bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical Performance of a Hydrostatic Foil Bearing
Authors: Carpino Marc , Peng Jih-Ping
Published: June 2008

The performance of a hydrostatic foil journal bearing operating with an incompressible fluid is discussed. In the configuration considered here, pressurized lubricant fluid is supplied through capillaries to recessed pockets on the surface of the journal. The foil is treated as a perfectly flexible, inextensible shell by a three dimensional finite element model. The pressure distribution is predicted by a finite element formulation of the incompressible Reynolds equation. Results are presented which demonstrate that the foil structure enhances load support while increasing lubricant film thickness. In addition, results indicate that membrane effects are essential in the structural model.
Static Analysis of Flying Characteristics of the Head Slider by Using an Optimization Technique
Authors: Choi Dong-Hoon , Yoon Sang-Joon
Published: June 2008

This paper suggests a method to predict the static solution for gas-lubricated slider bearings in a hard disk drive (HDD) by using an optimization technique. The modified Reynolds equation for the hydrodynamic lubrication theory under the slip condition is used to describe the compressible fluid flow within the gas bearing, and a factored implicit finite difference (FIFD) scheme is applied for pressure calculations. An optimization technique is employed to find the solution at which the forces and moments acting on the gas bearing are in a state of equilibrium. The proposed method is more general and robust than the conventional update schemes since it exploits the automatic update capability of the optimization technique instead of using an empirical approach, and may be more efficient than the transient FIFD approach since it directly searches for static solutions instead of tracking the time-dependent equations asymptotically to steady state. Results obtained for several typical head slider models are reported, showing the effectiveness of the proposed method.
Dynamic Flying Characteristics of Magnetic Head Slider With Dust
Authors: Tokuyama Mikio , Hirose Shinichi
Published: June 2008

The dust adhering to the taper of a slider changes the shape of its flying rail and decreases the efficiency of air lubrication that determines its flying height. The dynamic flying height characteristics of the slider with dust at its taper are numerically and experimentally examined. The numerical results show that a triangular accumulation of dust at the tapered portion of the slider degrades the motion-following performance of the slider as it covers the runout of the magnitude disk surface. A dust adhesion experiment revealed that dust at the tapered portion decreased the flying height and increased flying height fluctuations. The disk runout is considered a major reason for the fluctuation increase.
Effects of a Single Bump or Dent in Time Dependent Thermal Line EHD Lubrication
Authors: Sadeghi Farshid , Kim Kyung-Hoon
Published: June 2008

A time-dependent thermal compressible elastohydrodynamic lubrication of line contact model has been developed to investigate the effects of a single bump or dent in heavily loaded rolling/sliding contacts. The results illustrate the transient behavior of the film thickness, pressure and temperature distributions as a bump or a dent travels through the contact. The multigrid multilevel technique was used to simultaneously solve the discretized time dependent Reynolds, elasticity and energy equations. The effects of various loads and speeds have been investigated. Results are presented for the nondimensional loads of W = 1.3 × 10−4, 2.3 × 10−4 and nondimensional speeds ranging from U = 1 × 10−11 to U = 10−10 under pure rolling and rolling/sliding conditions.
Transient Analysis of Surface Features in an EHL Line Contact in the Case of Sliding
Authors: Venner C. H. , Lubrecht A. A.
Published: June 2008

This paper investigates in detail the influence of two different surface topographies on the pressure distribution and film thickness profile of a highly loaded (maximum Hertzian pressure 2 GPa) line contact as a function of the slide to roll ratio. To accomplish this the transient Reynolds equation is solved both in space and time. The first feature under investigation is localized, a so-called indentation, the second one is global: waviness. The observed lack of synchronism in the extremes of pressure and film thickness is explained theoretically by analyzing the Reynolds equation. The minimum and average film thickness values in case of waviness are analyzed as a function of the slide to roll ratio, amplitude, and wavelength. Depending on the slide to roll ratio, the transient solutions may differ significantly from their stationary counterparts. In such cases, therefore, a transient analysis cannot be avoided.
Numerical Contact Model of a Smooth Ball on an Anisotropic Rough Surface
Authors: Poon C. Y. , Sayles R. S.
Published: June 2008

A numerical elastic-plastic contact model of a smooth ball on a directionally structured anisotropic rough surface is presented. The contact model is tested on three types of surface contact of a smooth ball on (i) a smooth surface, (ii) a sinusoidal surface, and (iii) a real rough surface. The validity of the model is proven by good agreement of the numerical result for the smooth surface with the Hertz analytical result. The contact of the sinusoidal surface shows that by the introduction of surface undulation in a regular pattern, the real pressure distribution follows the expected behavior where the contact pressure at the peak is maximum and the contact pressure at the valley is zero and the peak pressure decreases away from the ball center. The contact of the real rough surface shows the ability of the model to cope with the more practically realistic situation where the asperity heights are distributed randomly. The results of the rough surface contact analysis for different surface roughness are presented in a separate paper.
Motions of an Unstable Retainer in an Instrument Ball Bearing
Authors: Kingsbury E. , Walker R.
Published: June 2008

We made an experimental investigation of the motions of the retainer in an instrument ball bearing during stable operation and during squeal. Radial motions of the retainer were measured with two fiber-light probes mounted 90 physical degrees apart. A signal analyzer was used to determine the phasing and frequency content of the probe signals. During squeal, a high-frequency retainer motion was found to be superimposed on the normal retainer ball group rotation rate. This high-frequency motion, which we call whirl, is a rigid-body translation in a circle. Whirl direction is opposite to the race for outer-race rotation, but in the same direction for inner-race rotation. Whirl frequency is approximately proportional to ball spin rate. The observations agree with predictions made from a squeal model based on retainer-to-ball frictional coupling that was originally presented in 1965.
The Effect of Surface Irregularities on the Tribological Behavior of Steel Rollers Under Rolling-Sliding Contact
Authors: Horng Jeng Hour , Lin Jen Fin , Lee Ke Yang
Published: June 2008

Experiments were conducted utilizing a gear-cam adapter to simulate line-contact lubrication and wear. Roller specimens with various asperity heights and roughness patterns were riding on the roller plates and sliding over the two lands of the lower specimen. The experimental results reveal that the roughness pattern and the asperity height are of importance in determining the friction coefficient, whereas asperity height is more related to the wear rate. The temperature rise of the lubricant seems to be seldom affected by the foregoing factors, but is governed by the combined effect of applied load and driver’s rotational speed. Surface roughness with a transversely oriented pattern, irrespective of the asperity height, normally generates a smaller friction coefficient and a relatively larger wear rate compared to the other two roughness patterns. Rollers having a smooth surface (0.2 μm) do not always lead to the smallest friction, but usually produce the minimum wear rate. The operating conditions, including the increase in applied load, and the reduction of the driver’s rotational speed, along with the increase of asperity height, form the controlling factors of increasing wear rate.
An Improved Model for Nonplanar Contact Sliding in Ball Bearings
Authors: Tzenov P. I. , Sankar T. S.
Published: June 2008

This investigation focuses on the contact interaction between a ball and a torus ring as in ball bearings. The sliding velocity field at the nonplanar contact area is analyzed and the resulting friction force and moment are determined assuming same friction coefficient at any point of the contact area. The results are illustrated by an application case to an angular contact ball bearing. The procedure developed through an appropriate computer simulation is useful in ascertaining the steady-state motion of a ball in ball bearings, in friction-and-wear test machine, etc. The results contribute to a better understanding of the nature of friction phenomena and the ball motion controlled by these phenomena. They also provide data on sliding and amount of wear at the contact areas, as well as lubricant degradation.
Numerical Simulation of Engagement of Paper Based Wet Clutch Facing
Authors: Natsumeda Shinichi , Miyoshi Tatsuro
Published: June 2008

Theoretical analysis was carried out on the engagement process of a paper based wet clutch. The annular multidisk clutch in a SAE#2 tester was chosen as an example. The average flow model on partial hydrodynamic lubrication proposed by Patir and Cheng was employed for the analysis of squeeze motion taking the permeability, the compressive strain, and the asperity contact of paper facing into account. Simultaneously, the equations of heat conduction were solved to calculate the viscosity reduction due to frictional heat generation. The theoretical torque curve agreed qualitatively with the experimental one in spite of the many assumptions made in this analysis.
Thermal Effects Due to Surface Films in Sliding Contact
Authors: Vick Brian , Golan L. P. , Furey M. J.
Published: June 2008

The present work examines theoretically the influence of surface coatings on the temperatures produced by friction due to sliding contact. A generalized thermal model is developed which incorporates three-dimensional, transient heat transfer between layered media with thermal coupling at multiple, interacting contact patches. A solution technique based on a variation of the boundary element method is developed and utilized. The method allows for the solution of the distribution of frictional heat and the resulting temperature rise in an accurate yet numerically efficient manner. Results are presented showing the influence of film thickness, thermal properties, velocity, and contact area on the division of heat and surface temperature rise. The results show that a film with thermal properties different than those of the substrate can have a pronounced effect on the predicted temperature rise.
Numerical Simulation of Contact Pressure Evolution in Fretting
Authors: Johansson L.
Published: June 2008

In the present paper an algorithm for frictional contact between two elastic bodies is presented. The algorithm is applied to the calculation of the evolution of contact pressure between two elastic bodies when material is being removed by fretting. To this end Archard’s law of wear is implemented into the algorithm. It is noticed that the calculated pressures after a period of fretting differ considerably from the initial Hertz type pressures. Further, it is noted that numerical instabilities can occur in explicit type wear calculations, and a stability criterion is suggested.
Frictional Temperature Field and Wear Behavior of Steel 52100 With Different Microstructures
Authors: Wang You , Yan Mufu , Li Xiaodong , Lei Tingqun
Published: June 2008

In this paper, the frictional temperature fields and the wear resistance of steel 52100 with different microstructures during dry sliding were studied by wear testing and computer simulation for thermometric data, which were real-time recorded by a thermal video system, using a mathematical model of frictional temperature field. The results show that the wear resistances of different microstructures are in close relationship with the temperature fields in surface layers during sliding. It is suggested that, because the different microstructures possess different thermal conductivities, the different microstructures will exhibit different frictional temperature fields, which will affect the wear resistances themselves. The less the thermal conductivity, the higher is the surface temperature during sliding, the lower is the wear resistance too.
Surface Temperatures Generated With Ceramic Materials in Oscillating/Fretting Contact
Authors: Weick B. L. , Furey M. J. , Vick B.
Published: June 2008

This paper summarizes the results of a study of the tribological behavior of ceramic materials in unlubricated oscillating/fretting contact with particular emphasis on frictionally-generated surface temperatures. The study was carried out using an oscillating contact device coupled to an infrared microscope. The contact geometry consists of a stationary spherical specimen loaded against a thin sapphire optical flat driven by an electromagnetic shaker. With this system, measurements can be made of friction, wear, and surface temperature over a wide range of loads, frequencies, and vibration amplitudes. Four ceramic materials were investigated, namely zirconium oxide (zirconia), tungsten carbide, and two different forms of aluminum oxide (alumina and sapphire). The first important finding was that each material exhibited unique and characteristic tribological behavior. Instantaneous variations in friction and surface temperature occurring within a single cycle of oscillation (e.g., in less than 0.005 seconds) could readily be measured with this technique. By digitizing the surface temperature, friction, and velocity signals, comparisons are made in the time and frequency domains. Frequency content correlations are determined using Fourier transform techniques. In addition, instantaneous frictional heat generation rates are calculated using the digitized friction and velocity data. Based on a series of experiments at constant oscillation frequency and amplitude, a correlation appears to exist between wear and surface temperature for the ceramics studied. Sapphire-on-sapphire and zirconia-on-sapphire produced the highest wear and the highest surface temperature rises (ca. 130–140 K). Tungsten carbide-on-sapphire produced the lowest wear and lowest surface temperature rise, while alumina exhibited intermediate behavior. In all cases, the rate of frictional heat generation was relatively low. It may be that the rapid fluctuations in surface temperature under these conditions—with two major temperature peaks per cycle—could lead to a kind of thermal stress fatigue of the ceramics as a wear mechanism. The use of the IR microscope in the scanning mode, coupled with scanning electron microscopy of the wear scars and theoretical treatment of sub-divided areas, can shed light on the nature and distribution of real areas of contact.
A Digraph Approach to System Wear Evaluation and Analysis
Authors: Gandhi O. P. , Agrawal V. P.
Published: June 2008

System structure is important for analyzing wear of a system. In this paper, this aspect is considered at the component level and is modeled using digraph and matrix methods. Contributing wear factors and the degree of influence among these factors, which determine wear independence and wear process between mating components, are identified. This along with the wear measure of subsystems determines the system wear. The procedure develops wear functions for characterization and comparison of subsystems of different types. These are also useful for wear analysis and evaluation leading to an optimum selection of subsystems based on the wear. The procedure is also shown to be useful at the operating state of a system and subsystems. Step-by-step procedure for system wear evaluation is presented. Methodology is also demonstrated by an example.
Effect of TiO2 Addition of Oxidizing Catalytic Activity of an Al2O3 Slider for Magnetic Disks
Authors: Higuchi Shinsuke , Miyazaki Takeshi , Suzuki Yasutaka , Tanaka Hideaki , Matsuyama Iwao
Published: June 2008

Oxidizing catalytic activity of a slider is a factor which could affect wear of magnetic disks with a carbon overcoat. Al2O3 composites containing 2–50 mol% TiO2 were produced, which had different oxidizing catalytic activities but nearly the same hardness and thermal conductivity. Activation energy (Ec) for carbon oxidation when it is mixed with the composite was measured to get the oxidizing catalytic activity, and it was found that Ec changed from about 70 kJ/mol for Al2O3 to about 110 kJ/mol for Al2O3 containing 9.1–16.7 mol% TiO2. TiO2 addition increased and decreased Ec. The former was due to segregation of the Ti-Al-O phase at the Al2O3 grain boundary, which could inhibit the catalysis at the Al2O3 grain boundary. The latter was due to the unreacted TiO2 phase, which by itself has high catalytic activity. TiO2-Al2O3 sliders having different Ec were examined in sliding wear against a magnetic disk with a carbon overcoat using a pin-on-disk test system. It was found that wear rate of the carbon overcoat was lower when the slider had a larger Ec, i.e., lower catalytic activity. It was also found that wear particles of the carbon overcoat were likely to be larger with lower catalytic activity.
A Theoretical Analysis of Breakaway Friction Measurement
Authors: Li Yufeng , Menon Aric Kumaran
Published: June 2008

The transient response of an I-beam friction measurement system to a constant disk acceleration is theoretically analyzed. The friction measurement system is modeled as a single-degree-of-freedom system. It is found that the true breakaway friction is different from the measured breakaway friction, and that the true breakaway time is also different from the measured breakaway time. The system measurement error is evaluated as a function of the acceleration, the effective mass, the spring stiffness, as well as the static and kinetic coefficients of friction. A closed-form dimensionless solution is derived for the system measurement error. If the dynamic effect of the measurement system can be neglected by selecting an appropriate effective mass and acceleration, the maximum possible discretization error can be estimated as a dimensionless function of the discretization frequency, the breakaway friction, the spring stiffness, and the acceleration.
Advancements in the Performance of Aerodynamic Foil Journal Bearings: High Speed and Load Capability
Authors: Heshmat H.
Published: June 2008

An advanced-design, aerodynamic, air-lubricated foil journal bearing achieved a landmark speed of 2200 cps (132,000 rpm) and a major breakthrough in load performance of 673.5 kPa (97.7 psi). At 20°C (68°F) room temperature, normal ambient pressure, 995 cps (59,700 rpm) rotor speed, and with bearing projected pad area of 1081 mm2 (1.675 in2), the bearing demonstrated a load capacity of 727.8 N (163.6 lb). The bearing also exhibited low heat generation, with about 40°C (104°F) average side leakage temperature rise. For this demonstration, a highspeed spindle utilizing a pair of 35-mm (1.375-in.) bearings and supporting a test rotor with a mass of 1.545 kg (weighing 3.41 lb) and overall length of 211 mm (8.3 in.) was successfully taken to the limiting speed of the test apparatus. This speed was set by the maximum sound velocity (Mach 1) in the spindle’s turbine wheel. The rotor/bearing speed of 4.62 × 106 DN is beyond the capability of any advanced oil-lubricated ball bearings or conventional gas-lubricated bearings. The net result is a highly stable bearing at high operating speed. This paper presents the development of this air-lubricated foil journal bearing, the operational procedures used during testing, test results (dynamic analyses), and load performance characteristics.
Non-Newtonian Effects on the Static Characteristics of One-Dimensional Slider Bearings in the Inertial Flow Regime
Authors: Hashimoto H.
Published: June 2008

In this paper, the non-Newtonian effects of lubricants on the static characteristics of one-dimensional, high-speed slider bearings are examined theoretically by considering the fluid inertia effects. In the derivation of the modified Reynolds equation, the fluid inertia term in the momentum equation for the non-Newtonian lubricant films is averaged over the film thickness, and the Rabinowitsch empirical model is used as a constitutive equation for non-Newtonian fluids. Applying the modified Reynolds equation to the one-dimensional slider bearings and solving the equation analytically based on the perturbation technique, the film pressure, load carrying capacity, friction force, and inlet flow rate are obtained under various values of the dimensionless nonlinear factor and film thickness ratio. The combined effects of fluid inertia and non-Newtonian characteristics on these static characteristics of lubricants are discussed.
Elastohydrodynamic Lubrication With O/W Emulsions
Authors: Zhu D. , Biresaw G. , Clark S. J. , Kasun T. J.
Published: June 2008

This paper presents a set of experimental results of the EHL film thickness with oil-in-water (O/W) emulsions in a wide range of rolling speed for different oil concentrations and pH values. The O/W emulsions have wide applications in metal-forming and machining processes as well as hydraulic systems. However, their lubrication mechanisms are very complex and have not been fully understood. A newly developed high speed optical EHL rig was used to measure the film thickness and observe the two-phase flow around the EHL point and line contacts. Experimental observations indicate that phase inversion/oil pool formation mechanism around the inlet zone takes place only at very low speeds, which are most likely far below practical speed ranges for major industrial applications. When the speed is low, the lubricant film thickness is dominated by the bulk properties of oil phase, and can be estimated by the conventional EHL theory together with the consideration of starvation effect. After the speed exceeds a certain limit, called first critical speed, there is a transition region, where no stable oil pool is observed and the film thickness starts to decrease, or increases slightly then decreases. It is believed that in this transition region there is still a considerable amount of oil concentrated in the inlet zone, and this local oil concentration decreases as the speed increases. The film thickness appears to be dominated by the entrainment of oil-enriched two-phase lubricant in the inlet zone. The increase of film thickness is due to entraining effect and the decrease due to the increased oil phase starvation. If the speed is further increased exceeding a second critical speed, the film thickness will stop decreasing and start to increase again. In this high speed region the local oil concentration of entrained lubricant in the inlet zone is believed to become quite constant and close to that of the bulk lubricant supply. The film thickness, therefore, continuously increases for all of the tested line and point contact cases as the speed goes up, and is always significantly smaller than that of neat oil but larger than that of pure water. The destabilized emulsions with lower pH values can form more stable oil pools and considerably thicker films. This is because the oil droplets in these low pH emulsions can be more easily trapped and brought into the contact by the solid surfaces. However, for the tested emulsions, the oil pools still cannot survive reasonably high speeds.
Roughness Influence on Turbulent Flow Through Annular Seals
Authors: Lucas Victor , Danaila Sterian , Bonneau Olivier , Freˆne Jean
Published: June 2008

This paper deals with an analysis of turbulent flow in annular seals with rough surfaces. In this approach, our objectives are to develop a model of turbulence including surface roughness and to quantify the influence of surface roughness on turbulent flow. In this paper, in order to simplify the analysis, the inertial effects are neglected. These effects will be taken into account in a subsequent work. Consequently, this study is based on the solution of Reynolds equation. Turbulent flow is solved using Prandtl’s turbulent model with Van Driest’s mixing length expression. In Van Driest’s model, the mixing length depends on wall shear stress. However there are many numerical problems in evaluating this wall shear stress. Therefore, the goal of this work has been to use the local shear stress in the Van Driest’s model. This derived from the work of Elrod and Ng concerning Reichardt’s mixing length. The mixing length expression is then modified to introduce roughness effects. Then, the momentum equations are solved to evaluate the circumferential and axial velocity distributions as well as the turbulent viscosity μ1 (Boussinesq’s hypothesis) within the film. The coefficients of turbulence kx and kz, occurring in the generalized Reynolds’ equation, are then calculated as functions of the flow parameters. Reynolds’ equation is solved by using a finite centered difference method. Dynamic characteristics are calculated by exciting the system numerically, with displacement and velocity perturbations. The model of Van Driest using local shear stress and function of roughness has been compared (for smooth seals) to the Elrod and Ng theory. Some numerical results of the static and dynamic characteristics of a rough seal (with the same roughness on the rotor as on the stator) are presented. These results show the influence of roughness on the dynamic behavior of the shaft.
A Test Apparatus and Facility to Identify the Rotordynamic Coefficients of High-Speed Hydrostatic Bearings
Authors: Childs Dara , Hale Keith
Published: June 2008

A facility and apparatus are described which determine stiffness, damping, and added-mass rotordynamic coefficients plus steady-state operating characteristics of high speed hydrostatic journal bearings. The apparatus has a current top speed of 29800 rpm with a bearing diameter of 7.62 cm (3 in.). Purified warm water, 55°C (130°F), is used as a test fluid to achieve elevated Reynolds numbers during operation. The test-fluid pump yields a bearing maximum inlet pressure of 6.9 Mpa (1000 psi). Static load on the bearing is independently controlled and measured. Orthogonally mounted external shakers are used to excite the test stator in the direction of, and perpendicular to, the static load. The apparatus can independently calculate all rotordynamic coefficients at a given operating condition.
Rotordynamics of a Mechanical Face Seal Riding on a Flexible Shaft
Authors: Lee An Sung , Green Itzhak
Published: June 2008

A mechanical face seal is a triboelement intended to minimize leakage between a rotating shaft and a housing, while allowing the shaft to rotate as freely as possible. All dynamic analysis to date have concentrated on the seal itself. In reality, however, especially in high speed turbomachinery, shafts are made flexible and the dynamics of seals must be coupled with the dynamics of shafts. (Perhaps the dynamics of other triboelements, such as gears, bearings, etc., have to be included as well.) In this work the complex extended transfer matrix method is established to solve for the steady state response of a noncontacting flexibly mounted rotor mechanical face seal that rides on a flexible shaft. This method offers a complete dynamic analysis of a seal tribosystem, including effects of shaft inertia and slenderness, fluid film, secondary seal, flexibly mounted rotating element, and axial offset of the rotor center of mass. The results are then compared to those obtained from an analysis that implicitly assumed the shaft rigid. The comparison shows that shaft dynamics can greatly affect the seal performance even at relatively low speeds.
Stability and Bifurcation of Unbalanced Response of a Squeeze Film Damped Flexible Rotor
Authors: Zhao J. Y. , Linnett I. W. , McLean L. J.
Published: June 2008

The stability and bifurcation of the unbalance response of a squeeze film damper-mounted flexible rotor are investigated based on the assumption of an incompressible lubricant together with the short bearing approximation and the “π” film cavitation model. The unbalanced rotor response is determined by the trigonometric collocation method and the stability of these solutions is then investigated using the Floquet transition matrix method. Numerical examples are given for both concentric and eccentric damper operations. Jump phenomenon, subharmonic, and quasi-periodic vibrations are predicted for a range of bearing and unbalance parameters. The predicted jump phenomenon, subharmonic and quasi-periodic vibrations are further examined by using a numerical integration scheme to predict damper trajectories, calculate Poincare´ maps and power spectra. It is concluded that the introduction of unpressurized squeeze film dampers may promote undesirable nonsynchronous vibrations.
Effect of a Circumferential Feeding Groove on the Dynamic Force Response of a Short Squeeze Film Damper
Authors: Arauz G. L. , San Andres L.
Published: June 2008

The effect of a circumferential feeding groove on the dynamic force response of a short length, open end squeeze film damper is studied experimentally. Damper configurations with increasing groove depths and journal orbit radii were tested for several conditions of whirl frequency and lubricant viscosity. Significant levels of dynamic pressure were measured at the circumferential groove, and relatively large tangential (damping) forces are produced at the groove which contribute considerably to the damping characteristics of the SFD test articles. Radial forces of substantial magnitude are determined at the groove and at the thin film land where the squeeze film Reynolds number is typically less than 1. The circumferential groove is thought to induce an inertia like effect into the film land. The experimental results correlate well with the predictions from a groove volume-circumferential flow model developed.
Atomic-Scale Friction Measurements Using Friction Force Microscopy: Part I—General Principles and New Measurement Techniques
Authors: Ruan Ju-Ai , Bhushan Bharat
Published: June 2008

Friction force measurements using modified atomic force microscopy, called here Friction Force Microscopy (FFM), are becoming increasingly important in the understanding of fundamental mechanisms of friction, wear, and lubrication, and to study interfacial phenomena in micro- and nanostructures used in magnetic storage systems and Microelectromechanical Systems (MEMS). FFMs can be used to study engineering surfaces in dry or wet conditions. A review of existing designs of FFMs and methods of friction force measurements is presented. In terms of friction force measurements, there are important issues related to the basic operation and calibration of these instruments which have not been fully studied. A new method of measuring friction fore using a commercial FFM and a calibration procedure for conversion of measured data to normal and friction forces are presented. Microscale friction data of selected materials are presented and discussed in light of macro-friction measurements.
Atomic-Scale Friction Measurements Using Friction Force Microscopy: Part II—Application to Magnetic Media
Authors: Bhushan Bharat , Ruan Ju-Ai
Published: June 2008

Atomic Force/Friction Force Microscopes (AFM/FFM) were used to study tribological properties of metal-particle tapes with two roughnesses, Co-γFe2O3 tapes (unwiped and wiped), and unlubricated and lubricated thin-film magnetic rigid disks (as-polished and standard textured). Nanoindentation studies showed that the hardness of the tapes through the magnetic coating is not uniform. These results are consistent with the fact that the tape surface is a composite and is not homogeneous. Nanoscratch experiments performed on magnetic tapes using silicon nitride tips revealed that deformation and displacement of tape surface material occurred after one pass under light loads (~ 100 nN). A comparison between friction force profiles and the corresponding surface roughness profiles of all samples tested shows a poor correlation between localized values of friction and surface roughness. Detailed studies of friction and surface profiles demonstrate an excellent correlation between localized variation of the slope of the surface roughness along the sliding direction and the localized variation of friction. Micro-scale friction in magnetic media and natural diamond appears to be due to adhesive and ratchet (roughness) mechanisms. Directionality in the local variation of micro-scale friction data was observed as the samples were scanned in either direction, resulting from the scanning direction and the anisotropy in the surface topography. Micro-scale coefficient of friction is generally found to be smaller than the macro coefficient of friction as there may be less ploughing contribution in micro-scale measurements.
A Mass and Energy Conserving Finite Element Lubrication Algorithm
Authors: Kumar A. , Booker J. F.
Published: June 2008

A previous mass-conserving finite element cavitation model is extended to include energy conservation in the lubricant film. The extension introduces no additional iteration and very little additional computational effort.
Adiabatic Approximate Solution for Static and Dynamic Characteristics of Turbulent Partial Journal Bearings With Surface Roughness
Authors: Hashimoto H. , Mongkolwongrojn M.
Published: June 2008

Hydrodynamic bearings are generally used for a long term, so the bearing surfaces may be roughened for many reasons such as wear, impulsive damage, foreign particles, cavitation erosion, rust, and so on. Under the turbulent operating conditions of high speed bearings, the surface roughness may result in considerable increase in both film pressure and temperature. This paper describes an adiabatic approximate solution for the static and dynamic characteristics of 180 deg partial journal bearings with homogeneous surface roughness. Applying the modified lubrication equation and energy equation, considering the combined effects of turbulence and surface roughness, to the finite width 180 deg partial journal bearings, the static and dynamic characteristics such as pressure and temperature distributions, Sommerfeld number, attitude angle, spring and damping coefficients and whirl onset velocity are obtained numerically. In the numerical analysis of the temperature distribution, adiabatic boundary conditions are assumed and then the heat transfer effect to the journal and bearing-bush surfaces is omitted. The numerical results are indicated in graphic form for various relative roughness under the mean Reynolds number of Re = 5000 and 10,000. Moreover, some numerical results of static characteristics are compared with the experimental results.
Thermohydrodynamic Lubrication Analysis Incorporating Thermal Expansion Across the Film
Authors: Wang Nen-Zi , Seireg Ali A.
Published: June 2008

The study reported in this paper deals with the development of a thermohydrodynamic computational procedure for evaluating the pressure, temperature and velocity distributions in fluid films with fixed geometry between the stationary and moving bearing surfaces. The velocity variations and the heat generation are assumed to occur in a central zone with the same length and width as the bearing but with a significantly smaller thickness than the fluid film thickness. The thickness of the heat generation (shear) zone is developed empirically for the best fit with experimentally determined peak pressures for a journal bearing with a fixed film geometry operating in the laminar regime. A transient thermohydrodynamic computational model with a transformed rectangular computational domain is utilized. The analysis can be readily applied to any given film geometry. The computed distribution of the pressure in the film is in excellent agreement with the experimental findings for different oils and speeds. The developed procedure gives an analytical basis for explaining the “Fogy effect” where significant pressures can be generated in slider bearings with parallel surfaces as a result of the thermal expansion of the film in the direction of the thickness. The procedure confirms the experimentally determined square root relationship between the pressure and the sliding velocity reported in references [1–4]. The normalized pressure profiles computed for the different conditions of the journal bearings are identical to those obtained by isoviscous theory.
Computer Optimization of External Fixation and Initial Clamping of a Bushing With Respect to Static and Dynamic Properties and Journal Bearings
Authors: Kicin´ski J. , Haller R.
Published: June 2008

Various models of external fixation of the bushing, including models which take deformation of the fixation itself into account, have been presented in this work. Analysis of static and dynamic properties of the bearing and of the simple rotor-bearing system as dependent on the bush fixation variant has been performed. Investigations on the influence of preliminary assembly of the bush on the system properties have been performed as well. Calculations were based upon a complex thermoelastohydrodynamic model and a special computer program worked out for this purpose. Obtained results point to wide possibilities of improvement of the system properties by optimization of fixation and clamping the bush in the bearing supports.
Analysis of Aerodynamic Journal Bearings With Small Number of Herringbone Grooves by Finite Element Method
Authors: Bonneau D. , Absi J.
Published: June 2008

A numerical study of gas herringbone grooved journal bearings is presented for small number of grooves. The compressible Reynolds equation is solved by use of the Finite Element Method. The nonlinearity of the discretized equations is treated with the Newton-Raphson procedure. A comparison of the results for a smooth bearing with previously published results is made and the domain of validity of the Narrow Groove Theory is analyzed. Load capacity, attitude angle, and stiffness coefficients are given for various configurations: groove angle and thickness of grooves, bearing number, and that for both smooth and grooved member rotating.
Forces Due to a Magnetic Dipole Near a Sliding Conductor: Applications to Magnetic Levitation and Bearings
Authors: Simone Michelle , Tichy John
Published: June 2008

A conducting body moving with respect to a magnet experiences lift and drag forces from the eddy currents induced in the conductor. The force on the conductor is dependent on the relative velocity between the conductor and the magnet. In this study, we investigate the force dependence on magnetic Reynolds number, a dimensionless indicator of velocity. The Lorentz equation is used to predict the force on the conductor, given the spatial dependence of the eddy currents and magnetic induction vector inside the conductor. Maxwell’s equations, which govern the electromagnetic quantities, are reduced to a single convection-diffusion equation for the magnetic induction vector inside the conducting body. An integral solution which satisfies the governing equation and boundary conditions is used to obtain the eddy currents and magnetic field. For our model, both lift and drag forces increase sharply with Reynolds number, reach a maximum, and decrease with increasing Reynolds number to an asymptotic limit. We also find that skin depth, the depth to which the eddy currents decay inside the conductor, decreases with increasing Reynolds number. The relevance to magnetically supported high-speed vehicles and magnetic bearings is discussed.
Effect of the Rotor/Stator Interface Condition Including Contact Type, Geometry, and Material on the Performance of Ultrasonic Motor
Authors: Maeno Takashi , Bogy David B.
Published: June 2008

The purpose of this paper is to show the dynamic contact characteristics and the motor performance of several types of ultrasonic motors. First, we compare the performance of “intermittent” and “traveling” contact type ultrasonic motors by using the locally deforming rotor model. We show that the motor performances are the same when the shape of the stator’s amplitude, and the rotor’s interpenetration are the same. Then, we show that the performance of these two types of motors are different according to the effects of inertia and actual three dimensional displacement by using the FE (finite element) analysis. Finally, the effects of the teeth and a surface layer at the contact interface on the performance of the ultrasonic motor, including T-N (torque-speed) characteristics and friction loss, are analyzed.
The Application of Newton-Raphson Method to Thermal Elastohydrodynamic Lubrication of Line Contacts
Authors: Wolff R. , Kubo A.
Published: June 2008

The Newton-Raphson method was applied to solve the thermal EHD lubrication model of line contacts. By accounting for thermal effects in the Newton-Raphson scheme, a very stable numerical approach was obtained. Two models with viscosity constant and variable across the oil film were developed. The results under extremely heavy conditions of dimensionless load W = 52 * 10−5 (pH = 2 GPa) and dimensionless rolling velocity U = 20 * 10−11 are presented. They show that even for pure rolling, but under heavy load and high rolling velocity conditions, the thermal effects significantly reduce the minimum film thickness. The distributions of pressure, film thickness, and temperature for two rolling velocities and various loads are presented. They indicate that under high rolling velocity conditions the thermal effects have a strong influence on a pressure spike.
Higher-Order Multilevel Solvers for the EHL Line and Point Contact Problem
Authors: Venner C. H.
Published: June 2008

This paper addresses the development of efficient numerical solvers for EHL problems from a rather fundamental point of view. A work-accuracy exchange criterion is derived, that can be interpreted as setting a limit to the price paid in terms of computing time for a solution of a given accuracy. The criterion can serve as a guideline when reviewing or selecting a numerical solver and a discretization. Earlier developed multilevel solvers for the EHL line and circular contact problem are tested against this criterion. This test shows that, to satisfy the criterion a second-order accurate solver is needed for the point contact problem whereas the solver developed earlier used a first-order discretization. This situation arises more often in numerical analysis, i.e., a higher order discretization is desired when a lower order solver already exists. It is explained how in such a case the multigrid methodology provides an easy and straightforward way to obtain the desired higher order of approximation. This higher order is obtained at almost negligible extra work and without loss of stability. The approach was tested out by raising an existing first order multilevel solver for the EHL line contact problem to second order. Subsequently, it was used to obtain a second-order solver for the EHL circular contact problem. Results for both the line and circular contact problem are presented.
Numerical Simulation of a Transverse Ridge in a Circular EHL Contact Under Rolling/Sliding
Authors: Venner C. H. , Lubrecht A. A.
Published: June 2008

This paper investigates the influence of a transverse ridge on the film thickness in a circular EHL contact under rolling/sliding conditions. It is a numerical simulation of the optical EHL work of Kaneta et al. (1992). One of the purposes of this investigation is to check the validity of the algorithm and the Newtonian, isothermal lubricant assumption for film thickness predictions under these conditions (ph = 0.54 GPa). It will be shown that, both quantitatively, the film thickness on the central axis Y = 0, and qualitatively, the film thickness profile through “pseudo interference graphs”, the agreement between experiment and Newtonian isothermal theory is good. This supports the argument that the rheological and the thermal behavior of the fluid only slightly influence the film thickness and pressure distribution of the lightly loaded non-smooth contact case.
An Efficient Algorithm for Thermal Elastohydrodynamic Lubrication Under Rolling/Sliding Line Contacts
Authors: Hsu Chao-Ho , Lee Rong-Tsong
Published: June 2008

One of the most time-consuming routines in thermal EHL problem is the calculation of the surface temperature integral. Combining the multigrid technique and the Newton-Raphson method, a modified multilevel, multi-integration algorithm for this integral is developed that can reduce the computational complexity from O (n2) to O (n ln n) for the thermal EHL problem of rolling/sliding line contacts. The employed standard central difference approximation to the coupled Reynolds and energy equations can yield the maximum difference of mass flow flux within one percent. Effects of dimensionless load, dimensionless materials parameter, slip ratio, and thermal loading parameter on the minimum film thickness are investigated. Correlation formula of thermal reduction factor for the minimum film thickness is derived for a wide range of slip ratios, loads, thermal loading parameters, and materials parameters.
Elastohydrodynamic Lubrication at Impact Loading
Authors: Larsson Roland , Ho¨glund Erik
Published: June 2008

Experimental and theoretical studies of elastohydrodynamically lubricated contacts normally assume static or quasi-static conditions. Nonsteady conditions are, however, very common, e.g., in machine elements such as ball bearings, gears, and cam-follower mechanisms. In this paper, the case of a ball impacting a flat lubricated surface is investigated theoretically. This case implies transient conditions and the lubricating effect is due to pure squeeze action in the contact. Pressure and film thickness distributions are computed during impact and rebound. The results of the analysis show the effects of ball mass, initial impact velocity, lubricant properties, and the thickness of the applied lubricant layer on, for example, minimum film thickness, maximum impact force, and maximum pressure. Increasing impact velocity increases the minimum value of film thickness achieved during the total impact time. The damping capacity of the lubricating film is very high at low impact velocity and small ball mass. In fact, the damping is so high that no rebound occurs if the velocity or the ball mass are smaller than certain critical values. The thickness of the lubricant layer has very little influence on the results if it is thicker than a certain value. If the pressure-viscosity coefficient is increased, the film becomes thicker.
EHL of Coated Surfaces: Part II—Non-Newtonian Results
Authors: Elsharkawy A. A. , Hamrock B. J.
Published: June 2008

A complete non-Newtonian elastohydrodynamic lubrication solution for multilayered elastic solids is introduced in this paper. A modified form for the Reynolds equation was derived by incorporating the circular non-Newtonian fluid model associated with a limiting shear strength directly into the momentum equations that govern the instantaneous equilibrium of a fluid element inside the lubricated conjunction. The modified Reynolds equation, the elasticity equations of multilayered elastic half-space, the lubricant pressure-viscosity equation, the lubricant pressure-density equation, and the load equilibrium equation were solved simultaneously by using the system approach. The effects of the surface coating on pressure profiles, film shapes, and surface shear stress profiles are shown. Furthermore, the effects of coating thickness on the minimum film thickness and on the coefficient of friction are presented for different coating materials. The results show that for hard coatings non-Newtonian fluid effects on the pressure profiles and film shapes are significant because of the increase in the contact pressure.