Experimental results and analytical film thickness predictions in EHD rolling point contacts

In this work, we consider several types of lubricants—including non-Newtonian fluids—that were studied in EHD pure rolling point contacts under various operating conditions, leading us to explore a wide range of dimensionless parameters. The experimental results are compared with predictions given by the usual analytical EHL relationships and by more recently developed models. This broad comparison conducted with particular emphasis on minimum film thickness (h_m) showed a fair agreement between experimental data and a few predictions including some obtained from extended models. Commonly used elastohydrodynamic lubrication (EHL) models did not systematically give accurate h_m estimation, whereas minimum film thickness not only is a yield value but also serves as a key parameter in estimating lubrication regimes.     

Thermal point contact EHL analysis of rolling/sliding contacts with experimental comparison showing anomalous film shapes

The paper presents an experimental and numerical investigation of non-conformal lubricated contacts in which anomalous film shapes occur. The experiments were concerned with the contact between a steel ball and the plane surface of a glass disc at various slide-roll ratios. A paraffin base mineral oil was used as a lubricant and friction coefficients and film thicknesses were measured. It was found that for slide-roll ratios with the disk moving faster anomalous elastohydrodynamic lubrication (EHL) films were obtained characterized by a “dimple” in the central region of the contact. Numerical thermal-elastohydrodynamic analyses were carried out to simulate both film thickness and friction corresponding to the experimental conditions using Newtonian and Ree-Eyring rheological models. Initial results from this study suggest that neither of these lubricant models predict the correct detailed film shape and the experimental friction at the same time. An alternative lubricant model including both thermal and limiting shear stress effects (wall slippage) is currently under development.     

Experimental investigation of transient and thermal effects on lubricated non‐conformal contacts

In this work, thermal and transient effects on non‐conformal lubricated contacts are investigated through experimental analyses. Experiments between a ball and a plane surface of a disc are described. Friction coefficients and film thicknesses are measured (the film thickness only for the glass‐on‐steel contact). A paraffin base mineral oil is used as a lubricant. First experiments are carried out under steady‐state conditions. To include effects due to different thermal properties of contacting materials, a steel‐on‐steel and a glass‐on‐steel contact with different slide‐to‐roll ratios are tested. If the contacting materials have different thermal properties, as in the case of a glass‐on‐steel contact, thermal effects like the temperature–viscosity wedge action could clearly be shown. It is found that the friction coefficients are influenced by the slide‐to‐roll ratio and the thermal properties of the contacting materials. Under transient conditions, the entraining velocity is varied with a sinusoidal law. Squeeze effects explain ‘loops’ of friction and film thickness found also in previous works. The formation of friction loops is related to the measured film thickness differences. However, also under non‐steady‐state conditions, thermal effects, like the temperature–viscosity wedge action, influence the friction coefficients. Copyright © 2007 John Wiley & Sons, Ltd.     

Effect of wear on EHD film thickness in sliding contacts

A theoretical solution to the elastohydrodynamic (EHD) lubrication problem in sliding contacts, which takes into consideration the effect of the change in shape of the gap due to wear on the load‐carrying capacity, is presented. The model of such a contact is based on assumptions of Grubin and Ertel (von Mohrenstein). The resultant dimensionless Reynolds and film profile equations have been solved numerically for a number of cases with several values of thickness of the worn layer. Iteration of the EHD film thickness is performed by means of the secant method. Values of the calculated dimensionless film thickness are presented as a function of dimensionless wear. The conclusions concern the influence of the linear wear on the film thickness in heavily loaded sliding contacts. Copyright © 2006 John Wiley & Sons, Ltd.     

指定摩擦系数下富锌漆漆膜厚度的计算研究

对富锌漆接触面间的摩擦系数进行了测量,结果表明：不同处理方式的富锌漆接触面间的摩擦系数差异较大,并且摩擦系数与接触面积成正比。对某2 MW风机主轴与轮毂栓接面间异响的研究表明：喷砂后球型无机富锌漆接触面间的摩擦系数有时可接近于0。通过推导栓接面间的摩擦系数与粗糙度的数学解析表达式,结合球型无机富锌漆膜厚度与粗糙度在一定条件下近似相等的特性,找到了一种指定摩擦系数下富锌漆漆膜厚度的计算方法,保证了栓接面间在喷涂富锌漆后其摩擦系数的可靠性和可计算性。通过测量某塑性材料的摩擦系数,得到了一种简洁的估算该塑性材料的临界应力强度因子的方法。     

The behaviour of friction modifiers under boundary and mixed EHD conditions

The behaviour of a range of model and commercial friction modifiers (FMs) has been evaluated under elastohydrodynamic (EHD) and boundary lubrication conditions. Using a series of long‐chain carboxylic acids, it has been shown that measured boundary friction coefficients (BFCs) decrease with increasing chain length, unsaturation level, temperature, and concentration. Base oil polarity was found to have no effect under these conditions. Commercial oleate esters in synthetic base fluids gave lower BFCs than nitrogen‐containing compounds under the same conditions. This difference was observed over a range of concentrations and temperatures. The friction performance of formulated oils under mixed and full‐film EHD conditions was found to be dependent on FM, base oil, and detergent type. Under boundary conditions, friction was found to vary with FM type, but the effect of changing the base oil and the detergent system was negligible.     

Film thickness in point contacts under generalized Newtonian EHL conditions: Numerical and experimental analysis

The current paper contributes to the understanding of the behaviour of a smooth point EHL contact with a generalized Newtonian lubricant under pure rolling. The film thickness distribution was computed using a numerical simulation with measured rheological lubricant properties. The numerical predictions, obtained solving the generalized Reynolds equation were compared with film thicknesses measured in an optical ball-on-disc device. The numerical results correctly predict the absolute film thickness and the film thickness increase with rolling speed.     

Influence of load and lubricants on EHD film thickness and rolling-contact fatigue lives of AISI 52100 steel balls

This work has evaluated the influence of load and type of lubricant on the thickness of the elastohydrodynamic (EHD) film and the rolling-contact fatigue lives of AISI 52100 steel balls. The lubricants studied have various viscosities and included two mineral oils and five synthetic oils from three families. Firstly, the central film thickness was determined in order to predict the lubrication regime. The stress—number of cycles fatigue curves were then calculated by means of Weibull plots, and the fatigue mechanism was evaluated. The test machine used for the analysis was a Seta-Shell 1980 four-ball EP lubricant tester. The 12.7 mm diameter test balls were made from a single batch of carbon-vacuum-deoxidised AISI 52100 steel with hardness RC65. Elastohydrodynamic film thickness estimation was carried out using pressure—viscosity coefficients (a). In this study, to calculate (X), a new interferometric technique, ultrathin film interferometry, was employed to measure the film thickness. A practical method was developed for evaluating EHD, mixed film, and solid lubrication processes. Micrographic mapping and energy dispersive spectroscopy (EDS) were used to analyse the rolling track of the test balls.     

A unified shear-thinning treatment of both film thickness and traction in EHD

A conclusive demonstration has been provided that the nature of the shear-thinning, that affects both film thickness and traction in EHL contacts, follows the ordinary power-law rule that has been described by many empirical models of which Carreau is but one example. This was accomplished by accurate measurements in viscometers of the shear response of a PAO that possesses a very low critical stress for shear-thinning and accurate measurements in-contact of film thickness and traction under conditions which accentuate the shear-thinning effect. The in-contact central film thickness and traction were entirely predictable from the rheological properties obtained from viscometers using simple calculations. These data should be invaluable to researchers endeavoring to accurately simulate Hertz zone behavior since the shear-thinning rheology is extensively characterized and accurate in-contact data are available to test. In addition, a new model has been introduced that may be useful for the rheological characterization of mixtures.     

The significance and use of the friction coefficient

The quantity known as the friction coefficient (or ‘coefficient of friction’) has long been used in science and engineering. It is easy to define, but not easy to understand on a fundamental level. Conceptually defined as the ratio of two forces acting, respectively, perpendicular and parallel to an interface between two bodies under relative motion or impending relative motion, this dimensionless quantity turns out to be convenient for depicting the relative ease with which materials slide over one another under particular circumstances. Despite the fact that both static and kinetic friction coefficients can be measured with little difficulty under laboratory conditions, the time- and condition-dependent characteristics of friction coefficients associated with both clean and lubricated surfaces have proven exceedingly difficult to predict a priori from first principles.The shaky nature of friction's fundamental underpinnings, has not prevented investigators from compiling lists of friction coefficients and publishing them for general use. Problems often arise, however, when engineers attempt to use tabulated friction coefficients to solve specific problems in mechanical design or failure analysis. The systems-dependence of frictional behavior is sometimes ignored, leading to misapplication of published data. This is particularly true for applications in nano-technology and others that differ from typical laboratory size scales. This paper will review the measurement and use of static and kinetic friction coefficients, discuss their usefulness, and describe the sources of frictional resistances in terms of shear localization.     

Measurement of the influence of sliding velocity on oil film thickness in an elastohydrodynamic point contact

Experiments have been carried out to measure the film thickness between a sapphire disc and a polished steel ball when slippage is permitted between the two surfaces. The velocity sum of the two surfaces was kept constant during the measurements. The method used to determine the oil film thickness in the contact zone is optical interferometry. It was observed that the change in minimum film thickness was slight. When the sliding velocity was increased, the decrease in central film thickness was considerably larger than that of the minimum film thickness. In addition, the two side lobes, in which the minimum film thickness area occurs, were observed to become flatter as the sliding velocity was increased.     

The effect of frequency of vibration and humidity on the coefficient of friction

Variation of friction coefficient with the variation of frequency of vibration and relative humidity is investigated experimentally on a mild steel disc. A pin-on-disc apparatus having facility of vibrating the disc at different frequency and amplitude is used for the experiment. During the experiment, normal load, speed and relative humidity were varied. The surface conditions of the mild steel base plate were ‘as-turned’ and ‘as-ground’. It is found that friction co-efficient under no vibration condition is higher than that under vibration condition, and the values of friction co-efficient decrease with the increase of frequency of vibration. Similarly, the friction co-efficient decreases with the increase of relative humidity. It is also observed that the rate of reduction of friction co-efficient has a particular relationship with the frequency of vibration and the relative humidity.     

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.     

Measurement of lubricating film thickness,temperature and pressure in gear contacts with surface topography as a parameter

Thin film transducers, sputtered onto a tooth flank, are used to measure temperature, pressure and oil film thickness profiles in gear contact. The former two are resistance sensors, while the film thickness sensor functions as a capacitive transducer. Experiments were carried out with ground and hobbed gears. The measurements are repeated after the tooth flank has been polished. The differences of the measured values for the ‘rough’ and ‘smooth’ surfaces are used for comparisons between differently machined surfaces. An EHD simulation programme has been developed which calculates film thickness and contact pressure, also taking surface roughness into account.     

The effect of amplitude of vibration on the coefficient of friction for different materials

This work examines how friction coefficients are affected by amplitude of normal vibration at different frequencies. Variation of friction coefficient with the amplitude of normal vibration is investigated experimentally when mild steel pin slides on different types of material such as glass fiber reinforced plastic, cloth reinforced ebonite, polytetrafluoroethylene (PTFE), rubber and mild steel. For this, a pin-on-disc apparatus having facility of vibrating the test samples at different amplitudes and frequencies of vibration was designed and fabricated. During the experiments, the effects of sliding velocity, roughness, normal load and duration of rubbing were also investigated. Studies have shown that the friction coefficient decreases with the increase of amplitude of vibration within the observed range. The observed ranges of amplitude of vibration were 10–200 μm. In this study, it is also observed that the rate of reduction of friction coefficient has a particular relationship on the amplitude and frequency of vibration. The experimental results are compared with those available in the literature and simple physical explanations are provided.     

Friction and wear of copper samples in the steady friction state

The effect of sliding velocity and load on the friction and wear of Cu-steel pairs 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. Steady friction states were observed in the mixed EHL and BL regions, however two types of the ML region are revealed. The first type is the stable ML range. The second one is the range of unstable friction and wear when a decrease of the lubricant film leads to abrupt change of all controlled parameters. It was found that a transition to the unstable ML region occurs within a narrow range of Z parameter. Wear modes in the lubrication regions were studied. Deformation hardening in the lubricant regimes is discussed.     

Development of an improved method for investigating the frictional properties of lubricants under transient EHD conditions

In the design and evaluation of mechanical system performance it is important to know the frictional qualities of the lubricant. Without correct numerical treatment of the lubricant during simulations of large systems, e.g., drive trains in trucks and buses, the results will, to a large extent, be inaccurate. However, obtaining detailed information places demands on the test equipment as the events are both transient and highly loaded. Under quasi‐static conditions, forces are measured with force transducers, but in elasto‐hydrodynamically lubricated conjunctions, where pressures are so high that the surrounding surfaces deform elastically, this cannot be done without permanently damaging the equipment. The conceptual design of the test equipment must therefore incorporate the measuring process in transient conditions (loading‐unloading times of 200–500 μs) being performed in real time, and allow extreme pressures of up to 3 GPa without component destruction. One way to obtain accurate friction data successfully is to apply a concentrated force pulse to a non‐instrumented surface and to measure the response from that pulse elsewhere. The development of a measurement technique, the Lulea ball and bar apparatus, which utilises wave propagation theory, is presented in this paper. An oblique impact on a robust end plate on a rod was used to generate both non‐dispersive compression waves and dispersive flexural waves. The normal force created by the axial wave was measured using strain gauges, while the transverse force was derived from the fast Fourier transforms of two lateral acceleration histories, using dynamic beam theory. The relation between the normal and tangential force histories showed the frictional properties at the impact as a function of time. A variety of lubricants was also studied at Hertzian pressures of up to 2.5 GPa, and the development of the method and results are presented. Experiments indicate that different lubricants exhibit different frictional properties and that the resolution in the test equipment is sufficient to indicate this.     

Friction, wear and structure of Cu samples in the lubricated steady friction state

Friction and wear of copper rubbed with lubrication in wide range of loads and sliding velocities were studied. The results of friction and wear experiments are presented as the Stribeck curve where the boundary lubrication (BL), mixed (ML) and elasto-hydrodynamic lubrication (EHL) regions are considered. The structural state of subsurface layers in different lubricant regions is studied by X-ray photoelectron spectroscopy, optical, transmission and scanning microscopy analysis. Dislocation density of dislocations in EHL and BL lubricant regimes was determined. Nanohardness at thin surface layers rubbed under different lubricant regimes is compared. The dominant friction and wear mechanisms in different lubrications regions are discussed.     

The influence of soot and dispersant on ZDDP film thickness and friction

The effect of dispersed soot in engine oils is an increasingly important issue in terms of both engine durability and fuel efficiency. Using carbon black as a soot analogue, a study has been carried out to investigate the main factors that determine the impact of soot on friction and ZDDP film formation in formulated oils. It has been found that dispersed carbon black can rapidly remove ZDDP reaction films by abrasion. However, this removal can be prevented or limited by the choice of an optimal dispersant additive.     

The influence of surface roughness and the contact pressure distribution on friction in rolling/sliding contacts

A numerical contact model is used to study the influence of surface roughness and the pressure distribution on the frictional behaviour in rolling/sliding contacts. Double-crowned roller surfaces are measured and used as input for the contact analysis. The contact pressure distribution is calculated for dry static contacts and the results are compared with friction measurements in a lubricated rolling/sliding contact made with a rough friction test rig. The mean pressure is suggested as a parameter that can be used to predict the influence of surface roughness on the friction coefficient in such contacts. The results show two important properties of the friction coefficient for the friction regime studied in this paper: (1) there is a linear decrease in friction coefficient as a function of the slide-to-roll ratio, and (2) the friction coefficient increases linearly with increasing mean contact pressure up to a maximum limit above which the friction coefficient is constant. The absolute deviation of experimental results from the derived theory is for most cases within 0.005.