bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparative Evaluation of MoS2 and WS2 as Powder Lubricants in High Speed, Multi-Pad Journal Bearings
Authors: Higgs C. Fred , Heshmat Crystal A. , Heshmat Hooshang
Published: February 2008

As part of a program to develop solid/powder-lubricated journal bearings, a comparative evaluation has been performed to aid in determining whether MoS2 and WS2 powder are suitable lubricants for high-speed, extreme-environment multi-pad journal bearings. Plots of traction coefficients, friction, frictional power loss, and bearing pad temperature are presented as a means for comparing various powder lubricants. This paper primarily focuses on experiments carried out on a three-pad journal bearing and a disk-on-disk tribometer. Results showed that MoS2 traction curves resemble that of SAE 10 synthetic oil. Unlike liquid lubricants, powder films have a limiting shear strength property. Once the powder reaches this limiting value, the maximum traction coefficient is limited and the powder essentially shears along sliding walls. Experimental traction data shows evidence of this property in various powders. The thermal performance of the bearing was evaluated at speeds up to 30,000 rpm and loads up to 236 N. Although WS2 displayed constant friction coefficient and low temperature with increasing dimensionless load, MoS2 exhibited frictional behavior resembling that of a hydrodynamic lubricating film. In this paper, an attempt has been made to provide a criterion for the selection of solid lubricants for use in those tribosystems that may be operated in a high speed/load regime (i.e., high strain rates) as an alternative yard stick to conventional comparative approaches.
Solving Elastic Contact Between Rough Surfaces as an Unconstrained Strain Energy Minimization by Using CGM and FFT Techniques
Authors: Ai Xiaolan , Sawamiphakdi Krich
Published: February 2008

An accurate, robust, and efficient method for elastic contact of rough surfaces is presented. The method converts the contact problem into an unconstrained energy minimization problem. The equality load equilibrium constraint and the inequality complementary constraints are placed in the objective function and enforced simultaneously in search of a feasible solution. Conjugate gradient method is used in conjunction with FFT technique to solve the unconstrained energy minimization problem. To overcome periodical extension error associated with discrete Fourier transform when solving a nonperiodical contact problem, the solution is decomposed into two parts: a smooth part and a fluctuation part. The smooth part is solved analytically and the fluctuation part is solved by the proposed method. The net load or surface traction of the fluctuation part is set to zero to minimize the periodical extension error. Numerical examples demonstrate that the proposed method is accurate, robust, and efficient.
The Effect of Waviness on Vibrations Associated Witli Ball Bearings
Authors: Aktu¨rk Nizami
Published: February 2008

In this paper, the radial and axial vibrations of a rigid shaft supported by a pair of angular contact ball bearings is studied. The effect of bearing running surface waviness on the vibration of the shaft is investigated. A computer program was developed to simulate inner race, outer race, and rolling surface waviness with the results presented in time and frequency domains. Results obtained from the similation programme are quantatively in good aggrement with various authors’ experimental researches.
Sound of Linear Guideway Type Recirculating Linear Ball Bearings
Authors: Ohta Hiroyuki
Published: February 2008

The sound characteristics of linear guideway type (LGT) recirculating linear ball bearing driven at a constant linear velocity was studied. The measurement results of the sound characteristics showed that the LGT recirculating linear ball bearing has a sound property between line sound source and point sound source, and the measured overall sound pressure level increases with increasing the linear velocity of the carriage. The rate of increase of the overall sound pressure level to the linear velocity was analyzed combining the deduction of Yamada et al. and the results of previous studies on the collision sound. The results of this analysis match roughly the rate of increase of the measured overall sound pressure level to the linear velocity. We also observed the main peaks in the sound spectra of the LGT recirculating linear ball bearing itself. As a result, it was made clear that the main peaks are caused by the lower rolling natural vibration of the carriage, the pitching natural vibration of the carriage, and the vertical natural vibration of the carriage. In addition, the frequencies of the main peaks can be estimated by using presented expressions fRL, fP, and fv.
Transient EHL Solutions With Interfacial Slip
Authors: Ehret P. , Dowson D. , Taylor C. M.
Published: February 2008

Simulations are carried out of an experimental procedure in which a ball impacts a thin layer of lubricant on a disk to form a dimple of entrapped lubricant. Thereafter, pure sliding conditions are imposed to release the entrapped lubricant from the contact. Conditions of slip between the core of the lubricant and the wall are investigated to explain the slower release of the lubricant observed in pure sliding conditions when the ball alone is in motion. A slip value up to 63 percent at 1 GPa enables numerical simulations to match the time of release of the entrapped lubricant measured experimentally.
Three-Dimensional EHD Behavior of the Engine Block/Crankshaft Assembly for a Four Cylinder Inline Automotive Engine
Authors: Garnier Thierry , Bonneau Dominique , Grente Christian
Published: February 2008

Detailed formulations are presented for an EHD model that studies all the engine main bearings, considering the bearings, housing and crankshaft elastic deformations in connection with the hydrodynamic pressure fields as well as the quasi-static loads balance for this complex kinematic system. The calculation process uses the finite element method and the Newton-Raphson method for the numerical analysis. The elastic crankshaft behavior is quantified by a standard stiffness matrix. With two structural analysis steps, a condensed housing stiffness matrix is obtained to reduce the computing time. The transient evolution of the cavitation area is evaluated with Murty’s algorithm. This model is utilized to study a four cylinder inline engine.
A Theoretical Model of Micro-Pool Lubrication in Metal Forming
Authors: Lo Sy-Wei , Wilson William R. D.
Published: February 2008

A model of a secondary hydrodynamic lubrication mechanism, which is called micro-pool or micro-plasto hydrodynamic lubrication, has been developed. It shows that, with sufficiently high viscosity and sliding speed, the lubricant trapped in the micro-pools between the tool and workpiece can be drawn into the interface. The friction force is either increased or decreased, depending on the viscosity and sliding speed. Without bulk stretching, the product of the lubricant viscosity and sliding velocity can be used as an index to indicate whether or not micro-pool lubrication will occur. Stretching the workpiece may make a strong influence not only on the thickness of the permeating film but also on the asperity contact area.
Roughness Generated in Surface Grinding of Metals
Authors: Bobji M. S. , Venkatesh K. , Biswas S. K.
Published: February 2008

When a metal is surface ground the roughness generated is the summation of a function of the wheel roughness and the roughness due to wheel attrition and damage to the workpiece. We identify this function here as a maximum envelope profile, which is fractal within certain cut off wavelengths determined by the dressing conditions of the wheel. Estimating the global displacement of the binder-grit-workpiece system from the maximum envelope power spectra, we determine the plastic indentation of the workpiece at characteristic length scales using simple contact-mechanical calculation. The estimated roughness corresponds well with that recorded experimentally for hard steel, copper, titanium and aluminium.
New Hypothesis of Friction and Its Application to Design of a Railway Wheelset
Authors: Fridberg A. , Vinnik L.
Published: February 2008

A new hypothesis for friction forces between two elastic bodies is proposed. The hypothesis is based on contact mechanics problem. The study concentrates on the problem of a railway wheel rolling on rail under tractive effort and normal load. The effect of friction force in developing adhesion is considered. Based on the proposed hypothesis, new design of a railway wheelset has been developed and tested on Moscow Metro and tramcar.
Sliding Wear of Aluminum-Silicon Carbide Metal Matrix Composites
Authors: Constantin V. , Scheed L. , Masounave J.
Published: February 2008

The sliding wear of an aluminum matrix composite, reinforced with different volume fraction of particles, against a stainless-steel slider has been studied. In dry conditions, i.e., unlubricated tests, the pairs (slider and specimen), wear. When rubbing against an aluminum alloy (unreinforced), the slider does not wear but the aluminum alloy wears quickly by adhesion. In dry conditions, both slider and composite wear, but there is a minimum wear rate for this pair at a critical volume fraction of reinforcing particles. Under lubricated conditions, the situation changes dramatically. The composite no longer wears, but the slider wears very quickly. Under water, results are a compromise between the two previous situations, dry and lubricated. These results are explained by a simple, descriptive mechanism, which takes in account both the effect of the shear rate, due to the sliding action in the composite, and the abrasive effect of the particles. A general relationship, which describes the effect of the applied pressure and volume fraction of particles in the composite, is proposed.
Effects of Synchronous Vibration of Bearing on Stability of Externally Pressurized Air Journal Bearings
Authors: Lee Jeong-Bae , Kim Kyung-Woong
Published: February 2008

An active control of bearing is proposed as a new method to improve the stability characteristics of the externally pressurized air journal bearing and the results of numerical investigations of the stability characteristics of the actively controlled air journal bearing are presented. The synchronous control, where the bearing center whirls with the same frequency as the journal center with respect to the position in the steady state, is proposed as a control algorithm of the active air bearing. The step jump method is used to calculate the locus of the journal center which whirls or vibrates relative to the whirling bearing. The stability characteristics of the high speed rotor supported by actively controlled bearing system for zero steady-state eccentricity are investigated for various phase difference between the bearing and journal. It is shown that the stability of the air bearing can be greatly increased by the controlled synchronous motion of the bearing, and there is an optimum phase difference, which gives the maximum stability limit of the system.
Rotordynamic Evaluation of a Near-Tangential-lnjection Hybrid Bearing
Authors: Laurant Franck , Childs Dara W.
Published: February 2008

Given the inherent DN and assembly limitations of rolling-element bearings, research is underway to develop hybrid bearings (combining hydrostatic and hydrodynamic effects) for their replacement. Hybrid bearings develop cross-coupled stiffness coefficients due to fluid rotation, leading to predictions of onset speeds of instability and potential limitations in their range of application. Injecting fluid into a bearing recess against rotation, versus the customary radial injection, can reduce the circumferential flow and the cross-coupled-stiffness coefficients, and increase the margin of stability. Test results are presented here for a hybrid bearing with against-rotation injection. The bearing has a 76.4 mm diameter with LID = 1, and CrIR = 0.001. Data are presented for 55°C water at three speeds out to 25000 rpm and three pressures out to 7.0 MPa. Compared to a radial-injection hybrid bearing, experiments show injection against rotation enhances stability, yielding reductions of cross-coupled stiffness coefficients and whirl frequency ratios. However, increased flow rate and a drop of effective stiffness with increasing speed adversely affect the bearing performance. The prediction code developed by San Andres (1995) includes angled-orifice injection. The code correctly predicts trends, but at low speed, measured cross-coupled stiffness coefficients are positive, versus a prediction of larger negative values.
Oil Film Thickness in Engine Connecting-Rod Bearing With Consideration of Thermal Effects: Comparison Between Theory and Experiment
Authors: Cho Myung-Rae , Han Dong-Chul , Choi Jae-Kwon
Published: February 2008

The aim of this paper is to study the thermal ejfects on minimum oil film, thickness in a connecting-rod bearing. A comparison between the theoretical and experimental results of minimum oil film thickness in a connecting-rod bearing is presented. The transient energy equation is solved by adapting the adiahatic boundary condition for considering the thermal effects. The minimum oil film thicknesses are measured by the total capacitance method. The minimum oil film thickness over an engine cycle estimated with consideration of thermal effects is closer to that measures than that estimated by using the iso-viscous lubrication theory.
An EHL Analysis of an All-Metal Viscoelastic High-Pressure Seal
Authors: Xu H. P. , Wong P. L. , Zhang Z. M.
Published: February 2008

An EHL (elasto-hydrodynamic lubrication) analysis of an all-metal viscoelastic high-pressure seal is presented. The fluid flow is assumed to be laminar and isothermal, and its inertial effect is neglected. Deformation of the cylinder and plunger is governed by Lame’s formula for a thick-walled cylinder. The pressure-viscosity and pressure-density relationships of a working fluid are assumed to satisfy the Burus and Dowson-Higginson formulas, respectively. The leakage rate of the seal decreases almost exponentially with an increase in the working pressure, while its minimum film thickness can remain at about seventy percent of its nominal value at high working pressures. The radial stiffness increases significantly with an increase in the working pressure, i.e., stable operations of the seal can be expected.
Fractal Analysis of Hard Disk Surface Roughness and Correlation With Static and Low-Speed Friction
Authors: Kennedy F. E. , Brown C. A. , Kolodny J. , Sheldon B. M.
Published: February 2008

The purpose of this work was to use new scale-sensitive fractal parameters to characterize the surface topography of computer hard disks and to correlate the fractal parameters with measured start-up friction (or stiction) behavior of textured hard disks. The topographies of glass disks with three different surface textures and aluminum disks with two different textures were measured using an atomic force microscope. Two parameters derived from area-scale fractal analysis by the patchwork method were chosen to characterize the surface topography: smooth-rough crossover and area-scale fractal complexity. The two fractal parameters were found to be characteristic of the disks’ texture; disks which had been processed in the same batch had similar values of complexity and smooth-rough crossover, but those values differed for disks subjected to a different texturing process. Friction behavior of unlubricated glass disks and lubricated aluminum disks was determined at startup and during low speed drag testing. The results show a high correlation between the smooth-rough crossover parameter and start-up friction (stiction) for both lubricated and unlubricated disks. Additional tests showed that the area-scale fractal parameter is related to low speed sliding friction. The friction results for the unlubricated glass disks were dependent on humidity and roughness in a manner that is consistent with a meniscus model of stiction.
Thermal Elastohydrodynamic Lubrication of Heavily Loaded Line Contacts—An Efficient Inlet Zone Analysis
Authors: Ghosh Mihir K. , Pandey Raj K.
Published: February 2008

An inlet zone analysis of TEHD lubrication of heavily loaded line contacts has been done using a computationally efficient and accurate numerical method based on Lobatto quadrature developed by Elrod and Brewe (1986). The results under extremely heavy conditions of dimensionless load W = 5.2*10−4 (pH = 2.0 GPa) and dimensionless rolling velocity U = 2.0*10−10(50 m/s) are presented. Significant reduction in thermal reduction factor (film thickness) at high rolling speeds relative to isothermal conditions have been observed. The results of the present work have been compared with the results of Wilson and Sheu (1983) and Hsu and Lee (1994). A correction formula of the thermal reduction factor for the minimum film thickness has been derived for a range of thermal loading parameters, loads, and slip ratios.
Film Thickness in Starved EHL Point Contacts
Authors: Chevalier F. , Lubrecht A. A. , Cann P. M. E. , Colin F. , Dalmaz G.
Published: February 2008

This paper presents a numerical study of the effects of inlet supply starvation on film thickness in EHL point contacts. Generally this problem is treated using the position of the inlet meniscus as the governing parameter; however, it is difficult to measure this in real applications. Thus, in this paper an alternative approach is adopted whereby the amount of oil present on the surfaces is used to define the degree of starvation. It is this property which determines both meniscus position and film thickness reduction. The effect of subsequent overrollings on film thickness decay can also be evaluated. In the simplest case a constant lubricant inlet film thickness in the Y direction is assumed and the film thickness distribution is computed as a function of the oil available. This yields an equation predicting the film thickness reduction, with respect to the fully flooded value, from the amount of lubricant initially available on the surface, as a function of the number of overrollings n. However, the constant inlet film thickness does not give a realistic description of starvation for all conditions. Some experimental studies show that the combination of side flow and replenishment action can generate large differences in local oil supply and that the side reservoirs play an important role in this replenishment mechanism. Thus the contact centre can be fully starved whilst the contact sides remain well lubricated. In these cases, a complete analysis with a realistic inlet distribution has been carried out and the numerical results agree well with experimental findings.
Analysis of Load Distributions Within Solid and Hollow Roller Bearings
Authors: Zhao Hua
Published: February 2008

The virtual contact loading method is further developed in this paper tostudy the multi-body contact phenomena within solid and hollow roller bearings. The distributive virtual contact loads are used to simulate the contact constraint forces on each contact surface. The deepest penetration criterion is proposed to select and determine the contact nodes in an orderly way, so as to ensure the local iteration process converged and to increase the computational efficiency. The load distributions within the solid and hollow roller bearings have been studied in more detail and some reasonable results are obtained.
Effect of Rate of Viscosity Variation on the Performance of Journal Bearings
Authors: Vijayaraghavan D. , Brewe D. E.
Published: February 2008

Variations in viscosity within the film is known to have a pronounced effect on journal bearing performance. Lubricants of the same grade with the same viscosity at a particular temperature and ambient pressure can have different viscosity values at other temperatures and pressures. The effect of rate of viscosity variation due to the changes in temperature and pressure on the performance of an axial grooved journal bearing is the focus of this investigation. Roelands’ model on viscosity-temperature-pressure relationship of lubricants is adopted. Cavitation modelling with regard to thermal effects and mass conservation are carefully considered. A total THD model with heat transfer within the fluid film and heat dissipation to the ambient through the journal and bush, is utilized. For different rates of viscosity variation with respect to temperature and pressure, comparison of performance parameters and typical temperature distributions are presented.
A Study of Subambient Pressure Tri-Pad Sliders Using Acoustic Emission
Authors: Khurshudov A. G. , Talke F. E.
Published: February 2008

Acoustic emission is used to study the contact behavior of subambient pressure tri-pad sliders during start/stop and constant speed operation. The contact force at the slider/disk interface is determined as a function of velocity and the dependence of contact force on flying height is investigated. The results indicate that contact forces for typical subambient pressure tri-pad sliders are on the order of a few mN.
Cutting Force Variation Due to Wear of Multi-Layer Ceramic Coated Tools
Authors: Cho S.-S. , Komvopoulos K.
Published: February 2008

Cutting forces were measured during turning AISI 4340 steel with uncoated, two-layer coated (TiC/Al2O3), and three-layer coated (TiC/Al2O3/TiN) cemented WC-Co inserts possessing chip-breaker grooves in order to investigate the progression of tool wear. The variations of cutting force signals are interpreted in light of predominant wear mechanisms. A characteristic feature of wearing coated tools is shown to be the rapid increase of the radial and axial cutting forces when the tool life is reached. This behavior is associated with both the increase of the real contact area on the nose and the flank face due to coating delamination and the relatively less severe wear on the rake face. Normalizing the cutting forces by those of unworn tools yields a single curve for each cutting force. The instant at which either the radial or the axial force deviates from these curves determines the wear life of coated tools. The good agreement between results for the wear life of coated tools obtained from cutting force and nose wear measurements suggests that the wear life of multilayer ceramic coated tools can be accurately determined by monitoring the cutting force response.
Thermohydrodynamic Seizure: Experimental and Theoretical Analysis
Authors: Jang J. Y. , Khonsari M. M. , Pascovici M. D.
Published: February 2008

An experimental apparatus was designed and tested to study the thermally induced seizure in bearing. The setup consists of a simple, unloaded journal bearing configuration which lends itself to useful physical interpretation without the complexities that are present in the control of flow rates and eccentricity in a loaded journal bearing in which the clearance would vary with time. The motor was fitted with a current limiter which stopped the motor when the torque exceeded a certain limit. Experiments revealed that with this particular system, the journal speed undergoes a significant reduction with time until the operation is halted by the current limiter, which signifies the occurrence of a seizure. The time of seizure is appreciably influenced by this behavior. A parallel theoretical analysis, which takes into account the speed variation with time, was developed. The analysis includes the derivation of the appropriate governing equations which involve the transient analysis of flow velocity, heat transfer, and thermomechanical expansion of the surfaces, together with the numerical solution. The results of the simulations compare favorably to those obtained experimentally both in trend and magnitude. Finally, general behavior of the system in terms of its time-to-seizure characteristic is illustrated through a series of parametric studies.
Stability Analysis of Mechanical Seals With Two Flexibly Mounted Rotors
Authors: Wileman J. , Green I.
Published: February 2008

Dynamic stability is investigated for a mechanical seal configuration in which both seal elements are flexibly mounted to independently rotating shafts. The analysis is applicable to systems with both counterrotating and corotating shafts. The fluid film effects are modeled using rotor dynamic coefficients, and the equations of motion are presented including the dynamic properties of the flexible support. A closed-form solution for the stability criteria is presented for the simplifled case in which the support damping is neglected. A method is presented for obtaining the stability threshold of the general case, including support damping. This method allows instant determination of the stability threshold for a fully-defined seal design. A parametric study of an example seal is presented to illustrate the method and to examine the effects of various parameters in the seal design upon the stability threshold. The fluid film properties in the example seal are shown to affect stability much more than the support properties. Rotors having the form of short disks are shown to benefit from gyroscopic effects which give them a larger range of stable operating speeds than long rotors. For seals with one long rotor, counterrotating operation is shown to be superior because the increased fluid stiffness transfers restoring moments from the short rotor to the long.
Simulation of the Time-Dependent Wear and Surface Accumulation Behavior of Particle-Filled Polymer Composites
Authors: Blanchet Thierry A. , Han Sung Won
Published: February 2008

A simulation has been developed to model the transient wear of particle-filled polymer composites as a function of sliding distance. All inputs are parameters of physical significance, including filler bulk volume fraction, specific wear rate (relative to that of the matrix), and contact pressure. Run-in wear behavior is simulated by consideration of the accumulation of wear-resistant filler particles and the formation of a volume fraction profile near the composite sliding surface, facilitated by matrix cold flow. Simulation outputs include time-dependent volume fraction profile, and composite wear rate and wear volume. The simulation may be used for evaluation of candidate materials for applications in which nonsteady-state run-in wear effects are important, as well as a guide for the engineering of composite surfaces with graded volume fraction profiles that may provide resistance to initial transient wear contributions.
Effect of Three-Dimensional Random Surface Roughness on Fatigue Life of a Lubricated Contact
Authors: Ai Xiaolan
Published: February 2008

A means of evaluating the surface roughness effect on contact fatigue life has been proposed. To account for stress variations caused by random surface roughness, an effective stress concept based on damage accumulation theory was employed. A point EHL analysis along with a comprehensive interior stress analysis has been performed to obtain the effective stress field under lubricated conditions. Numerical simulations were performed for surfaces produced by different finishing processes. Results show that surface roughness can cause significant stress variations in the near-surface. As a result, the effective stress at the near-surface is increased. The increased effective stress is responsible for the life reduction of the contact. Life reduction factors for contact surfaces with different finishing processes were compared.
Experimental Study on Static and Dynamic Characteristics of Annular Plain Seals With Porous Materials
Authors: Kaneko S. , Kamei H. , Yanagisawa Y. , Kawahara H.
Published: February 2008

Static and dynamic characteristics are experimentally investigated for annular plain seals with porous materials applied to the seal surface by insertion into the middle of the seal. Experimental results show that annular plain seals with porous materials have a higher leakage flow rate, larger main stiffness coefficients, and smaller cross-coupled stiffness coefficients and main damping coefficients than conventional annular plain seals with solid surfaces. In the porous seals, an increase of approximately 30 percent in the leakage flow rate and reduction of the same amount in the main damping coefficients are obtained, whereas the main stiffness coefficients for the porous seals are four to six times as much as those for the solid seals due to the increase in the hydrostatic force induced by a function of the hydrostatic porous bearing. This suggests that the quantitative effects of the porous materials on the main stiffness coefficients are much more significant than the effects on the leakage flow rate and the other dynamic coefficients. The larger main stiffness coefficients for the porous seals yield larger radial reaction force for a small concentric whirling motion, which would contribute to rotor stability from the viewpoint of increasing speed limits due to a stiffer rotor support.
Elastohydrodynamic Film Forming With Shear Thinning Liquids
Authors: Bair Scott
Published: February 2008

Recent advances in high pressure rheometry have elucidated the shear response of liquid lubricants at the high shear stress characteristic of the traction generating region of lubricated concentrated contacts. These new measurement techniques are used to characterize the shear response of shear thinning liquids at low (<10 MPa) shear stress. A recently developed numerical scheme for calculating film thickness is extended to accommodate sliding. Film thickness predictions are compared with measurements using shear thinning liquids including a polymer/mineral oil blend, a highly elastic liquid, and synthetic base oils. Useful insights are provided concerning the effects of pressure-viscosity behavior for Newtonian liquids, sliding, and starvation for non-Newtonian liquids and the relevant shear stress for film forming.
A Thermohydrodynamic Lubrication Analysis to Design Large Two-Pad Journal Bearing With Cooling Ditches
Authors: Taniguchi S. , Makino T. , Ozawa Y. , Ichimura T.
Published: February 2008

A newly developed large two-pad journal bearing with cooling ditches and viscous pump for turbines of electric utilities was analyzed theoretically. The turbulent thermohydrodynamic lubrication analysis was done in connection with the flow analysis of oil circulating system through the cooling ditches and the viscous pump. The complicated simultaneous equations concerned with the oil flow in the bearing were solved and used to design a bearing of 535 mm diameter which will make its appearance in a near future. Full size bearing tests were carried out to check the bearing performance and viscous pump performance and to confirm the accuracy of the calculation method. Good agreement was obtained between the theoretical and experimental results.
Finite Element Analysis of Herringbone Groove Journal Bearings: A Parametric Study
Authors: Zirkelback Nicole , San Andre´s Luis
Published: February 2008

Currently, the herringbone groove journal bearing (HGJB) has important applications in miniature rotating machines such as those found in the computer information storage industry. Grooves scribed on either the rotating or stationary member of the bearing pump the lubricating fluid inward thus generating support stiffness and improving its dynamic stability when operating concentrically. The narrow groove theory (NGT), traditionally adopted to model the concentric operation of these bearings, is limited to bearings with a large number of grooves. A finite element analysis is introduced for prediction of the static and rotordynamic forced response in HGJBs with finite numbers of grooves. Results from this analysis are then compared to available experimental data as well as to estimates from the NGT. A bearing geometry parametric study is then conducted to determine optimum rotordynamic force coefficients. A discussion on the temporal variation of the bearing reaction forces and force coefficients for a rotating journal with a small number of grooves is also presented. These changes can be significant at high operating eccentricities, possibly inducing a parametric excitation in rotating systems employing this type of bearing.
The Role of Interstitial Fluid Pressurization and Surface Porosities on the Boundary Friction of Articular Cartilage
Authors: Ateshian G. A. , Wang Huiqun , Lai W. M.
Published: February 2008

Articular cartilage is the remarkable bearing material of diarthrodial joints. Experimental measurements of its friction coefficient under various configurations have demonstrated that it is load-dependent, velocity-dependent, and time-dependent, and it can vary from values as low as 0.002 to as high as 0.3 or greater. Yet, many studies have suggested that these frictional properties are not dependent upon the viscosity of synovial fluid. In this paper, a theoretical formulation of a boundary friction model for articular cartilage is described and verified directly against experimental results in the configuration of confined compression stress-relaxation. The mathematical formulation of the friction model can potentially explain many of the experimentally observed frictional responses in relation to the pressurization of the interstitial fluid inside cartilage during joint loading, and the equilibrium friction coefficient which prevails in the absence of such pressurization. In this proposed model, it is also hypothesized that surface porosities play a role in the regulation of the frictional response of cartilage. The good agreement between theoretical predictions and experimental results of this study provide support for the proposed boundary friction formulation.
Effects of Film Temperature on Piston-Ring Lubrication for Refrigeration Compressors Considering Surface Roughness
Authors: Nakai H. , Ino N. , Hashimoto H.
Published: February 2008

This paper describes a theoretical model for piston-ring lubrication considering the combined effects of surface roughness and oil film temperature variation for refrigeration compressors. In the model, the piston-ring is treated as a one-dimensional dynamically loaded bearing with combined sliding and squeezing motion. The one-dimensional modified Reynolds equation, based on the average flow model by Patir and Cheng, is used to determine the pressure distribution, and the one-dimensional energy equation, considering the heat generated due to contact of asperities, is applied to calculate the oil film temperature distribution. In the analysis of the modified Reynolds equation, the flooded condition and Reynolds condition are employed at the leading edge and trailing edge of piston-ring, respectively. On the other hand, in the analysis of the modified energy equation, a constant temperature equivalent to the cylinder wall temperature is assumed at the leading edge. From numerical results of the minimum film thickness, pressure and temperature distributions and friction force, the combined effects of surface roughness and oil film temperature variation on these lubrication characteristics are clarified.
Solution of the Rarefied Gas Lubrication Equation Using an Additive Correction Based Multigrid Control Volume Method
Authors: Hu Yong , Bogy David B.
Published: February 2008

The ever shortening product cycle for magnetic recording disk drives demands a fast and accurate numerical prediction of the slider’s flying characteristics during the design stage. A computationally efficient multigrid control volume method is developed in this paper for the solution of the very high bearing number and shaped rail air bearing problems. The control volume schemes for discretizing the Reynolds lubrication equation are based on various convection-diffusion formulations, while the solution of the resulting discretization equations is accelerated by an additive correction based multigrid method. A comparison study using the 50 percent tripad and Headway AAB sliders demonstrates a significant improvement in the solver’s performance as compared to the single-grid method.
Dynamic Instabilities in the Sliding of Two Layered Elastic Half-Spaces
Authors: Adams G. G.
Published: February 2008

Two flat layered elastic half-spaces, of different material properties, are pressed together and slide against each other with a constant coefficient of friction. Although a nominally steady-state solution exists, an analysis of the dynamic motion yields complex eigenvalues with positive real parts, i.e., a flutter instability. These results demonstrate that self-excited (unstable) motion occurs for a wide range of material combinations. The physical mechanism responsible for this instability is that of slip-wave destabilization. The influence of the properties of the layers on the destabilization of sliding motion is investigated. These dynamic instabilities lead either to regions of stick-slip or to areas of loss-of-contact. Finally the dynamic stresses at the interfaces between the layers and the semi-infinite bodies are determined and compared to the nominally steady-state stresses. These dynamic stresses are expected to play an important role in delamination.
Static Friction of Contacting Real Surfaces in the Presence of Sub-Boundary Lubrication
Authors: Polycarpou A. A. , Etsion Izhak
Published: February 2008

A model for calculating the static friction coefficient of contacting real (rough) surfaces in the presence of very thin liquid films (sub-boundary lubrication) is developed. The liquid has a very high affinity for the surfaces and its thickness is of the order of the surface roughness average. An extension of the Greenwood and Williamson (GW) asperity model and an improved Derjaguin, Muller and Toporov (DMT) adhesion model are utilized for calculating the contact and adhesion forces, respectively. The effects of the liquid film thickness and the surface topography on the static friction coefficient are investigated. A critical film thickness is found above which the friction coefficient increases sharply. The critical thickness depends on the surface roughness and the external normal load. This phenomenon is more profound for very smooth surfaces and small normal loads, in agreement with published experimental work on magnetic hard disk interfaces.
Temperature Rise Simulation of Three-Dimensional Rough Surfaces in Mixed Lubricated Contact
Authors: Qiu Liangheng , Cheng Herbert S.
Published: February 2008

A numerical simulation of the temperature rise for a three-dimensional rough surface sliding against a smooth surface in mixed lubricated contact has been developed. The effects of lubricant film friction and solid asperity friction are considered in the simulation. The moving grid method, which greatly reduces the required computer memory size and computing time, is used to solve the coefficient matrix of temperature equations. The time-dependent surface temperature rise at very small subregions is obtained. Different friction coefficients for lubricant shearing, surface film shearing and dry solid asperity contact are used to simulate the change of frictional heat in mixed lubricated contact. A critical temperature criterion is used to determine whether the friction coefficient is controlled by lubricant film, surface film, or dry solid asperity contact. Solutions for different contact conditions are presented for verification of the present simulation
Sliding Friction and Heat Generation in Pressurized U-Cup Seals Under Low-Speed Reciprocating Conditions
Authors: Ovaert T. C. , Badger J. A.
Published: February 2008

A test apparatus was designed and constructed to investigate the friction and heat generation characteristics of sliding polymeric U-cup seals used in high-pressure nitrogen gas springs. Tests were run at three speeds; and friction and temperature data were obtained. Seal specimens were examined under a scanning electron microscope before and after each test. Evidence of degradation and thermal softening of the seal material was found for the highest relative speed tests where elevated temperatures were measured. In addition, a semi-empirical transient finite-difference thermal analysis of the rod and the high-pressure chamber in the test apparatus was also developed. Numerical simulations of the three tests were performed using measured friction force data, and temperature predictions from the model were compared with those measured at different locations in the test apparatus. Predicted values exhibited reasonable agreement with the measured values, particularly under steady-state conditions. The variations observed for certain transient conditions appeared to be due to overestimated convective heat transfer coefficients at lower speeds, and due to the transient nature of thermal resistances between the seal cartridge and the high-pressure chamber, which may vary with a change in temperature.
On the Maximum Allowable Loads in the Thermo-Elastohydrodynamic Lubrication
Authors: Rodkiewicz C. M. , Huang P.
Published: February 2008

Thermo-elastohydrodynamic lubrication is considered. A numerical procedure for obtaining the maximum allowable load is presented. The maximum allowable load is defined as the load which, for a given set of parameters, when exceeded will cause either change of the lubrication character, or collapse of the fluid film if the film thickness was large relative to the surface roughness. The procedure is illustrated by applying it to the infinitely wide thrust bearings lubricated by the Newtonian or power-law fluid.
Nanotribological Properties of Organic Boundary Lubricants: Langmuir Films Versus Self-Assembled Monolayers
Authors: Bliznyuk Valery N. , Everson Mark P. , Tsukruk Vladimir V.
Published: February 2008

Frictional characteristics of several types of boundary lubricants were tested using scanning probe microscopy (SPM). These include Langmuir monolayers of stearic acids (STA), their cadmium salts (STCd), self-assembling monolayers (SAMs) of alkylchlorsilanes, and complexes of STA with rigid naphthoylene benzimidazole (x-NBI) fragments. We observed that a Langmuir monolayer deposited on a silicon surface had a very low friction coefficient against a silicon nitride tip (about 0.01–0.05) but also low mechanical stability. SAMs were found to be much more stable but had the drawback of growth in the friction coefficient at high sliding velocities. Composite NBI/STA monolayers were much more stable and were not damaged by the highest normal load applied. The frictional behavior of different monolayers was analyzed in relation to their structural organization (the type of tethering to the surface and packing density). We introduced a figure of merit (FOM) parameter which allowed comparison of frictional properties of very different lubricant materials to those of the supporting substrate. For Langmuir monolayers the FOM increased strongly with surface packing density whereas for SAMs and x-NBI/STA complexes it possessed a maximum at surface densities in the range 3.5–4.5 molecules per nm2. Because of the possibility of tailoring the surface packing density of aliphatic tails in the complexes, they are a promising alternative to both LB films and SAMs. For such composite monolayers, the surface packing density can be optimized to give a desired frictional behavior.
An Investigation of New Tribological Variables in Friction Modeling of Bulk Metal Forming
Authors: Lo Sy-Wei , Horng Tzu-Chern
Published: February 2008

The present “realistic” friction model is examined by a series of upsetting tests. Specimens made of different aluminum alloys and prepared by various processes display diverse results. Severe surface roughening takes place in some cases using A6061T4 bar manufactured by hot extrusion. It results in a lubrication condition which is very different from the theoretical prediction. From hardness tests and microstructure observations, it is found that there is no obvious correlation between the hardness and the roughening process for A6061T4 alloy. The severe roughening is attributed to the distortion of the coarse grains recrystallized during the preprocess, that is, the hot extrusion. This is rare in sheet metal forming where most workpieces are prepared from cold rolled sheet metals. The orange peel appearance not only changes the lubrication mechanics, but also makes the surface finish of the product unacceptable. Since the existing friction model fails to predict such phenomena, surface roughening and all its related factors must be considered as new, important tribological variables in the friction modeling of bulk metal forming.
Flash Temperature Distributions at the Head-Disk Interface in Hard Disk Drives
Authors: Leo H-L. , Sinclair G. B.
Published: February 2008

The flash temperatures which can occur at a head-disk interface are simulated with a model which permits the two contacting surfaces to be of comparable roughness and recognizes the thermally distinct overcoat on the disk. The model is fairly readily implemented, as is demonstrated by using it to generate some representative statistics for the heat partition and temperature distribution at an interface.
The Effect of Web Permeability and Side Leakage on the Air Film Height Between a Roller and Web
Authors: Ducotey K. S. , Good J. K.
Published: February 2008

Air entrained between a web and roller can cause a loss in traction that can affect web quality. The entrained air causes an air layer to form which separates the web from the roller. Insufficient traction exists at this point and an idler roller will be unable to be driven by the web. Other applications, however, such as newsprint moving around a turnbar, require complete clearance. An equation for predicting the air film height between a permeable web and roller was developed using foil bearing theory. The separation distance (h) between the roller and web is a function of the roller radius (R), web tension (T), air viscosity (η), summation of the web and roller velocities (U), and the web permeability (α). The air film height was found to decrease linearly around the circumference (θ) of the roller in the constant pressure region. Therefore, the air film height can be expressed simply as, h=0.643R[(6ηU)/T]2/3−2[(αT)/U]θ The slope of the air film height is a function of the web/roller velocity, web tension, and the permeability of the paper. A correction factor for side leakage was also incorporated into the result.
Tilting-Pad Journal Bearings With Electronic Radial Oil Injection
Authors: Santos I. F. , Russo F. H.
Published: February 2008

This paper gives a theoretical treatment of the problem of journal bearings modeling connected to electronic oil injection into the bearing gap. The feasibility of influencing the static behavior of hydrodynamic forces by means of such oil injection is investigated. The lubricant is injected into the bearing gap by two mechanisms of lubrication: the conventional hydrodynamic lubrication and through orifices distributed along the bearing surface (active lubrication in the radial direction). By controlling the pressure of the oil injection, it is possible to get large variations in the active hydrodynamic forces; such effects could be useful for reducing vibrations in rotating machines.
A Nonlinear Dynamic Model With Confidence Bounds for Hydrodynamic Bearings
Authors: Chu Chin S. , Wood Kristin L. , Busch-Vishniac Ilene J.
Published: February 2008

In conventional rotordynamic modeling, hydrodynamic bearings are often characterized by a set of linear stiffness and damping coefficients obtained from a first-order Taylor series expansion of bearing reactions. Theoretically, these coefficients are only valid for small amplitude motion about an equilibrium position. In this paper, a nonlinear dynamic model that overcomes the small amplitude assumption in the conventional linear analysis is described. By including higher-order terms in the bearing reaction expansion, nonlinearity in the oil film forces for large amplitude motion can be captured and represented by a set of nonlinear stiffness and damping coefficients. These coefficients are functions of static bearing displacement. A finite difference approach is described and is used to solve for these coefficients. The stated model is applied to a conventional slider bearing and a mechanical smart slider bearing that experiences large variations in load. Error assessment is performed numerically on the higher-order solutions to determine an acceptable displacement bound for the higher order coefficients.