Showing papers in "Journal of Tribology-transactions of The Asme in 2005"

State of the art in Laser Surface Texturing

Abstract: Surface texturing has emerged in the last decade as a viable option of surface engineering resulting in significant improvement in load capacity, wear resistance, friction coefficient etc. of tribological mechanical components. Various techniques can be employed for surface texturing but Laser Surface Texturing (LST) is probably the most advanced so far. LST produces a very large number of micro-dimples on the surface and each of these micro-dimples can serve either as a micro-hydrodynamic bearing in cases of full or mixed lubrication, a micro-reservoir for lubricant in cases of starved lubrication conditions, or a micro-trap for wear debris in either lubricated or dry sliding. The present paper reviews the current effort being made world wide on surface texturing in general and on laser surface texturing in particular. It presents the state of the art in LST and the potential of this technology in various lubricated applications like mechanical seals, piston rings and thrust bearings. The paper also describes some fundamental on going research around the world with LST.

A Finite Element Study of Elasto-Plastic Hemispherical Contact Against a Rigid Flat

Abstract: This work presents a finite element study of elasto-plastic hemispherical contact. The results are normalized such that they are valid for macro contacts (e.g., rolling element bearings) and micro contacts (e.g., asperity contact), although micro-scale surface characteristics such as grain boundaries are not considered. The material is modeled as elastic-perfectly plastic. The numerical results are compared to other existing models of spherical contact, including the fully plastic truncation model (often attributed to Abbott and Firestone) and the perfectly elastic case (known as the Hertz contact). This work finds that the fully plastic average contact pressure, or hardness, commonly approximated to be a constant factor of about three times the yield strength, actually varies with the deformed contact geometry, which in turn is dependent upon the material properties (e.g., yield strength). The current work expands on previous works by including these effects and explaining them theoretically. Experimental and analytical results have also been shown to compare well with the current work. The results are fit by empirical formulations for a wide range of interferences (displacements which cause normal contact between the sphere and rigid flat) and materials for use in other applications.

Improving Tribological Performance of Piston Rings by Partial Surface Texturing

Abstract: An analytical model is developed to study the potential use of part ial laser surface texturing (LST) for reducing the friction between a piston ring and cylinder liner. The hydrodynamic pressure distribution and the time dependent clearance between the pi ston ring and cylinder liner are obtained from a simultaneous solution of the Reynolds equation and the ring equation of motion in the radial direction. The time behavior of the fric tion force is calculated from the shear stresses in the viscous fluid film and the time de pendent clearance. An intensive parametric investigation is performed to identify the m ain parameters of the problem. The optimum LST parameters such as dimples depth, texture area density and textured portion of the nominal contact surface of the piston ring are evaluated.

Two-Dimensional CFD-Analysis of Micro-Patterned Surfaces in Hydrodynamic Lubrication

Abstract: Results of a numerical study of the influence of micro-patterned surfaces in hydrodynamic lubrication of two parallel walls are reported. Two types of parameterized grooves with the same order of d ...

Virtual Texturing: Modeling the Performance of Lubricated Contacts of Engineered Surfaces

Abstract: Engineering practices and analyses have indicated that surface textures and topography may significantly affect the tribological performance of contact interfaces. Such an influence may be complicated and difficult to be captured with only a few statistic surface parameters. The need for further improvement of the performance and life of machine elements requires that surface topography and textures he optimized. The utilization of a numerical tool to determine the basic geometric aspects of surface textures may be named a virtual texturing technology, with which surface optimization may start from patterned surfaces where topography can be precisely quantified and the relationship between textures and lubrication performance can be numerically established. Presented in this paper are the concept of the virtual texturing technology, models involved, and a preliminary exploration of the relationship between a dimpled texture design and the mixed lubrication characteristics for a typical counterformal contact. The dimple influence area and the number of interruption are found to be two key factors for designing dimpled surfaces for counterformal contact lubrication. It is demonstrated that virtual texturing is able to provide comparative information and directions for innovative surface design and optimization.

The Difficulty of Measuring Low Friction: Uncertainty Analysis for Friction Coefficient Measurements

Abstract: The experimental evaluation of friction coefficient is a common laboratory procedure; however, the corresponding measurement uncertainty is not widely discussed. This manuscript examines the experimental uncertainty associated with friction measurements by following the guidelines prescribed in international standards. The uncertainty contributors identified in this analysis include load cell calibration, load cell voltage measurement, and instrument geometry. A series of 20 tests, carried out under nominally identical conditions, was performed using a reciprocating pin-on-disk tribometer. A comparison between the experimental standard deviation and uncertainty analysis results is provided.

CFD analysis of a low friction pocketed pad bearing

Abstract: A CFD method has been applied to model lubricant flow behavior within linear pad bearings having large, closed pockets or recesses. The study shows that the presence of closed pockets can result in a significant reduction in bearing friction coefficient and that there are two different origins for this, depending on the bearing convergence ratio. At high convergence ratios, as used in conventional thrust hearings, a pocket located in the high-pressure region of the bearing produces a reduction in local shear stress and thus friction. This friction reduction is larger than the reduction in load support resulting from the presence of the pocket so there is a net overall reduction in friction coefficient. In low convergence ratio bearings, each pocket also acts as an effectively-independent step bearing and thereby generates a higher local pressure than would otherwise he the case. This results in the overall hearing having enhanced load support and thus a reduced friction coefficient. This effect is particularly large at very low convergence ratios when cavitation occurs in the pocket inlet.

A Finite Element Study of the Residual Stress and Deformation in Hemispherical Contacts

Abstract: This work presents a finite element model (FEM) of the residual stresses and strains that are formed after an elastoplastic hemispherical contact is unloaded. The material is modeled as elastic perfectly plastic and follows the von Mises yield criterion. The FEM produces contours for the normalized axial and radial displacements as functions of the removed interference depth and location on the surface of the hemisphere. Contour plots of the von Mises stress and other stress components are also presented to show the formation of the residual stress distribution with increasing plastic deformation. This work shows that high residual von Mises stresses appear in the material pileup near the edge of the contact area after complete unloading. Values are defined for the minimum normalized interference, that when removed, results in plastic residual stresses. This work also defines an interference at which the maximum residual stress transitions from a location below the contact region and along the axis of symmetry to one near to the surface at the edge of the contact radius (within the pileup).

Numerical Simulation for Three Dimensional Elastic-Plastic Contact with Hardening Behavior

Abstract: An elastic-plastic contact (EPC) solution and code is developed using a modified semi-analytical method. The indentation tests with different hardening behavior are simulated by using the developed EPC code. The distributions of contact pressure, residual stress and plastic strain are obtained and compared with the results of the finite element method models without hardening. Some techniques, such as fast Fourier transform and fast convergence method, are used to increase the computation speed.

Finite Element Analysis of High Contact Ratio Spur Gears in Mesh

Abstract: High contact ratio gears have been demonstrated to provide significant advantages for decreasing tooth root and contact stresses with potential flow-on benefits for increased load carrying capacity. Previous investigations with high contact ratio gears have involved analytical, numerical and experimental aspects. Much of the earlier numerical work using FEA was limited in its usefulness due to several factors; (i) the difficulty in predicting load sharing over roll angles covering two or three teeth simultaneously in mesh, (ii) the difficulty for the analysis to obtain quality results when modelling Hertzian contact deflection simultaneously with the bending, shear and angular deflections, and (iii) the problem of primary unconstrained body motion when (long) profile modifications were applied. This paper presents methods and results for overcoming these difficulties with recent computer hardware and software improvements. Particular developments discussed include the use of FE analysis of High Contact Ratio Gears in mesh and the results obtained when adaptive meshing is used. The details of transmission error, combined torsional mesh stiffness, load-sharing ratio, contact stress and tooth root stress against various input loads over a complete mesh cycle are also given. Results with various tooth profile modifications are presented and comparisons between the results show evidence for the optimal profile modification expected to gain the maximum benefit of HCRG.

Analysis of Mixed Lubrication Effects in Simulated Gear Tooth Contacts

Abstract: The paper presents results obtained using a transient analysis technique for point contact elastohydrodynamic lubrication (EHL) problems based on a formulation that couples the elastic and hydrodynamic equations. Results are presented for transverse ground surfaces in elliptical point contact that show severe film thinning and asperity contact at the transverse limits of the contact area. This thinning is caused by transverse leakage of the lubricant from the contact in the remaining deep valley features between the surfaces. A comparison is also made between the point contact results on the entrainment center line and the equivalent line contact analysis. The extent of asperity contact is shown to be dependent on the Hertzian contact aspect ratio. It is also shown that transverse waviness (superimposed on the roughness) of even relatively small amplitude can lead to large increases in asperity contact rates over all waviness peaks in the contact.

An Average Flow Model of the Reynolds Roughness Including a Mass-Flow Preserving Cavitation Model

Abstract: An average Reynolds equation for predicting the effects of deterministic periodic roughness, taking Jakobsson, Floberg, and Olsson mass flow preserving cavitation model into account, is introduced based upon the double scale analysis approach. This average Reynolds equation can be used both for a microscopic interasperity cavitation and a macroscopic one. The validity of such a model is verified by numerical experiments both for one-dimensional and two-dimensional roughness patterns.

Numerical Analysis of a Journal Bearing With a Heterogeneous Slip/No-Slip Surface

Abstract: The no-slip boundary condition is part of the foundation of the traditional lubrication theory. It states that fluid adjacent to a solid boundary has zero velocity relative to the solid surface. For most practical applications, the no-slip boundary condition is a good model for predicting fluid behavior. However, recent experimental research has found that for certain engineered surfaces the no-slip boundary condition is not valid. Measured velocity profiles show that slip occurs at the interface. In the present study, the effect of an engineered slip/no-slip surface on journal bearing performance is examined. A heterogeneous pattern, in which slip occurs in certain regions and is absent in others, is applied to the bearing surface. Fluid slip is assumed to occur according to the Navier relation. Analysis is performed numerically using a mass conserving algorithm for the solution of the Reynolds equation. Load carrying capacity, side leakage rate, and friction force are evaluated. In addition, results are presented in the form of Raimondi and Boyd graphs. It is found that the judicious application of slip to a journal bearing’s surface can lead to improved bearing performance.

Contact Analyses for Bodies With Frictional Heating and Plastic Behavior

Abstract: The stress field within machine components is an important indicator for contact failures. Since both thermal stresses due to frictional heating and plasticity are significant in engineering application, it is critical to predict the total stress field. In this work, the steady-state thermal effect is considered and a thermo-elastic-plastic contact model is developed. The model is applicable for rolling and/or sliding contact problem, as far as small equivalent plastic strain hypothesis is respected. Influence coefficients for surface normal displacement, temperature, and strain and stress tensors are used with the discrete convolution and fast Fourier transform algorithm. The single-loop conjugate gradient iteration scheme is also applied to achieve fast convergence speed. Simulations are presented for several academic examples ranging from elastic to thermo-elastic-plastic. The thermo-elastic-plastic analyses show that the heat factor in a contact situation has significant effect not only on the critical Hertzian pressure and on the pressure distribution, but also on the magnitude and depth of the maximum von Mises stress during loading and the residual ones found after unloading.

A Material Removal Rate Model Considering Interfacial Micro-Contact Wear Behavior for Chemical Mechanical Polishing

Abstract: Chemical Mechanical Polishing (CMP) is a highly effective technique for planarizing wafer surfaces. Consequently, considerable research has been conducted into its associated material removal mechanisms. The present study proposes a CMP material removal rate model based upon a micro-contact model which considers the effects of the abrasive particles located between the polishing interfaces, thereby the down force applied on the wafer is carried both by the deformation of the polishing pad asperities and by the penetration of the abrasive particles.. It is shown that the current theoretical results are in good agreement with the experimental data published previously. In addition to such operational parameters as the applied down force, the present study also considers consumable parameters rarely investigated by previous models based on the Preston equation, including wafer surface hardness, slurry particle size, and slurry concentration. This study also provides physical insights into the interfacial phenomena not discussed by previous models, which ignored the effects of abrasive particles between the polishing interfaces during force balancing.

Non-Newtonian Thermal Analyses of Point EHL Contacts Using the Eyring Model

Abstract: A non-Newtonian numerical solution system for the thermal elastohydrodynamic lubrication (EHL) problems in point contacts has been developed. The Eyring rheology model has been used to describe the non-Newtonian flow of the lubricant. An effective viscosity has been defined for the Eyring fluid. The Newtonian solver can be applied easily to the non-Newtonian problems when the viscosity of the Newtonian fluid is replaced by the effective viscosity. A novel technique for the determination of the effective viscosity is proposed. Numerical solutions for the conventional point contact and normally crossing cylinders contact problems are presented and the effects of the entraining velocity, the load, the slide-roll ratio, and the characteristic shear stress of the Eyring fluid on the lubricating performance are discussed. The results indicate that the non-Newtonian thermal EHL theory predicts more realistic film temperatures and traction coefficients.

A Compact Model for Spherical Rough Contacts

Abstract: A new model is developed that considers the effect of roughness on the elastic contact of spherical bodies. A general pressure distribution is proposed that encompasses the contact of rough spheres and yields the Hertzian theory for ideally smooth surfaces. A new parameter, nondimensional maximum contact pressure, is introduced and it is shown that this is the key parameter that controls the contact. The results of the present study are presented in the form of compact relationships. These relationships are compared against the experimental data collected by others and good agreement is observed.

Simulation of wear through mass balance in a dry contact

Abstract: Numerical investigations are carried out to simulate wear and the analysis of these simulations leads to proposing an original new wear law that takes into account interfacial particles in the contact. A 3D Discrete Element Model is presented that simulates the detachment of particles, their flow in the contact and their ejection. It shows that a layer of detached particles can be formed at the interface, separating the solids in contact. The simulations show how influential the contact geometry and the properties of the interfacial particles are in studying wear. The processes of material degradation and particle ejection are then studied separately. Their physical behavior is analyzed and simple analytical expressions are proposed. Consideration of the mass balance of the contact provides an analytical law for wear, involving the fate of the detached particles. Classical wear laws (such as Archard's law), assuming that no particle stays in the contact, appear to be a limit case of this model.

An Experimental Study of Bearing and Thread Friction in Fasteners

Abstract: An experimental procedure is proposed for studying the underhead and thread friction in fasteners. The effective bearing friction radius, the underhead friction coefficient, and the thread friction coefficient are experimentally determined for fasteners with standard hexagonal heads and for flanged head fasteners. Hence, greater accuracy has been achieved in determining the value of the torque components that are consumed in overcoming friction in threaded fasteners. This would lead to a more reliable torque-tension correlation and would enhance the safety and quality of bolted assemblies. A design of experiment procedure is presented in order to investigate the effect of fastener material class, the thread pitch, and the fastener size on thread friction coefficient. For the underhead bearing friction, an experimental model is presented in order to determine the effect of the radii ratio of the contact area on the bearing friction radius.

An Elastoplastic Microasperity Contact Model for Metallic Materials

Abstract: In the present study, the formulas for the asperity contact loads (F ec and F pc ) corresponding to the critical interferences at the inception of elastoplastic and fully plastic deformations are employed to establish their relation with the ratio of these two critical interferences (δ ec and δ pc ). The critical interference ratio (δ pc /δ ec ) can thus be expressed as a function of the critical contact load ratio, (F pc /F ec ), whose value was obtained from the experimental results of metallic materials. The interference (δ pc ) corresponding to the inception of fully plastic deformation can thus be determined. The dimensionless analyses of asperity contact area, average contact pressure, and contact load in the elastic and fully plastic regime reveals that these parameters in the elastoplastic regime can be expressed in power form and to be as a function of dimensionless interference (δ/δ ec ). The coefficients and exponents of the power form expressions can be determined by the boundary conditions set at the two ends of this regime. Four models are proposed in this study to compare the contact behaviors in the elastoplastic regime. The applications in contact of rough surfaces are also presented and discussed.

Quantifying Multidirectional Sliding Motions in Total Knee Replacements

Abstract: This manuscript describes a formulaic method to predict the severity of crossing motions experienced by ultra-high-molecular-weight polyethylene (UHMWPE) under sliding conditions across a bearing surface. A statistical model for evaluating the distribution of sliding about this orientation is described. This model compares favorably to published experimental data on UHMWPE under multidirectional pin-on-disk testing. These algorithms are applied to the tibial component of a total knee replacement using patient-specific kinematics for gait and stair-rise activities collected using fluoroscopy. A dynamic contact model is used to calculate contact pressures and slip velocities on individual surface elements from this kinematic data. The most significant degree of crossing motion intensity was observed in the lateral compartments for both gait and stair-rise activities. This coincided with the location of maximum tribological intensity. The maximum crossing motions are characteristic of 4 and 9 deg of bidirectional crossing motion for gait and stair, respectively.

Survey and Performance Assessment of Solution Methods for Elastic Rough Contact Problems

Abstract: Recent developments in the solution of models of rough contact have demonstrated the advantages of fast matrix multiplication methods in increasing the size of problem that can usefully be solved. However, to date, no attempt has been made to compare these methods with existing approaches. In this paper a survey of solution methods reveals that three main classes of solution method have been developed: active set methods with direct matrix solution; active set methods with iterative matrix solvers; iterative methods based on fast matrix multiplications. The best features of each class are assembled from the literature and combined to formulate a best-possible algorithm in each class. These algorithms are implemented and compared, with emphasis on their solution speed, accuracy and memory requirements. For smaller problems with fewer than 2000 grid points in contact, the direct matrix solution method proves to be most efficient, and is always the most accurate. However, the method is limited by memory requirements as the contact region increases. For large problems with more than 2000 grid points in contact, the multilevel multisummation approach requires less memory, and performs faster, but gives errors in predicted pressure and contact region of approximately 0.5%. The active set method with iterative matrix solver requires less memory than direct matrix solution, is more accurate than the multilevel multisummation approach, but is always slower than one of the other methods.

Relationship Between Temperature Distribution in EHL Film and Dimple Formation

Abstract: This paper describes an experimental study on dimple formation under elastohydrodynamic lubrication (EHL) conditions. The oil film thickness between a ball surface and a glass disk was measured using optical interferometry, and the temperatures of both the surfaces and of the oil film averaged across it were measured using an infrared emission technique. It was found that the temperature profile across the oil film varies abruptly along the sliding direction, and the Couette flow decreases due to the viscosity wedge action as the oil flow is close to the dimple zone. The maximum temperature rise in the dimple zone sometimes reached over 400 K, thus, the phase transition of the oil from liauid to glass may not occur.

Experimental Evaluation of Piston-Assembly Friction Under Motored and Fired Conditions in a Gasoline Engine

Abstract: Piston-assembly friction measurement has been carried out on a single-cylinder gasoline engine using the IMEP (indicated mean effective pressure) method at realistic engine speeds and loads without any major engine modifications. Instantaneous and mean piston-assembly friction were measured under motored and fired conditions at different lubricant temperatures. The forces acting on the piston assembly were carefully determined by measuring the cylinder pressure, crankshaft angular velocity, and strain in the connecting rod. The difference between the resulting gas pressure, inertia, and connecting rod axial forces acting on the piston yields the piston-assembly friction. To achieve this with confidence, an advanced instrumentation, telemetry, and data acquisition system was designed and developed, giving special attention to the synchronization and simultaneous sampling of analog and digital channels. Experiments are reported for piston-assembly friction at a range of engine operating conditions with different lubricant formulations, with and without a friction modifier.

Experimental Investigation of the Elastic-Plastic Contact Area and Static Friction of a Sphere on Flat

Abstract: An experimental investigation is presented to evaluate recently published models for the contact and sliding inception of a deformable sphere loaded against a smooth rigid flat. The effects of the normal load on the contact area, junction growth, and the static friction force in the elastic-plastic contact regime are presented. Very good correlation is found between the predicted and measured contact area. A dramatic decrease of the static friction coefficient with increasing normal loading is observed, similar to the trend predicted by the model. The quantitative agreement is, however, less satisfying. Some possible reasons for the poor agreement are pointed out.

Third Body Influence on Thermal Friction Contact Problems: Application to Braking

Abstract: The aim of this study is to evaluate the temperature and the heat distribution into the two components of a disc brake system by combining macroscopic and microscopic effects. The major difficulty of the thermal problem is to determine how the heat is generated and how it is distributed in the two components in contact during transient state. Contrary to classical approaches assuming equal temperature at the contact surfaces, a contact interface is introduced in the model as a thin layer of third body with uniform volumic heat generation. This micro-macro model gives original indications on the temperatures near the contact surfaces, on the thermal gradients between the two components and on the heat partition between the two bodies during the braking time. Comparison with classical thermal models is discussed.

The effect of periodic textures on the static characteristics of thrust bearings

Abstract: Given a bearing of some specified macroscopic shape, what is the effect of texturing its surfaces uniformly? Experimental and numerical investigations on this question have recently been pursued, which we complement here with a mathematical analysis based on a seemingly novel combination of homogenization techniques and perturbation analysis. The flow is assumed governed by the Reynolds equation, with cavitation effects disregarded, and the texture length is assumed much smaller than the bearing's length. The results, which hold true for small-amplitude periodic textures and in the limit of vanishing period, can be summarized as follows: (a) The texture that maximizes the load for a given minimum clearance is no texture at all (i.e., the untextured shape); and (b) the texture that minimizes the friction coefficient is again the untextured shape.

Experimental and Numerical Investigation of Dynamic Instability in the Head Disk Interface at Proximity

Abstract: As the flying height decreases to achieve greater areal density in hard disk drives, different proximity forces act on the air bearing slider, which results in fly height modulation and instability. Identifying and characterizing these forces has become important for achieving a stable fly height at proximity. One way to study these forces is by examining the fly height hysteresis, which is a result of many constituent phenomena. The difference in the touchdown and takeoff rpm (hysteresis) was monitored for different slider designs, varying the humidity and lubricant thickness of the disks, and the sliders were monitored for lubricant pickup while the disks were examined for lubricant depletion and modulation. Correlation was established between the observed hysteresis and different possible constituent phenomena. One such phenomenon was identified as the Intermolecular Force from the correlation between the lubricant thickness and the touchdown velocity. Simulations using 3D dynamic simulation software explain the experimental trends.

Vibration Monitoring and Damage Quantification of Faulty Ball Bearings

Abstract: More often than not, the rolling element bearings in rotating machinery are the mechanical components that are first prone to premature failure. Early warning of an impending bearing failure is vital to the safety and reliability of high-speed turbomachinery. Presently, vibration monitoring is one of the most applied procedures in on-line damage and failure monitoring of rolling element bearings. This paper presents results from an experimental rotor-bearing test rig with quantified damage induced in the supporting rolling element bearings. Both good and damaged radial and tapered ball bearings are used in this study. The vibration signatures due to damage at the ball elements and the inner race of the bearing are also examined. Vibration signature analyzing schemes such as frequency domain analysis, and chaotic vibration analysis (modified Poincare diagrams) are applied and their effectiveness in pinpoint damage are compared in this study. The size/level of the damage is corroborated with the vibration amplitudes to provide quantification criteria for bearing progressive failure prediction. Based on the results from this study, it is shown that the use of the modified Poincare map, based on the relative carrier speed, can provide an effective way for identification and quantification of bearing damage in rolling element bearings.

An Experimental Study of Journal Bearing Lubrication Effected by Journal Misalignment as a Result of Shaft Deformation Under Load

Abstract: Journal bearing friction pair system is one of the most general and essential parts used in various mechanical devices. A special test bench is developed for the study on lubrication performance of cylindrical journal bearings. The effect of journal misalignment as a result of shaft bending under load is studied. The results show obvious changes at distribution and value of oil film pressure, oil film thickness and oil temperature of journal bearing due to journal misalignment. The higher the load on the shaft, the larger the journal misalignment resulted from shaft deformation, the more obvious effect on lubrication performance of journal bearing.