bluuebunny/crossref_metadata_2025 huggingface dataset:
Considerations of a Simplified Analysis of Transducer Dynamics for Measuring Friction
Authors: Streator J. L.
Published: June 2008

In many tribological investigations, the measurement of friction is complicated by the vibrational characteristics of the transducer. In particular, when a mechanical force transducer is excited, the static calibration factor no longer provides the friction force in the interface. In a recent study (Streator and Bogy, 1992), the problem of determining the friction force from a measurement of the periodic bending strain in a cantilever-mass transducer was solved using the equations of Euler-Bernoulli beam theory. The method showed that modelling the transducer as a single degree-of-freedom (DOF) system could lead to inaccuracies depending on the frequency content of the friction force, and depending on whether a measurement of beam strain or mass displacement was available. Employing the Euler-Bernoulli analysis, however, requires substantially greater analytical and computational effort. In the present work a two DOF model is considered which represents a compromise between the simplicity of the single DOF model and the accuracy of the more thorough analysis. Force computations are made for a simulated slider/disk tribological contact at different frequencies of disk rotation. One set of computations is performed based on knowledge of the beam strain and the other based on knowledge of the mass displacement. It is found that the friction force computed from the two DOF model compares well with that of the Euler-Bernoulli analysis for an extended range of frequencies as against the single DOF model, but loses accuracy at high frequencies. Nevertheless, if used in a careful manner, the two DOF model offers an improved method of determining dynamic friction forces for certain applications.
Entry Flow and Pressure Jump in Submerged Multi-Pad Bearings and Grooved Bearings
Authors: Mori A. , Makino T. , Mori H.
Published: June 2008

Numerical studies are conducted to investigate the flow and pressure fields along the runner surface in submerged multi-pad and/or multi-grooved bearings under high Reynolds number. The cavity region between the pads and/or the oil feed groove is modified into rectangular shape. The bearing gap region is modified into linearly tapered one. The two-dimensional full Navier-Stokes equations are solved consistently for both regions by means of the penalty function formulation and the Galerkin finite element method incorporated with the Newton-Raphson linearization. General feature of the inertia effects on the entry flow and pressure jump is illustrated under various conditions. The results are used to improve the former simplified approach, which is based on the quasi-fully developed gap flow linked to the cavity flow of boundary layer type by using conservation equations of mass and mechanical energy.
Wear Debris Formation and Agglomeration
Authors: Oktay S. Tu¨rker , Suh Nam P.
Published: June 2008

The hypothesis that one of the primary roles of an effective lubricant is to prevent wear particle agglomeration, thus reduce the plowing of the interfaces by the particles, and lower the frictional force, has been tested through a series of experiments and modeling. The friction coefficient, and the interfacial separation of the sliding surfaces due to entrapped wear particles, were measured on a number of sliding pairs in both dry and lubricated sliding. The results showed that the particle size, even during a single sliding test, did not remain constant but increased as sliding progressed. This increase in the wear particle size was found to be due not to the formation of larger wear particles but to the agglomeration of the small wear particles produced during sliding. The formation of large wear particle agglomerates caused an increase in the amount of plowing and often a concurrent increase in the friction coefficient. When one of the sliding surfaces was textured with undulations, no wear particle agglomeration was observed. The newly formed wear particles simply got entrapped in surface microgrooves and were immediately removed from the sliding interface before their subsequent growth by agglomeration. It was further shown that “good” lubricants prevented agglomeration, thus the friction coefficient remained more or less at the initial low value. Both the dry and lubricated sliding test results are presented and discussed in light of the analytical models based on the plastic deformation of the individual wear particles and the slip-line field analysis of the wear particle agglomerate.
Compliant Foil Bearing Structural Stiffness Analysis: Part I—Theoretical Model Including Strip and Variable Bump Foil Geometry
Authors: Ku C.-P. Roger , Heshmat H.
Published: June 2008

This paper presents a theoretical model of corrugated foil strip (bump foil) deformation in compliant foil bearings and dampers. The friction forces between bump foils and the housing or the top foil, local interaction forces, variable load distributions, and bump geometries are taken into consideration. Following the trend of earlier published experimental data, the bumps near the fixed end have a much higher predicted stiffness (lower deflection) than those near the free end. Higher friction coefficients tend to increase stiffness and may pin down bumps near the fixed end. An increase in the friction coefficient between the top foil and the bump is a more effective method of achieving both Coulomb damping and higher stiffness. In addition to bump geometry, the load distribution profile greatly influences bump stiffness. A follow-up paper will present the experimental verification and discuss the comparison between theoretical and experimental results.
Mechanism of Mild to Severe Wear Transition in Alpha-Alumina
Authors: Jahanmir S. , Dong X.
Published: June 2008

Friction and wear experiments were conducted on high purity alpha-alumina sliding against a similar material in air under different contact loads and at temperatures ranging from 23°C to 900°C. Experimental results indicate that tribochemical reactions between water vapor and alpha-alumina at room temperature produce aluminum hydroxide which results in relatively low coefficients of friction and low wear rates. Both the coefficient of friction and the wear rate of alumina were low at intermediate temperatures (200°C to 800°C), if the contact stress was below a threshold value. Above this load, wear occurred by fracture, the wear coefficient exceeded a value of 10−4, and the coefficient of friction increased to 0.90. At 900°C, the coefficient of friction was 0.40 and the wear coefficient was reduced to a value less than 10−6, because of the formation of a silicon-rich layer on the wear track. A contact mechanics model based on linear elastic fracture mechanics indicated that propagation of cracks from pre-existing flaws controls the onset of catastrophic wear in the intermediate temperature range.
Magnetic Head-Media Interface Temperatures—Part 3: Application to Rigid Disks
Authors: Bhushan B.
Published: June 2008

A thermal analysis has been used to predict transient temperature rises at a typical head-particulate-disk interface and a head-thin-film-disk interface. Thermal properties of the various thin-films used in the construction of magnetic rigid disks are measured. Average and maximum transient temperature rises for the assumed head-particulate-disk interface over the contact area are 34 and 44°C, respectively for an Al2O3-TiC slider. If the exposed magnetic particles or alumina particles contact the slider surface, the transient temperature rise could be more than 1000°C. Average and maximum transient temperature rises for the assumed head-thin-film-disk interface over the contact area are 56 and 81°C, respectively for an Al2O3-TiC slider and 77 and 110°C, respectively for an Mn-Zn ferrite slider. The durations of asperity contact generally are less than 100 ns. The thermal gradients perpendicular to the sliding surfaces are very large (a temperature drop of 90 percent in a depth of typically less than a contact diameter or less than a micron).
Analysis for Incompressible Flow in Annular Pressure Seals
Authors: Simon F. , Freˆne J.
Published: June 2008

An analysis is developed to calculate the static and dynamic characteristics of annular eccentric seals. Effects of inertia forces in the film, tapered geometry and rotor misalignment are taken into account. Derivation of the governing equations for incompressible flow is based on the Navier-Stokes equations, the continuity equation and a turbulence model using the nonlinear analysis developed by Elrod and Ng. The inlet boundary conditions define the initial swirl and the pressure drop due to the fluid acceleration. Perturbation of the flow variables yields a set of zeroth-order and first-order equations. Integration of the zeroth-order equations yields the steady-state solution which defines the seal leakage, the static load and the moment of misalignment. The eccentric and misaligned rotordynamic coefficients are obtained by integration of the first-order pressure equations. Comparisons are made between the stiffness, damping and inertia coefficients derived herein and both theoretical results based on other models and experimental data which were previously published.
Normal Impact Model of Rough Surfaces
Authors: Chang Wen-Ruey , Ling Frederick F.
Published: June 2008

An elastic-plastic impact model for spheres is introduced as the basis to study the normal impact of rough surfaces. Statistics is applied to arrive at the ensemble behavior of many unit events alluded above, allowing the investigation of surface roughness effects. Dissipation of kinetic energy increases such as surface roughness, material compliance, and impact velocity is increased. The rebound velocity is shown to be dependent on surface topography and material properties, in addition to impact velocity.
Nonlinear Transient and Frequency Response Analysis of a Hydrodynamic Journal Bearing
Authors: Choy F. K. , Braun M. J. , Hu Y.
Published: June 2008

The traditional approach is to characterize the behavior and performance of fluid film hydrodynamic journal bearings by means of linearized bearing analysis. The objective of this paper is to examine the nonlinear characteristics of the said journal bearing. Results based on bearing stiffness characteristics, steady and transient vibration orbits, and frequency response functions obtained from both linear and nonlinear bearing simulations are compared with each other. Conclusions are drawn from the results obtained from a prototypical bearing configuration used as an example in this analysis.
Analysis of Multiple Fatigue Cracks—Part I: Theory
Authors: Dubourg M. C. , Villechaise B.
Published: June 2008

This paper analyzes the effects of multiple cracks situated in the contact zone vicinity of an elastic isotropic component, modeled as a half-plane. Friction between the crack faces is taken into account using Coulomb’s law. Straight arbitrarily oriented cracks are considered. Any contact condition can be modeled between the crack faces as well as any loading condition over the half-plane surface, including complete loading cycles. The method has been tested for up to 5 cracks and shows no limitation in crack number. Further, the method is general as no prior assumptions concerning the state of the crack, i.e., the slip-stick-open configurations along the crack are required. The stress intensity factors (SIFs) calculated for two crack configuration are compared with those obtained for single cracks.
Analysis of Multiple Fatigue Cracks—Part II: Results
Authors: Dubourg M. C. , Godet M. , Villechaise B.
Published: June 2008

A semianalytical model of multiple fatigue crack analysis in sliding contact is developed. Linear elastic fracture mechanics is applied. Frictional resistance between crack faces is taken into account. Five crack interaction mechanisms have been identified. Load transfer between cracks can cause both significant increases and drops in stress intensity factors both in mode I and II. The interaction depends on the distance between cracks, their relative position with respect to the loading zone, and the interfacial crack coefficient of friction.
Theory of Viscoplastic Lubrication: Part 1—Basic Relationships
Authors: Huang X. B. , Wilson W. R. D.
Published: June 2008

Basic flow relationships are developed for three typical non-Newtonian lubricant models: the pseudo-plastic model, the circular viscoplastic model, and the ideal viscoplastic model. It is found that the conventional no-slip boundary conditions are inadequate to fully treat the latter two models. Conclusions regarding the appropriate boundary conditions for any viscoplastic model and the form of the resulting Reynolds equations are presented.
Theory of Viscoplastic Lubrication: Part 2—Applications
Authors: Wilson W. R. D. , Huang X. B.
Published: June 2008

The Reynolds equations for viscoplastic lubricants can be quite complex, involving multiple branches which depend on the form of the boundary conditions invoked in their development. Methods of visualizing and understanding the branching problem are explored. This information is used, together with the Reynolds equations developed in the companion paper, in analyses of the lubrication of a wide slider bearing and a metalforming inlet zone. The results of these analyses are used to draw general conclusions regarding the influence of different types of non-Newtonian behavior.
Numerical Analysis of the Flow Field Within Lip Seals Containing Microundulations
Authors: Salant R. F.
Published: June 2008

The flow field within the lubricating film of a rotating shaft lip seal containing microundulations is analyzed numerically. The results demonstrate that the action of the microundulations can prevent leakage through the seal. The effects on leakage rate of shaft speed, undulation amplitude and wavelength, shear deformation of the undulations, flattening of the undulations, and axial lip profile are presented.
Surface Modification and Measurement Using a Scanning Tunneling Microscope With a Diamond Tip
Authors: Bogy D. B.
Published: June 2008

Scanning Tunneling Microscopy is used to modify and measure the surface of magnetic media disks. A very rugged diamond tip allows continued scanning after it has severely scratched or punched the surface. Three techniques are used. First a manual method of penetrating the surface using a stand-alone head makes a scratch of essentially uncontrollable length and depth. Then the normal head is used to cause surface penetration by removing the bias voltage while scanning. Better control is obtained as regards the location and depth of the indentation. Excellent control of indentation location and depth can be obtained by using a new software developed by the STM manufacturer to push the tip into the surface with the piezoelectric scanner. The control of the indentations and their subsequent measurement may make the STM a useful tool as a hardness tester for ultra-thin films, on the order of a few tens of nanometers.
Performance Characteristics of Shrouded Rayleigh-Step and Spiral Groove Viscous Pumps
Authors: Sato Yuichi , Knight J. D.
Published: June 2008

In this paper the optimum geometrical configurations of a shrouded Rayleigh-step and a spiral groove viscous pump are analyzed. Then their performance characteristics are compared. The geometries and film shapes of these viscous pumps are optimized with respect to flow rate and the design parameters are summarized graphically. The flow rate of the optimized spiral groove viscous pump is approximately 5–10 percent greater than that of the optimized shrouded Rayleigh-step viscous pump.
An Experimental Study of the Parameters That Determine Slider-Disk Contacts During Dynamic Load-Unload
Authors: Jeong T. G. , Bogy D. B.
Published: June 2008

The parameters that appear to determine if slider-disk contact occurs during dynamic loading are the relative loading velocity at the instant of load and the initial pitch and roll of the slider at its unloaded state. A dual-beam LDV system is employed in this study to measure the displacement, pitch, and roll during dynamic loading for five different standard 3380-type sliders in order to investigate the effects of the initial pitch and roll. The effects of the initial height and control loading speed are also examined by using the dual-beam LDV and acoustic emission (AE). Based on the experimental results, we propose several slider-disk contact criteria. Among them, the criterion based on the initial diagonal slope and the peak loading velocity gives a simple way to predict if slider-disk contacts will occur, but it overlooks the effect of the signs of the initial pitch and roll. A three-dimensional criterion based on the peak velocity and the initial pitch and roll angles appears to be more reliable. Using either criterion, it is found that high peak loading velocity causes slider-disk contact, and sliders with larger initial pitch or roll angles are more likely to hit the disk.
Analysis and Optimization of Semicircular and Straight Lobe Viscous Pumps
Authors: Mainland M. , Green I.
Published: June 2008

A viscous pump design that is capable of producing higher flow rates than a viscous pump previously introduced by Etsion and Yaier (1988) is developed and investigated. The key to the new design is straight lobes as opposed to the semicircular lobes of the previous design. The geometry of the straight lobes does not lend itself to as straightforward a solution as the semicircular lobe analysis. Approximations are made and then verified using upper and lower shape factor bounds and finite element analyses. Lobe geometry for each lobe design is optimized to produce maximum pumping capacity. The results of the optimization show that the straight lobe pump is theoretically superior to the semicircular lobe pump.
Wear Analysis Using a Quick Stop Device
Authors: Furze D. C. , Griffiths B. J. , Carrey M. C.
Published: June 2008

This paper describes a new type of wear test rig which is based on a quick stop principle. The advantages of using such a rig and the associated testing method are described with respect to the analysis of prow formation.
An Analytical and Experimental Investigation of Ball Bearing Retainer Instabilities
Authors: Boesiger E. A. , Donley A. D. , Loewenthal S.
Published: June 2008

Factors contributing to the dynamic instability of ball retainers in precision bearings were investigated. A real-time, rolling-element bearing dynamics simulation was written (PADRE - Planar Analysis of a Dynamic Retainer) for rapid evaluation of key bearing design factors. This code permits economic screening of a large number of bearing geometric parameters as a prelude to other more comprehensive dynamic bearing codes. The simulation uses a recently-developed integration package to increase computational speed and accuracy, and also employs a simplified lubricant traction model due to Johnson and Tevaarwerk. Tests with precision angular-contact ball bearings verified important features of this analysis including friction threshold for instability, retainer motion, and the instability frequency. Retainer instability was found to be highly sensitive to retainer-ball friction and exhibited a constant characteristic frequency, which was independent of speed, external vibration, radial load, and preload.
A Complete Solution for Thermal-Elastohydrodynamic Lubrication of Line Contacts Using Circular Non-Newtonian Fluid Model
Authors: Hsiao Hsing-Sen S. , Hamrock Bernard J.
Published: June 2008

A complete solution is obtained for elastohydrodynamically lubricated conjunctions in line contacts considering the effects of temperature and the non-Newtonian characteristics of lubricants with limiting shear strength. The complete fast approach is used to solve the thermal Reynolds equation by using the complete circular non-Newtonian fluid model and considering both velocity and stress boundary conditions. The reason and the occasion to incorporate stress boundary conditions for the circular model are discussed. A conservative form of the energy equation is developed by using the finite control volume approach. Analytical solutions for solid surface temperatures that consider two-dimensional heat flow within the solids are used. A straightforward finite difference method, successive over-relaxation by lines, is employed to solve the energy equation. Results of thermal effects on film shape, pressure profile, streamlines, and friction coefficient are presented.
A Numerical Analysis for Piston Skirts in Mixed Lubrication—Part I: Basic Modeling
Authors: Zhu Dong , Cheng Herbert S. , Arai Takayuki , Hamai Kyugo
Published: June 2008

This paper presents a mathematical model for piston skirts in mixed lubrication. It takes into account the effects of surface waviness, roughness, piston skirt surface profile, bulk elastic deformation and thermal distortion of both piston skirts and cylinder bore on piston motion, lubrication and friction. The corresponding computer program developed can be used to calculate the entire piston trajectory and the hydrodynamic and contact friction forces as functions of crack angle under engine running conditions. This paper is the first part of a series of two papers. It gives basic information and some preliminary results. The second part will include the major results and discussions, focused on the influences of elastic and thermal deformations.
On Extending the Narrow Spiral-Groove Theory to Configurations of Interest in Seals
Authors: Constantinescu V. N. , Galetuse S.
Published: June 2008

The existing narrow-groove theory is used for evaluating leakage flow for various configurations which might be of interest in seal design, including variable angle, film thickness and groove/ridge length ratio.
Normal and Angular Motions at Rough Planar Contacts During Sliding With Friction
Authors: Hess D. P. , Soom A.
Published: June 2008

The planar dynamics of a rough block in nominally stationary or sliding contact with a counter-surface is studied in this work. Starting with the Greenwood-Williamson model of a rough surface, the analysis of elastic contact deflections is extended to accommodate angular as well as normal motions. The real area of contact and the normal contact force are obtained in terms of the relative approach and orientation of the surfaces. It is shown that angular and normal motions at frictional contacts are generally coupled. The contact area and normal contact force are shown to be nonlinearly related to the normal and angular motions. However, the contact area remains proportional to the normal load, even in the presence of angular motions. When the friction force is assumed to be proportional to the real area of contact, the coefficient of sliding friction will be unchanged by small relative rotations between the sliding bodies. Based on this contact and friction model, the nonlinear equations of motion that describe the planar contact vibrations of a sliding block can be written directly. Although a detailed analysis of the stability and response characteristics of these nonlinear equations is beyond the scope of the present work, a limited comparison of calculations and measurements taken on both stationary and sliding blocks indicate that the small amplitude contact vibrations are reasonably well captured by the model developed in this work.
Experimental Study of Tilting-Pad Journal Bearings—Comparison With Theoretical Thermoelastohydrodynamic Results
Authors: Fillon Michel , Bligoud Jean-Claude , Freˆne Jean
Published: June 2008

Operating characteristics of four-shoe tilting-pad journal bearings of 100 mm diameter and 70 mm length are determined on an experimental device. The load, between pad configuration, varies from 0 to 10,000 N and the rotational speed is up to 4000 rpm. Forty thermocouples are used in order to measure bearing element temperatures (babbitt, shaft, housing and oil baths). The influence of operating conditions and preload ratio on bearing performances are studied. Comparison between theoretical and experimental results is presented. The theoretical model is also performed on a large tilting-pad journal bearing which was investigated experimentally by other authors.
Non-Leaking Lip Seals: A Roughness Effect Study
Authors: To̸nder K. , Salant R.
Published: June 2008

An analysis of roughness effects on leakage of lip seals is presented. The generation of fine axially oriented striations has been seen to occur on some lip seals after a certain time of operation, Mu¨ller and Ott (1984). Kammu¨ller (1986) and Mu¨ller (1987) have suggested that under running conditions, the striations may be distorted due to differential shear stresses leading to a channelling or pumping action of the grooves. This may balance the flow due to the pressure difference over the seal and thus suppress leakage. The roughness theory of the first author has been combined with the roughness pattern distortion theory of the second author. The resulting model is seen to predict a complete leakage suppression under realistic conditions.
Stiffness and Damping of an Eddy Current Magnetic Bearing
Authors: Ting Leehua , Tichy John
Published: June 2008

The perturbation method, aided by symbolic computation software, is employed to solve Maxwell’s equations to obtain theoretical predictions of forces and stiffness and damping coefficients for a simplified eddy current magnetic bearing. The results show that this kind of bearing has low stiffness and extremely low damping compared to conventional bearings. In fact, the damping is probably negative in most cases. In addition, the cross-coupled stiffness is relatively high, further contributing to rotor-dynamic problems. Despite these drawbacks, if advanced high temperature superconducting materials become practical, eddy current magnetic bearings may be useful in many applications.
The Elastic Field for Spherical Hertzian Contact Including Sliding Friction for Transverse Isotropy
Authors: Hanson M. T.
Published: June 2008

This paper gives closed-form expressions in terms of elementary functions for the title problem of spherical Hertzian contact of elastic bodies possessing transverse isotropy. Traction in the contact region is also included in the form of Coulomb friction; thus the shear stress is proportional to the contact pressure. The present expressions derived here by integration of the point force Green’s functions are simpler and easier to apply than equivalent expressions which have previously been given.
Two Reference Time Scales for Studying the Dynamic Cavitation of Liquid Films
Authors: Sun D. C. , Brewe D. E.
Published: June 2008

Two formulas, one for the characteristic time of filling a void with the vapor of the surrounding liquid, and one of filling the void by diffusion of the dissolved gas in the liquid, are derived. By comparing these time scales with that of the dynamic operation of oil film bearings, it is concluded that the evaporation process is usually fast enough to fill the cavitation bubble with oil vapor; whereas the diffusion process is much too slow for the dissolved air to liberate itself and enter the cavitation bubble. These results imply that the formation of a two phase fluid in dynamically loaded bearings, as often reported in the literature, is caused by air entrainment. They further indicate a way to simplify the treatment of the dynamic problem of bubble evolution.
Surface Roughness Effects in an EHL Line Contact
Authors: Venner C. H. , ten Napel W. E.
Published: June 2008

In this paper the influence of surface roughness on the pressure profile and film thickness in a steady state EHL line contact is investigated using input from an actually measured roughness profile in the calculations. Pressure profiles and film shapes for different load conditions are shown. The presented results strongly indicate that in the steady state situation considered here a significant deformation of the roughness profile occurs. As a result the often used λ parameter being the ratio of film thickness and standard deviation of the roughness (h/σ) with σ based on the undeformed roughness profile may give misleading information as far as the effect of the roughness on pressure and film shape is concerned.
A Method of “Separation of Variables” for the Solution of Laminar Boundary-Layer Equations of Narrow-Channel Flows
Authors: Al-Bender F. , Van Brussel H.
Published: June 2008

A method of solution for laminar channel flow is established using the von Mises transformation followed by the “separation” of the main flow velocity into amplitude and profile functions. The known boundary conditions on the latter enable a generalized solution that yields, through a parametric relation between auxiliary parameters (the characteristics), a system of ordinary differential equations for the velocity amplitude and the pressure. The result is a powerful semi-analytical method which is very easy to implement for a variety of internal flow configurations. The method is, in essence, a downstream general solution which may be extended upstream to the singular limit of a uniform main flow velocity distribution at the channel entrance. The regular solution for uniform initial velocity distribution cannot thus be obtained. Comparisons with other solutions show only qualitative agreement, for reasons which are discussed, whereas agreement with experimental results, made in a separate publication, is remarkably good.
Symmetric Radial Laminar Channel Flow With Particular Reference to Aerostatic Bearings
Authors: Al-Bender F. , Van Brussel H.
Published: June 2008

After a short survey of the different methods and formulas used to determine the pressure distribution in radial (converging or diverging) flow between nominally parallel discs, the method of “separation of variables,” established in reference [1], is applied to the problem, especially the case pertaining to centrally fed circular aerostatic bearings. The results are compared extensively with experimental data from various sources and the agreement is found to be remarkably good, suggesting that a laminar flow model is sufficient in characterizing the flow over a wide range of Reynolds number values.
A Thermo-Hydrodynamic Analysis of a Mechanical Face Seal
Authors: Pascovici M. D. , Etsion I.
Published: June 2008

A thermo-hydrodynamic analysis is performed for a face-to-face double seal configuration. Temperature and viscosity variations both across and along the sealing gap are considered and realistic boundary conditions are considered. The energy equation is solved analytically and the radial temperature variation is presented by an implicit equation. This approach enables analytical parametric investigation and gives better understanding of the effects of various parameters on the seal’s thermal behavior.
Measurement and Simulation of Partial Contact at the Head/Tape Interface
Authors: Lacey C. , Talke F. E.
Published: June 2008

Partial contact at the head/tape interface is investigated as a function of changes in the head/tape spacing due to asperity compression during contact. A model of head/tape contact, based on measurements of average contact pressure versus average head/tape spacing, is developed and incorporated into a numerical simulation of the head/tape interface. Numerical calculations of head/tape spacing with partial contact are verified by interferometric measurements.
The Effect of Trialkyl Phosphites and Other Oil Additives on the Boundary Lubrication of Ceramics: Friction of Silicon-Based Ceramics
Authors: Demizu Kei , Ishigaki Hiroyuki , Kakutani Hideo , Kobayashi Fukuzo
Published: June 2008

In order to examine the fundamental boundary lubrication properties of ceramics, reciprocating friction experiments of silicon based ceramics such as silicon carbide and silicon nitride were conducted with trialkyl phosphites and other oil additives. When ceramics were slid against ceramics, trialkyl phosphites with long carbon chains reduced the friction of silicon nitride markedly; the friction coefficients decreased with an increase in the carbon chain length. Other oil additives, however, did not greatly affect the friction. When ceramics were slid against metals, additives containing chlorine or sulfur increased friction of certain sliding couples. On the other hand, a trialkyl phosphite reduced friction and the friction coefficients increased with an increase in the maximum Hertzian contact pressure.
Analysis of Short Squeeze Film Dampers With a Central Groove
Authors: San Andres Luis A.
Published: June 2008

A novel analysis for the dynamic force response of a squeeze film damper with a central feeding groove considers the dynamic flow interaction between the squeeze film lands and the feeding groove. For small amplitude centered motions and based on the short bearing model, corrected values for the damping and inertia force coefficients are determined. Correlations with existing experimental evidence is excellent. Analytical results show that the grooved-damper behaves at low frequencies as a single land damper. Dynamic force coefficients are determined to be frequency dependent. Analytical predictions show that the combined action of fluid inertia and groove volume—liquid compressibility affects the force coefficients for dynamic excitation at large frequencies.
AFM Imaging, Roughness Analysis and Contact Mechanics of Magnetic Tape and Head Surfaces
Authors: Oden P. I. , Majumdar A. , Bhushan B. , Padmanabhan A. , Graham J. J.
Published: June 2008

Roughness measurements of a magnetic tape, a biaxially oriented poly (ethylene terephthalate) (PET) substrate, a tape head and a rigid-disk slider were made by an atomic force microscope (AFM) and a non-contact optical profiler (NOP). The lateral resolution of the surface topographs ranges from 1 μm (for NOP) down to 1 nm (for AFM). The AFM images show submicron features of the surface that are characteristic of the manufacturing processes. Some of the statistical roughness parameters conventionally used in theories of contact mechanics showed strong dependence on instrument resolution. This suggests that, firstly, roughness measured by NOP at resolutions larger than 1 μm cannot be used to study tribology at sub-micrometer scales and, secondly, a scale-independent characterization by fractal geometry is necessary. Fractal analysis of the tape surface reveals two regimes of roughness demarcated by a scale of 0.1 μm corresponding to the size of magnetic particles. The fractal behavior explains the dependence of the rms height, slope and curvature on the instrument resolution. The predictions of real area of contact suggest that nanometer-scale asperities tend to deform plastically whereas micrometer-scale ones deform elastically.
Assessing Adsorption of Conventional Friction Modifying Molecules by Relative Contact Potential Difference Measurements
Authors: Beltzer M.
Published: June 2008

A contact potential difference device based on the vibrating capacitor method of Zisman (1932) was used to assess adsorption and film formation at metal-petroleum interfaces. Measurement of changes in contact potential due to material adsorbed on low surface area metallic substrates is an attractive and convenient method for obtaining information about the adsorbed layer such as the adsorption isotherm and energetics. The adsorption iostherms of methyl stearate, stearic, oleic, and elaidic acids on copper and steel have been determined by the vibrating capacitor method.
Labyrinth Seal Rotordynamic Forces Using a Three-Dimensional Navier-Stokes Code
Authors: Rhode D. L. , Hensel S. J. , Guidry M. J.
Published: June 2008

A finite difference method for determining rotordynamic forces on an eccentric whirling labyrinth cavity has been developed. A coordinate-transformation was applied to the Reynolds time-averaged Navier-Stokes equations in order to use the modified bipolar coordinate system. The SIMPLER algorithm with QUICK differencing and the high Reynolds number k–ε turbulence model are used to compute the complex turbulent flowfield. A circular whirl orbit about the geometric center of the housing was specified for simplicity. The new model was tested against the rotordynamic force measurements, and close agreement was found. For the cases considered, the radial and tangential force components become rotor dynamically less desirable with increasing inlet swirl. Also, circumferential pressure variations are included for enhanced insight into the flowfield.
Crack Propagation in Rolling Line Contacts
Authors: Salehizadeh H. , Saka N.
Published: June 2008

The stress intensity factors for short straight and branched subsurface cracks subjected to a Hertzian loading are calculated by the finite element method. The effect of crack face friction on stress intensity factors is considered for both straight and branched cracks. The calculations show that the straight crack is subjected to pure mode II loading, whereas the branched crack is subjected to both mode I and mode II, with ΔKI/ΔKII < 0.25. Although KI is small, it strongly influences KII by keeping the branched crack faces apart. Based on the ΔKII values and Paris’s crack growth model, the number of stress reversals required to grow a crack in a rolling component from an initial threshold length to the final spalling length was estimated. It was found that the crack propagation period is small compared with the expected bearing fatigue life. Therefore, crack propagation is not the rate controlling factor in the fatigue failure of bearings operating under normal loading levels.
Effect of Some Material, Manufacturing, and Operating Variables on the Friction Coefficient in OCTG Connections
Authors: Carper H. J. , Ertas A. , Issa J. , Cuvalci O.
Published: June 2008

Results are presented from an experimental study conducted to determine friction coefficient behavior in OCTG connections. The study was conducted using the OCTG friction and galling tester at Texas Tech University. The tester employs tubular pin and box specimen pairs fabricated from 73-mm (2 7/8 in.) diameter OCTG tubing. Friction coefficient was determined for test specimens made of material grades L80 and J55 at loads of up to 620 kN (140,000 lb) acting on a surface area of up to 645 mm2 (1 in.2) with sliding velocities of 0.53 and 1.78 cm/s (1 and 3.5 ft/min), and nominal surface roughnesses of 0.37, 1.2, and 1.5 micron (15, 47, and 60 μin.) Ra. An API Modified thread compound was used for all tests. When plotted versus the applied load, the friction coefficient behavior is seen to be highly nonlinear. In addition to load, the variables of sliding velocity, material, and surface roughness are shown to have an effect. Preliminary experiments show that the test surface geometry also has a significant effect on the friction coefficient.
Thermal Elastohydrodynamic Lubrication of Rolling/Sliding Line Contacts
Authors: Wolff R. , Nonaka T. , Kubo A. , Matsuo K.
Published: June 2008

The solution of thermal elastohydrodynamic lubrication of rolling/sliding line contacts has been obtained. The Newton-Raphson technique was used to solve the simultaneous system of Reynolds and elasticity equations. The energy equation with boundary conditions was solved by the finite-difference method. Two models were developed: one with a constant viscosity across the oil film and another with a variable viscosity across the oil film. Different viscosity formulas such as modified WLF, Roelands, and Barus can be used in these models. Viscosity measurements were also performed over wide ranges of pressure and temperature. A very good fitting of experimentally measured viscosity by modified WLF formula was obtained. The oil film shape and minimum film thickness were calculated for pure rolling and high slip. For high slip and high rolling velocity, a tapered wedge shape of EHL film (in the longitudinal direction) was obtained. These results show a good correlation with measurements reported in other papers. They show that there is a significant influence of temperature on the oil film shape.
Friction-Factor Characteristics for Narrow Channels With Honeycomb Surfaces
Authors: Ha T. W. , Morrison G. L. , Childs D. W.
Published: June 2008

The experimental determination of friction-factors for the flow of air in a narrow channel lined with various honeycomb geometries has been carried out. Test results show that, generally, the friction-factor is nearly constant or slightly decreases as the Reynolds number increases, a characteristic common to turbulent flow in pipes. However, in some test geometries this trend is remarkably different. The friction factor dramatically drops and then rises as the Reynolds number increases. This phenomenon can be characterized as a “friction-factor jump.” Further investigations of the acoustic spectrum and friction-factor measurements for a broad range of Reynolds numbers indicate that the “friction-factor jump” phenomenon is accompanied by an onset of a normal mode resonance excited coherent flow fluctuation structure, which occurs at Reynolds number of the order of 104. The purpose of this paper is not to present the friction-factor data but to explain the friction-factor-jump phenomenon and friction-factor characteristics.
Friction-Factor Data for Flat-Plate Tests of Smooth and Honeycomb Surfaces
Authors: Ha T. W. , Childs Dara W.
Published: June 2008

Friction-factors for honeycomb surfaces are measured with a flat plate tester. The flat plate test apparatus is described and a method is discussed for determining the friction-factor experimentally. The friction-factor model is developed for the flat plate test based on the Fanno Line Flow. The comparisons of the friction-factor are plotted for smooth surface and twelve-honeycomb surfaces with three-clearances, 6.9 bar to 17.9 bar range of inlet pressure, and 5,000 to 130,000 range of the Reynolds number. The optimum geometries for the maximum friction-factor are found as a function of cell width to cell depth and clearance to cell width ratios.
Parametric Variational Principle of Viscoplastic Lubrication Model
Authors: Wu C. W. , Zhong W. X. , Qian L. X. , Hu L. C. , Sun S. M.
Published: June 2008

An effective mathematical method to deal with the viscoplastic lubrication model is presented here by applying the parametric variational principle (Zhong, 1985). The plastic slippage of lubricant would occur at the lubricated surfaces under isothermal lubrication. The boundary tangential velocity, therefore, can be taken as the parametric vector (or the control vector) in variational process. The mathematical model of the original problem is finally reduced to a complementarity problem. The algorithm presented in this paper is simple and reliable and shows promise in engineering application.
On the Effects of Roughness Orientation—A Mapping and Double Scale Analysis of Flow Factors
Authors: Lo Sy-Wei
Published: June 2008

A comprehensive approach for computing the lubricant flow as functions of the roughness orientation is presented. By using a mapping function, a periodic, anisotropic surface is transformed into an isotropic one. The transformed isotropic roughness has only one characteristic wavelength, so that a double scale analysis based on this wavelength of roughness can be developed to calculate the flow factors introduced in the average Reynolds equation. In addition, the Bhushan number instead of the conventional Peklenik number is used to describe the surface property more clearly. The new analysis provides an accurate evaluation of the roughness effects with only a second order approximation. The agreement is excellent even for incipient surface contact. An explicit relationship between the nondiagonal flow factors, which are associated with the pumping flux, and the diagonal ones have been established. It has been found that the maximum pumping action takes place when the asperities are oriented at 45 degrees to the direction of pressure gradient or sliding motion. The pumping effect caused by the relative sliding is about 50 percent greater than the effect caused by the pressure gradient. Strictly speaking, the asperities oriented at 45 degrees to the sliding direction will give different flow factors for different Bhushan numbers. This differs from some prior studies for periodic or area-distributed random roughness. Marked difference in flow factors due to the difference of asperity orientation is also observed. It suggests that caution should be used when the average Reynolds equation is applied, such as in the case of axisymmetric punching where various asperity orientations exist.
Analysis of Turbulent Hydrostatic Bearings With a Barotropic Cryogenic Fluid
Authors: San Andres Luis A.
Published: June 2008

Hydrostatic journal bearings handling low viscosity cryogenic liquids operate under extreme conditions of external pressurization. The regime of operation of these bearings is fully turbulent with large fluid inertia effects. Furthermore, cryogenic liquid propeties depend strongly on the local pressure and affect the bearing leakage and force response. The turbulent flow of a barotropic cryogenic fluid in a hydrostatic bearing is considered. Advection fluid inertia, recess-edge fluid inertia and liquid compressibility, and bearing surface roughnesses are included in the analysis. Numerical results for a liquid hydrogen hydrostatic bearing show that an incompressible fluid model overpredicts flow rate and force coefficients when compared to the more realistic barotropic fluid model.
Slipper Balance in Axial Piston Pumps and Motors
Authors: Koc E. , Hooke C. J. , Li K. Y.
Published: June 2008

The paper presents the results of an experimental and theoretical investigation of the effect of clamping ratio and orifice size on the performance of slippers in axial piston pumps and motors. Polishing of the running face to a slightly convex form appears to be essential for successful operation under all conditions. It is shown that slippers operate perfectly satisfactorily with the orifice blanked and, indeed, are generally most stable in this condition. Introduction of the orifice in under-clamped slippers increases the clearance. However it can also destabilise the slipper resulting in the slipper becoming sensitive to the effect of tilting couples. The effect on overclamped designs is more complex depending on the precise value of clamping ratio and on the width of the slipper land.
Experimental Study of Dynamic Fluid Forces and Moments for a Long Annular Seal
Authors: Kanemori Yuji , Iwatsubo Takuzo
Published: June 2008

The dynamic fluid reaction forces and moments of a long annular seal were experimentally studied. A new testing apparatus was constructed in order to measure restoring forces and moments caused by the cylindrical whirling motion of a rotor. Experiments were conducted at various rotor speeds, whirling speeds, and with different pressure drops across the long seal. The rotor and outer cylinder were set in concentric alignment. The tangential force component was derived from the measured fluid force in order to determine the unstable threshold. Moments developed by the long seal were also obtained. Restoring forces and moments were expressed as stiffness, damping coefficients and added mass. These coefficients were then compared with Childs’ theory and found to be compatible.
Optical Interferometric Observations of the Effects of a Bump on Point Contact EHL
Authors: Kaneta M. , Sakai T. , Nishikawa H.
Published: June 2008

The effects of surface kinematic conditions on micro-elastohydrodynamic lubrication (micro-EHL) are investigated under rolling and/or sliding point contact conditions using the optical interferometry technique. A long bump of chromium sputtered on the surface of a highly polished ball is used as a model asperity. It is shown that the film thickness distribution or the elastic deformation of the bump is influenced significantly by the surface kinematic conditions and the orientation of the bump. An interesting phenomenon is also found when contacting surfaces move with different speeds; the thin film formed on a transversely oriented bump existing at the entrance of the contact travels through the contact region at the average speed of the surfaces. The experimental results obtained qualitatively confirm published numerical simulations.