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RESEARCH PAPERS

J. Tribol. 1997;119(1):1-7. doi:10.1115/1.2832459.

A refined method for interpreting the Vickers composite microhardness measurement for multilayer materials having layers of arbitrary plating thickness is first presented. The position of an “effective substrate” is found using the concept of the “plastic boundary,” and the depth-wise deformation of each layer is considered in a double-iterative procedure which converges fast. This computational method is then extended from pyramidal indenters to conical and spherical indenters (e.g., Meyer’s). For its confirmation, experimental investigations are carried out for two configurations of Cu/Ni/Au sandwiches, using different diameter spherical indenters and spherical tipped cones, through and well above the microhardness load range. The general rules for composite action are established.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):8-17. doi:10.1115/1.2832485.

Turning experiments were performed with cemented WC-Co cutting tools coated with two-layer and three-layer overcoats of TiC/Al2 O3 and TiC/Al2 O3 /TiN, respectively. For comparison, uncoated WC-Co tools were also tested under similar cutting conditions. The predominant wear mechanisms of the various ceramic overcoats and cemented WC-Co were investigated using surface profilometry, scanning electron microscopy, and energy dispersive X-ray analysis. Representative results of the tool wear behavior are presented, and the significance of each ceramic layer on the overall tool wear resistance is interpreted in light of the identified dominant wear mechanisms. Delamination wear characterized by the propagation and linkage of surface, subsurface, and interfacial cracks, abrasion, surface plastic shearing, plucking of carbide grains, and dissolution/diffusion are shown to occur depending on the tool material. These wear processes are not mutually exclusive; they may occur simultaneously at different positions on the same tool surface. Based on nose wear data, correlations between wear lives of coated and uncoated tools and feedrate are established.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):18-25. doi:10.1115/1.2832457.

A formulation of friction force in the interface of a friction pair is developed considering the mechanical components arising from the elastic and plastic deformations of the asperities and the chemical components represented by the adhesive forces between local contact regions. The results relate the normal load and dry-friction force to the relative normal and tangential velocities of a friction pair as a function of asperity deformations and adhesive forces. It is shown that the important parameter in the relationship between normal load and the dry-friction force is the projection of the contact area in normal and tangential directions to the mean planes of contacting surfaces rather than the contact area itself. The two forms of dry-friction force derived from the statements of energy balance at the interface allow alternate approaches to modelling of the friction between interacting rough surfaces.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):26-30. doi:10.1115/1.2832475.

The mechanical properties of SiC films grown via C60 precursors were determined using atomic force microscopy (AFM). Conventional silicon nitride and diamond-tipped steel AFM cantilevers were employed to determine the film hardness, friction coefficient, and elastic modulus. The hardness is found to be 26 GPa by nanoindentation of the film with a Berkovich diamond tip. The friction coefficient for the silicon nitride tip on the SiC film is about one half to one third that for silicon nitride sliding on a silicon substrate. By combining nanoindentation and AFM measurements an elastic modulus of ~300 GPa is estimated for these SiC films.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):31-35. doi:10.1115/1.2832476.

Titanium pins were slid against flat surfaces of alumina disks under dry condition and a normal load of 50N at sliding speeds varying from 0.1 to 4 ms−1 . Estimated strain rate and temperature distribution in the subsurface when superposed on a microstructure evolution map in temperature-strain rate space give the microstructural response of the material to different sliding velocities at different subsurface depths. The map was obtained by conducting uniaxial compression tests. The experimentally observed variation in wear rate with sliding velocity was found to have a qualitative correspondence with the predicted variation of microstructure in the near surface region.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):36-42. doi:10.1115/1.2832477.

Fretting-wear and fretting-fatigue loadings can both result in wear (material loss) and in crack nucleation and propagation (fatigue process). This paper deals with cracking induced by small amplitude displacements in the case of aeronautic aluminium alloys. The two sets of fretting maps are introduced: running condition fretting map is composed of partial slip (sticking), mixed fretting and gross sliding regime; material response fretting map is associated with two macro-degradation modes. Crack nucleation and propagation are analysed for every fretting regime. The mixed fretting regime appeared most detrimental with regards to fatigue cracking. Slip amplitude and normal load main effects discussed for fretting wear can be used to justify the fatigue limit decrease often obtained for fretting fatigue experiments.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):43-48. doi:10.1115/1.2832478.

A statistical methodology is presented for predicting drive performance based on fundamental static friction (stiction) measurements. The technique allows the prediction of drive stiction and dynamic friction failures, based on component level spin-stand measurements. We discuss both the fundamental measurement of component stiction and the interpretation of the results as applied to an actual disk drive. The component measurements examine the effects of acceleration, filtering and sampling. It is shown that motor acceleration and the electronic configuration of the test stand affect the stiction measurement, but by proper electronic and mechanical designs this effect can be reduced to an insignificant quantity. The interpretation of component results considers incorporating the effect of multiple heads in a drive, and a statistical model is devised that accounts for both static and dynamic friction variation, along with motor/driver variations. One can predict the probability of a single drive failure or the failure rates of a population of drives, either new or after extensive field exposure. We show that a poorly characterized measurement compared to an arithmetic average of the available motor torque may predict drive failure rates in error by several orders of magnitude.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):49-56. doi:10.1115/1.2832479.

This paper summarizes the modeling and control of hybrid squeeze film dampers (HSFDs) for active control of vibration of rotors exhibiting multiple modes. In a recent paper (El-Shafei and Hathout, 1994), it was shown that the automatically controlled HSFD based on feedback on rotor speed can be a very efficient device for active control of rotor vibration. It was shown that this closed-loop, on-off control strategy results in a much improved behavior of the rotor system. The previous investigation was performed on a Jeffcott rotor model. The model was simple and fluid inertia effects were not taken into consideration. In this paper, major strides were made in both the modeling of the rotor and the HSFD. Modal analysis was implemented in the dynamic analysis of the squeeze film damper supported rotor in a novel and unique manner of performing modal analysis on nonlinear rotor systems. This allowed the modeling of any number of modes using modal analysis and hence to verify the capability of the HSFD to control multiple modes. Also, fluid inertia forces were considered in our model for the HSFD due to their direct influence in changing the behavior of the damper (El-Shafei and Crandall, 1991). A complete mathematical model of this open-loop system is developed and is implemented on a digital computer. Finally, based on the feedback on speed, the closed-loop behavior was studied from both steady-state and transient points of view and showed an overall enhanced behavior for the rotor system.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):57-63. doi:10.1115/1.2832480.

This paper describes a combined theoretical and experimental investigation of the eight oil film stiffness and damping coefficients for a novel low impedance hydrodynamic bearing. The novel design incorporates a recess in the bearing surface which is connected to a standard commercial gas bag accumulator; this arrangement reduces the oil film dynamic stiffness and leads to improved machine response and stability. A finite difference method was used to solve Reynolds equation and yield the pressure distribution in the bearing oil film. Integration of the pressure profile then enabled the fluid film forces to be evaluated. A perturbation technique was used to determine the dynamic pressure components, and hence to determine the eight oil film stiffness and damping coefficients. Experimental data was obtained from a laboratory test rig in which a test bearing, floating on a rotating shaft, was excited by a multi-frequency force signal. Measurements of the resulting relative movement between bearing and journal enabled the oil film coefficients to be measured. The results of the work show good agreement between theoretical and experimental data, and indicate that the oil film impedance of the novel design is considerably lower than that of a conventional bearing.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):64-70. doi:10.1115/1.2832481.

A flexible disk, with small initial warpage/skew, is spinning in close proximity to a stationary baseplate. The partial differential equation for the disk deflection is coupled to the Reynolds equation of the stabilizing air-film. Disk warpage/skew produces a small change in the deflection which rotates with the disk. These deflections are obtained by linearizing the coupled equations about the axisymmetric configuration corresponding to a perfect disk. Numerical solutions are obtained and compared for different values of rotational speed and air-film thickness. The results show that among the three skewed/warped disks modeled, the skewed disk (i.e., the plane of the disk is skewed with respect to its axis of rotation) produces the largest deflection change (axial runout). With the effect of a point-contact head included, the existence of disk warpage/skew causes the head to produce a spatially-fixed harmonically varying force. The total disk motion is determined by superposition of the deflection pattern fixed on the disk and the space-fixed head-induced vibration. The disk pitch angle variation at the head is obtained and the results are compared for various values of the rotational speed and air-film thickness.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):71-75. doi:10.1115/1.2832482.

For the analysis of gas lubricated bearings, Reynolds equation is available in a number of forms—the original version, first-order modified, second-order modified, one-and-a-half order modified, and Boltzmann-Reynolds. All of these variants are asymptotically examined for instances in which local contact occurs in a bearing. These asymptotics serve to identify the order of the respective singularities in pressures, and thus imply the possible consequences of using a particular version of Reynolds equation in the presence of contact.

Topics: Bearings , Equations
Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):76-84. doi:10.1115/1.2832483.

We have demonstrated earlier that for laminar, isothermal flow of the lubricant in the non-cavitating film of long journal bearings, inertia has negligible effect on the load-carrying capacity and influences only the stability characteristics of the bearing. The question we pose in the present paper is: “will these conclusions remain valid for nonisothermal flow, or will lubricant inertia and dissipation interact and result in significant changes in bearing performance?” The results obtained here assert that the effect of lubricant inertia on load-carrying capacity remains negligible, irrespective of the rate of dissipation. The stability of the bearing is, however, affected by lubricant inertia. These results, although obtained here for long bearings and noncavitating films, are believed to be applicable to some practical bearing operations and suggest that for these, bearing load may be calculated from classical, i.e., noninertial theory.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):85-90. doi:10.1115/1.2832484.

A finite element perturbation approach to the prediction of foil bearing stiffness and damping coefficients is presented. The fluid lubricant is modeled as a simple barotropic fluid which is described by the Reynolds equation. The structural model includes membrane, bending, and elastic foundation effects in a general geometry. The equivalent viscous damping of the Coulomb friction caused by the foil relative motion is included in the structural calculation. Bearing stiffness and damping coefficients are predicted for an air-lubricated foil bearing with a corrugated sub-foil. The effects of the bearing number, bearing compliance, sub-foil Coulomb friction, and foil membrane stiffness on the bearing dynamic coefficients are discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):91-99. doi:10.1115/1.2832489.

This paper is an extension of an earlier work in which the present authors demonstrated the application of the differential quadrature method (DQM) to the steady-state analysis of incompressible and compressible lubrication problems. In the present work, the DQM is applied to the transient-state analysis of compressible lubrication problems. For this purpose, the analysis of gas-lubricated plain journal bearings under the conditions of nonuniform journal rotation is considered. The computed results from the solutions of the reference problem included in the paper provide a comparison of the convergence characteristics and computational efficiency of the differential quadrature and finite element methods.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):100-106. doi:10.1115/1.2832442.

A theoretical study of a submerged oil journal bearing is made considering surface roughness and thermal effects. The total load-supporting ability under such condition is due to the thermohydrodynamic as well as the asperity contact pressure. The effect of surface roughness and viscosity-temperature dependency on hydrodynamic pressure has been found by solving the average Reynolds equation, energy equation and heat conduction equations simultaneously. The cavitation model of Jacobsson-Floberg has been modified to take the surface roughness effects into consideration. A parametric study of steady-state behavior has been carried out. Finally, the isothermal, thermohydrodynamic, and contact loads for a model bearing have been calculated, assuming the surface height distribution as Gaussian.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):107-111. doi:10.1115/1.2832443.

The mechanisms that generate adhesion forces in liquid lubricants are studied under various experimental conditions. These forces occur between two surfaces when they are detached in the normal direction under static boundary lubrication conditions. The adhesion force is not influenced by the speed at which the upper specimen is pulled up, but is influenced by the viscosity of the lubricant. The adhesion force under boundary lubrication is much greater than that under hydrodynamic lubrication, and it is closely related to the compressibility of the lubricant.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):112-125. doi:10.1115/1.2832444.

In the present paper an isothermal elastohydrodynamic problem for a lightly loaded lubricated point contact of elastic bodies is formulated and studied numerically. Mathematical formulation of the problem is based on a steady nonlinear system of integrodifferential equations: Reynolds’ equation, equations of elasticity, boundary conditions for pressure, and the equilibrium condition. Nonlinearity of the problem is caused by nonlinearity of the Reynolds equation and the boundary conditions for pressure describing a free boundary. The inlet contact boundary is considered to be known and located close to the center of the contact. In order to determine the location of the free boundary (exit boundary of the contact region) the problem is formulated as a problem of complementarity by Kostreva (1984a) and Oh (1984). The dimensionless system of equations and inequalities for the elastohydrodynamic lubrication (EHL) problem is solved using the Newton-Raphson method. The inlet boundary of a contact region is considered to have a complex irregular shape (the inlet oil meniscus has some deep notches) which is taken into account while deriving the finite-difference equations. The effect of the shape and location of the inlet oil meniscus on the lubrication film thickness, pressure, gap, sliding and rolling frictional and subsurface stress distributions are considered. Some numerical results are presented for pressure, gap, frictional and subsurface stress distributions in EHL contact. These numerical results show that variations in the inlet meniscus shape and location may cause significant qualitative and quantitative changes in distributions of parameters of a lubricated contact. For instance, the maximum values of pressure may change by 20 percent, and for rolling frictional stress by 100 percent and even more.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):126-131. doi:10.1115/1.2832445.

This paper, the first of two companion papers, presents a model for investigating structural vibrations in rolling element bearings. The analytical formulation accounts for tangential and radial motions of the rolling elements, as well as the cage, the inner and the outer races. The contacts between the rolling elements and races are treated as nonlinear springs whose stiffnesses are obtained by application of the equation for Hertzian elastic contact deformation. The derivation of the equations of motion is facilitated by assuming that only rolling contact exists between the races and rolling elements. Application of Lagrange’s equations leads to a system of nonlinear ordinary differential equations governing the motion of the bearing system. These equations are then solved using the Runge-Kutta integration technique. Using the formulation in the second part—“A Nonlinear Model for Structural Vibrations in Rolling Element Bearings: Part II—Simulation and Results,” a number of effects on bearing structural vibrations are studied. This work is unique from previous studies in that the model simulates vibration from intrinsic properties and constituent elements of the bearing, and takes into account every contact region within the bearing, representing it by a nonlinear spring.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):132-141. doi:10.1115/1.2832446.

This paper describes an experimental and theoretical investigation of a four-pocket, oil-fed, orifice-compensated hydrostatic bearing including the hybrid effects of journal rotation. The test apparatus incorporates a double-spool-shaft spindle which permits independent control over the journal spin speed and the frequency of an adjustable-magnitude circular orbit, for both forward and backward whirling. This configuration yields data that enables determination of the full linear anisotropic rotordynamic model. The dynamic force measurements were made simultaneously with two independent systems, one with piezoelectric load cells and the other with strain gage load cells. Theoretical predictions are made for the same configuration and operating conditions as the test matrix using a finite-difference solver of Reynolds lubrication equation. The computational results agree well with test results, theoretical predictions of stiffness and damping coefficients are typically within thirty percent of the experimental results.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):143-148. doi:10.1115/1.2832450.

A simple mathematical model for the engagement of rough, permeable, grooved wet clutches has been developed and used to determine the effect of various input parameters (applied load, grooved area, and friction material permeability) on engagement. The model includes the effects of surface roughness according to Patir and Cheng (1978), friction material permeability according to Natsumeda and Miyoshi (1994) and Beavars and Joseph (1967), and grooving in the friction material according to a new approximation. The approach reduces the system of Reynolds and force balance equations to a single, first-order differential equation in film thickness and time. A line searching algorithm, exploiting the low computational cost of function evaluations for the new model, is used to find the set of input parameter combinations yielding the same engagement characteristics. This set of design points is presented as an engagement isosurface in the parameter space (Fapp , Φ̂, Ared ). The isosurface implicitly gives information about engagement time, and it shows regions in which the desired engagement characteristics cannot be achieved. The input parameters are classified as those affecting the transient portion of engagement and those affecting the steady-state portion.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):149-155. doi:10.1115/1.2832451.

Systematic analyses are presented to reveal the mechanism of multi-valued friction behavior in lubricated sliding contacts with time-varying velocities. The analyses are based on the theoretical results generated by a mixed-film friction model developed in this paper for line contacts. The model, which integrates theories of transient elastohydrodynamics and asperity contact mechanics, is validated by comparing its results with published experimental data. The results and the subsequent analyses disclose that strong multi-valued friction behavior can only be generated in the mixed-film lubrication regime with simultaneous presence of significant asperity contacts and hydrodynamic squeeze. Principal factors which influence the magnitude of dynamic friction are investigated in the paper. Being instructive to the design of tribocontacts in precise-motion control systems, the analyses suggest means to minimize the undesirable multi-valued friction behavior through proper selection of system parameters.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):156-162. doi:10.1115/1.2832452.

An analytical frequency domain solution is obtained using the spatial Fourier transform for thermal and thermoelastic fields due to an arbitrary heat source or thermal distribution moving at constant speed over the surface of an insulated, traction free elastic half space. Conversions between the space and frequency domains for the input and output are performed efficiently and robustly using FFT techniques. The method is validated by comparison to the analytical result for the moving line heat source in which it is shown that numerical evaluation of the analytical solution is problematic for large speeds or distances from the heat source. The utility of the method is illustrated on the constant patch moving heat source and discretely distributed multiple heat sources known as the “hot spot” problem. It is shown, through several examples, that the effect of hot spots on surface displacement and tangential stress is small. Finally, this conclusion is generalized by quantifying the frequency domain solution for the moving heat source problem as a low pass filter.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):163-170. doi:10.1115/1.2832453.

Polishing is a finishing process in which a smooth work surface is produced by rubbing it against a polishing block with an abrasive slurry interspersed between them. A model has been developed to estimate the temperature rise of the work surface in polishing. In this model, the forces acting on an abrasive particle are derived from a mechanistic analysis of abrasive-workpiece contacts. The heat generated at a contact is taken as the product of the friction force and the relative sliding velocity between the abrasive and the work surface. For calculating the heat flux transferred into the workpiece, each of the abrasive-workpiece contacts is modeled as a hardness indentation of the work material by a conical indenter. The moving heat source analyses of Jaeger and Blok are then applied to estimate the fraction of the heat flux flowing into the workpiece, and the maximum and average temperature rise of the work surface. Calculations of the work surface temperature rise are made for the polishing of steel, soda-lime glass, and ceramics. These show that the work surface temperature rise in polishing is quite small, typically much less than 200°C, and substantially less than in grinding. The low values calculated for the work surface temperature rise are shown to be consistent with many observations pertaining to the mechanical state of polished surfaces. The effect of polishing process variables on the work surface temperature rise is analyzed.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):171-178. doi:10.1115/1.2832454.

Pressure distributions in the oil film of a porous journal bearing are investigated theoretically and experimentally under hydrodynamic lubrication conditions. The circumferential boundary condition for the oil-film pressure is obtained by applying an integral momentum equation to the oil-film region in the bearing clearance. The oil-film pressure distributions are numerically solved using this momentum equation and taking into consideration the balance between oil fed into the clearance and that lost from it. The present analysis shows the occurrence of a negative film pressure before the trailing end of the oil-film region. The experimental results confirm the existence of this negative film pressure. Furthermore, the angular position of the trailing end of the oil-film region obtained in the present analysis moves toward the downstream region, yielding better agreement with the measured and calculated film regions than was found in our previous analysis based on the quasi-Reynolds boundary condition.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):179-187. doi:10.1115/1.2832455.

Hydrostatic/hydrodynamic (hybrid) journal bearings handling process liquids have limited dynamic stability characteristics and their application as support elements to high speed flexible rotating systems is severely restricted. Measurements on water hybrid bearings with angled orifice injection have demonstrated improved rotordynamic performance with virtual elimination of cross-coupled stiffness coefficients and null or negative whirl frequency ratios. A bulk-flow model for prediction of the static performance and force coefficients of hybrid bearings with angled orifice injection is advanced. The analysis reveals that the fluid momentum exchange at the orifice discharge produces a pressure rise in the hydrostatic recess which retards the shear flow induced by journal rotation, and thus, reduces cross-coupling forces. The predictions from the model are compared with experimental measurements for a 45 deg angled orifice injection, 5 recess, water hydrostatic bearing operating at 10.2, 17.4, and 24.6 krpm and with supply pressures of 4, 5.5 and 7 MPa. The correlations include recess pressures, flow rates, and rotordynamic force coefficients at the journal centered position. An application example for a liquid oxygen hybrid bearing also demonstrates the advantages of tangential orifice injection on the rotordynamic coefficients and stability indicator for forward whirl motions, and without performance degradation on direct stiffness and damping coefficients.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):188-192. doi:10.1115/1.2832456.

The misalignment between the journal and the bearing in a rotor-bearing system may be due to manufacturing error, elastic deflection, thermal expansion etc. In the present work, the eight linearized stiffness and damping coefficients of the cylindrical and three lobe bearings are identified at different levels of bearing misalignment (twisting misalignment) and at different speeds of the rotor. The identification method used here needs FRFs (Frequency Response Functions) obtained by the measurements and the finite element method. The twisting misalignment changes the stiffness and damping coefficients in the vertical and horizontal directions. In the case of three lobe bearings, for 0.7 degree of misalignment, the stiffness in the vertical direction is increased by about 12 percent.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):193-199. doi:10.1115/1.2832458.

The step jump method was developed approximately three decades ago to help determine the stability of gas lubricated triboelements. In the approach, the force contribution from the gas layer is characterized by its step response, which is the transient force response resulting from pressure diffusion in the gas film after a step increase in film thickness. The procedure is broadened by implementing Duhamel’s theorem to yield the system characteristic equation. Since its inception in the literature, the step response has been approximated in the equations of motion using a series of Laguerre polynomials, which allows for a closed form analysis. This paper will prove that using Laguerre polynomials can violate the second law of thermodynamics, and a test case will show that stability results predicted by this approach can be inaccurate. It will be proven that a mathematical correlation exists between the dynamic behavior of the gas film and the dynamic behavior of a linear viscoelastic medium. This correlation is advantageous since much of the viscoelastic theory can be applied to the dynamic analysis of gas lubricated triboelements.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):200-204. doi:10.1115/1.2832460.

The dynamic behavior of a mechanical face seal with two flexibly mounted rotors is investigated. The equations of motion are derived using linearized rotor dynamic coefficients to model the dynamic behavior of the fluid film. The equations are shown to be linear in the inertial reference with harmonic forcing functions which result from the initial misalignment of the flexible supports. A method for obtaining the steady-state response in the system is derived by transforming the equations of motion into reference frames which rotate with the shafts. The resulting equations contain constant forcing functions and can be readily solved for the magnitude of the steady-state response. The method presented allows a rapid determination of the steady-state misalignment of a seal without resorting to numerical modeling.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):205-210. doi:10.1115/1.2832461.

Previous numerical simulations and experimental observations have shown that the meniscus in a rotary lip seal will be ingested into the sealing zone when the shaft speed exceeds a critical value. The present numerical analysis shows that once the meniscus is ingested, multiple equilibrium operating points exist, so the steady-state operating characteristics of the seal will depend on the history of the seal as well as on the steady-state operating conditions and seal properties. The analysis also shows that if the meniscus moves too close to the liquid-side of the seal, asperity contact between the lip and the shaft will occur.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):211-216. doi:10.1115/1.2832462.

Micro-inertia effects of surface roughness on hydrodynamic lubrication are analyzed in the light of similitude principles, viz. a newly conceived reduced Reynolds number and the classical parameter of relative roughness. In particular, the dynamic properties of laminar sheet flow in a two-dimensional channel between a sinusoidal wall and a flat wall are studied. FEM solutions of the Navier-Stokes equation are compared with corresponding experimental findings. The latter are gathered in an especially designed laminar-flow wind tunnel. Conclusions are drawn concerning the roughness sensitivity of laminar thin-film flows.

Commentary by Dr. Valentin Fuster
J. Tribol. 1997;119(1):217-226. doi:10.1115/1.2832464.

This paper describes a new method for calculating the solvation pressure that acts between solid surfaces when the surfaces approach each other to within a very small distance in a liquid medium. Solvation pressure is calculated by solving the transformed Ornstein-Zernike equation for hard-spheres in a two-phase system with Perram’s method and using the Derjaguin approximation. Furthermore, the authors apply the new method to the elastohydrodynamic lubrication problem in which the film thickness is very small and solvation force and van der Waals force cannot be neglected. It will be shown that the calculation results agree well with experimental data. The results are then compared with two conventional solvation pressure models proposed so far, namely, Chan and Horn’s model, and, Jang and Tichy’s model. It is found that these two models neglect the elastic deformation of solid surface when obtaining the experimental parameter used in their models; thus they overestimate the solvation pressure resulting in the prediction of larger film thickness than the experiments.

Commentary by Dr. Valentin Fuster

DISCUSSIONS

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