Film-forming Characteristics of Grease in Point Contact under Swaying Motions

The technique of relative optical interference intensity (ROII) and simple numerical calculations were used to investigate the lubricating behavior of grease lubricant films in the rolling direction under swaying motions (acceleration/deceleration). Experimental results indicate that at a same entrainment velocity of the inlet, the central film thickness under deceleration is larger than that under acceleration. The minimum central film thickness in one swaying cycle does not occur at the moment of zero entrainment, but at the initial period of acceleration. At the moment of zero entrainment, the central film is thicker than its peripheries, and the value of the central film thickness increases with increases in the changing rate of the entrainment velocity. It is thought that the transient behaviors of the grease lubricant film deviate from those in steady state conditions. The profiles of the transient film thickness and the approximate thicknesses of elastohydrodynamic contact in the rolling direction calculated by using a simple numerical method are supported by the experimental results. The numerical method can also be used to explain the behavior of the grease lubricating film under non-steady state conditions. An erratum to this article can be found at     

Effect of Surface Velocity Directions on Elastohydrodynamic Film Shape

This article is focused on the effects of the angle between lubricant entrainment velocity and sliding velocity on elastohydrodynamic film thickness distribution. Thin-film colorimetric interferometry was used to evaluate the film thickness distribution in smooth glass–steel contacts to provide basic data on the effects of the slide–roll ratio and the direction of sliding with respect to entrainment velocity. It was observed that as the sliding perpendicular to the entrainment velocity increased, the overall film thickness was reduced and asymmetry of the film profile with respect to the direction of the entrainment velocity increased. The asymmetry of the film profile with respect to the direction of the entrainment velocity increased with the entrainment speed or the overall film thickness. When the speed of the glass disk was larger than that of the steel ball, a dimple was formed even if there was a difference in direction between the entrainment and sliding velocities. A part of the dimple was exhausted from the elastohydrodynamic lubrication (EHL) conjunction as the angle between the entrainment and sliding velocities approached 90°.     

具有中央凸起的点接触弹流润滑特性研究

建立具有中央凸起的点接触弹流润滑控制方程,并采用多重网格法及多重网格积分法进行数值求解;比较有凸起表面和光滑表面下的压力及膜厚曲线,讨论载荷及卷吸速度对压力分布及油膜形状的影响。结果表明：具有中央凸起时在接触中心附近,压力经历了急剧升高、骤然下降、再升高的一个波动过程;最小膜厚出现在接触中心,且接触中心前面产生了一个凹陷;增大卷吸速度或减小载荷都使得膜厚曲线整体升高,最小膜厚随着卷吸速度的增大而增大,载荷几乎不影响最小膜厚;载荷增大使得最大压力增大,但中心局部压力波动范围变化很小;增大卷吸速度使得最大压力和中心局部压力波动范围都减小。     

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.     

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。     

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。     

Effect of chemical structure on film‐forming properties of seed oils

The film thickness of seven seed oils and two petroleum‐based oils of varying chemical structure was investigated by the method of optical interferometry. The measured film thickness (h_meas) was then compared to that calculated using the Hamrock–Dowson (H‐D) equation. The result showed that the H‐D equation adequately and without exception predicted h_meas in the high entrainment speed region but not in the low entrainment speed region. In order to understand these deviations further, the chemical structural variabilities of the oils were quantified using empirical parameters, and its effect on film thickness in the low entrainment speed region was examined. The result of this examination indicated that, in the low entrainment speed region, the film thickness of seed oils (i) rarely correlates with viscosity as proposed by the H‐D equation, (ii) increases with decreasing polarity of the oil, and (iii) increases with decreasing degree of unsaturation of the oils. Published in 2008 by John Wiley and Sons, Ltd.     

变卷吸速度的点接触热弹流润滑分析

非稳态热弹流润滑一直是研究的热点和难点,针对变卷吸速度的点接触热弹性流体动力润滑问题,利用多重网格积分法,得到变卷吸速度的点接触热弹性流体动力润滑完全数值解。结果表明:卷吸速度的变化会引起油膜压力、膜厚和温度的变化;当卷吸速度变化到一定值时,在接触区会产生油膜的凹陷;凹陷的产生可用“温度-粘度楔”机制解释。     

Influence of surface roughness features on mixed‐film lubrication

An optical technique (three‐dimensional spacer layer imaging) has been developed to map accurately lubricant film thickness in thin‐film elastohydrodynamic (EHD) contacts. This experimental technique has been used to study the influence of surface roughness features, asperity height, and slope on EHD film thickness and pressure. Single ridges transverse to the entrainment direction were used to represent asperities. It was found that the ridges with lower slopes generate films of greater minimum thickness. Below a certain entrainment speed, the minimum film thickness declined at a rate dependent on the ridge slope. At low speeds, the ridges with higher slopes entrapped a larger volume of lubricant ahead of the ridge and along the entrainment direction. For all speeds, the highest ridges entrapped the most lubricant. Both ridge slope and ridge height had a negligible effect on mean film thickness in the contact. Asperity pressure increased with higher ridge slope, but was not influenced by entrainment speed. An increase in pressure was found where lubricant is entrapped upstream of a ridge.     

Film Thickness of Mechanically Worked Lubricating Grease at Very Low Speeds

Several different types of commercially available greases, aged in a roll stability tester, have been tested in a ball-on-disc machine where the film thickness was measured using the interferometry method. The grease film thickness at very low speed is thicker than the base oil film thickness due to the contribution of the thickener in this regime, which practically vanishes at higher speeds. The (mechanical) aging process reduces the thickener material, decreasing the grease film thickness at very low speeds (entrainment speed less than 0.01 m/s) with aging time, whereas no significant difference was found at higher speeds where the grease film thickness still approached the calculated base oil film thickness.     

变卷吸速度过程对直齿圆柱齿轮热混合润滑的影响

以渐开线齿轮为研究对象,建立考虑时变效应的齿轮混合润滑模型,计算得到卷吸速度随时间变化的直齿圆柱齿轮的混合润滑数值解,并分析以不同加速度起动和制动过程对齿轮润滑油膜形状和压力的变化。结果表明:起动过程开始时,由于速度较低,整个齿轮润滑系统处于混合润滑状态,随着速度的不断增加,边界润滑效应逐渐消失,接触区也由边界润滑区进入弹流润滑区,并且加速度值越大,边界润滑就会提前结束,进入全膜润滑状态;由于速度在很短时间内变化,必须考虑时变效应,变速过程及变速过程结束后的一段时间都存在时变效应;起动和制动过程的油膜压力和膜厚变化是不可逆的。     

摆线针轮行星传动瞬态弹流润滑分析

以弹流润滑理论为基础,利用多重网格法,针对啮合点当量曲率半径、卷吸速度、载荷等时变因素,对摆线针轮行星传动机构啮合齿廓各点的压力及油膜厚度进行数值计算,并与等温稳态计算结果进行比较。结果表明:时变因素对膜厚影响显著,对压力分布影响很小。考虑时变时,在啮合周期的前半段,中心膜厚和最小膜厚值都要低于稳态值,而在啮合的后半段则相反;其中中心膜厚值在啮合开始后迅速下降然后维持在一定水平,最后升高,而最小膜厚值在啮合开始后很快下降然后立刻上升。而在稳态情形下,中心膜厚值和最小膜厚值都在啮合的前期和末期出现过极小值现象。     

基于推力轴承结构的润滑膜厚与摩擦因数测量系统

开发一种基于推力轴承结构的润滑膜厚与摩擦因数测量系统。该测量系统在较低速度下可实现膜厚与摩擦因数的同步测量，在较高速度下可通过保持架固定和玻璃盘回转模式实现润滑油膜测量，通过保持架自由回转和玻璃盘固定模式实现摩擦因数的测量。通过测量不同载荷下的润滑油膜厚度随速度变化曲线，以及与单点接触的测量结果进行定量对比，验证了该测量系统的可靠性。测量得到的摩擦因数曲线表明了滚动体打滑现象的存在。该测量装置为润滑剂特性和滚动轴承润滑特性研究提供了一种评价方法。     

面接触润滑条件下表面形貌对动压润滑的影响

利用课题组自主研发的面接触光干涉油膜厚度测量系统,对表面凹槽滑块的流体动压润滑油膜厚度进行了试验测量.试验中以静止的微型凹槽滑块平面和旋转的光学透明圆盘平面构成润滑副,且两润滑平面始终保持平行;在载荷固定的条件下,对油膜厚度—速度曲线进行测量.结果表明凹槽的宽度、深度、方向和位置等因素对油膜厚度有着重要影响.     

Influence of temperature distributions in EHL film on its thickness under high slip ratio conditions

This paper describes the influence of temperature rise of oil film in Hertzian contact area on the film thickness or profile under high slip ratio conditions. Temperatures of both surfaces of a ball and a disk as well as average temperature across the oil film were measured by means of an improved infrared. Two kinds of optical band-pass filters were used to separate the radiations from the ball surface and the oil film through an infrared transparent disk made of sapphire glass. In case of temperature measurement of the disk surface, another sapphire glass disk was coated with 300 nm chromium layer on the contact surface to radiate the infrared from the disk surface and also to intercept the radiation from the ball surface and the oil film. Temperature profiles across the oil film were estimated by assuming a parabolic profile with the measured three kinds of temperatures.For case within 200% in slip ratio, both minimum and central film thickness decreased under constant entrainment velocity as slip ratio increased. Measured film shapes were not flat at central Hertzian contact region under high-slip condition and differed from the results by the conventional EHD theory assuming constant viscosity in the direction of film thickness. The profile of Couette flow varied due to the distribution of oil film temperature in thickness direction. The viscosity wedge action, that is the variation of the profile of Couette flow causes reduction of film thickness or deformation of film profile. For case over 200% in slip ratio, the relation between central film thickness and slip ratio under constant relative slide speed had a great difference from the results calculated from the formula presented by Chittenden et al.     

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.     

自旋对角接触球轴承弹流润滑与油膜刚度的影响

根据角接触球轴承自旋运动特征,同时考虑弹流润滑效应,建立角接触球轴承考虑自旋运动的弹流润滑模型;采用多重网格法求解弹性变形,利用有限差分法迭代求解雷诺方程,得到较为精确的数值解;分析不同赫兹接触压力、滚道表面粗糙度下自旋对角接触球轴承弹流润滑和油膜刚度的影响。结果表明：考虑自旋时随着Hertz接触压力、自旋角速度增大,油膜厚度减小,油膜压力增大,油膜承压区域呈细长状,并向接触中心靠近;随着滚道表面粗糙度幅值增大,油膜压力和膜厚均出现了波动,且考虑自旋运动时,轴承油膜厚度明显减小,油膜局部压力峰值更大;随着卷吸速度、润滑油黏度增大,油膜刚度减小,而考虑自旋运动时油膜刚度值更大;随着自旋角速度增大,油膜刚度逐渐增大。     

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.     

急停问题的椭圆接触弹流润滑分析

将机械急停时滚动轴承、齿轮等的弹流润滑油膜的渐变过程简化为从稳恒状态突然转化为恒载荷纯挤压状态,然后随时间的推移挤压效应逐渐消失的过程,建立了椭圆接触瞬态弹流润滑模型。假设润滑油为牛顿流体,在等温条件下用多重网格技术进行了动态数值求解,讨论了急停前卷吸速度分别沿椭圆接触区的长轴和短轴方向时,不同参数条件下的残留弹流润滑膜的压力和膜厚随时间的变化规律。数值计算结果表明,急停后润滑油会逐渐被挤出接触区,因此残留油膜只能保持很短的时间;较高的Hertz接触压力会通过增加润滑油粘度而延长残余油膜的维持时间,但不能根本上改变上述变化趋势,卷吸速度的方向也不能改变上述变化趋势。     

剪切应力影响下近壁微液膜测量技术及波动特性研究

基于尺度分离理论,近壁微液膜波动特性对临界热负荷的产生有至关重要的影响。针对水平管内分层流动近壁微液膜在气流剪切应力作用下的波动特性进行研究,分析声学法,射线法,电学法和光学法等不同检测方法在近壁薄液膜厚度测量上的应用,并比较各种方法的优缺点。最终采用光学法,即利用光谱共焦位移传感器,对不同气、液流速条件下近壁微液膜进行测量,分析剪切夹带对液膜厚度变化的影响规律,获得液膜撕裂的临界条件。研究结果表明：微液膜平均厚度在气流剪切夹带影响下随气速的增大而减小。由于液滴夹带现象影响程度的不同,在不同气、液流速条件下,试验段出口处液膜平均厚度液膜呈现线性或非线性的变化趋势。气流剪切应力增大时,液膜厚度超过临界厚度即发生撕裂现象,液膜撕裂存在随机性,当壁面条件一定时,临界液膜厚度不随气、液流速的变化而变化,但在高气、液流速条件下液膜波动加剧。