bluuebunny/crossref_metadata_2025 huggingface dataset:
Stiffness of Magnetic Bearings Subjected to Combined Static and Dynamic Loads
Authors: Rao D. K. , Brown G. V. , Lewis P. , Hurley J.
Published: June 2008

This paper investigates the stiffness of a magnetic bearing that is subjected to the combined action of static and dynamic loads. Since their sum cannot exceed the saturation load, a large static load will imply that the bearing can carry only a small dynamic load. This smaller dynamic load together with the practical vibration amplitude define a practical upper bound to the dynamic stiffness. This paper also presents approximate design formulas and curves for this stiffness capacity as a function of the ratio of dynamic and static loads. In addition, it indicates that vibrations larger than a certain gap fraction can destabilize the magnetic bearing. This gap fraction, called the critical gap fraction, depends on the dynamic and static load ratio. For example, if the dynamic load is half of the static load, the use of more than 25 percent of gap can destabilize the bearing.
Determination of Forces in a Magnetic Bearing Actuator: Numerical Computation With Comparison to Experiment
Authors: Knight J. D. , Xia Z. , McCaul E. , Hacker H.
Published: June 2008

Calculations of the forces exerted on a journal by a magnetic bearing actuator are presented, along with comparisons to experimentally measured forces. The calculations are based on two-dimensional solutions for the flux distribution in the metal parts and free space, using finite but constant permeability in the metals. Above a relative permeability of 104 the effects of changes in permeability are negligible, but below 104 decreases in permeability cause significant decreases in the force. The calculated forces are shown to depend on the metal permeability more strongly when the journal is displaced from its centered position. The predicted forces in the principal attractive direction are in good agreement with experiment when a relatively low value of permeability is chosen. The forces measured normal to the axis of symmetry when the journal is displaced from that axis, however, are significantly higher than predicted by theory, even with a value of relative permeability larger than 5000. These results indicate a need for futher work including nonlinear permeability distributions.
Analysis of Hydrostatic Journal Bearings With End Seals
Authors: San Andres L. A.
Published: June 2008

An approximate analysis for the pressure field and dynamic force coefficients in turbulent flow, centered hydrostatic journal bearings (HJBs) with fluid inertia and liquid compressibility effects is presented. Results from the analysis show that HJBs with end seals have increased damping, better dynamic stability characteristics, as well as lower flow rates, than conventional HJBs. However, hydrostatic (direct) stiffness may be lost if excessively tight end seals are used. End seals are shown to compensate for the effect of liquid compressibility within the recess volume, and prescribe a net reduction in the whirl frequency ratio for hybrid operation. Hydrostatic squeeze film dampers (HSFD) with end seals are shown to be a viable alternative in applications where a tight control of the bearing leakage is important such as in jet-engines. Furthermore, HSFDs with end seals could be used as an active device to control the available damping in a typical application.
The Influence of Oil Supply Pressure on the Behavior of a Rigid Symmetrical Balanced Rotor in Journal Bearings With Circumferential Feeding
Authors: Capone G. , Russo M. , Russo R.
Published: June 2008

A theoretical-experimental investigation was carried out on the behavior of a rigid symmetrical balanced rotor in short, cylindrical, circumferentially fed journal bearings under low load conditions. The influence of supply pressure on bearing static characteristics and oil film instability threshold was evaluated for the above system. The experimental results are shown along with the theoretical predictions of the static characteristics and limit stability curves for different values of the ratio between supply pressure and specific load.
Theoretical Analysis of Capacitive Effect of Roller Bearings on Repeated Starts and Stops of a Machine Operating Under the Influence of Shaft Voltages
Authors: Prashad Har
Published: June 2008

A theoretical analysis has been carried out to study the capacitive effects of roller bearings on repeated starts and stops of a machine operating under the influence of shaft voltages. The analysis gives the time required for the charge accumulation and increase of charge with time on the bearing surfaces based on bearing capacitance, resistance of film thickness and the shaft voltage. Also, it investigates the effect of gradual leakage of the accumulated charges with time as the shaft voltage falls as soon as the power supply to the machine is switched off. This paper gives an approach to determine ratio of the number of shaft revolutions required for charge accumulation and gradual discharge of the accumulated charges on the bearing surfaces depending on bearing to shaft voltage. Also, number of repeated starts and stops before initiation of craters on roller track of races are established to restrict the deterioration and damage of bearings.
Elastic Finite Element Analysis of Multi-Asperity Contacts
Authors: Komvopoulos K. , Choi D.-H.
Published: June 2008

The plane-strain contact problem of an elastic half-space indented by a nominally flat rigid surface having a finite number of regularly spaced cylindrical asperities is investigated using the finite element method to gain an understanding of the interactions in multi-asperity contacts. The significance of the number and spacing of asperities on the contact behavior at the center and edges of the interfacial region is examined. Subsurface stress fields of multi-asperity contacts are presented for various asperity distributions and indentation depths. Asperity interaction effects are quantified in terms of representative parameters, such as the maximum contact pressure, normal load, and maximum von Mises equivalent stress, normalized with similar quantities of the single-asperity contact problem. These nondimensional parameters are principally affected by the spacing and radius of asperities and secondarily by the indentation depth. Significant deviations from the single-asperity Hertzian solution may be encountered, especially in the neighborhood of asperity contacts, because of the unloading and superposition mechanisms which depend on the distance and radius of asperities and indentation depth. The finite element results are in fair qualitative agreement with the phenomenological behavior and analytical predictions.
A Transient Model for Micro-Elastohydrodynamic Lubrication With Three-Dimensional Irregularities
Authors: Ai Xiaolan , Cheng Herbert S. , Zheng Linqing
Published: June 2008

A transient model for micro-elastohydrodynamic lubrication with three-dimensional asperities in line contacts is presented in this paper. To take into account the effect of non-Newtonian behavior of lubricant, the Ree-Eyring viscous constitutive equation is employed in deriving Reynolds equation. The numerical solution of this model is based on an implicit finite difference scheme with under relaxation. Numerical simulation results show that the pressure bump caused by an asperity or asperities depends not only on asperity height and orientation but also on asperity dimension. The increase in asperity dimension decreases the pressure bump. The decrease in load and increase in sliding speed increases pressure bump.
Finite Element Analysis of Elastoplastic Indentation: Part I—Homogeneous Media
Authors: Montmitonnet P. , Edlinger M. L. , Felder E.
Published: June 2008

Indentation analysis is performed using a finite element model. The material is supposed elastoplastic, the spherical indenter is elastic. Elastic indentation with friction is first analyzed. It leads to a discussion of the formulation of the friction law and the effect of a sliding/sticking threshold. Elastoplastic analysis of the indentation of a homogeneous medium is conducted till unloading, suggesting the occurrence of replastification during unloading.
Isoviscous Hydrodynamic Lubrication of Deep Drawing and Its Comparison With Experiment
Authors: Mahdavian S. M. , Shao Z. M.
Published: June 2008

An analytical model for the isoviscous hydrodynamic lubrication of deep drawing is developed. The model covers the complete drawing stages from sheet metal blank to the drawn cup-shaped product. Equations to calculate the film thickness, radial, and drawing stresses related to the blank holder squeezing action are presented. Theoretical predictions of both film thickness and drawing force ratio for lubricated, low carbon steel are compared with experimental measurements.
Measurement and Prediction of the Surface Temperature in Polymer Gears and Its Relationship to Gear Wear
Authors: Hooke C. J. , Mao K. , Walton D. , Breeds A. R. , Kukureka S. N.
Published: June 2008

A design of a four square gear test rig that allows the wear of polymer and composite gears to be monitored continuously during operation is described. The wear behavior of three typical gear materials is examined and it is shown that the wear characteristics differ greatly. For Acetal there is a sharp rise in wear as the transmitted torque is increased, effectively limiting the torque that can be transmitted by an Acetal gear pair. This wear transition is shown to be associated with the maximum surface temperature of the gear reaching the melting point of Acetal. This provides an indirect method of checking the accuracy of existing models for the maximum surface temperatures in polymer gears. Very large discrepancies are found. A simple, alternative model for the average and maximum gear temperatrues is developed and is shown to predict the wear transition with far greater accuracy.
Experimental Pressures and Film Forces in a Squeeze Film Damper
Authors: Arauz G. L. , San Andres L. A.
Published: June 2008

The effect of whirl frequency and lubricant viscosity on the dynamic pressures and force response of an open end and a partially sealed squeeze film dampers (SFD) with a radial clearance of 0.38 mm is determined experimentally. The experiments are carried out in a damper test rig executing circular centered orbits and for whirl frequencies ranging from 33 to 83 Hz. The experimental results show that the sealed SFD configuration produces larger tangential forces than the open end SFD. The tangential (damping) force increases linearly with increasing whirl frequency. For this radial clearance fluid inertia effects in the damper are found to be negligible since the squeeze film Reynolds number is less than 1.20. Cavitation was observed in both damper configurations at high frequencies and high lubricant viscosities. This condition limited the rate of increment of the damping (tangential) force with increasing frequency and reduced the radial force when lubricant viscosity increased.
Competition Between Fatigue Crack Propagation and Wear
Authors: Fan H. , Keer L. M. , Cheng W. , Cheng H. S.
Published: June 2008

Based on a semi-empirical derivation of the Paris fatigue law, the fatigue crack length a is related to the yield limit or flow stress, which ultimately is related to the hardness of the material. The analysis considers together the cyclic loading, which tends to increase the surface crack length, and the wear, which tends to decrease the crack length at the surface, and shows that under certain conditions a stable crack length may be developed. Experiments conducted on two test groups ((i) Rc = 58.5 and (ii) Rc = 62.7) tend to support the present analysis.
Sectional Micro-Elastohydrodynamic Lubrication
Authors: Huang P. , Wen S. Z.
Published: June 2008

A method has been introduced to solve micro-EHL problems, with the discontinuous pressure distribution caused by high surface roughnesses. Treatments of inner boundary conditions, flow continuity, and deformation compatibility are given in detail. Examples of different sectional micro-EHL problems show that the method is efficient. The solutions of micro-EHL problems with the small film ratio are presented and discussed.
An Experimental Investigation Into the Thermal Behavior of a Three Lobe Profile Bore Bearing
Authors: Basri S. B. , Gethin D. T.
Published: June 2008

The paper describes an experimental investigation into the thermal behavior of a three lobe profile bore bearing. Three geometries are considered, a symmetric, tilted and tilted with siderail geometry. The results are presented in a format which enables convenient comparison with predicted data and confirms the severity of the thermal excursions in the loaded lobe. Global behavior is also presented which demonstrates trends similar to those for a cylindrical geometry. The results confirm the effectiveness of the side rail in reducing lubricant sideflow while having only a marginal effect on thermal excursions.
Finite Element Analysis of Elastoplastic Indentation: Part II—Application to Hard Coatings
Authors: Montmitonnet P. , Edlinger M. L. , Felder E.
Published: June 2008

A finite element model previously described is used to analyze indentation of hard-face materials. The materials (substrate and coating) are supposed elastoplastic, the spherical indenter is elastic. The state of stress during indentation (including unloading) of hard-face materials (chromium layer on steel) is investigated in details. The tensile stresses which might lead to failure are especially discussed, and related with experiments showing fracture or delamination of the coating. The model also shows the influence of film thickness on surface and interface stresses.
Experimental Versus Theoretical Characteristics of a High-Speed Hybrid (Combination Hydrostatic and Hydrodynamic) Bearing
Authors: Kurtin K. Alan , Childs D. , San Andres Luis , Hale K.
Published: June 2008

The high-speed test facility designed and installed at Texas A&M to study water lubricated journal bearings has been successfully used to test statically an orifice compensated five-recess-hybrid (combination hydrostatic and hydrodynamic) bearing for two radial clearance configurations. Measurements of relative-bearing position, torque, recess pressure, flow rate, and temperature were made at speeds from 10,000 to 25,000 rpm and supply pressures of 6.89 MPa (1,000 psi), 5.52 MPa (800 psi), and 4.14 MPa (600 psi). For speeds of 10,000 and 17,500 rpm, the bearing load capacity was also investigated. A pitching instability of the bearing limited the number of test cases. A 2-dimensional, bulk-flow, Navier-Stokes numerical analysis program was used for all theoretical performance predictions. Orifice discharge coefficients used in the program were calculated from measured flow and pressure data. Reynolds numbers for flow within the bearing lands due to shaft rotation and recess pressurization ranged from 6700 to 16,500. Predictions sensitivity to ±10 percent changes in the input parameters was investigated. Results showed that performance prediction sensitivities are high for changes in discharge coefficients and negligible for changes in relative roughness. The numerical predictions of relative bearing position, recess pressure, flowrate, and torque are very accurate, provided the selected orifice discharge coefficients are correct.
The Determination of Flash Temperature in Intermittent Magnetic Head/Disk Contacts Using Magnetoresistive Heads: Part I—Model and Laser Simulation
Authors: Li Yufeng , Kumaran Aric R.
Published: June 2008

The feasibility of using a magnetoresistive magnetic head to determine the flash temperature of intermittent magnetic head/disk contacts is investigated. A finite difference model is developed to study the effects of the magnetoresistive sensor height, the contact power intensity and the contact duration on the temperature response characteristics of the magnetoresistive sensor. A pulsed neody-minum:yttrium-aluminum-garnet laser is used as the heat source to simulate the contact and verify the model. The simulation results agree well with the theoretical model. Based on the theoretical model and laser simulation, a temperature sensitivity coefficient ζ is proposed to determine the surface temperature rise.
The Determination of Flash Temperature in Intermittent Magnetic Head/Disk Contacts Using Magnetoresistive Heads: Part II—Experimental Investigation
Authors: Li Yufeng , Kumaran Aric R.
Published: June 2008

The flash temperature during intermittent head/disk contacts was measured using specially fabricated magnetoresistive heads. Microasperities were intentionally created on disk substrates, and ZrO2 was sputtered on disc surfaces as overcoat. It was found that with a disk spinning speed of 20 m/s, flash temperature could reach 50 to 100°C for transient contact (head was moving in disk radial direction), but less than 40°C for dwell on track contact (head was stationary). The contact times were in the range from submicroseconds to several microseconds, contact widths on the order of several micrometers, and temperature sensitivity coefficients in the range from 0.15 to 0.7 for a 2.5–μ high magnetoresistive sensor.
Application of Average Flow Model to Thin Film Gas Lubrication
Authors: Makino T. , Morohoshi S. , Taniguchi S.
Published: June 2008

The flow factors for the average Reynolds equation introduced by Patir and Cheng (1978, 1979) are extended to be valid for thin film gas lubrication. The effects of molecular mean free-path on the roughness-induced flow factors are included on the assumption that the local compressibility is small. The derivation of flow factors is carried out by means of the perturbation approach developed by Tripp (1983). The results are expressed in terms of Knudsen number, Peklenik parameter and nondimensional film thickness defined as the ratio of average film thickness and standard deviation of composite roughness. Two-dimensional roughness effects on the load-carrying capacity of a gas lubricated finite slider are also investigated.
The Minimum Film Thickness in Line Contacts During Reversal of Entrainment
Authors: Hooke C. J.
Published: June 2008

In contacts, such as cams, non-involute gears and shaft seals, where the direction of entrainment reverses during the operating cycle, the minimum film thickness is typically found just after the reversal. This paper shows that this minimum film thickness is determined by the rate of change of the entraining velocity and by the fluid and surface properties. For line contacts, four regimes of lubrication are found—as for the steady-state situation—and expressions for the film thickness in each regime are developed. This enables an outline design chart for the minimum film thickness to be constructed. It is shown that this information, together with the steady-state predictions is sufficient to determine the variation of film thickness with time in most situations where load, radius of curvature, and entraining velocity vary.
Temperature Rise at the Sliding Contact Interface for a Coated Semi-Infinite Body
Authors: Tian X. , Kennedy F. E.
Published: June 2008

In this paper, a three-dimensional model of a semi-infinite layered body is used to predict steady-state maximum surface temperature rise at the sliding contact interface for the entire range of Peclet number. A set of semi-empirical solutions for maximum surface temperature problems of sliding layered bodies is obtained by using integral transform, finite element, heuristic and multivariable regression techniques. Two dimensionless parameters, A and Dp, which relate to coating thickness, contact size, sliding speed and thermal properties of both coating and substrate materials, are found to be the critical factors determining the effect of surface film on the surface temperature rise at a sliding contact interface. A semi-empirical solution for maximum surface temperature problems of homogeneous bodies, which covers the whole range of Peclet number, is also obtained.
Calculation of Stiffness and Damping Coefficients for Elastically Supported Gas Foil Bearings
Authors: Peng J.-P. , Carpino M.
Published: June 2008

The stiffness and damping coefficients of an elastically supported gas foil bearing are calculated. A perfect gas is used as the lubricant, and its behavior is described by the Reynolds equation. The structural model consists only of an elastic foundation. The fluid equations and the structural equations are coupled. A perturbation method is used to obtain the linearized dynamic coefficient equations. A finite difference formulation has been developed to solve for the four stiffness and the four damping coefficients. The effect of the bearing compliance on the dynamic coefficients is discussed in this paper.
The Role of Relative Humidity, Surface Roughness and Liquid Build-Up on Static Friction Behavior of the Head/Disk Interface
Authors: Tian Hong , Matsudaira Takeo
Published: June 2008

Stiction at the head/disk interface has become one of the major concerns as smoother surfaces are required to achieve lower flying heights of magnetic heads over magnetic disks. In this paper, static friction forces on three types of disk samples with different surface roughness values were measured at various relative humidities. It was found that static friction coefficients were well correlated with total thickness of liquid (lubricant and adsorbed water) at the head/disk interface. The experimental data also agreed fairly well with the calculated values based on a proposed stiction model. It is implied in the stiction model that the bearing ratio or the shape of asperity height distribution, especially the part of high asperities, determines the stiction force. Moreover, long-term stiction was investigated on the unlubricated disk surfaces at 80 percent relative humidity and on the lubricated disks at 5 percent relative humidity to separate the effects of water build-up and lubricant build-up at the head/disk interface. It appears that long-term stiction occurs only when enough mobile lubricant is present and the thickness of liquid at the head/disk interface is close to a critical thickness value which is related to surface roughness values.
Three-Dimensional Temperature Distribution in EHD Lubrication: Part II—Point Contact and Numerical Formulation
Authors: Kim Kyung-Hoon , Sadeghi Farshid
Published: June 2008

A numerical study of Newtonian thermal elastohydrodynamic lubrication (EHD) of rolling/sliding point contacts has been conducted. The two-dimensional Reynolds, elasticity and the three-dimensional energy equations were solved simultaneously to obtain the pressure, film thickness and temperature distribution within the lubricant film. The control volume approach was employed to discretize the differential equations and the multi-level multi-grid technique was used to simultaneously solve them. The discretized equations, as well as the nonorthogonal coordinate transformation used for the solution of the energy equation, are described. The pressure, film thickness and the temperature distributions, within the lubricant film at different loads, slip conditions and ellipticity parameters are presented.
Lubrication With Binary Mixtures: Liquid-Liquid Emulsion
Authors: Al-Sharif A. , Chamniprasart K. , Rajagopal K. R. , Szeri A. Z.
Published: June 2008

There are numerous instances of technical importance in which multicomponent lubricants are utilized either by design or by necessity. In many of these cases one of the components is a liquid while the other component is a gas, as in squeeze film dampers during high frequency operation, or both components are liquids, as in the oil-water emulsion used in metal forming processes. In this paper our objective is to develop a self-consistent theory of hydrodynamic lubrication with bicomponent, liquid-liquid lubricants. The basic scientific method utilized is the continuum theory of mixtures. In this, first instance we test the model on journal bearings lubricated with water-in-oil and oil-in-water emulsions.
High-Speed Frictional Heating of an Elastic Medium With a Near-Surface Horizontal Line Crack
Authors: Chen T. Y. , Ju Frederick D.
Published: June 2008

This paper studies the effect of high-speed frictional heating over the surface of an elastic material, which has a near surface horizontal line crack. The frictional heating is represented by a high-speed moving heat source, since the mechanical loading effect is much smaller than the thermal effect in the resulting stress field. Finite difference methods are employed to compute the temperature field and the displacement field, taking into consideration the characteristic singularity at the crack tip. The temperature field solutions are first computed, using the method of heat balance. The thermo-mechanical solutions follow with particular interest in the vicinity of the line crack as represented by the stress intensity factors at the crack tip. It was found that both the open mode and the shear mode occur, as a result of the excitation of the moving thermal load. The paper also presents effects on the stress intensity factors from varying the thermal and the mechanical properties of the medium, and the location of the line crack from the wear surface. The depth at which the maximum thermal stress occurs is an exponential function of the Peclet number, as in the cases when there is no defect in the wear material and when there is a near surface cavity. Albeit, the “critical depth” reduces with increasing Peclet number and severity of the defect.
Dynamic Force and Moment Coefficients for Short Length Annular Seals
Authors: San Andres Luis
Published: June 2008

Close form expressions for the dynamic force and moment coefficients in short length annular pressure seals operating at the concentric and aligned position are derived. The analysis considers fully developed turbulent flow within the seal and determines a set of ordinary differential equations for the bulk-flow field due to perturbations in rotor displacements and angular motions. The flow equations are solved exactly for seals of short length where dynamic variations in circumferential velocity are neglected. The analytical solution derived is simple and reasonably accurate for seals of length to diameter ratios (L/D) as large as 0.5 as comparisons with results from full-scale numerical solutions show. The formulae presented are practical for use in preliminary design stages and parametric studies of dynamic seal performance.
The Measurement of Surface Temperature in Dry or Lubricated Sliding
Authors: Dinc¸ O. S. , Ettles C. M. , Calabrese S. J. , Scarton H. A.
Published: June 2008

This paper describes the development and calibration of an original test apparatus to study the effect of frictional heating on lubricant effectiveness. The configuration used the Seebeck effect between a stainless steel ball and a disk of tool steel. Good agreement was found between the average temperature rise indicated by the thermoelectric voltage and Archard’s models. Surface temperature rises approaching 200°C were obtained in lubricated contact and an upper bound temperature of 830°C was found in dry contact. The likely effect of track heating was assessed by analysis and, for this configuration, was found to be negligible. A slight lag was found between a local increase of friction due to the passage of an asperity and the thermal response.
Simultaneous Pressure Measurement and High-Speed Photography Study of Cavitation in a Dynamically Loaded Journal Bearing
Authors: Sun D. C. , Brewe D. E. , Abel P. B.
Published: June 2008

Cavitation of the oil film in a dynamically loaded journal bearing was studied using high-speed photography and pressure measurement simultaneously. Comparison of the visual and pressure data provided considerable insight into the occurrence and non-occurrence of cavitation. It was found that (1), cavitation typically occurred in the form of one bubble with the pressure in the cavitation bubble close to the absolute zero; and (2), for cavitation-producing operating conditions, cavitation did not always occur; with the oil film then supporting a tensile stress.
The Effects of Relative Angular Motions on Friction at Rough Planar Contacts
Authors: Hess D. P. , Soom A.
Published: June 2008

Changes in friction due to angular motions at rough planar contacts are investigated using a compliant contact model that isolates an angular degree-of-freedom. Expressions are developed that relate the real area of contact, the contact forces and the line-of-action of the resultant normal contact force to the angular displacement for both periodic and random rough surfaces. We assume that the friction force is proportional to the real area of (elastic) contact. For a periodic rough surface, consisting of an array of hemispherical asperities of equal height and radius, the friction force is shown to be independent of angular displacement. The normal force increases and its line-of-action shifts away from the center of the contact as angular displacements increase. Therefore, the coefficient of friction decreases with angular displacement. In contrast, for a randomly rough surface, the contact area and normal contact force are shown to be non-linearly dependent on angular displacement, but remain proportional to each other in the presence of relative angular motion. Therefore, for the randomly rough surfaces the coefficient of friction is independent of angular motion.
Dynamic Analysis of Gas Lubricated Reciprocating Ringless Pistons—Basic Modeling
Authors: Gommed K. , Etsion I.
Published: June 2008

A mathematical model is developed for analyzing the dynamics of gas lubricated ringless pistons. It takes into account the effects of piston-cylinder geometry, dynamic properties of piston and connecting rod, and operating conditions. A corresponding computer program is described which enables to obtain the piston trajectory and evaluate its stability. Preliminary results are presented for a reference case of a cryocooler which uses a stirling cycle design where helium gas is compressed by a motor driven piston. It is found that the connecting rod inertia plays an important role in stabilizing the piston motion. It is also found that the stability of piston motion strongly depends on the thermohydrodynamic condition of operation. Under isothermal condition the piston can be stable while under adiabatic condition its dynamic equilibrium may be unstable.
Experimental Study of Stationary Head-Disk Contact in Magnetic Disk Drives
Authors: Gitis N. V. , Sonnenfeld R.
Published: June 2008

Magnetic disk drives are vulnerable to “stiction” a contact phenomenon in which static friction greatly exceeds kinetic friction. Stiction tends to increase dramatically with time of stationary contact. Thus, processes leading to stiction in the stationary head-disk contact are an important subject of tribological studies. Measurements of the static friction force are accompanied in this work by measurements of the capacitance between a resting slider and a thin-film rigid disk. Capacitance provides a sensitive measure of the surface separation and allows in-situ monitoring of the stationary contact. The experiments have shown that if the disk is covered with a thin film of a mobile lubricant, the adhesion growth with time measurably pulls the slider nearer to the plane of the disk (“slider microdescent”), and both the stiction and capacitance increase. If there is no mobile lubricant film, there is no growth of either stiction or capacitance. Thus, the stiction phenomenon is connected strongly with the slider microdescent. Their time dependence is likely governed by the capillary force increase caused by redistribution of the mobile lubricant toward the newly formed menisci.
A Study of the Thermal Characteristics of the Leading Edge Groove and Conventional Tilting Pad Journal Bearings
Authors: Dmochowski W. , Brockwell K. , DeCamillo S. , Mikula A.
Published: June 2008

In this paper dealing with the tilting pad journal bearing, experimental results are presented which show that, at higher shaft speeds, the leading-edge-groove (LEG) design has significantly lower operating temperatures to those of the conventional design of tilting pad journal bearing. Subsequent theoretical analysis has shown that this reduction in pad operating temperature is the result of feeding cool oil directly to the leading edge of the pad. This has the effect of reducing the amount of hot oil carried over from one pad to the next.
A Direct Analysis of Two-Dimensional Elastic-Plastic Rolling Contact
Authors: Yu M. , Moran B. , Keer L. M.
Published: June 2008

A direct approach for elastic-plastic analysis and shakedown is presented and its application to a two-dimensional rolling contact problem is demonstrated. The direct approach consists of an operator split technique, which transforms the elastic-plastic problem into a purely elastic problem and a residual problem with prescribed eigenstrains. The eigenstrains are determined using an incremental projection method which is valid for nonproportional loading and both elastic and plastic shakedown. The residual problem is solved analytically and also by using a finite element procedure which can be readily generalized to more difficult problems such as three-dimensional rolling point contact. The direct analysis employs linear-kinematic-hardening plastic behavior and thus either elastic or plastic shakedown is assured, however, the phenomenon of ratchetting which can lead to incremental collapse, cannot be treated within the present framework. Results are compared with full elastic-plastic finite element calculations and a step-by-step numerical scheme for elastic-plastic analysis. Good agreement between the methods is observed. Furthermore, the direct method results in substantial savings in computational effort over full elastic-plastic finite element calculations and is shown to be a straightforward and efficient method for obtaining the steady state (shakedown) solution in the analysis of rolling and/or sliding contact.
A Numerical Solution for Dry Sliding Line Contact of Multilayered Elastic Bodies
Authors: Elsharkawy A. A. , Hamrock B. J.
Published: June 2008

A dry sliding contact analysis for two elastic bodies coated with multiple layers is presented. Green’s functions for a unit normal load and a unit tangential load for the generalized plane strain problem are derived. The contact problem is reduced to an integral equation in which the contact pressure profile is unknown. A systematic and efficient numerical model for the developed integral equation is also introduced. The results for two different cases are presented and discussed.
The Effect of Two-Dimensional Surface Roughness on the Load-Carrying Capacity of a Thin Compressible Gas Film
Authors: White J. W.
Published: June 2008

The influence of two-dimensional surface roughness on a very low clearance gas bearing slider is analyzed for conditions typical of current and near term hard disk data storage products. A model lubrication equation including finite width effects, compressibility and molecular slip is first developed and expressed in terms of the product variable, Z = PH. The analytical solution is then obtained by a combined variation of parameters and perturbation expansion method. Results obtained show that the product variable, which is proportional to longitudinal mass flow per unit depth, is not influenced by the details of surface roughness but is instead influenced by the statistics of the roughness distribution. The gas film pressure, however, is found to be a function of both the roughness details and statistics. The bearing load support is increased by the surface roughness, with the roughness first influencing the load through a second order effect.
Lubrication With Binary Mixtures: Bubbly Oil
Authors: Chamniprasart K. , Al-Sharif A. , Rajagopal K. R. , Szeri A. Z.
Published: June 2008

Mixtures do not exhibit Newtonian behavior even when their individual components themselves are Newtonian, and thus classical lubrication theory is not applicable to bearings lubricated with mixtures. Our objective is to extend hydrodynamic lubrication theory to lubrication with mixtures of a Newtonian liquid and an ideal gas. To this end, we make use of the theory of interacting continua and derive two coupled non-linear equations in component densities. The extended theory is tested on finite journal bearings: our results for pressure distribution show good qualitative agreement with the experimental data of Braun et al.
The Response of Balls Undergoing Oscillatory Motion: Crossing From Boundary to Mixed Lubrication Regimes
Authors: Lovell M. R. , Khonsari M. M. , Marangoni R. D.
Published: June 2008

There are many applications where determining the response of a slowly oscillating ball bearing becomes crucial. Many instrument pointing mechanisms, in particular those used for aerospace applications, contain ball bearings which sinusoidally oscillate at very slow rates over small angles. Prediction of the frictional response of these bearings is essential to designers, as the friction torque which they develop is an important factor for controlling space instruments. The friction torque associated with the motion of sinusoidally oscillating ball bearings has been found to trace out hysteresis loops. These loops can be separated into two regions: the steady rolling region and the pre-rolling region. The steady rolling friction torque, Ts, characterizes the steady rolling region, while the rest slope, σ, characterizes the pre-rolling region. The speed of a ball bearing in sinusoidal motion varies from rest to a maximum velocity, changing velocity at instantaneous increments. It is found that while moving toward this maximum velocity, a bearing may cross from boundary to mixed lubrication regimes. As a result, the prediction and interpretation of σ and Ts in sinusoidal oscillating systems becomes more difficult than their counterparts in constant rate systems, which ordinarily operate in only one lubrication regime. To establish the velocity boundaries associated with the onset of different lubrication regimes, a series of experiments were conducted at a constant rotation rate. Starting at the ultra-low-speed of .01 deg/s, the angular velocity was gradually increased to 72 deg/s. In this velocity range, the balls traveled from boundary lubrication, crossing the mixed lubrication regime, into the elastohydrodynamic lubrication regime. Sinusoidal hysteresis curves were also generated experimentally and characterized. The rest slope and steady rolling friction torque were investigated in both rolling regions using two different lubricants.
Thermal Effects in Cryogenic Liquid Annular Seals—Part I: Theory and Approximate Solution
Authors: Yang Zhou , San Andres Luis , Childs Dara W.
Published: June 2008

A thermohydrodynamic (THD) analysis is introduced for calculation of the performance characteristics of cryogenic liquid annular seals in the turbulent flow regime. A full-inertial bulk-flow model is advanced for momentum conservation and energy transport. The liquid material properties depend on the local absolute pressure and temperature. Heat flow to the rotor and stator is modeled by bulk-flow heat transfer coefficients. An approximate analytical solution is obtained to the governing equations when the seal operates at a steady-state and concentric condition. The temperature-rise in the fluid film of a cryogenic liquid seal is found to be composed of four sources due to viscous dissipation, pressure extrusion work, surface heat transfer and kinetic energy variation. For incompressible adiabatic flows, the fluid temperature rises linearly along the axial direction. The approximate analytical solution provides a useful tool for preliminary design and a better understanding of seal performance. Full numerical predictions of load, leakage, temperature, and rotordynamic coefficients for a high speed liquid oxygen seal are given in Part II to show the importance of thermal effects on seal performance. The accuracy of the approximate concentric seal analysis is then demonstrated by comparison to the results from the full numerical solution.
Thermal Effects in Cryogenic Liquid Annular Seals—Part II: Numerical Solution and Results
Authors: San Andres Luis , Yang Zhou , Childs Dara W.
Published: June 2008

A numerical thermohydrodynamic model for solution of the nonlinear differential equations describing the turbulent flow of a single-phase, cryogenic liquid annular seal is presented. A perturbation method is used for calculation of the zeroth and first-order flow solutions to determine the seal steady-state response and dynamic force coefficients. Numerical predictions for a high speed liquid oxygen seal with a roughened stator surface show that, relative to an isothermal-barotropic fluidmodel, the adiabatic THD solution presents a significant temperature-rise with a profound effect on the liquid material properties and seal performance characteristics. The calculations show the onset of two-phase flow conditions at moderate eccentricities despite the large pressure differential across the seal. Comparisons of the static performance characteristics for the reference seal show excellent agreement between the approximate analytical solution developed in Part I and the full numerical results.
Cavitation Erosion of Cobalt Alloy Coatings Containing Tungsten Carbide Particles in Hydrochloric and Sulphuric Corrosive Media
Authors: Jun Wei , Fu-Xing Wang , Yin-Qian Cheng , Nan-Ping Chen
Published: June 2008

Cavitation erosion tests of composite coatings based on vacuum fusion sintered cobalt alloy containing tungsten carbide particles were carried out in 30 percent HCl and 50 percent H2SO4 solutions. The technique used included an ultrasonic vibratory apparatus at 30°C, 25μm amplitude and 30 kHz frequency. Weight loss was measured with an analytical balance and the microstructure was observed with SEM. The test results showed that the cavitation erosion resistance of the composite coatings was increased by increasing the tungsten carbide content. The cavitation erosion is mainly caused by removal of the matrix material. The steady-state erosion rates have a linear relationship with the volume fraction of the tungsten carbide phase.
Modified Reynolds Equation for Ultra-Thin Film Gas Lubrication Using 1.5-Order Slip-Flow Model and Considering Surface Accommodation Coefficient
Authors: Mitsuya Y.
Published: June 2008

A 1.5-order modified Reynolds equation for solving the ultra-thin film gas lubrication problem is derived by using an accurate higher-order slip-flow model. This model features two key differences from the current second-order slip-flow model. One is the involvement of an accommodation coefficient for momentum. The other is that the coefficient of the second-order slip-flow term is 4/9 times smaller than that for the current model. From the physical consideration of momentum transfer, the accommodation coefficient is found to have no affect on the second-order slip-flow term. Numerical calculations using the 1.5-order modified Reynolds equation are performed. The results are compared with those obtained using three kinds of currently employed modified Reynolds equations: those employing the first- and second-order slip-flow models and those utilizing the Boltzmann equation. These comparisons confirm that the present modified Reynolds equation provides intermediate characteristics between those derived from the first- and second-order slip-flow models, and produces an approximation closer to the exact solution resulting from the Boltzmann-Reynolds equation.
Study on Tribology of Silicon Nitride Ceramic Tappet
Authors: Wang Fu-Xing , Cheng Yin-Qian , Haung Lin-Heng
Published: June 2008

In order to elucidate the tribological behavior of the Si3N4 ceramic used in automobile engine, a series of tests was carried out to measure the friction coefficient and wear resistance of Si3N4 during lubricated sliding against metals. A Timken type test machine and a spring loaded wear test machine were used for this study. Tests were also conducted with metal against metal pairs for comparison. The simulation tests of hot-pressed Si3N4 Ceramic tappet against metal cam pairs were conducted on a single cam-follower test machine. The test results showed that not only that the Si3N4 ceramic has excellent wear resistance, but also that the wear of the metal parts in contact with the Si3N4 ceramic decreased. These results were supported by engine bench test results.
Static Characteristics of a Rectangular Hydrostatic Thrust Bearing With a Self-Controlled Restrictor Employing a Floating Disk
Authors: Yoshimoto S. , Anno Y. , Fujimura M.
Published: June 2008

This paper proposes a new type of a self-controlled restrictor which can achieve a very high bearing stiffness in hydrostatic bearings. This self-controlled restrictor employs a floating disk to control the mass flow rate of the oil entering the bearing clearance according to changes of the applied load. Furthermore, a hydrostatic bearing with this restrictor can theoretically achieve an infinite stiffness when the mass of a floating disk is assumed to be zero. The static characteristics of a rectangular hydrostatic thrust bearing with this self-controlled restrictor are theoretically and experimentally investigated. It was consequently shown that the proposed hydrostatic thrust bearing can achieve a very high stiffness (nearly infinite stiffness) in a very wide range of applied load independent of supply pressure.
Thermal and Inertia Effects in Hydrodynamic Lubrication of Rollers by a Power Law Fluid Considering Cavitation
Authors: Prasad D. , Singh P. , Sinha Prawal
Published: June 2008

A theoretical aspect of hydrodynamic lubrication of two symmetric rollers by power law fluids is analyzed. The effect of fluid convective inertia, which is significant in case of high speed bearing, is taken into account. The effect of hydrodynamic pressure and temperature on the lubricant consistency m is assumed to vary with pressure and the mean temperature. The squeezing motion of the surfaces is also incorporated along with inertia and thermal effects. The Reynolds equation and the energy equation (with convection and conduction), which are coupled through m, are solved simultaneously. Various bearing characteristics such as pressure, temperature, load and drag etc. are obtained and a comparison between results (with and without inertia) is also made. It is noted that the effect of inertia is to increase pressure, temperature, load and drag etc. and to displace the position of pressure peak slightly towards the center line of contact of the rollers. An attempt is also made to study the variation of film thickness with load, speed, Eckert number, pressure, and temperature viscosity exponents.
The Elastic Field for Spherical Hertzian Contact of Isotropic Bodies Revisited: Some Alternative Expressions
Authors: Hanson M. T. , Johnson T.
Published: June 2008

Closed-form expressions in terms of elementary functions are derived for the elastic field resulting from spherical Hertz contact of isotropic bodies. Shear traction is also included using a Coulomb friction law; thus the shear stress in the contact region is equal to the contact pressure multiplied by a friction coefficient. This paper provides alternative expressions to those recently given by Hamilton (1983) and Sackfield and Hills (1983a). Two methods are outlined for obtaining the present solution and the complete solution for displacements and stresses are given for both normal and tangential loading in terms of just two distorted length parameters. The elastic field is written in a complex notation allowing the expressions to be put in a compact form. This also allows the expressions for sliding in two directions to be written as simply as for sliding in one direction.