Showing papers in "Tribology Transactions in 2005"

Experimental Investigation of Partial Laser Surface Texturing for Piston-Ring Friction Reduction

Abstract: An experimental study is presented to evaluate the effect of partial laser surface texturing (LST) on friction reduction in piston rings. In a previous study, 30% friction reduction was obtained with full LST where the full width of the piston ring is textured with a very large number of microdimples that act individually as microhydrodynamic bearings. In partial LST, only a portion of the piston-ring width is textured with high dimple density, producing a “collective” effect of the dimples that provides an equivalent converging clearance even with nominally parallel mating surfaces. Experimental results obtained with flat and parallel test specimens with partial LST are presented, confirming a previously published theoretical model and the advantage of partial over full LST. Friction reduction by LST with actual production-crowned piston rings and cylinder liner segments is not straightforward and needs further investigation. Presented at the STLE Annual Meeting in Las Vegas, Nevada May 15-19, 2005 Revie...

Tribological Sensitivity of PTFE/Alumina Nanocomposites to a Range of Traditional Surface Finishes

Abstract: Wear tests were performed with polytetrafluoroethylene (PTFE) + Al 2 O 3 nanocomposites on various manufactured surfaces to determine whether or not the wear resistance of these nanocomposites is a strong function of surface preparation. Four different surface finishes of grade 304 stainless steel counterfaces were used: electropolished (R q = 88 nm), lapped (R q = 161 nm), wet-sanded (R q = 390 nm), and dry-sanded (R q = 578 nm). PTFE + Al 2 O 3 nanocomposites made from powders of roughly 2-20 μm PTFE (matrix) and ∼44 nm Al 2 O 3 (filler) were prepared at filler weight percentages of 0, 1, 5, and 10% and tested on each surface finish. Additionally, 5 wt% 44-nm nanocomposites were compared to identically prepared 5 wt% 80- and 500-nm Al 2 O 3 filled PTFE composites on each surface. Friction coefficients were between 0.12 and 0.19 and wear rates decreased from K = 810 × 10− 6 mm 3 /(Nm) for the 5 wt% 500-nm alumina-filled PTFE on the dry-sanded surface to K = 0.8 × 10− 6 mm 3 /(Nm) for the 5 wt% 80-nm fill...

Study of Zinc Dialkydithiophosphate Antiwear Film Formation and Removal Processes, Part I: Experimental

Abstract: Two recent trends in engine oil formulation are a progressive reduction in phosphorus concentration so as to reduce its impact on the de-NO x catalyst, and an increase in dispersant concentration to control the level of lubricant viscosity increase over extended drain intervals. Unfortunately, both of these trends make it more difficult to generate and retain effective antiwear films on lubricated surfaces and both thus strengthen the need to understand the processes by which antiwear films are formed, and removed, during rubbing. This article and its companion outline a study of the kinetics of antiwear film growth and removal. In Part I, a test method for monitoring antiwear film thickness during rolling/sliding is described and employed to explore how various factors, including operating temperature, antiwear additive type and concentration, and the presence of dispersant, influence both the formation and removal of the tribofilms formed by the antiwear additive zinc dialkyldithiophosphate (ZDDP). Part...

Study of Zinc Dialkyldithiophosphate Antiwear Film Formation and Removal Processes, Part II: Kinetic Model

Abstract: Zinc dialkyldithiophosphate (ZDDP) film thickness measurements made using in situ ultrathin-film interferometry and described in Part I of this two-part paper (Fujita, et al. [1]), have been used to develop and test kinetic models of antiwear film formation and removal. The main component of ZDDP film formation involves the gradual coverage of the surfaces by thick, discrete islands of film material. This process can be modeled by combining a simple coverage model in which the rate of film formation is proportional to the fraction of surface not yet covered, with an induction period. The process of film removal can be modeled by assuming that the rate of film loss is proportional to the fourth power of the coverage or film thickness. The combination of these film formation and removal rate equations is able to predict the complex, transient maximum film-forming behavior of secondary ZDDP as well as the process of film formation by primary ZDDP and the removal of antiwear film by dispersant additive. Prese...

Finish Machining of Nickel-Base Inconel 718 Alloy with Coated Carbide Tool under Conventional and High-Pressure Coolant Supplies

Abstract: Single-point turning of Inconel 718 alloy with commercially available Physical Vapour Deposition (PVD)-coated carbide tools under conventional and high-pressure coolant supplies up to 20.3 MPa was carried out. Tool life, surface roughness (Ra), tool wear, and component forces were recorded and analyzed. The test results show that acceptable surface finish and improved tool life can be achieved when machining Inconel 718 with high coolant pressures. The highest improvement in tool life (349%) was achieved when machining with 11 MPa coolant supply pressure at higher speed conditions of 60 m · min−1. Machining with coolant pressures in excess of 11 MPa at cutting speeds up to 40 m · min−1 lowered tool life more than when machining under conventional coolant flow at a feed rate of 0.1 mm · rev−1. This suggests that there is a critical coolant pressure under which the cutting tools performed better under high-pressure coolant supplies. Cutting forces increased with increasing cutting speed due probably to reac...

Ultrananocrystalline Diamond Film as a Wear-Resistant and Protective Coating for Mechanical Seal Applications

Abstract: Mechanical shaft seals used in pumps are critically important to the safe operation of the paper, pulp, and chemical process industry, as well as petroleum and nuclear power plants. Specifically, these seals prevent the leakage of toxic gases and hazardous chemicals to the environment and final products from the rotating equipment used in manufacturing processes. Diamond coatings have the potential to provide negligible wear, ultralow friction, and high corrosion resistance for the sliding surfaces of mechanical seals, because diamond exhibits outstanding tribological, physical, and chemical properties. However, diamond coatings produced by conventional chemical vapor deposition (CVD) exhibit high surface roughness (Ra ≥ 1 μm), which results in high wear of the seal counterface, leading to premature seal failure. To avoid this problem, we have developed an ultrananocrystalline diamond (UNCD) film formed by a unique CH4/Ar microwave plasma CVD method. This method yields extremely smooth diamond coatings wi...

Dynamic modeling and wear-based remaining useful life prediction of high power clutch systems

Abstract: A model-based technique is presented for remaining useful life (RUL) prediction of highly dynamic, high-power dry clutch systems by combining physicsbased simulation and wear-prediction models. Primary load and engagement shear drivers (i.e., torque, speed, and clutch surface temperature) are modeled using a first principles approach. An extension of Archard’s law is presented in which life usage is predicted by using multiple stochastic models to determine a wear coefficient for each applicable wear mechanism. These models consider the physical wear process, including debris-particle and protective-layer formation, using parameters such as surface roughness, particle size, and surface temperature. These stochastic variables are evaluated in a probabilistic framework, using statistical methods such as Monte Carlo and importance sampling, which consider both measurement and modeling uncertainty. Confidence interval prognostic results are provided to predict the RUL of the clutch throughout its limited life in near-real time.

Bearing Friction Torque in Bolted Joints

Abstract: Formulas are developed for calculation of the effective radius of the bearing friction forces on the rotating contact surface in threaded fasteners. These formulas provide a more accurate estimation of the underhead bearing friction torque component in threaded fastener applications. This enhances the reliability, safety, and quality of bolted assemblies, especially in critical applications. It is well known that the torque-tension correlation in threaded fasteners, and the resulting joint clamping force, is highly sensitive to friction torque components: under the turning head and between threads. This analysis focuses on the bearing friction torque component under the turning head of a threaded fastener. Furthermore, it analyzes the error contained in the current practice when an approximate value, equal to the mean contact surface radius, is used instead of the actual bearing radius. New formulas for the bearing friction radius are developed for a mathematical model of a bolted joint using four differe...

Reynolds' Model Suitability in Simulating Rayleigh Step Bearing Thermohydrodynamic Problems

Abstract: A Reynolds-based numerical thermohydrodynamic analysis, performed on a discontinuous geometric domain, is unable to correctly predict the flow field in the close vicinity of the discontinuity. This limitation, due to the very hypotheses used to derive the Reynolds equation, explains the lack of studies treating such configurations. A Navier-Stokes–based model seems appropriate but its complexity and its requirements in terms of computing time are somewhat prohibitive. To compare the two models we have developed two independent, finite-volume-based numerical algorithms. They are applied to a simple two-dimensional geometric configuration (Rayleigh step bearing). This will allow us to prove that, for the configurations commonly encountered in hydrodynamic lubrication, the errors introduced by the Reynolds model are highly localized. The overall performance and running parameters are correctly determined with both models, rendering unnecessary the complex Navier-Stokes–based solution. Presented at the STLE A...

Analysis of High-Speed Intershaft Cylindrical Roller Bearing with Flexible Rings

Abstract: For high-precision mechanisms such as machine tools or gas-turbine engines, which operate at extreme conditions, it is particularly important to accurately predict the behavior of the included bearing. This prediction includes, among other things, its load distribution, stiffness, and power dissipation. Although shaft speeds tend to increase, rings and shaft walls are becoming thinner due to size and weight constraints. Thus, bearing behavior is no longer independent of the housing and ring stiffness. This paper focuses on the problem of elastic ring deformation and certain behaviors of high-speed intershaft cylindrical roller bearings such as heat dissipation, contact pressure, and risk of bearing failure due to scuffing. The paper presents an analytical method to account for the structural deformation of the rings based on Roark's formulas. The elastic deformation of thin cylindrical rings has been introduced in the set of displacement and load equations that describes the bearing equilibrium. A correla...

The effect of argon-enriched environment in high-speed machining of titanium alloy

Abstract: A major factor hindering the machinability of titanium alloys is their tendency to react with most cutting tool materials, thereby encouraging solution wear during machining. Machining in an inert environment is envisaged to minimize chemical reaction at the tool-chip and tool-workpiece interfaces when machining commercially available titanium alloys at higher cutting conditions. This article presents the results of machining trials carried out with uncoated carbide (ISO K10 grade) tools in an argon-enriched environment at cutting conditions typical of finish turning operations. Comparative trials were carried out at the same cutting conditions under conventional coolant supply. Results of the machining trials show that machining in an argon-enriched environment gave lower tool life relative to conventional coolant supply. Nose wear was the dominant tool-failure mode in all the cutting conditions investigated. Argon is a poor conductor of heat; thus, heat generated during machining tends to concentrate in...

Tribological Studies on Scuffing Due to the Influence of Carbon Dioxide Used as a Refrigerant in Compressors

Abstract: Because hydrofluorocarbon (HFC) refrigerants in air-conditioning systems are known to have a negative effect on the environment, carbon dioxide (CO2) is a candidate as a replacement refrigerant. Research work related to CO2 as a refrigerant has been focused primarily on its thermodynamic performance, whereas work in the area of tribology related to carbon dioxide is absent. In this study, the effects of CO2 used as a refrigerant on the tribological behavior of surf aces in contact in such systems were investigated. Controlled experiments were performed at constant loads in environments of CO2 and the conventional HFC refrigerant, R134a, as well as under conditions of step-increasing loads in the presence of refrigerant (CO2 or R134a) and polyalkylene glycol lubricant. The experiments were performed on a high-pressure tribometer that is particularly suited for tribological testing of compressor contact interfaces. The tribological behavior of contacting surfaces in a CO2 environment was nearly identical to...

Transmission Electron Microscopy of Boundary-Lubricated Bearing Surfaces. Part II: Mineral Oil Lubricant with Sulfur-and Phosphorus-Containing Gear Oil Additives

Abstract: Transmission electron microscopy (TEM) was performed on cross-sectional samples of tapered roller bearing cone surfaces that were tested at two levels of local boundary lubrication severity, Λ ∼ 1.1 and 0.3. Unlike our previously reported work in which a base mineral oil was used, the bearing tests were conducted in mineral oil with sulfur- and phosphorus-containing gear oil additives. Structural and compositional characterization of undetached antiwear surface layers on the base steel (cone raceway) revealed that the films contained crystalline and amorphous regions. A sharp interface (<∼10 nm) that separated the surface layer and base steel was imaged. The surface layer for the cone tested at Λ ∼ 1.1 consisted of Fe, O, and P, whereas that for the cone tested at Λ ∼ 0.3 consisted of Fe, O, P, C, Ca, and S. Various TEM analytical techniques were used to study the segregation of these elements throughout the antiwear surface layer volume.

Elastohydrodynamic Analysis of a Rotary Lip Seal Using Flow Factors

Abstract: An elastohydrodynamic analysis of a rotary lip seal is performed numerically, incorporating both the fluid mechanics of the lubricating film and the elastic deformation of the lip. Asperities on the lip surface dominate the behavior of the flow field in the lubricating film and are taken into account through the use of flow factors. Because previous analyses treated those asperities deterministically, they required very large computation times. The present approach is much less computationally intensive because the asperities are treated statistically. Because cavitation and asperity orientation play important roles, these are taken into account in the computation of the flow factors. An asperity distortion analysis is introduced to model the complex variations in the asperity distribution on the surface of the lip. Results of the analysis show how the operating parameters of the seal and the characteristics of the asperities affect such seal characteristics as the thickness of the lubricating film, rever...

Hydrodynamic performance of a thrust bearing with micropatterned pads

Abstract: The primary objective of this study was to investigate the influence of surface texturing on hydrodynamic lubrication of tilting-pad thrust bearings in terms of bearing power loss, operating temperature, and oil-film thickness. For this purpose, the working faces of six thrust pads from a 228.6-mm-outer-diameter bearing were textured. The textured surface consisted of a system of crossing channels of less than 10 μm in depth. Tests were conducted with a VG68 mineral turbine oil supplied to the bearings at a constant temperature of 50°C and flow rate of 15 L/min. The following parameters were measured: frictional torque, pad and collar temperatures, oil-film thickness, and pressure profiles along two circumferential lines. No significant change in collar and pad temperature could be observed when the patterned bearing was used. However, the textured bearing showed a tendency to exhibit lower power loss especially when an optimum oil flow supply rate was used. At the same time, inlet and outlet film thickne...

Influence of Lubricant Additive and Surface Texture on the Sliding Friction Characteristics of Steel under Varying Speeds Ranging from Ultralow to Moderate

Abstract: A newly developed tribometer that undergoes significant changes in sliding speed, ranging from ultralow (5 μm/s) to moderate (17 cm/s), was used to study the lubricated friction characteristic of steel. In this study, the friction characteristics of stearic acid-formulated oil were studied to clarify the effects of surface roughness or surface roughness texture on friction. Several kinds of specimens having isotropic and anisotropic surface roughness with different textures were used. For an isotropic surface, a rougher surface resulted in low friction under low-speed conditions. The same surface produced high friction under high-speed conditions, where macroscopic hydrodynamic action was predominant. Remarkably less friction was observed in the transverse than in the longitudinal direction when the specimen had anisotropic roughness. This difference was particularly notable under ultra-low-speed conditions. Two other parameters of skewness and kurtosis of roughness distributions show that low friction wa...

Thermofluids Considerations and the Dynamic Behavior of a Finger Seal Assembly

Abstract: Finger seals represent a compliant seal configuration. What differentiates and makes them preferable to the brush seals is their potential hydrodynamic lifting capabilities, and thus their noncontacting nature. The fingers' compliance allows both axial and radial adjustment to rotor excursions without damage to the integrity of the seal. The work to be presented here concerns the mapping of the thermofluid and dynamic behavior of a repetitive section of the newly proposed design of a two-layer finger seal. The assembly contains four high-pressure and four low-pressure fingers arranged axially in a staggered configuration and subject to an axial pressure drop. The numerical three-dimensional temperature and pressure results were obtained using a customized Navier-Stokes–based commercial package, CFD-ACE+. The results were obtained in a parametric fashion where the high-pressure side, the speed of rotation, and the heat transfer coefficient are the controlling parameters; the gas compressibility and the vis...

Elastohydrodynamically Lubricated Ball Bearings with Couple-Stress Fluids, Part I: Steady-State Analysis

Abstract: Conventional elastohydrodynamic lubrication (EHL) analysis of point contacts is extended to include couple-stress effects in lubricants blended with polymer additives. A transient pressure differential equation, generally referred to as a modified Reynolds equation, is derived from the Stokes microcontinuum theory and solved using the finite difference method with a successive over-relaxation scheme. The solution is obtained under isothermal conditions, assuming a suitable exponential relation of pressure-viscosity variation. A nondimensional couple-stress parameter, which can be considered the molecular length of the additives in the lubricant, is used in the analysis. From the results obtained, the influence of the couple-stress parameter on the EHL point contacts is apparent and cannot be neglected. Lubricants with couple stresses provide an increase in the load-carrying capacity and reduction in friction coefficient as compared to Newtonian lubricants. Empirical formulas for the calculation of central...

Experimental Technique to Study Tangential-to-Normal Contact Load Ratio

Abstract: This paper considers the mechanical interaction due to surface roughness and examines the surface theories using the classical definition of coefficient of friction: the tangential-to-normal load ratio. The postulation for maximum static friction is used to experimentally evaluate the contact models. For this purpose, a pin-on-disk test apparatus is employed with the capability of measuring tangential and normal forces for a frictional contact. The tests involve pairs of disks and specimens, that is, steel-on-steel and aluminum-on-aluminum contacts. In each case, profilometer measurements are performed on the disk and the Greenwood and Williamson parameters, are determined. Using the parameters, the theoretical estimates of normal and tangential loads are obtained. The theoretical values of tangential-to-normal contact load ratios are compared with those obtained from measurements for various applied normal loads. The tests utilizing a pin-on-disk apparatus showed a partial agreement between the experimen...

Thermohydrodynamic Modeling of Leading-Edge Groove Bearings under Starvation Condition

Abstract: A recent trend in industry is to use directly lubricated bearings with reduced oil flow to achieve minimum power consumption. However, too much oil flow reduction would cause bearing failure. This work presents a comprehensive thermohydrodynamic algorithm that predicts the performance of the leading-edge groove bearing under low oil flow and starvation conditions. Because of starvation, more analyses are added and extra uncertainties are inevitably introduced in the theoretical models. The influence of starvation on pad temperature, power loss, journal eccentricity, attitude angle, and dynamic coefficients are investigated and discussed. The numerical results are compared with published experimental data with generally satisfactory agreement.

A Limiting Solution for the Dependence of Film Thickness on Velocity in EHL Contacts with Very Thin Films

Abstract: In recent years there have been substantial improvements in the capabilities of numerical modeling of elastohydrodynamic lubricant (EHL) films and it is now possible to analyze a very wide range of conditions rather than needing to rely on extrapolation using classical film thickness regression equations such as those of Dowson and Higginson. However, a new controversy has arisen concerning the film thickness-velocity dependence in EHL contacts at very low speeds and high loads, with some predictions showing a film thickness much less than that predicted by the classical equations. The present article applies the well-established limiting analysis, first presented by Grubin-Ertel, to the inlet of the EHL contact. It is shown that when the load is high and the speed is low (and the pressure gradient is very high in the inlet) an accurate resolution of the inlet pressure rise is critical for the determination of the film thickness. Discretization errors of this type might be responsible for discrepancies be...

Experimental Investigation of the Effect of Speed and Load on Film Thickness in Elastohydrodynamic Contact

Abstract: Recent numerical work has suggested that the elastohydrodynamic film thickness present in smooth-surface, high-pressure, lubricated contacts at low speed parameter conditions may be much thinner than previously predicted, effectively collapsing at very low values of speed parameter. To test this suggestion, central elastohydrodynamic film thickness measurements have been made on three base fluids over a wide range of speed and contact pressure conditions in point contact. None of the fluids tested has shown any evidence of film collapse at low speeds even at high pressure. Instead, the observed behavior has been found to conform closely to the Hamrock-Dowson classical regression equation.

P-S-N/P-F-L Curve Approach Using Three-Parameter Weibull Distribution for Life and Fatigue Analysis of Structural and Rolling Contact Components

Abstract: In almost all cases of the estimation of fatigue limit for structural materials and bearing steels, the practice of using S-N curves obtained by plotting the applied stress against the number of stress cycles has been continuing to date It usually takes into account the probabilistic approach of dealing with the stress or force life for only median life Therefore, a new approach to life estimation and the fatigue limit concept is proposed by using a three-parameter Weibull distribution consisting of a minimum life as the third parameter with a normalized constant Weibull slope m (Shimizu [4]) The values of m obtained in the previous paper from life tests for bearing steel were found to be in the range of m = 13 to 18 In this article, a constant mean value, = 15, is used for the analysis and is considered to be independent of the life distributions for any given experimental stress levels It is made clear that the stress or load life exponent of the P-S-N/P-F-L (probabilistic stress/force life) cur

Experimental results for a heat-sink mechanical seal

Abstract: This work presents experimental results for a heat-sink mechanical seal installed in a 1 x 1.5 x 6 in. ANSI centrifugal water pump. The heat-sink seal is constructed of a stainless steel substrate with an electrodeposited pin fin micro-heat sink located within 3 mm of the end face. Each pin has a ten-sided polygon cross section with a flat-to-flat diameter of 675 μm, a height of 856 μm, and a 300-μm edge-to-edge spacing. The end face is coated with a WC thin film that forms the wear surface for the rotating ring. The heat-sink seal's effectiveness is demonstrated in a significant reduction of both the seal interface and seal chamber temperature when compared to a similar industry standard seal. The heat-sink seal removes 750 W of heat and reduces the stationary ring temperature by 80° C under dead-head conditions using only 1.6 W of coolant pumping power.

Current Status of Hybrid Bearing Damage Detection

Abstract: Advances in material development and processing have led to the introduction of ceramic hybrid bearings for many applications. The introduction of silicon nitride hybrid bearings into the high pressure oxidizer turbopump, on the space shuttle main engine, led NASA to solve a highly persistent and troublesome bearing problem. Hybrid bearings consist of ceramic balls and steel races. The majority of hybrid bearings utilize Si3N4 balls. The aerospace industry is currently studying the use of hybrid bearings and naturally the failure modes of these bearings become an issue in light of the limited data available. In today s turbine engines and helicopter transmissions, the health of the bearings is detected by the properties of the debris found in the lubrication line when damage begins to occur. Current oil debris sensor technology relies on the magnetic properties of the debris to detect damage. Since the ceramic rolling elements of hybrid bearings have no metallic properties, a new sensing system must be developed to indicate the system health if ceramic components are to be safely implemented in aerospace applications. The ceramic oil debris sensor must be capable of detecting ceramic and metallic component damage with sufficient reliability and forewarning to prevent a catastrophic failure. The objective of this research is to provide a background summary on what is currently known about hybrid bearing failure modes and to report preliminary results on the detection of silicon nitride debris, in oil, using a commercial particle counter.

Identification of Journal Bearing Force Coefficients under High Dynamic Loading Centered Static Operation

Abstract: Lightly damped rotor bearing systems experience large amplitudes of vibration when traversing critical speeds. Bearing linearized force coefficients, strictly valid for minute motions about an equilibrium position, may not be reliable for design or troubleshooting in rotordynamics predictive analyses. Experiments assessing the dynamic forced response of a plain journal bearing undergoing large orbital motions due to single-frequency excitation forces were conducted in a test rig. The short test bearing of slenderness ratio L/D = 0.25 has a nominal radial clearance of 0.127 mm (5 mils). Tests were conducted at three rotor speeds (900, 1800, and 2700 rpm), three feed pressures (1, 3, and 6 psig), and three excitation frequencies (15, 30, and 45 Hz). Baseline bearing motions due to shaft runout are recorded and subtracted in the parameter identification procedure. The forces exerted on the bearing induce large orbital motions with peak amplitudes exceeding 50% of the nominal bearing clearance. Identified cro...

Time-dependent film formation from ZnDTPs and nonphosphorus antiwear agents

Abstract: Electrical contact resistance (ECR) studies, X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and X-ray absorption near-edge structure spectroscopy (XANES) were carried out on specimens run with oils containing 0.05% phosphorus as either primary zinc dialkyldithio-phosphate (ZnDTP) or secondary ZnDTP in Group II base oil. A series of progressively longer ECR experiments were run on each ZnDTP. At the end of each run in the series, the ball was removed and preserved for surface analysis. The surface analyses were designed to observe chemical species deposited on the surface and within the deposited films. The observation of surface phenomena at different intervening times during the ECR experiment, allowing for characterization of the maturing antiwear film, was the distinct feature of these experiments. In general, short ECR experiments gave poorer films than long ECR experiments. Atomic concentrations versus depth were determined from AES. Quite strikingly, the antiwear films fo...

Synergistic Effects of Boron-, Sulfur-, and Phosphorus-Containing Lubricants in Boundary Lubrication of Steel Surfaces

Abstract: The effectiveness of various combinations of borate-, sulfur-, and phosphorus-containing additives blended in gear oil to form antiwear tribofilms on steel surfaces sliding in the boundary lubrication regime was investigated experimentally. The formation of protective tribofilms and their tribochemical activity in the temperature range of 32–100°C were analyzed in terms of coefficient of friction and contact voltage results. The antiwear performance obtained with each blend was evaluated by wear volume calculations based on surface profilometry measurements. Scanning electron microscopy studies provided insight into the dominant wear mechanisms at various temperatures. The tribological performance of the tribofilms and associated wear mechanisms were found to depend strongly on the type of additive(s), tribofilm composition, and temperature. This study has revealed that sulfide-dominated tribofilms produce lower friction, whereas borate- and phosphate-dominated tribofilms are more effective in increasing ...

Fixed-Geometry, Hydrodynamic Bearing with Enhanced Stability Characteristics

Abstract: This article presents the results of a successful bearing optimization study aimed at identifying a fixed-geometry, hydro-dynamic journal bearing that does not suffer from the low load instability typical of this class of bearings. This goal was met through optimization of a fairly simple objective function based on the rigid rotor whirl-speed ratio, using a constrained, nonlinear algorithm based on sequential quadratic programming. In the interests of reducing computational time, a two-dimensional isoviscous formulation of the Reynolds equation was used for this work. The equation was solved using a finite element approach. This article includes a discussion of the optimization approach, the finite element solution approach, the resulting bearing design, and its performance characteristics. It concludes with an application example comparing the optimized bearing's predicted performance to a tilting-pad bearing's predicted performance for a centrifugal compressor-like rotor. The mismatch between shaft and...

Numerical Investigation on the Effects of Machining-Induced White Layer during Rolling Contact

Abstract: It is well known that a thin phase-transformed white layer can be formed on component surfaces produced by hard machining. However, it is not clear as to how the white layer affects component performance, for example, in rolling contact fatigue. This study aims to determine the effects of white layer and associated residual stress on rolling contact stresses and strains. It is nearly impossible for an experimental study to identify the effects of white layer alone on rolling contact. Furthermore, small-scale contact stresses and strains (less than 30 μm) of the phase-transformed region are difficult to measure using the current experimental techniques. Therefore, a finite element analysis simulation model of rolling contact incorporating machining-induced surface integrity has been developed in this study. Three cases were investigated to decouple the effects of surface integrity factors: surface with white layer only, surface with residual stress only, and surface with white layer and residual stress. Th...