Investigating the Effect of Surface Finish on Mixed EHL in Rolling and Rolling-Sliding Contacts

Surface finish may significantly affect the lubrication performance of a tribological interface through the influence of topography on micro/nanoscale fluid flows around localized contacts at surface asperities. This paper aims to study the mixed lubrication performance of a group of engineered surfaces, including turned, isotropically finished, ground, and dimpled surfaces, under different operation conditions by means of a deterministic mixed elastohydrodynamic lubrication (EHL) model. The honed surface was used to mate with other surfaces. The results indicate that a longitudinal contact ellipse favors longitudinally oriented mating surface roughness and that a transverse contact ellipse, as well as a line contact, prefers a transversely orientated mating surface roughness for lubrication enhancement.     

表面粗糙度对点接触弹流润滑性能的影响

本文应用多重网格法对椭圆接触的部分膜弹流问题进行了完全数值求解，计算中采用了Ｐａｔｉｒ－ｃｈｅｎｇ的平均流量模型及Ｇｅｅｎｗｏｏｄ和Ｔｒｉｐｐ的表面微峰弹性接触模型。在大量数值计算结果的基础上分析了表面粗糙度大小及纹理方向对点弹流的油膜厚度及微峰接触载荷的影响。     

Wavelength Dependence of Micro-EHL Pressure Rippling with Random Surface-Roughness Profiles

This paper studies line-contact elastohydrodynamic lubrication (EHL) between two rough surfaces of random roughness profiles. A transient micro-EHL model is used to simulate the problems. Numerical results are obtained for a range of practical operating conditions and for relatively small specific film thicknesses (i.e., the A-values). The study reveals the wavelength dependence of micro-EHL pressure rippling. For the same A-value, the size of the pressure rippling is shown to be significantly smaller than those predicted earlier using more idealized system models. Furthermore, the pressure distribution is essentially unaffected by the short-wavelength components in the random surface-roughness profiles, except under near rolling conditions. It is shown that lubricant non-Newtonian shear-thinning is the mechanism that suppresses the short-wavelength pressure ripples. With a Newtonian lubricant, sharp pressure rippling is generated by the small-scale surface roughness textures. Since EHL lubricants exhibit significant shear-thinning and since the surfaces possess random roughness profiles, the results reported in this paper reflect a main feature that may prevail in realistic EHL conjunctions. Future work will establish a theoretical basis for the numerical analysis conducted in this research.     

粗糙表面等温瞬态点接触弹流润滑问题直接迭代解法比较

在考虑接触表面粗糙度的情况下,采用SOR、Ф变换法、q变换法3种直接迭代解法求解瞬态Reynolds方程。结果表明,SOR法和q变换法收敛速度较快,但在载荷变大或粗糙表面等情形下,较难获得正确的结果,而Ф变换法变换关系式较为复杂,但因采用较精确的Roelands黏压关系式,容易得到理想的结果。因此在数值求解过程中,在轻载或者润滑表面粗糙度影响较小的情况下,可以采用SOR或者q变换法,同时增加网格和采取较低的松弛因子,改善SOR或者q变换法的数值不稳定性,从而较快获得收敛解,在载荷较大或者润滑表面粗糙度影响较大的情况下,应该优先采用Ф变换法。     

点接触弹流润滑条件下表面弹性变形研究

为研究弹流润滑条件下点接触表面变形及其对表面性能的影响,采用激光微织构法制备了一组不同形貌参数的滚动轴承滚道表面试件,基于表面频谱分析和弹流润滑快速算法（幅值缩减法）分析了试件在不同工况参数下的弹性变形。使用ISO 25178三维形貌参数体系对变形前后表面进行表征,研究了点接触弹流润滑状态下表面形貌的弹性变形与载荷、转速的关系。研究结果表明,弹流接触使表面形貌发生显著的弹性变形,载荷、转速等工况参数对变形量影响较大,表面弹性变形使得形貌参数发生显著变化。     

Investigation of Oil Additives for Boundary Lubrication of Railroad Journal Bearings

The boundary friction properties of the lubricated system of steel sliding on lead-base babbitt were investigated as a phase of the Association of American Railroads' sponsored project on freight car hot boxes. A friction pendulum was developed for determining the coefficients of boundary friction as a function of temperature with the specified metals lubricated with various lube oil additive blends.

A group of general relationships dealing with additives for the steel-babbitt system was developed. It is concluded that the extreme pressure agents containing sulfur or chlorine are generally unsuited for railroad journal bearing use. Lubricity additives which function by simple physical adsorption were found to be ineffective at moderate or high temperatures existing in railroad operations. Those lubricity additives which chemisorb to metal surfaces and form a close-packed layer were found to be effective antifriction agents.     

A Multiscale Framework for EHL and Micro-EHL

In this article, a heterogeneous multiscale method is introduced to analyze the microelastohydrodynamic lubrication (micro-EHL) of bearings with topological features. Two scales are adopted in the analysis: the large-scale simulations describe the entire bearing domain, and the small-scale simulations describe the fluid–structure interaction (FSI) at the small-scale features. Conservation of mass and momentum of the lubricant and the bearing's elastic deformation are solved for. The relationship between the pressure gradient and mass flow is obtained from homogenized small-scale FSI simulations and applied on a global scale via a scattered data interpolation method. When the micro structure is periodic the exact model at micro scale is replaced by an effective derived equation, i.e., homogenized model. The elastic deformation of the textured bearing surface is addressed at both the large and small scales, by decomposing the displacement influence matrix into the diagonal terms and nondiagonal terms (sorted at the small scale and large scale, respectively).

The multiscale method was demonstrated as being capable of modeling the global pressure and film thickness for a bearing with surface texture while maintaining the accuracy of the small-scale modeling features. The illustrative geometry was that of a linear converging pad bearing in two dimensions. The solutions were compared with those obtained using lubrication theory for the smooth surface case, and good agreement was obtained. The method was then demonstrated for geometries incorporating topographical features.     

Prediction of Real Rough Surface Deformation in Pure Rolling EHL Contact: Comparison with Experiment

The ability to predict the in-contact deformation of surface topography is very important for the design of machine components with respect to minimizing the friction and wear of rubbing surfaces. In this study the amplitude attenuation principle is verified as a simple tool for this purpose. Measured lubricant film profiles are compared with prediction based on this principle. From the results obtained it appears that the amplitude attenuation principle provides reasonable estimation of the deformation of rubbing surfaces that can be used for the prediction of in-contact behavior of surface roughness. Good agreement was obtained under pure rolling conditions, which provides a good initial point for the other studies under rolling/sliding conditions where the wear of rubbing surfaces is of key importance.     

Surface Roughness Effects on Metallic Contact and Friction

A study was made of surface roughness effects on metallic contact and friction in the transition zone between hydrodynamic and boundary lubrication. The system used was one of pure sliding and relatively high contact stress, namely a fixed steel ball riding on a rotating steel cylinder.

It was found that very smooth and very rough surfaces gave less metallic contact than surfaces of intermediate roughness; very smooth surfaces also gave less friction.

Four different types of antiwear/antifriction additives (including tricresyl phosphate) were studied and although they were found to reduce metallic contact and friction, they had little effect in reducing surface roughness. Rather, the additives merely slowed down the wearing-in process of the base oil. Thus, the “chemical polishing” mechanism advanced for the antiwear behavior of tricresyl phosphate appears to be incorrect.

With rough surfaces, the improvement in load-carrying capacity with increasing viscosity was less than that shown previously with smooth surfaces. Also, oils with a large pressure-viscosity coefficient did not show the expected beneficial effect with rougher surfaces.     

齿面粗糙度对低速重载齿轮磨损性能的影响

本文用自制磨损实验机对低速重载齿轮进行了滑动磨损模拟实验,通过实验研究了齿面粗糙度对低速重载齿轮磨损性能的影响,并得出了有益的结论。     

Coupled Effects of Fractal Roughness and Self-Lubricating Composite Porosity on Lubrication and Wear

The lubricant characteristics of porous self-lubricating composites with a realistic rough surface are incorporated into an improved elastohydrodynamic model. The evolved model demonstrates that the wear rate can be measured by examining the lubricant distribution at various fractal dimensions and porosities. The results show that the physical nature of the rough surface topography and the composite's physical properties must be understood, because the relative contact area is enlarged and friction forces are increased by the increase in the fractal dimension and the porosity. It is obvious that the method can significantly improve the lubricant properties to avoid wear by controlling these two coupled effects. The research also indicates that optimization of the design the microstructure of the porous self-lubricating composite should focus on the porosity based on the wear rather than the amount of lubricant.     

The Scale Effect of Roughness on Hydrodynamic Contact Friction

Multistage abrasive finishing processes (grinding, polishing, honing, etc.) are commonly used to produce the geometrical properties of a surface to meet its technical functionalities in the operating characteristics of contacting parts in friction, relating to their durability and reliability (running-in performance, wear resistance, load-carrying capacity, etc.). Coarse abrasive grits followed progressively finer ones are used, which leads to a multiscale stratified surface texture.

In this article, a numerical model of elastohydrodynamic (EHD) contact coupled to a multiscale surface texture model was developed that allows tracking the scale effect of surface features and their interactions on friction performance and lubricant flow under hydrodynamic lubrication conditions. Because the simulation model has as an input the surface topography and to overcome the variability in surface finish formation, textured surfaces at different stages of the finishing process were simulated (virtual texturing method). Surface topography can be decomposed into two principal components: superficial roughness and valleys. Superficial roughness was modeled using a fractal model and a scaling factor was introduced to model valley patterns. The results show the relationship between friction and surface scales.     

Effect of EHL Contact Conditions on the Behavior of Traction Fluids

New infinitely variable transmission (IVT) systems are under development for the automotive industry as a means to achieving significant fuel economy benefits. These systems rely on the lubricating fluid to transmit the drive train loads across the interface of the transmission components. This requires the development of new fluids that exhibit high traction properties under elastohydrodynamic lubrication (EHL) conditions. However, it has been reported recently that the traction performance of some fluids can reduce dramatically as temperature is reduced. This may place severe operational limits on IVT systems and suggests that the low-temperature traction properties of fluids for these systems should be studied in order to understand the mechanism for the observed reduction in traction.

The work reported here is an experimental study aimed at identifying whether low temperature traction reduction is related to a fundamental change in rheological behavior specific to the fluids tested or to more generic changes in the EHL contact conditions. A series of model experiments were performed using a mini traction machine (MTM) on three high-viscosity polybutene samples. The results have been mapped against previously reported non-dimensional parameters used to identify different EHL regimes. The results show that dramatic reductions in traction occur when the contact transitions from the rigid piezo-viscous (RP) toward the rigid iso-viscous (RI) region. Similar results were also found for two other high-viscosity fluids of different molecular structure and lower traction properties. The results support the hypothesis that the reduction in traction observed at low temperature is due to a change in EHL contact conditions rather than being solely due to a change in the rheological performance of the test fluids.     

Effects of the Lubricant Piezo-Viscous Properties on EHL Line and Point Contact Problems

This paper presents the application of the free-volume viscosity model in a Newtonian elastohydrodynamic line and point contact simulation using a more effective multigrid approach. According to recent experimental studies using high pressure viscometers, the free volume-based pressure–viscosity relationship closely represents the realistic piezo-viscous behavior for the high pressure typically encountered in elastohydrodynamic applications [1]. The effects of different pressure–viscosity relationships, including the exponential model, the Roelands model, and the free-volume model are investigated through an example with poly-alpha-olefin lubricant. It is found that the real pressure–viscosity behavior predicted by the free-volume model yields a higher viscosity at the low-pressure area, which results in a larger central film thickness. The fact that film thickness is formed mainly by the entraining action at the inlet area significantly weighs the importance of viscosity variation from different models in this area. The inlet area is a low-pressure area, and accordingly, the real viscosity of the lubricant predicted by Doolittle model undergoes a rapid increase in a convex function, being apparently larger than the Roelands one. Furthermore, the Doolittle model leads to higher pressure spike amplitude than that observed using the Roelands model. To solve the problem, a full multigrid approach has been used upon the assumptions of isothermal condition. Multigrid is more effective because it uses coarser grid levels to remove errors of different frequencies, which could be more quickly smoothed away than those on simply the fine grid alone. The developed coarse grid correction cycle proves to be an efficient tool to solve the EHL problem for a wide range of load conditions.     

Experimental Study of Lubricant Film Thickness Behavior in the Vicinity of Real Asperities Passing through Lubricated Contact

This article presents an experimental study of the influence of real surface micro-geometry on the film thickness in a circular elastohydrodynamic (EHD) contact formed between a real, random, rough surface of steel ball and smooth glass disk. Phase shifting interferometry was used to measure in situ initial undeformed rough surface profiles, whereas thin film colorimetric interferometry provided accurate information about micro-EHD film thickness behavior over a wide range of rolling speeds. Two real roughness features were studied in detail—a 56-nm-high ridge and a 90-nm-deep groove, both transversely oriented to the direction of surface motion. It was shown that the ridge is heavily deformed in a loaded contact and its height increases with increasing rolling speed. The asperity tip film thickness behavior is quite similar to the contact average film thickness when the film thickness is higher than the undeformed ridge height. However, below this limit the film is thicker than what the EHD theory predicts. For the groove, a local reduction in film thickness at the leading edge was observed. When the groove is passing through the EHD conjunction, it maintains its undeformed shape. The behavior of both roughness features studied shows good agreement with previous experimental observations conducted using an artificially produced ridge and groove.     

Analysis of the Fretting Conditions in a Contact Between an Epoxy Thermoset and a Glass Counterface

The fretting conditions in a contact between an epoxy thermoset and a glass counterface have been investigated using a specific device which allows in situ observation of the contact area. The critical displacement for transition from partial slip to gross slip conditions was investigated by the in situ detection of the micro-displacements and by the analysis of the fretting loops. Experimental results were in good relation with the theoretical predictions derived from Mindlin's approach of incipient sliding. Depending on the loading conditions, a progressive change from gross slip to partial slip conditions was found to occur during the early stages of the fretting loading, i.e., before the appearance of any macroscopic damage such as cracking or particle detachment. These fretting conditions were synthesized in a fretting map giving the boundary between various fretting regimes as a function of the normal load, the imposed displacement and the number of cycles.     

表面粗糙度测量和精度计算方法的探讨

零件表面粗糙度测量和精度评定中,通常是用Rz参数反映表面粗糙度,虽然它是个最或然值,但作为精度分析来讲,它不能很充分地反映表面粗糙度的偶然误差特性。为此提出一种以观测值改正数求零件表面粗糙度的观测值中误差方法,它能更明确地区分表面粗糙度同一系列中的差异。     

Effect of Surface Roughness on Hydrodynamic Lubrication of Slider Bearings

The effect of surface roughness on the performance of hydrodynamic slider bearings is studied. A generalized form of surface roughness characterized by a stochastic random variable with non-zero mean, variance and skewness is assumed to define the bearing surface topography. Various film shapes such as: plane slider, exponential, secant and hyperbolic are considered. The results are obtained for the general lubricant film shape in integral form which are numerically computed for the shapes under consideration. The results are presented both graphically as well as in tabular form. The performance of a rough bearing can be considered in terms of an identical smooth bearing with an equivalent film thickness. It is observed, for the lubricant film shapes under consideration, that the increasing positive values of α, σ and ε decrease the load carrying capacity, frictional force and temperature rise while it increases the coefficient of friction. Increasing positive values of α and ε shift the center of pressure towards the outlet edge. For negative values of α, the increasing value reverses the trend of the effect on performance characteristics which is in conformity with the physical aspects of the problem. A similar trend is observed in case of the effect of negative values of ε. Thus, a negatively skewed surface roughness modifies the performance of the slider bearings whereas the performance of a bearing suffers on account of positively skewed surface roughness. Moreover, it is noticed that in the case of exponential and hyperbolic slider bearings the effect of increasing values of σ is more pronounced whereas in case of plane slider and secant shaped slider this effect is marginal.     

A New Roughness Parameter Including Hardness and Contact Frequency Effects on Lubricated Rolling/Sliding Wear

Surface roughness, roughness arrangement, film thickness, material hardness, and run-in process have significant effects on the lubricated rolling/sliding wear of mechanical components such as gears and bearings. In conventional analysis, a film thickness parameter is calculated by a geometric approach to study the wear resistance of a contact system without considering the effects of material hardness and run-in process. Although the conventional parameter is simple, it does not correlate with some experimental observations. In this work, a new roughness parameter is developed for the prediction of lubricated rolling/sliding wear. Surface roughness will be adjusted by its hardness and contact frequency. The calculation results are consistent with four groups of experimental data. It is proved that the conventional models can be derived as a special case of the new model when two contact surfaces have the same properties. The new model can be used in the optimal design and manufacturing of mechanical interfaces to reduce lubricated rolling/sliding wear.     

表面粗糙度对圆柱滚子轴承弹流润滑性能的影响

为了揭示表面粗糙度对圆柱滚子轴承线接触稳态弹流润滑性能的影响,本文建立了具有表面粗糙度的圆柱滚子轴承弹流润滑模型,并推导出了摩擦系数方程;采用有限差分法求解了圆柱滚子轴承的弹流润滑性能,并分析了余弦粗糙度幅值、波长和纹理角度对圆柱滚子轴承弹流润滑性能的影响.数值结果表明:随着粗糙度幅值的增大,油膜厚度和油膜压力在粗糙度...