An Investigation into Grease Behavior in Thermal EHL Circular Contacts

The behaviors of two lithium lubricating greases were investigated under EHL circular contact through measurements of traction coefficients on a self-made rig in which the contact was continuously fed with fresh grease. The average values of Erying shear stress and shear modulus of the two lithium greases were obtained from traction experimental data using this rig. Based on the Evans-Johnson model and thermal analysis, we calculated the values of shear stress and traction coefficients of the two greases. The results show that the calculated traction coefficients agree fairly well with the measured data.     

A Film Thickness Correction Formula for Double-Newtonian Shear-Thinning in Rolling EHL Circular Contacts

Lubricants which contain a polymeric thickener will often display a second Newtonian plateau in measured flow curves. Like other manifestations of shear-dependent viscosity, this shear response will lead to an inaccurate prediction when the classical film-thickness formulas are employed. A correction formula has been developed from numerical experiments for a range of parameters of the double-Newtonian modified Carreau equation. The parameters of this shear-thinning model were selected from measurements for real lubricants obtained in Couette viscometers and a capillary viscometer. In addition, a full EHL film thickness formula has been derived from the same numerical experiments. The correction formula and the full formula were successfully validated using published film thickness data and published viscosity data for an EHL reference liquid, a polymer solution. Clearly, viscometer measurements of shear-dependent viscosity which contain the inflection leading to the second Newtonian are essential for a film-thickness calculation when a high-molecular-weight component of the lubricant is present.     

Boundary Conditions for Elastohydrodynamics of Circular Point Contacts

The paper presents the solution of an elastohydrodynamic point contact condition using inlet and outlet lubricant entrainment with partial counter-flow. The inlet and outlet boundaries are determined using potential flow analysis for the pure rolling of contiguous surfaces. This shows that Swift–Stieber boundary conditions best conform to the observed partial counter-flow at the inlet conjunction, satisfying the compatibility condition. For the outlet region, the same is true when Prandtl–Hopkins boundary conditions are employed. Using these boundary conditions, the predictions conform closely to the measured pressure distribution using a deposited pressure-sensitive micro-transducer in a ball-to-flat race contact. Furthermore, the predicted conjunctional shape closely conforms to the often observed characteristic keyhole conjunction through optical interferometry. The combined numerical–experimental analysis with realistic boundary conditions described here has not hitherto been reported in the literature.     

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。     

Full Film, Boundary Lubrication and Tribochemistry in Steel Circular Contacts Lubricated with Glycerol

In this article, the lubricating properties of pure glycerol are investigated under both mild and severe EHL regimes. Amazingly low friction coefficients (about 0.01) are reported by experiments in thick film regimes compared to traditional base oils. EHL calculations of film thickness and friction (including thermal effects) predict friction coefficients that are twice those actually found for glycerol. Chemical analysis of glycerol before and after the friction tests were performed by NMR and Karl Fischer methods, and they reveal that water is produced by tribochemical reaction as well as other species like aldehydes. This finding is in agreement with a corrosion pattern observed inside the wear scars of the steel samples. This study provides an explanation to the anomalously low friction observed in the thick film regime. In fact, water produced in the lubricant decreases traction forces due to the drastic decrease of the viscosity of glycerol with water addition.     

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。     

人工关节弹流润滑供液条件分析

在人工关节弹流润滑中,如果润滑剂供应不足,则会出现乏液现象.分析了球坐标系下不同的供液膜厚、载荷及径向间隙对乏液润滑行为的影响.结果表明:当供液膜厚较小时,压力区也较小,乏液现象较严重,但润滑液利用率较高.当载荷逐渐增大时,压力区增大,润滑液膜产生的压力变大,液膜起始处膜厚值也较大.径向间隙越大,压力区越小,润滑液膜产生的压力越高,液膜起始处的膜厚值也越大,但起始位置更接近于接触区中心.     

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。     

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.     

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.     

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。     

Investigating the Effect of Surface Finish on Mixed EHL in Rolling and Rolling-Sliding Contacts

Surface finish may significantly affect the lubrication performance of a tribological interface through the influence of topography on micro/nanoscale fluid flows around localized contacts at surface asperities. This paper aims to study the mixed lubrication performance of a group of engineered surfaces, including turned, isotropically finished, ground, and dimpled surfaces, under different operation conditions by means of a deterministic mixed elastohydrodynamic lubrication (EHL) model. The honed surface was used to mate with other surfaces. The results indicate that a longitudinal contact ellipse favors longitudinally oriented mating surface roughness and that a transverse contact ellipse, as well as a line contact, prefers a transversely orientated mating surface roughness for lubrication enhancement.     

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.     

界面滑移条件下点接触Stribeck曲线的实验研究

采用高黏度聚异丁烯润滑油,在光学弹流实验机上考察球-盘接触纯滑条件下的摩擦因数随卷吸速度和载荷的变化。结果表明,随着卷吸速度的增加,球-盘接触副进入弹流润滑并向流体动压润滑转变的过程中,摩擦因数并不像传统的Stribeck曲线一样,随着卷吸速度的增加而单调增加,而是呈现先上升、后下降、再上升的趋势,其中摩擦因数下降时的起始速度大致为凹陷出现的速度,摩擦因数再上升时的速度大致是润滑进入动压润滑的速度。初步论证界面滑移为产生上述波动的主要原因。     

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。     

点接触润滑状态转化的实验观察

利用球-盘接触润滑油膜厚度的光干涉测量法,通过卷吸速度或载荷的改变实验研究了弹性流体动力润滑与流体动力润滑转化过程中油膜厚度的变化规律。实验结果显示这2种润滑状态之间存在明显的过渡区。与已有的理论一致,在弹性流体动力润滑区和流体动力润滑区,油膜厚度与卷吸速度或载荷在对数坐标中呈直线关系。在两者的过渡区,固体表面的弹性变形量随卷吸速度或载荷的变化发生明显的变化,油膜厚度与速度或载荷的关系不再为对数坐标中简单的线性关系。使用已有的润滑状态区理论不能得到实验观测到的润滑状态的转化过渡区。     

Effects of Ionic Liquid Additive Concentration on Scuffing and Wear in Oil-Starved EHL Contacts

Abstract

We show how various concentrations ranging from 0 to 5.0?wt% of the ionic liquid (IL) trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate added to a fully formulated aviation DOD-PRF-85734 gearbox lubricant affects the properties of the lubricant, including scuffing resistance under starved lubrication conditions, coefficient of friction (COF), wear rate, and lubricant chemical stability. Specimen surfaces were then characterized using scanning electron microscopy, energy-dispersive spectroscopy, and auger electron spectroscopy. Scuffing and wear properties were found to generally improve with IL concentration, which we hypothesize is due to the presence of a phosphorous-rich film observed on specimens exposed to IL.     

The Behavior of Surface Roughness in EHL Contacts Under Small Slide to Roll Ratios

The amplitude attenuation theory represents a possible way how to predict the tribological performance of a surface having general roughness. There are applications of this theory for pure rolling and rolling?Csliding conditions having different governing parameters. Obviously, pure rolling case is much simpler and it was already experimentally verified. Conversely, rolling?Csliding conditions represent much complex case and its solution is still under development. Nevertheless, a lot of machine components operate under the conditions where only small slide to roll ratios are present. This study was carried out to find the reasonable validity of the pure rolling model for the applications where small amount of sliding is present. The behavior of roughness was observed under conditions of small sliding speed using optical tribometer for film thickness mapping. The results were analyzed and compared with the predictions according the amplitude attenuation theory for both pure rolling and rolling?Csliding cases. The influence of slide to roll ratio on roughness profile shape was studied and differences to the pure rolling profile were evaluated. It was concluded that the pure rolling model can be applied with reasonable precision within the range of slide to roll ratios from ?0.15 to 0.1. The correlation coefficients are better than 0.85, the RMS value changes are less than 3?%, and bearing area curves are almost unaffected.     

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.