Occurrence of Wall Slip in Elastohydrodynamic Lubrication Contacts

Preliminary experimental work has been carried out to identify some of the boundary slip phenomena of highly pressurised polybutenes in an elastohydrodynamic lubrication (EHL) conjunction. The movement of the oil is signified using an entrapment that can be readily formed by the impact of a steel ball against a layer of oil on a glass block in an optical EHL test apparatus. The post-impact lateral movement of the entrapment was investigated under the conditions: (i) pure rolling, (ii) pure glass block sliding (steel ball stationary) and (iii) pure ball sliding (glass block stationary). It was observed that under pure rolling the entrapped oil travels within the contact region at the entrainment speed, which is correlated with EHL theory. Under pure glass block sliding conditions, the speed of the entrapped oil core is less than the entrainment speed, and in the extreme cases, this core can be nearly stationary. Under pure ball sliding conditions, the oil core moves at a speed greater than the entrainment speed. The observation indicates that the oil/steel ball interface can sustain higher shear stress than the oil/glass (chromium coated) interface and there is a boundary slip in terms of relative sliding at the latter interface under the experimental conditions. Furthermore, the amount of slip increases with an increase in the pressure. These experiments provide evidence of the existence of wall slippage, which leads to the abnormal EHL film profile characterised with an inlet dimple as reported earlier.     

载荷对凹陷表面的动态微弹流润滑特征的影响

研究了表面凹坑在不同接触载荷和滚滑率的情况下,通过弹流润滑区时对油膜压力和形状产生的干扰现象。根据对微弹流润滑特征分析发现,只有在表面存在相对滑动的条件下,进入弹流接触区的表面凹坑才随接触载荷 的增加被逐渐地压平变浅;而在纯滚动条件下,进入弹流接触区的表面凹坑深度几乎与载荷大小无关保持不变;所有条件下因表面凹坑进入弹流接触区产生的油膜干扰凹陷深度与载荷大小无关而保持一个稳定的值。     

Localized Boundary Effect on Elastohydrodynamic Lubricated Film Shape

The study aimed to determine the formation of an adsorption film at elastohydrodynamic lubricated (EHL) contacts and its effects on EHL film shape and friction. Experiments were conducted on an optical EHL test rig with surfaces of different surface energies. A new type of “abnormal” EHL film shape characterized with three dimples in the inlet of the contact was obtained in pure ball sliding experiments with long-chain polybutene. The adsorption layer was inferred to be the main cause for the “tri-dimple” phenomenon. The “tri-dimple” formation was examined. Under a fixed speed, a single inlet dimple gradually broke into three dimples with increasing number of ball rotation, and it happened with slight increase in friction force. Three zones, namely a central and two lateral zones, of the contact were classified and characterized with different levels of influence on the adsorption layer.     

Elastohydrodynamic properties of water in oil emulsions

Elastohydrodynamic (EHL) film thicknesses of emulsions have been measured in a rolling point contact using an optical interference method. Both water in oil and oil in water emulsions have been studied.For water in oil emulsions the experimental data suggest that EHL film thickness is almost independent of water concentration and also particle size distribution, although the bulk viscosity of the emulsions is heavily dependent upon these two variables. It is concluded that the EHL properties of such emulsions are determined almost entirely by the EHL properties of the pure oil.For oil in water emulsions, negligible EHL film formation was observed.     

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.     

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。     

变速点、线接触副弹流润滑油膜的观测

应用光干涉方法,在自制的光弹流试验机上分别对纯滚动条件下点接触和线接触形成的弹流油膜进行变卷吸速度实验,并进行油膜测量。结果表明,在卷吸速度为零时都有封油现象的出现。随着卷吸速度的增加,油膜厚度增加,点接触形成的弹流油膜具有典型的马蹄形特征,线接触形成的弹流油膜在接触区端部有类似点接触的马蹄形收缩。要达到同样的最大赫兹接触压力,施加在线接触实验上的载荷要比施加在点接触实验上的载荷大40倍左右。     

Starved Grease Lubrication of Rolling Contacts

Many grease lubricated roller bearings operate in the starved elastohydrodynamic (EHL) regime where there is a limited supply of lubricant to the contact (1). Under these conditions the film thickness drops to a fraction of the fully flooded value (2) and, thus, it is difficult to predict lubrication performance, or bearing life, from conventional EHL models. In this regime film thickness depends on the ability of the grease to replenish the track rather than the usual EHL considerations. The conventional view of grease lubrication is that base oil bleeds from the bulk reservoir close to the track, replenishing the inlet and forming a fluid EHL film (3). Resupply, under starved conditions, will thus depend on both operating conditions and grease parameters. The aim of this paper is to evaluate the influence of these parameters on starved lubrication in a rolling contact. Starved film thickness has been measured for a series of greases and the results have been compared to the fully flooded values. These show that the degree of starvation increases with increasing rolling speed, base oil viscosity and thickener content but decreases at higher temperatures. In many cases an increase in absolute film thickness is obtained when moving from high viscosity base oil to a low one, this result is the reverse of normally accepted EHL rules. Taking the fully flooded film thickness as a guide to lubrication performance is therefore not valid as grease film thickness in the starved regime is determined by local replenishment rather than bulk rheological properties.     

Influence of spinning on the rolling EHL films

Spinning cannot be ignored in some elastohydrodynamically lubricated contacts. In this paper, spinning is incorporated into an elastohydrodynamic lubrication (EHL) contact of pure rolling and its influences on EHL films are studied. Results show that with increase in spinning, the symmetry of the film shape gets lost, and the minimum film thicknesses, located respectively at the two side-lobes, decrease and show more dependence on loads. The speed indices of the film thickness at the side lobes are higher than those of the classical EHL theory predicted. Numerical work has also been carried out to clarify the experiment measurements.     

Rolling Contact Fatigue Performance of Vibro-Mechanical Textured Surfaces

The rolling contact fatigue (RCF) performance of vibro-mechanical textured surfaces in a point elastohydrodynamic lubrication (EHL) condition is investigated. Two dimple designs, small (100 μ m × 100 μ m) and large (240 μ m × 100 μ m), are compared with a nontextured sample. Experimental RCF tests show that the textured surfaces exhibit a significantly reduced number of cycles to failure compared with the nontextured sample for the high load, pure rolling conditions evaluated. In order to understand these results, numerical models are used to calculate the lubrication and contact pressure conditions and the subsurface stress distribution. The fatigue failure trends observed experimentally are compared with the simulation results with good agreement. It is determined that RCF performance is related to the presence and size of the generated dimple.     

Effect of fluid rheology on the thermal EHL under ZEV in line contact

In this study, thermal EHL performances in line contact under zero entrainment velocity (ZEV) are investigated theoretically by employing Newtonian and Ree–Eyring fluid models. From high to modest surface velocity, both rheology models predict large classical surface dimple and the depth of the surface dimple increases with decrease of the surface velocity. However, if the surface velocity is further decreased, a smaller centralized dimple is obtained by using Ree–Eyring model similar to those point contact oil film shape observed in optical interferometric experiments. At lower surface velocity, the Newtonian model shows too poor convergence to predict such film shape. At last, the influence of the applied load on the centralized dimple is also studied.     

Asymptotic analysis of a lubricated heavily loaded spinning and rolling ball in a parabolic raceway

In the paper, the previously developed asymptotic approach to solution of the steady isothermal problem of elastohydrodynamic lubrication (EHL) for heavily loaded point contacts is applied to a lubricated point contact with rolling and spinning. It is shown that the whole contact region can be subdivided into three subregions. The central region can be subdivided into the Hertzian region and two adjacent boundary layers — the inlet and exit zones. The main results of the paper are threefold: (i) it is shown that in the central parts of the inlet and exit zones, the mechanisms and the equations controlling the behaviour of the lubrication contact parameters in heavily loaded point and line EHL contacts are identical, (ii) asymptotically precise formulas for the central and exit lubrication film thickness for pre‐critical and over‐critical starved and fully flooded lubrication regimes are analytically derived, and (iii) the inlet and exit zone asymptotically valid equations are uniform across all steady heavily loaded line and point EHL contacts for lubricants with the same rheology. These asymptotically valid equations were analysed and numerically solved in previously published work based on the stable methods utilising the specific regularisation approach developed for lubricated line contacts. Cases of pre‐critical and over‐critical lubrication regimes are considered. The formulas for the lubrication film thickness for pre‐critical and over‐critical starved and fully flooded lubrication regimes allow for simple analysis of the film thickness as a function of spinning angular speed, angle of the entrained lubricant and other pertinent contact characteristics. Copyright © 2013 John Wiley & Sons, Ltd.     

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.     

对数母线滚子光弹流润滑的实验研究

为了研究对数母线滚子凸度量对滚子摩擦副润滑油膜的影响 ,本文在自制的有限长线接触光干涉弹流试验机上 ,测量了中载、富油润滑和纯滚动工况下对数修形圆锥滚子与玻璃盘之间的油膜形状与膜厚。实验结果表明对数母线轮廓滚子的凸度量对其端部的油膜厚度和膜形分布有较大的影响 ,在给定的工况条件下 ,存在一个使滚子轴向油膜厚度最为均匀的最佳凸度量 ,此最佳凸度量比在相同工况条件下用静弹性接触力学方法求得的最佳凸度量大     

Film pressure distributions in point contacts predicted by thermal EHL analyses

In general, lubricated rolling/sliding contact fatigue problems have been investigated by assuming Hertzian contact pressure distributions. In this paper, thermal EHL analyses in consideration of the variations in oil properties in all directions within the film have been carried out under conditions of circular and elliptical contacts. It has been found that the actual film pressure distributions differ markedly from the Hertzian pressure distribution depending on the thermal conductivities of both contacting surfaces, slide–roll ratios and viscosity–pressure coefficients.     

Study of Boundary Slippage Using Movement of a Post-Impact EHL Dimple Under Conditions of Pure Sliding and Zero Entrainment Velocity

A well-recognized phenomenon of typical traction tests of elastohydrodynamic lubrication (EHL) contacts is finite maximum traction at increasing speeds, which led to the postulation that the limiting shear stress of liquid lubricants, a high-pressure rheological property, existed. If slippage occurs at the oil–solid boundary, the limiting traction measured is not necessarily an intrinsic property of the lubricant but rather a function of interfacial properties between the bounding solid surface and the lubricant. A recent report presented experimental evidence of boundary slippage at EHL contacts using a simple methodology based on differences in the speed of oil entrapment and the apparent entrainment. The reported experiments were carried out under pure sliding conditions. The phenomenon may also be explained by internal slippage in the bulk fluid film because of the limiting shear stress of the lubricant. To clarify this, similar experiments were repeated under zero entrainment velocity (ZEV) conditions. Evidence of the highly pressurized lubricant at the center of the oil entrapment sliding against the solid bounding surface was obtained. The purpose of this article is to discuss whether the slippage is attributed to the limiting shear stress of the oil or the critical shear stress of the oil/solid interfaces, and how to differentiate the magnitudes of the critical shear stress of the two bounding surfaces in a conventional optical EHL test rig.     

ELASTOHYDRODYNAMIC LUBRICATION OF LOGARITHMIC PROFILE ROLLER CONTACTS

0 INTRODUCTIONThe profiled roller contacts are widely used inheavy radial load and high rotational speed conditionsfOr overcoming the problems of edge effects caused bytheir finite length and by misal ig nment. E x tensivemathematical analyses['] and computer ..lc.lati...[2Jhave shown that the most favorable effect in respectof edge stress reduction is obtained when thegeneratrix of roller contacts corresPOnds to a certainlogarithmic function. But the theoretical basis of theseodculatbo …     

Oil film stiffness and damping in an elastohydrodynamic lubrication line contact-vibration

An elastohydrodynamic lubrication line contact-vibration model is proposed to study the stiffness and damping of the oil film existed in the EHL contact region. An initial mutual approach between interacting surfaces, which deviates from the steady-state balanced position, is assumed under the free contact-vibration to predict the response of the mutual approach. An inertia term, which represents the acceleration of the gap motion, is added to the classical force balance equation to form the equation of motion of the mutual approach. Response of the mutual approach is solved based upon the solving of the contact-dynamic model. The oil stiffness is calculated according to the natural frequency of the response under damped and non-damped conditions, the latter of which represents dry contact conditions. The oil film damping is calculated in terms of the principle of the energy conservation which utilizes the whole history of response compared with the log decrement method. Effect of the normal load, the rolling speed and the amplitude of the regular sinusoidal surface waviness on the oil film stiffness, the contact stiffness and the oil film damping are studied. The study provides an insight on the oil film dynamic characteristics of lubricated contact-vibration problems which appear in gears, bearings, etc. The results show that the oil film damping factor decreases with the increasing normal load as well as the increasing rolling speed. The oil film stiffness increases with the normal load and decreases with the rolling speed. Also, compared to the contact stiffness, the oil film stiffness makes less contribution to the total stiffness. The surface waviness amplitude has little effect on the oil film stiffness and the oil film damping.     

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.     

零卷吸点接触热弹流润滑的反常现象分析

应用Ree-Eyring流体，求得了零卷吸工况点接触热弹流润滑的完全数值解，分析了凹陷深度随表面速度的变化规律。结果表明，在低速工况下，Ree-Eyring非牛顿流体模型能更合理地解释此种润滑现象的特征，固体内沿X、Y方向的热传导不容忽视。