bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of Frequency on the Fretting Conditions in a Contact Between PMMA and a Rigid Counterface
Authors: Krichen A. , Kharrat M. , Chateauminois A.
Published: February 2008

The effects of frequency on the fretting conditions in a glass/PMMA contact have been investigated using experimental analysis and numerical simulations. For partial slip conditions, the changes in the shape of the fretting cycles giving the tangential load as a function of the imposed displacement have been interpreted on the basis of changes in the dynamic modulus of the PMMA as a function of frequency. Using the numerical simulations, the values of the PMMA’s modulus at the various frequencies were determined from the analysis of the fretting cycles. For gross slip conditions, the emphasis was placed on the determination of the range of velocity associated with stick-slip processes. This information has been summarized in a fretting map giving the initial fretting condition as a function of normal load, displacement amplitude, and frequency.
Surface Roughness Effects in Hydrodynamic Lubrication Involving the Mixture of Two Fluids
Authors: Li Wang-Long
Published: February 2008

In this paper, the relations expressing the effects of surface roughness and homogeneous mixture (the mixture of power-law fluid inserted into a Newtonian fluid) on the roughness-induced flow factors are derived. A coordinate transformation is utilized to simplify the derivation. By using the perturbation approach incorporated with Green function technique, the flow factors and shear stress factors are derived and expressed as functions of the volume fraction of the power-law fluid in the mixture (υ p), the viscosity ratio of the power-law fluid to that of the Newtonian fluid (N or μp*), the flow behavior index of the power-law fluid (η), the Peklenik numbers (γi) and the standard deviations (σl) of each surface. A form of the average Reynolds equation is then obtained. It is shown that a number of currently available models are special cases of the theory presented here. Finally, the performance of a journal bearing is discussed.
Vibration Analysis of Statically Indeterminate Rotors With Hydrodynamic Bearings
Authors: Feng N. S. , Hahn E. J.
Published: February 2008

In statically indeterminate rotor bearings systems, where the rotor is supported by one or more hydrodynamic bearings, the reactions at each hydrodynamic bearing, and hence its stiffness and damping properties depend not only on the bearing type, the operating conditions and the bearing dimensions but also on the relative lateral alignment between the journal and the bearing housing; the alignment, therefore, has a significant influence on the system stability and unbalance response. Additional complications arise if nonsymmetric bearing types such as elliptic or tilting pad bearings are present. An iterative procedure is outlined which enables the bearing reactions to be determined at any speed, thereby enabling even large systems such as turbomachinery to be rapidly analyzed in conjunction with existing linear rotor bearing vibration analysis software. Sample numerical examples show how misalignment and bearing type can affect the natural frequencies, the stability threshold, and the unbalance response of such statically indeterminate systems.
Fatigue Prediction of Thin Hard Coatings on the Steel Races of Hybrid Bearings Used in High Speed Machine Tool Spindles
Authors: Bouzakis K.-D. , Vidakis N. , Mitsi S.
Published: February 2008

The rotational speed requirements of high speed spindles led to the development of angular contact hybrid bearings with ceramic balls and PVD coated steel races. The present paper describes the determination and verification of critical coating fatigue stresses as well as their application in coating fatigue calculations of hybrid bearing steel races. The fatigue limits of low temperature deposited PVD coatings were determined by the application of the impact test and its FEM simulation and validated through their successful application to the prediction of coating life in rolling contact fatigue tests of coated specimens. Furthermore, a computer program that performs the quasi-static simulation of bearing operation yields the necessary kinematic and dynamic parameters for a FEM simulation of the stress field occurring in coated rings. For the investigated bearings, an adequate fatigue performance of their coated races was computationally exhibited. The PVD coated hybrid bearings illustrated the predicted behavior in long duration tests, conducted in full scale test rigs.
Equilibrium Analysis of Finite Width Tension Dominated Foil Bearings
Authors: Lakshmikumaran A. V. , Wickert J. A.
Published: February 2008

A foil bearing is formed when a flexible material passes over a stationary rigid surface and entrains a thin layer of air that lubricates the relative motion. Some applications include the transport of magnetic tape, and the manufacturing of paper and metal sheets. In each case, the transverse displacement of the moving medium couples with the air film’s pressure, requiring simultaneous solution of the governing elastic and lubrication equations. Such simulations can become computationally intensive, particularly when the model incorporates the finite width effects of side flow and cross-web deformation. Of primary focus in this paper is the development of an efficient numerical algorithm for such simulations. Two improvements in that regard are discussed: inexact Newton iteration and adaptive nodal point placement. In two case studies adapted from the literature, implementation of these techniques offered roughly five-fold improvements in computational effort when the equilibrium film thickness, air pressure, and web displacement were calculated. The key advantages of the approach are a more rapid convergence rate, and the opportunity to use fewer degrees of freedom in describing the pressure and film thickness since the nodes are assigned automatically during iteration to regions where the solution is most rapidly changing.
The Study of Three-Dimensional Analysis Techniques and Automatic Classification Systems for Wear Particles
Authors: Peng Z. , Kirk T. B.
Published: February 2008

Although the morphology of wear debris generated in a machine has a direct relationship to wear processes and machine condition, studying wear particles for machine condition monitoring has not been widely applied in Industry as it is time consuming and requires certain expertise of analysts. To overcome these obstacles, automatic wear particle analysis and identification systems need to be developed. In this paper, laser scanning confocal microscopy has been used to obtain three-dimensional images of metallic wear particles. An analysis system has been developed and applied to study the boundary morphology and surface topography of the wear debris. After conducting the image analysis procedure and selecting critical criteria from dozens of available parameters, neural networks and grey systems have been investigated to classify unknown patterns using the numerical descriptors. It is demonstrated that the combination of the image analysis system and automatic classification systems has provided an automatic package for wear particle study which may be applied to industrial applications in the future.
Application of the Response Surface Method to the Tribological Analysis of a Medium-Carbon Steel Under Mild-Oxidational Wear
Authors: Chou Chau Chang , Lin Jen Fin
Published: February 2008

Instead of using the conventional oxidation theory to depict a disk’s wear rate as a function of contact temperature, the response surface method (RSM) is herein introduced to relieve the one-factor-at-a-time defect in portraying tribological characteristics. By means of a central composite design technique, fewer operating conditions are needed to establish expressions for the wear rate parameter, the contact temperature and the friction coefficient as a function of sliding speed and applied load. A second degree polynomial was used to represent a curved surface which fits the experimental data. In addition to results for the designated operating conditions, wear rate parameters and contact temperatures obtained from the polynomials were compared with the experimental results. The activation energy in the wear rate expression can be derived from a function of sliding speed, applied load, and contact temperature. The experimental data for the wear rate parameter can be described by smooth curves, instead of two different straight lines in two temperature divisions.
Thermal Elastohydrodynamic Model of a Radial Lip Seal—Part I: Analysis and Base Results
Authors: Day Kevin , Salant Richard F.
Published: February 2008

A numerical thermal elastohydrodynamic model of a radial lip seal, with a flooded air side, has been constructed. The shaft surface is modeled as perfectly smooth, while the lip microgeometry is modeled as a uniform distribution of asperities with initially circular cross-sections. The asperities can deform circumferentially as the bulk lip material shears. Both the viscosity of the fluid and the elastic modulus of the lip are temperature dependent. Model predictions include the pressure distribution in the lubricating film under the lip, the film thickness distribution, the cavitation distribution, the pumping rate, and the lip temperature distribution.
Solving EHL Problems Using Iterative, Multigrid, and Homotopy Methods
Authors: Nurgat Elyas , Berzins Martin , Scales Laurence
Published: February 2008

The numerical solution of ElastoHydrodynamic Lubrication (EHL) point contact problems requires the solution of highly nonlinear systems of equations which pose a formidable computational challenge. Multigrid methods provide one efficient approach. EHL problems solved using a single grid and multigrid will be compared and contrasted with a homotopy method which works on the concept of deforming one problem into another by the continuous variation of a single parameter. Both the multigrid and the single grid method employ a new relaxation scheme. Numerical results on demanding test problems will be used to compare these methods and suggestions for future developments to produce robust solvers will be made.
Dependence of the Coefficient of Friction on the Sliding Conditions in the High Velocity Range
Authors: Molinari A. , Estrin Y. , Mercier S.
Published: February 2008

The velocity, normal pressure, and slider size dependence of the coefficient of dry friction of metals in the range of high sliding velocities (V ≥ 1 m/s) is investigated theoretically. Failure of the adhesive junctions by adiabatic shear banding is considered as the underlying process. The concept of asperity shearing by the adiabatic shear banding mechanism represents a new approach to unlubricated high velocity friction. Analytical solutions of a coupled thermomechanical problem are given for two constitutive relations. Numerical solutions for steel-on-steel friction showing a decrease of the coefficient of friction with the sliding velocity for different normal pressures are presented. The model is considered to be adequate in the velocity range of 1–10 m/s where friction enhanced oxidation or surface melting are believed not to interfere with the asperity shearing process.
Numerical Analysis of Static Characteristics at Start of Operation in Porous Journal Bearings With Sealed Ends
Authors: Kaneko Satoru , Takabatake Hiroyuki , Ito Kanya
Published: February 2008

Static characteristics at the start of the operation are theoretically investigated in a porous journal bearing with sealed ends lubricated only by the oil initially provided within its pores. This is a preliminary study for estimating the variation of these characteristics with running time. A simple analytical model of the mixed lubrication regime is proposed on the basis of the assumption that the external forces acting on the journal, i.e., the applied static load, the oil-film force and the force at the boundary friction part, are balanced. Numerical results show that air penetrates into the porous matrix at the oil-film rupture zone due to negative pressure in the porous matrix; this causes the reduction of oil content within the porous matrix and contributes to formation of the oil film in the bearing clearance. The oil leakage from the porous matrix induced by the air penetration suggests that, even if hydrodynamic lubrication conditions are possible at the start of operation, the lubrication mode will become mixed or boundary lubrication conditions with running time. The numerical data on the static characteristics are presented in graphical form, illustrating the effects of the Sommerfeld number in the hydrodynamic and mixed lubrication regimes.
Orbit Induced Journal Temperature Variation in Hydrodynamic Bearings
Authors: Gomiciaga R. , Keogh P. S.
Published: February 2008

A method for evaluating the asymmetric heat input into the synchronously vibrating journal of a hydrodynamic bearing is presented. CFD techniques are used to analyze the dynamic flow and heat transport in the lubricant film and the heat input into the journal is obtained by orbit time averaging. The procedure is applied to a two-inlet circular bearing with backward and forward circular whirl journal orbits over a range of rotational speeds. The steady asymmetric heat input into the journal is found to have a sinusoidal component, which causes a steady surface temperature differential across the journal. The maximum temperature on the journal surface is always upstream of the minimum film point for forward whirl orbits and downstream for backward whirl orbits. The results provide a greater understanding of the nature of rotor thermal bending.
Tribological Study of Chemical States of Protective Oxide Film Formed on Steel After Sliding in Humid Atmosphere and in Aqueous Solutions
Authors: Saito T. , Imada Y. , Sugita K. , Honda F.
Published: February 2008

A tribological study of low-carbon steel was carried out in humid atmospheres and in corrosive aqueous solutions (water and 1 percent H2O2 solution), to determine the chemical states of the protective oxide film and its tribological properties. The chemical states of the oxide film on the slid steel surface were determined using an electron probe microanalyzer. The analytical results showed that the Fe3O4 layer effectively reduced the friction coefficient, whereas FeO and Fe2O3 did not. The chemical states and compositions of the oxide film were attributed to the sliding conditions which induced the oxidation tribologically. And the oxide forms remained stable until analysis even in the aqueous solutions. Friction work was a simple and useful parameter for classifying corrosive and mechanical wear on the slid surface.
Influence of Mechanical and Chemical Factors on Transition Between Severe and Mild Wear in Saline Solution
Authors: Goto H.
Published: February 2008

Pin-on-disk wear tests of carbon steels in saline solution were carried out at high loads to study the effects of mechanical and chemical factors on the transition between severe and mild wear. The factors were load, presliding time, concentration of saline solution, dissolved oxygen content, and applied potential for cathodic protection. Severe wear and seizure appear at low concentration levels of saline solution and the transition takes place at a certain concentration level of saline solution. The concentration level for the transition is higher under argon-saturated condition than under air-saturated condition. Mild wear is predominant over the whole range of dissolved oxygen content from 0.5 to 18 ppm in 0.5 wt percent saline solution and the wear rate decreases with decreasing dissolved oxygen content. The mild wear occurs in the range from 2 to 18 ppm in 0.01 wt percent saline solution, whereas the wear mode moves to severe wear below 2 ppm. Mild wear predominates under incomplete cathodic protection between −0.50 and −0.80 V (versus Ag/AgCl in 0.5 wt percent saline solution. The applied potential for the transition from initial to steady wear is more negative under higher dissolved oxygen content condition. Complete severe wear occurs below −0.90 V (versus Ag/AgCl) with a risk of seizure.
Apparent Multi-Valued Friction in Lubricated Contact Systems From Tangential Dynamics
Authors: Hess D. P.
Published: February 2008

Multi-valued friction is often observed in lubricated contact systems operating under unsteady sliding conditions. It the past, this behavior has been modeled using a measured time delay between changes in sliding velocity and resulting changes in friction. More recent work has revealed that fluctuations in normal separation are the principal cause of this deviation from steady friction behavior. This paper demonstrates that tangential contact dynamics can result in apparent deviations from steady friction which can resemble true deviations found from fluctuations in normal separation. Two different dynamic models are considered. One model accommodates the tangential rider dynamics of a friction test apparatus. The other model considers the prospect of tangential dynamics associated with asperity contacts.
Three-Dimensional Residual Stress Effects on the Fatigue Crack Initiation in Rails
Authors: Yu Chau-Cho , Keer L. M. , Steele R. K.
Published: February 2008

A rail is simulated as an elastic-plastic kinematic hardening steel having a T-shaped cross section and finite length in the rolling direction. The direct analysis method and the finite element method are combined to solve for the steady-state residual stress/strains and cyclic plastic strain amplitudes. In the present analysis, some special loading cases are investigated. Results show that tensile residual stresses occur deep in a rail head over a large area of the substrate. These phenomena, which are observed in rail steel after service, cannot be deduced from a half plane or half space analysis.
A New Theoretical Model for Analysis of the Fluid Film in the Cavitation Zone of a Journal Bearing
Authors: Mistry K. , Biswas S. , Athre K.
Published: February 2008

Recent studies of the cavitation zone in the journal bearing indicate that a sublayer of lubricant adheres to the journal surface. Based on this observation, it is proposed in the present work that centrifugal force and surface tension should be considered as the primary cause for the lubricant film to be in equilibrium in the cavitation zone. Using this concept a cavitation model is proposed. The effect of the supply pressure on fluid film reformation is also studied. The proposed model is applied to study the cavitation zone of: (1) a journal bearing with lubricant entry along the line of centers and (2) a journal bearing with inlet groove located along the load line. It is found that the thickness of the lubricant layer adhering to the journal surface is an exponential function of the speed while the eccentricity ratio rather than speed has a predominant effect on the width of the streamers. The supply pressure has a remarkable effect on the film reformation when the lubricant supply is along the line of centers, with the possibility of film reformation at the supply groove, which does not happen when the lubricant inlet is located along the load line.
The Effects of Number of Balls and Preload on Vibrations Associated With Ball Bearings
Authors: Aktu¨rk N. , Uneeb M. , Gohar R.
Published: February 2008

In this paper, the radial and axial vibrations of a rigid shaft supported by a pair of angular contact ball bearings are studied. The effect on the shaft vibration of varying the preload and the number of balls in the bearings is investigated when the bearings are perfect. For perfect bearings the vibrations occur at the ball passage frequency. The amplitudes of these vibrations are shown to be considerably reduced if the preload and number of balls are correctly selected. All results show that the vibration characteristics of the shaft and its bearings change when the bearings operate in different regions of their nonlinear load-deflection characteristics. This implies that the preload and the number of balls in a ball bearing are two of the important governing parameters affecting its dynamic behavior and have to be considered at the design stage.
Porosity Modeling of Brush Seals
Authors: Chew J. W. , Hogg S. I.
Published: February 2008

A new model for predicting leakage flows through the bristle pack of brush seals is developed. In the model, the bristle pack is treated as a porous medium. Good agreement is demonstrated between predictions from a one-dimensional form of the model and a wide range of experimental data available from the literature, for seals with a bristle pack to rotor interference fit. The results demonstrate that both viscous and inviscid effects contribute significantly to the drag on the bristles within the pack. The model uses a linear superposition of viscous and inertial losses, with a resistance coefficient assigned to each contribution. Formulas that have been deduced for flow in packed beds, are adapted for use in assigning values to the resistance coefficients in the one-dimensional model. Finally, extension of the method to multiple dimensions is discussed, with a view to incorporating the model into a CFD code to form a general predictive capability for brush seal flows.
The Effect of Asperity Array Geometry on Friction and Pull-Off Force
Authors: Ando Yasuhisa , Ino Jiro
Published: February 2008

The friction and pull-off forces between regular asperity arrays with various heights on a silicon wafer and a scanning probe of an atomic force microscope (AFM) were measured. We used two-dimensional periodic asperity arrays. The arrays were created by using a focused ion beam (FIB) to mill patterns on a silicon plate and on a platinum layer deposited on a silicon plate. For both materials, the distance between adjacent peaks was about 240 nm and the groove depth ranged from about 3 to 49 nm. The probe of the AFM was a square flat, 0.7 × 0.7 μm2. For the silicon array, the pull-off force decreased with increasing groove depth and was proportional to the radius of curvature of the asperity. The friction force also decreased with asperity height and was proportional to both the asperity curvature and the pull-off force. For the platinum asperity array, although both the pull-off and friction forces also decreased with groove depth, the friction coefficient (calculated by dividing the friction force by the pull-off force) was about half that of the silicon asperity array.
Friction Force, Contact Resistance, and Lubricant Shear Behavior at the Magnetic Head-Disk Interface During Starting
Authors: Wang S. , Komvopoulos K.
Published: February 2008

During the starting operation of magnetic rigid disks, a stiction phenomenon characterized by a high friction force may be encountered due to the smoothness of the contacting surfaces and the small thickness of the lubricant film. Since friction measurement using a force transducer yields a signal proportional to the slider displacement, the real friction force at the head-disk interface cannot be measured directly. In the present study, a dynamic data analysis scheme is developed to obtain the real friction force as a function of time based on the measured apparent friction force. Electric contact resistance measurements demonstrate that the transition from static to kinetic friction occurs before the apparent friction force reaches a maximum value. Assuming a constant acceleration of the disk, the relative slip velocity at the contact interface is obtained as a function of time. The relationship between the shear stress and shear strain rate for a relatively thick lubricant film is found to be approximately linear up to a critical value of the shear stress. Due to the extremely high shear strain rates, the maximum real friction force can be significantly greater than the maximum static friction force.
Application of Boltzmann Statistical Mechanics in the Validation of the Gaussian Summit-Height Distribution in Rough Surfaces
Authors: Leung M. , Hsieh C. K. , Goswami D. Y.
Published: February 2008

In theoretical modeling of contact mechanics, a homogeneously, isotropically rough surface is usually assumed to be a flat plane covered with asperities of a Gaussian summit-height distribution. This assumption yields satisfactory results between theoretical predictions and experimental measurements of the physical characteristics, such as thermal/electrical contact conductance and friction coefficient. However, lack of theoretical basis of this assumption motivates further study in surface modeling. This paper presents a theoretical investigation by statistical mechanics to determine surface roughness in terms of the most probable distribution of surface asperities. Based upon the surface roughness measurements as statistical constraints, the Boltzmann statistical model derives a distribution equivalent to Gaussian. The Boltzmann statistical mechanics derivation in this paper provides a rigorous validation of the Gaussian summit-height assumption presently in use for study of rough surfaces.
A Uniform Analytical Approach for Ball and Roller Bearings Calculations
Authors: Houpert L.
Published: February 2008

A novel analytical approach is proposed, uniform for ball and roller bearings, which takes into account 5 relative race displacements (3 translations: dx, dy, and dz, and 2 tilting angles: dθy and dθz) to provide simple analytical relationships for calculating directly the resulting 3 bearing loads: Fx, Fy, and Fz as well as the 2 tilting moments: My and Mz. A full coupling between all these displacements and forces is considered. The maximum rolling element load Qmax, the load distribution Q(Ψ) and the 3-D rolling element load distribution dQ(Ψ, x′) at each roller-race contact slice are also given analytically. It will therefore be possible for bearing users, willing to study for themselves a complete statically indeterminate systems including shafts, housing and bearings, to do such calculations using accurate nonlinear bearing forces-displacements relationships suggested in this paper and to predict easily bearing and other system components performances. This approach can also be implemented in any nonlinear Finite Element Analysis (F.E.A.) package for describing a bearing element connecting the shaft to the housing for example. It completes, therefore, the F.E.A. library of elements.
Numerical Simulation of Traction in Rolling/Sliding Contacts
Authors: Wang Jiugen , Tan Jianrong
Published: February 2008

A new model of traction in elastohydrodynamically lubricated contact is presented to study shear stress distributions. Results of full numerical analyses of elliptical contacts under rolling and sliding are presented. This study is confined to elastohydrodynamically lubricated contacts of relatively low load. Effects of dimensionless parameters such as speed, normal load, and elliptical parameter and coefficient of limiting shear stress on shear stress distributions have been analyzed. Moreover, the influence of slide-roll ratio on visualized shear stress in EHD contacts has been studied. It has been found that the higher slide-roll ratios induce higher maximum shear stress. Shear stresses in fluid film and those on surfaces vary with many factors that reveal the mechanism of traction in EHD conjunction zones.
Surface Roughening and Contact Behavior in Forming of Aluminum Sheet
Authors: Lo Sy-Wei , Horng Tzu-Chern
Published: February 2008

The evolution of the surface topography and tool-workpiece contact behavior of an aluminum sheet during plastic deformation has been investigated by a series of indentation tests for various loads, strain rates, and bulk strains. A simple formulation for estimating the roughness of the indented surface and a model of asperity flattening have been proposed. Dynamic balance between the flattening and roughening processes, such as changes of asperity angle and spacing, is considered. Theoretical predictions of the variation of real area of contact with strain show good agreement with experiments. It is found that the very different frictional behavior of aluminum compared with steel sheets is due to the serious change of roughness, including asperity angle and spacing. When the angle is small, the increases in asperity angle and spacing have strong effect on increasing the contact area ratio. However, when the asperity angle is large, increased asperity angle has a significant contribution to the resistance to flattening. The asperity flattening velocity and the increasing rate of contact area ratio arise with the prestrain; however, they are not affected by the strain rate significantly. From FFT analysis, low frequency components (below 10 cycle/mm) due to the rotation of single grains or grain groups are observed on free surface during plastic straining. While in the contact area, in all cases, the low frequency components generated by plastic deformation are compressed.
Analytical Approximations in Modeling Contacting Rough Surfaces
Authors: Polycarpou Andreas A. , Etsion Izhak
Published: February 2008

A critical examination of the analytical solution presented in the classic paper of Greenwood and Williamson (1966), (GW) on the statistical modeling of nominally flat contacting rough surfaces is undertaken in this study. It is found that using the GW simple exponential distribution to approximate the usually Gaussian height distribution of the asperities is inadequate for most practical cases. Some other exponential type approximations are suggested, which approximate the Gaussian distribution more accurately, and still enable closed form solutions for the real area of contact, the contact load, and the number of contacting asperities. The best-modified exponential approximation is then used in the case of elastic-plastic contacts of Chang et al. (1987) (CEB model), to obtain closed-form solutions, which favorably compare with the numerical results using the Gaussian distribution.
Plowing Friction Under Harmonic Normal Loads
Authors: Hess D. P.
Published: February 2008

The influence of harmonic normal loads on sliding friction is investigated through analysis of contacts consisting of conical and spherical sliders of hard materials on softer metal surfaces. Friction for such contacts is assumed to result from a plowing component and a shearing component. Calculations and experiments show that the coefficient of friction is essentially independent of normal load for contacts with conical sliders. However, for spherical sliders the relation between the coefficient of friction and normal load is highly nonlinear. In the presence of harmonic variations in normal load, this non-linearity causes a shift in the average coefficient of friction. For ideal lubricated contacts, the shearing component of friction is very small and for this case, it is shown that the maximum average reduction in the coefficient of friction is ten percent. When the shearing component is more significant, as with dry contacts, the shift is less. For example, when the shear strength is one-sixth the hardness of the softer material, the maximum average reduction in the coefficient of friction is five percent.
Mathematical Model for Gas Bearing With Holes of Tangential Supply
Authors: Liu L Q. , Chen C. Z.
Published: February 2008

To investigate the dynamic characteristics of gas bearings with holes of tangential supply (TS bearing), drawing on the modified Reynolds equations proposed by Mori, we present new governing equations and their reasonable boundary conditions. Using this mathematical model, the inertia effect of the supplied gas on the aerodynamic film force can be evaluated properly. The governing equations are solved numerically using Finite Element Method (FEM), and the pressure distribution of the gas in the bearing, the critical whirl ratio and so on, are calculated for a typical design. Some results for a cylindrical journal bearing (CJ bearing) and ordinary bearing with holes of radial supply (RS bearing) are also provided for comparison.
Step Response Characteristics of Hydrostatic Journal Bearings With Self-Controlled Restrictors Employing a Floating Disk
Authors: Yoshimoto S. , Kikuchi K.
Published: February 2008

This paper describes the step response characteristics of hydrostatic journal bearings with self-controlled restrictors employing a floating disk. This type of bearing can achieve very high static stiffness by controlling the mass flow rate of the fluid entering the bearing clearance using a floating disk. Many design parameters such as supply pressure, viscosity, the magnitude of the step load and the imposed static load affect the step response characteristics of the proposed bearing. Therefore, the influences of each design parameter on the step response characteristics are theoretically investigated in this paper. Furthermore, the theoretical results are compared with the experimental results in order to verify the theoretical predictions. It is subsequently found that the proposed bearing consistently shows a stable step response irrespective of the step-load direction.
Stiffness and Damping Evaluation of Air Bearing Sliders and New Designs With High Damping
Authors: Zengi Q. H. , Bogy D. B.
Published: February 2008

We apply the dynamic simulation and modal analysis method to analyze the dynamic properties of slider-air bearings. First, the theoretical background and proposed methods are described. Then, five basic types, one of which is first proposed in this paper, of the air bearing surfaces (ABS) are briefly discussed. The dynamic properties of the sliders are investigated, and compared with each other. It is found that a negative pressure slider has the highest stiffness and lowest damping, the TPC and two newly proposed sliders demonstrate higher damping. Finally, the general ABS design problem is briefly discussed. A new advanced slider is designed, analyzed, and compared with the other sliders. The air bearing of the new slider design has larger stiffness and the highest damping of those studied.
Comparison of Predictions With Test Results for Rotordynamic Coefficients of a Four-Pocket Gas Damper Seal
Authors: Li Jiming , Ransom David , San Andre´s Luis , Vance John
Published: February 2008

Experiments and field applications have demonstrated that multiple-pocket gas damper seals effectively eliminate subsynchronous vibration and attenuate imbalance response at the critical speeds in turbomachinery. A one-control volume, turbulent bulk-flow model for the prediction of the seal leakage and rotordynamic force coefficients of centered multiple-pocket damper seals is hereby detailed. Comparisons of numerical predictions with experimental force coefficients for a four-pocket damper seal are presented. The bulk-flow model and experiments indicate the seal direct stiffness and damping force coefficients are insensitive to journal speed while the cross-coupled stiffnesses increase slightly. However, the current model overpredicts the direct damping coefficient and underpredicts the direct stiffness coefficient for increasing test pressure ratios. Computed results show that the force coefficients of multiple-pocket gas damper seals are also functions of the rotor excitation frequency.
Calculation of Dynamic Coefficients in a Hydrodynamic Bearing Considering Five Degrees of Freedom for a General Rotor-Bearing System
Authors: Jang G. H. , Kim Y. J.
Published: February 2008

A complete method is presented to calculate the stiffness and the damping coefficients in a hydrodynamic bearing considering five degrees of freedom for a general rotor-bearing system. Perturbation equations are obtained from Reynolds equation by assuming the small amplitude motion of a bearing center, and are solved by the finite element method. Their characteristics due to eccentricity and misalignment are investigated for herringbone groove journal and thrust bearings in the spindle motor of a hard disk drive. This research shows that the dynamic coefficients increase with increasing the misalignment as well as the eccentricity due to the wedge effect. It also shows that the moment coefficients, which have been neglected in most of the previous analyses, are of significant magnitude in a journal bearing and have even bigger values for the thrust bearing when they are compared with the ball bearing in the same type of a spindle motor.
A Method of Reducing Windage Power Loss in a Laser Scanner Mirror by Using the Pumping Effect of Herringbone-Grooved Gas Journal Bearings
Authors: Yoshimoto S. , Takahashi A.
Published: February 2008

This paper proposes a newly developed laser scanner motor with low power consumption, operating at a high speed of rotation. The principle of the scanner motor is that of a herringbone-grooved journal bearing functioning as a viscous vacuum pump. The windage power loss of a polygon mirror can be reduced because the air inside the housing is pumped out by herringbone-grooved viscous vacuum pumps. The performance of the proposed laser scanner motor was measured under various conditions. It was subsequently found that the proposed scanner motor of a laser beam had much reduced power consumption and higher scanning accuracy than the conventional scanner motor in which the rotor is rotated in the air.
Excitation Frequency Effects on the Stiffness and Damping Coefficients of a Five-Pad Tilting Pad Journal Bearing
Authors: Ha Hyun Cheon , Yang Seong Heon
Published: February 2008

An experimental study is performed to investigate the frequency effects of the excitation force on the linear stiffness and damping coefficients of a LOP (load on pad) type five-pad tilting pad journal bearing with the diameter of 300.91 mm and the length of 149.80 mm. The main parameter of interest in the present work is excitation frequency to shake the test hearing. The excitation frequency is controlled independently, using orthogonally mounted hydraulic exciters, as follows: 1) excitation frequency ratio in the x-axis direction νx = 0.5, 2) excitation frequency ratio in the y-axis direction νy = 0.6, 0.7, 0.8, 0.9. The magnitude of the excitation force is controlled to make sure that the test hearing has a linear behavior during the test. The relative movement between the bearing and shaft, and the acceleration of the bearing casing are measured as a function of excitation frequency using the different values of bearing load and shaft speed. Measurements show that the variation of excitation, frequency has quite a little effect on both stiffness and damping coefficients. The stiffness coefficients of the five-pad tilting pad journal bearing slightly decrease as the excitation frequency ratio increases, while the damping coefficients slightly increase with excitation frequency ratio, especially in the case of lower speed and higher load. Both direct stiffness and damping coefficients in the direction of bearing load decrease with an increase of shaft speed, but increase with the bearing load.
Direct and Indirect Approaches at the Plasto-Hydrodynamic Lubrication Problem. Application to an Industrial Ironing Process
Authors: Cahouet V. , Baillet L. , Meurisse M. H. , Bou-Sai¨d B.
Published: February 2008

A finite element fluid-structure coupling is developed in order to simulate a lubricated forming process, A dynamic explicit code is used for elasto-plastic deformation calculations and normal and tangential contact conditions are handled using a dynamic projection method. Two complementary approaches to the hydrodynamic lubrication problem are proposed: a classical method using the direct solution of the inverse hydrodynamic problem and an original indirect method based on the so called “optimal control theory.” The applicability of the code developed is proved by the numerical simulation of an industrial ironing process. Results of the two methods are compared in terms of friction forces and film thickness profiles. The distribution of local friction coefficients is also obtained.
A 3D Conservative Model for Self-Lubricated Porous Journal Bearings in a Hydrodynamic Steady State
Authors: Meurisse Marie-He´le`ne , Giudicelli Bernard
Published: February 2008

Successive steps in constructing a realistic conservative 3D model for hydrodynamic porous bearings are presented. They consist in iterative comparisons between theoretical results and industrial knowledge. This model is based on Darcy’s law for the porous bearing and an adaptation of Elrod’s model to a generalized Reynolds’s equation for the flow in the gap between the shaft and the bearing. Capillary effects at the outer surface of the bearing, and pore obstruction at its inner surface, leading to loss of permeability in the region where film thickness is minimal, are highlighted as phenomena which can prevent bearing leakage. They have been taken into account in the boundary conditions of the proposed model.
Analyses of Heat-Pipe Cooled Isothermal Journal Bearings
Authors: Wang Qian , Chen Gang , Cao Yiding
Published: February 2008

An isothermal journal bearing that incorporates heat-pipe cooling technology has been developed. The heat pipe can spread frictional heat rapidly along the bearing circumference, resulting in a uniform temperature distribution in the bearing with a low peak temperature and stable transient thermal performance. A numerical model has been developed for the new bearing to facilitate a thorough understanding of its performance as well as an optimal bearing design. The heat pipe in the bearing is modeled as a heat conductor whose effective thermal conductance is determined through the correlation between the numerical results and experimental data. The heat transfer coefficients at bearing boundaries are obtained with the assistance of experimental measurements and calculations using semi-empirical correlations. Good agreement is observed between the analytical and experimental results. Once the analytical model is validated, a parametric study is conducted for the performance of new bearings with different configurations and materials. The analytical results further confirm that the isothermal journal bearing developed has the ability to battle frictional-heat-induced problems, which can significantly benefit both bearing operation and failure prevention.
Ramp-Load Dynamics of Proximity Recording Air Bearing Sliders in Magnetic Hard Disk Drive
Authors: Hu Yong
Published: February 2008

The revival of dynamic load/unload (L/UL) technology forces us to rethink the air bearing design philosophy, which has traditionally been established for contact start/stop applications. Reliably loading a slider onto a full-rotating disk imposes its own requirements on the slider air bearing designs. This paper addresses the unique design requirements of dynamic L/UL technology, through an investigation of the air bearing characteristics of two proximity recording sliders during a dynamic load process. While the slider/disk contact force is employed as a key indicator of the reliability of the dynamic load mechanism, the air bearing suction force and squeeze flow effect are used to characterize the slider’s dynamics during loading. The effects of the slider’s loading velocity, pitch and roll static attitudes on its dynamic load performance are simulated. In comparison to the positive pressure air bearings, both the enlarged air bearing surface and shallowly recessed cavities of the subambient pressure air bearing sliders generate more squeeze flow, resulting in a rapid development of the air bearing lifting force at a higher attitude. This often leads to a more reliable dynamic load performance. The impact of the air bearing suction force on the slider’s dynamics during loading is determined by the suction force center. A towards-leading-edge suction force not only induces a negative pitch motion during the early stage, but also prolongs the pitch-up process. Both effects can result in a head crash for the slider with a large negative pitch static attitude. In summary, the subambient pressure air bearing sliders that feature the enlarged leading air bearing surface and towards-trailing-edge suction cavities with small recess depth offer a fast pitch-up load performance.
Lubricant Permeation From Micro Oil Pits Under Intimate Contact Condition
Authors: Lo Sy-Wei , Horng Tzu-Chern
Published: February 2008

In the situation of serious tool-workpiece contact when a smooth surface is desired in metal forming, the lubricant is either enclosed within the micro pits or infiltrates the contact plateaus region. The evolution of surface topography and the interfacial shear stress is accordingly altered. A quantitative analysis has been developed based on a series of compression-sliding tests and three-dimensional measurements of work-piece surface. It is found that the lubricant in the oil pits seeps into the contact region under the present experimental conditions. The fractional area of oil pit is a linear function of the average surface separation. The deformation of the oil pits is in the mode of “centripetal flow” where the opening of the pit is diminished while the depth of the oil pit is basically kept constant during the deformation. The specific permeation flux, which is defined as the nondimensional permeation flux per unit relative sliding speed and width of oil pit is an important index for the tendency of permeation. The variation of the specific permeation flux reveals that the lubricant permeation increases with sliding speed and average surface separation. After adopting the permeation model, it is found that the slipping between tool surface and permeating lubricant might exist. The average film thickness of the infiltrating oil increases with increasing sliding velocity and less serious asperity contact.
Thermoelastic Instability Including Surface Roughness Effects
Authors: Jang J. Y. , Khonsari M. M.
Published: February 2008

A thermoelastic instability (TEI) analysis is presented for determining the threshold of instability in a friction pair consisting of a conducting surface rubbing against a rough surface with very low thermal conductivity. The theory predicts that TEI is initiated when the operating speed exceeds a limiting value of U* given by: U*/Us=1+Λℑ′ where Us is identical to the critical equation developed by Burton (1980) for highly loaded surfaces in contact defined as: Us=2Ωks/fαE. The dimensionless parameters on the right-hand-side of the above equation emerge from the consideration of the surface roughness and are defined as Λ = σΩE/2χ, and ℑ′ = Ho/F′. The results indicate that the magnitude of the applied load and the surface roughness characteristics tend to play an important role in the prediction of the onset of TEI instability.
Wear Properties of Plasma Nitrided Steel in Dry Sliding Conditions
Authors: Podgornik B. , Vizˇintin J.
Published: February 2008

In our study, we investigated the wear, mechanical and tribological properties of plasma and pulse plasma nitrided AISI 4140 steel compared to hardened steel. Contact surfaces of the samples were fully characterized using metallographic, SEM microscopic, microhardness, and profilometric techniques, before and after wear testing. Wear tests were performed on a pin-on-disk wear testing machine in which nitrided pins were mated to hardened ball bearing steel disks under dry sliding conditions. Results show only slight improvement of the tribological properties of AISI 4140 steel after plasma and pulse plasma nitriding compared to hardening. However, improvements become more evident when testing conditions become more severe. It was also found that compound layer should be removed from the nitrided surface to avoid impairment of the tribological properties by fracture of hard and brittle compound layer followed by the formation of hard abrasive particles.