Showing papers in "Journal of Tribology-transactions of The Asme in 2006"

The Stribeck Curve: Experimental Results and Theoretical Prediction

Abstract: The Stribeck curve plays an important role in identifying boundary, mixed, elastohydrodynamic, and hydrodynamic lubrication regimes. Recent advances in elastohydrodynamic lubrication together with rough surface interaction have made it possible to develop a methodology for predicting the trend of the Stribeck curve. In this paper; we report the results of a series of experiments performed on a journal bearing together with a theoretical prediction of the Stribeck-type behavior. Various loads and oil temperatures are considered. The comparison between the experimental results with a mixed elastohydrodynamic lubrication model for line contacts is indicative of good agreement.

Vibration Response of Rolling Element Bearings in a Rotor Bearing System to a Local Defect Under Radial Load

Abstract: In the present investigation, a theoretical model has been developed to obtain the vibration response due to a localized defect in various bearing elements in a rotor-bearing system under radial load conditions. The rotor-bearing system has been modeled as a three degrees-of-freedom system. The model predicts significant components at the harmonics of characteristic defect frequency for a defect on the particular bearing element. In the case of a defect on the inner race or a rolling element, the model predicts sidebands about the peaks at defect frequencies, at multiples of shaft and cage frequencies, respectively. The model has also predicted some additional components at harmonics of shaft and cage frequencies due to a local defect on the inner race and a rolling element, respectively. The expressions for all these spectral components have also been derived. Typical numerical results for an NJ 204 bearing have been obtained and plotted. The amplitude of the component at defect frequency, for an outer race defect, is found to be much higher as compared to those due to inner race defect or a rolling element defect of the same size and under similar conditions of load and speed. The results of vibration measurements on roller bearings with simulated local defects have also been presented to experimentally validate the theoretical model proposed. It can be observed from the results that the spectral components predicted by the theoretical model find significant presence in the experimental spectra. Comparison of the normalized analytical values of the spectral components with their experimental values shows fair agreement for most of the cases considered. Probable area of the generated excitation pulses has been calculated and the effects of pulse area variation on the experimental results have been studied.

A Review of Dry Particulate Lubrication: Powder and Granular Materials

Abstract: Research efforts related to dry particulates in sliding contacts are reviewed. In the tribology community, there are primarily two types of dry particulate lubricants that are studied—granular and powder. Granular lubricants usually refer to dry, cohesionless, hard particles which transfer momentum and accommodate surface velocity differences through shearing and rolling at low shear rates, and collisions at high shear rates. Powder lubricants refer to dry, cohesive, soft particles which accommodate surface velocity differences mostly by adhering to surfaces and shearing in the bulk medium, in a manner similar to hydrodynamic fluids. Spanning the past five decades, this review proposes a classification system for the scientific works in the dry particulate tribology literature in terms of theory, experiments, and numerical simulations. It also suggests that these works can be further categorized based on their tribosystem geometry—annular, parallel, and converging.Copyright © 2006 by ASME

Effects of Differential Scheme and Mesh Density on EHL Film Thickness in Point Contacts

Abstract: This paper investigates the effects of differential scheme and mesh density on elastohydrodynamic lubrication (EHL) film thickness based on a full numerical solution with a semi-system approach. The solution variation with different schemes and mesh sizes is revealed based on a set of numerical cases in a wide range of central film thickness from several hundred nanometers down to a few nanometers. It is observed that when the film is thick, the effects of differential schemes and mesh density are not significant. However, if the film becomes ultra-thin, e.g., below 10‐20 nanometers, the influence of mesh density and differential schemes becomes more significant, and a proper dense mesh and differential scheme may be highly desirable. The present study also indicates that the solutions from the 1st-order backward scheme give the largest film thickness among all the solutions from different schemes at the same mesh size. DOI: 10.1115/1.2194916

Numerical Model of a Reciprocating Hydraulic Rod Seal

Abstract: A numerical model of an elastomeric reciprocating hydraulic rod seal has been constructed. The model consists of coupled fluid mechanics, deformation mechanics and contact mechanics analyses, with an iterative computational procedure. The fluid mechanics analysis consists of the solution of the Reynolds equation, using flow factors to account for surface roughness. Deformation of the seal is computed through the use of influence coefficients, obtained from an off-line finite element analysis. The contact mechanics analysis uses the Greenwood and Williamson model. The seal model is used to predict leakage rate, friction force, fluid and contact pressure distributions, and film thickness distribution. Results for a typical seal show that the seal operates with mixed lubrication, and the seal roughness plays an important role in determining whether or not the seal leaks.Copyright © 2006 by ASME

A comparison of contact modeling utilizing statistical and fractal approaches

Abstract: Statistical and fractal approaches for characterizing surface topography have been used widely in contact mechanics. In the present study, a comparison is conducted between contact mechanics results obtained with statistical and fractal approaches to characterize surface topography. Specifically, a three-dimensional fractal surface was generated and statistical surface parameters were extracted using different sampling resolutions. Contact mechanics simulations were performed using the simulated fractal surface and statistical surfaces represented by the extracted statistical surface parameters. Purely elastic contact (Hertz) is studied in order to eliminate any possible influence of the individual asperity mechanical response on the obtained results. Therefore, differences in the simulated contact area and load can be related solely to the different approach employed for surface characterization.

A Low Friction Bearing Based on Liquid Slip at the Wall

Abstract: In recent years it has been shown experimentally by a number of workers that simple, Newtonian liquids can slip against solid surfaces when the latter are both very smooth and lyophobic. It has also been shown theoretically how, based on a half-wetted bearing principle, this phenomenon may be used to significantly reduce friction in lubricated sliding contacts and thus make possible the hydrodynamic lubrication of very low load contacts. This paper describes the experimental validation of this concept. A low load bearing is constructed and the influence of surface roughness and the wetting properties of the surfaces on friction are investigated over a wide range of sliding speeds. It is shown that liquid slip can be used to considerably reduce friction in full film, hydrodynamic conditions.

Elastic-Plastic Contact Between Rough Surfaces: Proposal for a Wear or Running-In Model

Abstract: A semi-analytical thermo-elastic-plastic contact model has been recently developed and presented in a companion paper. The main advantage of this approach over the classical finite element method (FEM) is the treatment of transient problems with the use of fine meshing and the possibility of studying the effect of a surface defect on the surface deflection as well as on subsurface stress state. A return-mapping algorithm with an elastic predictor/plastic corrector scheme and a von Mises criterion is now used, which improves the plasticity loop. This improvement in the numerical algorithm increases the computing speed significantly and shows a much better convergence and accuracy. The contact model is validated through a comparison with the FEM results of Kogut and Etsion (2002, J. Appl. Mech., 69, pp. 657–662) which correspond to the axisymmetric contact between an elastic-perfectly plastic sphere and a rigid flat. A model for wear prediction based on the material removal during cyclic loading is then proposed. Results are presented, first, for initially smooth surfaces and, second, for rough surfaces. The effects of surface shear stress and hardening law are underlined.

A Thermohydrodynamic Analysis of Foil Journal Bearings

Abstract: A thermohydrodynamic model is developed for predicting the three-dimensional (3D) temperature field in an air-lubricated, compliant foil journal bearing. The model accounts for the compressibility and the viscosity-temperature characteristic of air and the compliance of the bearing surface. The results of numerical solutions are compared to published experimental measurements and reasonable agreement has been attained. Parametric studies covering a fairly wide range of operating speeds and load conditions were carried out to illustrate the usefulness of the model in terms of predicting the thermal performance of foil journal bearings.

Effect of Roughness Parameter and Grinding Angle on Coefficient of Friction When Sliding of Al–Mg Alloy Over EN8 Steel

Abstract: Surface topography of harder mating surface plays an important role in metal forming operations as it predominantly controls the frictional behavior at the interface In the present investigation, an inclined scratch tester was used to understand the effect of direction of surface grinding marks on interface friction and transfer layer formation EN8 steel flats were ground to attain different surface roughnesses with unidirectional grinding marks Al–Mg alloy pins were then scratched against the prepared EN8 steel flats The grinding angle (angle between direction of scratch and grinding marks) was varied between 0 deg and 90 deg during the scratch tests Scanning electron micrography of the contact surfaces revealed the transfer layer morphology The coefficient of friction and transfer layer formation were observed to depend primarily on the direction of grinding marks of the harder mating surface, and independent of the surface roughness of harder mating surface The grinding angle effect was attributed to the variation of plowing component of friction with grinding angle

Approximate Analytical Model for Hertzian Elliptical Contact Problems

Abstract: In rolling bearing analysis Hertzian contact theory is used to compute local contact stiffness. This theory does not have a closed form analytical solution and requires numerical calculations to obtain results. Using approximations of elliptical functions and with a mathematical study of Hertzian results, an empirical explicit formulation is proposed in this paper and allows us to obtain the dimensions, the displacement, and the contact stress with at least 0.003% precision and it can be applied to a large range of ellipticity of the contact surface.

A multiscale method modeling surface texture effects

Abstract: In this paper a multiscale method is presented that includes surface texture in a mixed lubrication journal bearing model. Recent publications have shown that the pressure generating effect of surface texture in bearings that operate in full film conditions may be the result of micro-cavitation and/or convective inertia. To include inertia effects, the Navier-Stokes equations have to be used instead of the Reynolds equation. It has been shown in earlier work [2] that the coupled 2D Reynolds and 3D structure deformation problem with partial contact resulting from the soft EHL journal bearing model is not easy to solve due to the strong nonlinear coupling, especially for soft surfaces. Therefore, replacing the 2D Reynolds equation by the 3D Navier-Stokes equations in this coupled problem will need an enormous amount of computing power that is not readily available nowadays. In this paper, the development of a micro-macro multiscale method is prescribed. The local (micro) flow effects for a single surface pocket are analysed using the Navier-Stokes equations and compared to the Reynolds solution for a similar smooth piece of surface. It is shown how flow factors can be derived and added to the macroscopic smooth flow problem, that is modelled by the 2D Reynolds equation. The flow factors are a function of the operating conditions such as the ratio between the film height and the pocket dimensions, the surface velocity and the pressure gradient over a surface texture unit cell. To account for an additional pressure build up in the texture cell due to inertia effects, a pressure gain is introduced at macroscopic level. The method also allows for micro-cavitation. Micro-cavitation occurs when the pressure variation due to surface texture is larger than the average pressure level at that particular bearing location. In contrast with the work of Patir and Cheng [4], where the micro-level is solved by the Reynolds equation, the Navier-Stokes equations are used at the micro-level. Depending on the texture geometry and film height, the Reynolds equation may become invalid. A second pocket-effect occurs when the pocket is located in the moving surface. In mixed lubrication, fluid can become trapped inside a pocket and squeezed out when the pocket is running into an area with higher contact load. To include this effect, an additional source term that represents the average fluid inflow due to the deformation of the surface around the pocket is added to the Reynolds equation at macro-level. The additional inflow is computed at micro-level by numerical solution of the surface deformation for a single pocket that is subject to a contact load. The pocket volume is a function of the contact pressure. It must be emphasized that before ready-to-use results can be presented, a large number of simulations to determine the flow factors and pressure gain as a function of the texture parameters and operating conditions have yet to be done. Before conclusions can be drawn, regarding the dominanant mechanism(s), the flow factors and pressure gain have to be added to the macro bearing model. In this paper, only a limited number of preliminary illustrative simulation results, calculating the flow factors for a single 2D texture geometry, are shown to give insight into the method.Copyright © 2006 by ASME

Static and Dynamic Characterisation of a Bump-Type Foil Bearing Structure

Abstract: The performance of Gas Foil Bearings (GFBs) relies on a coupling between a thin gas film and an elastic structure with dissipative characteristics. Due to the mechanical complexity of the structure, the evaluation of its stiffness and damping is still largely inaccurate if not arbitrary. The goal of this paper is to improve the understanding of the behavior of the bump type FB structure under static and dynamic loads. The structure was modeled with finite elements by using a commercial code. The code employed the large displacements theory and took into account the friction between the bumps and the support and between the bumps and the deformable top foil. Static simulations enabled the estimation of the static stiffness of each bump of a strip. These simulations evidence a lack of reliable analytical models that can be easily implemented in a FB prediction code. The models found in the literature tend to over-estimate the foil flexibility because most of them do not consider the interactions between bumps that seem to be highly important. The transient simulations allowed the estimation of the dynamic stiffness and the damping of a single bump of the FB structure. The presence of stick-slip in the structure is evidenced and hysteretic plots are obtained. The energy dissipation due to Coulomb friction is quantified in function of materials, excitation amplitude and frequency. Some energetic considerations allow the calculation of the equivalent viscous damping coefficient and the results are related to experimental data found in literature. The influence of the number of bumps is also briefly addressed.Copyright © 2006 by ASME

The Pressure-Viscosity Coefficient for Newtonian EHL Film Thickness With General Piezoviscous Response

Abstract: There has been a long-standing need for a piezoviscous parameter, αfilm , that together with the ambient viscosity, μ0 , will completely quantify the Newtonian rheology so that the film thickness for liquids that do not shear-thin in the inlet may be calculated as h = h(μ0 , αfilm , ...) regardless of the details of the pressure-viscosity response. It seems that Blok’s reciprocal asymptotic isoviscous pressure, α*, has certain advantages over the conventional pressure-viscosity coefficient that is poorly suited for this purpose. The first detailed review of piezoviscous models for low pressures is provided. A simulation code that is apparently stable for all realistic pressure-viscosity response was utilized with diverse piezoviscous models and model liquids to develop a satisfactory definition of αfilm that reads αfilm = [1 − exp(−3)]/03/α*μ (0) dpμ (p);1/α* = 0∞ μ (0) dp / μ (p). In the case of μ = μ0 exp(αp), αfilm = α and formulas are provided for other models.Copyright © 2006 by ASME

Experimental Characterization of Wheel-Rail Contact Patch Evolution

Abstract: The contact area and pressure distribution in a wheel/rail contact is essential information required in any fatigue or wear calculations to determine design life, re-grinding, and maintenance schedules. As wheel or rail wear or surface damage takes place the contact patch size and shape will change. This leads to a redistribution of the contact stresses. The aim of this work was to use ultrasound to nondestructively quantify the stress distribution in new, worn, and damaged wheel-rail contacts. The response of a wheel/rail interface to an ultrasonic wave can be modeled as a spring. If the contact pressure is high the interface is very stiff, with few air gaps, and allows the transmission of an ultrasonic sound wave. If the pressure is low, interfacial stiffness is lower and almost all the ultrasound is reflected. A quasistatic spring model was used to determine maps of contact stiffness from wheel/rail ultrasonic reflection data. Pressure was then determined using a parallel calibration experiment. Three different contacts were investigated; those resulting from unused, worn, and sand damaged wheel and rail specimens. Measured contact pressure distributions are compared to those determined using elastic analytical and numerical elastic-plastic solutions. Unused as-machined contact surfaces had similar contact areas to predicted elastic Hertzian solutions. However, within the contact patch, the numerical models better reproduced the stress distribution, as they incorporated real surface roughness effects. The worn surfaces were smoother and more conformal, resulting in a larger contact patch and lower contact stress. Sand damaged surfaces were extremely rough and resulted in highly fragmented contact regions and high local contact stress.

Modeling and Parametric Study of Torque in Open Clutch Plates

Abstract: The relative motion of the friction and separator plates in wet clutches during the disengaged mode causes viscous shear stresses in the fluid passing through the 100 microns gap. This results in a drag torque on both the disks that wastes energy and decreases fuel economy. The objective of the study is to develop an accurate mathematical model for the above problem with verification using FLUENT and experiments. Initially we two consider flat disks. The mathematical model calculates the drag torque on the disks and the 2D axisvmmetnc solver verifies the solution. The surface pressure distribution on the plates is also verified. Then, 3D models of one grooved and one flat disk are tested using CFD, experiments and an approximate 3D mathematical model. The number of grooves depth of groove and clearance between the disks are studied to understand their effect on the torque. The study determines the pressure field that eventually affects aeration incipience (not studied here). The results of the model, computations and experiments corroborate well in the single-phase regime.

A Comparison of Flattening and Indentation Approaches for Contact Mechanics Modeling of Single Asperity Contacts

Abstract: This study compares the flattening and indentation approaches for modeling single asperity contacts in order to reveal quantitatively their different behaviors in terms of the constitutive relationships for the contact parameters and deformation regimes. The comparison is performed with four empirical models recently developed for flattening and indentation based on the finite element method. In the elasto-plastic regime, the classic Hertz solution does not hold and, therefore, different mechanical behavior was obtained for flattening and indentation cases. Consequently, the contact condition and relative strength of mating surfaces should be considered when choosing between indentation or flattening models.

A Comprehensive Method to Predict Wear and to Define the Optimum Geometry of Fretting Surfaces

Abstract: This paper presents a fast and robust three-dimensional contact computation tool taking into account the effect of cyclic wear induced from fretting solicitations under the gross slip regime. The wear prediction is established on a friction-dissipated energy criteria. The material response is assumed elastic. The contact solver is based on the half-space assumption and the algorithm core is similar to the one originally proposed by Kalker (1990, Three Dimensional Elastic Bodies in Rolling Contact, Kluwer, Dordrecht) for normal loading. In the numerical procedure the center of pressure may be imposed. The effect of surface shear stress is considered through a Coulomb friction coefficient. The conjugate gradient scheme presented by Polonsky and Keer (1999, Wear, 231, pp. 206-219) and an improved fast Fourier transform (FFT) acceleration technique similar to the one developed by Liu et al. (2000, Wear, 243, pp. 101-111) are used. Results for elementary geometries in the gross slip regime are presented. It is shown that the surface geometry influences the contact pressure and surface shear stress distributions found after each loading cycle. It is also shown that wear tends to be uniformly distributed. This process continuously modifies the micro- and macrogeometry of the rubbing surfaces, leading after a given number of cycles to (i) an optimum or ideal contact geometry and (ii) a prediction of wear.

Hybrid Flexure Pivot-Tilting Pad Gas Bearings: Analysis and Experimental Validation

Abstract: Gas film bearings offer unique advantages enabling successful deployment of high-speed micro-turbomachinery. Current applications encompass micro power generators, air cycle machines and turbo expanders. Mechanically complex gas foil bearings are in use; however, their excessive cost and lack of calibrated predictive tools deter their application to mass-produced oil-free turbochargers, for example. The present investigation advances the analysis and experimental validation of hybrid gas bearings with static and dynamic force characteristics desirable in high-speed turbomachinery. These characteristics are adequate load support, good stiffness and damping coefficients, low friction and wear during rotor startup and shutdown, and most importantly, enhanced rotordynamic stability at the operating speed. Hybrid (hydrostatic/hydrodynamic) flexure pivot-tilting pad bearings (FPTPBs) demonstrate superior static and dynamic forced performance than other geometries as evidenced in a high speed rotor-bearing test rig. A computational model including the effects of external pressurization predicts the rotordynamic coefficients of the test bearings and shows good correlation with measured force coefficients, thus lending credence to the predictive model. In general, direct stiffnesses increase with operating speed and external pressurization; while damping coefficients show an opposite behavior. Predicted mass flow rates validate the inherent restrictor type orifice flow model for external pressurization. Measured coast down rotor speeds demonstrate very low-friction operation with large system time constants. Estimated drag torques from the gas bearings validate indirectly the recorded system time constant.Copyright © 2006 by ASME

Foil Gas Bearing With Compression Springs: Analyses and Experiments

Abstract: A new foil gas bearing with spring bumps was constructed, analyzed, and tested. The new foil gas bearing uses a series of compression springs as compliant underlying structures instead of corrugated bump foils. Experiments on the stiffness of the spring bumps show an excellent agreement with an analytical model developed for the spring bumps. Load capacity, structural stiffness, and equivalent viscous damping (and structural loss factor) were measured to demonstrate the feasibility of the new foil bearing. Orbit and coast-down simulations using the calculated stiffness and measured structural loss factor indicate that the damping of underlying structure can suppress the maximum peak at the critical speed very effectively but not the onset of hydrodynamic rotor-bearing instability. However, the damping plays an important role in suppressing the subsynchronous vibrations under limit cycles. The observation is believed to be true with any air foil bearings with different types of elastic foundations.

Monitoring of lubricant film failure in a ball bearing using ultrasound

Abstract: A lubricant-film monitoring system for a conventional deep groove ball bearing (type 6016, shaft diameter 80 mm, ball diameter 12.7 mm) is described. A high-firequency (50 MHz) ultrasonic transducer is mounted on the static outer raceway of the bearing. The transducer is focused on the ball-raceway interface and used to measure the reflection coefficient of the lubricant in the "contact" ellipse between bearing components. The reflection coefficient characterizes the lubricant film and can be used to calculate its thickness. An accurate triggering system enables multiple reflection measurements to be made as each lubricated contact moves past the measurement location. Experiments are described in which bearings were deliberately caused to fail by the addition of acetone, water and sand to the lubricant. The ultrasonic reflection coefficient was monitored as a function of time as the failure occurred. Also monitored were the more standard parameters, temperature and vibration. The results indicate that the ultrasonic measurements are able to detect the failures before seizure. It is also observed that, when us,ed in parallel, these monitoring techniques offer the potential to diagnose the failure mechanism and hence improve predictions of remaining life.

Rotordynamics of Small Turbochargers Supported on Floating Ring Bearings: Highlights in Bearing Analysis and Experimental Validation

Abstract: Turbochargers (TCs) improve performance in internal combustion engines. Due to low production costs, TC assemblies are supported on floating ring bearings (FRBs). TCs show subsynchronous motions of significant amplitudes over a wide speed range. However, the subsynchronous whirl motions generally reach a limit cycle enabling continuous operation. The paper advances progress on the validation against measurements of linear and nonlinear rotordynamic models for predicting shaft motions of automotive TCs. A comprehensive thermohydrodynamic model predicts the floating ring speeds, inner and outer film temperatures and lubricant viscosity changes, clearances thermal growth, operating eccentricities for the floating ring and journal, and linearized force coefficients. A nonlinear rotordynamics program integrates the FRB lubrication model for prediction of system time responses under actual operating conditions. Measurements of shaft motion in a TC unit driven by pressurized air demonstrate typical oil-whirl induced instabilities and, due to poor lubricant conditions, locking of the floating rings at high shaft speeds. Nonlinear predictions are in good agreement with the measured total amplitude and subsynchronous frequencies when implementing the measured ring speeds into the computational model. The computational tools aid to accelerate TC prototype development and product troubleshooting.

Numerical Calculations of Pressure Distribution in the Bearing Clearance of Circular Aerostatic Thrust Bearings With a Single Air Supply Inlet

Abstract: This paper describes the pressure distribution in the bearing clearance of circular aerostatic thrust bearings with a single air supply inlet. For high air supply pressure, large bearing clearance and a relatively small bearing outer radius, it was believed that shock waves are caused and that a complex fluid flow structure is formed in the bearing clearance. Accordingly, analytical models based on the occurrence of shock wave in the bearing clearance have been proposed. Recently, very small aerostatic bearings have been used in various machine devices where the pressure distribution near the air inlets has a large influence on the bearing characteristics due to a short distance between air inlets and the bearing edge. In order to accurately predict various bearing characteristics for these kinds of bearings, a proper analytical model has to be established. However, it is very difficult to obtain the detailed information about the flow structure from flow visualization because of a very thin bearing clearance. Therefore, we calculated the flow field using computational fluid dynamics (CFD), which can solve the Navier-Stoke equations directly. It was found that the airflow just after entering the bearing clearance becomes turbulent in a region where relatively rapid pressure recovery occurs and that no shock wave is generated at the boundary between subsonic and supersonic flow. In addition, the numerical results presented show good agreement with experimental data.Copyright © 2006 by ASME

Viscosity Ratio Effect in the Emulsion Lubrication of Soft EHL Contact

Abstract: Many foodstuffs and personal care products consist of two-phase systems which, during use, are rubbed between compliant biosurfaces to form thin lubricating films. It is important to understand the nature and properties of the films thus formed since these contribute to the user's sensory perception, and thus appreciation, of the products concerned. In this paper, the lubrication properties of simple oil-in-aqueous phase emulsions are studied in a steel/elastomer "soft-EHL" contact. It is found that overall behavior is strongly dependent on the ratio of the viscosities of the two phases. When the viscosity of the dispersed oil phase is lower or comparable to that of the continuous aqueous phase, the latter enters the contact and controls,film formation and friction. However when the dispersed phase has viscosity at least four times larger than the dispersion medium, the former enters the contact and determines its tribological properties. This effect is believed occur because at high viscosity ratios the droplets are nondeformable and are thus forced into the contact inlet region, where collisions occur that result in shear-induced coalescence. Once a pool of viscous fluid is formed, the lower viscosity bulk fluid is unable to displace it because the viscous shear stress is too small, so the pool acts as a reservoir to supply the contact.

Low Friction and High Load Support Capacity of Slider Bearing With a Mixed Slip Surface

Abstract: The classical Reynolds theory reveals that a converging gap is the first necessary condition to generate a hydrodynamic pressure in a viscous fluid film confined between two solid surfaces with a relative sliding/rolling motion. For hundreds of years, the classical lubrication mechanics has been based on the frame of the Reynolds theory with no slip assumption. Recent studies show that a large boundary slip occurs on an ultrahydrophobic surface, which results in a very small friction drag. Unfortunately, such a slip surface also produces a small hydrodynamic pressure in a fluid film between two solid surfaces. This paper studies the lubrication behavior of infinite width slider bearings involving a mixed slip surface (MSS). The results of the study indicate that any geometrical wedges (gaps), i.e., a convergent wedge, a parallel gap, and even a divergent wedge, can generate hydrodynamic pressure in an infinite slider bearing with a mixed slip surface. It is found that with an MSS, the maximum fluid load support capacity occurs at a slightly divergent wedge (roughly parallel sliding gap) for an infinite width slider bearing, but not at a converging gap as what the classical Reynolds theory predicts. Surface optimisation of a parallel sliding gap with a slip surface can double the hydrodynamic load support and reduce the friction drag by half of what the Reynolds theory predicts for an optimal wedge of a traditional slider bearing.

Influence of Wear on the Behavior of a Two-Lobe Hydrodynamic Journal Bearing Subjected to Numerous Startups and Stops

Abstract: The behavior of the hydrodynamic journal bearings is now very well known because of the many experimental and numerical studies that have been carried out on the topic. This interest in two-lobe journal bearings is due to the fact that their simplicity, efficiency and low cost have led them to being widely used in industry. These mechanical components tend to be subjected to numerous start-ups and stops. During transient periods, direct contact between the journal and bearing induces high friction in the lubricated contact and hence wear of the lining. The aim of this work is, firstly, to present experimental data obtained on a journal lobed bearing subjected to numerous starts and stops. Then, a comparison is made between the measured bearing performance and numerical results, these being obtained on the assumption that the regime is a thermohydrodynamic (THD) one. The wear after more than 2,000 cycles was measured and used to generate numerical simulations. The aim here was to compare experimental data with theoretical results. It was observed that hydrodynamic pressure increases, whereas the temperature at the film/bush interface slightly decreases on both the upper and lower lobes. These trends are confirmed by the numerical simulations, theoretical results being very close to experimental data. The final value for wear was measured, the maximum being found to be located at an angular coordinate of 180° and reaching nearly 9 μm. The present study demonstrates that, for the case studied, while the bearing behavior is clearly affected by wear, the bearing still remains useable and safe.Copyright © 2006 by ASME

Mixed Elastohydrodynamic Lubrication in a Partial Journal Bearing—Comparison Between Deterministic and Stochastic Models

Abstract: The analysis of the mixed lubrication phenomena in journal and axial bearings represents nowadays the next step towards a better understanding of these devices, subjected to more and more severe operating conditions. While the theoretical bases required for an in-depth analysis of the mixed-lubrication regime have long been established, only small-scale numerical modeling was possible due to computing power limitations. This led to the appearance of averaging models, thus making it possible to generalize the trends observed in very small contacts, and to include them in large-scale numerical analyses. Unfortunately, a lack of experimental or numerical validations of these averaging models is observed, so that their reliability remains to be demonstrated. This paper proposes a deterministic numerical solution for the hydrodynamic component of the mixed-lubrication problem. The model is applicable to small partial journal bearings, having a few centimeters in width and diameter. Reynolds' equation is solved on a very thin mesh, and pad deformation due to hydrodynamic pressure is taken into account. Deformation due to contact pressure is neglected, which limits the applicability of the model in those cases where extended contact is present. The results obtained with this deterministic model are compared to the stochastic solution proposed by Patir and Cheng, in both hydrodynamic and elastohydrodynamic regimes. The rough surfaces used in this study are numerically generated (Gaussian) and are either isotropic or oriented, having different correlation lengths. It is shown that the stochastic model of Patir and Cheng correctly anticipates the influence of roughness over the pressure field, for different types of roughness. However, when compared to the smooth surface solution, the correction introduced by this model only partially compensates for the differences observed with a deterministic analysis.

Bifurcation Analysis of a Flexible Rotor Supported by Two Fluid-Film Journal Bearings

Abstract: The effects of rotor stiffness on the bifurcation regions of a flexible rotor supported by two identical fluid-film journal bearings are presented. It is shown that the rotor stiffness has a pronounced influence on the bifurcation characteristics at the instability threshold speed. For short bearings, two bifurcation regions exist if the dimensionless rotor stiffness K¯⩾4.3. On the other hand, three bifurcation regions exist if the dimensionless rotor stiffness K¯<4.3. Information is presented that allows one to easily predict both the instability threshold speed and its bifurcation type of a rotor-bearing system with any specific set of operating parameters. The results presented have been verified by laboratory experiments as well as several published results in the open literature. Several examples are presented to illustrate the application of the results for design purposes.

The Validity of the Reynolds Equation in Modeling Hydrostatic Effects in Gas Lubricated Textured Parallel Surfaces

Abstract: Microdimples generated by laser surface texturing (LST) can be used to enhance performance in hydrostatic gas-lubricated tribological components with parallel surfaces. The pressure distribution and load carrying capacity for a single three-dimensional dimple, representing the LST, were obtained via two different methods of analysis: a numerical solution of the exact full Navier-Stokes equations, and an approximate solution of the much simpler Reynolds equation. Comparison between the two solution methods illustrates that, despite potential large differences in local pressures, the differences in load carrying capacity, for realistic geometrical and physical parameters, are small. Even at large clearances of 5% of the dimple diameter and pressure ratios of 2.5 the error in the load carrying capacity is only about 15%. Thus, for a wide range of practical clearances and pressures, the -Simpler approximate Reynolds equation can safely be applied to yield reasonable predictions for the load carrying capacity of dimpled surfaces.

TEHD Analysis of Thrust Bearings With PTFE-Faced Pads

Abstract: Results of a combined theoretical and experimental investigation into the operation of thrust bearings with polytetrafluoroethylene (PTFE)-faced pads are reported. Bearing performance is analyzed i ...