Showing papers in "Tribology Transactions in 1997"

Effect of Carbide Grain Size on the Sliding and Abrasive Wear Behavior of Thermally Sprayed WC-Co Coatings

Abstract: The carbide size and cobalt content of thermally sprayed tungsten carbide/cobalt coatings (WC-Co) can influence their microstructure, fracture strength, friction response and abrasion resistance. In this paper, these properties have been determined for one commercially available and three experimental WC-17 wt.% Co thermally sprayed coatings. The experimental coatings were processed from starting powders containing median carbide size distributions of 1.2, 3.8 and 7.9μm, respectively. All the coatings were produced using a high velocity oxy-fuel (HVOF) spray process. The present results indicate that coatings with a higher percentage of finer carbide size distribution in the starting powder display a higher degree of decomposition of the WC phase to W2C phase and, consequently, display lower fracture toughness and abrasion resistance values. Unidirectional, unlubricated sliding wear tests did not reveal major differences in the sliding wear response of the coatings as a function of carbide size. The micro...

Preliminary Investigation of Neural Network Techniques to Predict Tribological Properties

Abstract: A complete evaluation of the tribological characteristics of a given material/mechanical system is a time-consuming operation since the friction and wear process is extremely systems-sensitive. As a result, experimental designs, i.e., Latin Square and Taguchi, have been implemented in an attempt to not only reduce the total number of experimental combinations needed to fully characterize a material/mechanical system, but also to acquire life data for a system without having to perform an actual life test. Unfortunately, these experimental designs still require a great deal of experimental testing and the output does not always produce meaningful information. In order to further reduce the amount of experimental testing required, this study employs a computer neural network model to investigate different material/mechanical systems. The work focuses on the modeling of the wear behavior, while showing the feasibility of using neural networks to predict life data. The model is capable of defining which input...

The Effect of Compositional Tailoring on the Thermal Expansion and Tribological Properties of PS300: A Solid Lubricant Composite Coating

Abstract: This paper describes a research program in which the goal is to alter the thermal expansion coefficient of a composite solid lubricant coating, PS300, by compositional tailoring. PS300 is a plasma sprayed coating consisting of chrome oxide, silver and barium fluoride/calcium fluoride eutectic in NiCr binder. By adjusting the composition, the thermal expansion coefficient can be altered, and hence chosen, to more closely match a selected substrate preventing coating spallation at extreme temperatures. Thermal expansion coefficients (CTE) for a variety of compositions were measured from 25 to 800 C using a commercial dilatometer. The CTE's ranged from 7.0 to 13 x lO(exp -6)/deg C depending on the binder content. Subsequent tribological testing of a modified composition indicated that friction and wear properties were relatively insensitive to compositional tailoring.

Friction and Wear Mechanisms of Smooth Diamond Films During Sliding in Air and Dry Nitrogen

Abstract: Under the influence of extreme contact pressure and high factional heating, the real contact areas of diamond films may undergo phase transformation and gradual wear during long-duration dry sliding contacts. The wear debris particles that accumulate at the contact interface can then dominate the long-term sliding friction and wear performance of these films. In this study, employing a combination of transmission electron microscopy, electron diffraction, Raman spectroscopy, and electron energy loss spectroscopy, the authors explored the structural chemistry of the diamond debris particles and the sliding contact interfaces of smooth diamond films (surface roughness: 20–40 nm, root mean square (RMS)) and described their friction and wear mechanisms in open air and dry nitrogen (N2). The results of tribological tests indicated that the friction coefficients of Si3N4 balls against smooth diamond films were 0.04 in dry N2 but 0.1–0.15 in air. Friction fluctuated substantially in dry N2, especially during lon...

Tribological Evaluation of PS300: A New Chrome Oxide-Based Solid Lubricant Coating Sliding Against Al2O3 from 25° to 650°C©

Abstract: This paper presents the tribological characteristics of Al2O3 sliding against PS300, a chrome oxide-based self-lubricating coaling. Al2O3, pins were slid against PS300-coated superalloy disks in air under a 4.9 N load at velocities of 1 to 8 m/s. At a sliding velocity of I m/s, friction ranged from 0.6 at 25° C to 0.2 at 650° C. Wear factors for the Al2O3 pins were in the 10−7mm3/N-m range and in the 10−5 mm3/N-m range for the PS300 coating. The test results suggest that increased surface temperature resulting from either frictional heating, generated by increased sliding velocity, or ambient heating caused a reduction in friction and wear of the sliding couple. Based upon these results, the tested material combination is a promising candidate for high temperature wear applications. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in San Francisco, California, October 13–17, 1996

The Control of Friction by Molecular Fractionation of Base Fluid Mixtures at Metal Surfaces

Abstract: The elastohydrodynamic film forming and friction properties of ester/hydrocarbon lubricant base fluid mixtures have been measured down to the very thin film regime. The film thickness results show that the near-surface viscosity, and thus elastohydrodynamic (EHD) film formation in the thin film region, is dominated by the Theological properties of the ester even when this has only low bulk concentration. This suggests that the ester in a blend, being the more polar component, concentrates close to the rubbing surfaces. The friction measurements show that this fractionation of ester close to the surface has a direct influence on the friction coefficient in the mixed lubrication regime. When a lubricant is blended from a higher viscosity ester and a lower viscosity hydrocarbon, the resultant enhancement of viscosity close to the surfaces, and thus of the EHD film thickness, produces a transition from boundary to EHD lubrication at lower speeds than would otherwise be the case. The converse is seen when a lo...

Elastohydrodynamic Lubrication Theory of Line Contacts: Couple Stress Fluid Model

Abstract: A theoretical study of the elastohydrodynamic lubrication (EHL) of line contacts with couple stress fluids as lubricants is presented. The paper mainly deals with the theoretical values of the central film thickness within the elastic contacts of rolling and sliding surfaces in terms of equivalent radius of a cylinder and a plane. The central film thickness is numerically calculated based on Grubin's theory of inlet builtup pressure and considering the effect of a couple stress parameter in complex fluids. In particular cases, film thickness and pressure distribution are also numerically calculated and graphically presented by considering the cavitation boundary condition. A comparative study of the couple stress behavior with the Newtonian behavior of lubricants has been made. As per experimental studies, the central film thickness in EHL contacts depends on a variety of lubricants under identical operational conditions. The present theoretical consideration with couple stress fluid indicates that the pr...

A mixed lubrication model of liquid/gas mechanical face seals

Abstract: A mixed lubrication model for axisymmetric seals, intended as a practical design tool, has been developed. The model considers such physical mechanisms as surface roughness effects on the film lubrication, elastic-plastic face contact, face deformation caused by pressure and contact forces, thermal deformation due to viscous and frictional heating phase change, and temperature and viscosity variations in the film. A numerical scheme that utilizes the influence coefficient method to calculate the face deformations has been developed. This scheme considerably reduces computation time while still maintaining the accuracy of the results. Numerical results obtained through parametric studies show good agreement with available test data. Presented at the 52nd Annual Meeting in Kansas City, Missouri May 18–22, 1997

Optimum design of high-speed, short journal bearings by mathematical programming

Abstract: Generally, the selection of design variables in bearing design is done by a trial and error method using many design charts. However, it is not so easy to successfully select optimum design variables by such a method, and a considerable amount of working time and cost is needed to complete the optimum design of bearings. In this paper, an optimum design procedure for high-speed, short journal bearings operated in both laminar and turbulent flow regimes is developed based on mathematical programming. Under the short bearing assumption, simplified closed-form design formulas are obtained for the eccentricity ratio, maximum film pressure, fluid film temperature rise, supply lubricant quantity and whirl onset velocity. The radial clearance, slenderness ratio and average viscosity of lubricant, which minimize the weighted sum of fluid film temperature rise and supply lubricant quantity for various combinations of journal rotational speed and applied load, are determined by successive quadratic programming, whi...

Optimizing Film Formation by Oil-in-Water Emulsions

Abstract: Oil-in-water (O/W) emulsions are widely used in metal rolling and cutting and also as fire-resistant hydraulic fluids. For both of these applications they require a reasonable degree of lubricating ability—in metal rolling to protect the surfaces against wear and seizure and to provide a controlled value of friction, and in hydraulics to limit the extent of wear, scuffing and fatigue of rubbing pump components. The lubricating properties of O/W emulsions are significantly poorer than those of most water-free oils and this deficiency can create service problems, such as excessive wear or rolling contact fatigue of hydraulic pumps or the uneven rolling or heat streaking of metal strip. Considerable work has gone into the design of O/W emulsions which have acceptable lubricating properties combined with satisfactory emulsion stability and corrosion resistance. Much of this work has been empirical, although there have also been a number of more fundamental studies of the film-forming and friction properties o...

On Mechanisms of Fatigue Life Enhancement by Surface Dents in Heavily Loaded Rolling Line Contacts

Abstract: This paper studies mechanisms of surface dents in enhancing the fatigue life of rolling bearings previously reported in Akamatsu et al. (1). First, transient micro-EHL analyses of heavily loaded contacts between rough surfaces with multiple dents are conducted under near rolling conditions. Contacts with various dent dimensions, dent arrangements under different loading and kinematic conditions are investigated. Results show that surface dents generate no favorable micro-EHL effects to enhance the contact fatigue life. Subsequent analyses, in conjunction with other published studies, suggest that the fatigue life enhancement likely comes from the reduced local traction at asperity contacts through the “oil pots” effects of the dents. The effects of the surface dents on contact fatigue life may depend on the lubrication regime in which the contact is operating being favorable in poor lubrication conditions but adverse in well-lubricated contacts. Since rolling bearings are usually designed to operate in a ...

Study on the Tribological Properties of Ultradispersed Diamond Containing Soot as an Oil Additive

Abstract: The ultradispersed diamond (UDD) powders synthesized by the detonation of explosives with negative oxygen balance are new nano-materials with practical value. In this paper, the detonation soot and UDD powders were used as an additive in paraffin oil. This kind of two-phase lubricant possesses excellent load-carrying capacity, antiwear and friction reduction properties. Under boundary lubricating condition, the detonation soot and UDD powders can easily penetrate into the rubbing surfaces owing to their nanoscale to reduce the wear and friction. The detonation soot is less effective than UDD powders. In addition to sliding, it is hypothesized that a nanoscale ball bearing effect is occurring in the UDD powders which reduces the wear and friction. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in San Francisco, California, October 13–17, 1996

Torque Transmission Characteristics of Automatic Transmission Wet Clutches: Experimental Results and Numerical Comparison

Abstract: A test rig has been designed to quantify the torque transmission characteristics of wet clutches. This apparatus is presented and its features discussed. A comparison is made between the torque measurements from the test rig and simulations using a previously developed mathematical model. Results are given for a variety of operating conditions. The data presented here validate the computational model as an accurate tool for wet clutch design and analysis. Important results include accurate torque transfer prediction capabilities, with the limiting factor being precise knowledge of the sliding friction coefficient μc. Another important conclusion of the work is that torque transfer during a single engagement event can be adequately predicted using the isothermal model presented here. The simplicity of the isothermal model, along with its computational efficiency, makes it attractive for wet clutch torque transfer prediction and wet clutch design. Presented at the 52nd Annual Meeting in Kansas City, Missour...

Wear Mechanism Analysis of Engine Exhaust Valve Seats with a Laboratory Simulator

Abstract: Engine valve seat wear affects engine performance. A common goal for both valve and engine manufacturers is to improve both valve quality and life. By performing tests on a simulator which is verified to be capable of simulating an engine operating environment, the wear resistance of various valve materials is ranked, and the wear mechanisms are investigated. Four commonly used exhaust valve materials, 23–8N, 21–4N Mod, Pyromet 31 and Stellite 6 (hardfacing), were tested with insert material made of Sil XB at an operating temperature of 538° C. Tests run for three million cycles showed that Pyromet 31 had the lowest wear rate and 21-4N Mod had the highest. Then, three valve materials, 23-8N, 21-4N Mod and Pyromet 31, were tested with insert material made of Eatonite 6 at an operating temperature of 649° C. These tests gave the same ranking as those tested with the Sil XB inserts. However, all three valve materials in the second group showed much less wear than those in the first group. The valve seat wear...

The Elastohydrodynamic Properties of Lubricants in Refrigerant Environments

Abstract: The elastohydrodynamic (EHD) film-forming properties of several refrigeration oils placed in pressurized refrigerant environments were investigated by an interferometric EHD tester equipped with a pressurized vessel. It has been shown that the oils with dissolved refrigerants formed EHD films at the point of contact in the same manner as lubricant base fluids. It was also found that greater amounts of dissolved refrigerant in the oil decreased not only the viscosity but also the viscosity-pressure coefficient of the oil. In this paper, the authors describe a method to predict the viscosity-pressure coefficient of oils with dissolved refrigerants. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in San Francisco, California, October 13–17, 1996

Performance of PVD-Coated Carbide Tools When Machining Ti-6Al-4V©

Abstract: This work investigates the performance of TiN and TiN/TiCN/TiN physical vapor deposition (PVD)-coated carbide tools in continuous turning of Ti-6Al-4V. Significant improvement in tool life was achieved when machining Ti-6Al-4V with both PVD-coated tools under the optimum cutting conditions. Notching at the depth of cut (DOC) was suppressed and flank wear, chipping and flaking of tool material on the rake face and/or at the nose of tools were the dominant failure modes. Tool life equations of the coated tools (when machining Ti-6Al-4V) were achieved by using a statistical regression analysis of the experimental data.

Residual stresses due to debris effects in EHL contacts

Abstract: Debris in lubricated contacts significantly reduces the contact fatigue life. The life reduction is due to the surface damage caused by debris denting and the subsequent overrolling of the dent in the EHL contact. High pressure spikes are generated due to the dent which will modify the contact stress profiles, leading to stress concentration at or near the surface. In this paper, the residual stresses caused by debris effects are investigated. The residual stresses originate from the debris denting process and from the overrolling process of the dent in EHL contacts. The finite element method was used to investigate the residual stresses due to each process and their combined effects on the internal stress distribution of an EHL contact. It was found that the residual stress from debris denting will increase the internal stresses in an EHL contact. However, the residual stresses from overrolling of dent will reduce the internal stresses. The residual stresses are largely dependent on the plastic modulus of the contacting materials and need to be considered when investigating the internal stresses in heavily loaded lubricated contacts.

Simulation of Asperity Contacts at the Head/Disk Interface of Tri-Pad Sliders During Steady-State Flying©

Abstract: A model for asperity contacts at the head/disk interface has been developed that allows for elastic asperity deformations based on an experimentally derived function for the contact pressure. The model predicts the distributed contact pressure, the contact force and the apparent area of contact on the air bearing surface of the slider. A second approach that assumes entirely rigid asperities is employed to verify the asymptotic behavior of the contact force as the asperity stiffness increases. Further results are reported for the elastic asperity method and the dependence of the contact force, contact area and flying height on various operating conditions is investigated. The comparison between numerical calculations and experimental results shows excellent qualitative agreement. Presented as a Society of Tribologlsts and Lubrication Engineers paper at the ASME/STLE Tribology Conference in San Francisco, California, October 13–17, 1996

The Performance of a Hybrid Ceramic Ball Bearing Under High Speed Conditions with the Under-Race Lubrication Method

Abstract: Large bore (150 mm) hybrid ceramic ball bearings and all-M50 steel bearings were tested with under-race lubrication to compare the heat generation and the temperature rise at speeds up to 2.25 million DN. Furthermore, oil shut-off tests were carried out with both bearings over 2.25 million DN. The experimental results of the heat generation for both bearings were nearly the same at an axial load of 19.6 kN. at 34.3 kN, the heat generation of the hybrid bearing was lower than that of the M50 steel bearing at low speed. The heat generation of both bearings gradually approached each other with increasing speed and became nearly equal at a speed of 15,000 rpm. The survivability of the hybrid bearing in the oil shut-off test was superior to that of the M50 bearing. These experimental results were explained by the calculation results using a computer analysis software which simulates the kinematics and the performance of ball bearings. Presented at the 52nd Annual Meeting in Kansas City, Missouri May 18–22, 1997

Optimization of Asperities for Laser-Textured Magnetic Disk Surfaces

Abstract: Increasing areal densities on the order of I Gbit/in2 in magnetic disk drives demands the use of supersmooth disk substrates which can, however, lead to a severe sticlion problem. A practical solution involves the creation of bumps in a dedicated landing zone near the disk inner diameter using a focused laser beam. The tribological performance of the laser-textured disk depends on the size and number of bumps on its surface. A large number of asperities (bumps) can result in high sticlion force. On the other hand, to prevent plastic deformation, the number of asperities must be greater than a certain number. A statistical contact model has been developed to generate design curves for estimating the critical number of asperities as a function of asperity radius which can prevent both plastic deformation and severe sticlion. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in San Francisco, California, October 13–17, 1996

An Experimental Investigation on the Misalignment Effects in Journal Bearings

Abstract: In this paper an investigation is made to evaluate the experimental performance of misaligned cylindrical and three-lobe journal bearings. A three-bearing double-disc rotor system with a test bearing in the middle and driven by a variable speed DC motor through an electromagnetic coupling is used. Various parameters which have been studied are friction through coastdown analysis, unbalance response and film thickness. The system damping also has been evaluated. The film thickness decreases, friction increases and system damping increases as the bearing misalignment increases. While the first, third and fourth harmonics vibration response is found to be almost steady with a increasing degree of misalignment, the second harmonics vibration response tends to decrease slightly. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in San Francisco, California, October 13–17, 1996

An average Reynolds equation for non-Newtonian fluid with application to the lubrication of the magnetic head-disk interface

Abstract: A closed, form of the average Reynolds equation is derived for non-Newtonian inelastic fluids with roughness effects taken into consideration. The derivation of flow factors is carried out by means of the perturbation approach with Green's function technique. A coordinate transformation is also utilized to make the equations similar with the Newtonian ones. The results of flow factors are expressed as functions of the Peklenik numbers and the standard deviations of roughness heights of each surface, as well as functions of the film thickness ratio Final manuscript approved November 10, 1995 and the flow behavior index of the power-law fluid of the lubrication film. One can find a critical value for Peklenik number (γcr) from the expressions of pressure flow factors. When γ > γcr(γ < γcr), the pressure flow factors increase as n increases (decreases). A modified Peklenik number is defined to reveal the interactions between roughness orientations and the non-Newtonian flow. Furthermore, an appropriate combi...

Increased performance of hybrid bearings with silicon nitride balls

Abstract: An experimental study on the performance of hybrid bearings in a lubrication environment where the oil contains metallic particles, such as found in industrial gearboxes and transmissions is conducted. Improvements in the wear resistance performance of hybrid bearings are reported and compared with the performance achieved by conventional steel bearings. The performance merits of hybrid bearings under commonly used grease lubrication conditions operating at high speed and temperature is also described. The mechanisms and advantages gained by adopting hybrid rolling-element bearings as a means of extending the service life of mechanical systems or machines operating in highly demanding environmental conditions are discussed. Presented at the 52nd Annual Meeting in Kansas City, Missouri May 18–22, 1997

Degradation of thin-film disk PFPE lubricants by ceramic powders

Abstract: Perfluoropolyether lubricants have been the lubricants of choice on thin-film magnetic disks because of their superior properties. However, studies have shown that they can degrade at lower temperatures in the presence of metal oxides such as Al2O3 and Fe2O3. In this paper, the degradation behavior of three widely used disk lubricants which have a Z-backbone structure were studied in the presence of the following ceramic powders: AlN, Al2O3, Fe2O3, SiC, Si3N4, SiO2, TiC, TiO2, and ZrO2. Experiments involved monitoring the weight loss of a continuously stirred mixture of lubricant containing 1 wt% of powder with lime, at a fixed temperature of 250° C. In the case of ZDOL 4000, there was a 10% loss in the first 60 minutes and then a very low and constant rate of weight loss. TiC, AlN and Al2O3were very aggressive in degrading the lubricant while SiC and Si3N4 had no effect at all. ZDOL 2000 exhibited a 60% loss in the first 100 minutes and then a low and constant rate of loss with time. ZrO2, Al2O3, TiC and...

Experimental Determination of the Thermal Characteristics of a Mechanical Seal and Its Operating Environment

Abstract: The thermal distortion of mechanical seal faces has a critical impact on the performance of the seal. Experimental work is discussed which quantifies the thermal characteristics of a mechanical seal and its operating environment. Experimental techniques are employed which measure the seal face torque, thermal gradients, and fluid flow patterns under the normal operating conditions of the seal. The experimental data is used to calculate the heat that is generated at the seal faces and the Nusselt number on the wetted surfaces on the stationary element. The experimentally determined Nusselt numbers are compared to experimental and empirical values that have been presented by other researchers. The effect of the seal geometry on the experimental Nusselt numbers is evaluated. Presented at the 52nd Annual Meeting in Kansas City, Missouri May 18–22, 1997

Rolling Contact Testing of Vapor Phase Lubricants–-Part I: Material Evaluation

Abstract: Material screening tests have been performed with four candidate vapor phase lubricants under rolling contact conditions at bearing test temperatures of 370°C. The lubricants evaluated were t-butyl phenyl phosphate (TBPP), polyalphaolefin (PAO), cyclophosphazene (X-1P), and PAO blended with 15 percent TBPP (PAO+). Factorial analyses were used to separate the effects of lubricant; bearing steel, T15 vs. thin dense chrome-coated T15; cage material, AISI 4340 vs. Monel 400; and ball material, T15 vs. NBD 200 Si3N4. Response variables included in the analyses were the bearing friction coefficient, wear rate, and life under accelerated conditions. The type of lubricant was significant at the 90 percent confidence level for all three response variables, with the PAO+ and TBPP showing the best results. In general, bearing materials containing Fe showed improved performance over materials which do not contain Fe at the bearing surface. Presented at the 51st Annual Meeting in Cincinnati, Ohio May 19–23, 1996

On the numerical analysis to the thermoelastohydrodynamic lubrication of a tilting pad inclusive of side leakage

Abstract: Numerical solutions are obtained for the thermoelastohydrodynamic lubrication of a tilting pad based on the three-dimensional flow of lubricant. For elastic deformation, the pad is idealized as a uniform plate with free boundaries so that a displacement coefficient matrix can be established by the conventional finite difference method. For pressure distribution, a fast Newton-Raphson's approach is used in solving the generalized Reynolds equation, film thickness equation, inlet pressure buildup equation, and the force balance and moment balance equations of the pad, simultaneously. For temperature distribution, the finite difference method is carried out with the help of a sweeping scheme.

Tribo-Characteristics of Cryogenic Hybrid Ceramic Ball Bearings for Rocket Turbopumps: Self-Lubricating Performance©

Abstract: Experiments were conducted to investigate and compare the self-lubricating performance of two types of modified, 25 mm-bore ball bearings: a hybrid bearing with Si3N4 ceramic balls, and an all-steel bearing. The modified bearings had a single outer land-guided retainer with elliptical pockets and a narrow axial pocket clearance to reduce wobbling of the retainer. The bearings were tested under high thrust loads to 2840 N and at speeds up to 50,000 rpm in liquid hydrogen (LH2), liquid oxygen (LO2), or liquid nitrogen (LN2). Testing showed that the bearing torque of the modified all-steel bearing was drastically decreased to about one-half that of the conventional double outer land-guided bearing at a speed of 50,000 rpm in LH2 Furthermore, the hybrid bearing had relatively stable variation of bearing torque and better load capacity than those of the all-steel bearing in both LH2 and NH2. However, in LO2, the all-steel bearing exhibited excellent performance with high load capacity at high speeds, contraste...

Topographic Characterization of Progressive Wear on Deep Drawing Dies

Abstract: The subject of this paper is an analysis of progressive, wear and other surface alteration processes which take place on deep drawing dies. The deep drawing process was simulated using strip test evaluation equipment. A theory on the influence of different surface alterations on the die surface topography was then, developed. The wear progress was quantified using conventional surface roughness parameters as well as a set of recently developed parameters based on the bearing curve and extended to thee dimensions. It was concluded from conventional surface roughness measurements that maximum wear takes place at angles of 20° and 70° from the vertical direction on the die. Furthermore, it was observed from three-dimensional measurements that the progressive wear is initialed by ploughing after only a few drawings and this mechanism continues until the die is worn out. In the investigations described in this work, the wear progress was interpreted using the. three-dimensional parameters Spk and Svk in terms ...

The Mechanism of Bearing Surface Fatigue–-Experiments and Theories

Abstract: This paper begins, with a discussion on the limitations of the bearing lubrication parameter, i.e., the Lambda ratio, as well as the variables which influence surface fatigue of rolling bearings, based on close examination of results of bearing tests and peeling tests. Since most rolling bearings in service operate in the “low Lambda” regime, the maximum stress in the microcontact of asperities can be much higher than that in the macrocontacts of rolling members. A localized residual stress field is generated as a result of plastic deformation in microcontact. This residual stress contains a tensile component, which is believed to be more detrimental than compressive stress. A new unified bearing surface (micropitting) failure theory is proposed based on the tensile (residual and applied) stress in the material. This theory can be used to quantify the effect of asperity contact maximum pressure, residual compressive stress, tensile hoop stress and hardness on pitting fatigue life. Presented at the 52nd An...