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RESEARCH PAPERS

J. Tribol. 1987;109(2):207-211. doi:10.1115/1.3261335.

A symposium was recently held at the National Science Foundation in which a small group of internationally recognized tribologists from academia, federal laboratories, and private industry was invited to discuss and recommend future directions for basic research in tribology. This symposium was followed by a session at the ASME/ASLE Tribology Conference (October 20–22, 1986, Pittsburgh, PA) where representatives from private industry were invited to assess the experts’ recommendations and express the research needs of their respective industries. Among the many research topics that were discussed, four specific areas were emphasized as being in need of immediate attention. They were: 1. predictive models for friction, wear and failure; 2. microscopic and chemical aspects of lubrication; 3. mechanisms and prevention of wear at a microscopic level; and, 4. materials and lubricants for high temperature applications. It was noted that many of these areas are extremely complex and could benefit by interdisciplinary teams collaborating in universities, federal laboratories, and the private sector. The participants expressed the view that there is a great need for the transfer of basic research results to the users in industry.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):215-222. doi:10.1115/1.3261341.

A new plate impact configuration is described for subjecting lubricants to simple shearing motion under uniform hydrostatic pressure. Thin layers (∼50 μm thick) of the lubricant are confined between two hard metallic plates which are subjected to impact by a parallel plate that is inclined relative to the direction of approach in order to induce both pressure and shear loading. Stress waves in the hard plates are monitored by laser interferometry; all measurements are made before unloading waves arrive from the periphery. For approximately one microsecond the compressed lubricant is subjected to a simple shearing motion and a continuous record of the shear stress and shear rate is obtained. Results presented for the lubricant 5P4E at pressures of 1-5 GPa and shear rates of approximately 9 × 105 s−1 show good agreement with available data on the dependence of the limiting shear stress on the confining pressure at lower pressures and lower shearing rates.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):223-231. doi:10.1115/1.3261342.

Lubricated and dry experiments on titanium and steel surfaces with and without TiN sputtered coatings of various thicknesses have been conducted. The significance of the layer thickness, interfacial “friction”, magnitudes of normal and tangential surface tractions, and the mechanical properties of the layer and of the substrate (e.g., elastic modulus and hardness) are critically examined. The conditions under which the deformation mode at the solid-solid contacts is elastic or plastic are addressed in light of the experimental evidence and a finite element analysis. It is shown that surfaces with very low friction, especially for unlubricated sliding, and practically zero wear rates can be obtained in both lubricated and dry sliding by coating the surfaces with sufficiently thick TiN layers. Removal of the protective TiN layer resulted in plowing, severe damage, and delamination.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):232-237. doi:10.1115/1.3261343.

This paper studies the rapid simple shearing flow of dry cohesionless metal powders contained between parallel rotating plates. In this study, an annular shear cell test apparatus was used; the dry metal powders are rapidly sheared by rotating one of the shear surfaces while the other shear surface remains fixed. Such a flow geometry is of interest to tribologists working in the area of dry or powder lubrication. The shear stress and normal stress on the stationary surface are measured as a function of the following parameters: shear surface boundary material and roughness, the shear-cell gap thickness, the shear-rate and the fractional solids content. Both the fractional solids content and the gap thickness are kept at prescribed values during stress measurements. In this experiment the metal powder tested is different from the shear transmission surface material; the effect on the measured normal and shear stress data are reported. The results show the dependence of the normal stress and the shear stress on the shear-rate, particle density and particle diameter. Likewise, a significant stress dependence on both the fractional solids content and the shear-cell gap thickness was observed.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):238-242. doi:10.1115/1.3261345.

The wear rate of cast iron and the corrosion potential (Ecorr) and current (Icorr) were measured in sulphuric acid in order to study the mechanism of corrosive wear of cast iron. The corrosive wear rate of cast iron was found to reach a maximum at around 35 wt. percent sulphuric acid concentration and increase with load and acid temperature. The increment of corrosion current, i.e., the dissolution rate of Fe ion, due to friction (ΔIcorr) was found to correlate with the corrosive wear of cast iron. It has been concluded that the combined action of the changed properties and profile of corroded rubbing surfaces and the mechanical destruction due to wear causes the drastic corrosive wear, which is much greater than the wear seen with either one alone.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):243-251. doi:10.1115/1.3261346.

A “generalized” thermal analysis is described to estimate the flash temperature during sliding when both surfaces are of more or less equal roughness or one surface is substantially smoother than the other. High- and low-speed cases are considered. The basic model includes surface-topography statistics, frictional conditions, and mechanical and thermal parameters. Temperature history during the life of an individual asperity contact is calculated, from which average temperatures of an asperity contact are calculated. Thermal interaction of neighboring asperity contacts is considered. Then, an analysis is presented to show how individual asperity temperatures should be averaged. Temperature variations perpendicular to the sliding surfaces are also analyzed. Throughout the analysis, closed-form equations are developed, which can be conveniently used in the design of any sliding interface.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):252-256. doi:10.1115/1.3261347.

An analytical model has been used to predict the interface temperature of a typical magnetic head-tape contact and of isolated (exposed) magnetic particles in contact with the head. Average and maximum interface temperatures for the assumed head-tape interface are about 7° and 10° C, respectively. If the exposed magnetic particles contact the head surface, the average and maximum temperture rises could be about 600° and 900° C, respectively. The duration of an asperity contact is about 2 to 4 μs and the thermal gradients perpendicular to the sliding surface are very large (a temperature drop of 90 percent in a depth of less than the radius of an asperity contact or a few micrometers). The predicted temperatures are compared with the temperatures previously measured using an infrared radiometric technique.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):257-263. doi:10.1115/1.3261348.

An elastic-plastic asperity model for analyzing the contact of rough surfaces is presented. The model is based on volume conservation of an asperity control volume during plastic deformation. Numerical results obtained from this model are compared with other existing models that are either purely elastic or purely plastic. It is shown that these models are limiting cases of the more general elastic-plastic model presented here. Some of the results obtained deviate appreciably from previous analyses which do not consider asperity volume conservation.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):264-270. doi:10.1115/1.3261349.

An easily programmed method is proposed for translating the rms height (Rq ) and rms slope (Δq) determined using a profile measuring instrument, into more readily interpreted measures of functional severity such as the density of plastic contacts or the mean real contact pressure. The method involves estimation from the ratio Rq /Δq, of the exponent k of an assumed power function relation between the profile spectrum and the spatial frequency. Having estimated k, the mean square curvature is computed analytically and used together with Rq and Δq to determine the three input variables needed for the Greenwood-Williamson (GW) microcontact model. The GW model is then used to compute, as a function of the separation of two rough surfaces, the contact density, the plastic contact density, the mean load per unit area and the mean load per unit of real contact area. The mean square curvature estimated in this manner is compared to the directly measured mean square curvature for 12 distinct surface types. The values compared quite favorably (within 25 percent) for three of the specimens which included a bearing ball and the ground inner ring rolling path of a cylindrical roller bearing. The discrepancies exceeded a factor of 3 for three other specimens. The microcontact model output computed using both measured and estimated mean square curvature values showed that some output variables, e.g., plastic contact density, are more discrepant than the estimated and measured curvature values. Other output variables of the microcontact model, in particular, the mean real pressure, attenuate the discrepancies. The mean real pressures computed using the calculated and measured curvatures, were within 30 percent for all but three specimens. The maximum discrepancy observed was 55 percent. The results are sufficiently encouraging and the methodology so easy to apply, to commend the practice of routinely supplementing profile measurement data with microcontact model output.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):271-275. doi:10.1115/1.3261350.

A thin gas film wide wedge bearing with rough translating surface is studied both numerically and analytically. The fluctuating inlet flow area causes a symmetric wave to be generated from the inlet boundary which travels at half the translating surface velocity, possesses a wavelength half that of the surface roughness, and which decays in amplitude in the direction of motion. Since the wave is out of phase with the surface roughness, it has essentially no influence on the load. The load generated varies monotonically with gas bearing number and at a finite bearing number is less than the load developed with the rough stationary surface bearing.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):276-282. doi:10.1115/1.3261351.

This paper presents a study into the gas lubrication capability of an ultra-thin 0.025 μm film (converted value for ambient air film). The experimental results obtained using subambient helium as the lubricating film are compared with the calculated results using the modified Reynolds equation considering flow slippage due to the molecular mean free path effects. This comparison confirms that the slip flow model holds true within the range of the present experiments, and that the modified Reynolds equation is applicable for designing the computer flying heads operating at such thin spacing. The reason for the excellent agreement is discussed considering the locality of rarefaction effects on the lubricating surfaces and the anisotropy of these effects between the film thickness and the slider width.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):283-289. doi:10.1115/1.3261352.

A finite element model of the thermohydrodynamic behavior of a cylindrical bore bearing is presented. The predicted behavior is compared with experimental evidence where possible and favorable correlation is obtained. The analysis is extended to include thermoelastic behavior in both the shaft and bush and it is demonstrated that this mechanism can influence bearing behavior considerably.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):290-294. doi:10.1115/1.3261353.

This paper presents the results of an experimental investigation intended to observe cavitation in squeeze-film bearing dampers representative of those commonly found in aircraft gas turbine engines. Two different squeeze-film damper geometries were tested with both high-speed motion pictures and stroboscopic video recordings acquired at speeds up to 20,000 r/min. The results presented are limited to 8000 r/min due to the increased clarity of the photos acquired at the lower speeds and the similarity of trends at the higher speeds. Comparisons are also made with analysis formulated to handle the dynamics of the film rupture for the “short” damper case. The test results confirmed several of the commonly held “short” bearing assumptions (i.e., predominant axial flow and the effect of supply pressure and eccentricity on the cavitation zone). However, the test results demonstrated that significant flow reversals and film rupture were experienced in the feed/drain grooves in contradiction to the assumed boundary conditions. While agreement between analysis and test is of the right order of magnitude in predicting the cavitation zone shape and circumferential extent, current analyses do not adequately account for the observed variations in the boundaries and change in shape of the cavitation zone.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):296-300. doi:10.1115/1.3261355.

We study the steady-state hydrodynamic lubrication of a solid-solid sliding bearing with spontaneous fusion or sublimation of the solid slider substance. Lubrication is sustained by the fluid film produced by fusion or sublimation. Our analysis extends the recent literature on liquid film lubrication of a melting solid slider to the interesting case of gaseous film lubrication of a sublimating solid slider. The results are in the form of analytical expressions showing the explicit influence of every parameter of the problem, together with conditions specifying the range of validity, and conditions guaranteeing that fusion or sublimation develop the necessary supply of lubricant. For substances like water and bismuth that contract upon melting, we extend the results to the interesting range of conditions dominated by the effect of pressure on the melting temperature.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):301-306. doi:10.1115/1.3261356.

A theoretical analysis has been carried out to determine the stability characteristics of externally pressurized porous oil journal bearings of finite length considering the tangential velocity slip at the bearing-film interface. The stability curves have been drawn for different slip parameters, eccentricity ratios, slenderness ratios, bearing speed parameters etc.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):307-314. doi:10.1115/1.3261357.

The dynamic characteristic problems of turbulent journal bearings are theoretically analyzed. The oil film forces considering the turbulent effects are analytically obtained under the short bearing assumption. The linearized stability analysis is developed for a perfectly rigid rotor supported horizontally in the two identical aligned oil film journal bearings and the whirl onset velocities for rotor-bearing systems are determined for various Reynolds numbers. In addition, the nonlinear equations of motion for rotor-bearing systems are solved by the improved Euler’s method and the journal center trajectories are examined for both cases with and without out-of-balance forces. The results obtained from the short bearing theory are compared with those from the finite bearing theory for the length-to-diameter ratio λ = 0.25 and 0.5. It is found that the turbulence has significant effects on the dynamic behavior of rotor-bearing systems and the short bearing theory has an advantage of reducing a computation time required for the calculation of journal center trajectories with an acceptable accuracy.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):315-319. doi:10.1115/1.3261358.

This paper describes some interesting preliminary results obtained with tool steel pins sliding against a rotating sapphire disk without lubrication. It is shown (i) that mild (oxidational) wear of the pin surface can be obtained with this combination of materials, (ii) that the “hot-spots” between the pin and the disk surfaces can be seen and photographed for size analysis, and (iii) that the temperature of the hot-spots can be estimated from the photographs. Some scanning electron micrographs were also taken of the pin surfaces at the end of the experiments, the objective being to complement the optical analyses, and thereby provide information about the number, size, and temperature of the hot-spots. The revelance of these estimates to the mechanisms involved in mild (oxidational) wear is also discussed.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):321-329. doi:10.1115/1.3261361.

Based on the theory by Blok and Jaeger, simple equations have been obtained for the theoretical evaluation of flash temperatures, i.e., the temperatures of circular or elliptical contact spots between two homogeneous materials, when friction heat and/or Joule heat is evolved at the interface. The parameters appearing in the equations have been expressed in terms of experimental data, the coefficient of friction, known materials properties, and the ellipticity and number of contact spots. The equations are especially simple in the limiting cases of very high and of very low speeds, and were indeed known in these limits for circular, albeit not for elliptical contact spots. As an example, the flash temperatures of plastic contact spots on account of friction heat have been computed for (i) an electrical brush material sliding on copper, and (ii) a carbon steel sliding on itself. In these examples the dependence of the flash temperature on the velocity of the contact spots relative to either or both of the two sides has been investigated, wherein the effect of sliding rate on the flash temperature via the strain rate dependence of local hardness has been taken into consideration,- it is believed for the first time in any theoretical investigation of flash temperatures. In the high-speed case, in agreement with intuitive expectation, the numerical examples show that under otherwise same conditions and same macroscopic velocity, minimum flash temperature is attained when the spot moves relative to both sides. Typically, that minimum does not occur when the contact spot speed is one half of the relative speed on each of the two sides, however. Moreover, and somewhat unexpectedly, the results show that the previously neglected strain rate dependence of the hardness has a considerable effect.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):330-337. doi:10.1115/1.3261362.

Large plastic deformation caused by friction for high purity copper was investigated using the finite element method with an updated Lagrangian formulation. The phenomenological background of this large plastic deformation was studied with a scanning electron microscope, and the nucleation of voids similar to those obtained for copper rolled to over 50 percent reduction was observed. Void nucleation was found to correlate with the agglomeration of over-saturated vacancies formed under high plastic strains. The computer-simulation analyzed such heavy deformation with an equivalent stress greater than the tensile strength and with an equivalent plastic strain of 0.44. Crack propagation was discussed by computing the J-integrals.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):338-342. doi:10.1115/1.3261363.
Abstract
Topics: Tribology , Machining
Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):343-350. doi:10.1115/1.3261364.

A new compression-twist type friction testing machine has been developed to simulate the frictional conditions prevailing in metal forming. Twin indenter tools with different sliding length were used and mineral oils were tested under frictional sliding between mild steel and a tool steel. New indexes of boundary lubricity, oil-film strength, and anti-galling ability are proposed to evaluate lubricant performance.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):351-355. doi:10.1115/1.3261365.

A technique has been developed for modeling solid films in rolling element bearings. For a given bearing geometry an effective viscosity can be calculated for a solid film as a function of the film shear modulus and ball-race friction coefficient. The calculated effective viscosity can subsequently be used as an input to a numerical model of cage motion and stability. Results from a sample calculation of effective viscosity and prediction of cage stability for a turbopump bearing are presented for films of molybdenum disulfide (MoS2 ) and polytetrafluoroethylene (PTFE).

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):356-362. doi:10.1115/1.3261366.

The elastohydrodynamic lubrication of an automotive piston was analyzed. The important effects of thermal and pressure distortions were demonstrated by analyzing two piston skirts which have different axial profiles but are otherwise identical. Results show that a significant reduction in friction can be realized.

Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):363-370. doi:10.1115/1.3261367.

Piezoviscous Rigid (PVR) film thicknesses, rolling and sliding traction forces formulas for the line and point contact case have been derived. They have been used to study the Rolling Element (R.E.)–cage pocket contact mechanism. As the R.E. gets closer to the cage web, the lubrication regime change gradually from the PVR regime to Piezoviscous Elastic (PVE or EHD) regime when steel cages are used. If plastic cages are used, the lubrication regime moves from the Isoviscous Elastic (IVE) regime to EHD regime. For the conditions simulated herein, the transition occurs at typical film thickness values of the order of 0.5 micrometers and substantially large loads can be calculated in the R.E.–steel cage contact. They can be of the order of 100 N at the transition. In many circumstances, the PVR regime prevails and the braking moment due to hydrodynamic traction in the contact can be easily calculated. A tight osculation between the R.E. and the cage pocket will increase significantly the load and braking moment on the R.E. Starvation effects are also estimated and cause a reduction of the load, but not of the braking moment.

Commentary by Dr. Valentin Fuster

DISCUSSIONS

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

TECHNICAL BRIEFS

J. Tribol. 1987;109(2):372-373. doi:10.1115/1.3261370.
Abstract
Topics: Fluids , Lubricants , Traction
Commentary by Dr. Valentin Fuster
Commentary by Dr. Valentin Fuster

BOOK REVIEWS

J. Tribol. 1987;109(2):376. doi:10.1115/1.3261372.
FREE TO VIEW
Abstract
Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):376-378. doi:10.1115/1.3261373.
FREE TO VIEW
Abstract
Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):378-379. doi:10.1115/1.3261374.
FREE TO VIEW
Abstract
Commentary by Dr. Valentin Fuster
J. Tribol. 1987;109(2):379-380. doi:10.1115/1.3261375.
FREE TO VIEW
Abstract
Topics: Wear
Commentary by Dr. Valentin Fuster

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