Showing papers in "Journal of Tribology-transactions of The Asme in 2003"

Mechanism of Action of Colloidal Solid Dispersions

Abstract: In the past there has been considerable interest in the possibility of using liquid lubricantscontaining dispersed, solid particles in the 1–50 micron size range to reduce friction andwear. These particles are used in greases and some industrial oils. Researchers are nowdirecting their attention to the behavior of much smaller colloidal particles in the rangeof 5 nm to 200 nm diameter. Such systems are formally known as ‘‘colloidal sols’’ andhave been claimed to influence friction and wear. Further reasons for studying suchcolloidal particles is that they are present in soot-contaminated engine lubricating oils, aswear debris and as partially-soluble additives. Thus, the objective of the work derived inthis paper was to investigate the mechanism of action of colloidal solid particles in therange of 5 to 200 nm diameter in lubricating oils. Of particular interest was the effect ofslide-roll ratio on particle entrainment and the influence of the ratio of particle diameterto elastohydrodynamic lubricant film thickness on particles’ behavior. This study hasshown that in thin film contacts, colloid nanoparticles penetrate EHD contacts mainly bya mechanism of mechanical entrapment. It is found also that in rolling contacts at slowspeeds, colloids formed a boundary film of at least 1 or 2 times the particle size. This filminfluence friction and wear. However, this film is lost at high speed and the film thicknessreverts to the colloid-free fluid. The results of this study have enabled a mechanism oflubricating action by colloid sols to be derived. @DOI: 10.1115/1.1537752#

A Semi-Analytical Solution for the Sliding Inception of a Spherical Contact

Abstract: A finite element analysis, for an elastic perfectly plastic sphere normally loaded by a rigid flat, is combined with an approximate analytical solution to evaluate the maximum tangential load (static friction) that can be supported by the spherical contact at the inception of sliding. Sliding inception is treated as a failure mechanism based on plastic yield rather than a Coulomb friction law with a certain friction coefficient. Two different failure modes are identified, either on the contact area or below it, depending on the elastic-plastic status of the normal preloading. A limiting normal preload is found above which the contact cannot support any additional tangential load. Simple analytical expressions for an internal static friction coefficient are presented for both the elastic and the elastic-plastic regimes.

Theoretical Analysis of the Incompressible Laminar Flow in a Macro-Roughness Cell

Abstract: The present work deals with the analysis of the incompressible laminar shear driven flow in a channel of which one of the walls carries a macro roughness pattern while the opposite one has a parallel velocity. The problem is discussed from the standpoint of lubrication theory and it is shown that the usual simplified models as the Reynolds or the Stokes equations are not applicable. Numerical results are presented for three types of two dimensional macro-roughness and two versions of a three dimensional one. It is shown that a pressure generation effect occurs with increasing the relative importance of convective inertia. Previous analyses found in the literature discussed only the increase of the shear stress due to the presence of the macro roughness but the lift effect due to the pressure generation has never been enlightened up to now. It is further discussed that, extrapolated to a very large number of macro roughness characterizing a textured surface, this new effect could be added to the other lift generating mechanisms of the lubrication theory. It could thus bring a different light on inertia effects stemming from the use of textured surfaces.

Elastohydrodynamics and Mechanics of Rectangular Elastomeric Seals for Reciprocating Piston Rods

Abstract: 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 far 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.

Effect of Lubricant Protein Concentration on the Wear of Ultra-High Molecular Weight Polyethylene Sliding Against a CoCr Counterface

Abstract: In the wear testing of prosthetic joints, the optimal lubricant protein concentration is disputed. The effect of protein concentration of calf serum based lubricant on the wear of ultra-high molecular weight polyethylene against CoCr was studied with a 12-station, circularly translating pin-on-disk device. The wear factor first steeply increased with increasing concentration, reached a peak at 10-20 mg/ml, and then slowly decreased. Below 20 mg/ml, the wear mechanisms were not entirely representative of clinical wear. Above this value, the morphology of the UHMWPE wear surface resembled that of retrieved cups. The results indicated that the concentration should not be below 20 mg/ml. The scope of this recommendation is discussed.

The Generalized Newtonian Fluid Model and Elastohydrodynamic Film Thickness

Abstract: 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 I 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.

An Elliptical Microcontact Model Considering Elastic, Elastoplastic, and Plastic Deformation

Abstract: This study developed an elastic-plastic microcontact model that considers the elliptical contact of surface asperities. In the elastoplastic regime, the relations of the mean contact pressure and contact area of asperity to its contact interference are modeled considering the continuity and smoothness of variables across different modes of deformation. Results obtained from this model are compared with other existing models such as that calculated by the GW, CEB, Zhao and Horng models. The elliptic contact model and circular contact model can deviate considerably in regard to the separation and real area of contact.

On the Frictional Damping Characterization of Compliant Bump Foils

Abstract: High-speed rotor systems use either fluid film or rolling element bearing supports, depending upon their design and operating constraints. Regardless of bearing type used, these systems require specific bearing and support stiffness and damping characteristics to achieve the desired stable and low vibration operation. Building upon the technology of thin metallic corrugated bump foils presently used in a particular class of film riding hydrodynamic bearings, a novel corrugated bump foil damped mount is introduced which provides stiffness and clamping for application with rolling element bearings. These damping elements are capable of operating at elevated temperatures where implementation of conventional squeeze film dampers is ruled out. The frictional damping results from micro-slip motions between the bump foils and the mating surfaces. A semi-empirical model, based on a one degree of freedom model was developed in which damping is replaced by an equivalent frictional force in order to gain insight into the dynamic friction coefficient of the individual damping element interfaces. Experimental results, obtained in the form of hysteresis loops were compared to the developed model with good agreement. The variation in damping and dynamic coefficient of friction was found to be dependent primarily upon three factors : vibration frequency. amplitude of motion and applied static load. These parameters were tested within the range of 50-1400 Hz, 2.54-12.7 micron and 45-135 N, respectively. The tests were conducted at room and 538°C ambient temperatures under both dry and vapor phase lubricated conditions. Using the resulting empirical data, several bearing dampers were designed, built and tested in a small, high-speed gas turbine engine simulator. The tested novel foil dampers were capable of operating reliably under extremely high levels of shaft imbalance (i.e., 320 times greater than the air bearing supported with specification of 0.0002 oz-in) even while operating at temperatures to 560°C. These results show the great potential for wide application of these dampers on gas turbine engines and high-speed rotating machinery.

A Scale-Dependent Model for Multi-Asperity Contact and Friction

Abstract: As loading forces decrease in applications such as MEMS and NEMS devices, the size of the asperity contacts which comprise the real contact area tend to decrease into the nano scale regime. This reduction in size of the contacts is only partially offset by the nominally increased smoothness of these contacting surfaces. Because the friction force depends on the real area of contact, it is important to understand how the material and topographical properties of surfaces contribute to friction forces at this nano scale. In this investigation, the single asperity nano contact model of Hurtado and Kim is incorporated into a multi-asperity model for contact and friction which includes the effect of asperity adhesion forces using the Maugis-Dugdale model. The model spans the range from nano-scale to micro-scale to macro-scale contacts. Three key dimensionless parameters have been identified which represent combinations of surface roughness measures, Burgers vector length, surface energy, and elastic properties. Results are given for the friction coefficient versus normal force, the normal and friction forces versus separation. and the pull-off force for various values of these key parameters.

Measurement of Temperature Field in Surface Grinding Using Infra-Red (IR) Imaging System

Abstract: An experimental technique is described for measuring the temperature field in a workpiece during surface grinding. The technique involves measurement of the radiation emitted by a side of the workpiece immediately adjoining the wheel-workpiece contact region using a Charge-Coupled Device (CCD) based Infra-Red imaging system. By using an appropriate calibration procedure, measured radiation values are converted to temperatures. Novel aspects of the experimental technique are full-field measurement of temperature at high spatial and temporal resolution, high sensitivity and non-intrusive measurement. The repeatability of temperature measurement is found to be very good. The experiments have provided an accurate estimate of the surface and sub-surface temperatures in the workpiece. Furthermore, by grinding along a taper with a continuously increasing depth of cut, the effect of material removal rate on temperature field has been characterized. Measurements of the temperature field in taper grinding have been found to correlate well with those made in conventional constant depth grinding thereby, establishing taper grinding as a viable, accelerated test for studying grinding temperatures. Full-field measurements of workpiece temperature should facilitate study of thermal damage and multi-scale validation of thermal models in grinding.

Design Consideration of Contact/Near-Contact Sliders Based on a Rough Surface Contact Model

Abstract: We numerically investigated contact characteristics of a contact pad with a rough disk surface and the possibility of contact/near-contact sliders, using a single-degree-of-freedom (I-DOF) slider and a random wavy surface model with random roughness. Contact characteristics of a contact pad are numerically calculated based on a modified Greenwood-Williamson model, considering the bulk deformation of the surface due to all other asperity contact forces. It was found that contact stiffness and other characteristics are mainly determined by asperity contact, to the extent that the contact pad penetrates into the upper standard deviation of asperity peak height. However, the contact stiffness tends to approach a constant value as the pad penetrates into the average asperity height because the bulk deformation becomes predominant. From the numerical simulations of a 1-DOF air bearing slider model in contact and near-contact regimes over a random wavy surface with random roughness, a typical example of design condition of disk surface waviness in terms of the tracking ability and wear durability are shown, and the possibility and difficulty of a contact/near-contact slider is discussed. Finally, we analyzed meniscus effects on the contact characteristics and found a hysteresis process of the touch down and take off of a slider due to the meniscus force.

Indentation Analysis of Elastic-Plastic Homogeneous and Layered Media: Criteria for Determining the Real Material Hardness

Abstract: A hardness analysis based on finite element simulation results and contact constitutive models is presented for both homogeneous and layered elastic-plastic media. The analysis provides criteria for obtaining the real material hardness from indentation experiments performed with spherical indenters. Emphasis is given on the estimation of the hardness of thin surface layers. The critical (maximum) interference distance that yields an insignificant effect of the substrate deformation on the estimation of the layer hardness is determined from the variation of the equivalent hardness of the layered medium with the interference distance (indentation depth). A relation between hardness yield strength, and elastic modulus. derived from finite element simulations of a homogeneous half-space indented by a rigid sphere, is used in conjunction with a previously developed contact constitutive model for layered media to determine the minimum interference distance needed to produce sufficient plasticity in order to ensure accurate measurement of the material hardness. An analytical approach for estimating the layer hardness front indentations performed on layered media is presented and its applicability is demonstrated in light of finite element indentation results for an elastic-perfectly plastic layered medium with a hard surface layer.

Three-Dimensional Finite Element Analysis of Elastic-Plastic Layered Media Under Thermomechanical Surface Loading

Abstract: 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 Mixes 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.

A Lubricant Model Considering Wall-Slip in EHL Line Contacts

Abstract: A wall-slip model including limiting shear stress and the occurrence of slip at the interfaces between the lubricant and the adjacent surfaces is presented. The lubricant model is applied to EHL line contacts using smooth surfaces and isothermal conditions. The main part of the model concerns the lubricant velocities at the surfaces that are decoupled from the corresponding surface velocity giving two new variables in the EHL equations. The lubricant velocities at the surfaces are related to the corresponding shear stresses. As long as the value of the shear stress is below the limiting shear stress, the lubricant velocity is equal to the surface velocity. However when the shear stress reaches the limiting shear stress, interfacial slip appears and the lubricant velocity differs from the surface velocity. Some initial results are presented and compared to a Newtonian analysis. (Less)

Finite Element Simulation of Dynamic Instabilities in Frictional Sliding Contact

Abstract: This paper describes tools for numerical modeling which enable the understanding of the appearance of vibrations of a structure generated by the frictional contact between two bodies (the excitation source being friction). The dynamic finite element code PLASTD is used to reproduce transitory phenomena generated at the contact interface. This code includes contact and friction algorithms based upon a formulation which uses Lagrange multipliers. A numerical study of the dynamic response of a 2D mechanical model composed of a deformable body in relative translation and unilateral contact with Coulomb friction with a rigid surface is presented. The steady sliding solution is generically unstable and leads to a dynamic response which leads to the generation of instabilities characterized by the appearance of steady-state pulses. It is important to notice that those instabilities appear even with a constant friction coefficient of Coulomb. These simulations provided the local contact conditions (kinematics, tribological state, contact stresses, etc). The kinematics shows the existence of local impacts and sliding at high frequencies. Furthermore, local contact normal stress is found to be much higher than that expected for a smooth surface. Finally, a 3D simulation of brakes is carried out, focusing on the vibrations of the disk and the brake pad which produce noise and are due to the interface instabilities.

TEHD lubrication of Mechanical Face Seals in stable tracking mode. Part 2: Parametric study

Abstract: In this paper the influence of the design and operating parameters on the TEHD behavior of Mechanical Face Seals (MFS), in steady dynamic tracking mode, is analyzed for two different types of applications First, an extensive parametric analysis of a typical MFS with very low leakage is presented Then, the influence of rotational speed, sealed fluid temperature and pressure, rings materials, shape, waviness and misalignment of the rotor, are successively examined The use of an original dimensionless parametric analysis leads to a very simple and overall description of the results It is shown that ignoring the thermoelastic distortions of the rings could be misleading as far as the evaluation of MFS performance is concerned In the final part, a hydrostatic MFS with a very large gap and flow rate is studied The increase of the rotational speed induces a progressively turbulent radial flow In this case, it is shown that neither thermal effects nor fluid flow regime significantly affect seal behavior

The Effects of Three-Dimensional Model Surface Roughness Features on Lubricant Film Thickness in EHL Contacts

Abstract: The Spacer Layer Imaging method has been used to investigate the influence of three-dimensional roughness features on the thickness and shape of elastohydrodynamic (EHL) films. An array of near-hemispherical bumps was employed to represent asperities. A micro-EHL film developed at the bumps whose orientation depended on that of the inlet boundary at the location at which the bump had entered the contact. Rolling-sliding conditions induced a micro-EHL film with a classical horseshoe shape at the bumps. The flow of lubricant around the bumps appeared to differ between thin and thick films.

Semi-Analytical Dynamic Analysis of Spiral-Grooved Mechanical Gas Face Seals

Abstract: A novel semi-analytical formulation is presented for the linearized dynamic analysis of spiral-grooved mechanical gas face seals. The linearized rotordynamic properties of the gas film are numerically computed and then represented analytically by a constitutive model consisting of a cosine modified Prony series. The cosine modification enables the Prony series to characterize the gas film properties of face seals in applications with large compressibility numbers. The gas film correspondence principle is then employed to couple the constitutive model to the dynamics of the mechanical face seal. Closed-form solutions are presented for the transient natural response to initial velocity conditions, the steady-state response to rotor runout and initial stator misalignment, the transmissibility ratios, and the stability threshold. Results from the closed-form solutions are all within a few percent of the results from a full nonlinear numerical simulation. @DOI: 10.1115/1.1510876#

New Experimental Results of a Single Ridge Passing Through an EHL Conjunction

Abstract: The micro-elastohydrodynamic lubrication of a single transverse ridge is revisited using an experimental technique, which combines an optical interferometry technique and a high-speed color video camera. The purpose of this study is to augment prior experimental analyses, by providing a complete and detailed history of the ridge associated with changes in film thickness as it passes through a high-pressure conjunction. An enhanced experimental procedure has been developed to enable an automatic analysis of the interferograms. In particular, the methodology allows abrupt changes in film thickness and rapid variations of interference orders to be taken into account. The observations presented in this paper exhibit interesting and fascinating features that have not been previously reported. In particular, it is observed that under rolling/sliding conditions the ridge undergoes further deformations as it proceeds to the exit to the contact. In addition, there appears to be an important contribution of pressure flow to the transport of lubricant and, contrary to current understanding, entrapped lubricant is seen to accompany the ridge as it passes through the contact, therefore appearing not to move at the entraining velocity.

New First and Second Order Slip Models for the Compressible Reynolds Equation

Abstract: In the original derivations of the first order and the second order slip models of the generalised Reynolds equation in the literature [3,4], a length scale equal to the mean free path of the gas molecules was used in a Taylor series expansion of the mean velocity field. The coefficients of the correction terms in the derived lubrication equation depend on that length scale. This choice of the length scale is arbitrary to some extent. In this paper, new first order and the second order slip models are derived using a somewhat more physical approach, in which the requirement that the expansion length scale be the mean free path is relaxed. In this approach the momentum transfer rate across each surface element is obtained by summing up the contributions from each group of molecules impinging on the surface at an angle θ to the surface normal within a solid angle dω. The new second order slip lubrication equation appears to be preferable to the original one when the inverse Knudsen number is small, and it is free of any contact pressure singularity, whereas the new first order slip model continues to contain the unacceptable pressure singularity in the limit as the spacing approaches zero, as does the original first order model.

Effect of Surface Patterning on Contact Deformation of Elastic-Plastic Layered Media

Abstract: 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.

Characteristics and Pattern of Plasma Generated at Sliding Contact

Abstract: To verify the tribo-microplasma concept proposed by Nakayama, who suggested that a microplasma is generated in the gap of a sliding contact due to electrical discharge of the ambient gas in the electric field caused by tribo-charging, we challenged to observe directly the tribo-microplasma and to measure spectral characteristics of the emitted photons. In experiments to observe plasma image (with a diamond hemispherical pin sliding on a sapphire disk) it was found that the plasma was generated in the several micrometer gap of the sliding contact. The plasma had a shape of an ellipse with a tail, surrounding the contact and spread to the rear of the sliding contact. The plasma image observed through the UV transmittable filter (UV image) had a horseshoe pattern, while the IR image had a shape of a ring on the ellipse. The strongest UV emission was observed in the center of the horseshoe pattern outside the sliding contact, while the IR photon image showed that the most intense emission occurred at the sliding contact. The electrical discharge origin of the photon triboemission was proved by comparing spectra of tribophotons with spectra of photons emitted from plasma by electrical discharge in parallel electrodes in various gases. The results showed that the spectra of photons emitted from the sliding contact and those of gas-discharge completely coincided for all gases tested, i.e., dry air N 2 , O 2 , H 2 , He, CH 4 , C 2 H 4 and C 3 H 8 , except peaks originated from the excited atoms of the sliding surfaces. It was concluded that microplasma is produced by electrical breakdown of ambient gas at sliding contact. These results corroborate the tribomicroplasma concept.

TEHD Lubrication of Mechanical Face Seals in Stable Tracking Mode: Part 1—Numerical Model and Experiments

Abstract: After a short presentation of state-of-the-art experimental studies on the thermal behavior of non-contacting face seals, the literature about numerical models for thermal effects is investigated. Next, the geometry, kinematics and dynamics of a steady state three-dimensional model are developed. Simplified Navier-Stokes equations, a generalized Reynolds equation and an energy equation with proper boundary conditions are established for flow regimes, varying from laminar to turbulent. The numerical computer code for solving the governing equations is presented and representative results are shown. It is demonstrated that face distortions strongly modify the seals' thermal behavior. An original test rig has been developed in order to ensure full fluid film conditions. This apparatus and the experimental procedure are described. The ability of the numerical model to simulate real configurations is also illustrated. Theoretical and experimental results are in good agreement. Yet an improved model of heat transfer on boundaries is still needed.

Effects of Air Bearing Stiffness on a Hard Disk Drive Subject to Shock and Vibration

Abstract: A finite element model of a hard disk drive (HDD) is developed to investigate the transient response of an operational HDD subject to shock and vibration. The air bearing stiffness of the head disk interface is determined from a finite element solution of the Reynolds equation and approximated with linear springs. The structural response is analyzed for several types of sliders with a wide range of air bearing stiffness. Results show the response of the head-disk interface subject to shock and the modes excited by vertical and lateral vibrations of the HDD.

Experimental and Theoretical Study of Instantaneous Engine Valve Train Friction

Abstract: A new method has been developed to directly measure valve train friction as a function of crank angle using specially designed timing belt pulley torque transducers fitted to the inlet and exhaust camshafts of a single-cylinder gasoline engine. Simultaneous and instantaneous friction torque of both the inlet and exhaust camshafts at any engine speed can be measured, with no apparent detrimental effect of timing belt loading on the output reading. Experiments are reported for valve train friction at a range of motored engine operating conditions with different lubricant formulations, with and without a friction modifier These are compared with the predictions of an existing valve train friction model based upon elastohydrodynamic lubrication theory. Measured friction decreased with increasing engine speed but increased with increasing oil temperature and the fuel economy benefit of friction modifiers was observed. The model yielded similar magnitudes of friction at medium engine speeds and above but predicted much lower friction with high oil temperatures at low speed. Comparison of theory and experiments also suggests that some oil may leak from hydraulic lash adjusters during the cam event with a consequent reduction in geometric torque.

Stability Analysis of a Hydrodynamic Journal Bearing With Rotating Herringbone Grooves

Abstract: This paper presents an analytical method to investigate the stability of a hydrodynamic journal bearing with rotating herringbone grooves. The dynamic coefficients of the hydrodynamic journal bearing are calculated using the FEM and the perturbation method. The linear equations of motion can be represented as a parametrically excited system because the dynamic coefficients have time-varying components due to the rotating grooves, even in the steady state. Their solution can be assumed as a Fourier series expansion so that the equations of motion can be rewritten as simultaneous algebraic equations with respect to the Fourier coefficients. Then, stability can be determined by solving Hill's infinite determinant of these algebraic equations. The validity of this research is proved by the comparison of the stability chart with the time response of the whirl radius obtained from the equations of motion. This research shows that the instability of the hydrodynamic journal bearing with rotating herringbone grooves increases with increasing eccentricity and with decreasing groove number, which play the major roles in increasing the average and variation of stiffness coefficients, respectively. It also shows that a high rotational speed is another source of instability by increasing the stiffness coefficients without changing the damping coefficients.

Test Results for PTFE-Faced Thrust Pads, With Direct Comparison Against Babbitt-Faced Pads and Correlation With Analysis

Abstract: The use of PTFE-faced pads in large vertical axis hydro-generators was pioneered in Russia in the 1970s, prompted by a series of failures of conventional babbitt-faced bearings. Some advantages claimed include higher specific loading, lower power loss and the omission of oil-lift facilities. There is strong interest in the Industry concerning this material, but limited data are available on actual performance. Some results from extensive testing of PTFE-faced pads are given, for two sizes of pad. These are compared directly size-for-size with results for babbitt bearings of nominally the same area. The power losses for the two types of bearing were found to be almost identical. Some of the effects observed during testing are described and discussed, including the effect of creep. The test results are compared with predictions using the GENMAT analysis software A method of allowing for creep in numerical modeling is discussed.

Compression of a Single Transverse Ridge in a Circular Elastohydrodynamic Contact

Abstract: A detailed experimental study has been made of the behavior of a 100 nm high transversely oriented ridge in an elastohydrodynamic (EHD) contact. Ultra-thin film interferometry has been used to measure film profiles accurately over a very wide range of lubricant film thicknesses, from a few nanometers up to nearly one micron. This enables the recovery of the amplitude of the inlet perturbation geometry with increasing EHD film thickness to be quantified and compared with numerical predictions. In pure rolling under very thin film conditions, corresponding to a smooth surface EHD film thickness of 10 nm, the surfaces near the ridge were squashed down, leading to a constriction in the film of only about 9 percent of the height of the un-deformed ridge. As the EHD film thickness increased, this deformation recovered until the ridge constriction regained about 90 percent of its original height at film thicknesses of about 1 μm. However this relatively rapid recovery only occurred in pure rolling and is attributed to the local perturbation of film convergence which the ridge generates while in the inlet region. This propagates through the contact at the mean speed of the surfaces and-in pure rolling-acts to diminish the effect of local squeeze. When sliding was present, the ridge remained almost fully deformed even when the mean film thickness was as much as twice the height of the original ridge. In this case, the ridge travels through the contact at a different speed from the mean of the two surfaces. The consequent decoupling of the ridge and the convergence perturbation results in a large local pressure due to squeeze which acts to inhibit recovery of the ridge. The general trend of the behavior of the lubricated ridge is shown to be in good agreement with earlier theoretical results.

Transient Thermoelastic Stress Fields in a Half-Space

Abstract: 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.

Effect of Residual Stress in Surface Layer on Contact Deformation of Elastic-Plastic Layered Media

Abstract: The effect of residual stress in the surface layer on the deformation of elastic-plastic layered media due to indentation and sliding contact loading and unloading was analyzed with the finite element method. A three-dimensional finite element model of a rigid sphere interacting with a deformable layered medium was developed, and its accuracy was evaluated by contrasting finite element results with analytical solutions for the surface stresses of an elastic homogeneous half-space subjected to normal and friction surface traction. Deformation of the layered medium is interpreted in terms of the dependence of the von Mises equivalent stress, first principal stress, and equivalent plastic strain on the magnitudes of residual stress and coefficient of friction. The effect of residual stress on the propensity for yielding and cracking in the layered medium is discussed in the context of results for the maximum Mises and tensile stresses and the evolution of plasticity in the subsurface. It is shown that the optimum residual stress in the surface layer depends on the type of contact loading (indentation or sliding), coefficient of friction, and dominant deformation mode in the layer (i.e., plastic deformation or cracking).