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RESEARCH PAPERS

J. Tribol. 1987;109(3):381-386. doi:10.1115/1.3261450.

Tribological knowledge is widely scattered and inconvenient of access for practicing engineers. This results in suboptimal utilization of tribological knowledge in many engineering decisions. As a remedy, ASME has embarked on a project to create a comprehensive tribology database system accessible by desktop computer. Since tribological design methodology is as important as hard data in generating optimal engineering decisions, an artificial intelligence (AI) based expert system is projected to serve as a methodology guide for the database user. This paper describes the content and structure concept of the tribology database system “entry module,” which is to comprise this expert system.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(3):388-394. doi:10.1115/1.3261453.

For modelling the turbulent flow in a seal the Navier-Stokes equations in connection with a turbulence model (k-ε-model) are solved by a finite-difference method. A motion of the shaft around the centered position is assumed. After calculating the corresponding flow field and the pressure distribution, the rotordynamic coefficients of the seal can be determined. These coefficients are compared with results obtained by using the bulk flow theory of Childs [1] and with experimental results.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(3):395-401. doi:10.1115/1.3261454.

A method of analysis is described treating starvation in finite thrust bearing pads. A variable-size finite difference mesh is used to represent the two-dimensional temperature and pressure fields. A combination of Newton-Raphson iteration, direct iteration, and column matrix methods are used to solve for the start-of-film and minimum film thickness as well as the coupled two-dimensional energy and Reynolds equations. A parametric study describes the performance characteristics of the tapered land thrust bearing (flowrates, extent of fluid film, temperature rises, load capacity and torque) for different minimum film thicknesses and levels of starvation. This study considered variations in the geometrical parameters such as pad aspect ratio (L/R2 =1/3, 1/2, 2/3) and extent of the pad (β=27, 42, and 57 deg) with an optimum taper ratio (β1 /β=0.8). It is found that the effects of starvation are fairly small near the flooded condition but accelerate rapidly below the 50 percent starvation level. The start of the film (θ1 ) depends mostly on the level of starvation, and is essentially independent of the geometrical parameters, operating conditions or film thickness.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(3):405-414. doi:10.1115/1.3261459.

The goal set forth here is to continue the work started by Braun et al. [11] and present an integrated analysis of the behavior of the two row, 20 staggered pockets, hydrostatic cryogenic bearing used by the turbopumps of the space shuttle main engine (SSME). The variable properties Reynolds equation is fully coupled with the 2-D fluid film energy equation [η = η(P, T)]. The 3-D equations of the shaft and bushing model the boundary conditions of the fluid film energy equation. The effects of shaft eccentricity, angular velocity and inertia pressure drops at pocket edge are incorporated in the model. Their effects on the bearing fluid properties, load carrying capacity, mass flow, pressure, velocity and temperature form the ultimate object of this paper.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(3):417-426. doi:10.1115/1.3261462.

An analysis using finite difference method and small amplitude perturbation technique has been presented to evaluate the frequency dependent stiffness and damping coefficients of multirecess hydrostatic journal bearings including the effect of shaft rotation that results in hybrid operation. Recess volume fluid compressibility effect that results in the frequency dependent dynamic coefficients have been taken into account. Results for direct and cross stiffness coefficients, direct and cross damping coefficients are presented for a capillary compensated bearing with deep recesses. Frequency effects are shown in terms of dimensionless squeeze number (σ) for various recess volume parameters (γ) for different eccentricity ratios (ε0 ) and dimensionless speed parameter (Λ). It has been found that the dynamic coefficients are very drastically altered within a very useful range of frequency parameter (σ) and recess volume parameter (γ) resulting in increased direct stiffness coefficient and a substantially decreased direct damping coefficients. Journal speed parameter (Λ) results in substantial magnitude of cross stiffness and cross damping parameters. However, the cross damping coefficient is usually small in comparison to direct damping coefficient.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(3):427-431. doi:10.1115/1.3261463.

Lubricant fluxes and lubrication forces have been calculated for statistical realizations of a piecewise planar slider running over a smooth base with an incompressible lubricant. The statistics of the response are not simply related to those of the roughness. The standard deviation of the force increases approximately as the fourth power of the roughness deviation for probable contact situations.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(3):432-436. doi:10.1115/1.3261466.

A set of computational results of the point contact EHL problem under heavily loaded condition is presented in this paper, and the numerical scheme for the inverse solution of the Reynolds Equation is developed. By using a deformation matrix to calculate the local elastic deformation and its inverse matrix to modify the pressure distribution, both the convergence and the stability of the numerical method are satisfactory. The examples of calculation adopted have been extended from Hamrock and Dowson’s cases into the operating region of heavy load. The results obtained have shown the effects of speed, load, and choice of materials on the film thickness, and have proved Hamrock and Dowson’s formulae for estimating film thicknesses to be accurate under heavily loaded condition.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(3):437-443. doi:10.1115/1.3261467.

Detailed and accurate film thickness and pressure profiles have been calculated for point contacts at moderate and high loads, using a multigrid method. The influence of the compressibility of the lubricant and of the number of nodal points on the calculated minimum film thickness and maximum spike pressure have been examined. The required computing time is two orders of magnitude less, compared with the calculations using “classical” iterative methods.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(3):444-450. doi:10.1115/1.3261468.

A recently proposed fatigue life model for rolling bearings has been applied to the study of lifetime reduction under conditions conducive to microspalling. The presence of a spike in the EHD pressure distribution produces large shear stresses localized very close to the surface which may account for early failure. This paper describes a parametric study of the effect of such spikes. Accurate stress fields in the volume are calculated for simulated pressure spikes of different height, width and position relative to a Hertzian pressure distribution, as well as for different lubricant traction coefficients and film thicknesses. Despite the high stress concentrations in the surface layers, reductions in life predicted by the model are modest. Typically, the pressure spike may halve the life, with the implication that subsurface fatigue still dominates. In corroboration of this prediction, preliminary experimental work designed to reproduce microspalling conditions shows that microindents due to overrolling particles are a much more common form of surface damage than microspalling.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(3):452-461. doi:10.1115/1.3261471.

With hard metal balls of 1 mm diameter, small indentations are pressed into the inner ring of a 6205 deep groove ball bearing in the middle of the track. Running the bearing over a short period of time, under a suitable load causes the dents to change shape. The deformed dent shape is theoretically transposed to a particular 7205B angular contact ball bearing geometry in the middle of its running track, and the possibility of theoretical 3-D analysis is explored. As a first step, pressure distributions in the ball-raceway contacts are calculated with a theoretical 3-D model, with due consideration of the indentations. This is followed by computation of subsurface stresses by means of another mathematical 3-D model. Finally, fatigue life is analyzed by integrating a fatigue (survival probability) function over the endangered volume in the inner ring, using a general 3-D model featuring a fatigue stress limit. As observed in experiments with similar dents, reported in the literature, the analysis predicts considerable reduction in fatigue life. Also, the maxima of fatigue stress and fatigue function are predicted in the experimentally observed region of initiation of failure.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(3):462-469. doi:10.1115/1.3261472.

Contact stresses are one of the most important parameters in the analysis of a contact problem found for instance, in the design of gears and roller bearings. In this work the influence of geometrical surface imperfections on the normal pressure distribution in the contact is studied. A variational formulation based on the principle of complementary virtual work is used to solve the normal contact problem. The normal contact between two elastic half-spaces is considered, as the contact surface is small when compared to the dimensions of the contacting bodies. Results are presented to determine the influence of surface roughness, wavelength, and amplitude on the normal pressure distribution.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(3):471-477. doi:10.1115/1.3261475.

Conventional formula for calculating the running torque of tapered roller bearings often showed discrepancy from actual running torque, particularly under axial load. Therefore, an equation was formulated based on the knowledge of EHL rolling resistance and EHL oil film thickness. Careful examination of actual bearing running torque suggested the load dependency of EHL rolling resistance which previous theory did not include. Such load effect was confirmed by means of two disc machine and the equation was partly corrected. A new running torque formula of a tapered roller bearing under axial load was proposed and good agreement with actual bearing torque was confirmed.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(3):479-485. doi:10.1115/1.3261478.

Presented here is a method of solution for three-dimensional contact involving cylindrical bodies without using the half-space assumption. The solution involves the numerical iterative technique to determine the contact pressure in various cells discretizing a prescribed area on the cylindrical surface, sufficiently large to cover the contact. The iteration is carried out using the flexibility matrix which is the radial deflections at various cell centers due to a uniformly distributed unit load acting in a small cell on the cylindrical surfaces. Numerical results have been obtained for (1) the closely conformed contact of a journal and a shaft and (2) the low conformity contact between a roller and a raceway. The results consist of contact pressure distribution, as well as elastic approach between the two bodies in contact.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(3):487-495. doi:10.1115/1.3261481.

The study reported in this paper deals with the development of a generalized and efficient computer-based model for parametric evaluation of heat partition and transient temperatures in dry and lubricated layered concentrated contacts. The program utilizes finite differences with the alternating direction implicit method. It is capable of treating this general class of problems provided that the heat generation distribution and the layer properties and thicknesses are known.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(3):496-501. doi:10.1115/1.3261482.

The computer-based model described in Part I is utilized in this paper to develop dimensionless relationships for lubricated unlayered contacts and dry layered contacts. Because of the recent interest in tribological surface coating these relationships can be used to provide parametric evaluations of heat partition and temperature rise in the contacts under different coating parameters and operating conditions.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(3):503-510. doi:10.1115/1.3261486.

This paper discusses the results of a series of experiments to measure the pressure and temperature distribution between a pair of elastohydrodynamically lubricated (EHD) disks. Three lubricants have been studied: a synthetic paraffinic oil (XRM-109F), a polyphenyl ether (OS-124), and a traction fluid (Santotrac 50). Pressure and temperature readings throughout the contact are presented as three-dimensional plots. Simultaneous measurements of the EHD film thickness, and the tractive force between disks under slip conditions are also included. The variables are disk load and slip at a fixed surface speed. The highest temperatures were measured with the traction fluid which recorded a temperature rise of 50 degrees C through the contact.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(3):512-517. doi:10.1115/1.3261489.

A two dimensional numerical solution to the problem of thermal elastohydrodynamic lubrication of rolling/sliding contacts was obtained using a finite difference formulation. The technique involves the simultaneous solution of the thermal Reynolds’ equation, the elasticity equation, and the two dimensional energy equation. A pressure and temperature dependent viscosity for a synthetic paraffinic hydrocarbon lubricant (XRM-109F) was considered in the solution of the Reynolds’ and energy equations. The experimental pressure and surface temperature measurements obtained by Dow and Kannel [1] were used in evaluating the results of the numerical analysis for the cases of pure rolling and slip conditions.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(3):519-523. doi:10.1115/1.3261492.

An approximate method for mid-film temperature and traction force in elastohydrodynamic (EHD) lubrication of sliding contacts is obtained. The results are obtained at the point of maximum pressure where the pressure gradient is zero. The results indicate excellent correlation exists between this approximate method and results of a previous paper by the authors [1].

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(3):525-530. doi:10.1115/1.3261495.

The term “white-layer” refers to hard layers of material which resist etching and appear featureless and white under the microscope. This paper presents some of the publications on white layer formed during machining and deformation processes. The mechanisms of white layer generation are discussed and to the three traditional mechanisms (usually associated with wear) of rapid heating and cooling, surface reaction and plastic deformation are added pressure, strain rate and cooling rate.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(3):531-536. doi:10.1115/1.3261496.

Experimental investigation of the wear behavior of Magnesia Partially Stabilized Zirconia (Mg-PSZ) rubbed against itself showed that up to three orders of magnitude increase in the wear resistance can be achieved in a particular temperature range that depends on both the sliding speed and the ambient temperature. XRD analysis revealed that thermally induced phase transformation takes place on the frictional interface. Surface analysis show that wear rates at maximum wear resistance are controlled by the crack generation kinetics rather than by crack propagation kinetics. The plastic strain before fracture varies with temperature. The maximum plastic strain was observed at the temperature of maximum wear resistance. A phenomenological model is presented that provides an explanation for the wear temperature behavior of Mg-PSZ. The model is based on the following chain of events that takes place on the frictional interface: spatial overheating of the surface areas, phase transformation of the overheated areas, cooling, volume expansion, and development of a compressive stress field in the near surface volumes.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(3):537-543. doi:10.1115/1.3261499.

The mathematical framework for a sliding friction model for run-in and other tribological transitions is presented. The semiempirical model was developed to portray the commonly observed shapes, durations, and variations in kinetic friction coefficient versus sliding time curves. Terms in the model involve material properties and physical interface conditions such as transfer, debris accumulation, and surface roughness. The forms of individual terms are adjustable through the use of systemspecific scaling parameters in order to provide enough modeling flexibility to treat a variety of possible tribological conditions. Effects of such conditions as lubrication efficiency loss over time, and temperature build-up can be incorporated by modification of appropriate terms. Illustrative plots using the framework with several combined contributions are compared with experimental data from previous work. The basic framework of the model can be further developed to incorporate sub-models for specific sliding friction contributions and, in so doing, reduce the number of empirical system parameters required to model actual tribosystem behavior.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(3):545-551. doi:10.1115/1.3261502.

The friction between a real engineering surface composed of many micro-asperities and a surface covered with soft metal film is discussed. First, the frictional coefficient shows a remarkable load dependency, when a hard single protuberance with a small radius of curvature is slid on a soft metal surface. This load dependence originates from the ploughing effect induced by the cooperation of contact pressure and shearing resistance of the soft metal film deforming on the protuberance surface. Based on this result, the effect of the real engineering surface topography on the frictional properties is studied.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(3):552-559. doi:10.1115/1.3261503.

A study of the mechanics of ice friction was performed in which various theoretical models were compared to experimental results. Analytical models indicate that ice friction is affected by load, speed, ambient temperature, and thermal properties of the counterface material. All four parameters were varied and the results compared to analytical models with additional study using regression analysis. Conclusions indicate that under some conditions the friction of ice is subject to a thermal control mechanism.

Commentary by Dr. Valentin Fuster

DISCUSSIONS

Commentary by Dr. Valentin Fuster
Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(3):431. doi:10.1115/1.3261465.
FREE TO VIEW
Abstract
Commentary by Dr. Valentin Fuster

TECHNICAL BRIEFS

J. Tribol. 1987;109(3):562-566. doi:10.1115/1.3261506.

A theoretical and experimental study has been made for a film repture in hydrodynamic lubrication. A model is proposed on boundary conditions at the film rupture point. It contains a pressure correction term as a parameter, which simplifies that derived by Coyne and Elrod, and the so-called separation boundary condition. Some experiments have been conducted for a flow in a cylinder-plane geometry. It is found that numerical results using the present model agree reasonably well with the present and previous experimental data.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(3):566-567. doi:10.1115/1.3261507.
Abstract
Commentary by Dr. Valentin Fuster

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