Non-Newtonian effects on the dynamic characteristics of one-dimensional slider bearings: Rabinowitsch fluid model

The non-Newtonian effects of an isothermal incompressibe laminar-flow lubricant on the dynamic stiffness and damping characteristics of one-dimensional slider bearings are theoretically examined. On the basis of Rabinowitsch fluid (cubic equation) model, the modified Reynolds equation considering bearing-squeeze action is derived to take into account the transient motion of the slider, and the non-Newtonian properties of lubricants. Applying a small perturbation technique, both the steady-state performance and the dynamic characteristics are evaluated. According to the results, the steady film pressure, load-carrying capacity, and the dynamic stiffness and damping behaviors are significantly affected by the values of the dimensionless nonlinear factor accounting for non-Newtonian effects, the wedge parameter of a slider profile and the squeeze number of bearing-squeeze action.     

Effect of surface roughness on static and dynamic characteristics of MHD couple stress lubrication of parabolic slider bearing

The effect of surface roughness on static and dynamic characteristics of parabolic slider bearing lubricated with couple stress fluid in the presence of magnetic field is theoretically analysed in this paper. The modified stochastic MHD couple stress Reynolds-type equation is derived on the basis of Christensen stochastic theory and considered two types of roughness pattern namely longitudinal and transverse. Expressions for steady pressure and load, dynamic stiffness and damping coefficients are obtained for both roughness patterns. Compared to smooth surface, transverse roughness pattern provides larger load-carrying capacity, dynamic stiffness and damping coefficients, whereas longitudinal roughness pattern has adverse effects. The presence of couple stress and applied magnetic field improves the bearing performance.     

Squeeze film lubrication between circular stepped plates: Rabinowitsch fluid model

In this paper, a theoretical analysis on the squeeze film characteristics between circular stepped plates lubricated with Rabinowitsch fluid is presented. By using Rabinowitsch fluid model, the modified Reynolds type equation is derived to study the dilatant and pseudoplastic nature of the fluid in comparison with Newtonian fluid. The closed form solution is obtained by using perturbation method. According to the results obtained, the load-carrying capacity and squeeze film time increases for dilatant fluids as compared to the corresponding Newtonian fluids whereas the reverse trend is observed for pseudoplastic fluids. Further, it is observed that the response time decreases as the step height increases.     

Combined effects of surface roughness and viscosity variation due to additives on long journal bearing

Abstract

The combined effects of surface roughness and viscosity variation due to additives on long journal bearing are analytically studied. The variation in viscosity along the film thickness is considered. The presence of solid particles in the lubricant is an increased effective viscosity, which increases the load carrying capacity and decreases the frictional coefficient, whereas the viscosity variation tends to decrease both the load carrying capacity and coefficient of friction for non-micropolar fluid case. The modified Reynolds type equation for surface roughness has been derived on the basis of Eringen’s micropolar fluid theory. The generalised stochastic random variable with non-zero mean, variance and skewness is assumed to mathematically model the surface roughness on the bearing surface. Numerical results were obtained for the fluid film pressure, load carrying capacity and the coefficient of friction. It is observed that the combined effect is to increase the load carrying capacity and to decrease the coefficient of friction, which improves the performance of the bearing.     

Effect of surface roughness on the hydrodynamic lubrication of porous step-slider bearings with couple stress fluids

This paper presents the theoretical study of the effect of surface roughness on the hydrodynamic lubrication of porous step-slider bearings. A more general form of surface roughness is mathematically modeled by a stochastic random variable with non-zero mean, variance and skewness. The generalized average Reynolds-type equation is derived for the rough porous step-slider bearing lubricated with Stokes couple stress fluid. The closed-form expressions for the mean load carrying capacity, frictional force and the coefficient of friction are obtained. The performance of the rough porous step-slider bearing is compared with a corresponding smooth porous step-slider bearing. The numerical computations of the results show that the negatively skewed surface roughness pattern increases the load carrying capacity and decreases the coefficient of friction whereas the adverse effects were found for the positively skewed surface roughness pattern.     

Effect of environment humidity and temperature on stationary and transient flying responses of air bearing slider

It is well known that the environment humidity and temperature have a significant influence on the flying height of an air bearing slider. However, not many research papers address this topic, especially when the transient flying response is considered. This paper studies the influences of the environment humidity and temperature on both the stationary and transient flying responses of slider by simulation. A slider design for the thermal protrusion application is addressed. The reason for causing the drop of the air bearing pressure is discussed, and the methods for decreasing the drop are proposed. It is observed that the environment humidity and temperature may determine whether the slider is in full flying state or in partial flying/partial dragging state, when the slider is released from a certain height. The reason may be due to the high humidity and temperature which weakens the air bearing. As a result, the air bearing becomes not strong enough to support well the full flying of slider when the influence of the intermolecular force is significant. Slider vibrations for the full flying case and the partial flying/partial dragging case are analyzed in frequency domain, and the slider vibration frequencies are discussed. It shows that the environment temperature and humidity have significant effects on both the stationary and transient flying responses of the slider.     

A new model for combined Couette and Poiseuille flows in the transverse groove of a plane inclined slider bearing

Although extensive research has been performed on the grooving of hydrodynamic bearings, there is much to be done on the flow interactions near to and in axial grooving. This work was initiated when the pressure boundary conditions were unknown for the case of a journal bearing with multiple axial grooves each being fed by an external lubricant source. This work is a forerunner to a more extensive research programme using journal bearings. A new technique has been devised to calculate the pressure profile across and along an axial groove. There is a strong pressure flow along the groove and Couette and pressure flow in the direction of sliding. Good correlation between experimental results and theoretical predictions for a wide range of operating conditions has been achieved.     

Calculated journal bearing lubrication of non-Newtonian medium with surface roughness effects

To analyze the effects of non-Newtonian lubricants and surface roughness in journal bearing lubrication, a modified Reynolds equation is derived. In the equation, differential viscosity and the first normal stress function are defined to specify the rheological properties of non-Newtonian medium. Flow factors are used to specify the effect of surface roughness. The modified Reynolds equation is numerically calculated using super over relaxation method. Numerical results of the lubrication show that the differential viscosity is the principal non-Newtonian property affecting the lubrication, it is determined by the material parameters of the lubricant and is affected by the shearing rate. Under its effect, the load capacity of non-Newtonian lubricants is not always higher or lower than that of Newtonian lubricants’. The effect of the first normal stress difference increases under the conditions of dynamic loading. Surface roughness shows an obvious effect on load capacity when it is greater than one-tenth of the film thickness, and the surface with longitudinal pattern affects the load capacity most. However, surface roughness has less effect on lubrication results than lubricants’ non-Newtonian property. __________ Translated from Tribology, 2005, 25(6) (in Chinese)     

超声振动切削超薄壁精密零件的粗糙度试验研究

对直径为 47.75mm壁厚为 0 .8～ 1 .5mm的照相机导向镜筒零件进行普通和超声车削试验 ,研究了各切削参数对此类超薄壁零件表面粗糙度的影响规律 ,也试验研究了超声振动切削时切削液及车刀对表面粗糙度的影响     

Mechanism studies of the multiple flying states of the air bearing slider

The multiple flying states of a negative pressure air bearing slider is demonstrated and verified by simulation. It is observed that a slider may have one to three balanced flying states under certain conditions. One of these states is the intended flying state, while the other two are much higher and could make the read/write operations unreliable. It is observed that there is no suction force when the slider is at the highest flying state, but there is a small or no suction force in the median flying state. It is noticed that these three states could interchange with each other under certain circumstances. Some aspects of their physical meanings and characteristics are discussed. Simulations were done for loading forces of 2.5 and 1.5 g, respectively. Negative air bearing stiffness is found in certain regions, and the regions could affect the load/unload performance of the slider.     

高速车削工件表面形貌及其粗糙度特征的试验研究

借助于扫描电镜照片、已加工样品表面形貌轮廓描绘和试验数据处理等手段,对高速车削工件已加工表面形貌与其表面粗糙度之间的关系以及它们的形成特征进行了分析研究.研究结果表明,切削速度和被切削材料的硬度是决定高速车削过程中被切削层材料变形和已加工表面形貌及其表面粗糙度形成的主要因素,随着被切削材料硬度和切削速度的提高,工件已加工表面质量在一定程度上得到了改善.在已加工表面上出现了犁垄和高速加工所特有的熔融金属涂抹现象,由此决定着已加工表面粗糙度值的变化.     

基于机器视觉的零件表面粗糙度测量

机器视觉测量是近年来测量领域中的新测量技术,是以光学为基础,融电子技术、计算机技术、图像处理技术为一体的测量系统。介绍一种基于机器视觉进行表面粗糙度测量的方法。     

Influence of Non-Gaussian-Distributed Surface Roughness on the Static Performance of Slider Bearings

An investigation on full fluid film lubrication assuming non-Gaussian-distributed surface roughness has been conducted. A typical slider bearing is chosen as the research object. Edgeworth expansion containing skewness and kurtosis is introduced to approximate the roughness density function. Using Christensen's stochastic roughness model, effects of roughness direction and non-Gaussian distribution properties on the working performance of slider bearings have been quantitatively analyzed. The results show that the variation range of working performance caused by skewness and kurtosis is up to 9.82%. Effects of skewness and kurtosis on the performance are nontrivial and should not be neglected. Compared to the Gaussian distribution, Edgeworth series can represent physical rough surfaces more accurately. Though the effects of skewness and kurtosis on the working performance are complicated, some laws can still be found. For example, large skewness decreases the load capacity, whereas large kurtosis reduces friction force. According to the results, we can choose certain machining processes that can produce the desired surface roughness.     

Morphometric assessment of surface conformation as an estimate of roughness

One of the standard methods for assessing the roughness of a material subjected to wear uses the surface arithmetic means and root-mean-square deviation. However, these parameters often do not provide a qualitative assessment of the difference in materials worn under the same conditions of load and elapsed time. The profile and surface roughness parameters are frequently inconsistent. Such measurements are also required to determine the wear characteristics of various materials under different conditions. A morphometric assessment of wear characteristics, based on the surface area fraction of localized deviations in the surface texture and stress fractures, is provided, and clearly indicates the observed difference.     

高速铣削过程中表面粗糙度变化规律的试验研究

在高速铣削试验的基础上,研究切削速度与进给量对加工表面粗糙度的影响。     

机械零件表面粗糙度的影响与选择

从摩擦学的角度出发 ,阐明对于配合表面只取Ra值时的不足 ,建议应当再增加一个表面纹理加工方向 ,以此确保配合表面的润滑效果 ,减少磨损 ,提高使用寿命。     

Linear stability of short journal bearings with consideration of flow rheology and surface roughness

The combined effects of surface roughness and flow rheology on the linear stability of a rigid rotor supported on short-length journal bearings are analyzed. The modified Reynolds equation and the rotor motion equation are linearized about the equilibrium position and the fluid film is modeled as spring and damping elements. From the characteristic equation, the instability threshold is then obtained with various surface roughness parameters (standard derivation, σ, and Peklenik number, γ), flow rheology (power-law index, n) and eccentricity ratios (). The results show that the size of the stability regions of different roughness patterns has the following characteristics: longitudinal (γ>1)>isotropic (γ=1)>smooth>transverse (γ<1). The stability of the bearing system deteriorates with decreasing power-law index. Moreover, there are crossing points in the vicinity of =0.45 among the curves of dimensionless speed parameter ( ) associated with various power-law indices and surface roughness parameters.     

Effect of real longitudinal surface roughness on lubrication film formation within line elastohydrodynamic contact

In many applications (e.g. roller, barrel or needle bearings) surface features exhibit longitudinal alignment to the direction of motion. These features are produced by surface finishing techniques in the circumferential direction and are associated with line or very wide elliptical contact geometries. In such a case, the contact length in the direction of motion is considerably shorter comparing its width and the effect of a longitudinal roughness could significantly influence the lubrication film formation. Recent experimental studies have indicated less severe effect of a longitudinal roughness on lubrication film formation in the comparisons with that observed with transversely orientated roughness caused by the inlet perturbation. Nevertheless, these experimental studies have been focused on the behaviour of artificially produced asperities within a circular contact. The quantitative experimental study of longitudinal real surface roughness within a line contact has not been realized yet. That is why, in this study, the line contact formed between a steel tapered roller and glass disc is observed within an optical test rig and the effects of real surface roughness on lubrication film formation are studied. Experiments carried out under pure rolling conditions have shown that the depth is the key parameter that influences the effect on the film thickness. If the roughness features are shallow, the lubrication film shape within the contact follows the shape of the surface closely. However, the groove having only about 800 nm in depth divided the line contact into two parts that behave as two separate line contacts. Such an effect can increase the risk of the wear of rubbing surfaces as the lubrication film thickness between the real machine components can be significantly lower than expected.