds方程,给出深腔压力边界条件和流量边界条件,对不同偏心率下不同含气率的内、外油膜压力场进行了分析。计算表明,深腔气穴使动静压浮环径向轴承的内、外油膜的压力峰值下降,在小偏心率下影响较为明显,对内油膜压力峰值的影响大于外油膜。     

小孔节流深浅腔动静压轴承承载特性解析研究

小孔节流深浅腔动静压轴承是一种采用小孔节流器实现节流作用及浅腔实现二次节流作用的动静压混合轴承。针对现有理论不能解析研究油腔结构参数及工作参数对承载特性影响规律的不足,以及计算流体力学数值仿真软件计算时周期长,而不便于工程设计人员应用的缺点,基于油腔压强分段线性化的思想,建立分析小孔节流深浅腔动静压轴承的油腔压强、承载能力、静刚度、进油流量及温升等承载特性的解析方法。进而以该方法研究动静压轴承的供油压强、主轴转速、进油孔径、浅腔深度、初始油膜厚度等参数对轴承承载特性的影响规律。研究发现,在其他结构参数及工作参数一定的条件下,浅腔深度为初始油膜厚度的2～3倍时,轴承刚度接近最大、温升接近最低。通过油腔压强的解析值与试验值的比较,证实了该方法的有效性和研究结果的正确性。     

The behaviour of hydrostatic pads with grooved lands

A hydrostatic pad is usually made up of a recess surrounded by a land. Viscous fluid is supplied under pressure to the recess. The land, being separated from the bearing surface by a relatively small clearance, will act as the hydraulic impedance needed to separate the required bearing pressure inside the recess from the pressure of the surrounding environment. If the pad is moved relative to the bearing surface, the film of fluid in the clearance, being viscous, will be sheared. This shearing action will initiate viscous shear stresses between the fluid layers and hence viscous drag between the moving pad and the bearing surface. The lands of the pad, having a much smaller clearance from the bearing surface, will be subjected to a much higher drag force than the recess. The power required to overcome such a drag force, and cause the required motion of the pad relative to the bearing surface, will be transformed mainly into heat. Sometimes, especially under high relative speeds and with small clearances, the generated heat can be detrimental to the bearing action, and if excessive, may lead to bearing failure.     

磁头承载面结构层深度对气膜润滑性能的影响

采用理论分析和数值模拟相结合的方法分析了磁头承载面结构层深度变化对气膜性能的影响.结果表明:磁头的基层和次景层深度变化对气膜的正负压承载区域的压力分布产生明显的影响.其中次景层深度的变化主要对正压产生影响,随着次景层深度的不断增加,产生正压的滑块区域压力都明显下降,特别是磁头尾端滑块处的气膜压力变化最大;而基层深度的变化对正压和负压分布都产生影响,随着基层深度变化产生正压的滑块区域压力变化较大,表现更为明显.由于磁头结构层深度变化对正、负压力分布和总气浮力的作用,从而影响气膜承载力性能.在实际磁头结构设计中可以通过优化设计确定最佳各结构层深度,达到增加磁存储容量的目的.     

An experimental investigation into the performance of externally pressurized rectangular air bearings

An experimental programme which was designed to investigate the performance characteristics of externally pressurized rectangular air bearings considering the effects of pad geometry and inlet gas conditions is presented. Three forms of rectangular bearings were considered: one arrangement consisted of continuous plane surfaces, the second had a recessed pad and a plane bed and the third had a plane bed and a recessed pad with a step in the bearing area following the recess.

The results revealed that the recess length has a dominant effect on the pressure distribution in the air film. The effect of pressure depression is obvious at extremely small clearances and supply pressures. Such an effect is exaggerated when a step is introduced in the pad surface. The introduction of the step improves the load-carrying capacity of the bearing as long as the bearing is free from shock-wave formation.     

Turbulent lubrication of externally pressurized bearings including convective inertia effects

A numerical solution is developed for the equations governing the turbulent lubrication of externally pressurized circular bearings. The eddy viscosity is based on the nonlinear theory proposed by Elrod and Ng. The effects of fluid convective inertia tobether with these of lubricant acceleration at the recess edge are included in the analysis. Typical graphs showing the influence of recess radius ratio, film thickness ratio, pressure ratio, and method of compensation on the bearing performance—pressure distribution, load, flow rate, power consumption, and stiffness—are presented. Inertia effects are shown to increase or decrease the load factor, flow factor, and power factor, depending upon the values of recess radius ratio and film thickness ratio.     

Porous inclined stepped composite bearings with micropolar fluid

Abstract

This paper describes the theoretical analysis of the effect of micropolar fluid on the lubrication characteristics of porous inclined stepped composite bearing. The lubricant with additives in the film region and also in the porous region is modelled as Eringen's micropolar fluid, which is characterised by the presence of suspended rigid particles with microstructures. The generalised Reynolds type equation is derived for the most general porous bearing configuration (porous composite bearings) lubricated with micropolar fluid. The closed form expressions are obtained for the fluid film pressure, load carrying capacity, frictional force and coefficient of friction. These expressions can be utilised to obtain the performance characteristics of four different bearing systems, namely, porous plane inclined slider, porous composite tapered land bearing, porous stepped bearing and composite porous tapered concave bearing. It is observed that the micropolar fluid lubricants provide an increased load carrying capacity and decreased coefficient of friction as compared to the corresponding Newtonian case.     

A Method for the Measurement of Hydrodynamic Oil Films Using Ultrasonic Reflection

The measurement of the thickness of an oil film in a lubricated component is essential information for performance monitoring and control. In this work, a new method for oil film thickness measurement, based on the reflection of ultrasound, is evaluated for use in fluid film journal bearing applications. An ultrasonic wave will be partially reflected when it strikes a thin layer between two solid media. The proportion of the wave reflected depends on the thickness of the layer and its acoustic properties. A simple quasi-static spring model shows how the reflection depends on the stiffness of the layer alone. This method has been first evaluated using flat plates separated by a film of oil, and then used in the measurement of oil films in a hydrodynamic journal bearing. A transducer is mounted on the outside of the journal and a pulse propagated through the shell. The pulse is reflected back at the oil film and received by the same transducer. The amplitude of the reflected wave is processed in the frequency domain. The spring model is then used to determine the oil film stiffness that can be readily converted to film thickness. Whilst the reflected amplitude of the wave is dependent on the frequency component, the measured film thickness is not; this indicates that the quasi-static assumption holds. Measurements of the lubricant film generated in a simple journal bearing have been taken over a range of loads and speeds. The results are compared with predictions from classical hydrodynamic lubrication theory. The technique has also been used to measure oil film thickness during transient loading events. The response time is rapid and film thickness variation due to step changes in load and oil feed pressure can be clearly observed.     

Characteristics of a hybrid journal bearing with one recess: Part 2: Thermal analysis

A thermal analysis is made of a fluid-film hybrid journal bearing with one recess under steady-state and adiabatic conditions. The coupled Reynolds equation and energy equation are solved simultaneously by a finite difference method with a successive under-relaxation method. The results for isothermal conditions with effective temperature and feed oil temperature are also obtained for comparison. The load capacity for the adiabatic condition is lower than that for the isothermal condition due to temperature gradients as well as the higher temperature distribution in the high pressure zone. Moreover, the high pressure recess works as a cooling source, so that the maximum temperature occurs at the edges of the bearing and near the position of minimum film thickness. The side flow decreases as the eccentricity increases in the adiabatic condition. This is caused by the rapid decrease in film thickness when eccentricity increases, which dominates the effect of pressure-induced flow.     

Experimental assessment of hydrostatic thrust bearing performance

This work presents an experimental study to assess the performance characteristics of hydrostatic thrust bearings. A test rig is designed for testing two configurations of hydrostatic bearings compensated by a capillary tube, one with a mid-circular recess and the other with multi-sectors (four recesses of different recess radii ratios). The performance characteristics have been measured, namely, oil film thickness, recess pressure, pressure distribution and oil flow rate. A specially devised computer program using an iterative technique has been adopted to compute numerically the pressure distribution and predict other performance characteristics. Good agreement has been obtained between predicted theoretical performance and that experimentally measured. The results demonstrate that the bearing recess size and location have a great influence on the performance of the hydrostatic thrust bearing.     

Laminar Flow in a Recess of a Hydrostatic Bearing

The flow in a recess of a hydrostatic journal bearing is studied in detail. The Navier-Stokes equations for the laminar flow of an incompressible liquid are solved numerically in a two-dimensional plane of a typical bearing recess. Pressure- and shear-induced flows, as well as a combination of these two flow conditions, are analyzed. Recess friction, pressure-ram effects at discontinuities in the flow region, and film entrance pressure loss effects are calculated. Entrance pressure loss coefficients over a forward-facing step are presented as functions of the mean flow Reynolds number for pure-pressure and shear-induced laminar flows.     

The effect of the recess shape on performance analysis of the gas-lubricated bearing in optical lithography

Gas-lubricated bearings are widely used in the optical lithographic manufacturing of wafers to realize nearly zero friction for the motion of the stage. As the extreme precise positioning accuracy of these equipments has reached nanometer level, and almost approaches the physical limit, the effect of the recess shape of the gas bearings cannot be neglected any longer. This paper studies the effect of the recessed shape of the bearing on the performance analysis through numerical method. The pressure distribution, as well as the loading capacity and mass flow rate for the case of different shape (no pocketed bearing, rectangular shape and spherical shape bearing, respectively) of the bearing is achieved with the well-known CFD-control element procedures in Quad meshes. The results show that the average loading capacity reduces as the following sequence: rectangular recessed bearing, spherical recessed bearing and non-recessed bearing. The mass flow rate in the bearing with spherical recess is smaller than that of with rectangular recess, and bigger than that of with non-recess. Vortexes, which can cause higher temperature in gas film, are also found in the spherical recess or rectangular recess. It means that only the bearing with non-recess is fitted to use in ultra-high precision equipments.     

工况对盾构机主驱动轴承润滑特性的影响

以某隧道工程实际工况条件为例,建立盾构机主驱动轴承载荷分布计算模型和等温线接触弹流润滑模型,通过数值分析得到极限工况和占比99.9%的工况条件下盾构机主驱动轴承的油膜厚度及油膜压力分布;依据实际工况条件分析不同工况对轴承油膜厚度、油膜压力的影响规律,以及滚子所处位置不同时滚子负载与油膜压力和膜厚之间的变化关系。结果表明：不同工况下主轴承油膜厚度、油膜压力分布规律相似,均出现二次峰值;同一工况下,随着滚子于主轴承所处位置不同,油膜压力及膜厚最值随滚子负载的增大而减小;同一位置处二者最值随主轴承受力的增大而减小。     

Performance of an externally pressurized rectangular gas bearing under constant effective recess pressure

The present study predicts the performance of an externally pressurized rectangular recessed gas bearing under different operating conditions. The analysis makes use of simplifying assumptions and an empirical formula which relates the recess pressure to the bearing supply pressure. A comparison between the theoretical results obtained and available experimental data shows qualitative agreement. There are some deviations in pressure distribution and load-carrying capacity especially at greater film thickness and supply pressures. The reasons for these deviations are discussed.     

偏心率对高速动静压轴承动特性影响

本文分析了水润滑,五腔动静压轴承偏心率对轴承静动性能的影响。轴承量在高压力、高转速下使用。计算表明,在中小偏心下,轴承动力系统为常数;而在大偏心率下,由于流体膜空穴和油腔位置的影响,轴承动特性有较大变化。同时考虑了轴承系统稳定性。     

静压孔位置对油膜支承可倾瓦轴承承载性能的影响分析

为了提高汽轮机转子系统中支承轴承的油膜刚度,以三瓦油膜支承可倾瓦轴承为研究对象,研究静压孔相对位置对轴承承载性能的影响规律。建立了油膜支承可倾瓦轴承油膜润滑模型,并运用计算流体动力学方法数值求解三维N-S方程,揭示了不同静压孔相对位置下轴承压力分布、最小膜厚、偏心率、刚度等性能参数的变化规律。分析结果表明：在载荷为890 N的情况下,改变孔的位置可以提高轴承油膜刚度;当静压孔相对位置γ=5°左右时,孔位置接近油膜最大压力分布区,与γ=0°时相比,最小膜厚和偏心率分别减小9.8%和48%,主刚度kyy、kxx接近原结构的1.4倍和1.1倍,此时静压孔位置为相对最优位置区域。依据分析结果开发了新型油膜支承可倾瓦轴承（γ=5°）,通过试验对比分析了普通滑动轴承与新型油膜支承可倾瓦轴承的综合性能,结果表明,高转速时所开发的新型油膜支承可倾瓦轴承具有更好的承载性能与减振性能。研究结果对油膜支承可倾瓦轴承的性能分析具有一定的参考价值,设计轴承静压孔时可根据油膜压力分布规律对其优化以提高轴承性能。     

Effects of operating conditions and supply hole diameter on the performance of a rectangular aerostatic bearing

The present study is an experimental investigation of the performance characteristics of an externally pressurized rectangular recessed gas bearing operating under different working conditions. It includes the measurements of pressure, load carrying capacity, and mass flow rate. The experiments covered a wide range of film thicknesses and supply pressures. In addition, pads of different supply hole diameters were used. The results obtained from the present investigation show that the effect of varying the supply hole diameter on bearing performance is significant. Increasing the supply hole diameter to bearing length ratio has the effect of increasing the recess pressure and load carrying capacity. Further, as the supply pressure and/or film thickness increases the dimensionless load carrying capacity decreases and the lubricant mass flow rate increases.