Elastohydrodynamic Lubrication of Circular Contacts at Impact Loading with Generalized Newtonian Lubricants

In this article, pure squeeze elastohydrodynamic lubrication (EHL) motion of circular contacts with power law model lubricant is explored at impact loading. The coupled transient modified Reynolds, the elasticity deformation, and the ball motion equations are solved simultaneously, thus obtaining the transient pressure profiles, film shapes, normal squeeze velocities, and accelerations. The simulation results reveal that the greater the flow index (n), the earlier the pressure spike and the dimple form, while the maximum pressure and the film thickness increase, and the diameter of the dimple, the maximum value of the impact force, the rebounding velocity, and the acceleration decrease. Further, this analysis numerically demonstrates that the contact central pressure for a ball impacting and rebounding from a lubricated surface reached two peaks during the total impact period. As the flow index increases, the primary and the secondary peak increase, and the first and second peaks form earlier; as the total impact time decreases. Moreover, the phase shift between the time of the peak value of the squeeze acceleration and the zero value of the squeeze velocity increase with increasing flow index.     

线接触热弹流润滑的数值分析

基于Ree—Eyring流变模型，建立线接触热弹流润滑方程，通过数值计算得出了载荷参数、速度参数、材料参数和滑滚比对于二次压力峰、最小油膜厚度和最大油膜温度的重要影响。     

Elastohydrodynamic Lubrication of Rough Surfaces under Oscillatory Line Contact with Non-Newtonian Lubricant

This paper presents the results of a transient analysis of elastohydrodynamic lubrication (EHL) of two parallel cylinders in line contact with a non-Newtonian lubricant under oscillatory motion. Effects of the transverse harmonic surface roughness are also investigated in the numerical simulation. The time-dependent Reynolds equation uses a power law model for viscosity. The simultaneous system of modified Reynolds equation and elasticity equation with initial conditions was solved using the multigrid, multilevel method with full approximation technique. The film thickness and the pressure profiles were determined for smooth and rough surfaces in the oscillatory EHL conjunctions, and the film thickness predictions were verified experimentally.

For an increase in the applied load on the cylinders or a decrease in the lubricant viscosity, there is a reduction in the minimum film thickness, as expected. The predicted film thickness for smooth surfaces is slightly higher than the film thickness obtained experimentally, owing primarily to cavitation that occurred in the experiments. The lubricant film under oscillatory motion becomes very thin near the ends of the contact when the velocity goes to zero as the motion direction changes, but a squeeze film effect keeps the fluid film thickness from decreasing to zero. This is especially true for surfaces of low elastic modulus. Harmonic surface roughness and the viscosity and power law index of the non-Newtonian lubricant all have significant effects on the film thickness and pressure profile between the cylinders under oscillatory motion.     

Compression Heating and Cooling in Elastohydrodynamic Contacts

In this study, the infrared temperature mapping technique, originally developed by Sanborn and Winer (Trans ASME J Tribol 93:262–271, 1971) and extended by Spikes et al. (Tribol Lett 17(3):593–605, 2004), has been made more sensitive and used to study the temperature rise of elastohydrodynamic contacts in pure rolling. Under such conditions lubricant shear heating within the contact is considered negligible and this allows temperature changes due to lubricant compression to be investigated. Pure rolling surface temperature distributions have been obtained for contacts lubricated with a range of lubricants, included a group I, and group II mineral oil, a polyalphaolefin (group IV), the traction fluid Santotrac 50 and 5P4E, a five-ring polyphenyl-ether. Resulting maps show the temperature rise in the contact increases in the inlet due to compression heating and then decreases and in most cases becomes negative in the exit region due to the effect of decompression. Temperature changes increase with entrainment speed but in the current tests are always very small, and less than 1 °C. Contact temperature rises from compression were compared to those from sliding contacts (where a slide-roll ratio of 0.5 was applied). Here the contribution to the contact temperature from compression is shown to decrease dramatically with entrainment speed. The lubricant 5P4E is found to behave differently from other lubricants tested in that it showed a peak in temperature at the outlet. This effect becomes more pronounced with increasing speed, and has tentatively been attributed to a phase change in the exit region. Using moving heat source theory, the measured temperature distributions have been converted to maps showing rate of heat input into each surface and the latter compared with theory. Qualitative agreement between theory and experiment is found, and a more accurate theoretical comparison is the subject of ongoing study.     

Measurements of Temperature Distributions around Longitudinally Grooved Rough Surfaces in Sliding Elastohydrodynamic Point Contacts

This paper describes the temperature measurements in the EHL conjunction area comprising a longitudinally grooved steel ball and a sapphire disk under high slip conditions. The authors measured the temperatures of the oil film as well as both the disk and ball surfaces; furthermore, they estimated the temperature profile across the oil film by means of experimental values. The experimental results show that the temperature of the grooved ball surface increased considerably compared with that of a non-grooved ball. The temperatures of the faster surface for the grooved ball became sensitive to the slip ratio, whereas that for the non-grooved surface was almost constant. The temperature distribution had a higher value at the land zones and a lower one at the grooved zones. The temperature rise in the grooved zones varied qualitatively depending on the thermal conditions of both the sliding surfaces.     

零卷吸点接触热弹流润滑的反常现象分析

应用Ree-Eyring流体，求得了零卷吸工况点接触热弹流润滑的完全数值解，分析了凹陷深度随表面速度的变化规律。结果表明，在低速工况下，Ree-Eyring非牛顿流体模型能更合理地解释此种润滑现象的特征，固体内沿X、Y方向的热传导不容忽视。     

Molecular dynamics characterization of thin film viscosity for EHL simulation

Molecular simulations were used to characterize changes in lubricant viscosity that may occur during thin film elastohydrodynamic lubrication (EHL). Molecular dynamics simulations were performed at variable wall speed and film thickness such that the effects of both parameters could be evaluated. Using this approach it was found that the viscosity of thin films under large shear is subject to both shear thinning and oscillation with film thickness. A composite model was developed that incorporated both effects. The expected impact that this model might have on an EHL interface was evaluated using a continuum simulation. An overall decrease in viscosity with some oscillation near the interface edges was predicted due to the molecularly modeled thin film effects.     

点接触弹流润滑入口凹陷的速度域

利用等效黏度将circular流变模型整合进通用Reynolds方程,获得点接触弹流润滑入口凹陷的控制方程。采用多重网格法及多重网格积分法进行数值求解,研究卷吸速度变化导致的入口凹陷变化过程,讨论载荷、黏度、黏压系数、钢球半径及弹性模量对入口凹陷速度域的影响。结果表明：随着卷吸速度的增大,入口凹陷深度从0增大到最大值,然后再减小到0;入口凹陷出现的卷吸速度随着载荷、黏度、黏压系数及弹性模量的增大而减小,随着钢球半径的增大而增大;入口凹陷消失的卷吸速度及入口凹陷的速度域都随着载荷及钢球半径的增大而增大,随着黏度、黏压系数及弹性模量的增大而减小。     

点接触弹流润滑供油条件退化的乏油分析

在点接触弹流润滑中,如果不能及时补充新油,则接触区的供油条件会随着润滑次数而退化。分析了供油油膜厚度、中心膜厚、最小膜厚和润滑油膜压力区形成位置与润滑次数的关系。结果表明:润滑开始时,由于供油油膜厚度较大,系统处于充分供油状态;随着润滑次数的增加,有一部分油从两侧泄漏,系统逐渐转到乏油状态,供油油膜厚度、中心膜厚和最小膜厚均逐渐变小,压力区形成位置则逐渐向Hertz接触区靠近;最终供油油膜厚度、中心膜厚和最小膜厚趋于定值,压力区趋于Hertz接触区,从而达到一种稳定乏油状态。     

粗糙峰和粗糙谷对直齿圆柱齿轮热弹流润滑的影响

利用多重网格技术,研究了表面粗糙度对渐开线直齿圆柱齿轮非牛顿热弹流润滑的影响。从同一啮合线和不同啮合时刻两方面,分别讨论了粗糙峰和粗糙谷对直齿圆柱齿轮弹流润滑的影响,并且将这些解与光滑解进行比较。结果表明,粗糙峰和粗糙谷分别给接触区内带来了压力的单峰和双峰,但对膜厚的影响却不大。     

Experimental Investigation of Elastohydrodynamic (EHD) Film Thickness Behavior at High Speeds

At very high speeds, elastohydrodynamic (EHD) films may be considerably thinner than is predicted by classical isothermal regression equations such as that due to Dowson and Hamrock. This may arise because of viscous dissipation, shear thinning, frictional heating or starvation.

In this article, the contact between a steel ball and a glass disc over an entrainment speed ranging from 0.05 m s^?1 to 20 m s^?1 was studied. Two sets of tests were performed. In the preliminary testing, the disc was driven at speeds of up to 20 m s^?1 and the ball was driven by tractive rolling against the disc, its speed being determined using a magnetic method. After all possible explanations for the reduction in film thickness at high speeds were considered, it was shown that the results, which fall well below classical predictions, are consistent with inlet shear heating at the observed sliding speeds.

Another set of tests was then performed, with both disc and ball driven separately, so that the accuracy of the shear heating theory for different types of oils and at different sliding conditions could be assessed. It was found that the thermal correction factor predicts the trend of film thickness behavior well for the oils tested and is particularly accurate at certain slide–roll ratios (depending on the type of oil). Experimental data were also used to obtain improved coefficients for the correction factor for different types of oil to achieve better prediction of film thickness at high speed throughout the whole range of slide–roll ratios.     

涂层材料热特性参数对点接触弹流润滑的影响

为探究涂层材料热特性参数对点接触弹流润滑的影响,选择3种不同方法制备的类金刚石（DLC）涂层和氧化锆陶瓷涂层,构建考虑涂层热特性的点接触弹流润滑模型,分析涂层材料、涂层厚度和润滑剂的流变性对接触区润滑性能的影响。结果表明：在弹流润滑状态下具有不同热特性的4种表面涂层导致了膜厚的差异,固体表面温度及润滑区温度场会随涂层热惯性变化;热惯性最小的DLC涂层加在快速运动表面能获得更高的膜厚;随着涂层厚度的增加,会引起固体表面的温度升高,使摩擦因数降低;非牛顿流体对压力、膜厚的影响很小,但与牛顿流体相比,能获得相对较低的温度。在弹流润滑状态下,涂层覆在快表面对于减小摩擦、提高膜厚是有益的。     

椭圆接触纯自旋问题的弹流润滑数值分析

研究了椭圆接触纯自旋问题的等温弹流润滑,用多重网格法求得了完全数值解,研究了速度、载荷、椭圆比和计算区域对压力和膜厚的影响。结果表明,在轻载、高速、大椭圆比条件下,椭圆接触的纯自旋运动可产生润滑膜,油膜的形状和压力分布都和经典弹流润滑截然不同;椭球的自旋速度、载荷、椭圆比以及承载域的大小都对压力和膜厚有很大的影响。     

内燃机凸轮-滚轮型接触副弹流润滑分析

基于某内燃机凸轮-滚轮型机构,建立相应的接触副弹流润滑数值模型,得到凸轮旋转周期内运动副的完整润滑状态,并分析滚轮凸度、润滑油黏度及凸轮-滚轮间打滑现象的影响。结果表明：一个周期内,凸轮-滚轮接触副的润滑状态可分为波动期和平稳期,与凸轮升程的改变规律相对应;滚轮凸度会影响接触副的润滑状态,且接触区压力分布对其十分敏感;提高润滑油黏度在一定程度上可以起到优化接触区压力分布,改善润滑状态的效果;凸轮-滚轮间打滑现象则会降低接触区成膜厚度,尤其是对润滑油温升和摩擦因数的影响更为显著。     

连续波状粗糙度对直齿轮热弹流润滑的影响

工程实践中没有理想光滑的表面,在齿轮弹流润滑中,油膜的厚度通常与某些切削工艺形成的金属表面粗糙度处于同一数量级,所以表面粗糙度对齿轮弹流润滑的影响是不应该忽略的。在考虑不同啮合点处的曲率半径、卷吸速度、轮齿载荷随时间变化的基础上,考虑轮齿表面连续波状粗糙度对弹流润滑的影响,利用多重网格技术求得齿轮瞬态微观热弹流润滑的完全数值解。结果表明,连续波状粗糙度会造成齿轮瞬态弹流润滑的油膜压力和温升产生振荡,并使最小膜厚变薄,最高压力变大,最大温升增大。轮齿间振荡的高压和高温会造成齿轮振动疲劳破坏,所以连续的波状粗糙度对齿轮的润滑是不利的。     

Elastohydrodynamic Analysis of a Rotary Lip Seal Using Flow Factors

An elastohydrodynamic analysis of a rotary lip seal is performed numerically, incorporating both the fluid mechanics of the lubricating film and the elastic deformation of the lip. Asperities on the lip surface dominate the behavior of the flow field in the lubricating film and are taken into account through the use of flow factors. Because previous analyses treated those asperities deterministically, they required very large computation times. The present approach is much less computationally intensive because the asperities are treated statistically. Because cavitation and asperity orientation play important roles, these are taken into account in the computation of the flow factors. An asperity distortion analysis is introduced to model the complex variations in the asperity distribution on the surface of the lip. Results of the analysis show how the operating parameters of the seal and the characteristics of the asperities affect such seal characteristics as the thickness of the lubricating film, reverse pumping rate, power dissipation, and liftoff speed.     

螺杆挤出机润滑间隙内温度入口效应

在不同条例上计算了润滑间隙内温度入口长度，并用它作为衡量温度入口效应的定量指标；分析了各有关参数和温度入口效应之间的关系，本文的结论有助于认识热对流对间隙内温度分布及间隙流产生的润滑作用的影响。     

Thermal effects in elastohydrodynamic lubrication of line contacts using a non-Newtonian lubricant

A Reynolds' equation, using Winer's viscoplastic model to express the non-Newtonian fluid property, is derived for line-contact EHL problems. The numerical solutions are obtained to the incorporated Reynolds', elasticity, and energy equations for pressure, film thickness, and temperature distribution between two surfaces simultaneously having rolling and sliding motions. The results are presented for thermal non-Newtonian lubrication, to observe the difference between Winer's equation and Trachman's expression on temperature distribution, pressure, and film thickness. The variation in friction coefficient with slip shear rate is in agreement with other experimental data.     

In Situ Pressure and Film Thickness Measurements in Rolling/Sliding Lubricated Point Contacts

The physical conditions—pressure, temperature, shear stress—generated in an elastohydrodynamic contact govern the rheological behavior of the lubricant within the contact, and thus its tribological performance.This paper presents in situ measurements of pressure and film thickness in EHD point contacts using respectively Raman microspectrometry and differential colorimetric interferometry. A model lubricant—a polyphenylether oil (5P4E)—is used. The influence of load, temperature and sliding speed at different mean entrainment speeds on pressure and film thickness distributions is investigated. The analysis is based upon the relative values taken by the Moes dimensionless parameters M and L. In all cases, the pressure peak tends to increase and to move towards the center of the contact when the slide to roll ratio increases. When the sliding speed reaches large values (100 to 180%), several cases are encountered: When M > L, the pressure peak softens and moves towards the outlet; The Petrusevich peak disappears and the pressure profile remains almost constant as M and L are close to each other; When M < L, the peak disappears along with a strong modification of the pressure distribution. The maximum value is significantly reduced and the area of contact increases.Measured film thickness profiles confirm these tendencies. Increasing the slide to roll ratio leads to an important film thickness reduction and modifies the position of the constriction. Furthermore, film increase localized between the contact center and the constriction region is observed. This indicates the presence of important thermal effects within the contact and is discussed in the light of recently published papers.