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RESEARCH PAPERS

J. Tribol. 1986;108(4):497-501. doi:10.1115/1.3261252.

Experts estimate that in 1978 over four quadrillion Btu of energy were lost in the United States because of simple friction and wear. The Energy Conversion and Utilization Technology Program (ECUT) in the U.S. Department of Energy commissioned six surveys from various experts in the field of tribology to learn about the causes of these energy losses and how to reduce them. The surveys included: 1) identification of typical tribology energy sinks in industry, 2) reduction of tribological losses in utilities and transportation, 3) tribological research needed for advanced heat engines, 4) energy conservation potential of new surface modifications, 5) identification of current tribology work sponsored by the government, and 6) an assessment of industrial research needs. A summary of the major findings of each survey is included in this paper.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(4):502-513. doi:10.1115/1.3261253.

Analytical and experimental studies were conducted on oxidized and nonoxidized pure aluminum, OFHC copper, and electroplated chromium to investigate the role of surface oxide layers in boundary lubrication. The effects of the thickness of the oxide layers, the elastic moduli of the oxide and the metal, and the normal surface traction have been addressed. In addition, several possible failure mechanisms of both thin and thick oxide films have been proposed. The experimental results have shown that low coefficients of friction, about 0.1 or less, and especially low wear can be obtained in boundary-lubricated sliding if the metal surfaces are protected from plastic deformation by sufficiently thick oxide layers. Scanning electron microscopy has shown that when the oxide layers are not ruptured, the wear of the surfaces is negligibly small. In this case, the oxide-oxide contacts deform primarily elastically and the predominant friction mechanism is the shear of the lubricant film. Based on this evidence, a theoretical model for friction was proposed and the agreement between theoretical and experimental coefficients of friction was reasonably good. Disruption of the oxide layers during sliding, however, was found to result in plastic deformation and plowing of the surfaces.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(4):514-521. doi:10.1115/1.3261254.

An investigation was conducted to determine the friction and wear characteristics of hot-pressed silicon nitride. Sliding produced wear debris and a damaged surface. The physical and crystallographic morphology of surfaces was compared with that of diamond ground surfaces. Wear tests were done with pin-on-disk apparatus at a load of 10N with various sliding speeds to 780 mm/s, and in four different environments which included in dry nitrogen, in air at humidities of 50 percent RH and 90 percent RH, and in distilled water. The results of the wear experiments indicated that residual α-silicon nitride was transformed into β-silicon nitride. Adsorbed water appeared to enhance plastic flow of the surface and reduced both the wear rate and friction. A second investigation was conducted to correlate the coefficient of friction with the fracture toughness of silicon nitride, silicon carbide, aluminum oxide and zirconium oxide. The friction experiments were done in reciprocating sliding, using spherical diamonds. Two tip radii, 0.005 mm and 0.1 mm were used over a range of load of 0.1 to 3N and a speed of 0.17 mm/s. The coefficient of friction was found to be inversely correlated with fracture toughness of all four ceramics in several conditions. Frictional anisotropy was also observed in the hot-pressed silicon nitride.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(4):522-526. doi:10.1115/1.3261255.

Wear tests in dry rolling contact were carried out at room temperature on five ceramic materials, such as silicon nitride, silicon carbide, cermet, titania, and alumina. The results showed that wear rate of silicon nitride was smaller than any of the other ceramic materials and bearing steel. Observations of worn surface and wear debris revealed that ceramic materials have two types of wear, one related to real contact area and another related to Hertzian contact area. It was also found that brittle fracture dominates the wear process of ceramic materials in dry rolling contact. Based on the experimental results, wear rate of ceramic materials was expressed with a new nondimensional parameter.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(4):527-533. doi:10.1115/1.3261256.

The stresses and strains in a semi-infinite homogeneous body indented by a rigid sphere is a classical problem first investigated by Hertz. The special case of a thin elastic layer on a rigid substrate indented by a rigid sphere is considered here using a finite element approach. This is a problem of interest in the hardness testing of thin sheet material and in engineering design when the applied load is axially directed. When the applied load has a frictional (tangential) component, the results are of particular interest in tribology. The study is divided into two parts. The first considers purely axial loading while the second part, presented in a companion paper, considers a combination of axial and tangential loading. Results in both cases are compared with the well known Hertz solution.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(4):534-539. doi:10.1115/1.3261257.

Deformation and elastic stress characteristics are considered for a thin elastic film on a rigid substrate when loaded by a rigid cylindrical indentor in conjunction with different amounts of friction. Friction tends to move the point of maximum shear stress from below the loaded surface into this surface, particularly when the elastic film is thin relative to the width of contact. Increased friction also increases the magnitude of the horizontal tensile stress (σx ) in the free top surface and moves it closer to the trailing edge of the indentor. Friction has a rather negligible influence on the depth of deformation for a given loading condition but substantially increases the magnitude of the peak normal stress in the loaded surface and moves it toward the leading edge of the indentor.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(4):540-544. doi:10.1115/1.3261258.

The influence of the residual stresses attending the plastic deformation of a rim on the cyclic crack growth driving force produced by repeated two-dimensional rolling contacts is evaluated. The residual stresses are estimated for disks and rings with different geometries. Values of the Mode II and Mode I stress intensity range, ΔKI and ΔKII , are derived for small, subsurface cracks from the variations in the stress intensity factors with position of the contact for rolling in the absence of surface shear tractions. The calculations take into account (i) steady-state contact stresses generated by a relative peak pressure of po /k = 5; (ii) the radial and circumferential residual stresses, (iii) the frictional resistance on the crack faces; (iv) the crack inclination, and (v) the relative ratios of the inner and outer ring radii. A prediction of the crack initiation and growth rate for high strength steel is illustrated.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(4):545-550. doi:10.1115/1.3261259.

A method is described for the calculation of the film thicknesses in soft, highly deformed contacts for situations where the entrainment velocity is not constant. Two particular results are presented. It is shown that, where there is a rapid reversal of motion, the steady state analysis remains acceptable. However, for a contact reciprocating with a sinusoidal motion, it does not, and here the minimum film thickness occurs at the end of the stroke. The minimum film thickness lies at the end of the contact furthermost from the area swept during the stroke and can only be determined by a dynamic analysis.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(4):551-556. doi:10.1115/1.3261260.

Film thickness and pressure profiles have been calculated for line contacts at moderate and high loads, using a Multigrid method. Influence of the compressibility of the lubricant on the minimum film thickness and on the pressure spike has been examined. The required computing time is an order of magnitude less than when using the previous methods.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(4):557-564. doi:10.1115/1.3261263.

The elastohydrodynamic problem is revisited in terms of an extended variational formulation, where the corresponding functional exhibits minimum properties in the solution neighborhood. Such features are exploited in the development of a relaxation-type solver. The numerical results indicate that the convergence rate of the proposed relaxation scheme becomes increasingly poor as the solution of the elastohydrodynamic problem is approached. A polyalgorithm based on a combination between relaxation-type and Newton-type schemes is proposed. The numerical experiments referred to various sealing profiles of increasing foundation compliance show that the proposed procedure is particularly advantageous in the case of soft lubricated contacts.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(4):565-569. doi:10.1115/1.3261264.

A simple capillary viscometer was used to demonstrate how one obtains the true shear stress-shear rate rheological properties of a polymer solution. The methodology, described in this paper, shows how the pressure drop caused by the friction of the liquid flowing through the tube is separated from the pressure drops associated with entrance and exit effects and the elastic energy of the polymer solution.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(4):571-577. doi:10.1115/1.3261267.

In order to evaluate the relationship between limiting shear strength and pressure for a lubricant an experimental apparatus has been built. Pressures up to 2.2 GPa and temperatures up to 200°C are attainable simultaneously. Thus the limiting shear strength-pressure relationship has been surveyed in a wide range. Several types of natural and synthetic lubricants have been tested. The results show that all the mineral oils tested behave in quite a similar way. Synthetic lubricants do not show this behaviour, but the relationship between limiting shear strength and pressure depends strongly upon the chemical base of the synthetic lubricants. The behaviour of grease does not differ from that of liquid lubricants. The pressures at which the lubricants change from a liquid to a solid behaviour have also been measured.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(4):579-583. doi:10.1115/1.3261270.

Measurements of viscoelastic parameters as a function of pressure and temperature are reported for the m-bis (m-phenoxy phenoxy) benzene. Infinite frequency shear modulus is compared with the limiting shear strength data obtained from the literature under the same pressure and temperature conditions.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(4):584-588. doi:10.1115/1.3261271.

This two part paper presents the experimental observation and numerical simulation of the dynamic response of self-acting gas-lubricated slider bearings used to maintain the sub-micron spacings between the Read/Write transducers and the rotating disks in magnetic recording disk files. In this Part I, a novel laser Doppler interferometric technique is used to measure the transient motions of the R/W head as it encounters an artificially introduced crater-like surface imperfection and a sputtered rectangular step.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(4):589-593. doi:10.1115/1.3261272.

This two part paper presents the experimental observation and numerical simulation of the dynamic response of self-acting gas-lubricated slider bearings used to maintain the sub-micron spacings between the Read/Write transducers and the rotating disks in magnetic recording disk files. In this Part II, a factored implicit finite difference scheme is used to integrate the Reynolds lubrication equation, which describes the isothermal compressible fluid flow within the bearing region, and a fourth order Runge-Kutta method is used to solve the equations of motion, which describe the slider dynamics. Using this numerical model, the theoretical slider response due to a rectangular step in the disk surface is obtained. Excellent correlation is observed between theory and experiment. Results are presented to illustrate the effects of step size, step location, and surface velocity on the dynamic performance of slider bearings.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(4):594-597. doi:10.1115/1.3261273.

The sealing effects of squeeze air film were analyzed experimentally and theoretically. The air flow rate and the sealed pressure were measured in a squeeze face seal. The air flow rate can be expressed as the difference between the flow rate by the pumping and the flow rate by the leakage. The air flow rate by the pumping increases proportionally to the square of the vibration amplitude of the surface, as does the sealed pressure. The air flow rate by the leakage increases proportionally to the pressure difference between the vessel pressure and the ambient pressure. The experimental results showed good agreement with the theoretical results.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(4):598-603. doi:10.1115/1.3261274.

The mixed-lubrication problem is formulated as a nonlinear generalized complementarity problem in which the pressure acting on the load-bearing surface is taken as the unknown, and the lubricant flow and the gap between the surfaces are taken as its complements. An iterative method was developed to find the solution, which satisfies the complementarity condition that at each point on the load-bearing surface, the pressure or at least one of its complements, is zero at all times. Moreover, the pressure and its complements satisfy non-negativity constraints. The solution intrinsically decomposes the load-bearing surface into three distinct subregions: solid-to-solid contact, hydrodynamically lubricated contact, and no contact (or cavitation). It is shown that the mixed-lubrication formulation degenerates into the special cases of hydrodynamic or solid-to-solid contacts under appropriate load and speed conditions. A journal bearing with elastic support is analyzed to illustrate the method of solution. The transition of the lubrication mode from pure hydrodynamic contact to mixed contact is demonstrated.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(4):605-609. doi:10.1115/1.3261277.

Test results are presented for nine annular seals which use a roughened stator and smooth rotor to yield an increase in net damping as compared to conventional smooth-rotor / smooth-stator annular seals. Round-hole roughness patterns are used to achieve the desired stator roughness. The results presented demonstrate that the maximum net damping is achieved by (a) a hole pattern which takes up about 34 percent of the surface area, and (b) hole depths which are about three times the radial clearances. When compared to a smooth seal, the optimum configuration increases net damping by 37 percent, while reducing leakage by 46 percent and direct stiffness by 23 percent. Comparisons of experiment to theory are generally satisfactory for hole-patterns near the optimum area ratio of 34 percent. However, the theory is unsatisfactory for seals which have a substantially higher percentage of the surface area taken up by holes.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(4):612-619. doi:10.1115/1.3261282.

Mobility and impedance tensors are introduced for full journal bearings. These tensors may replace the well known mobility and impedance vectors. Since tensors apply to arbitrary systems of reference, coordinates rotating with the sleeve, i.e. fixed coordinates, will be introduced and will henceforth replace the unidirectional system in current use. As a consequence, the restriction to full journal bearing applications which was necessary up to now may be withdrawn. The mobility- and impedance methods, as derived for full journal bearings, from now on apply equally well to partial-arc bearings. The mobility- and impedance tensor descriptions, needed in applications of the methods, can be derived in a straightforward manner form the generally applied vector descriptions for full journal bearings. Descriptions for partial-arc bearings will also be presented.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(4):621-626. doi:10.1115/1.3261285.

The temperature distributions in full circular bearings were measured in a test rig. The effects of journal speed, lubricant viscosity, and clearance ratio on the maximum bearing temperature and its location were discussed. The results were compared with the theoretical analysis by the present authors and good agreement was obtained over the wide range of operating conditions. The maximum bearing temperature is found to increase considerably with the increase of speed or lubricant viscosity and also with the decrease of clearance ratio. Its angular position is found to vary with speed and clearance ratio. These phenomena can be explained by the characteristics of maximum film temperature in the oil film.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(4):628-637. doi:10.1115/1.3261288.

A theoretical investigation is made of the evolution of a vapor bubble for a submerged journal bearing under dynamically loaded conditions. The solution to the Reynolds equation is determined numerically using a control volume method (Elrod algorithm). This method conserves mass throughout the computational domain including the liquid-vapor interface which may or may not be in motion relative to the minimum film line. An ADI (Alternating Direction Implicit) method is used to effect the time march. Excellent agreement was found with the experimental work of Jakobsson and Floberg for stationary cavitation. Predictions of bubble life for nonstationary cavitation compare reasonably well with that measured by Jacobson and Hamrock using high-speed photography. A comparison study was performed to determine some of the consequences of applying a nonconservative theory to a dynamic problem. A complete dynamic cycle of a journal whirling in a circular path was chosen for the basis of comparison. Significant differences were observed in the load components near the end of the cycle. In each case, onset of cavitation was observed followed by bubble growth and subsequent collapse. More complete details of this phenomena are illustrated with the use of perspective graphic plots depicting the associated pressure distribution and region of cavitation with position and motion of the journal within the housing.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(4):639-644. doi:10.1115/1.3261292.

This paper records observations of the behavior of an oil film subject to an oscillatory squeeze motion of its containing surfaces. In the experimental apparatus, the square upper surface oscillated at a frequency within the range 5–45 Hz and contained two pressure transducers. A fixed transparent lower surface facilitated viewing of cavitation patterns and their position relative to the pressure transducers. A computer controlled technique enabled these patterns to be photographed at any selected point in the oscillatory cycle, and synchronized with the corresponding instantaneous oil film pressure and thickness. The effect is given of vibratory amplitude, frequency and initial oil film thickness upon the steady force generated by the oscillatory squeeze motion. A previously identified cavitation regime has been shown to be more complex than hitherto supposed. Four sub-regimes have been tentatively identified. Their characteristics are described, together with photographs of typical sequences of cavitation patterns in each subregime, at identified times in the pressure and film thickness cycle. The effects of surrounding oil depth upon the vibratory amplitude at which cavitation first appears, is described. Descriptions are given of the sub-regime appearing at onset, and any changes thereto appearing with further increases in vibratory amplitude.

Commentary by Dr. Valentin Fuster
J. Tribol. 1986;108(4):645-654. doi:10.1115/1.3261295.

An analysis for hydrodynamic, non-Newtonian lubrication of misaligned journal bearings is given. The hydrodynamic load-carrying capacity for partial arc journal bearings lubricated by power-law, non-Newtonian fluids is calculated for small valves of the bearing aspect ratios. These results are compared with: numerical solutions to the non-Newtonian modified Reynolds equation, with Ocvirk’s experimental results for misaligned bearings, and with other numerical simulations. The cavitation (i.e., film rupture) boundary location is calculated using the Reynolds’ free-surface, boundary condition.

Commentary by Dr. Valentin Fuster

DISCUSSIONS

Commentary by Dr. Valentin Fuster

TECHNICAL BRIEFS

J. Tribol. 1986;108(4):655-658. doi:10.1115/1.3261296.

The problem of plane elastic contact between a symmetrical indentor and a half-plane is addressed. The form of the contacting profile of the indentor is represented in terms of Chebyshev polynomials, and the resulting stress-field is deduced, for both static and sliding contact. It is shown that by making the profile somewhat flatter than a cylinder a large load may be sustained without yielding. Practical implications of the result, including profiles needed to attain optimal contact conditions, are discussed.

Commentary by Dr. Valentin Fuster

BOOK REVIEWS

J. Tribol. 1986;108(4):659. doi:10.1115/1.3261297.
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Abstract
Commentary by Dr. Valentin Fuster

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