加载条件下零卷吸凹陷油膜分布的试验研究

为探究加载过程中零卷吸工况下油膜的变化情况,使用球-盘光干涉试验机进行PAO100润滑油润滑下的摩擦实验。实验过程中通过伺服电机驱动钢球和蓝宝石盘以等值反向速度稳定转动,同时匀速加载;采用工业相机拍摄球-盘之间的油膜图像,实验后使用双光干涉法测量接触区中截面油膜厚度。实验发现：在加载情况下,当表面速度较低时且载荷较小条件下,油膜规律性不强,只有当载荷增加到一定程度,“温度-黏度楔”效应才会起作用,形成中央凹陷油膜;中央凹陷出现的时间随着表面速度的增加而提前;在速度较高条件下,中央小凹陷会迅速演化为大凹陷;与稳态结果的对比显示,时变效应会延迟“温度-黏度楔”效应的作用。     

滚子副润滑状态转变的光干涉试验研究

利用光干涉测量技术,测量了滚子-盘有限长线接触副的润滑油膜形状和厚度,研究了滚子副的润滑状态随载荷、速度转变的规律。结果表明,接触区卷吸速度增加或载荷减小,使得滚子-盘接触副润滑状态逐渐由弹流润滑转变为流体动力润滑,且在两种润滑状态转变过程中存在过渡状态;由载荷变化引起流体动力润滑状态转变为弹流润滑状态过程中,接触区表面发生了弹性变形,使得接触区的油膜厚度增加。速度变化使滚子-盘接触处于流体动力润滑状态时,油膜出口颈缩消失,最小膜厚位置由出口颈缩处移至接触区中心,油膜光干涉图关于滚子轴线对称。     

基于电容法与光干涉法相结合的油膜测量方法

通过将电容法膜厚测量仪耦合在球-盘点接触光干涉试验台上,搭建油膜厚度测量装置。通过对目标球-盘接触副采取合理的导电措施以及台架绝缘设施来保证润滑油膜电信号的提取,该装置可实现相同工况下膜厚度值及其相应的电信号(如油膜分压值和电容值)。在纯滚动接触情况下,分别对油润滑和脂润滑下的油膜进行测量,得到光干涉膜厚、油膜分压值和电容值随随卷吸速度的变化规律,并分析接触副电容随膜厚的变化。结果显示,随卷吸速度的增加光干涉膜厚升高而油膜分压值和电容值减小,电容值随着膜厚的增加而逐渐降低。实验结果初步验证了该测量系统的可行性,可为后续实际接触副内润滑状态的评估提供方案。     

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。     

零卷吸条件下滚子摩擦副弹流特性的实验研究

采用线接触光弹流实验装置,在摆动工况下研究滚子在零卷吸速度条件下的弹流特性,探讨不同周期同一载荷下滚子摩擦副在零卷吸速度时油膜厚度的变化情况,以及载荷对零卷吸速度下滚子弹流特性的影响。结果表明:在滚子转速近零卷吸速度时,挤压效应起主导作用,油膜被封在接触区内,形成凹陷,并且该处油膜较为稀薄;在往复运动工况下,滚子周期性运动的次数影响零卷吸速度时滚子的油膜厚度,在起动瞬间滚子中部的油膜厚度最小,随着摆动次数的增加,滚子中部的油膜厚度逐渐增加,多次摆动后,油膜将达到相对稳定的厚度;载荷对滚子零卷吸速度下的弹流特性影响较大,随载荷的增大接触区增大,滚子端部最小油膜厚度变小,滚子端部边缘效应增大。     

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。     

乏油对反向滑动条件下45号钢磨损性能的影响

使用光干涉技术,在球盘点接触试验台上,在零卷吸速度条件下,观察不同乏油程度下,接触区凹陷的变化情况;通过球-盘磨损试验机,研究反向滑动工况下45#钢在干接触和不同乏油程度条件下的表面损伤行为以及时间、速度和载荷对磨损的影响。结果表明:零卷吸速度条件下,乏油会导致"温度-黏度楔"作用的减弱,以及油膜凹陷的消失,因此会增加金属表面接触和磨损的发生;在反向滑动工况下,金属表面的损伤随乏油程度的增加而加剧,干接触对钢盘和钢球表面造成的磨损最为严重;在乏油工况时,工作时间、表面相对速度以及载荷的增大都会加剧磨损,磨损机制主要表现为疲劳磨损和黏着磨损。     

纯滑动椭圆接触条件下急停对弹流润滑表面凹陷现象的影响

在光干涉滚子-盘试验机上,研究在纯滑动椭圆接触条件下急停对弹流润滑表面凹陷现象的影响,通过改变载荷与速度的大小,观察"温度-黏度楔"凹陷油膜在急停过程中向挤压凹陷油膜的转化情况,并使用双光干涉图像处理软件对油膜中截面膜厚进行测量。实验结果表明:载荷增加时,"温度-黏度楔"凹陷出现的时间与凹陷到达接触区中心的时间均缩短;当速度不同时,急停过程中"温度-黏度楔"凹陷的产生时间不同,但都会向接触区中心移动;随稳态速度的增加,在速度急剧降低为0的阶段,初始油膜厚度高,油膜厚度下降较快;在速度为0的恒载荷纯挤压阶段,油膜厚度下降较慢。     

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。     

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。     

Movement of Entrapped Oil Under Pure Rolling Conditions

This article presents a study on the movement of an oil entrapment (or impact dimple) in a pure rolling elastohydrodynamic lubricated (EHL) contact. The oil entrapment was formed by impacting a steel ball against a lubricated glass disc. The contact was then activated under pure rolling conditions, and the movement of the entrapped oil was visualized by optical interferometry. It was found that during the movement of the dimple within the EHL contact, there exists a critical value for the displacement of dimple core. For the displacement of the dimple core less than the critical value, the dimple moves at the entrainment velocity and the film thickness of dimple core remains almost constant. For displacement beyond the critical value, the dimple slows down and its depth decreases rapidly. The effects of influential factors such as speed, initial dimple depth, load, and initial gap size were theoretically and experimentally investigated.     

In situ pressure measurements in dimpled elastohydrodynamic sliding contacts by Raman microspectroscopy

In situ pressure measurements within dimples formed in glass-steel point contacts under various sliding conditions were performed using Raman microspectroscopy. Experiments were conducted using a ball-on-disc type apparatus in which a circular contact is formed between a rotating glass disc and a rotating steel ball. Film thickness distributions were measured by duochromatic optical interferometry. Polyphenyl ether oil (5P4E) which has a high pressure-viscosity coefficient was used to produce a dimple in the contact area. Experimental results show that the pressure increases locally in the dimple zone and the pressure profile changes, accompanying the changes of the dimple location as a function of the slide-to-roll ratio. The maximum pressure is located downstream the maximum height of the dimple. The rheological response of the oil film under dimple occurrence conditions is discussed.     

The influence of the slide-to-roll ratio on the friction coefficient and film thickness of EHD point contacts under steady state and transient conditions

The effect of different slide-to-roll ratios has been experimentally investigated under steady state and transient conditions using a steel ball in contact with the plane surface of a glass disc. Under transient conditions the entraining velocity has been varied with a sinusoidal law at two different frequencies. Measurements of the friction force and film thickness using optical interferometry have been made.For the same working conditions, different friction coefficient trends found for positive and negative slide-to-roll ratios can be related to different film thickness values and shapes. The combination of different thermal effects could be a possible explanation for the obtained results.     

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。     

链条套筒与销轴铰链副冲击载荷-往复运动的弹流润滑数值模拟

数值模拟链传动中销轴与套筒之间的定载荷和变载荷弹流润滑接触问题,套筒相对于销轴做纯滑动往复运动。定载荷是假定往复运动过程中载荷恒定不变;变载荷是假定链节在啮入和啮出链轮过程中存在的冲击载荷按正弦函数规律变化。比较在定载荷和变载荷加载条件下线接触往复运动工况的弹性润滑油膜变化情况,分析在动载荷加载条件下不同行程长度对弹性流体动力润滑特性的影响。研究发现,动载荷对油膜的压力、膜厚影响较大:随着动载荷的增加,油膜中压力急剧增大,膜厚减小;但加载方式对摩擦因数的影响不大;在相同的加载方式下,随着行程长度的增加,油膜压力减小,中心膜厚和最小膜厚显著增加。     

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.     

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.     

Numerical analysis of TEHL line contact problem under reciprocating motion

This paper presents a full numerical analysis to simulate the thermal elastohydrodynamic lubrication (TEHL) of steel–steel line contact problem under reciprocating motion. The equation system is solved using multigrid techniques. General tribological behaviors of TEHL under reciprocating motion are explained. Comparison between thermal and isothermal results reveals the importance of thermal effect in prediction of the traction coefficient and film thickness. The influences of frequency, stroke length, and applied load on the variations of film thickness, pressure and traction coefficient during one working cycle are discussed. Furthermore, the influence of slide–roll ratio on tribo-characteristics of oil film under same entraining velocity is revealed.     

Measurement of the influence of sliding velocity on oil film thickness in an elastohydrodynamic point contact

Experiments have been carried out to measure the film thickness between a sapphire disc and a polished steel ball when slippage is permitted between the two surfaces. The velocity sum of the two surfaces was kept constant during the measurements. The method used to determine the oil film thickness in the contact zone is optical interferometry. It was observed that the change in minimum film thickness was slight. When the sliding velocity was increased, the decrease in central film thickness was considerably larger than that of the minimum film thickness. In addition, the two side lobes, in which the minimum film thickness area occurs, were observed to become flatter as the sliding velocity was increased.