Effect of Interfacial Properties on EHL Under Pure Sliding Conditions

This paper presents an experimental study of the effect of boundary slip on the lubricating film shape and friction of an elastohydrodynamic lubrication (EHL) contact under isothermal conditions. Ball and disc pure sliding experiments were carried out with a high viscosity polybutene oil using a conventional optical EHL test rig. The film shape and friction were measured simultaneously. The results obtained from two discs with different coatings were compared. One disc was coated only with Cr, the partially reflective layer, and the other had an extra layer of SiO₂ coating on top. When running under mild conditions of low load and speed, there was no evidence of any boundary slip effect. However, when the load increased, the Cr-coated disc produced lower film thickness and friction than the SiO₂-coated disc. The Cr-coated surface had a larger contact angle, i.e., smaller surface energy, than the SiO₂ surface, which reflects the weak bonding between the molecules of the surface and the lubricant. The study concludes that surfaces with low surface energy promote boundary slip at the EHL contact, leading to a reduction in friction and film thickness.     

EHL Performance of the Lubricant With Shear Strength: Part I — Boundary Slippage and Film Failure

This paper analytically investigates the isothermal line contact elastohydrodynamic lubrication of three lubricants with much different shear strengths under the nondimensional operating parameters of w = 2.15e-4 and U = 2.53e-10 applying the lubricant ideal viscoplastic rheological model. The boundary slippage of the low-shear-strength lubricant occurring in the EHL inlet zone was found and results in a much thinner film compared to the classical EHL theory prediction. The film boundary slippage and its growth with the slide/roll ratio variation of tile low-shear\- strength lubricant exhibit special phenomena, which are much different from those of the high-shear-strength lubricant. The easy occurrence of film failure in concentrated contact in the case of high sliding speed, heavy load, large slide/roll ratio, and low-shear-strength lubricant was concluded due to the severe friction heating on the surface conjunction and the lubricant thermal desorption on tile lubricant/surface boundary. The EHL film failure mechanism was further recognized.     

Thermal point contact EHL analysis of rolling/sliding contacts with experimental comparison showing anomalous film shapes

The paper presents an experimental and numerical investigation of non-conformal lubricated contacts in which anomalous film shapes occur. The experiments were concerned with the contact between a steel ball and the plane surface of a glass disc at various slide-roll ratios. A paraffin base mineral oil was used as a lubricant and friction coefficients and film thicknesses were measured. It was found that for slide-roll ratios with the disk moving faster anomalous elastohydrodynamic lubrication (EHL) films were obtained characterized by a “dimple” in the central region of the contact. Numerical thermal-elastohydrodynamic analyses were carried out to simulate both film thickness and friction corresponding to the experimental conditions using Newtonian and Ree-Eyring rheological models. Initial results from this study suggest that neither of these lubricant models predict the correct detailed film shape and the experimental friction at the same time. An alternative lubricant model including both thermal and limiting shear stress effects (wall slippage) is currently under development.     

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.     

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。     

Asymptotic analysis of lubricated heavily loaded point contacts with skewed direction of entrained lubricant

The paper is devoted to the application of the early developed asymptotic approach to solution of the steady isothermal problem of elastohydrodynamic lubrication (EHL) for heavily loaded point contacts with skewed direction of entrained lubricant. It is shown that the whole contact region can be subdivided into three subregions: the central one, which is far away from the other two regions occupied by the ends of the horseshoe‐shaped pressure/gap distribution. The central region, in turn, can be subdivided into the Hertzian region and two adjacent boundary layers — the inlet and exit zones. Moreover, in the central region in the inlet and exit zones, the EHL problem can be reduced to asymptotically valid equations identical to the ones obtained in the inlet and exit zones of heavily loaded line EHL contacts. These equations can be analytically analysed and numerically solved on the basis of the stable methods using a specific regularization approach, which were developed for lubricated line contacts. Cases of pre‐critical and over‐critical lubrication regimes are considered. The by‐product of this asymptotic analysis is an easy analytical derivation of formulas for the lubrication film thickness for pre‐critical and over‐critical starved and fully flooded lubrication regimes. The latter allows for simple analysis of the film thickness as a function of the contact eccentricity and the direction of the entrained lubricant at the inlet in the contact. Copyright © 2013 John Wiley & Sons, Ltd.     

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.     

Thin film elastohydrodynamic lubrication—a power-law fluid model

A 1-D modified Reynolds equation for power-law fluid is derived from the viscous adsorption theory for thin film elastohydrodynamic lubrication (TFEHL). The lubricating film between solid surfaces is modeled as three fixed layers, which are two adsorption layers on each surface and a middle layer between them. The comparisons between classical non-Newtonian EHL and non-Newtonian TFEHL are discussed. Results show that the TFEHL model can reasonably calculate the pressure distribution, the film thickness, the velocity distribution and the average viscosity. The thickness and viscosity of the adsorption layer and the flow index influence the lubrication characteristics of the contact conjunction significantly. The film thickness increases with the increase of flow index. As the flow index becomes greater, the dimple in the film shape moves towards the center of the contacts. The effect of flow index produces an obvious difference in the pressure distribution. The greater the flow index, the greater the pressure spike, and the pressure spike tends to move toward the center. The larger the flow index, the more the velocity varies in both the middle layer and adsorption layers along the Z-axis. The greater the thickness and viscosity of the adsorption layer and the flow index, the greater the deviation in central film thickness versus speed between EHL model and TFEHL model produced in the very thin film regime.     

多点接触乏油弹流润滑模型及试验研究

为探讨多点接触乏油弹流润滑机制,基于球与滚道接触区域的排油和补油平衡,建立适用于不同润滑状态的油膜厚度计算模型,可以计算从充分供油、乏油到干涸乏油的中心膜厚以及油膜不平衡时中心膜厚随滚动次数的衰减。利用自制的球-盘接触光干涉弹流试验装置,通过安装双镜筒同时获取相邻球的油膜图像,研究多点接触中相邻球的轨道重合和不重合时前球尾迹对后球油膜图像和中心膜厚的影响。结果表明:乏油润滑条件下,前后球的轨道不重合时轨道之间可相互补油;前后球的轨道重合时,在给定供油条件下,随着滚动线速度增加,入口弯液面逐渐靠近接触区域,中心油膜厚度增加,与相同工况下乏油润滑模型计算的膜厚对比吻合较好,验证了所建乏油润滑模型的正确性。     

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.     

Asymptotic analysis of lubricated heavily loaded point contacts

The paper is devoted to the development of an asymptotic approach to solution of the steady isothermal problem of elastohydrodynamic lubrication (EHL) for heavily loaded point contacts. It is shown that the whole contact region can be subdivided into three subregions: the central one that is adjacent to the other two regions occupied by the ends of the horseshoe‐shaped pressure/gap distribution zone. The central region, in turn, can be subdivided into the Hertzian region and its adjacent inlet and exit zones that, in turn, are adjacent to the inlet and exit boundaries of the contact, respectively. Moreover, in the central region, in the inlet and exit zones of heavily loaded point EHL contact, the EHL problem can be reduced to asymptotically valid equations identical to the ones obtained in the inlet and exit zones of heavily loaded line EHL contacts. The latter means that many of the well‐known properties of heavily loaded line EHL contacts are also valid for heavily loaded point EHL contacts. These asymptotically valid equations can be analysed and numerically solved based on the stable methods using a specific regularisation approach that were developed for lubricated line contacts. Cases of pre‐critical and over‐critical lubrication regimes are considered. The by‐product of this asymptotic analysis is an easy analytical derivation of formulas for the lubrication film thickness for pre‐critical and over‐critical lubrication regimes. The method is validated by the results of some experimental and numerical studies published by a number of researches. Copyright © 2013 John Wiley & Sons, Ltd.     

滑动条件下弹流润滑启动过程的实验观察

使用光干涉动态油膜厚度测量系统对不同启动条件下聚丁烯润滑油弹流油膜的形成过程进行了实验观测。结果表明,在纯滑动条件下,由于界面滑移弹流油膜存在反常的入口凹陷;卷吸速度相等时,较大的启动加速度产生较大的界面滑移,诱发较大的入口凹陷;不同的启动加速度,入口区的油膜形状和最小油膜厚度的变化也不相同。     

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.     

Operating Limits for Acoustic Measurement of Rolling Bearing Oil Film Thickness

An ultrasonic pulse striking a thin layer of liquid trapped between solid bodies will be partially reflected. The proportion reflected is a function of the layer stiffness, which in turn depends on the film thickness and its bulk modulus. In this work, measurements of reflection have been used to determine the thickness of oil films in elastohydrodynamic lubricated (EHL) contacts. A very thin liquid layer behaves like a spring when struck by an ultrasonic pulse. A simple quasi-static spring model can be used to determine the proportion of the ultrasonic waves reflected.

Experiments have been performed on a model EHL contact between a ball and a flat surface. A transducer is mounted above the contact such that the ultrasonic wave is focused onto the oil film. The reflected signals are captured and passed to a PC for processing. Fourier analysis gives the reflection spectrum that is then used to determine the stiffness of the liquid layer and hence its thickness. In further testing, an ultrasonic transducer has been mounted in the housing of a deep-groove ball bearing to measure the film generated at the outer raceway as each ball passes. Results from both the ball-flat and ball bearing measurements agree well with steady-state theoretical EHL predictions. The limits of the measuring technique, in terms of the measurable rolling bearing size and operating parameters, have been investigated.     

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.     

椭圆接触弹流润滑油膜形状的实验研究

采用多光束干涉测量技术,在自制光弹流实验机上进行了椭圆接触弹流润滑油膜形状的实验测量,观察了椭圆接触区短轴与卷吸方向之间的夹角θ、速度、施加载荷等对油膜形状的影响。结果表明：夹角θ较小时,油膜厚度整体上更大,接触区较窄,入口区油膜更陡峭;低速时,夹角及载荷基本不影响膜厚;高速、轻载时,夹角θ对膜厚影响更显著;载荷及夹角越大,动压油膜越难建立。     

The investigation of oil film thickness in heavily-loaded contact

An experimental study of the shape and thickness of the oil film during rolling in a thrust ball bearing has been carried out by the interference method.The experimental results showed good agreement with theory. Oil film thickness was affected mainly by the rolling velocity, viscosity of oil and maximum Hertzian stress. The groove radius had no effect on the film thickness. With increase of rolling velocity the film thickness increases and then reduces sharply owing to temperature rise and the non-Newtonian properties of the lubricant. A qualitative similarity was derived from the experimentally observed dimensionless shapes of the film and of the dimensionless theoretical shapes of the oil film for the lubricant in the non-Newtonian state. The flat “squashed” contact area diminished and disappeared with rise in velocity, which agreed with theoretical predictions.Good agreement was found between the theoretical and the experimental values of the oil film thickness and the friction coefficients for a ball sliding on a plane. Values of relaxation time for oil agree with values observed by the vibration method.The interference method is proposed to estimate the relation of the relaxation time for lubricants to the pressure and temperature up to maximum Hertzian pressures of 14,000 kg/cm². Experimental studies by the interference method and the solution of the non-isothermal hydrodynamic contact problem for liquids both in the Newtonian and non-Newtonian state provide a method of calculation of the friction coefficient.     

The Effect of the Electric Double Layer on Very Thin Thermal Elastohydrodynamic Lubricating Film

The effect of the electric double layer (EDL) of friction surface on lubrication is significant under the condition of very thin lubricating film. This article presents a theoretical evaluation concerning the effect of the EDL on the film thickness and the pressure distribution of the elastohydrodynamic lubricating water film. These numerical analyses are based on the modified Reynolds equation that considers the EDL. Owing to the temperature risen readily in elastohydrodynamic lubrication (EHL) contact area, the influence of the temperature rise on the EDL effect was also investigated. The analysis results show that the EDL leads to a noticeable increase in the film thickness but has few influences on the pressure. Further, the analytical comparisons between isothermal and thermal conditions reveal that the temperature rise in the contact area weakens the effect of the EDL on the EHL film. Overall, consideration of the EDL effect gives a thicker EHL film, but once the temperature rise in the EHL regime is taken into account, the film thickness is correspondingly decreased.     

The Influence of DLC Coating on EHL Friction Coefficient

High hardness, high elastic modulus, low friction characteristics, high wear and corrosion resistance, chemical inertness, and thermal stability are factors that make diamond-like carbon (DLC) coatings the subject of many studies. For the same reasons they also seem suitable for use in, amongst others, machine components and cutting tools. While most studies in the literature focus on the influence of coatings on wear and friction in boundary lubrication and pure sliding contacts, few studies can be found concerning rolling and sliding elastohydrodynamic lubrication (EHL) friction, especially in the mixed and full film regime. In this article tests are carried out in a Wedeven Associates Machine tribotester where an uncoated ball and disc pair is compared to the case of coated ball against uncoated disc, coated disc against uncoated ball, and coated disc against coated ball. The tests are conducted at two different temperatures and over a broad range of slide-to-roll ratios and entrainment speeds. The results are presented as friction maps as introduced in previous work (Bj?rling et al. in J Eng Tribol 225(7):671, 2011). Furthermore a numerical simulation model is developed to investigate if there is a possibility that the hard, thin DLC coating is affecting the friction coefficient in an EHL contact due to thermal effects caused by the different thermal properties of the coating compared to the substrate. The experimental results show a reduction in friction coefficient in the full film regime when DLC-coated surfaces are used. The biggest reduction is found when both surfaces are coated, followed by the case when either ball or disc is coated. The thermal simulation model shows a substantial increase of the lubricant film temperature compared to uncoated surfaces when both surfaces are coated with DLC. The reduction in friction coefficient when coating either only the ball or the disc are almost the same, lower than when coating both the surfaces but still higher than the uncoated case. The findings above indicate that it is reasonable to conclude that thermal effects are a likely cause for the decrease in coefficient of friction when operating under full film conditions, and in the mixed lubrication regime when DLC-coated surfaces are used.     

Lubricant film formation properties of alkyl imidazolium tetrafluoroborate and hexafluorophosphate ionic liquids

The film thickness and friction properties of four imidazolium ionic liquids (1-butyl-3-methyl tetrafluoroborate and hexafluorophosphate and 1-hexyl-3-methyl tetrafluoroborate and hexafluorophosphate) were measured for mixed rolling-sliding conditions and the results compared to an additised mineral oil. Film thickness results showed that three of the fluids demonstrated classical EHL behaviour; however, the 1-butyl-3-methyl fluids gave anomalously thick, time-dependent films at low speeds (<0.3 m/s). Post-test inspection of the specimens revealed a loosely bound brown film deposited in the track. Film formation appeared to originate in the bulk fluid where brown “fibrous” agglomerations were observed. These were flocculated by shear flow and deposited in the track after passing through the contact. Overall the RTIL friction coefficients were less than the mineral oil for all conditions investigated. In the absence of thick film formation all RTILs gave a similar friction coefficient of 0.03 in the boundary regime, which is thought to be due to electrical double layer formation. In the fluid film regime traction was determined by the nature of the anion.