A Transient Thermal Model for Mixed-Film Contacts

A thermal model is presented in this paper for deterministic calculations of the contact temperatures in mixed-film lubrication. A first-principal approach is taken to obtain the temperature results by solving transient energy equations in a domain consisting of three solid-film-solid regions. Various modeling techniques are used to feature model simplicity and to achieve a proper balance among solution accuracy, model robustness and computational efficiency. This thermal model may be incorporated into other deterministic isothermal mixed-film models to form a complete base model for mixed-film lubrication. It provides a platform upon which to model other effects such as lubricant additives, asperity plastic deformation and surface coating.     

Effect of Slide-to-Roll Ratio on Interior Stresses Around a Dent in EHL Contacts

The present, study extends the transient EHL point contact model and subsurface stress field calculation model to examine the influence of a surface dent on interior stresses in an EHL point contact under various slide-to-roll conditions. Results revealed that under the pure rolling condition the effect of a surface dent on the stresses is quite negligible. The presence of a shallow surface dent is unlikely to reduce the contact fatigue, life so long as pure rolling motion and good lubrication conditions are maintained. Unfortunately, the same cannot be said of the contact if it is operating in the boundary lubrication regime.

When sliding was introduced, the surface indentation generated significantly high pressure spikes with a strong directional preference. These high-pressure spikes cause severe stress concentrations either below the trailing edge of the dent, if it moves faster than the opposing surface, or below the leading edge of the dent, if it moves slower than the opposing surface. The maximum von Mises stress moved close to the surface and significantly increased in value as compared to the smooth surface solution. For the case of simple sliding, the maximum von Mises stress is even greater than the value calculated for the boundary lubrication case.

In regard to maximum tensile principle stresses, the presence of a dent increased the stresses only marginally over the smooth surface solutions. It is unlikely that surface indentation would significantly reduce the contact fatigue life due to Mode 1-type crack initiation.     

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.     

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.     

纯滚动集中接触润滑入口区热效应的数值分析

在对入口区膜厚形状和压力分布的简化处理前提下，采用数值分析方法全面地分析了纯滚动集中接触润滑入口区的热效应，得到了的结论与其它学者的试验及分析结果较为一致。     

重载下非牛顿流体线接触弹性流体动力润滑的数值解

对重载下线接触流体润滑时 ,润滑剂的流变学作用和表面变形进行了理论分析 ,从而导出了非牛顿流体的模型 ,指数模型就是其中之一。当指数 n增大时 ,油膜厚度随之上升 ,并且油膜破裂点向接触区中心移动 ,同时润滑油的等效粘度也随之上升 ,当 n=1时计算结果与牛顿流体基本一致。     

Grease Degradation in Rolling Element Bearings

Grease is degraded during use in rolling element bearings and as a result the lubrication performance can deteriorate. Under severe conditions this can result in lubrication failure and, thus, the grease life will effectively limit the bearing life. At present there is a lack of detailed information regarding the changes that occur in the grease and the way in which this degradation affects lubrication performance and failure.

This paper reports an initial study into grease degradation in bearings. The aim of the work was to characterize the changes that occur to the chemical and physical properties during use. A series of bearing tests using the modified DIN 51 806 test designated R2F(M) have been carried out using two greases: additized and non-additized. The tests have been run for different temperature and speed conditions for up to 300 hours. The aim was to examine the grease during normal running rather than after failure. At the end of the tests the bearings were dismantled and grease taken from different parts of the bearing for infrared spectroscopic analysis. This technique can characterize the degree of oxidation or degradation of the grease both in the bulk sample and from thin grease layers remaining on the bearing surfaces.

The analysis has shown that the condition of the grease varies depending on the distribution within the bearing. The lubricant remaining in the cage pocket region was heavily degraded and contained very little thickener. The grease on the seals contained different amounts of thickener depending on the seal position. The lubricant remaining on the inner raceway surface was predominately base oil although there was some thickener present. These results are discussed in the light of proposed bearing lubrication mechanisms.     

Modeling of Dynamic Friction in Lubricated Line Contacts for Precise Motion Control

A simple dynamic friction model for an elastohydrodynamic lubrication sliding and rolling line contact has been developed. This model uses the technique introduced earlier by Harnoy and Friedland (1). The model includes low-velocity regions where friction is a combination of contact and elastohydrodynamic friction. The study shows that the lime-variable friction is not only a function of instantaneous sliding velocity, but is also a memory function of the velocity history. Simulation of the model for an oscillating velocity exhibits similar hysteresis effects in friction-velocity curves as observed earlier in several experimental studies. The model can be useful for friction compensation to enhance the precision of motion in control systems.     

Theoretical and Experimental Investigations on EHL Point Contacts with Different Entrainment Velocity Directions

Film thickness prediction plays an important role in evaluating the performance and durability of machine elements under elastohydrodynamic lubrication (EHL). However, current formulae may not be appropriate for general conditions occurring in real contacts. This study investigates the effect of different lubricant entrainment velocity angles on film thickness distribution. For this purpose, a steady-state isothermal EHL model is used under a wide range of parameter sets including varying sum velocity, contact pressure, and sum velocity angle. Considerable differences in the trend of the central film thickness with respect to the lubricant entrainment velocity angle for low and high loaded contacts are shown. The results are compared with experimental measurements by means of an optical ball-on-disc tribometer and a twin-disc machine using capacitance method. Good agreement between numerical results and experimental measurements was found.     

Mapping Shear Stress in Elastohydrodynamic Contacts

A new method has been, devised for investigating the theological properties of lubricant films in two-dimensional EHD contacts. A lubricated, sliding contact is produced between a sapphire flat and a steel ball. Thermal infrared emission microscopy is then employed to obtain 2-D maps of the variation of temperature rise due to friction across the contact. These maps are then used in conjunction with moving heal source theory to produce maps of energy dissipation and thus shear strength, of the lubricant film across the contact.

A series of mixtures of two lubricants, one giving high traction and one with low traction, have been studied using this technique to investigate the influence of lubricant, blending on shear stress and traction.     

一种简化的点接触热弹流模型及其完全数值解

本文提出了一个简化的点接触热弹流计算模型并进行了完全数值求解。与现有的完全数值模型相比，采用本模型进行求解具有计算量小，精度适中的特点，更适合工程计算的要求。文中对多种工况下点接触热弹流的摩擦牵引性能和表面闪温特性进行了数值分析。     

Tribological Behavior of Some Candidate Advanced Space Lubricants

Performance of a variety of space lubricants was compared under boundary and elastohydrodynamic lubrication (EHL). The types of fluids studied were naphthenic mineral oil, paraffinic mineral oil, polyalphaolefin, and silahydrocarbon. The silahydrocarbon and the polyalphaolefin lubricants exhibited lower traction under similar conditions. A specific additive package increased the traction of the polyalphaolefin. Volatility characteristics of some of these fluids were also studied.     

Prediction of Real Rough Surface Deformation in Pure Rolling EHL Contact: Comparison with Experiment

The ability to predict the in-contact deformation of surface topography is very important for the design of machine components with respect to minimizing the friction and wear of rubbing surfaces. In this study the amplitude attenuation principle is verified as a simple tool for this purpose. Measured lubricant film profiles are compared with prediction based on this principle. From the results obtained it appears that the amplitude attenuation principle provides reasonable estimation of the deformation of rubbing surfaces that can be used for the prediction of in-contact behavior of surface roughness. Good agreement was obtained under pure rolling conditions, which provides a good initial point for the other studies under rolling/sliding conditions where the wear of rubbing surfaces is of key importance.     

Temperature Analysis in Lubricated Simple Sliding Rough Contacts

A temperature analysis of dry sliding fully plastic contact is extended to calculate the asperity temperatures between a sliding lubricated rigid smooth plane and a stationary elastic rough surface. First, surface roughness is generated numerically to have a Gaussian height distribution and a bilinear autocorrelation function. Lai and Cheng's elastic rough contact computer program is then used to determine the asperity contact loads and geometries of real contact areas. Assuming different frictional coefficients for shearing the lubricant film at the noncontact areas, shearing the surface film at the asperity contacts and shearing the oxide film as the asperity temperature exceeds a critical temperature, asperity temperature distributions can be calculated. Eight cases in Durkee and Cheng's scuffing tests of lubricated simple sliding rough contacts are simulated by using 20 computer-generated rough surfaces. The results show that scuffing is correlated to high-temperature asperities which are above the material-softening temperature.     

A New Method for Measuring the Film Thickness of Mixed Lubrication in Line Contacts

In the present paper, an optical method, called frustrated total reflection (FTR), for measuring the film thickness of mixed lubrication in line contacts is proposed. The principles of FTR are analyzed. The relationship between the reflectivity and the film thickness is derived. The measuring methods and procedures are described.     

An Investigation of Seal Materials for High-Temperature Application

Novel rotating seal materials were developed by powder metallurgy techniques for potential aircraft applications at high speeds and high temperatures.

A systematic wear study without lubrication included several commercially available materials and the following types of experimental materials: (a) pure refractory hard metals, (b) binary alloys of refractory hard metals bonded with nickel and, (c) ternary alloys of refractory hard metals bonded with nickel and infiltrated with silver.

Two ternary alloys, containing nickel bonded WB or CrN and infiltrated with silver, showed superior wear qualities against either tool steel or a nickel—chromium—iron alloy at a sliding speed of 29,000 ft/min under a 14 lb/in² load and at ambient temperatures as high as 1300 F. A commercial titanium carbide composition showed excellent wear characteristics in contact with an identical composition at a sliding speed of 14,000 ft/min under a 16 lb/in² load and at an ambient temperature of 1050 F.     

Measurements of Temperature Distributions around Longitudinally Grooved Rough Surfaces in Sliding Elastohydrodynamic Point Contacts

This paper describes the temperature measurements in the EHL conjunction area comprising a longitudinally grooved steel ball and a sapphire disk under high slip conditions. The authors measured the temperatures of the oil film as well as both the disk and ball surfaces; furthermore, they estimated the temperature profile across the oil film by means of experimental values. The experimental results show that the temperature of the grooved ball surface increased considerably compared with that of a non-grooved ball. The temperatures of the faster surface for the grooved ball became sensitive to the slip ratio, whereas that for the non-grooved surface was almost constant. The temperature distribution had a higher value at the land zones and a lower one at the grooved zones. The temperature rise in the grooved zones varied qualitatively depending on the thermal conditions of both the sliding surfaces.     

An Investigation of Lubricant Film Thickness in Sliding Compliant Contacts

An optical interferometric technique has been used to investigate fluid film thickness in sliding, isoviscous elastohydrodynamic contacts (I-EHL). Monochromatic two-beam interferometry has been employed to map lubricant film thickness across a range of applied loads and entrainment speeds. The contact was formed between an elastomer sphere and plain glass disc, illuminated under red light, λ= 630 nm. Experimental work has employed sunflower oil and glycerol/water solutions as the test lubricants, due to their similar refractive indices and varying viscosity. A black-and-white-image-intensified camera has been employed to capture interference images and a computer processing technique used to analyse these images, pixel by pixel, and create film thickness maps based on their gray-scale intensity representations. Comparison of film thickness results to theoretical models shows reasonable qualitative agreement. Experimental results show both a reduced horseshoe, which is limited to the rear of the contact, and wedge-shaped film thickness profile within the Hertzian contact region. This is unlike conventional hard EHL contacts where the horseshoe-shaped pressure constriction extends around the contact toward the inlet. Experimental results suggest that film thickness profiles take on a convergent wedge shape similar to that used in many hydrodynamic bearings. It is likely that this wedge is largely responsible for generating fluid pressure and therefore the load-carrying capacity of the contact.     

Accurate Measurements of Pressure-Viscosity Behavior in Lubricants

The piezoviscous effect is accurately characterized for a number of representative lubricating oil base stocks and a simple, well-defined hydrocarbon. Procedures for the generation of accurate pressure-viscosity coefficients are outlined. The reciprocal asymptotic isoviscous pressure may be measured to within perhaps 1%. The piezoviscous effect may be important to automotive fuel economy. The departure from exponential behavior at very low pressure becomes more pronounced at high temperature.     

Film Formation by Colloidal Overbased Detergents in Lubricated Contacts

It has been reported in the literature that overbased detergents can possess good antifriction and antiwear properties although the origins of these properties are not fully clear. In practice, over-based detergents are colloidal dispersions and this may be important in determining their properties and mechanism of action.

In the current study, the lubricating properties of commercial, overbased magnesium and calcium sulfonates were measured in thin film, lubricated conditions and compared to a neutral sulfonate additive. A range of techniques was employed to evaluate the tribological performance of solutions of these additives. Film thickness measurements were carried out using optical interferometry and in-contact visualization, while friction and wear measurements complemented the study.

It has been found that, when operating in thin film conditions, overbased detergents deposit solid-like boundary films on the rubbing surfaces. These films form in both rolling and mixed rolling/sliding conditions and, unlike many other colloidally-formed boundary films, are able to survive in high speed, thick film conditions. During formation, the film rapidly reaches a thickness corresponding to one colloid particle diameter, between 10 and 20 nm. After prolonged rubbing, however, the film thickness reaches the equivalent of three particle diameters. No such thick boundary films are observed with the neutral sulfonate.

The boundary films formed by overbased detergents produce a significant reduction in wear. However, for the very smooth surfaces used in this study, they also result in an effective roughening of the very smooth surfaces studied. This leads to an increase in friction in the intermediate speed region by promoting solid-solid contact in thin fluid film conditions.