Effect of surface roughness on elastohydrodynamic traction: Part 1

The effect of surface roughness on traction performance was experimentally studied using a two‐roller tester. The nature of the contact was investigated using electrical resistance and electrical capacitance methods. Increased shear stress was observed in the viscous region, which could be attributed to a higher average shear rate and to greater viscosity under EHD contact conditions due to pressure fluctuations caused by the surface texture.     

单轨车辆耦合动力学模型与振动响应特性

为了研究单轨车辆在设计车速范围内的振动响应特性,进而评价单轨车辆运行平稳性与舒适性能,分析了单轨车辆走行轮、导向轮、稳定轮和转向架中央牵引装置及车体间的拓扑关系,利用Hamilton方程构建了包含走行轮与轨道梁顶部,导向轮、稳定轮与轨道梁侧部等三向轮轨接触对的单轨车辆空间耦合系统动力学模型,并对单轨的PC轨道梁走行面及轨道梁左、右两侧部轨面的不平度进行了数值模拟。基于空间耦合动力学模型,以轨道不平度为激励源,设定车辆以某一恒定速度在直线轨道上运行,获取了单轨车辆的振动响应特性,并与实车测试结果对比分析,验证了单轨空间耦合动力学模型的正确性。在此基础之上,获取了单轨车辆不同设计车速下的振动响应特性,对单轨车辆的运行平稳性和舒适性进行了评价。结果表明,单轨车辆的舒适性能优良,运行平稳性能处于优秀等级。     

Effect of Tangential Traction and Roughness on Crack Initiation/Propagation During Rolling Contact

Rolling-fatigue tests of 0.45 percent carbon steel rollers were carried out using a four-roller-type rolling-contact fatigue tester. Tangential traction and surface roughness of the harder mating rollers were varied and their effect was studied. The results of the study indicate that the fatigue life decreases when traction is applied in the same direction as that of rolling. When the direction of traction is reversed, the life increases over that obtained with zero traction. The roughness of harder mating rollers also has a marked influences on life. The smoother the mating roller, the longer the life. Microscopic observation of specimens revealed that the initiation of cracks during the early stages of life is more strongly influenced by the surface roughness, while the propagation of these cracks in the latter stages is affected mainly by the tangential traction.     

Tribological behaviour of steel‐alumina sliding pairs under boundary lubrication conditions

The tribological behaviour of oil‐lubricated steel‐alumina sliding pairs was investigated using a ball‐on‐disc tribometer at room temperature. Commercial bearing balls of 10 mm diameter were mated to 99.7% Al₂O₃ discs, and additive‐free mineral oil was fed into the contact area. The sliding speed and the applied normal load were varied, and the initial surface roughness of the Al₂O₃ disc was altered using different polishing and grinding procedures. The results showed that the surface roughness of the ceramic discs dominated the tribological behaviour under the given experimental conditions. The sliding speed as well as the normal load showed less effect on the friction behaviour, but the amount of wear depended strongly on the normal load. From the results it was concluded that improvement of the surface roughness and optimised surface machining of the ceramic material can be essential for improving the tribological performance for boundary‐lubricated steel‐ceramic sliding pairs.     

表面粗糙度对金属平垫泄漏率的影响

为探究金属垫片表面形貌对泄漏率的影响,通过三维形貌仪对不同表面粗糙度的Cu、Al和316L金属垫片进行局部扫描,构建出真实法兰垫片的微观接触模型。在此基础上,进行静力学和计算流体力学分析,通过对流体域设置不同的边界条件,考察不同介质压力下,表面粗糙度和接触压力对泄漏率的影响。仿真结果表明：3种材质的垫片均呈现表面粗糙度越大,则泄漏率越大的趋势;其中,Al垫片的泄漏率对表面粗糙度最为敏感,316L垫片的泄漏率受表面粗糙度的影响最小;当表面粗糙度一定时,施加的接触压力越大,泄漏率越低,但当接触压力达到一定值后,增大接触压力对降低泄漏率的影响将越来越小。     

Twin disc micropitting tests

Gears are important power transmission components and their efficiency and durability strongly depend on their tribological performance. The demand for higher efficiency has further increased the requirements for controlled tribological performance of gear contacts. One of the typical tribological failures that gears experience is micropitting, which is a fatigue failure of the surface strongly associated with low lubricant film thickness. Micropitting is claimed to be the dominant life limitation for ground hardened gears operating under modest conditions. In this work the factors affecting micropitting performance of gears were studied. Twin disc experiments were carried out to investigate the effect of surface roughness, lubricant type and surface treatment on micropitting performance of two case hardening steels. The test results showed that the roughness of the gear has a major impact on the micropitting performance. Also the use of DLC coating or carbonitriding surface treatment and the use of ester type lubricant had a beneficial effect compared to untreated or mineral oil lubricated surfaces.     

Increase of Traction Coefficient due to Surface Microtexture

Increasing the traction coefficient of a traction drive system is a key factor in obtaining a smaller, lighter unit and also greater torque capacity. This study focused on the microtexture of the rolling elements, and effect of microtexture was examined with the aim of improving the traction coefficient in the viscous region. Three textures—dimple, transverse, and longitudinal—were examined using a 4-roller tester that enabled tests to be conducted under high pressure and high rolling speed. As a result, it was found that the longitudinal surface texture is the best for improving the traction coefficient. The results obtained with EHL analysis showed that only the surface texture with longitudinal grooves increased the traction coefficient, just as in the tests conducted with the 4-roller tester. The longitudinal surface texture was optimized using the 4-roller tester. The test results made it clear that the groove depth, groove pitch, and also the radius of curvature of the convex portion of the rolling elements are important parameters of the longitudinal grooves for improving the traction coefficient while assuring high durability at the same time. An attempt was then made to increase the traction coefficient of an actual CVT variator by applying the optimized longitudinally grooved microtexture to the traction surfaces. The test results show that the traction coefficient can be increased without sacrificing durability by optimizing the surface microtexture.     

CFD analysis of performance change in accordance with inner surface roughness of a double-entry centrifugal pump

In this study, the performance change of a double suction centrifugal pump (Q = 60 m³/min, H = 97 m) was analyzed using Computational fluid dynamics (CFD) to investigate the effects of internal surface roughness of pump components. The calculated performance of the pump using CFD is in strong agreement with the experimental results, which used a smooth wall case. In terms of the predicted total pump efficiency with the surface roughness case, the CFD results indicate that the pump efficiency is reduced by approximately 3.0 %. CFD results reveal that the most significant roughness effect on the pump components is that of the impeller, whereas the smallest effect is that of the inlet casing. Furthermore, the CFD results demonstrate that the pump performance is strongly dependent on the outward shroud surface roughness of the impeller.     

Behavior of Traction Oils under Impact Loads

The squeeze film forming ability of traction fluids is studied under impact load by a falling bearing steel ball against a stationary cylindrical surface or flat anvil made of mild steel, bearing steel, or aluminum. The effect of the pressure-viscosity coefficient and of the viscosity is investigated for plastic and elastic impact.

For soft materials, oils with high pressure-viscosity coefficients maintain much greater surface separation under impact load. The time fraction of separation by the oil film increases linearly with the product of the pressure-viscosity coefficient and the yield pressure of the anvil material. It decreases with an increase in surface roughness. Similar results are observed under impact on stationary or rotating surfaces. These results can be used to estimate the solidification behavior of traction oils. In the case of the elastic impact for hard materials, the coefficient of restitution of the ball is influenced mainly by the squeeze film effect, which is governed by viscosity itself.     

表面粗糙度对配流副润滑特性的影响

为研究表面粗糙度对轴向液压柱塞泵马达配流副润滑特性的影响,引入Weierstrass-Mandelbort分形函数,对不同幅值的表面粗糙度的表面形貌进行二维和三维模拟,建立考虑表面粗糙度的流固热耦合下的配流副油膜润滑模型,采用中差分形式的有限差分法和松弛迭代法对其进行数值求解,并分析油膜厚度、油膜压力、油膜承载力、摩擦因数等性能参数随着表面粗糙度幅值变化的规律。通过盘-盘形式的配流盘-缸体摩擦磨损试验,得到不同幅值的表面粗糙度下配流副摩擦因数,对所建立的数学模型进行验证。数值计算结果表明,表面粗糙度幅值的增大会引起油膜承载力增大,但也会引起最大油膜压力和摩擦因数的增大,导致摩擦性能下降。摩擦磨损试验发现,表面粗糙度增大,配流盘表面摩擦磨损情况加剧,配流副润滑性能和耐磨性能整体降低。因此在配流盘表面加工处理中,应适当降低其表面粗糙度。     

Thermal analysis of lubricated three-dimensional contact bodies considering interface roughness

Surface roughness and thermal action are of remarkable importance in the lubrication performance of mechanical components, especially in extreme conditions. However, available studies mainly focus on the full-film lubrication conditions without considering temperature rise and real 3D surface roughness due to the complexity of surface topography and temperature characteristics. Moreover, studies on the interfacial thermal behaviors of 3D rough surface lubricated contact in an extended range of working conditions remain limited. In this paper, a deterministic mixed thermal elastohydrodynamic lubrication model considering real 3D surface roughness and thermal effects is proposed. In this model, pressure and temperature are coupled with each other, the computation of elastic deformation is accelerated through the discrete convolution and fast Fourier transform method, the temperature field is calculated with the column sweeping technique, and the semi-system method is introduced to improve convergence and numerical stability under severe conditions. The model is validated by comparing its results with available published numerical and experimental results. The thermal behaviors of the contact interface are studied in a wide range of working conditions. The influences of surface roughness and thermal effect on lubrication performance are revealed. The results show that the proposed model can be used as a powerful analysis tool for lubrication performance and temperature prediction in various heavy-load, high-speed lubricated components over a wide range of lubrication conditions.     

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.     

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.     

Investigation of micro-milling process parameters for surface roughness and milling depth

The main objective of this study is to investigate the micro-milling performance of the AISI H13 with different process parameters namely laser power, scan speed, frequency, and fill spacing using 30W fiber laser marking machine and to find the optimal operation conditions for minimum surface roughness and maximum milling depth. The 108 different combinations occurred with the interaction of each level of the parameters used in this study. Therefore, the main contribution of this paper to the related literature is that it produces new evidence regarding the effects of the multi-scan times on both surface roughness and milling depth. The experimental results are showed that 0.03?mm of fill spacing, the highest scan speed (800?mm/s), lowest frequency (20?kHz), and laser power (60%) produced better surface roughness, which is 1.75???m. The deeper cavity on the geometry is obtained under the experimental combination as 200?mm/s of scan speed, 0.02?mm of fill spacing, 60% of laser power, and 40?kHz of frequency, which is 195???m. The regression analysis was used to develop a mathematical model and determine the effect of process parameters on the surface roughness and milling depth. The results of subsequent tests verifies regression models.     

CRH3型动车组牵引传动系统的直接转矩控制研究

为了研究高速动车组的具体模型结构及其牵引传动控制系统,采用直接转矩控制(DTC)系统控制异步牵引电机,建立了CRH3型高速动车组牵引传动系统的Matlab/simulink仿真模型,整流部分由四象限脉冲整流器输出3 000 V左右平滑的直流电压,逆变部分由直接转矩控制系统驱动牵引电机。该系统能根据CRH3型动车组的牵引、制动曲线模拟动车组牵引、惰行、制动等各种运行工况。整个运行过程中系统的谐波较小,电压/电流相位差基本能保持在同相或反相运行,功率因数基本接近1;在网压波动和突然失电情况下,机车也基本能保持恒速运行。最后,将该模型仿真结果与京津城际高速铁路的部分数据进行了对比验证。研究结果表明,该模型建模基本正确,同时系统具有良好的稳态和动态性能,验证了直接转矩控制方法的有效性。     

Hydrodynamic analysis of rough curved pivoted porous slider bearings with couple stress fluid

Abstract

The purpose of this paper is to study the effect of surface roughness on the performance of curved pivoted porous slider bearings lubricated with couple stress fluid. The modified B–J slip boundary condition is utilised at the porous/fluid film interface to derive the Reynolds type equation for the problem under consideration. To mathematically model the surface roughness due to non-uniform rubbing of bearing surfaces, a stochastic random variable with non-zero mean, variance and skewness is considered. The closed form solution is obtained for the averaged Reynolds equation, and the compact expressions for the mean fluid film pressure mean load carrying capacity, frictional force and the centre of pressure are obtained. The numerical computations of the results show the improved performance due to the couplestresses and the presence of negatively skewed surface roughness. However, the presence of porous facing and positively skewed surface roughness affects the performance of the pivoted porous slider bearing.     

Surface roughness effects in porous inclined stepped composite bearings lubricated with micropolar fluid

In this paper, the effect of surface roughness on the performance characteristic of porous inclined stepped composite bearings is studied. A generalised form of surface roughness is mathematically modelled by a stochastic random variable with non‐zero mean, variance and skewness. The generalised average Reynolds‐type equation is derived for the rough porous inclined stepped composite bearings with micropolar fluid. The closed‐form expressions are obtained for the fluid film pressure, load‐carrying capacity and frictional force. The results are presented for three different types of bearing system. The numerical computations of the results show that the negatively skewed surface roughness pattern increases fluid film pressure and load‐carrying capacity and decreases the coefficient of friction, whereas adverse effects were found for the positively skewed surface roughness pattern. Further, the rough porous inclined stepped composite bearing provides the largest load‐carrying capacity and the least coefficient of friction as compared with the porous plane slider and porous composite tapered concave bearings. Copyright © 2012 John Wiley & Sons, Ltd.     

计入润滑油粘压效应和表面形貌的倾斜轴颈轴承润滑分析

提出了轴颈倾斜情况下的轴承油膜厚度方程。采用平均Reynolds方程，分析了倾斜轴颈轴承的润滑性能。通过不考虑和考虑润滑油粘压效应的轴承润滑性能对比，验证了分析倾斜轴颈轴承润滑性能时计入润滑油粘压效应的重要性；通过光滑轴承与粗糙轴承润滑性能的对比，验证了分析倾斜轴颈轴承润滑性能时考虑表面粗糙度影响的必要性。计算了不同偏心率、轴颈倾斜角、表面粗糙度、表面方向参数下的轴承润滑性能，结果表明，表面粗糙度在最小油膜厚度较小时对倾斜轴颈轴承润滑性能产生影响，而表面方向参数在最小油膜厚度与综合粗糙度的比值较小时会对倾斜轴颈轴承润滑性能产生显著影响。     

PERFORMANCE SIMULATION OF VEHICLES EQUIPPED WITH TRACTION DRIVE CVTS

0 INTRODUCTIONVehicles equipped with continuously variable transmission(CVT) have the potential of better fuel economy than vehicleswith conventional stepped ratio transmissions. Although the mar-ket penetration of CVT vehicles today is still insignifican…     

Traction Characteristics of Siloxanes with Aryl and Cyclohexyl Branches

The molecular structures, rheological properties, and friction coefficients of several new siloxane-based polymers were studied to explore their traction characteristics. The molecular structures including branch content were established by nuclear magnetic resonance spectroscopy, while the molecular mass distributions were determined by gel permeation chromatography. Density, viscosity, elastohydrodynamic film formation, and friction were investigated over a temperature range of 303–398 K. Film thickness and friction measurements were studied under the conditions that are representative of boundary, mixed, and full-film lubrication regimes, aiming at maximizing traction performance and temperature stability by simultaneous optimization of the size and content of ring-shaped branch structures. This study provides quantitative insight into the effect of siloxane molecular structure on the tribological performance for traction drive applications such as continuously variable transmissions.