齿轮瞬态弹流润滑的多重网格数值分析

利用多重网格技术，考虑了轮齿上的载荷、啮合点的卷吸速度与综合曲率半径随啮合线的变化，求得了齿轮传动瞬态弹流润滑的完全数值解。揭示出最小膜厚、最大压力沿啮合线的变化，讨论了齿轮传动比对压力与膜厚的影响．给出了几个导致润滑失效的危险啮合点的压力与膜厚分布。     

脂润滑轮毂轴承弹流润滑数值分析

基于Ostwald模型建立脂润滑控制方程,运用多重网格法求得等温线接触脂润滑弹性流体动力润滑数值解,得到钢球-沟道的压力分布、油膜形状及最小油膜厚度。针对轿车轮毂轴承的典型应用工况条件,分析工况参数对油膜压力分布和油膜形状的影响。结果表明:脂润滑弹流膜具有与油润滑膜相同的二次压力峰和出口颈缩现象。在轿车轮毂轴承可能的承载条件下,随着载荷的减小,二次压力峰的高度降低,其位置向入口区移动;一定承载条件下,速度增加时,膜厚相应增加,油膜的平行部分缩短,二次压力峰的高度增加,其位置也向入口区移动;一定承载和卷吸速度下,润滑脂流变参数增大时,二次压力峰的高度升高,其位置向入口区移动,膜厚相应增加。     

FB2型凸轮/平面挺柱的摩擦性能数值分析

通过运动学和动力学分析得到FB2型内燃机凸轮/平底挺柱摩擦副在一个周期内的综合曲率半径、卷吸速度和载荷变化情况,在内燃机凸轮/从动件的瞬态热弹性流体动力润滑数值仿真平台上研究FB2型凸轮与平底挺柱摩擦副的摩擦学性能.分析结果表明在凸轮的基圆段和缓冲段部分,摩擦因数在0.015～0.017之间,接触区中心的油膜厚度在0.9μm左右;在凸轮桃尖附近摩擦因数高达0.079;在卷吸速度为0的两个位置由于温度楔效应和挤压效应中心油膜厚度仍维持在0.313和0.270μm.     

内燃机排气凸轮/挺柱机构的瞬态热弹性流体动力润滑研究

在对内燃机排气凸轮/挺柱机构进行运动学和动力学分析的基础上,采用多重网格法和多重网格积分技术对该摩擦副进行了热弹性流体动力润滑研究。结果表明:凸轮的一个旋转周期内的不同瞬时摩擦副的润滑状况存在显著的差别,压力、膜厚等相关参数都表现出剧烈的变化;最小膜厚和最高接触压力出现在凸轮挑尖处;卷吸速度为零的2个工作点之间是凸轮-挺柱润滑的危险区;摩擦造成的润滑油温升超过40℃。     

油润滑对微动摩擦特性影响的研究

研究了钢摩擦副在油润滑工况下不同位移幅值对微动润滑摩擦特性的影响,分析了表面形貌。研究表明微动过程中表面之间的油介质会被驱除出接触区域,而滑动过程中表面之间的油介质始终保持在接触区域;润滑油的存在对接触区域起到了遮盖的作用,减少了氧化反应;由于油的流动性,在微动过程中容易再次渗透到接触区域,降低了表面摩擦与磨损。     

表面粗糙度对发动机活塞环-缸套润滑状态及摩擦功耗的影响

本文应用“平均”形式的Reynolds方程研究了表面粗糙度对发动机活塞环-缸套润滑状态的影响,分析研究了活塞环-缸套间的混合润滑效应。在研究中考虑了温度效应的影响,改善了以往混合润滑模型中的不足之处。还联系实际分析了S195柴油机环组的润滑状态,定量地给出了S195柴油机环组在整个工作循环过程中的摩擦功耗值。     

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。     

润滑条件对M50钢摩擦磨损性能的影响

使用重载往复摩擦磨损试验仪对比分析300 N载荷下不同润滑状态对M50钢摩擦磨损性能的影响,使用扫描电子显微镜(SEM)、三维形貌轮廓仪、能谱仪(EDS)测量磨损表面形貌和成分,并对试样磨损后的截面组织进行分析。结果表明,在从乏油润滑到干摩擦的过程中摩擦因数发生突变,并在进入干摩擦后趋于稳定,磨损率先增大后减小。磨损机理从乏油润滑的磨粒磨损转变为干摩擦的氧化磨损和黏着磨损,且随着干摩擦的进行氧化加深,黏着磨损加剧,磨痕的截面组织细化,塑性变形加剧,在干摩擦阶段出现裂纹。     

周向安装位置对水润滑塑料合金轴承摩擦因数的影响

由于水润滑塑料合金轴承带有纵向润滑沟槽,因而其周向安装位置影响轴承的摩擦性能。为了研究润滑沟槽对其摩擦因数的影响,通过实验方法,研究了不同安装角度对摩擦因数的影响,从而确定了其合理的周向安装位置。     

方向性微孔对润滑状态转化的影响

基于平均流量模型和微凸体接触模型,研究混合润滑状态下织构表面的摩擦特性,通过数值求解得到Stribeck曲线,分析法向载荷、润滑油黏度、表面粗糙度、方向因子和倾斜角对摩擦因数及名义摩擦副间隙等摩擦性能参数的影响规律。结果表明：混合润滑条件下,随着载荷的减小或润滑油黏度的增大,摩擦因数减小,名义摩擦副间隙增大,混合润滑转变为流体润滑时的临界转速降低;随着表面粗糙度的增大,摩擦因数和名义摩擦副间隙均增大,临界转速升高;随着倾斜角的减小或方向因子的增大,摩擦因数减小,名义摩擦副间隙增大,并且倾斜角越小,临界转速越低。     

Surface Roughness Effects on Metallic Contact and Friction

A study was made of surface roughness effects on metallic contact and friction in the transition zone between hydrodynamic and boundary lubrication. The system used was one of pure sliding and relatively high contact stress, namely a fixed steel ball riding on a rotating steel cylinder.

It was found that very smooth and very rough surfaces gave less metallic contact than surfaces of intermediate roughness; very smooth surfaces also gave less friction.

Four different types of antiwear/antifriction additives (including tricresyl phosphate) were studied and although they were found to reduce metallic contact and friction, they had little effect in reducing surface roughness. Rather, the additives merely slowed down the wearing-in process of the base oil. Thus, the “chemical polishing” mechanism advanced for the antiwear behavior of tricresyl phosphate appears to be incorrect.

With rough surfaces, the improvement in load-carrying capacity with increasing viscosity was less than that shown previously with smooth surfaces. Also, oils with a large pressure-viscosity coefficient did not show the expected beneficial effect with rougher surfaces.     

Friction Dependence on Surface Roughness for Castor Oil Lubricated NiTi Alloy Sliding on Steel

Abstract

NiTi alloys are highly resistant to corrosion with hardness and elasticity properties suitable for bearing applications. However, lubrication of this material is still being explored since many commercial lubricants are designed to function through chemical and physical interactions with steel surfaces. Previous studies suggested castor oil as a natural lubricant for NiTi and reported friction coefficients less than 0.01, i.e. superlubricity, when sliding steel against 60NiTi (60wt.%?Ni, 40?wt.%?Ti). Here we explore the robustness of those findings by characterizing the change in friction coefficient as a function of contact pressure and surface roughness. It is found that friction is extremely sensitive to surface roughness, such that ultra-low friction could only be achieved with surfaces having roughness less than tens of nanometers, conditions which may not be achievable in some practical bearing applications.     

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.     

混合润滑下粗糙表面的接触刚度研究*

针对润滑状态下结合面的接触刚度问题,建立一种混合润滑状态下粗糙表面接触刚度等效薄层模型,将接触界面的总刚度等效为固体接触刚度和润滑剂接触刚度之和,研究不同实际接触面积下的表面形貌和润滑剂类型对法向接触刚度的影响,并讨论固体刚度和润滑剂刚度占总法向刚度的比例。结果表明：粗糙界面的法向接触刚度随法向载荷的增加而增加,且混合润滑状态下的接触刚度大于干接触条件下的接触刚度;在初始接触时,法向接触刚度敏感地依赖于润滑性能;随着实际接触面积的增大,表面形貌对接触刚度的影响变得更加明显。     

润滑点接触粗糙表面滑动摩擦因数的实验研究

利用多功能微摩擦试验机,在轻载荷条件下测量了具有横向纹理圆盘表面的摩擦因数,得到了包括流体润滑、混合润滑和边界润滑完整的Stribeck曲线。考虑了载荷和时间对实验结果的影响,对不同粗糙度表面的Stribeck曲线进行了比较。实验的初步结果表明:增加载荷与实验时间,会获得更加稳定的实验结果;表面越粗糙,从混合润滑向流体润滑转换的临界速度越大;表面越光滑,Stribeck曲线的“谷底”越明显。     

The Combined Role of Soot Aggregation and Surface Effect on the Friction of a Lubricated Contact

One of the considered research paths to reduce friction loss consists in optimizing the interactions between surfaces and lubricants. The latter may significantly change with the lubricant ageing. In this framework, the tribological behaviour of aged formulated lubricant is analysed for various low-speed reciprocating motions and with different nature of surfaces. This paper focuses on soot aggregate formation processes in a lubricated contact and on their correlation to friction. Although no aggregates have been observed in pure rolling conditions, pure sliding conditions may lead to the appearance of aggregates moving through the contact as a function of the nature of the surfaces. The analysis of their displacement within the contact is used to discuss their interactions with the surfaces. Moreover, we show that the velocity and the dwell time of the aggregate depend on the sliding speed. The morphology of these aggregates evolves over time, affecting friction behaviour. An additive law combining a contribution from the shear of the aggregates with another one due to the shear of a thin lubricant film surrounding the aggregate is then proposed to interpret friction origin and friction evolution with time of shear. The aggregate motion also varies with the nature of the surfaces: in particular, DLC–DLC couple reduces aggregation phenomena and maintains a low friction without apparent wear.     

Dynamic Friction Model of Lubricated Surfaces for Precise Motion Control

A model is developed to describe dynamic friction effects in lubricated surfaces. The model covers the hydrodynamic, mixed and boundary lubrication regions. The dynamic friction model can predict the friction force for time-varying velocity, and is useful in precise motion control. The model presented is for a short journal bearing, but can be extended to other geometries of sliding surfaces, such as point and line contacts or rolling element bearings. The friction is related to a time variable fluid film thickness, resulting from journal vibrations relative to the sleeve. The proposed model agrees qualitatively with experimental results for lubricated line contact. Both show similar hysteresis-type friction curves under oscillating velocity.     

A New Roughness Parameter Including Hardness and Contact Frequency Effects on Lubricated Rolling/Sliding Wear

Surface roughness, roughness arrangement, film thickness, material hardness, and run-in process have significant effects on the lubricated rolling/sliding wear of mechanical components such as gears and bearings. In conventional analysis, a film thickness parameter is calculated by a geometric approach to study the wear resistance of a contact system without considering the effects of material hardness and run-in process. Although the conventional parameter is simple, it does not correlate with some experimental observations. In this work, a new roughness parameter is developed for the prediction of lubricated rolling/sliding wear. Surface roughness will be adjusted by its hardness and contact frequency. The calculation results are consistent with four groups of experimental data. It is proved that the conventional models can be derived as a special case of the new model when two contact surfaces have the same properties. The new model can be used in the optimal design and manufacturing of mechanical interfaces to reduce lubricated rolling/sliding wear.     

表面凹坑对点接触脂润滑油膜特性的影响

通过数值计算方法,研究了点接触脂润滑摩擦副表面存在凹坑时油膜厚度和油膜压力的分布规律,并与光滑表面条件下的油膜特性进行了对比。研究结果表明:点接触脂润滑摩擦副表面存在凹坑时,在凹坑边缘位置会出现油膜压力峰,而在凹坑中心周围油膜压力值较低;靠近入口处一侧凹坑边缘位置油膜厚度出现"凹陷"现象,润滑脂流经凹坑时油膜厚度则会"跃升";凹坑引起油膜压力和油膜厚度分布的变化会随着凹坑位置的变化而相应的变化;表面凹坑不利于点接触脂润滑摩擦副的润滑。     

The Scale Effect of Roughness on Hydrodynamic Contact Friction

Multistage abrasive finishing processes (grinding, polishing, honing, etc.) are commonly used to produce the geometrical properties of a surface to meet its technical functionalities in the operating characteristics of contacting parts in friction, relating to their durability and reliability (running-in performance, wear resistance, load-carrying capacity, etc.). Coarse abrasive grits followed progressively finer ones are used, which leads to a multiscale stratified surface texture.

In this article, a numerical model of elastohydrodynamic (EHD) contact coupled to a multiscale surface texture model was developed that allows tracking the scale effect of surface features and their interactions on friction performance and lubricant flow under hydrodynamic lubrication conditions. Because the simulation model has as an input the surface topography and to overcome the variability in surface finish formation, textured surfaces at different stages of the finishing process were simulated (virtual texturing method). Surface topography can be decomposed into two principal components: superficial roughness and valleys. Superficial roughness was modeled using a fractal model and a scaling factor was introduced to model valley patterns. The results show the relationship between friction and surface scales.