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RESEARCH PAPERS

J. Tribol. 1998;120(1):1-7. doi:10.1115/1.2834181.

Variations in viscosity within the film is known to have a pronounced effect on journal bearing performance. Lubricants of the same grade with the same viscosity at a particular temperature and ambient pressure can have different viscosity values at other temperatures and pressures. The effect of rate of viscosity variation due to the changes in temperature and pressure on the performance of an axial grooved journal bearing is the focus of this investigation. Roelands’ model on viscosity-temperature-pressure relationship of lubricants is adopted. Cavitation modelling with regard to thermal effects and mass conservation are carefully considered. A total THD model with heat transfer within the fluid film and heat dissipation to the ambient through the journal and bush, is utilized. For different rates of viscosity variation with respect to temperature and pressure, comparison of performance parameters and typical temperature distributions are presented.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(1):8-15. doi:10.1115/1.2834196.

An experimental apparatus was designed and tested to study the thermally induced seizure in bearing. The setup consists of a simple, unloaded journal bearing configuration which lends itself to useful physical interpretation without the complexities that are present in the control of flow rates and eccentricity in a loaded journal bearing in which the clearance would vary with time. The motor was fitted with a current limiter which stopped the motor when the torque exceeded a certain limit. Experiments revealed that with this particular system, the journal speed undergoes a significant reduction with time until the operation is halted by the current limiter, which signifies the occurrence of a seizure. The time of seizure is appreciably influenced by this behavior. A parallel theoretical analysis, which takes into account the speed variation with time, was developed. The analysis includes the derivation of the appropriate governing equations which involve the transient analysis of flow velocity, heat transfer, and thermomechanical expansion of the surfaces, together with the numerical solution. The results of the simulations compare favorably to those obtained experimentally both in trend and magnitude. Finally, general behavior of the system in terms of its time-to-seizure characteristic is illustrated through a series of parametric studies.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(1):16-20. doi:10.1115/1.2834180.

An alternative average Reynolds equation for use under conditions of large fractional contact area is proposed. The flow factors for this form of the equation are calculated for a variety of longitudinal surfaces and the results are shown to be relatively insensitive to the initial height distribution. Pressure and shear flow factors for the Christensen height distribution and a variety of Peklenik surface pattern parameters are also derived from the work of Patir and Cheng, Lo and Tripp. These are represented by semi-empirical equations over the full range of contact conditions. The implications of the results, with respect to the lubrication of metal forming processes, is discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(1):21-27. doi:10.1115/1.2834182.

A matrix of specially formulated greases composed of perfluorinated polyalkylether (PFPE) oil and telomers of polytetrafluoroethylene (PTFE) particulate thickener were tested. The PFPE greases were chosen for this evaluation because their oils have a very low vapor pressure and they will not volatilize in the disk drive. Base oil viscosity, PTFE particle size and percentage of oil content were varied. The rheological properties of complex viscosity, storage and loss moduli, and loss factor were measured. Percent oil was determined using thermal gravimetric analysis. The thickener’s PTFE particle sizes were measured and their shapes were imaged using scanning electron microscopy. Bearing low speed torque and bearing noise tests were performed to evaluate the effect of the grease on bearing performance. The head settle track misregistration (TMR) was measured on disk drives to measure the effect of bearing lubrication on the servo performance. Generally, greases with high base oil viscosity had the lowest complex viscosity. Greases with highest viscosity PFPE oils had the highest torque and lowest noise. Greases with large PTFE particles had high loss factors and exhibited high torque and noise levels. High PTFE thickener to PFPE oil ratio (thicker grease) causes the bearings to have fluctuations in torque and noise levels. Actuator bearings lubricated with greases having higher viscosity oil had lower head settle TMR.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(1):28-34. doi:10.1115/1.2834183.

This series of three papers presents an experimental and numerical analysis of the friction between a head slider and a thin-film rigid disk with circumferential surface texture, under various humidities. In Part I, we develop a model of a cylindrical contact of a slider rail with strong anisotropic asperities under dry conditions. This model predicts that the real contact pressure is primarily determined by the height distribution and geometry of asperities on the disk, whereas it is very weakly dependent on the surface curvature of the rail. The result of the latter calculation suggests that the frictional coefficient is insensitive to variation in the head contact angle due to head pitching motion.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(1):35-41. doi:10.1115/1.2834184.

This series of three papers presents an experimental and numerical analysis of friction in a thin-film rigid disk with a circumferential surface texture, under various humidities. In Part II, we develop a theoretical formulation of the meniscus force of elliptical contact at isolated liquid bridges. This model predicts that the mean real pressure due to the meniscus force apparently decreases the net real contact pressure pr and increases the frictional coefficient. This effect becomes significant when the relative surface energy of the liquid 2γ/σ is comparable to pr , where σ is the standard deviation of asperity height distribution. When the disk is covered with lubricant and the liquid bridges are connected with each other, the solid noncontacting asperities play an important role in the formation of the meniscus force, and the relative lubricant thickness t/σ is the critical factor determining the friction.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(1):42-50. doi:10.1115/1.2834185.

The time dependence of static friction is an important aspect of tribological studies whereby the static friction increases with an increase in the time of stationary contact (rest static friction or rest stiction). This effect is commonly observed in computer disk drives especially at high humidities, in which the static friction increases rapidly with an increase in rest time between some head slider and the disk surfaces. For the first time, a comprehensive kinetic meniscus model is proposed to explain this phenomenon, both for a single asperity and multiple asperity contacts at a liquid mediated interface. It is found that the static friction increases up to a certain equilibrium time with a power law relationship after which it remains constant. The equilibrium time is dependent on the liquid film thickness, the liquid viscosity and the contact geometry. The model developed is applied to a rough textured disk, a polished disk and a super smooth disk. To study the effect of the scan size on the meniscus force, the static friction is calculated by using roughness parameters obtained from measurements at different scan sizes.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(1):54-59. doi:10.1115/1.2834190.

Acoustic emission is used to study the contact behavior of subambient pressure tri-pad sliders during start/stop and constant speed operation. The contact force at the slider/disk interface is determined as a function of velocity and the dependence of contact force on flying height is investigated. The results indicate that contact forces for typical subambient pressure tri-pad sliders are on the order of a few mN.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(1):60-68. doi:10.1115/1.2834191.

This paper presents a series of trials of direct detection of the transition process of a squeeze film, from the contacting state into the floating one. The material was initially placed on the surface of a vibrator constituting a squeeze air film. Three electrical trials were performed to certify whether the material could be elevated after the vibrator was excited. When this is done, clearance is expected between the surfaces of the test material and the vibrator. This may be regarded as the equivalent of a change in electrical resistance from almost zero to infinity, with a kind of parallel-plate capacitor being formed. First, detection through DC current was carried out. The experimental result, however, reveals that the resistance never does become infinite, although the equivalent contacting area calculated from this value is very small. Second, detection through DC voltage was performed by terminating the clearance with parallel resistance. The experimental results verify the appearance of the floating state. Third, the average clearance could be detected by regarding it as a parallel-plate type capacitor. Here a Langevin type piezoelectric transducer was used as the vibrator. Through these experimental results an important fact was observed: the waveform of the voltage applied to the transducer is closely related to the movement of the clearance in the time domain.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(1):69-74. doi:10.1115/1.2834192.

The Eulerian-Lagrangian approach is employed to simulate droplet trajectories due to the large-velocity gradient between two solid surfaces: a stationery block (slider) and a rotating plane (disk). Sudden expansion after the extremely small spacing will trap the particles in the open spaces. The fluid phase flowfield is obtained by solving Navier-Stokes equations with slip boundary correction in the Eulerian approach, and the droplet trajectories are calculated by integrating equations of motion with slip correction in the Lagrangian approach. Because of the extremely small spacing and the droplet size, Brownian motion effectively increases the probability of slider-head collisions, especially for extremely small particles. This study demonstrates that the effect due to particle size is the dominant factor in determining the probability of particle-slider collision, especially for particle sizes comparable with the air mean free path and the flowfield immediately adjacent to the solid surfaces. The results also show that lowering the flying height of the slider and increasing the disk velocity attracts the particles toward the gap between the disk and the slider.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(1):75-81. doi:10.1115/1.2834193.

Cutting forces were measured during turning AISI 4340 steel with uncoated, two-layer coated (TiC/Al2 O3 ), and three-layer coated (TiC/Al2 O3 /TiN) cemented WC-Co inserts possessing chip-breaker grooves in order to investigate the progression of tool wear. The variations of cutting force signals are interpreted in light of predominant wear mechanisms. A characteristic feature of wearing coated tools is shown to be the rapid increase of the radial and axial cutting forces when the tool life is reached. This behavior is associated with both the increase of the real contact area on the nose and the flank face due to coating delamination and the relatively less severe wear on the rake face. Normalizing the cutting forces by those of unworn tools yields a single curve for each cutting force. The instant at which either the radial or the axial force deviates from these curves determines the wear life of coated tools. The good agreement between results for the wear life of coated tools obtained from cutting force and nose wear measurements suggests that the wear life of multilayer ceramic coated tools can be accurately determined by monitoring the cutting force response.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(1):82-88. doi:10.1115/1.2834194.

An elastic-plastic microcontact model, that takes into account the directional nature of surface roughness, is proposed for elliptic contact spots between anisotropic rough surfaces. In addition, the plasticity index was modified to suit more general geometric contact shapes. This contact model, which expands the usefulness of the CEB model, is also utilized to determine the effect of effective radius ratio (γ) on microcontact behavior and to compare the results of this model and other models under different surface topographies. The results show that the elliptic contact model and circular contact model deviate considerably in regard to the separation (h), total real contact area (At ), plastic area (Ap ) and plasticity index (Ψ). The present model can be simplified to become other stochastic models.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(1):89-96. doi:10.1115/1.2834195.

The problem of sliding surface tractions on a porous biphasic layer with viscous interstitial fluid is solved, which can serve as the basis for solving problems of elastohydrodynamic lubrication of deformable porous layers bonded to a rigid substrate. A general small strain solution is derived for a layer of arbitrary thickness, using Fourier transform methods. The applied tractions, consisting of two normal components and two shear components, have arbitrary distributions and are assumed to slide at a constant speed along the layer surface. It is found that the solution to this problem is dependent on five nondimensional parameters; one of these, Rh , is the ratio of the sliding speed to the characteristic speed of fluid flow within the porous layer; another, τf , controls the transition between Darcy-like and Navier-Stokes flow. Results are presented for various values of these parameters, particularly in relation to the flux of fluid relative to the solid phase along the tangential and normal directions to the layer surface. Discussion of these results and their potential effect on the analysis of elastohydrodynamic lubrication of deformable porous bearings is provided.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(1):97-102. doi:10.1115/1.2834197.

In the present investigation a plate-impact pressure shear loading device is employed to study frictional characteristics of sliding interfaces subjected to step changes in normal pressure. The present experimental configuration represents a significant improvement over the conventional tribology experiments by allowing the control of interfacial tractions through the use of pressure-shear loading waves instead of manipulating actuator motion. Moreover, the experimental configuration allows critical frictional parameters such as the applied normal pressure, the interfacial slip resistance, and the interfacial slip velocity to be interpreted by using the framework of one-dimensional plane wave analysis. The experimental results, deduced from the response to step changes imposed on the normal pressure at the frictional interface, reinforce the importance of including frictional memory in the development of the rate-dependent state variable friction models. The scope of the above experiments include technologically important combinations of workpiece materials such as 4340VAR structural steel and a commercially available titanium alloy (Ti-6Al-4V), and tool materials such as tungsten based tool cermets (WC-Co alloys).

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(1):103-111. doi:10.1115/1.2834172.

The paper presents a lubrication analysis for the tooth contact of a proposed wormgear transmission. The information needed for the lubrication analysis has been mostly obtained from a previously published wormgear analysis. The information includes the geometry of the clearance between the meshing surfaces, the velocity of the worm surface relative to the gear surface, and the normal force acting on a gear tooth as it moves through the meshing zone. The lubrication analysis is carried out after a design of the oil supply configuration is made, that consists of a single transverse oil recess and a capillary tube flow restrictor. Under the predetermined normal force, the lubrication analysis is aimed at obtaining the needed supply pressure to separate the meshing surfaces by a minimum oil film thickness, which is prescribed to insure the establishment of fluid film lubrication at the contact. The lubrication analysis considers (1) the hybrid lubrication effect (combined hydrostatic action and hydrodynamic wedge and squeeze actions), (2) the temperature rise in the oil film flow and the restrictor flow, and (3) the pressure and temperature dependence of oil properties. Part I describes the formulation of the oil film flow problem (in discrete form) and the restrictor flow problem (in analytical form). The two problems are coupled through the conditions of flow continuity and energy balance in the oil recess.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(1):112-118. doi:10.1115/1.2834173.

Part 2 begins by describing the numerical solution procedures of the hybrid lubrication problem. Results of the computation are then presented that include the detailed pressure and temperature distribution in the oil film, the required supply pressure for maintaining the prescribed minimum oil film thickness, the fluid friction acting on the worm coil surface, the mass flow rate of supply oil, and the power loss associated with the restrictor flow. The feasibility of the hydrostatically lubricated wormgear transmission is discussed in light of these results.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(1):119-125. doi:10.1115/1.2834174.

An inlet zone analysis of TEHD lubrication of heavily loaded line contacts has been done using a computationally efficient and accurate numerical method based on Lobatto quadrature developed by Elrod and Brewe (1986). The results under extremely heavy conditions of dimensionless load W = 5.2*10−4 (pH = 2.0 GPa) and dimensionless rolling velocity U = 2.0*10−10 (50 m/s) are presented. Significant reduction in thermal reduction factor (film thickness) at high rolling speeds relative to isothermal conditions have been observed. The results of the present work have been compared with the results of Wilson and Sheu (1983) and Hsu and Lee (1994). A correction formula of the thermal reduction factor for the minimum film thickness has been derived for a range of thermal loading parameters, loads, and slip ratios.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(1):126-133. doi:10.1115/1.2834175.

This paper presents a numerical study of the effects of inlet supply starvation on film thickness in EHL point contacts. Generally this problem is treated using the position of the inlet meniscus as the governing parameter; however, it is difficult to measure this in real applications. Thus, in this paper an alternative approach is adopted whereby the amount of oil present on the surfaces is used to define the degree of starvation. It is this property which determines both meniscus position and film thickness reduction. The effect of subsequent overrollings on film thickness decay can also be evaluated. In the simplest case a constant lubricant inlet film thickness in the Y direction is assumed and the film thickness distribution is computed as a function of the oil available. This yields an equation predicting the film thickness reduction, with respect to the fully flooded value, from the amount of lubricant initially available on the surface, as a function of the number of overrollings n. However, the constant inlet film thickness does not give a realistic description of starvation for all conditions. Some experimental studies show that the combination of side flow and replenishment action can generate large differences in local oil supply and that the side reservoirs play an important role in this replenishment mechanism. Thus the contact centre can be fully starved whilst the contact sides remain well lubricated. In these cases, a complete analysis with a realistic inlet distribution has been carried out and the numerical results agree well with experimental findings.

Commentary by Dr. Valentin Fuster
J. Tribol. 1998;120(1):134-139. doi:10.1115/1.2834176.

The virtual contact loading method is further developed in this paper tostudy the multi-body contact phenomena within solid and hollow roller bearings. The distributive virtual contact loads are used to simulate the contact constraint forces on each contact surface. The deepest penetration criterion is proposed to select and determine the contact nodes in an orderly way, so as to ensure the local iteration process converged and to increase the computational efficiency. The load distributions within the solid and hollow roller bearings have been studied in more detail and some reasonable results are obtained.

Commentary by Dr. Valentin Fuster

DISCUSSIONS

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

TECHNICAL BRIEFS

J. Tribol. 1998;120(1):140-142. doi:10.1115/1.2834177.
Abstract
Commentary by Dr. Valentin Fuster

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