Boundary Film Formation by Viscosity Index Improvers

Ultrathin film interferometry has been used to measure the film-forming properties of a range of viscosity index improver (VII) solutions in rolling, concentrated contacts. It has been shown that some VIIs form boundary lubricating films of thickness 10 to 30 nm in contacts. These films result from the presence of highly concentrated and thus very viscous layers of polymer solution formed on the two rubbing solid surfaces by polymer adsorption. These boundary films are formed only by some types of VII and can persist up to temperatures in excess of 120°C. The possible implications of this type of boundary lubrication are discussed.     

滑动条件下弹流润滑的屈服膜厚与屈服边界

本文讨论了弹流润滑的屈服问题，从润滑剂的极限剪应力可以得到屈服膜厚的表达式。屈服膜厚是弹流润滑膜的下降 ，当利用拟合公式得到的膜厚小于屈服膜厚时，由于润滑已经处在非牛顿区，所拟全公式不再适用。本文还给出了用膜厚形式表示的屈服准则，并详细讨论了最大压力，平均速度和滑滚比对屈服膜厚的影响。     

Experimental Study of Lubricant Film Thickness Behavior in the Vicinity of Real Asperities Passing through Lubricated Contact

This article presents an experimental study of the influence of real surface micro-geometry on the film thickness in a circular elastohydrodynamic (EHD) contact formed between a real, random, rough surface of steel ball and smooth glass disk. Phase shifting interferometry was used to measure in situ initial undeformed rough surface profiles, whereas thin film colorimetric interferometry provided accurate information about micro-EHD film thickness behavior over a wide range of rolling speeds. Two real roughness features were studied in detail—a 56-nm-high ridge and a 90-nm-deep groove, both transversely oriented to the direction of surface motion. It was shown that the ridge is heavily deformed in a loaded contact and its height increases with increasing rolling speed. The asperity tip film thickness behavior is quite similar to the contact average film thickness when the film thickness is higher than the undeformed ridge height. However, below this limit the film is thicker than what the EHD theory predicts. For the groove, a local reduction in film thickness at the leading edge was observed. When the groove is passing through the EHD conjunction, it maintains its undeformed shape. The behavior of both roughness features studied shows good agreement with previous experimental observations conducted using an artificially produced ridge and groove.     

The Design of Boundary Film-Forming PMA Viscosity Modifiers

Previous research has shown that some viscosity modifier additives are able to adsorb from oil solution on to metal surfaces to produce thick, viscous boundary films. These films enhance lubricant film formation in slow-speed and high temperature conditions and thus produce a significant reduction in friction. This article describes a systematic study of this phenomenon, which makes use of the versatile nature of polymethacrylate (PMA) chemistry. Dispersant polymethacrylates with a range of different functionalities, molecular weights, and architectures have been synthesized using controlled radical polymerization techniques. The influence of each of these features on boundary film formation and friction has been explored using optical interferometry and friction versus speed measurement. From the results, guidelines have been developed for designing PMAs having optimal boundary lubricating and, thus, friction-reducing properties.     

无极性油润滑下摩擦面间边界膜的电特性

考察了无极性石蜡基油润滑下摩擦面间的自生电势及外加电压时摩擦面间边界润滑膜的电特性。用摩擦面的氧化作用解释了施加不同方向电压时边界润滑膜表现出的整流效果,并就如何有效利用该氧化作用提出建议。     

The Influence of Lubricant Upon EHD Film Behavior During Sudden Halting of Motion

Although steady state elastohydrodynamic (EHD) lubrication is quite well understood both from the theoretical and from the experimental point of view, studies of transient effects in EHD are currently far less developed. This paper describes an experimental investigation into EHD film behavior during sudden halting of motion. A technique has been devised which enables both central lubricant film thickness and film thickness profiles to be measured every millisecond during halting of a ball on flat, sliding contact. This has enabled detailed information of influence of lubricant on film collapse during halting to be obtained. It is shown that film collapse occurs in two stages. The first is a very rapid reduction in film thickness with only very small changes in film geometry and thus pressure distribution. This is followed, as soon as entrainment ceases, by the formation of a lubricant entrapment, and subsequent slow leakage of fluid from the central film region. This paper focussed on the formation of this entrapment and the influence of the rheological properties of the lubricant, i.e. viscosity and pressure-viscosity coefficient, on its development and behavior.     

Nonlinear Dynamic Analysis of Noncontacting Coned-Face Mechanical Seals

The complete nonlinear equations of motion of a flexibly mounted stator in a noncontacting coned-face mechanical seal are solved numerically. The solution utilizes a transient dynamic analysis and takes into account rotor axial runout and assembly tolerances in the form of initial stator misalignment. Cavitation of the fluid film is also accounted for. A parametric investigation is performed and the effect of various design parameters and operation conditions on the seal dynamics is presented and discussed. A critical shaft speed is found above which the seal becomes dynamically unstable. A critical rotor runout is found which, if exceeded, will cause seal failure due to local face rubbing contact. A comparison is made between the numerical results and those of a simpler analytical solution. It is found that the analytical solution is valid for most practical applications of mechanical seals.     

Experimental Study of Central and Minimum Elastohydrodynamic Film Thickness by Colorimetric Interferometry Technique

Recently developed colorimetric interferometry technique was used for the study of both minimum and central film thicknesses for a wide range of operating parameters. Over 300 film thickness maps were obtained for the combination of four values of the materials parameter G, five values of the load parameter W and many values of the speed parameter U. The use of a spacer layer extended the range of film thickness measurement down to 5 nm. An excellent agreement was found between experimental values and data obtained from numerical solution presented by Venner and ten Napel, especially for thin lubrication films. An increase in a speed exponent with increasing material parameter G was observed for both central and minimum film thicknesses. The minimum film thickness and, thereby, the ratio between central and minimum film thickness was confirmed to be of a stronger dependence on material and load dimensionless parameters than Hamrock and Dowson equations predict.     

磁流体润滑的BP神经网络模型

本文利用Matlab建立了磁流体边界润滑时,摩擦系数与外加载荷,磁场,纳米粒子浓度以及摩擦副材料硬度之间关系的BP神经网络模型,并利用L－M算法对网络进行了优化训练,所得到的网络收敛速度快,误差小。网络输出结果与实验结果相比较,具有较好的吻合性。     

Elastohydrodynamic Film Thickness Mapping by Computer Differential Colorimetry

This paper describes a new experimental technique for the study of elastohydrodynamic (EHD) lubricant films. This technique, which is based on the computer processing of EHD chromatic interferograms, uses a combination of image analysis and differential colorimetry for film thickness evaluation. This approach overcomes some major limitations of conventional optical interferometry and allows the precise mapping of lubricant film thickness distribution in EHD contacts, including transient and quasistatic phenomena. The technique has been used for the evaluation of chromatic interference patterns obtained from a conventional optical test rig for rolling point contacts. Three-dimensional representations of lubricant film thickness and shape with high accuracy and spatial resolution have been obtained. The technique's accuracy has been checked and a comparison with conventional monochromatic interferometry has been done for validation. The technique's resolution has been confirmed through the observation of local film thickening just before the EHD exit constriction for both pure rolling and sliding conditions.     

EHL Performance of the Lubricant With Shear Strength: Part I — Boundary Slippage and Film Failure

This paper analytically investigates the isothermal line contact elastohydrodynamic lubrication of three lubricants with much different shear strengths under the nondimensional operating parameters of w = 2.15e-4 and U = 2.53e-10 applying the lubricant ideal viscoplastic rheological model. The boundary slippage of the low-shear-strength lubricant occurring in the EHL inlet zone was found and results in a much thinner film compared to the classical EHL theory prediction. The film boundary slippage and its growth with the slide/roll ratio variation of tile low-shear\- strength lubricant exhibit special phenomena, which are much different from those of the high-shear-strength lubricant. The easy occurrence of film failure in concentrated contact in the case of high sliding speed, heavy load, large slide/roll ratio, and low-shear-strength lubricant was concluded due to the severe friction heating on the surface conjunction and the lubricant thermal desorption on tile lubricant/surface boundary. The EHL film failure mechanism was further recognized.     

Film Formation in EHL Point Contacts under Zero Entraining Velocity Conditions

The contacts of adjacent balls in a retainerless bearing are subjected to the zero entrainment velocity (ZEV). The existence of an effective elastohydrodynamic lubrication (EHL) film between contacts running under ZEV conditions has long been proven experimentally. However, the classical EHL theory predicts a zero film thickness under ZEV conditions. Mechanisms, such as the thermal viscosity wedge effect and immobile film theory, have been proposed to tentatively explain the phenomenon. However, detailed numerical results are needed to provide theoretical evidence for such film formations. This paper aims to simulate, based on the viscosity wedge mechanism, the film formation of EHL point contacts under ZEV conditions. Complete numerical solutions have been successfully obtained. The results show that the thermal viscosity wedge induces a concave film profile, instead of a parallel film (Hertzian) as postulated by some previous researchers. By the simulation solver developed, the variation of film thickness with loads, oil supply conditions and ellipticity parameters have been investigated. Some unique lubrication behaviors under ZEV conditions are demonstrated. Furthermore, preliminary quantitative comparisons with the latest optical EHL experiments are finished. Both results are in good correlation.     

Influence of Lubricant Properties on ARKL Temperature Rise and Transmission Efficiency

It has been suggested in the literature that the ability of a lubricant to produce low “end of test temperature” (EOTT) in the achsialrillenkugellager (ARKL)—that is, axial groove ball bearing—thrust bearing test is an indicator of its likely efficiency in vehicle transmissions. Based on this, the current study has used regression analysis to correlate the ARKL EOTT with a range of physical and friction properties of a set of 26 lubricants. The latter have been selected to span a wide variety of base oil and additives types. It has been found that the most important lubricant property in determining ARKL EOTT is the elastohydrodynamic (EHD) friction or traction coefficient at low slide–roll ratio. Lubricants with low EHD friction were found to give low EOTT. Two lubricant properties of secondary importance have been identified. One is the lubricant viscosity index (VI), where a high VI contributes to high EOTT. The second is the heat transfer capability of the fluid, where a high capability was found to correlate with lower EOTT.

The findings help clarify how low EOTT in the ARKL test can be achieved via lubricant composition. If the correlation between ARKL and transmissions that has been suggested in the literature is valid, then these findings are transferable to the design of high efficiency transmission lubricants.     

Rheology of Perfluoropolyalkylether Fluids in Elastohydrodynamic Lubrication

The rheological characteristics of two branched and two linear, commercial perfluoropolyalkylether (PFPAE) fluids were studied under high pressures and temperatures. The effects of branching and carbon-to-oxygen (C:O) ratio on the pressure-viscosity-temperature behavior and on the non-Newtonian behavior of these fluids were studied experimentally under high pressures and temperatures. The branching and the higher C:O ratio seemed to increase the pressure-viscosity coefficients of these fluids. The effects of the viscosity and the pressure-viscosity coefficient on the capabilities of these fluids to generate elastohydrodynamic lubrication (EHL) film thickness were studied and compared with experimental measurements. All of the fluids studied seemed to follow the Roelands viscosity model and classical EHL theory (1). The C:O ratio also influenced the temperature dependence of the limiting-shear-strength proportionality constant. The results show that for similar-viscosity fluids, the linear PFPAE with higher C:O ratio is most desirable for wide temperature use.     

Microprocessor Control of Running-in of Plain Bearings

Correct running-in procedures have a major influence on the subsequent life and performance of a plain bearing. With very large bearings, the process can be both time-consuming and expensive.

It is necessary to control the running-in conditions very carefully, since too thick a lubricant film, will retard the running-in, while too little lubrication will lead to overheating and possible damage or even seizure. Since the surface topography is changing continuously during running-in, the optimum lubrication condition also changes.

This paper describes a new bearing rig in which running-in parameters are monitored and fed to a microprocessor which, in turn, controls the loading and speed conditions. The objective is to obtain correct running-in, in the minimum possible time.     

Elastohydrodynamic Film Thickness and Tractions for Oil-in-Water Emulsions

Emulsions, consisting of a small volume of oil dispersed in water in the form of small particles, are popular lubricants for metal rolling and some machine design applications. A number of mechanisms have been suggested for the lubricating behavior of emulsions, among which plate-out, starvation, and dynamic concentration are of particular interest here. At low speeds, the emulsion provides essentially the same lubricating ability as neat oil for a point contact, consistent with plate-out. At some critical speed, the emulsion behavior departs from the neat oil, associated with starvation of the inlet zone. At a second critical speed, dynamic concentration becomes the important mechanism. This article measures the film thickness and traction coefficients of oil-in-water emulsions in the different regimes of behavior and compares the results to existing theoretical understanding. The effect of droplet size is isolated as a causative element in fluid film formation.     

Effect of Lubricant Viscosity and Type on Ball Fatigue Life

Two separate investigations were conducted to determine the effect of lubricants on the fatigue life of M-1 tool steel balls in the rolling contact fatigue spin rig. In the first investigation four paraffinic mineral oils with viscosities of 5 to 113 centistokes at the 100F test temperature were used. Longer life was obtained with more viscous oil, life varying approximately as the 0.2 power of lubricant viscosity. In the second investigation of methyl silicone, a paraffinic mineral oil, a sebacate, a water base glycol and an adipate, each of which had a viscosity of about 10 centistokes at the 100F test temperature, were used. The 10 per cent life was about 40 times as great with the silicone (best) as with the adipate (poorest). The life results correlated fairly well with the pressure viscosity characteristics as estimated from lubricants of the same base stocks.     

Surface Characterization and Experimental Design for Testing of a Radial Lip Seal

In radial lip seals, it is generally accepted that sealing occurs through microasperities that form on the surface of the elastomer. It is most often thought that these asperities form during the first few hours of operation due to the stresses and wear in the seal. Thus, the formation of these asperities is highly dependent upon the specific operating conditions of the seal. This article studies the size, shape, and quantity of the asperities that are present on a specific seal design, when run at a certain set of operating conditions. From this analysis it has been determined that the hydrodynamic pumping aids that are present on the seal significantly affect the formation of microasperities. Radial lip seal samples affected by pumping aids had more distinct microasperities, but these asperities are clustered on the edges of the sealing zone, which is not the case for samples unaffected by pumping aids.     

Flow Characteristics of Hydraulic Fluids of Different Viscosities II. Flow in Pumps: Internal Leakage and Loss of Efficiency

A new type of hydraulic fluid, the high-water type, is being developed for use in industrial equipment which is currently using oil or oil-like fluids. In order to determine which factors affect the flow properties of high-water hydraulic fluids, a series of solutions ranging in viscosity from that of a water-glycol fluid (55 mPa·s or 250 SUS) to that of a high-water fluid (3 mPa·s or 35 SUS) was formulated by varying the amount of thickening agent. From flow rates of these fluids at known pressure and viscosity, the efficiency (percent loss of flow) was calculated. As would be expected, internal leakage from the high-pressure side of the pump to the low-pressure side was much greater with the 3 mPa·s fluids than with the 50 mPa·s fluids. Increasing the pump size increased pump efficiency at a given viscosity.

Equations relating pressure, viscosity and flow for two types of internal leakage were developed by mathematical or graphical analysis of the experimental data.     

Elastohydrodynamically Lubricated Ball Bearings with Couple-Stress Fluids,Part I: Steady-State Analysis

Conventional elastohydrodynamic lubrication (EHL) analysis of point contacts is extended to include couple-stress effects in lubricants blended with polymer additives. A transient pressure differential equation, generally referred to as a modified Reynolds equation, is derived from the Stokes microcontinuum theory and solved using the finite difference method with a successive over-relaxation scheme. The solution is obtained under isothermal conditions, assuming a suitable exponential relation of pressure-viscosity variation. A nondimensional couple-stress parameter, which can be considered the molecular length of the additives in the lubricant, is used in the analysis. From the results obtained, the influence of the couple-stress parameter on the EHL point contacts is apparent and cannot be neglected. Lubricants with couple stresses provide an increase in the load-carrying capacity and reduction in friction coefficient as compared to Newtonian lubricants. Empirical formulas for the calculation of central and minimum film thicknesses of lubricated point contacts with couple-stress fluids are derived with the nonlinear least-squares curve-fitting technique using different numerically evaluated data. This may help to avoid the time-consuming numerical calculations.