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RESEARCH PAPERS

J. Tribol. 1994;116(3):401-407. doi:10.1115/1.2928852.

A three-dimensional model of axisymmetric moving solids submitted to nonaxisymmetric transient heat flux conditions is presented in this paper. Temperature fields are obtained using a new hybrid FFT-FEM method that combines Fourier transform techniques and finite element method. A fast Fourier form algorithm is used which leads to inexpensive computer time. Validation tests are presented. Efficiency of the method is demonstrated.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):409-414. doi:10.1115/1.2928855.

Thermoelastic instability in an automotive disk brake system is investigated experimentally under drag braking conditions. The onset of instability is clearly identifiable through the observation of nonuniformities in temperature measured using embedded thermocouples. A stability boundary is established in temperature/speed space, the critical temperature being attributable to temperature-dependence of the brake pad material properties. It is also found that the form of the resulting unstable perturbations or eigenfunctions changes depending upon the sliding speed and temperature.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):415-422. doi:10.1115/1.2928856.

A numerical analysis has been carried out for a three-dimensional frictional heating problem of a composite material, in which an asperity contact (heat source) moves across the boundary of the constituents of a composite. The mathematical model adopted here is that a surface asperity on a semi-infinite body slides on a composite which consists of two semi-infinite bodies. Expressions of temperature distribution both in the composite and on the moving surface are derived. Then the temperature distribution and its change with time are obtained by a numerical procedure. It is shown that the temperature rise caused by frictional heating remarkably depends on the thermal properties of constituents of the composite, and temperature distributions in the vicinity of the asperity contact both on the moving surface and in the composite rapidly change when the asperity passes over the boundary of the constituents. The effect of the frictional heating on the worn surface profile of a composite is also discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):423-429. doi:10.1115/1.2928857.

Ceramics are hard and brittle. Machining such materials is time-consuming, difficult, and expensive. Current machining technology requires stiff machine, high hardness tools, and small material removal rates to minimize surface damage. This study demonstrates the feasibility of a novel ceramic machining concept that utilizes chemical reactions at the tool-workpiece interface to reduce the stress and minimize the surface damage. A series of cutting tests using a diamond wheel on silicon nitride with different chemical compounds has been performed. The results demonstrate that by using different chemistries, the material removal rate and the surface finish of the machined ceramic can be significantly altered. Some halogenated hydrocarbons show a significant improvement over some commercial machining fluids currently in use.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):430-437. doi:10.1115/1.2928858.

The two-dimensional plane-strain sliding contact of a smooth rigid roller on a transverse ground rough surface is analyzed. The rough surface is idealized as an elastic half-space with periodic roughness modeled as cylindrical ridges oriented transverse to the sliding direction. The contact problem is solved using a numerical iterative method in which each asperity contact is treated as a micro-Hertz contact, and the exact treatment of asperity interaction is included. The subsurface stress field is calculated using Westergaard stress functions. The subsequent analysis compares the rough surface stress fields with the corresponding smooth Hertz contact to evaluate the influence of surface roughness and friction on the subsurface stress distributions. The results show that the real area of contact is less than the corresponding smooth surface Hertz contact area, and the magnitude of the actual localized maximum contact pressure is always greater than the corresponding smooth surface contact pressure. The asperity level subsurface effective stresses are greater in magnitude than the maximum subsurface stress due to the macro-Hertz contact for low coefficients of friction, and for high coefficients of friction the maximum effective stresses occur on the bulk material surface.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):439-444. doi:10.1115/1.2928861.

This paper addresses the wear characteristics of the mixing tube of an abrasive-waterjet nozzle. An effective nozzle material should possess high values of both hardness and toughness. The mixing tube, which is where the abrasives are mixed, accelerated, and focused with the high-pressure waterjet, is the component in the abrasive-water jet nozzle that receives the greatest wear. Accelerated wear tests were conducted on relatively soft (steel) mixing tubes using a typical soft abrasive (garnet sand) and on harder (tungsten carbide) tubes using a harder abrasive material (aluminum oxide). A wide range of candidate tool materials, including several carbides and ceramics, was also tested using actual machining parameters. The tungsten carbide grades exhibited greater longevity than the harder ceramics, such as boron carbide, when garnet abrasives were used. The reverse trend was observed with aluminum oxide abrasives. Wear trends suggest that the wear mechanisms along the mixing tube change from erosion by particle impact at the upstream sections to abrasion at the downstream sections. Linear cutting tests were also conducted on several candidate nozzle materials to gain more information related to wear performance. It was found, for example, that the binder in tungsten carbide, which controls these properties, is a critical factor that also controls the lifetime of tungsten carbide mixing tubes.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):445-453. doi:10.1115/1.2928862.

The role of surface roughness on the coefficient of friction and wear of polished CVD diamond films has been investigated. Diamond films grown on single crystal silicon (001) substrates by Hot Filament Chemical Vapor Deposition (HFCVD) process were polished by a chemomechanical process in which a diamond film was polished against another diamond film in the presence of a fused alkaline oxidizer at 320°C. Friction and wear properties of these polished films were measured at elevated temperatures and in the presence of various gaseous environments. The coefficient of friction of the polished diamond films was found to be about 0.09, which is very close to that of natural diamond (0.07). The wear rate of the mating alumina ball slid against a polished diamond film was also found to be comparable, when slid natural diamond.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):454-462. doi:10.1115/1.2928865.

Unlike polycrystalline diamond films, amorphous hydrogenated carbon (AHC) films can be deposited at room temperature, are amorphous in atomic structure, and form very smooth surfaces. Amorphous hydrogenated carbon film consists of very small 10–20 Å sp2 bonded (graphitic) clusters captured in a largely sp3 coordinated, partially hydrogenated random network of covalently bonded carbon. Because of the extreme stiffness of the carbon-carbon bond, this hydrocarbon composite, less dense even than graphite, exhibits hardness rivaling that of the hardest ceramics. We report a systematic study of the tribological characteristics of AHC films deposited on silicon substrates by radio frequency plasma assisted chemical vapor deposition. The friction and wear behavior of these films in sliding contact with a steel ball without any lubrication was evaluated as a function of film deposition conditions, contact stress, sliding speed, sliding distance, and relative humidity. The friction coefficient and the wear of both the contacting surfaces were found to increase with relative humidity. At low relative humidity, (a) the films exhibited friction coefficients in the range of 0.05–0.16 under a contact stress ranging from 0.83 to 1.66 GPa and a sliding speed ranging from 0.03 to 1 m/s and (b) the wear rates of the film and the steel ball were significantly lower than that of other hard coating, such as TiN or TiC, evaluated under similar conditions. These results are very encouraging for some tribological applications of AHC films.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):463-469. doi:10.1115/1.2928866.

Several retainer materials for high-temperature silicon nitride bearings to be used in air were examined with a high-temperature rolling three-ball tester. The “soft” or “self-lubricating” retainer materials (metal-bonded MoS2 -WS2 alloys, a carbon-carbon composite and graphite) tended to yield longer bearing life, lower silicon nitride wear rate and lower traction coefficients than the hard retainers (Fe-based sintered alloys, chromia and silicon nitride). The main problem in the use of the self-lubricating retainer materials was their rapid wear by tribo-oxidation at high temperatures.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):470-478. doi:10.1115/1.2928867.

The assumption of axisymmetry, employed by most of studies on piston ring lubrication, probably gives a too idealistic model for the real situation. A theoretical model for a nonaxisymmetrical analysis of piston ring lubrication has been established in the present study. When a piston ring with an arbitrary free shape is fitted into the cylinder bore, the determination of ring deflection and contact load has been modeled mathematically as a Linear Complementary Problem (LCP). By combining LCP solution with lubrication analysis, the film thickness and contact load distribution over the circumference are obtained, leading to a more realistic simulation for piston ring lubrication. The friction force between piston ring and cylinder bore is predicted by the mixed lubrication model including the effects of surface roughness and asperity contact. The static distortion of cylinder bore, gas pressure variation, and lubricant starvation are also considered in the simulation. Results show that the contact pattern and film thickness between piston ring and cylinder bore are not exactly axisymmetrical. The main reason for the nonuniform contact is the asymmetry of ring elasticity, the static distortion and dynamic load created by the secondary movement of piston skirt.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):479-487. doi:10.1115/1.2928868.

A previously developed dynamic model for two-phase face seals is extended to include turbulent flow. Thermal transients over the entire range of laminar and turbulent flow are considered for both parallel and tapered geometries. Inlet losses and choking are accounted for in the turbulent regime. Axial responses to perturbations from equilibrium are examined, and general criteria are discussed for predicting stable, unstable and bistable seal operation.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):489-498. doi:10.1115/1.2928871.

The mutual interaction effects of cylindrical and conical whirl on the dynamic fluid forces and moments, which act on a long annular seal, were studied experimentally. A whirling motion composed of cylindrical and conical whirls is actuated by intentionally giving the phase difference between the seal exit and inlet whirling movements. This whirling motion is believed to generate during actual pump running. The experiment was conducted by changing the phase difference, at various rotor speeds and with a pressure difference between the seal inlet and exit. The result of this study revealed that fluid forces and moments are greatly dependent on the phase difference of the whirl, namely the long seal has a significant coupling between displacements and rotations. Furthermore, dynamic fluid forces and moments were derived theoretically, assuming that total fluid force acting on the rotor could be determined by superposing fluid forces due to conical and cylindrical whirling movements. It was confirmed that the experimental results moderately agree with the theoretical values, if the rotor and seal are set in concentric alignment, the principle of superposition becomes applicable.

Topics: Force , Motion , Whirls , Fluids , Rotors , Pumps , Pressure
Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):499-507. doi:10.1115/1.2928872.

Squeeze film dampers introduce nonlinear motion dependent damper forces into otherwise linear rotor bearing systems, thereby considerably complicating their analysis. Noncircular orbit type dampers, such as unsupported or uncentralized dampers, have generally necessitated transient solutions, which are computationally prohibitive for design studies of large order systems, particularly for systems with low damping. By utilizing harmonic balance with appropriate condensation, it is possible to considerably reduce the number of simultaneous nonlinear equations inherent to this approach. The stability (linear) of the equilibrium solutions may be conveniently evaluated using Floquet theory, particularly if the damper force components are evaluated in fixed, rather than rotating, reference frames. The versatility of this technique is illustrated on systems of increasing complexity with and without damper centralizing springs. Of particular interest, is its applicability to unsupported systems illustrating how such systems can lift off and, with further increase in speed, the damper forces can be linearized about the orbit center.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):508-513. doi:10.1115/1.2928873.

Test results are presented for rotordynamic coefficients and leakage for three annular seals which use an anti-swirl self-injection concept to yield significant improvement in whirl frequency ratios as compared to smooth and damper seals. A new anti-swirl self-injection mechanism is achieved by deliberately machining self-injection holes inside the seal stator to partially divert inlet flow into the anti-swirl direction. The anti-swirl self-injection mechanism is used to achieve effective reduction of the tangential flow which is considered as a prime cause of rotor instability in high performance turbomachinery. Test results show that the self-injection mechanism significantly improves whirl frequency ratios; however, the leakage performance degrades due to the introduction of the self-injection mechanism. Through a series of tests, an optimum anti-swirl self-injection seal which uses a labyrinth stator surface with anti-axial flow injections is selected to obtain a significant improvement in the whirl frequency ratio as compared to a damper seal, while showing moderate leakage performance. The best whirl frequency ratio is achieved by an anti-swirl self-injection seal of 12 holes anti-swirl and 6 degree anti-leakage injection with a labyrinth surface configuration. When compared to a damper seal, the optimum configuration outperforms the whirl frequency ratio by a factor of 2.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):514-520. doi:10.1115/1.2928874.

The behavior of a flexible disk, spinning in close proximity to a warped/skewed stationary baseplate, is investigated. The governing partial differential equation for the disk deflection is coupled to the Reynolds equation of the air film. Four warped/ skewed baseplate configurations are modeled. The effects of baseplate warpage and skew on the steady-state configuration of the disk are determined by investigating small deviations away from the axisymmetric configuration of the disk corresponding to a perfect baseplate. Exponential Fourier series expansions in the circumferential direction, along with finite differences in the radial direction, are used. Numerical results are determined and compared for various values of the angular velocity and initial thicknesses of the air film. Among the three warpages considered, the saddle warped baseplate provides the largest change in disk deflection whereas the spherically warped baseplate gives the smallest change. The total response of the disk is obtained by superposition of the deflection change caused by the warped/skewed baseplate and the deflection obtained from the axisymmetric solution.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):521-527. doi:10.1115/1.2928875.

The bearing load of a plane inclined sector-shaped hydrodynamic thrust bearing, under simultaneous translation and transverse vibration, is measured experimentally. The results are used to evaluate the lubrication theory solutions. Consequently, both the influences of the unsteady film inertia, measured by the squeeze Reynolds number Res , and the convective film inertia, measured by the modified Reynolds number Re*, on load amplitude and phase are investigated. It is found that the inertia-neglected lubrication solutions underestimate: (1) the oscillatory component of the bearing load by 6.5 percent at Res = 1.0 and by 1.4 percent at Re* = 1.0, and (2) the mean component of the bearing load by 0.7 percent at Res = 1.0 and by 2.0 percent at Re* = 1.0 Moreover, the fluid inertia induces an equivalent negative spring force component which shifts the phase of the bearing load by 9.5 deg at Res =1.0 and by 4 deg at Re* = 1.0 as compared to the lubrication theory predictions. Hence it can be an important consideration when designing bearings for vibration control purposes.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):528-534. doi:10.1115/1.2928876.

The experimentally determined behavior of a short radial squeeze-film damper with no end seals, executing circular centered orbits, is discussed. Accurate circular orbits were achieved, for ε values in the range 0.1 and 0.8, by using digitally generated signals to drive two electromagnetic shakers. Radial and tangential dynamic fluid force coefficients were estimated from measurements of the applied forces and the orbit radii, using a simple algebraic method. Cavitation was found to occur when ε exceeded 0.5, at large orbit frequencies, and was the cause of an observed jump-up phenomenon. The magnitude of an oil stiffness effect, previously reported by the authors and confirmed by the present results, was found to depend significantly on the oil supply pressure. Its contribution to the total fluid force was of the same order as that from fluid inertia, in the case of small orbits (ε ≪ 1).

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):535-540. doi:10.1115/1.2928877.

The study of bearings subjected to impulsive loads have previously showed that inertia effects and surface accelerations play an important role in the bearing response. Although the lubricant was considered Newtonian, this assumption is no longer valid with modern lubricant. In industrial applications, mineral lubricants are added to several long soluble chains of polymer in order to conserve optimum properties under different operating conditions. The addition of these polymers results in the drop of viscosity under high shear-rate, in the range of 10−6 –10−8 s−1 . This study presents a continuation of previous works. It examines the influence of both effects, the decrease in viscosity and the fluid inertia, in a journal bearing under impulsive loads. Using the power-law model, the results show important differences in shaft responses compared to the Newtonian cases. Furthermore, in high shear-thinning effects, a reduction of lubricant capacity to absorb sudden dynamic loads is observed.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):541-548. doi:10.1115/1.2928878.

The prospect of contact and near-contact recording in magnetic hard disk files naturally leads to reduced flying heights of the read-write head over the rigid disk. To avoid dry contact at these low head-to-disk spacings, a lubricant should be used to minimize wear and maximize reliability. Since fluids generally have a much greater viscosity than air and very large shear rates develop under the slider, it is believed that a fully flooded interface can only be practically possible if the fluid possesses a non-Newtonian character with a significant amount of shear-thinning. In this paper, we present results from extensive numerical simulations of the fully flooded head-disk interface using the finite element technique. This approach has proven very successful in calculating a wide variation of slider geometries for various fluid nonlinearities.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):549-556. doi:10.1115/1.2928879.

Transient EHL analysis for line contacts with measured surface roughness is performed by using the multigrid method. Results show that the transient effect, induced by surface roughness, has a remarkable influence on pressure distribution and the film thickness profile. Pressure fluctuation increases with the relative sliding speed between the contact surfaces. For simple sliding with stationary surface roughness, the roughness profile is almost flattened. When the rough surface moves, the elastically deformed surface roughness, in the contact zone, increases with the moving speed. As the moving speed of surface roughness equals to or exceeds the rolling speed, the roughness of the deformed surface profile in the contact zone is close to the roughness of the undeformed roughness profile.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):559-568. doi:10.1115/1.2928883.

The thermal circular non-Newtonian model accompanied with three specialized models was used to study the mechanisms of film generation and traction reduction in EHL line contact conjunctions. Results revealed that the film generation capability is mainly controlled by the inlet zone pressure buildup and the inlet zone piezo-thickening. The diffusion time effect enhances the thermal thinning that reduces this capability. On the other hand, the piezoviscosity and the shear rate in the central contact zone are the main traction generation factors whereas shear skidding and thermal skidding are the traction reduction mechanisms. Results also showed that neglecting either viscous heating or the combination of shear thinning and shear stress reduction in formulating an EHL simulator is inaccurate for many cases.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):569-576. doi:10.1115/1.2928884.

A mathematical model for liquid lubricated strip rolling in the full-film regime is developed. The model combines slab plasticity, hydrodynamic lubrication and thermal analyses to relate local and global condition to process variables and material properties. The predictions of the model are compared with experimental measurements of outlet speed ratio, roll separating force and roll torque in rolling 1100-H14 aluminum with a viscous mineral oil and 5P4E polyphenyl ether as lubricants. The excellent agreement which is obtained provides powerful support of the validity of the model.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):577-587. doi:10.1115/1.2928885.

Based on a stress invariant hypothesis and a stress/strain relaxation procedure, an analytical approach is forwarded for approximate determination of residual stresses and strain accumulation in elastic-plastic stress analysis of rolling contact. For line rolling contact problems, the proposed method produces residual stress distributions in favorable agreement with the existing finite element findings. It constitutes a significant improvement over the Merwin-Johnson and the McDowell-Moyar methods established earlier. The proposed approach is employed to study combined rolling and sliding for selected materials, with special attention devoted to 1070 steel behavior. Normal load determines the subsurface residual stresses and the size of the subsurface plastic zone. On the other hand, the influence of tangential force penetrates to a depth of 0.3a, where a is the half width of the contact area, and has diminishing influence on the residual stresses beyond this thin layer. A two-surface plasticity model, commensurate with nonlinear kinematic hardening, is utilized in solution of incremental surface displacements with repeated rolling. It is demonstrated that a driven wheel undergoes greater plastic deformation than the driving wheel, suggesting that the driven wheel experiences enhanced fatigue damage. Furthermore, the calculated residual stresses are compared with the existing experimental data from the literature with exceptional agreements.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):588-596. doi:10.1115/1.2928886.

A theoretical model for the normal instantaneous squeeze film force for a finite length cylinder is developed in this paper. The model assumes large unidirectional cylinder motion along a sleeve diameter. Based on the assumption of a parabolic flow field, a normal squeeze film model for an infinitely long cylinder is first obtained. Combining the infinitely long model with side-leakage factors, a finite length model is then obtained. The model shows that the instantaneous squeeze film force consists of three position-dependent nonlinear terms: namely a viscous term, an unsteady inertia term and a convective inertia term. From experimental measurements using water and a clearance to radius ratio of 0.032, the viscous term of the theoretical model should be corrected by a factor involving the instantaneous squeeze film Reynolds number and the absolute value of instantaneous eccentricity. The synthesized squeeze force waveforms obtained using the corrected equation with averaged weighting coefficients agree very well with the experimental waveforms for eccentricity ratios up to 0.9 and a wide frequency range. The corrected equation is suitable for the calculation of the normal instantaneous squeeze film force given the instantaneous position, velocity, and acceleration of the cylinder center.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):597-605. doi:10.1115/1.2928887.

This paper describes a numerical model developed to predict the elastohydrodynamic (coupled solid-fluid) response of unit injector fuel systems. These systems consist of a concentric barrel and plunger with a small annular clearance. During operating (axial movement of the plunger), highly nonuniform pressure and clearance fields are developed which are strongly coupled with each other. The model simultaneously solves for the transient response of the fluid film pressure distribution and three different structural deformation components in a two-dimensional (axial-circumferential) domain. These structural components are the transverse bending of the plunger, radial expansion of the barrel, and radial growth of the plunger from a Poisson effect. The fluid film pressure distribution is governed by the transient Reynolds equation (i.e., lubrication theory) and the structural deformation components governed by linear elastic theory. Full account is taken of these hydrostatic, hydrodynamic, and squeeze-film forces generated in the fluid. The model has been applied to several injector designs. Results have been compared with known performance characteristics and have been found to be qualitatively accurate, in that locations of plunger/barrel contact, and potential for failure, have been accurately predicted.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):606-611. doi:10.1115/1.2928888.

The static characteristics of porous journal bearings under hydrodynamic lubrication conditions are theoretically investigated assuming that the oil is fed through their outside diameters under a small pressure. The angular extent of the oil film formed in the bearing clearance is numerically solved on the basis of the following postulate: when the oil film extent reaches steady state, the inflow of oil into the bearing clearance through the porous matrix due to the oil-feed pressure must make up for the oil leakage from the ends through the clearance gap and that into the porous matrix due to the hydrodynamic pressure in the film. Numerical results show that the dimensionless oil-feed pressure significantly influences the static characteristics. Experiments are also conducted for confirmation after the theoretical examination.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):612-620. doi:10.1115/1.2928889.

When calculating film thickness and friction in elastohydrodynamically lubricated contacts, assuming a non-Newtonian fluid, the lubricant limiting shear stress is an essential parameter. It influences minimum film thickness and determines traction in the contact. The limiting shear stress is pressure dependent according to the Johnson and Tevaarwerk equation:

τL = τ0 + γp
The limiting shear stress-pressure coefficient γ has in a previous screening investigation been shown to depend on several parameters: oil type, oil viscosity at + 40°C, maximum contact pressure and temperature. In the present investigation, the preliminary data is used together with response surface methodology. With these results in mind, further experiments are made and an empirical model is built. This paper presents a new model for γ which is valid for two types of oil (a polyalphaolefine with diester and a naphthenic oil) with different viscosities at +40°C. The model incorporates the influence of maximum contact pressure and oil temperature on γ. The measurements on which the model is based were carried out at temperatures ranging from −20 to + 110°C. The pressure range was 5.8–7 GPa and the shear rate was about 106 s−1 .

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):621-627. doi:10.1115/1.2928890.

A theoretical nonlinear analysis of tilting-pad journal bearings is presented for small and large unbalance loads under isothermal conditions. The radial displacements of internal pad surface due to pressure field are determined by a two-dimensional finite element method in order to define the actual film thickness. The influence of pad deformations on the journal orbit, on the minimum film thickness and on the maximum pressure is studied. The effects of pad displacements are to decrease the minimum film thickness and to increase the maximum pressure. The orbit amplitude is also increased by 20 percent for the large unbalance load compared to the one obtained for rigid pad.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):629-634. doi:10.1115/1.2928893.

This paper presents a rheological investigation conducted on three lubricants used in space applications under vacuum. Lubricant behavior was determined for representative pressures and temperatures. Relations between standardized methods and space requirements are discussed. Finally, the elastohydrodynamic load-carrying capacity of the three fluids in the Newtonian domain is computed over a wide temperature range and compared to those of two reference base oils.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):635-639. doi:10.1115/1.2928894.

Transient behavior of elastohydrodynamic (EHL) films caused by a transversely oriented groove passing through the EHL conjunction is directly observed using the optical interferometry technique. A steel ball on which the groove is formed is run against a smooth glass disk under conditions of rolling with sliding. It is shown that a local reduction in film thickness caused by the groove and its recovery depend strongly on shear flow dominating the EHL conjunction and side leakage along the groove. Surface kinematic conditions and the relative size of the groove with respect to the overall film thickness have also profound effects on the above phenomena.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):640-647. doi:10.1115/1.2928895.

A porous medium model is basically a macroscopic description of the fluid behavior and is considered more appropriate to the mixed lubrication regime. The present work has successfully combined the flow factor method, porous medium model and continuum percolation theory to investigate the flow phenomena in the mixed lubrication regime. Not only the lubricant flow can be solved but also the anisotropic percolation threshold can be obtained. Instead of the flow factor which is singular near h = 0, the lubricant flux itself is considered. A new definition of the pressure flow factor which is more appropriate to the mixed lubrication realm is suggested.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):648-653. doi:10.1115/1.2928896.

This paper describes a theory for the approximated analytical determination of pressure distribution in the gap of a generic finite length journal bearing in isothermal laminar lubrication conditions as the position assumed by the journal axis varies in a reference frame fixed in the bearing. The same theory is used to set up lubricated finite length journal bearing models in order to obtain an analytical description of the fluid film force, which is characterized by considerable prediction accuracy in the whole field defining the length-diameter ratio. In order to assess the model’s prediction accuracy, the static characteristics of the journal bearing are analytically determined. The theoretical results, presented in the form of diagrams, are compared with the corresponding results obtained numerically.

Commentary by Dr. Valentin Fuster
J. Tribol. 1994;116(3):654-657. doi:10.1115/1.2928897.

The theory of micropolar liquid lubrication (see Prakash and Sinha, 1975; Tipei, 1979; Singh and Sinha, 1982) takes into account only the increasing of effective viscosity in thin layers. Modern experiments (see Derjaguin et al., 1985) show that effective viscosity can increase or decrease and approaches to a certain limit (boundary viscosity), depending on the type of liquid and nature of the solid surface. A new generalized Reynolds equation that takes into account both these effects and also all possible situations in microrotation near the friction surface is derived in this work. An example using the equation for calculation of the journal bearing performance is given. It is shown that the friction coefficient can be sufficiently decreased without a noticeable change in the load capacity by regulation of interaction between micropolar lubricant and surfaces.

Commentary by Dr. Valentin Fuster

DISCUSSIONS

Commentary by Dr. Valentin Fuster
Commentary by Dr. Valentin Fuster

TECHNICAL BRIEFS

J. Tribol. 1994;116(3):658-660. doi:10.1115/1.2928898.

Rolling resistance model, developed based on rigid body assumption, is further analyzed in this paper using a simplified analytical model and experimental results. The analysis leads to the conclusion that the effect of the reaction forces is equivalent to a resultant having the point of contact located outside the zone of contact.

Commentary by Dr. Valentin Fuster

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