Effect of Slide-to-Roll Ratio on Interior Stresses Around a Dent in EHL Contacts

The present, study extends the transient EHL point contact model and subsurface stress field calculation model to examine the influence of a surface dent on interior stresses in an EHL point contact under various slide-to-roll conditions. Results revealed that under the pure rolling condition the effect of a surface dent on the stresses is quite negligible. The presence of a shallow surface dent is unlikely to reduce the contact fatigue, life so long as pure rolling motion and good lubrication conditions are maintained. Unfortunately, the same cannot be said of the contact if it is operating in the boundary lubrication regime.

When sliding was introduced, the surface indentation generated significantly high pressure spikes with a strong directional preference. These high-pressure spikes cause severe stress concentrations either below the trailing edge of the dent, if it moves faster than the opposing surface, or below the leading edge of the dent, if it moves slower than the opposing surface. The maximum von Mises stress moved close to the surface and significantly increased in value as compared to the smooth surface solution. For the case of simple sliding, the maximum von Mises stress is even greater than the value calculated for the boundary lubrication case.

In regard to maximum tensile principle stresses, the presence of a dent increased the stresses only marginally over the smooth surface solutions. It is unlikely that surface indentation would significantly reduce the contact fatigue life due to Mode 1-type crack initiation.     

表面横向粗糙峰对点接触热弹流润滑的影响

通过数值解研究丁固体表面的横向粗糙峰对钢球与玻璃盘形成的热弹流润滑接触区压力、膜厚以及温度分布的影响。计算中假设钢球是静止的．即滚滑比为2。结果表明在粗糙峰出现的位置上，压力、温度均有急剧的增加，并且固体表面的凹陷现象也变得更加复杂。     

凹坑边缘隆起对弹流油膜的影响

采用光束干涉技术研究纯滑条件下表面单凹坑及边缘隆起对弹流油膜的影响.实验结果表明,随着卷吸速度的增加,边缘隆起处在弹性恢复阶段时,阻碍润滑油的流动,在靠近隆起处形成了明显的凹陷;当隆起完全恢复后,其对润滑油流动的影响减弱,凹陷也随之消失.比较相同工况下凹坑表面和光滑表面出口油膜的特征,分析凹坑位置对油膜的影响,结果显示凹坑表面的出口膜厚略有增加;凹坑处在接触区中央位置时对油膜的影响最为明显.     

The role of fragility in EHL entrapment

Experimental measurements of time dependent film thickness in entrapped liquids, measurements of viscosity under pressure, and simulations using realistic pressure–viscosity models contribute to improved understanding of the mechanisms of entrapment formation and persistence. The ambient viscosity and pressure–viscosity coefficient affect entrapment only as much as they are predictors of behavior at much higher pressure. Fragile liquids, such as lubricating oils, experience rapid increase in sensitivity of dynamic properties to temperature and pressure as the glass transition is approached. The fragility property of lubricants appears to be of overwhelming importance to entrapment which experimental evidence indicates will reduce starting friction.     

Film Formation in EHL Point Contacts under Zero Entraining Velocity Conditions

The contacts of adjacent balls in a retainerless bearing are subjected to the zero entrainment velocity (ZEV). The existence of an effective elastohydrodynamic lubrication (EHL) film between contacts running under ZEV conditions has long been proven experimentally. However, the classical EHL theory predicts a zero film thickness under ZEV conditions. Mechanisms, such as the thermal viscosity wedge effect and immobile film theory, have been proposed to tentatively explain the phenomenon. However, detailed numerical results are needed to provide theoretical evidence for such film formations. This paper aims to simulate, based on the viscosity wedge mechanism, the film formation of EHL point contacts under ZEV conditions. Complete numerical solutions have been successfully obtained. The results show that the thermal viscosity wedge induces a concave film profile, instead of a parallel film (Hertzian) as postulated by some previous researchers. By the simulation solver developed, the variation of film thickness with loads, oil supply conditions and ellipticity parameters have been investigated. Some unique lubrication behaviors under ZEV conditions are demonstrated. Furthermore, preliminary quantitative comparisons with the latest optical EHL experiments are finished. Both results are in good correlation.     

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.     

重载点接触热弹流润滑中油膜温度分析

对HKD-1型航空润滑油进行了重载点接触热弹流计算和油膜温度分析,并进行了试验验证.结果表明:重载情况下,润滑油的非牛顿特性较显著;油膜温度随滑滚比的增加而增大,各种模型下得到的温度分布形状相似;采用Evans-Johnson模型预测的温度远小于Newton模型的预测温度,滑滚比较大时,Newton模型对温度的预测失真;油膜温度关于油膜厚度方向的中心平面对称分布,最大温升发生在该平面附近.     

滚子摩擦副弹流的闭合效应

以基于Ｊａｃｏｂｉ松弛迭代过程的改进的顺解法研究滚子摩擦副的弹流特性,揭示了摩擦副端部的闭合效应,它使最小油膜厚度出现在端部出口区,并形成油膜压力峰,端部的润滑特性对摩擦副的几何特性十分敏感,给出的工程对数设计展示了良好的弹流润滑特性。     

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。     

表面粗糙纹理对非牛顿热弹流润滑性能的影响

建立了考虑表面粗糙度的点接触热弹流润滑模型,假设滚滑工况下慢速运动的固体表面是粗糙的,而快速运动的固体表面绝对光滑.卷吸速度沿接触区短半轴方向,表面粗糙纹理以正弦函数来表征,润滑剂符合Ree-Eyring非牛顿流体粘性定律.通过完全数值求解,讨论了3种不同构成的粗糙纹理对弹流润滑性能的影响.其中2种是周期性时变问题,求解是从一个准稳态解开始逐个瞬时迭代来实现的.结果表明,粗糙纹理会使得接触区里对应处产生明显的局部高压,局部温升,油膜厚度也相应减小.而纵向粗糙度时润滑效果最差,因为具有更危险的最小膜厚值.     

Mapping Shear Stress in Elastohydrodynamic Contacts

A new method has been, devised for investigating the theological properties of lubricant films in two-dimensional EHD contacts. A lubricated, sliding contact is produced between a sapphire flat and a steel ball. Thermal infrared emission microscopy is then employed to obtain 2-D maps of the variation of temperature rise due to friction across the contact. These maps are then used in conjunction with moving heal source theory to produce maps of energy dissipation and thus shear strength, of the lubricant film across the contact.

A series of mixtures of two lubricants, one giving high traction and one with low traction, have been studied using this technique to investigate the influence of lubricant, blending on shear stress and traction.     

The Influence of Surface Dents and Grooves on Traction in Sliding EHD Point Contacts

Changes in traction, caused by dents and grooves on a highly polished ball, are investigated as these defects approach and go through sliding elastohydrodynamic (EHD) point contacts. The contacts are formed with the ball loaded against a transparent disk. The ball and thus the topographical features are held stationary at various locations in the vicinity and within the contact while the disk is rotating. It is shown that these topographical features can cause substantial changes in the traction when compared to traction obtained with smooth surfaces.     

表面划痕对滚子润滑性能的影响

对滚子表面划痕对润滑性能的影响进行研究。研究表明,划痕两侧将产生压力和温度升高及膜厚跌落,划痕中部将出现压力和温度跌落及膜厚增加,并且这一现象随划痕深度和宽度的增加趋于剧烈,将恶化滚子的应力状态;与划痕深度比,划痕宽度对划痕两侧最小膜厚的影响不显著。     

两波动表面间的椭圆接触热弹流润滑数值分析

研究滚滑工况下两波动表面间的椭圆接触微弹流润滑问题,建立两接触表面均有连续波动的粗糙度模型,求得热条件下的完全数值解。假设卷吸速度沿接触椭圆短轴方向,快速运动表面的速度是另一表面的4倍,表面纹理相似的两波动表面间的微弹流润滑大多是周期性时变问题,以准稳态解为初始条件,逐个周期求得时变热解。讨论不同方向的表面微观连续波动对润滑性能的影响,并将牛顿和非牛顿模型的数值结果进行比较。结果表明,波动表面间接触区里的压力、膜厚、温升等呈现特定的特征,两表面波动皆为纵向纹理时,润滑膜厚度最小润滑条件最恶劣;热效应和非牛顿效应在微弹流问题中都很明显。     

A Transient Thermal Model for Mixed-Film Contacts

A thermal model is presented in this paper for deterministic calculations of the contact temperatures in mixed-film lubrication. A first-principal approach is taken to obtain the temperature results by solving transient energy equations in a domain consisting of three solid-film-solid regions. Various modeling techniques are used to feature model simplicity and to achieve a proper balance among solution accuracy, model robustness and computational efficiency. This thermal model may be incorporated into other deterministic isothermal mixed-film models to form a complete base model for mixed-film lubrication. It provides a platform upon which to model other effects such as lubricant additives, asperity plastic deformation and surface coating.     

Effects of Surface Roughness on Point Contact EHL

The effects of orientation of surface roughness, entrainment (rolling) velocity, and slide/roll ratio on micro-elastohydrodynamic lubrication (micro-EHL) are investigated under pointcontact conditions using the optical interferometry technique. Long bumps with constant height and wavelength produced artificially on the surface of a highly polished steel ball are used as a model roughness. It is shown that the asperities are elastically deformed and the magnitude depends on the film factor A, defined by the ratio of the central film thickness based on smooth surfaces to the composite surface roughness, as well as the surface kinematic conditions and the orientation of the asperities. It is also found that a thin or thick oil film formed at the inlet of the contact by a moving rough surface travels through the contact region at a speed very close to the average speed of the contacting surfaces. The possible mechanism is discussed.     

一种抗燃液压液的研究

由乙二醇，去离子水，稠化剂，自制水溶性润滑剂，防锈剂和抗泡剂等组成的抗燃液压液，用作浇铸机的铝铸件加工，满足了使用要求。     

基于推力球轴承的点接触弹流润滑分析

研究了推力球轴承的钢球与滚道间形成的椭圆接触等温弹流润滑问题。据此问题建立的数学模型既有沿接触椭圆短轴方向的卷吸速度分量，又有沿其长轴方向的卷吸速度分量，且速度值处处各异。运用多重网格法求得了几组参数下的完全数值解，并分析了转动速度、中心距、椭圆比等参数对结果的影响。     

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.