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TECHNICAL PAPERS

J. Tribol. 2002;125(1):1-7. doi:10.1115/1.1509773.

The abrasiveness of hard carbon-containing thin films such as diamond-like carbon (DLC) and boron carbide (nominally B4C) towards steel is considered here. First, a remarkably simple experimentally observed power-law relationship between the abrasion rate of the coatings and the number of cycles is described. This relationship remains valid over at least 4 orders of magnitude of the number of cycles, with very little experimental scatter. Then possible models of wear are discussed. It is assumed that the dominant mechanism of steel wear is its mechanical abrasion by nano-scale asperities on the coating that have relatively large attack angles, i.e. by the so-called sharp asperities. Wear of coating is assumed to be mainly due to physical/chemical processes. Finally, models of the abrasion process for two basic cases are presented, namely a coated ball on a flat steel disk and a steel ball on a coated flat disk. The nominal contact region can be considered as constant in the former case, while in the latter case, the size of the region may be enlarged due to wear of the steel. These models of the abrasion process are based on the assumption of self-similar changes of the distribution function characterizing the statistical properties of patterns of scattered surface sharp asperities. It is shown that the power-law relationship for abrasion rate follows from the models.

Commentary by Dr. Valentin Fuster
J. Tribol. 2002;125(1):8-15. doi:10.1115/1.1481365.

A model is developed to study the tribological behavior of sliding micro-contacts. It provides a building block to the modeling of tribo-contacts in boundary lubrication. Three contact variables are calculated at the asperity-level by relating them to the state of contact and the state of asperity deformation. These variables include micro-contact friction force, load carrying capacity and flash temperature. The deformation of the contacting asperity is either elastic, elasto-plastic, or fully plastic. Furthermore, the asperity may be covered by the lubricant/additive molecules adsorbed on the surface, protected by a surface oxide layer or other chemical reaction films, or in direct contact with no boundary protection. The possibility of the contact in each of these three states is represented by a corresponding contact probability. A numerical method is developed to determine the contact state and contact variables in the course of an asperity-to-asperity collision. The asperity flash temperature, which governs the kinetics of lubricant/surface adsorption/desorption, is first calculated by integrating the Jaeger equation over the contact area and in time. Then, the probability of contact covered by an adsorbed film is determined using the Volmer adsorption isotherm, and the probability of contact protected by the oxide layer is estimated using a classical wear theory. For elastic/elasto-plastic deformation of the asperity, the friction coefficient is given by the linear combination of the friction coefficients of the three contact states with their contact probabilities as the weighting factors. For fully plastic deformation of the asperity, the contact pressure and friction force become dependent of each other. The shear stress is approximated by a linear function of the contact probabilities, and the contact pressure and friction coefficient then calculated. Meanwhile, the influence of fresh surface generation due to plastic flow on the contact probabilities is also modeled. Insights are provided into the asperity collision through numerical studies of a sample problem. In addition, parametric studies are carried out to analyze the effects of lubricant and surface parameters on the micro-contact severity and its load capacity.

Commentary by Dr. Valentin Fuster
J. Tribol. 2002;125(1):16-24. doi:10.1115/1.1501086.

A plane-strain finite element analysis for patterned elastic-plastic layered media was performed in order to elucidate the effect of surface geometry on the deformation and stress fields due to normal and sliding contact. Surface interaction between the layered media and a rigid asperity was modeled with special contact elements. Results for the contact pressure distribution, surface tensile stress, and subsurface equivalent plastic strain are presented for layered media with different meandered and sinusoidal surfaces. The significance of surface patterning on the deformation behavior is interpreted in terms of stress and strain results illustrative of the tendency for crack initiation and plastic deformation in the first two layers, where deformation is confined in all simulation cases. Relations for the contact pressure concentration factor and onset of yielding in the first (hard) layer are derived from finite element results for indented layered media with sinusoidal surface patterns. Predictions for the indentation depth at the onset of yielding based on the developed yield criterion are shown to be in good agreement with those obtained from finite element simulations.

Commentary by Dr. Valentin Fuster
J. Tribol. 2002;125(1):25-32. doi:10.1115/1.1509772.

A numerical simulation is performed to investigate the development of forces between a rigid sphere and an elastic half-space during normal, dynamic contact in the absence of friction. Of interest is to quantify the magnitude of forces that arise and to identify any sources of hysteresis between approach and separation, the latter being associated with energy dissipation. In the simulation a rigid sphere approaches and separates from an isotropic, linearly elastic half-space at a prescribed, constant speed. Surface forces are incorporated in the model by ascribing a surface interaction potential derived from the Lennard-Jones 6-12 intermolecular potential. Dynamical equations of motion for the interface are integrated numerically during the approach-separation event. During the approach phase, it is found that the magnitude of adhesive force is generally consistent with well-known static-equilibrium based analytical models (e.g., DMT and JKR), depending upon the strength of the interaction potential. However, during separation, the attractive force computed in this dynamic simulation may be several times higher than the predictions of the analytical models. Additionally, the maximum compressive forces attained during the contact process far exceed the predictions of Hertzian contact theory. The discrepancy between results of this simulation and those of the static-equilibrium analytical and numerical models indicate that dynamic interactions play a significant role in determining the development of contact forces. Moreover, dynamic effects persist even when the approach-separation speed of the sphere is small compared to the dilatation and shear wave speeds of the half-space.

Commentary by Dr. Valentin Fuster
J. Tribol. 2002;125(1):33-43. doi:10.1115/1.1501087.

Computing the thermoelastic stress field of a material subjected to frictional heating is essential for component failure prevention and life prediction. However, the analysis for three-dimensional thermoelastic stress field for tribological problems is not well developed. Furthermore, the pressure distribution due to rough surface contact is irregular; hence the frictional heating can hardly be described by an analytical expression. This paper presents a novel set of frequency-domain expressions (frequency response functions) of the thermoelastic stress field of a uniformly moving three-dimensional elastic half-space subjected to arbitrary transient frictional heating, where the velocity of the half-space, its magnitude and direction, can be an arbitrary function of time. General formulas are expressed in the form of time integrals, and important expressions for constant velocities are given for the transient-instantaneous, transient-continuous, and steady-state cases. The thermoelastic stress field inside a translating half-space with constant velocities are illustrated and discussed by using the discrete convolution and fast Fourier transform method when a parabolic type or an irregularly distributed heat source is applied.

Commentary by Dr. Valentin Fuster
J. Tribol. 2002;125(1):44-51. doi:10.1115/1.1506315.

Hot spotting and judder phenomena were observed in automotive aluminum drum brakes. A vehicle judder test schedule was developed to determine the critical speed for thermoelastic instability (TEI). The brake material properties relevant to the TEI analysis were measured as a function of temperature. The critical speeds for the brake systems with different drum materials were determined by the judder schedule and they are compared with the analytical predictions of Lee (2000). The brake drums and linings were then modified and tested in order to investigate its effects on the hot spotting and judder propensity. The design modifications include brake linings with a different compound, stress-relieved drums, linings with a convex or concave surface finish, three-segmented linings, and linings with a circumferential groove. The linings with a circumferencial groove effectively reduce the size of hot spots and the best judder rating was achieved.

Commentary by Dr. Valentin Fuster
J. Tribol. 2002;125(1):52-59. doi:10.1115/1.1497360.

The simultaneous effects of mechanical and thermal surface loadings on the deformation of layered media were analyzed with the finite element method. A three-dimensional model of an elastic sphere sliding over an elastic-plastic layered medium was developed and validated by comparing finite element results with analytical and numerical solutions for the stresses and temperature distribution at the surface of an elastic homogeneous half-space. The evolution of deformation in the layered medium due to thermomechanical surface loading is interpreted in light of the dependence of temperature, von Mises equivalent stress, first principal stress, and equivalent plastic strain on the layer thickness, Peclet number, and sliding distance. The propensity for plastic flow and microcracking in the layered medium is discussed in terms of the thickness and thermal properties of the layer, sliding speed, medium compliance, and normal load. It is shown that frictional shear traction and thermal loading promote stress intensification and plasticity, especially in the case of relatively thin layers exhibiting low thermal conductivity.

Commentary by Dr. Valentin Fuster
J. Tribol. 2002;125(1):60-69. doi:10.1115/1.1506316.

A numerical model was developed to study the sealing performance of rectangular elastomeric seals for reciprocating piston rods used in linear hydraulic actuators. The model takes into account a large number of parameters and has been applied in the study of seals for aircraft actuation assemblies in a broad range of temperatures (−55°C to +135°C) and sealed pressures (1–50 MPa or more). The model is used to calculate the contact pressures and film thickness maps as well as the leakage rates and friction for the dynamic or static contact between a seal and a reciprocating piston rod, aiming at the minimization of both the leakage and the wear of the seals.

Commentary by Dr. Valentin Fuster
J. Tribol. 2002;125(1):70-75. doi:10.1115/1.1504086.

The nature of real shear-thinning in elastohydrodynamic contacts is well-known from both experimental measurement and nonequilibrium molecular dynamics to follow a power-law. Shear-thinning will affect the film thickness when the Newtonian limit is low enough to occur in the inlet zone (less than about 1 MPa shear stress). Then kinetic theory tells us that film thinning should occur for molecular weight greater than 2000 kg/kmol. We present a review of generalized Newtonian models, flow curves for real lubricants and comparison of calculated and measured film thickness. The calculations utilize measurable liquid behavior, in contrast to most previous work.

Commentary by Dr. Valentin Fuster
J. Tribol. 2002;125(1):76-90. doi:10.1115/1.1481366.

In this paper, an isothermal study of the shut down process of elastohydrodynamic lubrication under a constant load is performed. The surface mean velocity is decreased linearly from the initial steady state value to zero. The details of the pressure and film thickness distributions in the contact area are discussed for the two stages of shut down process, namely the deceleration stage and the subsequent pure squeeze motion stage with zero entraining velocity. The nature of the balance between the pressure, the wedge and the squeeze terms in Reynolds equation enables an analytical prediction of the film thickness change on the symmetry line of the contact in the deceleration period, provided that the steady state central film thickness relationship with velocity is known. The results indicate that for a fixed deceleration rate, if the initial steady state surface mean velocity is large enough, the transient pressure and film thickness distributions in the deceleration period solely depend on the transient velocity. The pressure and film thickness at the end of the deceleration period are then the same and do not depend on the initial steady state velocity. From the same initial steady state velocity, larger deceleration rates provide higher central pressure increase, but also preserve a higher film thickness in the contact area at the end of the deceleration period. Later in the second stage when the axisymmetric pressure and film thickness patterns typical of pure squeeze motion form, the pressure distribution in the contact area resembles a Hertzian contact pressure profile with a higher maximum Hertzian pressure and a smaller Hertzian half contact width. As a result, the film thickness is close to a parabolic distribution in the contact area. The volume of the lubricant trapped in the contact area is then estimated using this parabolic film thickness profile.

Commentary by Dr. Valentin Fuster
J. Tribol. 2002;125(1):91-101. doi:10.1115/1.1504090.

This research presents an analytical model to investigate the stability due to the ball bearing waviness in a rotating system supported by two ball bearings. The stiffness of a ball bearing changes periodically due to the waviness in the rolling elements as the rotor rotates, and it can be calculated by differentiating the nonlinear contact forces. The linearized equations of motion can be represented as a parametrically excited system in the form of Mathieu’s equation, because the stiffness coefficients have time-varying components due to the waviness. Their solution can be assumed as a Fourier series expansion so that the equations of motion can be rewritten as the simultaneous algebraic equations with respect to the Fourier coefficients. Then, stability can be determined by solving Hill’s infinite determinant for these algebraic equations. The validity of this research is proven by comparing the stability chart with the time responses of the vibration model suggested by prior research. This research shows that the waviness in the ball bearing generates the time-varying component of the stiffness coefficient, whose frequency is called the frequency of the parametric excitation. It also shows that the instability takes place from the positions in which the ratio of the natural frequency to the frequency of the parametric excitation corresponds to i/2 (i=1,2,3,[[ellipsis]]).

Commentary by Dr. Valentin Fuster
J. Tribol. 2002;125(1):102-109. doi:10.1115/1.1494087.

Theoretical analysis and optical interferometry experiments are performed to investigate the dimple phenomena in thermal elastohydrodynamic lubrication (TEHL) of elliptical contacts under pure sliding conditions. The lubricant entrainment is along the major and minor axes of the Hertzian contact ellipse or at some intermediate angle. Good agreement is achieved between theoretical and experimental results and the surface dimple phenomena occurring in glass-steel conjunctions are explained by the temperature-viscosity wedge mechanism. The influence of the angle between the minor axis and the entrainment vector on the position and shape of the dimple, the central and minimum film thickness, the temperature distribution and the frictional coefficient is discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 2002;125(1):110-120. doi:10.1115/1.1504087.

Rotary viscous couplings with interleaved, perforated plates and viscous fluids are used in automotive systems to transmit torque. During operation, viscous dissipation raises fluid temperature, lowers fluid viscosity and causes the torque transmitted to drop monotonically to unusable levels. Couplings designed with certain plate geometry exhibit a reversal of the torque trend with temperature, and transmit increasingly high torque even under continuous operation. Such couplings achieve torque amplification factors in excess of twenty, compared to earlier couplings. This torque amplification phenomenon has been utilized by industry without fully understanding the mechanisms involved. A comprehensive theory is proposed to explain the complex sequence of events that results in this “anomalous,” but useful phenomenon. Mathematical models are developed for each interdependent process. A visual simulation tool is used to model the intricate dynamics inside the coupling. Results from the simulation model are compared with experimental findings. The various thermodynamic, hydrodynamic, structural and mechanical processes are delineated and tested with a combination of theoretical analysis, computational simulation and experimental observations. The proposed theory identifies, defines and explains the conditions necessary for initiating and sustaining the self-induced torque amplification. The hypotheses are validated by the reasonable agreement of the model with the test results.

Commentary by Dr. Valentin Fuster
J. Tribol. 2002;125(1):121-134. doi:10.1115/1.1506317.

Present understanding of the mechanisms of lubrication and the load carrying capacity of lubricant films mainly relies on models in which the Reynolds equation is used to describe the flow. The narrow gap assumption is a key element in its derivation from the Navier Stokes equations. However, the tendency in applications is that lubricated contacts have to operate at smaller film thickness levels, and because engineering surfaces are never perfectly smooth, locally in the film this narrow gap assumption may violated. In addition to this geometric limitation of the validity of the Reynolds equation may come a piezoviscous and compressibility related limitation. In this paper the accuracy of the predictions of the Reynolds model in relation to the local geometry of the gap is investigated. A numerical solution algorithm for the flow in a narrow gap has been developed based on the Stokes equations. For a model problem the differences between the pressure and velocity fields according to the Stokes model and the Reynolds equation have been investigated. The configuration entails a lower flat surface together with an upper surface (flat or parabolic) in which a local defect (single asperity) of known geometry has been embedded. It is investigated how the magnitude of the differences develops as a function of the geometric parameters of the film and the feature. Finally, it is discussed to what extend for these problems a perturbation approach can provide accurate corrections to be applied to the Reynolds solution.

Commentary by Dr. Valentin Fuster
J. Tribol. 2002;125(1):135-144. doi:10.1115/1.1467634.

This paper presents a systematic development of the theory of powder lubrication with the appropriate formalism based on the fundamentals of fluid mechanics. The theory is capable of predicting flow velocity, fluctuation (pseudo-temperature), powder volume fraction, and slip velocity at the boundaries. An extensive set of parametric simulations covering granular size, surface roughness, volumetric flow, load and speed are performed to gain insight into the performance of a powder lubricated thrust bearing. The results of the simulations are compared to published experimental results. Good agreement between the theoretical and experimental results attests to the capability of the model and its potential for design of powder lubricated bearings.

Commentary by Dr. Valentin Fuster
J. Tribol. 2002;125(1):145-151. doi:10.1115/1.1504091.

This paper proposes an analytical design sensitivity analysis (DSA) to topological parameters of MGL (molecular gas film lubrication) sliders by introducing an adjoint variable method. For the analysis of slider air bearings, we used the spatial discretization of the generalized lubrication equation based on a control volume formulation. The residual functions for inverse analysis of the slider are considered as the equality constraint functions. The slider rail heights of all grid cells are chosen as design variables since they are the topological parameters determining air bearing surface (ABS). Then, a complicated adjoint variable equation is formulated to directly handle the highly nonlinear asymmetric coefficient matrix and vector in the discrete system equations of slider air bearings. An alternating direction implicit (ADI) scheme is utilized to efficiently solve large-scale problem in special band storage. The simulation results of DSA are directly compared with those of finite-difference approximation (FDA) to show the effectiveness and accuracy of the proposed approach. The overall sensitivity distribution over the ABS is reported, and clearly shows to which section of the ABS the special attention should be given during the manufacturing process. It is demonstrated that the proposed method can reduce more than 99 percent of the CPU time in comparison with FDA, even though both methods give the same results.

Commentary by Dr. Valentin Fuster
J. Tribol. 2002;125(1):152-161. doi:10.1115/1.1510882.

This paper describes optimum air-bearing design of a tri-pad slider in terms of tracking ability to micro-waviness based on theoretical analysis of the two-degree-of-freedom slider model and the distributed and concentrated air-bearing stiffness model. Although a short tri-pad type slider was introduced through the load/unload technique, we point out that this type of slider is superior to the traditional rail type slider in terms of tracking ability to micro-waviness. More importantly, the distance between head-gap position and the rear air-bearing center should be made as small as possible. The spacing variation due to lower mode resonance can be eliminated if the positions of front and rear air-bearing centers are located at the center of percussion. The resonance amplitude of the higher order mode in spacing variation can be reduced if the length of the rear air-bearing pad is designed to be 1.2∼1.3 times the wavelength of the higher mode resonance frequency. Since the momental stiffness of the front air-bearing prevents the head-gap from tracking micro-waviness, the front air-bearing length should be made short or the ratio of rear to front air-bearing stiffness should be made large. If the resonance amplitude of the lower mode must be decreased, the front air-bearing length should be designed to be 1.2∼1.3 times the wavelength of the lower mode resonance frequency.

Commentary by Dr. Valentin Fuster
J. Tribol. 2002;125(1):162-167. doi:10.1115/1.1506314.

A new beam suspension model for load/unload simulations of magnetic hard disk drive sliders is presented. The model is applicable to the load/unload process as well as to the dynamic flying behavior of the slider on the disk. Suspension and gimbal are modeled with C1 continuous beam elements. The roll degree of freedom of the slider is decoupled from spacing and pitch. The numerical implementation of the model is described. Loading and unloading of a typical subambient pressure type slider is presented.

Commentary by Dr. Valentin Fuster
J. Tribol. 2002;125(1):168-180. doi:10.1115/1.1491977.

This paper introduces a novel method for preventing nozzle wear in abrasive water jets. It consists of using a porous nozzle, surrounded by a reservoir containing high-viscosity lubricant, which is exposed to the same driving pressure as the flow in the nozzle. The pressure difference across the porous medium, generated due to the high-speed flow in the nozzle, continuously forces lubricant through it. The resulting thin oil film forming on the walls of the nozzle protects the walls from the impact and shear caused by the abrasive particles. The porous nozzles were manufactured using Electric Discharge Machining and examined with Scanning Electron Microscopy. Two test facilities were used for evaluating the porous lubricated nozzles. The first was a two-dimensional facility, supporting a 145 μm wide nozzle with windows on both sides, which enabled visualization of the oil film and measurements of the liquid and abrasive-particle velocities using Particle Image Velocimetry. The measured slip velocities were also compared to computed values from a simple numerical model involving one-way coupling. The second facility used a 200 μm axisymmetric nozzle to determine the extent of nozzle wear under different conditions. We found that the presence of an oil film substantially reduced the extent of nozzle wear, from 111 percent of the diameter, when the nozzle was not lubricated, to 4 percent, when the oil viscosity was 1800 mm2 /s and its flow rate was 2.4 percent of the water flow (over the same period). The wear increased as the lubricant flow rate and viscosity decreased. The presence of the oil film also improved the coherence of the jet.

Commentary by Dr. Valentin Fuster
J. Tribol. 2002;125(1):181-186. doi:10.1115/1.1504089.

Scuffing or scoring is an important form of damage leading to component replacements in lubricated mechanical systems such as power drives, gears, bearings, cams and followers, piston rings etc. Since scuffing necessarily involves localized welding of asperities, suitable surface modifications or coating can impart a good resistance to scuffing. A new class of low temperature salt bath nitriding process provides good resistance to scuffing. The current work is on evaluation of the scuffing resistance of AISI 4340 steels imparted with two such treatments. Experimental investigations were conducted on Sursulf and Arcor treated AISI 4340 steel specimen using a pin on ring test system under boundary lubrication condition. Continuous monitoring of friction records and (near surface) bulk temperature were done under step loading. Limiting loads and load velocity relations were evaluated and using the data generated and a thermo-mechanical wear model, performance is indexed. Off line studies on surface finish, hardness variations and surface transformations were also accomplished. Some salient aspects of the investigation and data generated are presented here. Onset of scuffing was observed to be far delayed with pin specimen imparted with these treatments compared to hardening.

Commentary by Dr. Valentin Fuster
J. Tribol. 2002;125(1):187-192. doi:10.1115/1.1504092.

The evolution of the geometry of a simple two-dimensional circular cam as a result of wear is studied using three complementary approaches: a closed form analytical expression, a computer simulation, and the development of an experimental apparatus. Experiments were run for over 1.5 million cycles, and measurements of cam shape and follower motion were recorded and compared favorably to the predictions of both techniques. Errors associated with an accelerated computational approach are discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 2002;125(1):193-199. doi:10.1115/1.1506313.

Miniature devices including MEMS and the head disk interface in magnetic storage often include very smooth surfaces, typically having root-mean-square roughness, σ of the order of 10 nm or less. When such smooth surfaces contact, or come into proximity of each other, either in dry or wet environments, then strong intermolecular (adhesive) forces may arise. Such strong intermolecular forces may result in unacceptable and possibly catastrophic adhesion, stiction, friction and wear. In the present paper, a model termed sub-boundary lubrication (SBL) adhesion model is used to calculate the adhesion forces, and an elastic-plastic model is used to calculate the contact forces at typical MEMS interfaces. Several levels of surface roughness are investigated representing polished and as-deposited polysilicon films that are typically found in MEMS. The SBL adhesion model reveals the significance of the surface roughness on the adhesion and pull-off forces as the surfaces become smoother. The validity of using the SBL adhesion model to estimate the pull-off forces in miniature systems is further supported by direct comparison with experimental pull-off force measurements performed on silicon and gold interfaces. Finally, the significance of the interfacial forces as relate to the reliability of MEMS interfaces is discussed.

Commentary by Dr. Valentin Fuster

TECHNICAL NOTES

J. Tribol. 2002;125(1):200-203. doi:10.1115/1.1491979.

Excessive interfacial slip is one of the primary failure mechanisms in the cross wedge rolling (CWR) process. In order to predict operating conditions that lead to excess interfacial slip, an analytical method is presented for determining the critical rolling condition in a two-roll CWR operation. Based on a transverse section of the tool-workpiece contact interface, the method uses Bowden and Oxley’s friction models to obtain the critical rolling condition in CWR as a direct function of tool geometry and workpiece area reduction. The relative accuracy of the results obtained from each friction model is then ascertained and discussed by comparing the analytical results to experiments.

Topics: Friction , Wedges , Geometry
Commentary by Dr. Valentin Fuster
J. Tribol. 2002;125(1):203-206. doi:10.1115/1.1506319.

Osborne Reynolds’ classical paper on the theory of lubrication Reynolds (1886) produced the generalized Reynolds equation. For spherical bearing applications, the generalized Reynolds equation is transformed in order to obtain useful results when the hemispherical shell is not in a horizontal position. A new film thickness expression is also presented. These transformations permit the determination of pressure distributions and fluid film thickness for any orientation of the hemispherical shell including the horizontal position, for which the conventional description of Reynolds equation is well suited. The resulting equation in two-dimensional form, for an incompressible, variable viscosity fluid, with upper and lower sliding surfaces, in spherical coordinates, contains the inclination angle β, which accounts for non-horizontal positions of the shell.

J. Tribol. 2002;125(1):206-210. doi:10.1115/1.1506321.

Frene, J., 1997, Hydrodynamic Lubrication, Amsterdam: Elsevier, p. 75.Nikolakopoulos,  P. G., Papadopoulos,  C. A., 1998, “ Controllable High Speed Journal Bearings, Lubricated With Electro-Rheological Fluids: An Analytical and Experimental Approach,” Tribol. Int., TRBIBK31(5), pp. 225–234.trbTRBIBK0301-679XNoresson,  V., Ohlson,  N. G., 2001, “ A Critical Study of the Bingham Model in Squeeze-Flow Mode,” Mater. Des., MADSD222, pp. 651–658.8qqMADSD20264-1275Park,  W. C., Choi,  S. B., and Suh,  M. S., 1999, “ Material Characteristics of an ER Fluid and Its Influence on Damping Forces of an ER Damper,” Mater. Des., MADSD220, pp. 325–330.8qqMADSD20264-1275Leek,  T. H., Lingard,  S., Atkin,  R. J., and Bullough,  W. A., 1993, “ An Experimental Investigation of the Flow of an Electro-Rheological Fluid in a Rayleigh Step Bearing,” J. Phys. D, JPAPBE26, pp. 1592–1600.jpdJPAPBE0022-3727Choi,  S. B., Park,  D. W., and Cho,  M. S., 2001, “ Position Control of a Parallel Link Manipulator Using Electro-Rheological Valve Actuators,” Mechatronics, MECHER11, pp. 157–181.9fbMECHER0957-4158Wilson,  S. D. R., 1999, “ A Note on Thin-Layer Theory for Bingham Plastics,” J. Non-Newtonian Fluid Mech., JNFMDI85, pp. 29–33.jnfJNFMDI0377-0257Streeter, V. L., Wylie, E. B., Bedford, K. W., 1998, Fluid Mechanics, McGraw-Hill, p. 238.Hong, Y. P., 2000, “Turbulence Model on River Confluence and Its Application to Water Pollution Control in Three Gorges Reservior,” Ph.D. thesis, Tsinghua University, Beijing.Doormaal,  J. P., Raithby,  G. D., 1984, “ Enhancements of the Simple Method for Predicting Incompressible Fluid Flows,” Numer. Heat Transfer, NUHTD67, pp. 147–163.9awNUHTD60149-5720Braun,  M. J., Shou,  Y. M., and Choy,  F. K., 1994, “ Transient Flow Patterns and Pressures Characteristics in a Hydrostatic Pocket,” ASME J. Tribol., JOTRE9116(1), pp. 139–146.jtiJOTRE90742-4787Tao, W. O., Numerical Heat Transfer, Xi An Jiao Tong University Press, China, p. 145.Cameron, A., 1981, Basic Lubrication Theory, 3rd ed., Ellis Horwood Ltd., England, p. 77.

J. Tribol. 2002;125(1):210-214. doi:10.1115/1.1506320.

Using a mineral bright stock oil as lubricant, the dimple phenomena in the circular EHL contacts composed of a glass disk and a steel ball were investigated experimentally with the optical interferometry technique. The traction coefficient was also measured. The experimental results were compared quantitatively with the numerical results of the corresponding thermal Newtonian EHL solutions. The comparisons show that the Newtonian thermal EHL theory can explain very well the relationship between the dimple phenomena and the slide-roll ratio, but cannot predict accurately the depth of the dimple and the magnitude of the traction coefficient. It has also been found that the Newtonian flow model overestimates the effect of the temperature-viscosity wedge and consequently, a non-Newtonian flow model may be necessary for a better understanding of the EHL dimple phenomena.

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