Interferometry Measurement of Spin Effect on Sliding EHL

This paper describes interferometry measurement of the film profiles of sliding elastohydrodynamic lubrication (EHL) contacts with spin. In the custom-made EHL test rig, spin motion is introduced through adjusting the center offset of the ball-on-disc contact with respect to the disc rotation axis. A parameter, spin ratio S _sp, is employed to represent the spin level, which is defined as the ratio of the Hertzian contact radius to the center offset. Experimental results show that with spin the film shape is obviously skewed, and the film thicknesses at the two side lobes are no longer the same; therefore, the symmetry of the classical horseshoe film shape is lost. The film thickness dependences on entrainment speeds are significantly influenced by the spin ratio S _sp, and high spin ratios induce high speed indices. At a fixed spin ratio S _sp, with increasing sliding speeds the film thickness difference between the two side lobes becomes large, and the horseshoe film shape is more distorted. When applied loads are raised, more spin is introduced, film thickness decreases, and film shape is obviously twisted.     

Pressure fluctuations as grease soaps pass through an EHL contact

Soap lumps entering a grease-lubricated elastohydrodynamic lubrication (EHL) contact were traced as they passed through the contact by using optical interferometry in a standard Ball & Disc apparatus and a high-speed video camera with light enhancer. One specific soap lump could be traced when it was passing through the contact and sharp images of the passing lump could be captured. This paper presents a combined experimental and numerical approach to determine what is happening when such lumps pass through the contact. From the fringe pattern, obtained by optical interferometry, a film thickness map is created by using image processing. This is done for every time step (1 ms) as the lump is passing through the contact. These maps serve as input to a numerical computation of the pressure by assuming elastic deformation theory. Consequently, no assumptions about the rheological behaviour of the grease have to be made. Two greases were studied, based on the same synthetic poly-α-olefin but thickened with Li-12-OH and Li-complex respectively. It is seen that the soap thickened lumps can cause deep elastic indentations accompanied by large pressure fluctuations. The pressure level will in some cases be more than doubled due to lumps entering the contact region. The effects on noise level and operational life are also discussed.     

A Generalized Differential Colorimetric Interferometry Method: Extension to the Film Thickness Measurement of Any Point Contact Geometry

Whereas industrial elastohydrodynamic (EHD) contacts are generally noncircular, most experimental observations are made on sphere-on-plane conjunctions. The circular case is indeed a specific elliptical case, and it was widely investigated. The differential colorimetric interferometry (DCI) technique was often used to perform precise film thickness measurements in circular EHD contacts. From a single picture of the dynamic contact, it enables mapping the film thickness of the full conjunction, and postprocessing can be applied afterwards. Moreover, it is possible to record sequences at relatively high-frequency sampling. However, until now, the method could not be directly applied to noncircular conjunctions. In the present article, a generalized DCI method is proposed and assessed by several static and EHD validation cases for elliptical and torus-on-plane contact geometries. This new method no longer necessitates particular requirements on the contact shape while retaining the advantages of the original DCI method. It allows precise film thickness measurements in realistic industrial EHD contacts and opens the way for new experimental observations.     

自旋对弹流油膜影响的试验研究

利用自制的旋滑式光干涉弹流薄膜测量系统,对带有自旋的钢球—玻璃盘接触副形成的弹流油膜形状和厚度进行试验研究。采用新的方法来获得自旋,即通过调节接触副与玻璃盘旋转中心的距离改变自旋分量的大小。试验结果表明,自旋导致油膜厚度降低,油膜形状也失去了经典的马蹄形相对卷吸中心线的对称性。当卷吸速度增加时,油膜厚度增加,油膜形状的非对称性增强;载荷增加,油膜厚度减小,油膜形状的非对称性增强;偏心距增加,油膜整体厚度增加,两侧油膜厚度差别减小,油膜形状的对称程度增加。     

Oil Film Stiffness of Double Involute Gears Based on Thermal EHL Theory

Lubrication failure is one of the main failure forms of gear failure.Time varying meshing stiffness is an important factor affecting the dynamic behavior of gears.However,the influence of oil film stiffness is usually ignored in the research process.In this paper,according to the meshing characteristics of double involute gears,based on the non-New-tonian thermal EHL theory,a new calculation method of normal and tangential oil film stiffness for double involute gears is established by the idea of subsection method.The oil film stiffness difference between double involute gears and common involute gears is analyzed,and the influence of tooth waist order parameters,working conditions,and thermal effect on the oil film stiffness are studied.The results reveal that there are some differences between normal and tangential oil film stiffness between double involute gears and common involute gears,but there is little dif-ference.Compared with the torque,rotation speed and initial viscosity of the lubricating oil,the tooth waist order parameters have less influence on the oil film stiffness.Thermal effect has a certain influence on normal and tan-gential oil film stiffness,which indicates that the influence of thermal effect on the oil film can not be ignored.This research proposes a calculation method of normal and tangential oil film stiffness suitable for double involute gears,which provides a theoretical basis for improving the stability of the transmission.     

Measurement of Sub-Nanometer Lubricant Films Using Ultra-Thin Film Interferometry

The ultra-thin film interferometric method of measuring the thickness of very thin films in lubricated contacts has been refined so as to be able to measure films down to 0.3nm with a standard deviation of 0.15nm. The main remaining source of measurement variation for films below 3nm thick is the surface roughness of the contacting solids. This modified technique has been applied to study the film-forming properties of three fluids, hexadecane, a dilute solution of surfactant in hexadecane, and cyclohexane. Purified hexadecane shows a very slightly enhanced oil-film thickness below 1nm. The long-chain surfactant forms a boundary film 2nm thick. Cyclohexane behaves as though it forms a surface layer about 1nm thick with viscosity three times the bulk fluid viscosity.     

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。     

线接触弹流问题中油膜厚度的理论计算

给出了一种线接触弹流问题中油膜厚度的理论计算公式。通过与不同参数下的数值计算结果进行比较,表明理论解能较好地反映油膜厚度的大小。并且这种理论分析也给计算接触区外压力分布和乏油问题提供了一条解决途径。     

Effect of Sliding Direction on EHL Film Shape Under High Sliding Conditions

It is well known that a sliding speed influences a lubricant film thickness of elastohydrodynamic rolling–sliding contacts significantly. The effect of sliding is described quite well for unidirectional rolling and sliding; however, there are a limited number of papers dealing with sliding in different directions. This study describes how the sliding direction influences elastohydrodynamic film shape under high sliding conditions. An optical ball-on-disc tribometer together with thin-film colorimetric interferometry method is used for a film thickness measurement. The results show that the sliding direction influences lubricant film shape and the effect is connected with dimple phenomena. The temperature–viscosity wedge effect is discussed as a possible mechanism. The results are important for a film thickness prediction under high sliding conditions and provide experimental evidence for an extension of elastohydrodynamic lubrication (EHL) theory.     

表面粗糙度对点接触弹流润滑性能的影响

本文应用多重网格法对椭圆接触的部分膜弹流问题进行了完全数值求解，计算中采用了Ｐａｔｉｒ－ｃｈｅｎｇ的平均流量模型及Ｇｅｅｎｗｏｏｄ和Ｔｒｉｐｐ的表面微峰弹性接触模型。在大量数值计算结果的基础上分析了表面粗糙度大小及纹理方向对点弹流的油膜厚度及微峰接触载荷的影响。     

Basics of EHL for practical application

Elastohydrodynamic lubrication (EHL) is present in all lubricated components whose elements roll together, including gears, rolling bearing, cams and constant velocity joints. These are characterised by having very localised and thus very high pressure contact, of order 1 to 3 GPa, between the elements. Two important practical properties of EHL contacts are the lubricant film thickness and the friction, and lubricant and machine designers and users need to be able to predict both of these. In principle, they can be determined from full numerical solution of the elastohydrodynamic problem. However, this is quite difficult and time‐consuming and requires detailed knowledge of the rheology of the lubricant film at high pressure and shear rate. This paper is aimed at practising engineers and describes alternative approaches, i.e. how EHL can be applied to predict film thickness and friction in practical applications. Copyright © 2014 John Wiley & Sons, Ltd.     

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.     

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。     

倒角参数对滚子弹流润滑性能的影响研究

工程上常对滚子摩擦副的滚子进行端部倒角,倒角参数包括倒角长度与倒角半径。取润滑油为Eyring流体,用多重网格技术研究了此类滚子的弹流润滑性能,研究表明:最小膜厚位置可能出现在修形起始位置和滚子最边缘。当最小膜厚出现在修形起始位置时为正常工作状态;当最小膜厚出现在滚子最边缘时膜厚很薄,即将润滑失效。进而分别讨论了倒角长度和倒角半径对弹流润滑性能的影响,研究表明:应根据工况参数通过弹流计算设计端部倒角参数,以优化滚子的润滑状态和载荷分布,实现全膜润滑。     

Influence of spinning on the rolling EHL films

Spinning cannot be ignored in some elastohydrodynamically lubricated contacts. In this paper, spinning is incorporated into an elastohydrodynamic lubrication (EHL) contact of pure rolling and its influences on EHL films are studied. Results show that with increase in spinning, the symmetry of the film shape gets lost, and the minimum film thicknesses, located respectively at the two side-lobes, decrease and show more dependence on loads. The speed indices of the film thickness at the side lobes are higher than those of the classical EHL theory predicted. Numerical work has also been carried out to clarify the experiment measurements.     

An Experimental Validation of the Recently Discovered Scale Effect in Generalized Newtonian EHL

New quantitative numerical simulations of the elastohydrodynamic lubrication (EHL) film forming ability of generalized Newtonian liquids have elucidated a previously unrecognized property of EHL films. The dependency of the film thickness on the scale of the contact is greater when the viscosity is shear dependent within the inlet. Measurements of film thickness were performed in a ball on disc experiment using balls ranging from 5.5 to 35 mm in diameter. Three liquids were investigated with varying shear dependence in the range of stress important to film forming. The experimental results confirm the previous analytical findings. Numerical simulations using the measured viscosities show that the increased scale sensitivity is substantially the result of shear-thinning. However, the smallest scales produced films thinner than even the shear-dependent prediction, possibly indicating molecular degradation. It is quite likely that some machine components, which were designed using the effective viscous properties derived from a larger scale film thickness measurement, are operating with substantially lower film thickness than the designer had intended.     

表面速度异向下的点接触润滑油膜试验机研制及膜厚测量

弹流润滑领域中,润滑油膜的形态、厚度、摩擦力是研究接触区润滑状态,探究润滑机制极为重要的信息。研制表面速度异向下的点接触润滑油膜试验机,该试验机以多光束干涉法作为测量手段,结合弧形轨道实现接触副表面速度夹角的变化;使用旋转系统将接触副系统摩擦力转变为压力,利用高精度压力传感器及摩擦力合力公式,实现表面速度异向下的点接触摩擦力测量和计算。在限量供油条件下对表面速度异向的点接触润滑油膜形态与膜厚进行了观测,探究速度夹角对乏油状态改善及油膜形态变化的影响。结果表明：改变接触副表面速度方向,入口油池得到改善,中心膜厚增加。     

塑料与钢制齿轮啮合的弹流润滑研究

在油润滑条件下，对改性聚醚醚酮塑料齿轮与钢制齿轮啮合时的弹流润滑机理进行了数值模拟研究，并将计算结果与钢制齿轮副的接触点压力、润滑膜厚等进行了对比。结果表明，塑料与钢制齿轮的啮合点处存在弹流润滑油膜，且油膜要比钢制齿轮间的啮合油膜要厚；塑料对钢齿轮啮合点处的最大和中心油膜压力都比钢对钢齿轮的啮合压力明显降低。     

A Limiting Solution for the Dependence of Film Thickness on Velocity in EHL Contacts with Very Thin Films

In recent years there have been substantial improvements in the capabilities of numerical modeling of elastohydrodynamic lubricant (EHL) films and it is now possible to analyze a very wide range of conditions rather than needing to rely on extrapolation using classical film thickness regression equations such as those of Dowson and Higginson. However, a new controversy has arisen concerning the film thickness-velocity dependence in EHL contacts at very low speeds and high loads, with some predictions showing a film thickness much less than that predicted by the classical equations. The present article applies the well-established limiting analysis, first presented by Grubin-Ertel, to the inlet of the EHL contact. It is shown that when the load is high and the speed is low (and the pressure gradient is very high in the inlet) an accurate resolution of the inlet pressure rise is critical for the determination of the film thickness. Discretization errors of this type might be responsible for discrepancies between the classical equations and some recently published numerical predictions.     

对数母线滚子光弹流润滑的实验研究

为了研究对数母线滚子凸度量对滚子摩擦副润滑油膜的影响 ,本文在自制的有限长线接触光干涉弹流试验机上 ,测量了中载、富油润滑和纯滚动工况下对数修形圆锥滚子与玻璃盘之间的油膜形状与膜厚。实验结果表明对数母线轮廓滚子的凸度量对其端部的油膜厚度和膜形分布有较大的影响 ,在给定的工况条件下 ,存在一个使滚子轴向油膜厚度最为均匀的最佳凸度量 ,此最佳凸度量比在相同工况条件下用静弹性接触力学方法求得的最佳凸度量大