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EDITORIAL

J. Tribol. 1988;110(1):1. doi:10.1115/1.3261568.
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Abstract
Commentary by Dr. Valentin Fuster

RESEARCH PAPERS

J. Tribol. 1988;110(1):2-6. doi:10.1115/1.3261570.

Computer based rolling bearing analysis programs exist which provide models for load and stress distribution, EHD conditions, heat balance and fatigue life prediction for rolling bearings and machine assemblies comprising such bearings. Fatigue life prediction is generally based on the ANSI standard. This two-part paper offers correction factors for refining the life prediction by modeling effects of material and operating variables. The corrections can be applied to the ANSI-predicted life of a complete bearing or to predicted lives of stressed volume elements in the bearing, with subsequent summation over the stressed volumes. Formulations are presented with a view to their incorporation into dynamic bearing analysis computer programs. This first part of the paper comprises the general model, basic life calculation and the nomenclature for both parts of the paper.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):7-12. doi:10.1115/1.3261579.

Computer based rolling bearing analysis programs exist which provide models for load and stress distribution, EHD conditions, heat balance and fatigue life prediction for rolling bearings and machine assemblies comprising such bearings. Fatigue life prediction is generally based on the ANSI standard. This two-part paper offers correction factors for refining the life prediction by modeling effects of material and operating variables. The corrections can be applied to the ANSI-predicted life of a complete bearing or to predicted lives of stressed volume elements in the bearing, with subsequent summation over the stressed volumes. Formulations are presented with a view to their incorporation into dynamic bearing analysis computer programs. This second part of the paper comprises the equations for the correction factors, the complete life correction, an Appendix showing logic flow for the calculations, and figures illustrating the behavior of the correction factors.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):13-18. doi:10.1115/1.3261553.

Computer simulations of the dynamics performance of roller bearings, as obtained by the computer code ADORE, are used to model the infuence of race out-of-roundness, race taper and centrality of land on partly crowned rollers. Both oil and solid lubricated high-speed roller bearings are considered and the overall performance parameters, such as, bearing life, heat generation, cage interactions and race flange wear due to roller skew and tilt, are correlated to the magnitude of the various geometrical imperfections. Thus the strength of computer modeling approach to optimization of geometrical tolerances in cylindrical roller bearings is demonstrated.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):19-25. doi:10.1115/1.3261567.

Computer simulations of dynamic bearing performance, as obtained by the computer code ADORE, are used to develop correlations between key performance parameters and geometrical imperfections in both oil and solid lubricated ball bearings. The influence of race out-of-roundness, variation in groove curvatures and the variation in ball size, is modeled in terms of the effects on bearing life, heat generation and cage stability characteristics. The results demonstate the significance of the computer modeling approach to optimization of geometrical tolerances in practical bearing design.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):26-31. doi:10.1115/1.3261569.

Predictions of fatigue life from debris indented roller bearings are calculated by the methods described by Webster, Ioannides, and Sayles [9] and presented as a comparison to the actual fatigue history of the bearings. Research is also presented on the influence of debris type and geometry in relation to the formation of dents within nonconforming contacts. In particular, the concept of elastic conformity around entrained debris is studied in detail and it is shown that a critical debris aspect ratio may well exist which defines a boundary between debris that can damage the contact surfaces, and that which cannot.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):32-37. doi:10.1115/1.3261571.

In this paper a full numerical solution for the partial elastohydrodynamic lubrication in elliptical contacts is presented, and the procedure of computation is briefly described. The average Reynolds equation developed by Patir and Cheng, the elasticity equation, and the pressure-viscosity relationship are solved simultaneously. The asperity contacts are also taken into account by using the model contributed by Greenwood and Tripp. The distribution of surface irregularities is assumed to be Gaussian. The effects of various roughness parameters on the film thickness are investigated. Special attention is given to the surface pattern parameter and hydrodynamic roughness parameter.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):38-43. doi:10.1115/1.3261572.

It is known that the apparent viscosity of grease varies with service time and that grease starvation is important in EHL. However, these effects were often ignored in the research of EHL lubricated with grease. This paper deals with the thixotropy of grease and determines which value of rheological parameters can be used in numerical analysis. The starved boundary position and its cause are also discussed and observed experimentally. A steady starved boundary of grease was adopted in computing the film shape. Since in both calculation and experiment the thixororpy and grease starvation were considered, the numerical solution was quite consistent with the experimental data. This paper explains why even though the apparent viscosity of grease is always larger than that of its base oil, the film thickness of EHL lubricated with grease is equal to or lower than that with the oil alone.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):44-49. doi:10.1115/1.3261573.

The stresses, strains, and deformations produced by repeated, two-dimensional rolling-sliding contact are analyzed using a modified finite element model developed by Bhargava et al. [1]. Rolling and sliding are simulated by translating an appropriate set of normal and tangential surface tractions across an elastic-perfectly plastic half space. The study examines a peak-pressure-to-shear strength ratio of p o /k = 4.5 and normal to tangential force ratios of T /N = 0.20 and T /N = 0.17. The calculations describe the residual stresses, displacements and the continuing cyclic radial, shear and equivalent strains generated at various depths in the rim. The results are compared with previous calculations by Johnson and Jefferis [2] of rolling-sliding contact and with pure rolling. The present work predicts much higher deformations than previously calculated.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):50-56. doi:10.1115/1.3261574.

An improved DMT adhesion model in conjunction with an elastic-plastic contact model is used to study adhesion of contacting metallic rough surfaces. The effects of surface roughness and surface energy of adhesion on the pull-off force and on the significance of the adhesion force are investigated. It is shown that for clean surfaces the adhesion is quite large even for relatively rough surfaces. Adhesion is negligible only for contaminated rough surfaces or at very high external loading.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):57-63. doi:10.1115/1.3261575.

The friction force required to shear interface bonds of contacting metallic rough surfaces is calculated, taking into account the prestress condition of contacting asperities. The surfaces are modeled by a collection of spherical asperities with Gaussian height distribution. Previous analyses for adhesion force and contact load of such surfaces are used to obtain the static friction coefficient. It is shown that this coefficient is affected by material properties and surface topography, and that it actually depends on the external loading contrary to the classical law of friction.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):64-68. doi:10.1115/1.3261576.

This paper investigates the tribological properties of Ag film fabricated by ion enhanced deposition at atmospheric room temperature, whereby 150 keV Ar ion bombardment and Ag ion plating are performed simultaneously. In the absence of a lubricant, friction test apparatus employing ball-plate geometry is used. Also, properties such as film crystallinity, morphology and concentration profile are examined to study the high energy ion bombardment effect related to the film’s tribological properties. The activation of the mutual diffusion process which results from simultaneous ion bombardment during ion plating, as well as promotion of the crystallization in the ion bombarded film due to subsequent annealing, can greatly extend the mixing layer, and thus enhances the sliding life of the Ag film.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):69-72. doi:10.1115/1.3261577.

The stability of sliding has been studied, taking into account frictional heating effect and friction coefficient dependence upon the interface temperature and sliding velocity. The collective—thermal and mechanical—sliding instability has been found to exist; instability emergence conditions and dynamics (both in linear and nonlinear stages) have been determined. It is shown that both the threshold and the dynamics of thermofrictional instability differ qualitatively from the analogous characteristics of “stick-slip” phenomenon. Namely, the oscillational instability behavior due to the energy exchange between thermal and mechanical modes has been found to occur under certain initial conditions; the velocities range has been determined for which collective sliding instability may occur whereas the stick-slips would be not possible. The nonlinear analysis of instability evolution has been carried out for pairs with the negative thermal-frictional sliding characteristics, the final stage of sliding dynamics has been described. It is found that stable thermofrictional oscillations can occur on the nonlinear stage of sliding instability development; the oscillations frequency and amplitude have been determined. The possibility has been discussed of the experimental observation of new dynamical sliding phenomena at low temperatures.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):73-79. doi:10.1115/1.3261578.

Static and dynamic characteristics of load-on-type rolling-pad journal bearings with six pads have been studied theoretically and experimentally by considering the effects of both turbulence and viscous heat generation in the oil film. By concurrently solving a turbulent lubrication equation and an adiabatic energy equation, pressure and temperature distributions, load capacity, and friction force are obtained. Moreover, the linearlized spring and damping coefficients of oil film are calculated under the assumption of small displacements of the journal center. It is found that the load capacity and the friction force considerably increase due to turbulence, and the effects of heat generated under turbulent conditions are more pronounced than those under laminar conditions. Also clarified are that bearing characteristics are improved by preloading the lower-side two pads located before and after the lowest pad, and that the curvature radius of the pad insignificantly affects the characteristics. The experimental results on static characteristics agree well with the theoretical results.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):80-86. doi:10.1115/1.3261580.

The scuffing failure of a sliding contact is a complicated process, involving physics, chemistry, and mechanical science. No comprehensive theory has yet been formulated; the published work consists of studying each aspect independently. The objective of this paper is to present a thermoelastic instability theory of scuffing for elliptic contact between sliding surfaces. From this analylsis, the condition for which instability occurs is obtained. The approximate equation includes the effects of load, speed, coefficient of friction and material properties and can be used as an estimate to prevent scuffing failure. The results calculated from this equation are shown to provide satisfactory agreement with available experimental data.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):87-92. doi:10.1115/1.3261581.

Surface finishing of magnetic recording head materials, such as ferrites, by diamond grinding and lapping results in a residual compressive stress on the surface. Residual stresses alter the magnetic properties of the ferrite causing the recording head performance to deteriorate. Hence, they need to be minimized. This paper considers the role of two processing techniques—annealing and chemical lapping—in controlling residual stress in ferrites. The effect of these processing techniques on various mechanical and physical properties of finished ferrites and the mechanism of residual stress control are discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):93-99. doi:10.1115/1.3261582.

A new concept of a viscous pump is described and the pump performance is analyzed. A linear relation is found between the flow rate and discharge pressure. Optimization of the pump geometry for maximum flow rate is presented. The potential of the new pump is demonstrated by numerical examples for general and medical applications. Finally the effect of misalignment tolerances on pump performance is discussed. It is found that under the maximum possible misalignment flow rate is reduced by no more than 30 percent and discharge pressure by less than 54 percent.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):100-105. doi:10.1115/1.3261546.

A common attractive type active Magnetic Bearing (AMB) was designed and tested with three parallel feedback control loops. The feedback of the measured AMB journal displacement provides the bearing stiffness. Two special circuits, called Velocity Observer and Acceleration Observer, are formulated for estimating the AMB journal velocity and acceleration based on the same measured displacement without performing differentiation. The feedback control using the estimated velocity provides the AMB damping. The feedback control using the estimated acceleration creates a rotating force which cancels the imbalance force and other external disturbances. The rotating force control can be switched on or off in the designed speed range without causing rotor instability. This additional vibration suppression mechanism greatly enchances the versatility of AMB.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):106-111. doi:10.1115/1.3261547.

Microcontact models provide average values of the random interfacial load, area and pressure between rough contacting surfaces. They do not provide a measure of the variability about that average. Events of tribological importance, however, are likely to be dependent on extreme rather than average behavior conditions. In this paper Monte Carlo simulation is used to determine the 75th and 90th percentiles of three dimensionless random variables as a function of the dimensionless separation of two contacting rough surfaces. These values may be used to determine the corresponding percentiles under the Greenwood-Williamson microcontact model of the distributions of 1) real contact area fraction, 2) the radius of the microcontact area, 3) microcontact load, 4) the maximum microcontact pressure and 5) the asperity flash temperature under low speed sliding conditions. A numerical example illustrates the computations.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):112-118. doi:10.1115/1.3261548.

This paper clarified piston friction forces and conditions of lubrication in the high engine speed range through the improvement of piston friction measuring instruments. Measurements of piston friction forces of the two-ring package have been also done. It is found that the effect of the two-ring package on the reduction of friction forces is greater than expected by means of the oil starvation phenomenon.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):119-127. doi:10.1115/1.3261549.

The problem of identifying the dynamic coefficients relating to inertia, damping, and stiffness, in a single degree of freedom model of squeeze-film behavior, is discussed. It is demonstrated that two methods—referred to as “invariant imbedding” and “state variable filter”—are applicable, and that both yield a recursive scheme for sequentially estimating all three coefficients. The methods are compared through an analysis of simulated data, where the coefficient values are known a-priori. This is followed by a presentation of some typical results obtained from experimental tests, using both transient and forced response data. The comparisons indicate that the state variable filter method is much superior, with respect to both computational efficiency and accuracy.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):128-133. doi:10.1115/1.3261550.

An approximate solution technique for tilting pad journal bearings is presented. The method makes use of approximations to the axial pressure profile and the temperature profiles and includes heat transfer between the film and its boundaries. A second order profile is assumed to represent the temperature distribution across the film. The classical Reynolds equation is applied, using the viscosity based on the cross-film average of temperature. The transfer of heat in the pads is modeled as purely radial conduction, and the journal surface temperature is given by a circumferential average of the film temperatures. Results of calculations are presented for a sample case representative of an industrial compressor or turbine bearing and are compared to isothermal results. The dynamic characteristics in this case are found to vary by 10 to 35 percent for a change in lubricant inlet temperature of 28°C. Temperature distributions in the lubricant and on the surfaces of the pads are given.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):134-138. doi:10.1115/1.3261551.

The flexible foil slider bearing is an alternate approach to the design of low clearance slider bearings used in direct access storage devices. The flexible foil, or membrane design will be presented as a slider bearing with high film stiffness that can achieve competitive flying heights. The elastohydrodynamics of the membrane slider design will be examined. A steady-state analysis will examine the effects on membrane shape of various physical parameters such as membrane stiffness, and membrane surface backing pressure. A transient analysis will assess the fluid film stiffness of the membrane slider. The membrane slider design has impressive compatibility between the slider and the disk surface for single impulse disturbances as well as continuous or ripple disturbances.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):139-143. doi:10.1115/1.3261552.

The purpose of the present work is to study theoretically the conical whirl instability of unloaded self-acting porous gas journal bearings considering the tangential velocity slip at the bearing-film interface. The hydrodynamic pressure developed in the bearing clearance due to angular displacements of the journal at the midplane of the bearing is obtained by the simultaneous solution of equation of continuity in the porous medium and the modified Reynolds equations, satisfying the appropriate boundary conditions. With the help of the dynamic tilting stiffness and damping coefficients, stability characteristic is obtained. The effects of various parameters on conical stability parameter have been investigated for a nongyroscopic system.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):144-147. doi:10.1115/1.3261554.

The characteristics of the externally pressurized bearings under high Knudsen number conditions were investigated experimentally by the use of surface restriction bearings in a medium vacuum on the order of 0.1 kPa (10−3 atm.). The experimental results agreed well with the numerical results calculated from a generalized lubrication equation based on the Boltzmann equation. Therefore, it would appear that this generalized lubrication equation is valid even when flows are categorized into transition flow or free molecular flow.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):148-155. doi:10.1115/1.3261555.

This paper outlines an optical method utilizing laser interferometry for measuring the static and dynamic characteristics of self-acting gas-lubricated slider bearings. The method achieves a high accuracy of down to 1–2 nanometers and a wide frequency range of more than 100 kHz by suppressing the adverse effect of undesirable interference patterns through the adoption of optics which permit the laser beam to be focused into a fairly small cavity. Experimental results derived from this method in an ambient helium atmosphere confirm that the modified Reynolds equation can be applied to the prediction of slider flying characteristics within an experimental range of spacing down to 0.025 μm and a corresponding Knudsen number as high as 8. Furthermore, experiments designed to measure slider transient motion as it travels across rectangular grooves or bumps indicate that the measurement system achieves a higher response performance than do conventional systems.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):156-161. doi:10.1115/1.3261556.

The dynamic characteristics of a hydrostatic thrust air bearing with an actively controlled orifice restrictor are investigated theoretically and experimentally. Theoretical results show that the dynamic stiffness and damping coefficients can be increased simultaneously when the restrictor area is controlled with appropriate phase-lag to the change of air film thickness, that is, plate vibrations. Consequently stability of the bearing can be improved. Experimental results show fairly good agreement with theoretical ones.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):162-166. doi:10.1115/1.3261557.

A modified form of the author’s frequency-domain estimation algorithm has been applied to estimate the inertia and damping coefficients of an uncavitated squeeze-film bearing. These estimates are obtained in situ from measurements on a specially designed rig. The experiments provided further evidence that oil-film inertia coefficients may be significant and showed that theoretical predictions are inadequate.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):167-173. doi:10.1115/1.3261558.

The clearances in highly deformed contacts are formed in a narrow region on the upstream side, are convected through the contact with little change and emerge downstream where a restriction forms in a narrow exit zone. The paper describes how this behavior simplifies the lubrication analysis. Expressions for the minimum film thickness under point contacts are obtained for the elastic isoviscous regime.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):174-179. doi:10.1115/1.3261559.

This paper compares the LEG and pressurized controlled flow lubricant supply methods and evaluates their influence on the babbitt temperature and bearing power loss performance of a tilting pad, equalizing thrust bearing. The paper also presents new experimental temperature data from bidirectional testing of a unidirectional LEG bearing. The experimental data presented is from a 267 mm (10.5 in.) O.D. bearing, operating at shaft speeds up to 13000 rpm with applied loads that produced mean unit pressures of up to 3.45 MPa (500 psi). Conclusions are drawn based upon these test data.

Commentary by Dr. Valentin Fuster
J. Tribol. 1988;110(1):181-187. doi:10.1115/1.3261562.

This investigation deals with the stability characteristics of oil journal bearings, including the effect of elastic distortions in the bearing liner. Graphical results are presented for (1) steady-state load, (2) stiffness and damping coefficients, and (3) the stability. These results are given for various slenderness ratios, eccentricity ratios, and elasticity parameters. The lubricant is first assumed to be isoviscous. The analysis is then extended to the case of a pressure-dependent viscosity. It has been found that stability decreases with increase of the elasticity parameter of the bearing liner for heavily loaded bearings.

Commentary by Dr. Valentin Fuster

DISCUSSIONS

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