Showing papers in "Tribology Transactions in 1991"

The Measurement and Study of Very Thin Lubricant Films in Concentrated Contacts

Abstract: Optical interferometry is now a widely used technique for measuring the separating film thickness in model rolling and sliding elastohydrodynamic contacts. There are two limitations of the method as conventionally employed: first, it cannot easily be used to accurately measure films less than one quarter the wavelength of visible light, i.e. less than about 100 nm. Secondly, only certain, discrete thicknesses, spaced at least 50 nm apart can be determined. This paper describes work aimed at overcoming these limitations so as to make optical interferometry applicable to the study of boundary or very thin film elastohydrodynamic lubrication in rolling contacts. A combination of a solid spacer layer with spectrometric analysis of reflected light from the contact enables very thin lubricant films to be accurately measured. The approach is applied to the study of thin films formed in rolling contacts by low viscosity lubricants. Some anomalies in the relationship between film thickness and speed are found with...

Temperature Maps for Frictional Heating in Dry Sliding

Abstract: When contacting solids slide, work is done against friction. Almost all the frictional work appears as heat, generated at or very close to, the surface at which the two solids meet. The local increase in temperature influences both the friction and the rate of wear. In this paper, frictional heating is analyzed using a particularly simple framework. The analysis provides for a wide range of geometries for the two contacting solids, which may differ in their thermal and mechanical properties, and may not be in perfect contact with the heat-sinks to which they are attached. The results are presented as temperature maps which show the surface temperature of a sliding couple over a wide range of load and sliding velocity. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in Toronto, Ontario, Canada, October 8–10, 1990

Fluid-Film Lubrication Regimes Revisited

Abstract: Dimensionless film-thickness equations are provided for four fluid-film lubrication regimes found for nonconformal surfaces. Side-leakage effects are present in the equations. These regimes are isoviscous-rigid; piezoviscous-rigid, isoviscous-elastic, or soft EHL; and piezoviscous-elastic, or hard EHL. The influence or lack of influence of elastic and viscous effects is a factor that distinguishes these regimes. The film-thickness equation for the respected regimes come from earlier theoretical studies. Results are presented as a map of the lubrication regimes, with film thickness contours on a log-log grid of the viscosity and elasticity parameters for four values of the ellipticity parameter. Presented as a Society of Tribologlsts and Lubrication Engineers paper at the ASME/STLE Tribology Conference In Toronto, Ontario, Canada, October 8–10, 1990

The Rheology and Hydrodynamics of Dry Powder Lubrication

Abstract: This paper conceptualizes a powder lubrication mechanisms which closely resembles that of a hydrodynamic fluid film. Based on the observations of past investigations and on the author's experiments, it is postulated that a layered shearing of the compacted powder generates velocity, density, and temperature profiles akin to fluid film bearings. Thus, a lubricant consisting of a fine powder unserted either deliberately or one generated by the water of the mating surfaces, constitutes a viable lubricant that generates the required flows and pressures to prevent contact between the surfaces.

In Lubro Studies of Lubricants in EHD Contacts Using FTIR Absorption Spectroscopy

Abstract: Research into the physical and chemical processes occurring within lubricated concentrated contacts has traditionally been limited by the necessity to observe events out-of-contact, or under simulated conditions This paper describes the application of a new technique involving infrared microreflection absorption spectroscopy The method is applied to the direct determination of the conditions of pressure experienced by lubricants in contacts, to the formation of antiwear films by phosphorus additives and to the study of the alignment of molecules, including viscosity index improvers in contacts Presented at the 45th Annual Meeting In Denver, Colorado May 7–10, 1990

A friction model of partial-EHL contacts and its application to power loss in spur gears

Abstract: The major power losses in spur gears due to sliding and rolling friction have been analyzed by applying a simplified analytical friction model in the partial-elastohydrodynamic contacts where the friction is affected by both asperity interactions and hydrodynamic effects. Results based on this friction model suggest that the variations in both surface roughness and coefficient of asperity friction alter the traction coefficient appreciably. Reduction of the traction coefficient can improve the efficiency of gears. Presented at the 43rd Annual Meeting in Cleveland, Ohio May 9–12, 1988

Effect of Selected Chemical Compounds on the Lubrication of Silicon Nitride

Abstract: Successful use of advanced ceramics in many tribological applications requires an understanding of the physical, chemical, and mechanical properties of the material. Physical and mechanical data are relatively abundant for most ceramics. However, information on the chemical interactions of ceramics is scarce. This is especially true for chemical interactions with regard to lubrication of these materials. This paper investigates the influence of selected chemical compounds on the friction and wear of silicon nitride under boundary lubrication conditions. A ball-on-three-flat modification of the four-ball wear tester was utilized to evaluate the tribological characteristics of a hot pressed silicon nitride lubricated with a paraffinic base oil containing 1 weight percent additives. Friction, wear, and. film formation tendencies were observed for a range of oil soluble chemical compounds containing oxygen, sulfur, nitrogen, chlorine, and phosphorous. A wide range of additive response was observed. Friction c...

The Mechanism of Cavitation in Hydrodynamic Lubrication

Abstract: The paper offers a conceptual visualization of the mechanism of cavitation in the light of available experimental evidence. The experiments span steady, starved, and dynamic or transient lubrication and the resulting cavitation is postulated in terms of several basic parameters. Chief among these are: temperature of the surfaces, availability of the gaseous and vapor components and the occurrence of negative liquid stresses under transient conditions. Mode, shape, and stability of the resulting liquid-gaseous phase are seen to be a function of the prevailing operational conditions. Presented at the 35th STLE/ASME Tribology Conference In Fort Lauderdale, Florida October 16–19, 1989

Modeling of instabilities induced by cage clearances in ball bearings

Abstract: Generalized dynamic motion of balls and cage in a ball bearing are simulated by solving the differential equations of motion under prescribed operating conditions and bearing geometry. The general cage motion is parametrically evaluated as a function of clearances both in the ball pockets and at the guide lands. The design significance of the modeling approach is demonstrated by the prediction of critical clearances which trigger certain instabilities in the cage motion. In more practical terms, the correlation between cage clearances and instability defines a wear life for the bearing under the prescribed operating conditions. Presented at the 45th Annual Meeting in Denver, Colorado May 7–10, 1990

Hydrodynamic Lubrication and Bearing Behavior with Impulsive Loads

Abstract: The problem of hydrodynamic lubrication under conditions of suddenly applied loads is of practical importance and theoretical interest. For example, such conditions may occur if a turbine blade fractures. From a theoretical point of view, the classical theory of Reynolds, which is quasi-steady, may not be applicable. Reynolds' equation does not depend on bearing surface accelerations, which may be considerable, but only on bearing surfaces1 velocities. A theory is developed and compared to known exact solutions in the simple squeeze-film case. The theory is then applied to the case of a massive rotor simply supported by a short bearing, to which a large impulsive load is applied about the steady condition. Rotor trajectories are computed and in many instances a large difference is observed between the behavior predicted by the Reynolds equation and the improved theory. Results show that fluid inertia, not included in Reynolds' theory, has the desirable effect of suppressing the trajectory amplitude due to...

Friction and Wear Characteristics of Silicon Nitride-Graphite and Alumina-Graphite Composites©

Abstract: To take advantage of the beneficial properties of advanced ceramics, it is necessary that their friction coefficient be reduced to an acceptable value. One method for achieving this goal is incorporation of a solid lubricant phase in the ceramic matrix. In this study, ceramic-matrix composites were fabricated by drilling a series of small holes in alumina and silicon nitride, which were subsequently filled with NiCl2 intercalated graphite under high pressure. It was found that the addition of graphite to silicon nitride considerably reduces the friction coefficient, but alumina-graphite composites exhibit only a marginal reduction in friction coefficient compared to alumina. The reduction in friction coefficient for silicon nitride-graphite composite can be explained by the formation of transfer films consisting of a mixture of materials from both contacting surfaces. However, for alumina-graphite composites the graphite regions were completely covered with steel wear particles, inhibiting the formation o...

Analysis of Bearing Incidents in Aircraft Gas Turbine Mainshaft Bearings

Abstract: An examination of approximately 200 bearing incidents in current aircraft engines has shown that damage in the bearing is initiated at the surface. Twenty-one cases were studied in detail, and five typical incidents are presented here. The initial damage was produced by abrasive particles, dents, grinding scores, skidding, large carbides and corrosion pits. The first phase of the failure mechanism appeared to be by peeling, where microscopic regions about 0.13 mm (0.005 in) were removed from the surface, and the peels eventually developed into spalls. These mechanisms of failure are a consequence of the partial EHD lubrication utilized in these bearings, and the consequent sharing of the load by the asperities and the EHD film. Failures of this type were reproduced in lest bearings run under typical engine conditions, and these tests were used to study the peeling mechanism. Techniques for the minimization of surface-initiated fatigue are also discussed. Presented at the 45th Annual Meeting In Denver, Col...

Sliding Friction Mechanisms of Boundary-Lubricated Layered Surfaces: Part II—Theoretical Analysis

Abstract: A new friction theory of boundary-lubricated layered surfaces based on the qualitative aspects of the prevailing friction mechanisms and the phenomenological observations is presented. The cases of elastic and plastic contacts are investigated analytically, and mathematical friction models which yield relationships between the coefficient of friction and critical parameters are developed. For the case in which elastic deformation prevails at the asperity junctions and when shearing of the lubricant film is the predominant friction mechanism, a friction model which accounts for the shear properties of the lubricant, the ratio of the layer thickness to the size of the asperity contacts, and the elastic properties of the layer and substrate media is presented, and the concept of boundary lubrication “friction space” is introduced. Furthermore, a friction model for plastically deformed contacts which accounts for interfacial adhesion and plowing effects is derived. It is shown that the friction coefficient is...

Effect of Eccentricity on the Force Response of a Hybrid Bearing

Abstract: The effect of journal eccentricity on the static and dynamic performance of a water lubricated, five-recess hybrid bearing is presented. The hydrostatic bearing has been designed to operate at a high speed and with a large level of external pressurization. The operating conditions determine the flow in the bearing to be highly turbulent and strongly dominated by fluid inertia effects. The analysis covers the range of journal center displacements directed toward the middle of a recess and toward the mid-land portion between two consecutive recesses. Predicted dynamic force coefficients are constant for small to moderate eccentricities. For large eccentricities, fluid cavitation and recess position determine large changes in the bearing dynamic performance. The effect of fluid inertia force coefficients on the threshold speed of instability and whirl frequency ratio of a single mass flexible rotor is discussed.

In-Situ Electro-Charging for Friction Reduction and Wear Resistant Film Formation

Abstract: A technique has been developed to form friction-reducing and antiwear films in-situ by means of an electric field applied between rubbing metal surfaces. Using this in-situ charging technique, the effectiveness of zinc organodithiophosphate (ZDP) additives as antiwear and friction reducing agents can be enhanced by electrochemical reactions which form surface coatings. The coatings formed on sliding surfaces compared with uncharged surfaces reduce friction by up to 35 percent. In addition, during the sliding process, the films formed on the surfaces are replenished and protect the rubbing surfaces from wear. A steel plate subjected to rubbing and in-situ charging for four hours had a smooth surface and a yellowish reaction film; the one without charging was rougher and no yellowish reaction film was formed. The improved tribological characteristics are attributed to an increase in the electrochemical reactivity of the ZDP-mineral oil blends and the formation of phosphate or sulfate films on the metal surf...

The Effect of Different Lubrication Methods on the Performance of a Tilting-Pad Journal Bearing

Abstract: This paper contains the results of comparisons between the effects of three different lubrication methods, flooded, directed and leading-edge, on the performance of a tilting-pad journal bearing. Experimental pad temperatures and power loss data are presented for a 127 mm diameter, four pad bearing, operating over a range of shaft speeds up to 7000 rpm (46.6 m/s) and loads up to 22 kN (1.3 MPa). The experimental data indicate that temperature and power loss reductions are attainable with the use of the different lubrication methods. However, these are not as significant as those achieved with their use in tilting-pad thrust bearings.

Effects of oscillatory speed and mutual overlap on the tribological behavior of PTFE and selected PTFE-based self-lubricating composites

Abstract: The wear reducing actions of bronze, graphic powder and discontinuous glass fiber within a PTFE matrix have been demonstrated in oscillatory contact with stainless steel, at two different amplitudes, over a range of speeds. The secondary effects on the friction of the PTFE have also been investigated. The effects of oscillation amplitude on frictional surface morphology have been observed for these composites. The friction and wear of the unfilled PTFE matrix were both found to increase as speed increased, with wear going through a transition from a low- to high-rate regime. The speed at which this transition occurs increases with decreasing oscillatory amplitude.

Performance Characteristics of Cavitated Noncircular Journal Bearings in the Turbulent Flow Regime

Abstract: In this paper, the characteristics of noncircular bearings, considering the effects of turbulence and cavitation, are numerically analyzed. Four bearing profiles are considered: circular, worn-circular, two-lobe and elliptical. To model the turbulence, the linearized lubrication theory of Ng and Pan is used. Reynolds' equation is suitably modified to account for both turbulence and cavitation. The equation is incorporated into the Elrod cavitation algorithm. This numerical procedure, which conserves mass throughout the bearing, implicitly incorporates the JFO boundary conditions at rupture and reformation boundaries. JFO theory is considered to accurately model cavitation in moderately to heavily loaded bearings. Performance predictions are compared to results presented in a variety of earlier papers. Presented at the 45th Annual Meeting in Denver, Colorado May 7–10, 1990

Sliding Microindentation Fracture of Brittle Materials

Abstract: The sliding indentation of ceramics by sharp rigid indenters is considered. A diamond conical indenter is used to scribe borosilicate and fused-silica glasses. The coefficient of friction and the median vent crack depth are measured. It is found that active lubricants are useful for controlling the friction and the vent crack depths. The principles of fracture are used to calculate stress intensity factors for surface-breaking cracks loaded by normal and frictional scribing forces. The implications of the results to the evolution of grinding-induced cracks in ceramics is discussed.

Tribochemical Lubrication of Ceramics by Carbonaceous Vapors

Abstract: Friction coefficients of less than 0.02 and negligible wear have been attained and continuously maintained in a pin-on-disc apparatus between a sapphire pin and discs of Sialon, silicon carbide, zirconia or zirconialalumina as long as a carbonaceous gas or vapor, such as ethylene, propane, benzene, or 1-propanol, was supplied to the conjunction region. Mean contact pressures were around 300 MPa and bulk temperatures were varied between ambient and 700°C. For every ceramic and vapor combination some temperature ranges exhibited high friction and wear, others very low friction and wear. Raman and Auger spectroscopies of the carbonaceous deposits formed in the wear tracks show significant differences depending on the temperature range and the feed gases or vapors supplied. In general, friction and wear reduction occurred in two steps at different rates. The tribochemistry involved is likely to comprise hydrocarbon acid-catalytic cracking at low temperatures and pyrolysis at high temperatures. Presented as a ...

Flow Visualization and Quantitative Velocity and Pressure Measurements in Simulated Single and Double Brush Seals

Abstract: A method to visualize and characterize the complex flow fields in simulated single and double brush seals is presented. The brush seal configuration was tested in a gravity fed water tunnel and a pump driven pressurized oil tunnel. Visualization of the flow field revealed regions that are characteristically river jetting, vortical, crossflow and exist upstream, downstream or within the seal. Such flows are especially engendered by variations in fiber void that are spatial and temporal and affect changes in seal leakage and stability. While the effects of interface motion and cylindrical configuration have not been considered herein, it is believed that the observed flow fields and pressure drop patterns characterize flow phenomenology in brush seals. The axial pressure profiles upstream, across and downstream of the brush have been measured under a variety of inlet pressure conditions and the ensuing pressure maps are presented and discussed. Through the application of the Full Flow Field Tracking method developed by the authors, the flow patterns and the accompanying fluid velocities inside the brush were non-intrusively determined and graphically reconstructed.

Calibration of the Laser Fluorescence Technique Compared With Quantum Theory

Abstract: The quantum efficiency of the laser fluorescence technique decreases with increasing temperature. This paper reports detailed measurements of the decrease, for five different commercial oils. A comparison with the quantum theory of the decrease is presented. These results can be used to determine the degradation of a lubricant in the ring zone of a running engine, and to calibrate film thickness measurements on adiabalic engines. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in Toronto, Ontario, Canada, October 8–10, 1990

Study on Frictional Loss of Spur Gears (Concerning the Influence of Tooth Form, Load, Tooth Surface Roughness, and Lubricating Oil)©

Abstract: Nowadays, reducing frictional loss of gears to save energy is a big concern. There are many sources of printed information on this problem for gears, but most of them are practical specialized information and are not developed to give a generalized basic consideration. Since this kind of research has not been brought beyond such, a practical and specialized range, experiments were carried out to study the influences of tooth form, sliding velocity, load, tooth surface roughness, and viscosity of lubricating oils, and kinds of oils on the frictional loss of gears. The frictional loss of spur gears decreases with sliding velocity and the viscosity of lubricating oils, and increases with surface roughness. The frictional loss is very low when using a synthetic oil of the SHF type. The organic molybdenum friction modifier (FM) is also quite effective for reducing frictional loss. Finally, empirical formulas for calculating frictional loss were expressed in terms of tooth form, sliding velocity, tooth surface ...

A Comparison of Fatigue Failure Morphology Between TiN Coated and Uncoated Lubricated Rollers

Abstract: In this study two series of morphological fatigue tests were con-ducted with a two disk machine for TiN coated rollers as well as uncoated rollers. Surface examinations showed that 1 mu;m TiN coated surfaces underwent principally elastic deformation without coating debonding. However, uncoated surfaces were heavily deformed even after a shorter contact cycle. For the uncoated rollers, cross-sectional micrographs revealed the presence of many near-surface microcracks at an asperity scale nearly parallel to the surface. It is believed that these microcracks are initiated by asperity interaction. Metallographic examinations of the uncoated subsurface revealed a. thin layer of dark etching region corresponding to the microcrack failure region. For the coated roller, few cracks were found and the dark etching region was absent. The hard coating minimizes the surface distress and suppresses the initiation of microcracks. This explains why the fatigue life is enhanced with a thin hard coating. Cross-sectional tr...

Scale Effects in Steady-State Friction

Abstract: Sliding friction phenomena have long been the subject of study and modeling. The physical portrayal of the sliding contact upon which such models are based is often a function of the modeler's background, and may range from the atomic scale to the scale of more macroscopic surface roughness. Sometimes, quasi-static arguments are inappropriately applied to sliding situations. The current paper examines the nature of frictional variations during sliding contact. High recording-rate instrumentation was used to analyze detailed, point-to-point variations in friction force for self-mated, pin-on-disk experiments with alumina and aluminum specimens. Data indicate that friction mechanisms, even on a fine scale, must be modeled considering the whole tribosystem, not just the interfacial materials properties alone. The contact conditions stimulate the displacements in the tribocomponents, which in turn result in sensible mechanical output. It is physically unrealistic to develop sliding solid friction models which...

Measurements of Pressure Distributions and Force Coefficients in a Squeeze Film Damper Part II: Partially Sealed Configuration

Abstract: In Part I, a squeeze film damper (SFD) test rig and measurement procedures were explained, arid the experimental results obtained from an open ended damper were presented. In this paper, the experimental results measured from a partially sealed SFD test rig executing a circular centered orbit are presented and discussed. A serrated piston ring is installed at the damper exit. This device involves a new sealing concept which produces high damping values while allowing for oil flow to cool the damper. In the partially sealed damper, large cavitation regions are observed in the pressure fields at orbit radii e = 0.5 and e = 0.8. The cavitated pressure distributions and the corresponding force coefficients are compared with a cavitated bearing solution. The experimental results show the significance of fluid inertia and vapor cavitation in the operation of squeeze film dampers. Squeeze film Reynolds numbers tested reach up to Re = 50, spanning the range of contemporary applications. Presented as a Society of ...

Measurements of Pressure Distributions and Force Coefficients in a Squeeze Film Damper. Part 1: Fully Open Ended Configuration

Abstract: Experimental measurements of pressure distributions and force coefficients obtained from a squeeze film damper lest rig executing a circular centered orbit are presented The test rig has been designed to study the effect of fluid inertia on the pressure field and dynamic force response on a damper configuration with a relatively large clearance Past measurements of the squeeze film damper force characteristics have been carried out at squeeze film Reynolds numbers not exceeding a value equal to 10 In the present paper, following contemporary applications, operations at Reynolds numbers up to fifty are tested for CCO's with an orbit radius = 08 (Re ≤ 10 at e = 05) The results obtained from a fully open ended damper are presented in detail The effects of fluid inertia, cavitation and the open end geometry on the pressure distributions and force coefficients are discussed Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in Toronto, Ontario,

Surface Changes Around Large Raceway Indentations During Run-in of Tapered Roller Bearings

Abstract: The results of an experimental study are presented in which the raceway surfaces of tapered roller bearings were intentionally pre-dented with single, large indentations. Contact stress analysis results and the observed changes in the surface near the indentations during run-in are described. Depending on indentation size, extremely high initial contact stresses are predicted at the raised shoulders surrounding the indentations. After running-in for 7.58 million revolutions at a moderate load level, these shoulder stresses drop to approximately 4.1 GPa (600,000 psi) or less. Microcracking and micropitting are observed around the indentations, corresponding to the areas of stress elevation. The microcracking develops during the first 100,000 revolutions of the run-in period, and is often observed to follow the direction of the abrasive finishing marks on the raceway surface. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference In Toronto, Ontario, Can...

Modeling of Instabilities Induced by Cage Clearances in Cylindrical Roller Bearings

Abstract: The classical differential equations of motion for the rollers and cage, in a high-speed cylindrical roller bearing, are integrated under prescribed operating conditions and bearing geometry. The generalized cage motion is parametrically evaluated as a function of clearances both in the cage pockets and at the cage-to-race guide lands. Definite correlations between certain cage instabilities and the operating clearances provide practical guidance for optimization of cage clearances for a given performance specification. Presented at the 45th Annual Meeting in Denver, Colorado May 7–10, 1990

Friction Reduction From Electrochemically Deposited Films

Abstract: The lubricated sliding friction characteristics of both electrochemically precharged and uncharged piston ring specimens were determined m a laboratory friction apparatus. The iron phosphate films deposited on precharged piston ring specimens from a specific lubricant blend reduced both static and dynamic friction by as much as 25 to 35 percent. The mechanisms of friction reduction using electrochemically deposited films were investigated and interpreted in terms of surface roughness reduction and reaction film formation. The bulk metal properties influenced both the degree to which precharged films could be applied to a metal surface and the friction measured in a lubricated sliding contact. In addition, the friction characteristics and surface morphology of the iron phosphate films deposited on precharged piston ring specimens from a hydrocarbon-based medium were compared with those of traditional phosphate coatings made in an aqueous solution of phosphoric acid. The iron phosphate film deposited on pre...