Abnormal behavior of a hydrodynamic lubrication journal bearing caused by wall slip

Reynolds lubrication theory assumes that there is no wall slip on the interfaces between the solids and lubricant. During recent years, however, it is found that wall slip often happens. The present paper analyzes the wall slip occurring in a hydrodynamic lubrication journal bearing. If the two surfaces have the same adhesion property wall slip always decreases the oil film load support capacity. If there is wall slip over all of the lubricated surfaces, the hydrodynamic effect of the journal bearing vanishes, and no load support exists. If the two lubricated surfaces have different adhesion properties, the wall slip effect is more complex and may cause the journal bearing to operate in an instable manner. In order to avoid the wall slip, the limiting shear stress at the bearing surface should be higher than that at the journal surface.     

Influence of Wall Slip on the Dynamic Properties of a Rotor-Bearing System

Based on the limiting shear stress model, we used a multi-linearity finite element algorithm and quadratic programming technique to study the influence of wall slip (boundary slip) on the operation stability of a rigid rotor-bearing system. The shaft surface is designed as a no-slip surface. The bearing sleeve surface is designed as three types of surfaces: (a) no slip is allowed (traditional no-slip rotor-bearing system), (b) the entire sleeve surface has the same slip property (the homogeneous slip bearing), and (c) the sleeve surface is optimized to have an optimized slip zone (the optimized slip bearing). It is found that if the sleeve surface has a single slip property, the wall slip generally reduces the system operation stability, as well as the load-carrying capacity. However, if the sleeve surface is designed as the optimized slip surface, the wall slip enhances the system operation stability as well as the load-carrying capacity. Furthermore, the smaller the surface limiting shear stress, the better the dynamic stability and the higher the load-carrying capacity.     

The numerical analysis of the radial sleeve bearing with combined surface slip

In order to improve the carrying capacity and reduce the temperature rise in high speed and precise spindle systems, a combined surface radial sleeve bearing using the interfacial slip technique was discussed. An extended Reynolds equation was derived based on the modified slip length model considering the limiting shear stress. By means of the finite differential methods, the characteristic analysis and optimization of the slip region of the combined surface sleeve bearing were carried out, and it has been proved that there is still a considerable large load support in a parallel sliding gap. Comparing with the general journal bearing, the load capacity and end leakage rate of the combined surface sleeve bearing can be increased greatly and the load capacity can be increased by 1.75 times. The attitude angle, friction drag, temperature rise of the combined bearing can be decreased distinctly and the temperature rise can be decreased by 92.4%.     

滑动轴承二维流场的滑移现象研究

目前对于二维流场及复杂流场的界面滑移分析很少,根据螺旋油楔滑动轴承能使润滑剂产生周向和轴向二维流动的独特的结构特点,考虑周向和轴向两方向的滑移建立基于极限切应力的数学模型,并通过试验和理论对比验证模型的正确性。试验方面运用"目标速度跟踪法"证实了周向和轴向都存在滑移,获知随着供油压力的提高滑移速度有所提高,并且提出轴瓦和轴表面的极限切应力;理论方面运用有限差分法和试验测得的轴瓦和轴表面极限切应力,求解四种状态的广义雷诺方程,发现滑移发生在极限切应力大、间隙小和油膜的封油面区域;考虑界面滑移时,螺旋油楔滑动轴承的承载力和摩擦阻力有所降低;偏心率、螺旋角和转速的变化,影响着承载力和摩擦阻力降低的幅度。     

Hydro-Roll: A Novel Bearing Design with Superior Thermal Characteristics

A novel journal bearing design, called hydro-roll, which combines a hydrodynamic journal bearing with a rolling-element bearing in series, is presented. The hydro-roll principally operates as a rolling-element bearing at low speeds and as a hydrodynamic bearing at high speed. When properly designed, the hydro-roll can offer superior performance over the conventional hydrodynamic bearing in terms of both wear and thermal characteristics. The hydro-roll prevents the severe wear which occurs during the start-up of hydrodynamic journal bearings since it operates as a rolling-element bearing whenever the sliding friction is too high. This characteristic has the important advantage of reducing the risk of catastrophic failure by bearing seizure.

At a steady high speed, the hydro-roll operates as a hydrodynamic journal bearing while the sleeve together with the inner race of the rolling-element bearing turn at a low speed. This study focuses on the thermal analysis of a journal bearing with a rotating sleeve in comparison to an identical stationary sleeve. The results indicate that the sleeve rotation reduces the risk of bearing failure due to local overheating of the sleeve. The minimum sleeve speed required for this effect is evaluated.     

Boundary slip surface design for high speed water lubricated journal bearings

This paper presents the development of a numerical model for high speed and water lubricated journal bearings with different boundary slip arrangements. The effect of boundary slip and its possible mechanism are analyzed and discussed. The results suggest that a suitable combination of slip/no-slip surfaces on the sleeve of a journal bearing enables improvement of the tribological performance through (i) suppressing the occurrence of cavitation, (ii) enhancing the load bearing capacity, and (iii) reducing the interfacial friction between bearing sleeve and shaft. Such improvement becomes more significant for the bearings with smaller eccentricity ratio, smaller width and larger diameter.     

Numerical Analysis of a Slider Bearing with a Heterogeneous Slip/No-Slip Surface

The behavior of a fluid-film bearing depends on the boundary conditions at the interfaces between the liquid and the solid bearing surfaces. For almost all solid surfaces, the no-slip boundary condition applies. However, a number of researchers have recently found that slip can occur with specially engineered surfaces. These include molecularly smooth surfaces and surfaces with micron-scale patterns. By constructing an engineered heterogeneous surface on which slip occurs in certain regions and is absent in others, the flow in the liquid film of a bearing can be altered, and such characteristics as load support and friction can be improved. In the present study, a numerical analysis of a slider bearing with such an engineered slip/no-slip surface is analyzed. Slip is assumed to occur when a critical shear stress is exceeded and follows the Navier relation. The results show that with a critical shear stress of zero, a significant increase in load support and decrease in friction can be achieved with an appropriate surface pattern. With nonzero values of critical shear stress, an instability occurs over a range of speeds. At speeds above this range, the bearing behaves similar to the case with zero critical shear stress, while below this range it behaves like a conventional bearing.     

Dynamic Friction Model of Lubricated Surfaces for Precise Motion Control

A model is developed to describe dynamic friction effects in lubricated surfaces. The model covers the hydrodynamic, mixed and boundary lubrication regions. The dynamic friction model can predict the friction force for time-varying velocity, and is useful in precise motion control. The model presented is for a short journal bearing, but can be extended to other geometries of sliding surfaces, such as point and line contacts or rolling element bearings. The friction is related to a time variable fluid film thickness, resulting from journal vibrations relative to the sleeve. The proposed model agrees qualitatively with experimental results for lubricated line contact. Both show similar hysteresis-type friction curves under oscillating velocity.     

Theoretical Prediction of Journal Bearing Stability Characteristics Based on the Extent of the Slip Region on the Bearing Surface

Journal bearing performance depends on the boundary conditions at the interfaces between the fluid and the solid surfaces. In the derivation of the Reynolds equation used to predict the bearing performance, the no-slip boundary conditions of the fluid and the solid interfaces are used. Recent research has shown that a slip can occur on specially made surfaces, the conventional no-slip boundary conditions are not valid, and the Reynolds equation is no longer applicable. If the slip is allowed to occur in certain regions, the fluid flow in the bearing can be altered, and the bearing stability characteristics can be improved. In this article, the numerical stability analysis of a journal bearing based on the extent of the slip region on the bearing surface is analyzed. An extended Reynolds equation is derived based on the slip length model, using a no-slip boundary condition against the journal surface and the slip against the bearing surface. A linearized perturbation method is used to determine the stability limit of a rigid rotor supported on two symmetrical journal bearings. Using the linear stability analysis, the linearized stiffness and damping coefficients, the threshold speed, and the critical whirl ratio are evaluated. The effects of the slip parameter on the bearing stability performance are discussed. The results show that with a critical shear stress of zero, an increase in the stability threshold can be achieved with a higher value of the nondimensional slip length and a smaller extent of the slip region on the bearing surface.     

Squeeze fluid film of spherical hydrophobic surfaces with wall slip

Isothermal squeeze film flow of Newtonian fluid between spherical hydrophobic surfaces with wall slip is investigated using a limiting shear stress model and complementary algorithm. Wall slip velocity is controlled by the liquid–solid interface limiting shear stress. It is found that the wall slip dramatically decreases the hydrodynamic support force of the squeeze fluid film. In the case of large wall slip the hydrodynamic support force increases only slightly with the decrease in the film thickness. We find that wall slip decreases with increasing film thickness and limiting shear stress, but increases with increasing fluid viscosity and approaching velocity. An empirical equation is given for prediction of the fluid load support capacity. The possible effect of pressure on wall slip is also discussed. It is found that fluid pressure suppresses wall slip after the proportionality coefficient of limiting shear stress reaches a critical threshold. However, almost no effect is found when it is below this critical threshold. Good agreements exist between the present theoretical predictions and some existing experimental observations.     

Surface roughness effect on finite journal bearings with flexible porous liners

The effects of surface roughness on the static characteristics of finite porous journal bearings under hydrodynamic lubrication conditions are investigated in this paper. The well‐established Christensen stochastic theory of hydrodynamic lubrication of rough surfaces is used to incorporate the effects of surface roughness into the Reynolds equation. The analysis takes into account the flexibility of the porous liner by using a thin liner model. The effects of velocity slip at the surface of the porous medium are considered in the analysis by using the Beavers‐Joseph criterion. The mathematical model is then solved numerically by finite‐difference methods for mean hydrodynamic pressure, which in turn gives the hydrodynamic load. The effects of the surface roughness parameter, surface pattern, eccentricity ratio, length‐to‐diameter ratio, permeability parameter, and flexibility parameter on the hydrodynamic load‐carrying capacity, attitude angle, and friction factor are discussed.     

界面滑移对圆柱形凹坑织构滑动轴承摩擦力的影响

为研究不同的滑移情况对圆柱形凹坑织构滑动轴承摩擦力的影响,建立含有圆柱形凹坑织构的滑动轴承在不同界面滑移状态下的摩擦力计算模型,探究影响织构化滑动轴承摩擦力的参数,并借助ANSYS分析不同滑移情况下界面滑移对圆柱形凹坑织构滑动轴承摩擦力的影响规律。结果表明:织构化滑动轴承的摩擦力主要是由轴颈线速度、油膜滑移比、轴承的进出油口压力、织构处油膜压力、织构深度、油膜厚度和承载力决定;不同滑移情况下织构模型的摩擦力均小于无织构模型;且在上下表面均滑移时,圆柱形凹坑织构在出口位置时表现出最优的承载和减摩效果;适当地增加圆柱形凹坑织构的深度可以改善模型的摩擦性能,但是过深的凹坑织构并不能发挥出其性能。     

多叶动压气体滑动轴承静态特性的有限差分算法

多叶动压气体滑动轴承因其结构简单、摩擦阻力低、旋转精度高和无环境污染等优点,在高速离心分离机、空气压缩机和透平膨胀机等旋转机械中应用广泛。为探究多叶动压气体滑动轴承的静态性能,通过数学变换将三叶动压轴承的气体润滑Reynolds方程转化为标准偏微分方程形式,利用有限差分法和超松弛迭代法进行数值求解,研究气膜厚度和气膜压力分布、承载力、摩擦因数和质量流量等静态性能,随偏心率、预负荷系数、轴承数、长径比及瓦块分布位置的变化规律。结果表明：三叶轴承的承载力和轴颈表面摩擦因数随偏心率和长径比的增加而增加,而偏位角和质量流量随偏心率和预负荷系数的增加则呈现出相反的变化趋势;随着轴承数和预负荷系数的增大,承载力和摩擦因数显著提高,偏位角和质量流量则逐渐减小;瓦块分布位置对三叶动压气体滑动轴承的静态性能影响显著,其中瓦上承载方式的承载力、偏位角和质量流量明显高于瓦间承载方式。     

Surface roughness effects on thermo‐hydrodynamic lubrication of journal bearings lubricated with bubbly oil

In this paper, the combined effect of surface roughness and bubbles content on the hydrodynamic performance of journal bearings is studied. In the analysis, it is assumed that the bearing and shaft surfaces are covered with homogeneous isotropic roughness, the air bubbles are evenly distributed through the lubricant and the bubble size is very small. The modified Reynolds equation governing the pressure generation in the bearing gap for compressible fluid is solved simultaneously with the energy equation. Temperature and pressure distributions, coefficient of friction, bearing load capacity and attitude angle as affected by surface roughness, bubble content and some bearing parameters are presented. Results showed that the bearing load carrying capacity is higher at higher values of average roughness and higher bubble content as a direct consequence of the higher pressure values attained, and the average roughness and the bubbles content had no significant effect on the attitude angle. Copyright © 2006 John Wiley & Sons, Ltd.     

Experimental study of journal bearings with undulating journal surface

A journal bearing test rig was designed and constructed to test the behaviour of journals with wavy surfaces, the circumferential undulations being varied both in amplitude and in number. Results show that wavy journal surfaces may well enhance the load carrying capacity of a bearing. Moreover, surface undulations are shown to move the journal centre locus closer to the load line, ie cause a lower attitude angle. These effects are found to be more pronounced with larger wave amplitudes, and with higher numbers of waves around the journal circumference. In general, friction is found to be reduced with increase in surface wave amplitude.Good agreement is shown to exist between test results and a computer aided analysis conducted by the authors to predict wavy journals performance¹. It has been established that a wavy journal surface may, under certain conditions, display higher load capacity, lower friction and permit safer running of journal than bearings with perfectly smooth surfaces.     

Static Performance of Finite Hydrodynamic Journal Bearings Lubricated by Magnetic Fluids with Couple Stresses

Based upon the Stokes micro-continuum theory, the problem of lubrication of finite hydrodynamic journal bearing lubricated by magnetic fluids with couple stresses is investigated. By taking into account the couple stresses due to the microstructure additives and the magnetic effects due to the magnetization of the magnetic fluid, modified Reynolds equation is obtained. The effects of couple stresses are studied by defining the couple stress parameter L that can be considered as a measure of the chain length of the additive molecule. The magnetic effects of the magnetic fluid are investigated by the magnetic coefficient γ. Using the finite-difference technique and for different values of couple stress parameter and magnetic coefficient, the Reynolds equation is solved, and pressure distributions are obtained. The bearing static characteristics namely load carrying capacity, attitude angle, friction coefficient, and side leakage flow are determined. The results indicate that the influence of couple stresses and magnetic effects on the bearing characteristics are significantly apparent. It is concluded that fluids with couple stresses are better than Newtonian fluids. The improvement of the bearing characteristics is enhanced if the magnetic effects are present.     

An analytical approach on the tribological behaviour of pocketed slider bearings with boundary slip including cavitation

Slider bearing performance depends on the boundary conditions at the interface between the solid surfaces and the fluid. This paper presents the combined effect of pockets and boundary slip on the load support and friction of parallel sliding systems using analytical solutions for a simple pocketed bearing. The effect of cavitation was of particular interest with respect to the inlet suction mechanism. It was demonstrated that applying boundary slip in a pocketed slider bearing gives a reduction in load support compared with the textured bearing without wall slip. Adding slip over the whole surface could retard the presence of cavitation. The influence of boundary slip is explored, and was found to significantly affect the frictional behaviour. Copyright © 2016 John Wiley & Sons, Ltd.     

Texture shape effects on hydrodynamic journal bearing performances using mass-conserving numerical approach

ABSTRACT

It is a known fact that incorporating textures in the contact surfaces can significantly enhance bearing performances. The purpose of this paper is to outline the effects of texture bottom profiles and contour geometries on the performances of hydrodynamic textured journal bearings. The analysis was conducted using computational approach to test eight texture shapes: rectangular, cylindrical, spherical, triangular (TR, T1, T2, T3) and chevron. The steady-state Reynolds equation for modelling the hydrodynamic behaviour of thin viscous film was solved using finite difference technique and mass conservation algorithm (JFO boundary conditions), taking into account the presence of textures on both full film and cavitation regions. The comparison with the benchmark data shows good consistency and an enhancement in bearing performances (load carrying capacity and friction). The results clearly show that the mechanisms of wedge effect and micro-step bearing for the full/partial texturing feature are the main crucial parameters, where the convergent wedge effect present in T2 triangular texture shape can significantly enhance the load-carrying capacity, while the divergent wedge action causes a net load loss. Considering the right arrangement of textures on the contact surface, their surface contours can have a significant impact on the performance of hydrodynamic journal bearings at high eccentricity ratios.     

Numerical analysis of spiral oil wedge sleeve bearing including cavitation and wall slip effects

The modified Reynolds equation is established on the basis of critical shear stress model, in which the circumferential and axial wall slip of sleeve and journal surface is considered. Cavitation is treated using modified Elrod algorithm that simplifies the solution of modified Reynolds equation in the full‐film region. The modified Reynolds equations considering wall slip and cavitation effect for two‐dimensional sleeve bearing are established. The results show that wall slip decreases oil film pressure, carrying capacity, friction drag and temperature rise but increases end leakage and cavitation region. The obtained results using the mass‐conserving boundary condition are compared with the Reynolds boundary condition. Copyright © 2014 John Wiley & Sons, Ltd.     

Suitability of grooved configurations for bearings and seals

Suitably designed grooves and wedges on the surface of a plain hydrodynamic sleeve can modify the load-carrying capacity, the side leakage and other characteristics. If the sleeve is to be used as a hydrodynamic seal for turbogenerators to prevent leakage of cooling gas or for other engineering applications, it must have an unbroken oil film of extent 2Π. A plain sleeve with a very small eccentricity may provide an uninterrupted film but it will have a load capacity insufficient to support even its own weight and it will also be unstable.The effects of various groove and wedge configurations on the performance characteristics of a hydrodynamic sleeve were studied. The suitability of these configurations or combinations of them for bearings or seals was investigated.