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IN THIS ISSUE

### RESEARCH PAPERS

J. Tribol. 2005;128(2):221-229. doi:10.1115/1.2164469.

A new method is developed in the present study to determine the elastoplastic regime of a spherical asperity in terms of the interference of two contact surfaces. This method provides an efficient way to solve the problem of discontinuities often present in the reported solutions for the contact load and area or the gradients of these parameters obtained at either the inception or the end of the elastoplastic regime. The well-established solutions for the elastic regime and experimental data of metal materials using indentation tests are provided as the references to determine the errors of these contact parameters due to the use of the finite-element method. These numerical errors provide the basis to adjust the contact area and contact load of a rigid sphere in contact with a flat such that the dimensionless mean contact pressure $Pave∕Y$ ($Y$: the yielding strength) and the dimensionless contact load $Fpc∕Fec$ ($Fec$, $Fpc$: the contact loads corresponding to the inceptions of the elastoplastic and fully plastic regimes, respectively) reaches the criteria arising at the inception of the fully plastic regime, which are available from the reports of the indentation tests for metal materials. These two criteria are however not suitable for the present case of a rigid flat in contact with a deformable sphere. In the case of a rigid flat in contact with a deformable sphere, the proportions in the adjustments of these contact parameters are given individually the same as those arising in the indentation case. The elastoplastic regime for each of these two contact mechanisms can thus be determined independently. By assuming that the proportion of adjustment in the elastoplastic regime is a linear function, the discontinuities appearing in these contact parameters are absent from the two ends of the elastoplastic regime in the present study. These results are presented and compared with the published results.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):230-235. doi:10.1115/1.2164470.

In this paper we report the experimental investigation to evaluate the published models for the contact of a deformable sphere against a hard flat in the fully plastic contact regime. A new measurement method has been used to measure the contact area. The behavior of the mean contact pressure and the contact area as a function of the contact load are presented. Substantial differences are found between the measurements and the model predictions. A constant value of the mean contact pressure as the load increases is observed, however, the value is lower than the hardness, as often reported. The contact area is found to be a simple truncation of the sphere by a hard flat.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):236-244. doi:10.1115/1.2163360.

A semi-analytical thermo-elastic-plastic contact model has been recently developed and presented in a companion paper. The main advantage of this approach over the classical finite element method (FEM) is the treatment of transient problems with the use of fine meshing and the possibility of studying the effect of a surface defect on the surface deflection as well as on subsurface stress state. A return-mapping algorithm with an elastic predictor/plastic corrector scheme and a von Mises criterion is now used, which improves the plasticity loop. This improvement in the numerical algorithm increases the computing speed significantly and shows a much better convergence and accuracy. The contact model is validated through a comparison with the FEM results of Kogut and Etsion (2002, J. Appl. Mech., 69, pp. 657–662) which correspond to the axisymmetric contact between an elastic-perfectly plastic sphere and a rigid flat. A model for wear prediction based on the material removal during cyclic loading is then proposed. Results are presented, first, for initially smooth surfaces and, second, for rough surfaces. The effects of surface shear stress and hardening law are underlined.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):245-251. doi:10.1115/1.2162557.

This study investigates the effects of asperity interactions on the mean surface separation and the real contact area for rough surfaces with non-Gaussian height distributions. The effects of the asperity interactions on the local deformation behavior of a given microcontact are modeled using the Saint Venant principle and Love’s formula. The non-Gaussian rough surfaces are described by the Johnson translatory system. The results indicate that asperity interactions can significantly affect the mean separation of surfaces with non-Gaussian height distributions. The findings also reveal that the contact load and the real contact area of surfaces with non-Gaussian height distributions are significantly different from those of surfaces with Gaussian height distributions. This study uncovers that skewed surfaces tend to deform more elastically, which provides underlying physics for the long-time conventional wisdom and recent experimental data [Y. R. Jeng, 1996, Tribol. Trans., 39, 354–361;Y. R. Jeng, Z. W. Lin, and S. H. Shyo, 2004, ASME J. Tribol., 126, 620–625] that running-in surfaces have better wear resistance.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):252-261. doi:10.1115/1.2164467.

In the present investigation, a theoretical model has been developed to obtain the vibration response due to a localized defect in various bearing elements in a rotor-bearing system under radial load conditions. The rotor-bearing system has been modeled as a three degrees-of-freedom system. The model predicts significant components at the harmonics of characteristic defect frequency for a defect on the particular bearing element. In the case of a defect on the inner race or a rolling element, the model predicts sidebands about the peaks at defect frequencies, at multiples of shaft and cage frequencies, respectively. The model has also predicted some additional components at harmonics of shaft and cage frequencies due to a local defect on the inner race and a rolling element, respectively. The expressions for all these spectral components have also been derived. Typical numerical results for an NJ 204 bearing have been obtained and plotted. The amplitude of the component at defect frequency, for an outer race defect, is found to be much higher as compared to those due to inner race defect or a rolling element defect of the same size and under similar conditions of load and speed. The results of vibration measurements on roller bearings with simulated local defects have also been presented to experimentally validate the theoretical model proposed. It can be observed from the results that the spectral components predicted by the theoretical model find significant presence in the experimental spectra. Comparison of the normalized analytical values of the spectral components with their experimental values shows fair agreement for most of the cases considered. Probable area of the generated excitation pulses has been calculated and the effects of pulse area variation on the experimental results have been studied.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):262-266. doi:10.1115/1.2162177.

Traction drive makes oil film between two rollers, and power is transmitted by oil film shearing. It has the following characteristics. (1) Traction drive can be operated at low level of vibration and noise, so they are more suitable at higher speed rotations than gear. (2) Traction drive can change continuously the distance from the contact point of the rotating part to the axis of rotation; it is useful in continuously variable transmission (CVT). Generally-fixed-reduction-ratio-type traction drive is developed for the purpose of use by high-speed rotation taking advantage of the feature of characteristic (1). On the other hand, the authors have developed a micro drive system for transmission; a micro-traction-drive based on the structure of an angular ball bearing is advantageous over geared speed reducers, for small scale equipment requiring high numbers of revolutions. A micro-traction-drive is easily manufactured by modifying angular bearings and tapered roller bearings for which preload inner race and outer race act as thrust force. The driving force is transmitted by the contact of the retainer with the rolling element in the rotating direction. The test of the experimental model of micro-traction-drive using an angular ball bearing of $10mm$ inner diameter, $30mm$ outer diameter, and $9mm$ width was carried out. Power-absorbing-type test equipment was made and the input and output torque, number of revolutions, temperature, noise, and state of lubrication were measured. With the same test equipment, the micro-traction-drive was compared to the equivalent type planetary gear with outer diameter of $32mm$ on the market. In comparison with commercially available speed reducers, the planetary gear system, the newly developed micro-traction was found to bear superior performance in terms of allowable transmission torque, efficiency, noise, and other characteristics.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):267-274. doi:10.1115/1.2164463.

A new theoretical model for estimating the entrained air film thickness between a web and roller is presented for both impermeable and permeable webs. A simple curve fit formula for estimating the air film thickness, which considers the effects of air leakage from the web edges and air diffusion due to the permeability of web, was obtained based on a large number of simultaneous numerical solutions of the compressible Reynolds equation and the web equilibrium equation. The variation of air film thickness with roller velocity is measured for three typical webs: polyethylene terephthalate, coated paper, and newsprint. The effects of web permeability, web width, and web tension on the air film thickness are examined theoretically and experimentally for a wide range of roller velocity. Reasonable agreement is seen both quantitatively and qualitatively between the predicted and measured results. The validity of the formula for the first-order estimation of web-roller interface problems is verified experimentally.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):275-281. doi:10.1115/1.2162916.

The long-term success of a total knee replacement depends on the wear performance of a polyethylene bearing that separates a metal femoral component from a metal tibial tray. Although fixed bearing designs secure the polyethylene bearing to the tibial tray, mobile bearing knees allow the polyethylene to move relative to the tibial tray. This study has evaluated the wear performance of an intended articulation on the inferior surface of the LCS®-Rotating Platform mobile bearing by conducting clinically relevant tribological testing and comparing results to retrieved knee bearings. A retrieval analysis leads to the conclusion that third-body particles in the contact produce curvilinear scratches longer than the expected rotation of the knee on both the polyethylene bearing and the CoCr tibial tray. Tribological testing shows that polymethylmethacrylate (PMMA) bone cement particles produce worn surfaces most similar to retrievals. Porous-coating beads and bone debris also have the ability to damage both surfaces. Worn polyethylene surfaces from pin-on-flat tests show scratches longer than the excursion length, and “skipping marks”—pits spaced at smaller rotation intervals along a scratch—as observed in retrievals. These wear features suggest that a ratcheting mechanism, which moves the third-body particles further along the scratch with each cycle, may be responsible for the observed wear.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):282-290. doi:10.1115/1.2164465.

This paper focuses on the mechanism of starvation and the thermal and non-Newtonian behavior of starved elastohydrodynamic lubrication (EHL) in line contacts. It has been found that for a starved EHL line contact if the position of the oil-air meniscus is given as input parameter, the effective thickness of the available lubricant layers on the solid surfaces can be solved easily from the mass continuity condition, alternatively, if the later is given as input parameter, the former can also be determined easily. Numerical procedures were developed for both situations, and essentially the same solution can be obtained for the same parameters. In order to highlight the importance of the available oil layers, isothermal and Newtonian solutions were obtained first with multi-level techniques. The results show that as the inlet meniscus of the film moves far away from the contact the effective thickness of the oil layers upstream the meniscus gently reaches a certain value. This means very thin layers (around $1μm$ in thickness) of available lubricant films on the solid surfaces, provided the effective thickness is equal to or larger than this limitation, are enough to fill the gap downstream the meniscus and makes the contact work under a fully flooded condition. The relation between the inlet meniscus and the effective thickness of the available lubricant layers was further investigated by thermal and non-Newtonian solutions. For these solutions the lubricant was assumed to be a Ree-Eyring fluid. The pressures, film profiles and temperatures under fully flooded and starved conditions were obtained with the numerical technique developed previously. The traction coefficient of the starved contact is found to be larger than that of the fully flooded contact, the temperature in the starved EHL film, however, is found to be lower than the fully flooded contact. Some non-Newtonian results were compared with the corresponding Newtonian results.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):291-295. doi:10.1115/1.2162559.

Based on the theory of variational inequality, a rapid efficient algorithm for fluid force and its Jacobian matrix in journal bearing is presented in this paper. Primarily, to solve the fluid force is transformed to solve a set of linear algebraic equations with tri-diagonal coefficient matrices. Meanwhile, an amendatory direct-method is proposed to solve the united equations about fluid forces and their Jacobian matrices, rapidly and synchronously. The Reynolds boundary condition has to be satisfied automatically during the process. Secondly, the coefficient matrices, which are involved in the previous process, can be decomposed to an assembly of a part of relative with journal motion and a part of invariable matrix, which can be prepared in advance and be referred to later repeatedly. Through these measures, many redundant operations are avoided. The numerical examples show that, under the accuracy guaranteed, the algorithm in this paper can reduce computational time remarkably, which reveals that the current method has a good operational characteristic and practicability.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):296-311. doi:10.1115/1.2164464.

In this study the effect of water as a contaminant in lubricated contacts was analytically and experimentally investigated. A steel ball on glass disc apparatus was used to measure lubricant film thickness of pure oil and water in oil emulsions under various operating conditions. A steel ball on steel disc rig was used to measure friction as a function of various loads, slide to roll ratios and water in oil emulsions. A finite difference numerical model was developed using the continuum theory of mixtures and results were corroborated with the experimental measurements. Numerical results are in excellent agreement with the experimental results and indicate that water will flow around the contact. The experimental and analytical results suggest that for heavily loaded contacts water-in-oil emulsions perform essentially the same as pure oils.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):312-318. doi:10.1115/1.2162914.

Pocket-pads or steps are often used in journal bearing design, allowing improvement of the latter’s dynamic behavior. Similar “discontinuous” geometries are used in designing thrust bearing pads. A literature review shows that, to date, only isoviscous and adiabatic studies of such geometries have been performed. The present paper addresses this gap, proposing a complete thermohydrodynamic (THD) steady model, adapted to three-dimensional (3D) discontinuous geometries. The model is applied to the well-known geometry of a slider pocket bearing, operating with an incompressible viscous lubricant. A model based on the generalized Reynolds equation, with concentrated inertia effects, is used to determine the 2D pressure distribution. On this basis, a 3D field of velocities is constructed which, in turn, allows the resolution of the 3D energy equation. Using a variable-size grid improves the accuracy in the discontinuity region, allowing an evaluation of the magnitude of error induced by Reynolds assumptions. The equations are solved using the finite volume method. This ensures good convergence even when a significant reverse flow is present. Heat evacuation through the pad is taken into account by solving the Laplace equation with convective boundary conditions that are realistic. The runner’s temperature, assumed constant, is determined by imposing a zero value for the global heat flux balance. The constructed model gives the pressure distribution and velocity fields in the fluid, as well as the temperature distribution across the fluid and solid pad. Results show important transversal temperature gradients in the fluid, especially in the areas of minimal film thickness. This further justifies the use of a complete THD model such as the one employed.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):319-326. doi:10.1115/1.2162920.

The existing literature contains a number of disparities in assessing the effect of inlet oil temperature on the instability threshold speed in hydrodynamic journal bearings. Specifically, some papers have presented evidence that lowering the inlet oil temperature tends to have a stabilizing effect, whereas others have shown the opposite. No clear explanation has been offered for this phenomenon. This paper presents the results of a series of experiments that explain the nature of these disparities and sheds light on the effect of the inlet oil temperature on the instability threshold in hydrodynamic journal bearings.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):327-334. doi:10.1115/1.2162917.

A metal foil disk offers some of the best characteristics of both the hard disk and floppy disk for digital data storage. The current work defines an opposed slider air-bearing arrangement that provides advantages when used with a high-speed metal foil disk in either a fixed or removable format. Use is made of the fact that the opposing sliders interact through their influence on the flexible disk that is sandwiched between them. Asymmetry of opposing air bearings is created by etching the air-bearing pad opposite the recording element pad to a depth sufficient that the flying height and air film stiffness of the opposing pad reach desired levels. The result is an air-bearing interface with low flying height and high stiffness over the recording element directly opposed by a high flying height and low stiffness on the other side of the disk. This air-bearing interface was found to provide an enhanced dynamic flexibility to the metal foil disk when it is subjected to mechanical shock. As a result, the opposed slider arrangement with metal foil disk is able to avoid contact and impact when subjected to substantial levels of mechanical shock. Thus, wear and damage to slider and disk surfaces are reduced as well as the possibility of lost recorded data. This should make the metal foil disk a strong candidate as a rotating storage medium for mobile and portable applications where a shock environment is common. Computer simulation of the new air-bearing configuration will be presented and discussed. The current work is related to but distinct from that reported recently by White (2005, ASME J. Tribol., 127, pp. 522–529) for a Mylar disk.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):335-340. doi:10.1115/1.2162918.

This paper proposes a method to calculate the characteristics of a coupled fluid dynamic journal and thrust bearing of a hard disk drive (HDD) spindle motor. The governing equations for the journal and thrust bearings are the two-dimensional Reynolds equations in the $θz$ and $rθ$ planes, respectively. The finite element method is appropriately applied to analyze the coupled bearing under the conditions of the continuity of mass and pressure at the interface between the journal and thrust bearings. The pressure in the coupled bearing was calculated by applying the Reynolds boundary condition. The validity of this application was verified by comparing the analytical results of the flying height at various rotating speeds with experimental results. The characteristics of the coupled journal and thrust bearing were also investigated due to the Reynolds and Half-Sommerfeld boundary conditions and the coupled and separate analysis. This research demonstrates that the proposed method can accurately and realistically describe the coupled fluid dynamic bearing in a HDD system.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):341-344. doi:10.1115/1.2162551.

System identification methods have been used to study the response of a magnetic recording slider during contact with a scratch on the disk surface. In addition, the slider response was studied taking into account the effect of disk micro-waviness at various disk rotational speeds. The simulated slider response was compared with the measured slider dynamic behavior. Very good agreement was found between simulated and measured data. The flying height modulation of the slider, due to disk micro-waviness, was found to depend on disk velocity.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):345-350. doi:10.1115/1.2148419.

Microdimples generated by laser surface texturing (LST) can be used to enhance performance in hydrostatic gas-lubricated tribological components with parallel surfaces. The pressure distribution and load carrying capacity for a single three-dimensional dimple, representing the LST, were obtained via two different methods of analysis: a numerical solution of the exact full Navier-Stokes equations, and an approximate solution of the much simpler Reynolds equation. Comparison between the two solution methods illustrates that, despite potential large differences in local pressures, the differences in load carrying capacity, for realistic geometrical and physical parameters, are small. Even at large clearances of 5% of the dimple diameter and pressure ratios of 2.5 the error in the load carrying capacity is only about 15%. Thus, for a wide range of practical clearances and pressures, the simpler, approximate Reynolds equation can safely be applied to yield reasonable predictions for the load carrying capacity of dimpled surfaces.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):351-357. doi:10.1115/1.2162558.

This paper analyzes the performance of hydrodynamic journal bearings by applying the control-volume method to solve the average lubrication equation for grain flow. The grain characteristics, including the particle size and the grain-grain collision elasticity are investigated. The nondimensional load, attitude angle, friction coefficient, and side flow are explored for different eccentricity and diameter-to-width ratios. The numerical results provide an understanding of the performance of powder lubricated journal bearings and are consistent with the experimental findings of Heshmat and Brewe.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):358-364. doi:10.1115/1.2164466.

Liquid lubricants break down at extreme temperatures and promote stiction in micro-/nanoscale environments. Consequently, using flows of solid granular particles as a “dry” lubrication mechanism in sliding contacts was proposed because of their ability to carry loads and accommodate surface velocities. Granular flows are highly complex flows that in many ways act similar to fluids, yet are difficult to predict because they are not well understood. Granular flows are composed of discrete particles that display liquid and solid lubricant behavior with time. This work describes the usefulness of employing lattice-based cellular automata (CA), a deterministic rule-based mathematics approach, as a tool for modeling granular flows in tribological contacts. In the past work, granular flows have been modeled using the granular kinetic lubrication (GKL) continuum modeling approach. While the CA modeling approach is constructed entirely from rules, results are in good agreement with results from the GKL model benchmark results. Velocity results of the CA model capture the well-known slip behavior of granular flows near boundaries. Solid fraction results capture the well-known granular flow characteristic of a highly concentrated center region. CA results for slip versus roughness also agree with GKL theory.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):365-377. doi:10.1115/1.2162555.

An experiment was carried out to investigate the distribution of oil pressure within a squeeze film damper. The damper was made so that its operation turned out to be as simple as possible, in order to highlight the main causes of practical deviation from theoretical prediction, with particular reference to cavitating mechanisms and regardless of inertia effects. The journal of the damper was given an eccentric orbital precession, with adoption of two distinct values of the offset. A groove placed laterally to the film secured the oil feeding. An outlet plenum at the opposite side of the film was operated with two different levels of exposure to the ambient air. Observation of the oil pressure was restricted to the film section midway between the inlet and outlet border, by means of three piezoelectric transducers plus a strain gauge sensor. Theoretical prediction with a simple isoviscous short bearing uncavitated model was shown to be a significant reference for the experimental data. Analysis of the pressure levels and shape of the pressure waves made it possible to recognize operating conditions with the presence of tensile stresses and rupture of the film. The latter conditions were chiefly due to vapor cavitation. In many circumstances, spikes with tensile strength preceded the ruptured region. Air entrainment and its effects proved to be restricted at high frequency regimes with very low supply pressures and coexisted with vapor cavitation. The influence of moderate orbital distortion on pressure signals was highlighted. Significant differences in the pressure behavior from one sensor location to the other, for the same operating conditions, were frequently observed.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):378-387. doi:10.1115/1.2164468.

Test results are presented for laminar-flow seals that are representative of buffered-flow oil seals in centrifugal compressors. The seals are short $(L∕D≅0.21)$, with a diameter of $117mm$ and a clearance-to-radius ratio 0.0007. A smooth seal, a seal with one central groove, and a seal with three grooves were tested. Groove geometries employed are representative of industrial practice for compressors with a groove-depth to clearance ratio on the order of 6. Tests were conducted at 4000, 7000, and $10,000rpm$ shaft speed with delta pressures across the seals of 21, 45, and $69bars$. For all cases, the flow was laminar. The seals were tested from a centered position out to an eccentricity ratio of 0.7. Static data included leakage and equilibrium loci for a range of loads. Direct and cross-coupled stiffness and damping coefficients and direct mass coefficients were determined from dynamic tests. For the smooth seal, comparisons between measurements and predictions were reasonable for the direct and cross-coupled stiffness and damping coefficients; however, measured added mass coefficients were roughly ten times larger than predicted. Predictions for the grooved seals from a “deep-groove” model that assumed zero pressure oscillations in the grooves greatly over predicted the influence of the grooves. In a centered position, smooth and grooved seals had comparable leakage performance. At higher eccentricity ratios, the grooved seals leaked modestly more. For eccentricity ratios less than approximately 0.3, the grooved seals and the smooth seal had qualitatively similar static and dynamic characteristics. In terms of cross-coupled stiffness coefficients, the grooved seals were less stable than the smooth seal at eccentricity ratios greater than approximately 0.5 but had significantly lower cross-coupled coefficients at reduced eccentricity ratios between zero and 0.3. A grooved centered seal is more stable than a smooth centered seal. The smooth seal had higher damping than the grooved seals and had moderately better centering capabilities.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):388-395. doi:10.1115/1.2162552.

Experimental dynamic-stiffness-coefficient results are presented for a high-speed, lightly loaded, load-on-pad, flexible-pivot tilting-pad (FPTP) bearing. Results show that the real parts of the direct dynamic-stiffness are quadratic functions of the excitation frequency. Frequency independent $[M]$, $[K]$, and $[C]$ matrices can be used in place of frequency dependent $[K]$ and $[C]$ matrices to model the FPTP bearing for the conditions tested. The model reduction that results in moving from twelve degrees of freedom (three degrees of freedom for each of four pads) to two degrees of freedom in the bearing reaction model seems to account for most of the observed and predicted frequency dependency. Predictions indicate that pad and fluid inertia have a secondary impact for excitation frequencies out to synchronous frequency. Experimental results are compared to numerical predictions from models based on: (i) The Reynolds equation, and (ii) a Navier-Stokes (NS) equations bulk-flow model that retains the temporal and convective fluid inertia terms. The NS bulk-flow model results correlate better with experimental dynamic stiffness results, including added-mass terms. Both models underestimate the measured added-mass coefficients for the full excitation range; however, they do an adequate job for excitation frequencies up to synchronous frequency. The advantage of using a frequency-independent $[M]-[K]-[C]$ model is that rotordynamic stability calculations become noniterative and much quicker than for a frequency dependent $[K]-[C]$ model. However, these results only apply to this bearing at the conditions tested. Conventional tilting pad and/or FPTP bearings with different geometry and operating conditions (or even this FPTP bearing at higher loads) may require a frequency-dependent $[K]-[C]$ model.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):396-405. doi:10.1115/1.2164471.

An axisymetric numerical model of face seals operating with compressible fluids at high pressure is presented. Inertia terms are included using an averaged method and thermal effects are considered. The real behavior of gases at high pressure is taken into account. An original exit boundary condition is used to deal with choked flow. The model is validated by comparison with experimental data and analytical solutions. Finally, the influence of the operating conditions on the performance of a high-pressure gas face seal is analyzed. It is shown that when the flow is choked, the mass flow rate is reduced and the behavior of the seal becomes unstable.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):406-413. doi:10.1115/1.2162915.

The cross section of an elastomer seal for pneumatic actuator pistons was optimized to minimize the friction force exchanged with the cylinder bore. The new seal geometry was developed using a finite element numerical model which takes material nonlinearities and frictional contact with seat and cylinder bore surfaces into account. The friction force exchanged between the new seal and the pneumatic cylinder bore was measured experimentally using a suitable sensorized test bench.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):414-421. doi:10.1115/1.2162919.

Numerous works have been recently carried out to describe friction and lubrication in the tool-sheet interface. This phenomenon is of great importance for all forming processes, particularly, in the deep drawing operations where the rapid progress in the field of computing requires the predictive knowledge of friction coefficient behaviors. The present paper considers the dynamical modeling by a global approach of the tangential force evolution. All measurements come from a tribometer equipped for strip-drawing tests on lubricated steel sheets and aluminum sheets, as a function of applied normal load. Models are obtained from serial, parallel, or closed-loop association structures for linear transfer functions. Numerical values of delays, time constants, and gain are identified for each test order; complexities and structures are determined with a double optimization. For all situations, a stop criterion is defined to achieve the optimal model, which is reached for the steel sheets.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):422-430. doi:10.1115/1.2162553.

The relative motion of the friction and separator plates in wet clutches during the disengaged mode causes viscous shear stresses in the fluid passing through the $100microns$ gap. This results in a drag torque on both the disks that wastes energy and decreases fuel economy. The objective of the study is to develop an accurate mathematical model for the above problem with verification using FLUENT and experiments. Initially we two consider flat disks. The mathematical model calculates the drag torque on the disks and the 2D axisymmetric solver verifies the solution. The surface pressure distribution on the plates is also verified. Then, 3D models of one grooved and one flat disk are tested using CFD, experiments and an approximate 3D mathematical model. The number of grooves, depth of groove and clearance between the disks are studied to understand their effect on the torque. The study determines the pressure field that eventually affects aeration incipience (not studied here). The results of the model, computations and experiments corroborate well in the single-phase regime.

Commentary by Dr. Valentin Fuster

### TECHNICAL BRIEFS

J. Tribol. 2005;128(2):431-435. doi:10.1115/1.2162556.

An approximate analytical model is proposed to describe the process of separating a sphere from a flat in the presence of a Newtonian liquid, under conditions of constant force. The model is based on consideration of the effects of sphere inertia in characterizing the critical point of separation—namely, that there is a sudden onset of significant sphere acceleration. Good agreement is found between the predictions of the analytical model with those of a full numerical simulation.

Commentary by Dr. Valentin Fuster
J. Tribol. 2005;128(2):436-441. doi:10.1115/1.2162554.

This paper describes an experimental method, bio-ferrography, to separate ultrahigh molecular weight polyethylene (UHMWPE) wear debris, generated in hip simulators, from bovine serum lubricating fluid. A total of 54 experiments were performed in which an enzyme digestion “cocktail” was developed and used to clean the bovine serum samples of extraneous sugars, proteins, and lipids that interfere with the UHMWPE particle separation. Erbium chloride was used to marginally magnetize particles in the fluid prior to passing through the ferrographic device. The particles were captured and separated from the fluid by traversing the treated serum across a magnetic gap of a bio-ferrograph. Morphology of the captured and separated wear debris was compared with particles from samples of fluid filtered through a paper sieve arrangement with pores of $0.05micrometers$ in diameter. The UHMWPE wear debris collected using the described experimental method, were found to be between 0.1 and $20micrometers$ in diameter with spherical and pill-shaped particles. The filtered UHMWPE particles were in the same size range as the debris separated using bio-ferrography. To show that the experimental method captured UHMWPE particles, the spectra of the chemical composition of UHMWPE from an acetabular cup insert of a hip implant and of UHMWPE particles separated using bio-ferrography were compared and found to be the same. To further demonstrate that polyethylene could be captured and separated through the experimental method, manufactured polyethylene microspheres in the diameter range of $3–45micrometers$, were captured and separated using the bio-ferrographic process.

Commentary by Dr. Valentin Fuster

### ERRATUM

J. Tribol. 2006;128(2):442. doi:10.1115/1.2173641.
FREE TO VIEW
Material constants used in plasticity model
Commentary by Dr. Valentin Fuster