On thin film lubrication

Various phenomena are revealed under EHL and micro-EHL conditions, such as the properties of the lubricant under high pressure, traction, and the load-bearing capacity of the lubricant film, and are discussed in the present paper. A new lubrication regime, thin film lubrication, has been discussed. The theoretical and practical significance of research on thin film lubrication is elaborated. Finally, the characteristics describing thin film lubrication and its main research directions are suggested.     

Thin film hydrodynamic lubrication of flying heads in magnetic disk storages

Typical hydrodynamic lubrication problems commonly encountered in the ultrathin spacing between a computer flying head and a magnetic disk are reviewed. In magnetic disk storages, minimizing the spacing between the head and disk is essential to promote the largest possible increase in magnetic bit density. In the small (nearly 1.0 μm) spacing that has recently been attained, the rarefaction effects owing to the molecular mean free path become dominant. Specifically, in this paper the three governing equations resulting from the first- and second-order slip-flow models and from the linearized Boltzmann equation are compared. Next, some numerical approaches to eliminating the instability in pressure distribution in the high bearing number region are described. Surface roughness effects are also a principal concern in thin spacing. A mixed lubrication model which enables the analysis of the start/stop operation and the average film thickness theory for one- and two-dimensional roughnesses is summarized. Finally, from the viewpoint of practical head design, the slider dynamic characteristics and related slider design factors are discussed.     

Failure criteria in thin film lubrication with EP additives

The scuffing behaviour of sliding couples made from 12 NiCr 13 steel, lubricated with SAE 90 and SAE 20 W30 oils with and without additives, was studied as a function of relative sliding speed and bulk oil temperature. It was found that the scuffing load decreases almost inversely with sliding speed. Calculations which take into account the decrease in hardness at increasing temperature show a fairly good constancy of total contact temperature (bulk + flash temperature) at scuffing (i.e. values ranging from 550 ° to 650 °C for all oils and test conditions). At low speeds (up to 1 m s^?1) a well-defined increase in scuffing load was found when EP additives were used; at higher speeds this effect was found to have vanished completely. Variations in nominal contact pressure in the range 1 to 3 had no appreciable influence on the scuffing load, indicating that, in the present case, scuffing was associated with a transition from the boundary lubrication regime to the severe wear regime.     

Rolling contact fatigue using solid thin film lubrication

This paper addresses rolling contact fatigue (RCF) testing in ultra high vacuum (UHV) under high speed conditions. A ball–rod RCF test platform has been adapted for testing in UHV conditions that allows rapid accumulation of stress cycles, over 10 million cycles within 5 h of testing at 130 Hz rotation. The UHV environment and solid lubrication enables good vibration detection for the onset of spall. In this paper, approximately 0.2 μm of silver is applied to the balls and provides sufficient lubrication for up to 25 h of testing, or 50 million stress cycles in high vacuum at 130 Hz. Seventy-nine RCF tests using thin-film silver lubrication have been completed covering two ball sizes, and two rod and ball materials. 9.53 mm diameter Rex 20 steel and silicon-nitride (Si₃N₄) rods were tested against 7.94 mm diameter Rex 20 and 12.7 mm diameter M50 steel balls. It was found that ball size and material hardness did not affect the stress cycle life over a Hertzian contact stress range of 2.1–4.2 GPa and Rockwell C hardness range of 62–77. Rather, the key limiter to test length is lubrication depletion based on 79 tests and an average silver thickness of 0.2 μm. One of the two failure modes were observed for all tests: (i) early life spall of the silver coating, and (ii) depletion of silver lubrication followed by spall failure of both the ball and rod surfaces. A third-body storage model along with the Control Volume Fraction Coverage (CVFC) assumption and analysis was used to predict lubrication availability between asperities on the third body. There is good agreement between calculated and measured post-test lubrication thickness using the third-body storage model.     

A continuous viscosity model for thin film lubrication

The thickness of adsorbed molecular layers is the most critical factor in studying thin film lubrication, and is a key feature that distinguishes thin from thick film lubrication analysis. A method is presented that enables the adsorbed layer thickness to be calculated. This is based on adsorption theory and expressed in terms of molecular interaction energies. A continuous cross-gap viscosity model incorporating the layer thickness is introduced and used to calculate the load capacity and frictional characteristics of a simple bearing operating in the thin film regime.     

Measurements of friction in strip drawing under thin film lubrication

Strip drawing is used to investigate the friction behaviour under thin film lubrication in metal forming with plastic deformation. Friction coefficients are measured under a wide range of tribological conditions. The surface roughness is measured on an interferometric profilometer. The results show that the friction coefficient decreases with increasing oil film thickness h_w, as estimated using a formula appropriate for smooth tool and workpiece. Measurements of the surface topography show that change in friction is associated with a change in contact ratio between the tool and strip. The effect of strip reduction, strip roughness and die roughness on the friction coefficient is also investigated.     

季戊四醇油酸酯膜厚测量及薄膜润滑研究

运用光干涉法相对光强原理,通过纳米级润滑膜厚度测量仪测量了不同温度、不同载荷下纳米间隙中环境友好润滑剂季戊四醇油酸酯的油膜厚度。探究了膜厚与速度、载荷、温度之间的关系,观察了薄膜润滑现象。结果表明：在对数坐标系下,膜厚与速度具有一定的线性关系,在速度较高时,线性关系更强,润滑剂具有流体效应。载荷对膜厚的影响远小于速度对膜厚的影响,温度对膜厚的影响主要表现在温度越高时,润滑剂的黏度越低,润滑剂的膜厚越薄。     

弹流润滑条件下齿轮传动润滑油膜厚度影响参数研究

基于弹性流体动力润滑理论,利用Dowson-Higginson公式推导出渐开线直齿轮基本参数与最小油膜厚度计算关系式,并通过MATLAB软件编程绘制出相应的关系曲线图,分析了传动比、模数、重合度、齿宽系数等齿轮传动参数对齿轮副润滑油膜厚度的影响,从而揭示了齿轮传动参数与齿轮副润滑性能之间的关系,为弹流润滑条件下齿轮传动的设计提供了理论依据.     

Velocity analysis for layered viscosity model under thin film lubrication

The lubrication of bearings is directly affected by the flow properties of the liquid lubricant. One key feature of lubricant is its viscosity distribution in the bearing. The distribution is dependent on the adsorbent properties of the surface and the velocity distribution of the lubricant. This paper is only connected with the velocity analysis on the cross-gap flow. Because of increasing the speed, apart from increasing the temperature, the other features of lubricant will be changed. When fluid film is thin, the speed effects become important due to the increased shear rate. This paper describes the fluid velocity profiles produced for the case where layers of greater viscosity than the bulk occur on the bearing surfaces.     

Research on thin film lubrication: state of the art

Thin film lubrication (TFL) has been well studied as a new lubrication regime since 1990s. TFL is a transition lubrication regime between elastohydrodynamic lubrication (EHL) and boundary lubrication (BL), which has specific lubrication features. Present paper summarizes the advancements of researches on TFL, which involves the origin of the TFL concept, advancement of measuring techniques, TFL film-forming features, investigation on its mechanism, and computation theories.     

Failure criteria in thin film lubrication: Investigation of the different stages of film failure

The mechanisms of failure of sliding lubricated concentrated steel contacts have been studied using a newly developed tribometer which allows fast separation of the sliding surfaces at the different stages of film failure. The appearance of the corresponding contact topography was investigated by means of optical interference microscopy. The results indicate a gradual transition from full to partial elastohydrodynamic (EHD) lubrication as a function of increasing load and increasing bulk oil temperature until complete failure of the EHD film occurs at critical triplets of normal load, sliding velocity and bulk oil temperature.     

A finite element approach of thin film lubrication in circular EHD contacts

So far, most of the numerical studies concerning elastohydrodynamic point contact lubrication have been realized using finite differences. This paper describes a finite element approach for the modelling of both, thick and thin film lubrication in circular contacts under isothermal elastohydrodynamic regime. Multigrid techniques are used to accelerate the convergence and reduce calculation time. The use of the finite element method (FEM) offers two advantages: first, the existence of high level ready-to-use Finite Element commercial software which reduces the time spent in implementing the method and second, the easier adaptability to different and various physical models such as lubricant rheology, starvation effects, thermal effects, non-Newtonian behaviour, etc. The numerical model is presented in detail and an example of its adaptation taking into account non-Newtonian effects is shown. Finally, a comparison is made between Newtonian and non-Newtonian solutions, exhibiting differences obtained when using both models, especially at high shear rates where the film thickness and the friction coefficients are overestimated by a Newtonian approach.     

The influence of transverse roughness in thin film, mixed elastohydrodynamic lubrication

The Spacer Layer Imaging (SLIM) method has been used to investigate the influence of transverse roughness on the thickness and shape of elastohydrodynamic (EHD) films. The effects of entrainment speed and lubricant viscosity on film distribution are shown for three distinct asperity heights over a wide range of lambda ratio (ratio of lubricant film thickness, separating two contacting surfaces, to their combined RMS roughnesses). Subsequently, the behaviour of film distribution for a range of mixed rolling–sliding conditions is also studied for both thin and thick film conditions. This study provides an estimate of how and when transverse asperities decompress and an indication of conditions under which these asperities cease to affect lubricant film formation.     

Non-equilibrium theory of hydrodynamic lubrication

A new variational method is developed for the solution of the Navier Stokes equations which describe the unsteady flow between two infinite plates approaching or receding from each other symmetrically. The partial differential equations of the system are reduced to ordinary non-linear differential equations by using similarity transformations containing the Reynolds number as a parameter. The application of the governing principle of dissipative processes reduces the non-linear differential equation to an algebraic equation which can be solved for the required value of R and thus the flow characteristics can be studied. The velocity functions are obtained for various values of R ranging from 0.01 to 1.0.     

Modeling and analysis of interfacial electro-kinetic effects on thin film lubrication

At the interface between a solid and a following liquid, charge migration within the electric double layer causes an electro-viscous force to develop which manifests itself as an enhanced fluid viscosity. Based on the Poisson–Boltzmann equation, a new mathematical model of the electro-viscosity is developed. Numerical analysis is carried out taking into account the influence of the electric double layer on thin film lubrication. Analysis results show that the electro-viscosity leads to a significant increase in the film thickness in the thin film regime (below 100 nm).     

重载点接触热弹流润滑中油膜速度分析

对HKD-1型航空润滑油进行了重载点接触热弹流计算和不同滑滚比下油膜速度分析,并进行了试验验证.结果表明:油膜速度随滑滚比的增加而增大,油膜速度沿滚动方向和厚度方向越来越大,不同模型下得到的油膜速度分布形状不同.Evans-Johnson模型预测的速度分布较准确,牛顿模型预测误差较大.重载下,该润滑油表现为非牛顿特性.     

Failure of thin film lubrication — A detailed study of the lubricant film breakdown mechanism

The mechanism of breakdown of the lubricant film in a concentrated steel contact has been studied by examining friction-time diagrams as functions of sliding speed (ranging from 0.0007 to 0.7 m s^?1) and normal force on the contact. It was found that, even at the lower speed of 0.0007 m s^?1, the load-carrying capacity of the contact still depends strongly on lubricant viscosity. Over the whole range of speeds, lubricant film breakdown can be explained in terms of a major disturbance in the steady state condition, which is characteristic of (partial) elastohydrodynamic lubrication. In that condition, oxidation of microasperities which enter the surface zone keeps abreast of metal-to-metal junction formation.