弹流润滑入口凹陷现象的数值模拟

入口凹陷(inlet-dimple)是近年来弹性流体动力润滑(EHL)研究中出现的一类新油膜形状特征。使用Circular流变模型,利用数值分析,研究了滑滚比、载荷及卷吸速度等对凹陷的影响,并与部分已有实验结果进行了比较。初步结果显示,当滑动较大时,入口区油/固体界面处表观粘度剧烈下降,产生伪滑移,从而诱发了较大的压力梯度而形成入口凹陷。     

Thermal transient rough EHL line contact simulations by aid of computational fluid dynamics

Reynolds equation is the pre-dominantly used PDE for modelling the fluid flow or more accurately the fluid pressure in an elastohydrodynamic lubrication (EHL) contact. The equation is derived by combining the two conservation equations of momentum and continuity into a single equation for the fluid pressure. The numerical approach for theoretical investigations performed on EHL contacts in this work is somewhat different. The modelling of the fluid flow is based on a computational fluid dynamic (CFD) technique. The fluid flow is simulated by aid of the equations of momentum and continuity in a more complete form and when the thermodynamics is incorporated, the equation of energy. The aim of the investigation was to examine whether the CFD technique could be used to handle thermal transient rough EHL line contacts. It is shown that commercial CFD software can be modified to meet such requirements. The influence of thermal effects on the flow under sliding motion was investigated. The non-Newtonian model used in this work is the Ree-Eyring model. It is shown that the choice of the Eyring stress in the model influences flow in the contacts. If the thermal properties of the surrounding solids differ, it has been shown experimentally and theoretically that a dimple or increased central film thickness may appear in the EHL contacts. This work shows that the governing mechanisms that result in the dimple are also present in thermal transient rough EHL line contacts.     

挤压条件下弹流油膜的摩擦学现象研究

纯挤压条件下弹流油膜的研究已证实了油膜的中央凹陷。研究利用常载荷下的钢球在较小的初始间隙下冲击附有润滑油的玻璃盘,在接触区外围出现了外围凹陷,而不是熟知的中央凹陷。研究结果表明当初始的冲击间隙较大时,油膜的压力分布和厚度以及中心压力-时间曲线中的峰值和接近结束时的中央凹陷都与以前的自由落球问题相似。随着初始的冲击间隙减小,最大压力从接触区中心转换到圆周外围区,相应地,中央凹陷变得越来越不明显而在外围区出现了圆周凹陷,进一步的数值分析,发现当油膜足够厚时在小的初始间隙条件下也出现了外围凹陷。这主要是当初始的冲击间隙变小时,中央油膜厚度比大冲击间隙条件下润滑油被"冰冻"时薄很多,中央凹陷不明显;在外围区域,表面间隙很小,局部挤压效应变强,因此形成了外围凹陷。     

Movement of Entrapped Oil Under Pure Rolling Conditions

This article presents a study on the movement of an oil entrapment (or impact dimple) in a pure rolling elastohydrodynamic lubricated (EHL) contact. The oil entrapment was formed by impacting a steel ball against a lubricated glass disc. The contact was then activated under pure rolling conditions, and the movement of the entrapped oil was visualized by optical interferometry. It was found that during the movement of the dimple within the EHL contact, there exists a critical value for the displacement of dimple core. For the displacement of the dimple core less than the critical value, the dimple moves at the entrainment velocity and the film thickness of dimple core remains almost constant. For displacement beyond the critical value, the dimple slows down and its depth decreases rapidly. The effects of influential factors such as speed, initial dimple depth, load, and initial gap size were theoretically and experimentally investigated.     

Effect of Sliding Direction on EHL Film Shape Under High Sliding Conditions

It is well known that a sliding speed influences a lubricant film thickness of elastohydrodynamic rolling–sliding contacts significantly. The effect of sliding is described quite well for unidirectional rolling and sliding; however, there are a limited number of papers dealing with sliding in different directions. This study describes how the sliding direction influences elastohydrodynamic film shape under high sliding conditions. An optical ball-on-disc tribometer together with thin-film colorimetric interferometry method is used for a film thickness measurement. The results show that the sliding direction influences lubricant film shape and the effect is connected with dimple phenomena. The temperature–viscosity wedge effect is discussed as a possible mechanism. The results are important for a film thickness prediction under high sliding conditions and provide experimental evidence for an extension of elastohydrodynamic lubrication (EHL) theory.     

Experimental observation of a dimple-wedge elastohydrodynamic lubricating film

One of the main features of typical elastohydrodynamic lubricating (EHL) contacts is the unique horseshoe film shape, which can be readily observed by using interferometry and quite accurately modelled by the well-established EHL theory. However, an anomalous EHL film, characterized by a wedge shape together with a tiny dimple at the inlet region, is observed under pure sliding conditions with ultra slow speeds of 3–800 μm/s in an optical EHL test rig. The variations of the wedge and the inlet dimple with different sliding speeds and loads are investigated using a series of polybutene oils of high viscosities. It is found that the inclination of the wedge is dependent on sliding speeds, loads and oil viscosities. The dimple always occurs at the inlet. The appearance of an inlet dimple together with a wedge film shape is reported for the first time. The phenomenon can be attributed to a non-Newtonian characteristic of the lubricant: the limiting shear strength. Additionally, the influence of starvation on the film shape is also examined.     

Monochromatic image analysis of elastohydrodynamic lubrication film thickness by fringe intensity computation

Point contact film thickness in elastohydrodynamic lubrication (EHL) is analyzed by image processing method for the images from an optical interferometer with monochromatic incident light. Interference between the reflected lights both on half mirrorCr coating of glass disk and on super finished ball makes circular fringes depending on the contact conditions such as sliding velocity, applied load, viscosity-pressure characteristics and viscosity of lubricant under ambient pressure. In this situation the film thickness is regarded as the difference of optical paths between those reflected lights, which make dark and bright fringes with monochromatic incident light. The film thickness is computed by numbering the dark and bright fringe orders and the intensity (gray scale image) in each fringe regime is mapped to the corresponding film thickness. In this work, we developed a measuring technique for EHL film thickness by dividing the image patterns into two typical types under the condition of monochromatic incident light. During the image processing, the captured image is converted into digitally formatted data over the contact area without any loss of the image information of interferogram and it is also interpreted with consistency regardless of the observer’ s experimental experience. It is expected that the developed image processing method will provide a valuable basis to develop the image processing technique for color fringes, which is generally used for the measurement of relatively thin films in higher resolution.     

A novel approach to measure slip-length of thin lubricant films under high pressures

Abundant evidence of boundary slip at liquid/solid interfaces has been presented by experiments and theoretical analyses. Recent research reveals that the boundary slip effect of lubricants can be obvious in small confined gaps, even with simple Newtonian fluids. Therefore, it is appropriate to ask if there is boundary slip of thin highly pressurized lubricant films in EHL contacts, and, if so, how to quantify it. This paper proposes a method for measuring the slip-length of lubricant films within a small gap under high pressure. Measurement principles have been addressed in details in the paper. The idea makes use of the phenomenon whereby a tiny quantity of lubricant can be entrapped and forms a dimple film by impacting a steel ball against a lubricated glass plate. The slip-length can be extracted from the relative movement of the dimple against the solid surfaces. Case experiments are also presented to validate the feasibility of the method.     

Low viscosity shear heating in piston skirts EHL in the low initial engine start up speeds

Absence of elastohydrodynamic lubricating (EHL) film causes piston wear in low speed cold initial engine start up, while shearing of low viscosity lubricant in few cycles affects its load carrying capacity. Shear heating effects are incorporated in 2-D hydrodynamic and EHL model by solving 2-D heat equation. EHL pressures are calculated using inverse solution technique. Comparative analysis is based on viscous dissipation coupled with piston motion, changes in pressure, film thickness and viscosity. This study suggests that the increase in temperature varies with speed to affect piston eccentricities, viscosity and film thickness. This optimizes low start up speed for a few engine cycles.     

Lubricant Flow in an Elastohydrodynamic Contact Using Fluorescence

It is well-documented that parameters, such as film thickness and temperature in EHL contacts, can be measured experimentally using a range of techniques include optical interferometry, ultrasonics, capacitance and infrared emission. Considerably less is known, however, about the flow of lubricant through such contacts. Information about lubricant flow would greatly benefit the prediction of friction in machine components. This article describes initial steps to develop fluorescence as a means of observing lubricant flow. An EHL contact was produced between a steel ball and a glass disc and viewed using a fluorescence microscope. The entrained lubricant was dyed using a fluorescent species, so that when illuminated with laser light, a fluorescence intensity map could be viewed. When the contact was fully flooded with dyed lubricant, the fluorescence intensity within the contact correlated well with optical interferometric film thickness measurements under the same conditions. This suggests useful possibilities for mapping film thickness in contacts where conventional optical methods are impractical, such as between rough surfaces and within soft contacts. In order to observe how lubricant flows in an EHL contact, fluorescer-containing lubricant was placed on the out-of-contact track. The boundary between fluorescent and non-fluorescent lubricant was then entrained into the contact and the passage of the boundary through the contact was monitored.     

Evidence of lubricant slip on steel surface in EHL contact

The limiting traction provided by typical elastohydrodynamically lubricated (EHL) contacts leads to the postulation that liquid lubricants are subject to limiting shear stress, which is generally accepted as an intrinsic property of the lubricants. The results of recent optical EHL research show that lubricant at EHL contacts may slip on the Cr-coated glass surface under certain circumstances. This paper presents further evidence that high pressure EHL film can slip on a steel surface. Because the steel/steel contacts are common in typical traction drives and the interfaces are therefore oil/steel, the deduction of the limiting shear stress of lubricants from the measured limiting traction may simply reflect a property of the system should boundary slippage occur.     

圆柱滚子轴承弹流接触副刚度及阻尼系数研究

针对圆柱滚子轴承中滚子-滚道弹流接触副,建立有限长线接触非稳态弹流润滑模型,利用追赶法、快速傅里叶变换和Newmark技术数值求解接触副在自由振动下的衰减曲线。以刚体接近距离的变化作为判断接触副振动的标准,结合弹流润滑模型和有阻尼系统的自由振动模型,给出预测滚子-滚道弹流接触副动力学参数的方法,考察初始扰动量、润滑剂黏压系数、滚子长度和载荷对刚度和阻尼系数的影响。结果表明：小扰动下,初始扰动量大小对刚度和阻尼系数的影响可忽略不计;弹流润滑下的刚度小于干接触下的Hertz接触刚度;增加润滑剂黏压系数、滚子长径比和载荷,均可增大弹流接触副的刚度系数;阻尼系数随润滑剂黏压系数的增加而减小,随滚子长径比的增加而增加,随一定范围内载荷的增加而减小。     

往复运动条件下润滑油膜特性的实验观察

采用多光束干涉技术观察往复运动条件下润滑油膜的滑移及黏弹特性,研究振幅和频率对往复动态润滑弹流油膜的影响。结果表明:往复运动过程中,在特定时刻气穴的出现使油膜厚度逐渐减小,削弱了滑移程度;因润滑油的黏弹性而引起的运动滞后导致了油膜的非对称性;频率增大时,正行程末端时膜厚明显增大,油膜输送速度也随着增大;而负行程末端油膜受气穴的影响膜厚增大较慢;振幅(输入位移)增大时,正行程末端时油膜整体平移,而负行程末端入口凹陷呈现先变明显而后消失的现象。     

Theoretical and Experimental Investigations on EHL Point Contacts with Different Entrainment Velocity Directions

Film thickness prediction plays an important role in evaluating the performance and durability of machine elements under elastohydrodynamic lubrication (EHL). However, current formulae may not be appropriate for general conditions occurring in real contacts. This study investigates the effect of different lubricant entrainment velocity angles on film thickness distribution. For this purpose, a steady-state isothermal EHL model is used under a wide range of parameter sets including varying sum velocity, contact pressure, and sum velocity angle. Considerable differences in the trend of the central film thickness with respect to the lubricant entrainment velocity angle for low and high loaded contacts are shown. The results are compared with experimental measurements by means of an optical ball-on-disc tribometer and a twin-disc machine using capacitance method. Good agreement between numerical results and experimental measurements was found.     

虚拟弹性流体润滑实验台的研究

弹性流体动力润滑是摩擦学主要研究领域,由于弹流状态下润滑剂所处的特殊条件,其实验过程十分复杂,获取数据也非常困难。本文利用数值计算方法和计算机技术。在计算机上虚拟显示实验过程和结果,形象地表现出弹流实验的现象及特征,为弹流实验教学和研究提供了一种快速和有效的工具。     

Experimental Validation of Critical Temperature-Pressure Theory of Scuffing

A series of experiments was conducted for validating a newly developed theory of scuffing. The Critical Temperature-Pressure (CTP) theory is based on the physisorption behavior of lubricants and is capable of predicting the onset of scuffing failures over a wide range of operating conditions, including the contacts operating in the boundary lubrication and in the partial elastohydrodynamic lubrication (EHL) regimes. According to the CTP theory, failures occur when the contact temperature exceeds a certain critical value which is a function of the lubricant pressure generated by the hydrodynamic action of the EHL contact. A special device capable of simulating the ambient conditions of the partial EHL conjunctions (of contact temperature, pressure, and the lubricant pressure) was constructed. A ball-on-flat type wear tester was put inside a pressure vessel, completely immersed in a highly pressurized bath of mineral oil. The temperature on the flat specimen was gradually increased while the ball was slowly traversed. At a certain critical temperature, the friction force abruptly jumped indicating the incipiency of the lubrication breakdown. This experiment was repeated for several levels of hydrostatic pressure and the corresponding critical temperatures were obtained. The test results showed an excellent correlation with the newly developed CTP theory.     

The Prediction of Contact Pressure–Induced Film Thickness Decay in Starved Lubricated Rolling Bearings

Under starved conditions the thickness and distribution of the lubricant film in an elastohydrodynamically lubricated (EHL) contact is directly related to the distribution of lubricant on the track in the inlet to the contact. In starved lubricated rolling bearings this lubricant distribution is determined by many effects. The authors have developed a model to predict the oil lost from the track induced by EHL pressure with no replenishment. A complete bearing is modeled with multiple rolling element EHL contacts and with the applied load to the rolling elements varying along the circumference of the bearing. Results of the oil layer thickness on the track are presented for a ball bearing and a spherical roller bearing for different bearing loads and rotational speeds. The predicted layer thickness decay rate for a ball bearing is significantly larger than for a spherical roller bearing and the predicted effect of the bearing load on the decay rate is small compared to the effect of the rotational speed. The predicted decay periods due to the contact pressure effect are small compared to the observed (grease) life of bearings. The results show that a bearing cannot sustain an adequate layer of oil on the running track unless significant replenishment takes place.     

A Quantitative Solution for the Full Shear-Thinning EHL Point Contact Problem Including Traction

We present a realistic elastohydrodynamic lubrication (EHL) simulation in point contact using a Carreau-like model for the shear-thinning response and the Doolittle-Tait free-volume viscosity model for the piezoviscous response. The liquid lubricant modeled is a high-viscosity polyalphaolefin which has been shown by high-pressure viscometry to possess a relatively low threshold for shear-thinning as a single-component liquid lubricant. As a result, the measured EHL film thickness is about one-half of the Newtonian prediction. We derived and numerically solved the two-dimensional generalized Reynolds equation for the modified Carreau model based on Greenwood. In this simulation, viscosity was not treated as an adjustable parameter; the models used for the pressure and shear dependence of viscosity were obtained entirely from viscometer measurements. Truly remarkable agreement is found in the comparisons of simulation and experiment for traction coefficient and for film thickness in both pure rolling and sliding cases.     

零卷吸预跑合对一类反常弹流油膜的影响

在极低速纯滑动的光弹流实验中,采用高粘度聚丁烯润滑剂形成的弹流油膜会在入口区出现凹陷,该反常的入口凹陷与极限剪切应力/界面滑移有关。针对盘纯滑和球纯滑2种不同的运动条件进行了油膜形状的测量,分析了零卷吸预跑合对油膜形状的影响。结果表明,一般地,纯玻璃盘滑动和纯钢球滑动产生的此类反常的油膜形状并不相同;当对弹流接触副采用零卷吸预跑合处理(即钢球和玻璃盘在接触区以大小相同方向相反的速度运动)之后,纯玻璃盘滑动形成的油膜形状有较大变化,油膜厚度增加,入口的楔形斜度下降,此时纯玻璃盘滑动和纯钢球滑动产生的油膜形状差别减小,甚至相同。     

Elastohydrodynamic Lubrication of Circular Contacts at Impact Loading with Generalized Newtonian Lubricants

In this article, pure squeeze elastohydrodynamic lubrication (EHL) motion of circular contacts with power law model lubricant is explored at impact loading. The coupled transient modified Reynolds, the elasticity deformation, and the ball motion equations are solved simultaneously, thus obtaining the transient pressure profiles, film shapes, normal squeeze velocities, and accelerations. The simulation results reveal that the greater the flow index (n), the earlier the pressure spike and the dimple form, while the maximum pressure and the film thickness increase, and the diameter of the dimple, the maximum value of the impact force, the rebounding velocity, and the acceleration decrease. Further, this analysis numerically demonstrates that the contact central pressure for a ball impacting and rebounding from a lubricated surface reached two peaks during the total impact period. As the flow index increases, the primary and the secondary peak increase, and the first and second peaks form earlier; as the total impact time decreases. Moreover, the phase shift between the time of the peak value of the squeeze acceleration and the zero value of the squeeze velocity increase with increasing flow index.