高黏度齿轮油润滑状态转变的试验观察

利用双色光干涉润滑油膜测量技术,观察球-盘接触副内大黏度齿轮油润滑状态的转变过程,并对不同速度和载荷条件下润滑状态在不同区间内的转化进行定量分析。结果表明:在充分供油条件下,随卷吸速度增加,润滑油膜从弹流润滑状态向动压润滑状态转化,且2种润滑状态之间存在着明显过渡区间;而随着载荷的增加,润滑油膜从动压润滑向弹流润滑状态转化;在定量供油条件下,润滑油膜在弹流润滑区间内从富油润滑状态向乏油润滑状态转化。     

基于弹流润滑的圆锥滚子球基面曲率半径优化设计

根据圆锥滚子轴承的运动学原理,建立圆锥滚子球基面-内圈大挡边的运动学模型,得到圆锥滚子的自转速度、公转速度和圆锥滚子球基面-内圈大挡边接触区域的卷吸速度分布;建立圆锥滚子球基面-内圈大挡边弹流润滑的数学模型,计算分析在不同载荷和卷吸速度作用下,圆锥滚子球基面曲率半径对油膜厚度和摩擦因数的影响规律,并得到载荷与速度对圆锥滚子球基面最优化曲率半径的影响规律,即当其他工况条件不变时,圆锥滚子球基面所承受的载荷越大,其最优化的曲率半径就越大;速度越高,最优化的曲率半径则越小。     

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。     

表面磨损对非牛顿弹流润滑性能的影响

考虑磨损对润滑状态的影响,构建带有磨损带的非牛顿流体圆接触热弹流润滑模型,分析磨损半宽对油膜压力、厚度、温度和摩擦因数的影响,并与牛顿流体的热弹流解进行比较。结果表明,随着磨损半宽的增大,最小油膜厚度明显减小,且总是处于磨损带边缘附近,并向入口处移动;而中心膜厚随磨损半宽的增大线性增大;卷吸速度、载荷及滑滚比对膜厚随磨损半宽改变的影响不大;非牛顿效应对温度和摩擦因数有较大影响。     

面接触边界润滑模型研究

根据经典的热传导公式,求解面接触边界润滑条件下的接触温度,再根据吸附热和摩擦因数、接触温度之间的关系建立一种求解边界润滑摩擦因数的模型。利用模型计算一个实例,通过分析发现,在载荷较低时,摩擦因数随滑动速度或载荷的变化变动很小,载荷较高时,摩擦因数随滑动速度或载荷的增大而增加;摩擦因数随接触温度的增加而增加;相对油亏量和摩擦因数有着基本一致的变化规律。通过在自制滑块-摩擦盘机构上的实验证实,实验结果和理论结果有相似的变化规律。     

球-环接触下油性添加剂对聚α-烯烃油润滑性能的影响

为研究全膜润滑条件下油性添加剂对润滑性能的影响,利用球-环光弹流油膜测量试验台对PAO10 (聚α-烯烃)以及分别含有质量分数0.2%硬脂酸、 0.2%棕榈酸和0.2%肉豆蔻酸油性添加剂的PAO10 (分别记为PAOS、PAOP和PAOM)进行膜厚和摩擦因数的测量,研究在球-环接触下油性添加剂对PAO10油润滑性能的影响。结果表明:在全膜润滑状态下,硬脂酸、棕榈酸和肉豆蔻酸3种油性剂对PAO10油的膜厚和摩擦因数都有影响;随着卷吸速度的增加,膜厚随之增加, PAOM、PAOP和PAOS的膜厚小于PAO10膜厚,并且随着油性添加剂链长增加依次减小,其中PAOS膜厚最小;滑滚比对油性剂作用有明显影响,随着滑滚比增加油膜受到的剪切应力增加,导致油性剂吸附效应降低; PAOS、PAOP和PAOM膜厚和摩擦因数降低主要因为是油性剂在钢球表面形成的吸附膜具有疏油性,而且随着油性剂分子链长增加固液界面间的结合能力越弱。     

流变模型对弹流润滑数值解的影响

采用Carreau流变模型和Ree-Eyring流变模型,研究不同流变模型对黏度较低的Squalane润滑油弹流润滑数值解的影响。分别计算不同卷吸速度、不同滑滚比下Eyring流变模型、Carreau流变模型的摩擦因数,并与试验值进行比较,同时比较Eyring流变模型与Carreau流变模型的摩擦因数、油膜最高温度、中心膜厚及最小膜厚随滑滚比、卷吸速度和最大赫兹压力变化的数值解。结果表明:在滑滚比较小时Eyring流变模型的摩擦因数更加接近试验值,在滑滚比较大时Carreau流变模型的摩擦因数更接近试验值;滑滚比对不同流变模型之间数值解的差别没有影响;随着卷吸速度的增大,Eyring流变模型所对应的膜厚值逐渐高于Carreau流变模型,而油膜最高温度逐渐小于Carreau流变模型;随着最大赫兹压力的增大,Carreau流变模型的油膜最高温度及摩擦因数逐渐大于Eyring流变模型。研究表明,在温和工况下Eyring流变模型更适合Squalane润滑油的弹流分析。     

球-环接触摩擦力测量系统的研制*

研制一种球-环接触摩擦力测量系统,该系统可以模拟滚动轴承中滚动体与轴承外圈之间的接触,通过调节球和环的转速可以测得不同滑滚比下的摩擦力。介绍系统的组成和测量原理,选用PAO20和PAO40两种基础油进行测量试验,验证系统的准确性。使用该系统测得不同滑滚比、卷吸速度及黏度工况下的摩擦因数变化曲线。结果表明:摩擦因数随滑滚比的增大而缓慢增大直至平缓,随卷吸速度的增大而逐渐减小。该结果与经典的润滑理论的结果一致,验证了测量装置的准确性。     

基于Carreau流变模型的圆柱滚子轴承热混合润滑分析*

为了研究圆柱滚子轴承接触区的混合润滑性能,建立基于Carreau非牛顿流体的热混合润滑模型,求解非牛顿流体线接触热混合润滑数值解。研究滑滚比、卷吸速度及载荷对线接触混合润滑特性的影响,并与相同工况下牛顿流体热混合润滑的结果进行对比。结果表明：随着滑滚比、卷吸速度及载荷的增大,油膜温度都会升高,Carreau非牛顿流体的温度要低于牛顿流体的温度;油膜厚度随着滑滚比、载荷的增大而减小,随着卷吸速度的增大而增大,Carreau非牛顿流体与牛顿流体膜厚相差不大;随着滑滚比的增大,2种流体的平均摩擦因数均增大,随着卷吸速度和载荷的增大,2种流体的载荷比均减小。     

多工况滚子摩擦副弹流润滑实验装置的设计

为了更好地观测滚子摩擦副在不同卷吸速度下的弹流润滑现象,开展有限长线接触弹流润滑油膜成膜机制的实验研究,对原有的线接触光干涉实验装置进行改进,将原有的曲柄滑块机构设计成丝杠推杆,从而将旋转运动转变成直线运动。实验表明:改进后的实验装置可以有效地实现滚子卷吸运动速度大小和运动规律的变化,可以观测到非常清晰的油膜干涉图像,在滚子接触区域油膜呈现出较为明显的弹流特征,可为线接触弹流的理论研究提供可靠的实验数据。在改进后的实验装置上,对滚子在突然加速的工况下进行了实验研究,由于步进电机的特点,在突然加速的瞬间有极短的停顿,导致挤压效应较为明显,油膜被封在滚子的接触区域内形成凹陷,出现封油现象,当滚子卷吸速度增加时,油膜厚度也会随之增加。     

Experimental study on lubrication regime variation in point contacts

The change between elastohydrodynamic lubrication (EHL) and hydrodynamic lubrication (HL) under a wide range of entrainment speeds and applied loads was studied using an optical EHL apparatus. A log-log scale linear relationship was demonstrated in the two lubrication regions between the film thickness and the entrainment speed (or load). A transition region can be clearly discerned between these two regions in which the film thickness no longer bears a linear relationship with the entrainment speed (or load). It is shown that a piezoviscous effect can be distinguished in the HL region by the speed exponent or the load exponent, and that relative sliding has a significant influence on the transition region.     

The Influence of DLC Coating on EHL Friction Coefficient

High hardness, high elastic modulus, low friction characteristics, high wear and corrosion resistance, chemical inertness, and thermal stability are factors that make diamond-like carbon (DLC) coatings the subject of many studies. For the same reasons they also seem suitable for use in, amongst others, machine components and cutting tools. While most studies in the literature focus on the influence of coatings on wear and friction in boundary lubrication and pure sliding contacts, few studies can be found concerning rolling and sliding elastohydrodynamic lubrication (EHL) friction, especially in the mixed and full film regime. In this article tests are carried out in a Wedeven Associates Machine tribotester where an uncoated ball and disc pair is compared to the case of coated ball against uncoated disc, coated disc against uncoated ball, and coated disc against coated ball. The tests are conducted at two different temperatures and over a broad range of slide-to-roll ratios and entrainment speeds. The results are presented as friction maps as introduced in previous work (Bj?rling et al. in J Eng Tribol 225(7):671, 2011). Furthermore a numerical simulation model is developed to investigate if there is a possibility that the hard, thin DLC coating is affecting the friction coefficient in an EHL contact due to thermal effects caused by the different thermal properties of the coating compared to the substrate. The experimental results show a reduction in friction coefficient in the full film regime when DLC-coated surfaces are used. The biggest reduction is found when both surfaces are coated, followed by the case when either ball or disc is coated. The thermal simulation model shows a substantial increase of the lubricant film temperature compared to uncoated surfaces when both surfaces are coated with DLC. The reduction in friction coefficient when coating either only the ball or the disc are almost the same, lower than when coating both the surfaces but still higher than the uncoated case. The findings above indicate that it is reasonable to conclude that thermal effects are a likely cause for the decrease in coefficient of friction when operating under full film conditions, and in the mixed lubrication regime when DLC-coated surfaces are used.     

Thin-Film Rheology and Tribology of Magnetorheological Fluids in Isoviscous-EHL Contacts

This article is concerned with an investigation of the tribological performance of magnetorheological (MR) fluids in pure sliding soft-elastohydrodynamic lubrication (EHL) steel/polytetrafluoroethylene (PTFE) point contacts. The lubricating properties of MR fluids were measured in thin film, lubricated conditions using a ball-on-three-plates tribometer and compared to base fluids in the form of Stribeck curves. A range of techniques was employed to interpret the possible mechanisms of friction and wear of dispersed iron microparticles. The friction surfaces were investigated using optical microscopy, environmental scanning electron microscopy (ESEM), X-ray photoelectron spectroscopy, and energy-dispersive spectroscopy (EDS). In all cases investigated, the friction coefficient is found to strongly depend on the viscosity of the lubricant oil. In the case of low-viscosity liquids, iron microparticles are entrapped in the contact and plow the PTFE surfaces resulting in a sensibly constant friction coefficient. For intermediate viscosities, friction decreases at low speeds because of the so-called ??ball-bearing?? effect, and later, friction increases as particles become embedded in the PTFE matrix. Finally, for high-viscosity fluids, iron particles either accumulate around the rubbing zone as a barrier that reduces the supply of oil available to the contact for boundary lubrication or the particles indent PTFE surfaces.     

The Failure and Damage Mechanisms Under Friction of Copper in the EHL and Mixed EHL Regions

Recently, friction and wear behaviour of Copper (Cu)–steel pairs rubbed under different lubrication conditions were studied. The Stribeck curve was used to identify the different regimes of friction of copper with different virgin grain sizes: the elasto-hydrodynamic lubrication (EHL), mixed lubrication and boundary lubrication (BL) regimes. The aim of this work is the detailed analysis of the damage evolution under friction in the EHL and mixed EHL regions. The effects of load on the friction and the wear and damage mechanisms have been studied. The surfaces of Cu samples before and after friction have been analysed using SEM and AFM techniques as well as roughness and hardness measurements. It was shown that the mechanisms of damage and failure of Cu samples under friction in the EHL region are similar to the damage and failure mechanisms in Very High Cycle Fatigue. Friction in the EHL region is accompanied by initiation and coalescence of pores and microcracks. The effects of the coalescence of pores and microcracks observed in the EHL region are enlarged with a subsequent loading in the mixed EHL region. The effect of the loading rate on the transition from the EHL to BL region has been studied. It was shown that decreasing the loading rate increases significantly the load of the transition from the EHL to BL region. The pore and microcrack coalescence remain as the dominant damage mechanism under friction in the mixed EHL region with the low loading rate, while a lot of ploughing tracks and large delaminated regions appeared on the surface of Cu sample after friction with the higher loading rate.     

弹流润滑入口凹陷现象的数值模拟

入口凹陷(inlet-dimple)是近年来弹性流体动力润滑(EHL)研究中出现的一类新油膜形状特征。使用Circular流变模型,利用数值分析,研究了滑滚比、载荷及卷吸速度等对凹陷的影响,并与部分已有实验结果进行了比较。初步结果显示,当滑动较大时,入口区油/固体界面处表观粘度剧烈下降,产生伪滑移,从而诱发了较大的压力梯度而形成入口凹陷。     

Stribeck Curve Under Friction of Copper Samples in the Steady Friction State

The friction and wear of a pure copper block (99.98 wt% Cu) against a hardened steel disc were studied. The effect of sliding velocity and load on the friction coefficient and wear rate of Cu samples during steady tests was studied. Elasto-hydrodynamic (EHL), mixed (ML) and boundary lubrication (BL) regions were analyzed using the Stribeck curve. The lubrication number of Schipper, Z, was used in the analysis of the Stribeck curve. The transitions from one lubrication region to another are discussed. The mixed EHL region is characterized by stable low values of the friction coefficient, wear rate and temperature. Straight asperity contact is the dominant mechanism under friction of Cu–steel pair in the BL region. High-friction coefficients and wear rates, thin lubricant films and large wear grooves indicate straight asperity contact between rubbed surfaces in the BL region. Although the dominant mechanisms in the mixed EHL and BL regions are different in principle, a steady friction state is preserved in both cases. It is expected that the steady friction state in the BL and mixed EHL regions is associated with deformation and fracture of surface layers but these process occur at different scale levels. It was shown that under friction of Cu–steel pair, two types of ML regions are observed. The first is the stable steady friction of mixed EHL with low values of the friction coefficient and wear rate. The second type of the ML region is the region of unstable friction and wear when a decrease of lubricant film leads to a change of external (roughness, temperature, friction and wear) and internal (strain and stress) parameters. It was found out that a transition to the unstable ML region occurs within a narrow range of Z parameter under definite values of the load and sliding velocity.     

The Frictional Properties of Newtonian Fluids in Rolling–Sliding soft-EHL Contact

A combined experimental and numerical study has been carried out to explore friction in rolling–sliding, soft-EHL contact. Experimental work has employed corn syrup solutions of different concentrations in water to provide a range of lubricant viscosities and has measured Couette friction in mixed rolling–sliding conditions over a wide range of entrainment speeds. A Stribeck curve has been generated, ranging from the boundary to full film, isoviscous-elastic lubrication regime. In the latter regime, friction coefficient is approximately proportional to the product of (entrainment speed × viscosity) raised to the power 0.55. Numerical solution of the isoviscous-elastic lubrication regime has been used to derive predictive equations for both Couette and Poiseuille friction in circular, soft-EHL contacts. This shows that in soft-EHL the Poiseuille or “rolling” friction can have magnitude comparable to the Couette friction. The calculated Poiseuille friction coefficient can be predicted from non-dimensional load and speed using a simple power law expression similar to that used for film thickness. However accurate prediction of calculated Couette friction coefficient requires a two-term power law expression. Comparison of experimental and numerical Couette friction coefficients shows quite good agreement between the two, with a similar non-dimensional speed dependence, but slightly lower predicted than measured values.     

The Influence of Surface Roughness on the Lubrication Properties of Adsorbing and Non-Adsorbing Biopolymers

The lubrication properties of a glycoprotein (pig gastric mucin or PGM) and a high-molecular-weight hydrosoluble polymer (guar gum) have been studied. Friction has been measured over a wide range of entrainment speeds and Stribeck curves have been obtained spanning the boundary, mixed and hydrodynamic lubrication regimes. The adsorption properties of the polymers have also been assessed using evanescent wave spectroscopy. The results show that the polymer that adsorbs on solid surfaces is able to reduce friction in the boundary lubrication regime (PGM). Guar, which does not adsorb on surfaces, shows high friction in boundary lubrication but still promotes the onset of mixed lubrication; thus friction starts to fall from its boundary values at low speeds. These results can be explained in classical terms of entrainment of polymer solution into the thin film conjunction and associated shear thinning in the contact inlet. With roughened surfaces, a shift of the Stribeck curves towards high speed is observed.     

On the thermal elastohydrodynamic lubrication in opposite sliding circular contacts

Cases of elastohydrodynamic lubrication (EHL) point contacts running under opposite sliding conditions have been studied with consideration of the thermal effect for various loads and entrainment velocities. A thermal EHL solver has been developed and proven to be able to deal even with extreme cases under an infinite slide–roll ratio. Results show that film profiles featuring a dimple can be formed in the contact zone when the slide–roll ratio exceeds a certain level. Moreover, the present study provides theoretical evidence for the lubricating film build up in the case where the two bounding surfaces run with equal but opposite velocities. An effective lubricating film under zero entrainment speeds was experimentally proven by Dyson and Wilson [1] (Proc Instn Mech Engrs, 1968–1969 183(3P) 81) in the 1960's, which, however, cannot be explained by the isothermal EHL theory.     

Tribological Behavior of Self-mated Ti3SiC2 in Short-Chain n-Alcohols,Glycol and Glycerol under Boundary Lubrication

The tribological behavior of self-mated Ti₃SiC₂ in bath of alcohols are investigated at sliding velocity ranging from 0.005 to 0.3 m/s. The results show that the friction coefficient and wear rates of self-mated Ti₃SiC₂ are reduced greatly under lubrication of alcohols compared to that under dry condition. In alcohols, smooth worn surfaces of Ti₃SiC₂ can be obtained. The mechanical wear is inhibited and the oxidized Ti and Si species on the worn surface are TiO₂ and silica gel. The friction coefficients decrease with carbon numbers and sliding velocity in the n-alcohols. The friction coefficient decrease with sliding velocity in glycol and glycerol as well. The decrease is determined by the increase of the viscosity of alcohols. In glycerol, at 0.1 m/s and 5 N, the friction coefficient of self-mated Ti₃SiC₂ is 0.08 which is the lowest in this paper. The lubrication regimes are calculated according to classical lubrication theory. At velocity from 0.005 to 0.3 m/s, in all of the alcohols except glycerol, the λ are below 1 which indicates that the lubrication regime is BL. While in glycerol, the lubrication regimes vary from BL to BL and EHL, then to EHL as the velocity increases.