Showing papers in "Tribology Transactions in 2011"

A Review of Elasto-Hydrodynamic Lubrication Theory

Abstract: The development and understanding of elastohydrodynamic lubrication (EHL) can be traced back to the beginning of the previous century. However, it was not until 1949 that the first real solution of the problem was published. Since then, the technology has evolved enormously. In the current article a summary of these developments is given. Smooth surface EHL has become well established. Numerical methods, analytical solutions, and experimental techniques have become mature. Focus areas of research today are thermal EHL, starved EHL, friction (non-Newtonian lubricants), roughness, and grease. The scope of EHL is so wide that the authors needed select the topics of focus in this article. Therefore, in addition to the general overview of the areas of friction, analytical methods, starved EHL, and grease EHL are highlighted in this article.

Micropitting Modelling in Rolling–Sliding Contacts: Application to Rolling Bearings

Abstract: In this article an engineering approach is described to model micropitting in rolling–sliding, heavily loaded lubricated contacts. The competitive mechanism between surface fatigue and mild wear is captured in the present approach as well as the effects of deterministic surface microgeometry (e.g., roughness). The fatigue model is based on the Dang Van fatigue criterion and the mild wear model uses a modified Archard approach. The complete modeling scheme is validated experimentally first using laboratory-controlled conditions, where the surface topography is varied as well as the operating conditions in the contact. Then the model is applied to describe the behavior of full-bearing tests. The behavior of the model agrees well with the experimental observations, qualitatively.

On the Modeling of Elastic Contact between Rough Surfaces

Abstract: The contact force and the real contact area between rough surfaces are important in the prediction of friction, wear, adhesion, and electrical and thermal contact resistance. Over the last four decades various mathematical models have been developed. Built on very different assumptions and underlying mathematical frameworks, model agreement or effectiveness has never been thoroughly investigated. This work uses several measured profiles of real surfaces having vastly different roughness characteristics to predict contact areas and forces from various elastic contact models and contrast them to a deterministic fast Fourier transform (FFT)-based contact model. The latter is considered “exact” because surfaces are analyzed as they are measured, accounting for all peaks and valleys without compromise. Though measurement uncertainties and resolution issues prevail, the same surfaces are kept constant (i.e., are identical) for all models considered. Nonetheless, the effect of the data resolution of measured sur...

Studies of the Influence of Graphite and MoS2 on the Tribological Behaviors of Hybrid PTFE/Nomex Fabric Composite

Abstract: Hybrid polytetrafluoroethylene (PTFE)/Nomex (DuPont, USA) composite specimens were prepared with graphite and MoS2 as fillers. The friction and wear tests were conducted under dry sliding conditions at a speed of 0.26 m/s under various loads and environmental temperatures. The results showed that the incorporation of graphite was effective in reducing the wear of the hybrid PTFE/Nomex fabric composite, whereas the MoS2 filler was unfavorable for improving the wear resistance. X-ray photoelectron spectroscopy (XPS) results revealed that MoS2 was oxidized into MoO3 during sliding. The absence of the layered structure of MoO3 resulted in a higher wear rate of fabric composite. Differential scanning calorimetry (DSC) analysis of unfilled and graphite-filled composites and their debris indicated that graphite increased the thermal stability of the unfilled composite and therefore enhanced the wear resistance.

Wear-Resistant PTFE/SiO2 Nanoparticle Composite Films

Abstract: Polytetrafluoroethylene (PTFE) is a polymer that is well known for its exceptional tribological properties and, as such, it is commonly used to reduce the coefficient of friction between surfaces. In recent years it has also been established that by incorporating nanoparticle fillers in PTFE, it is possible to extend the polymer's life by reducing its wear rate. Although much study has been placed on bulk PTFE, very little study has been focused on thin films. This article demonstrates that SiO 2 nanoparticles can be used as a filler to significantly reduce the wear of PTFE thin films while also maintaining a low coefficient of friction. The wear resistance and coefficient of friction of PTFE/SiO 2 composite films on stainless steel substrates were tested using a linear reciprocating tribometer and compared to pure PTFE films and bare stainless steel to evaluate the benefit of incorporating the SiO 2 filler in the film. The composite films showed a significant improvement in wear resistance when compared ...

Three-Dimensional Thermohydrodynamic Analyses of Rayleigh Step Air Foil Thrust Bearing with Radially Arranged Bump Foils

Abstract: A three-dimensional (3D) thermohydrodynamic (THD) model for air foil thrust bearings (AFTBs) is presented. The nonisothermal Reynolds equation is solved using pressure boundary conditions at the cooling air plenum considering local temperature-dependent viscosity and density. Air film temperature is calculated using the 3D energy equation with thermal boundary conditions at the top foil, thrust runner, and top foil’s leading edge. The cooling air plenum distributes the cooling air to multiple radially arranged cooling channels. The plenum temperature and pressure are found from mass and energy balance equations applied to the plenum. Temperature fields of the top foil, bump foils, thrust disc runner, bearing plate, and cooling air channels are also solved through appropriate energy balance equations with their surroundings. A robust computational algorithm with multiple iteration loops was developed to find all the temperature fields. THD analyses were performed for AFTB with outer radius of 50 mm up to 1...

Evaluation of Vapor Pressure and Ultra-High Vacuum Tribological Properties of Ionic Liquids

Abstract: Ionic liquids are a class of salts that incorporate polyatomic anions and cations. These materials are typically viscous fluids at room temperature. The fluids are generally characterized as possessing negligible vapor pressures under ambient conditions. These beneficial properties have led us to study the effectiveness of ionic liquids containing both organic cations and anions for use as lubricants with space applications. This article presents vapor pressure measurements for four ionic liquids as well as friction coefficient data from a spiral orbit tribometer in the boundary lubrication regime under simulated space conditions using stainless steel tribocouples. In addition, we present the first tribological data on mixed ionic liquids and an ionic liquid additive. Postmortem infrared and Raman analyses of the balls and races indicates that the major degradation pathway for these organic ionic liquids is similar to those of other carbon-based liquid lubricants; that is, deterioration of the organic str...

Influence of Surface Roughness Topography on Properties of Water-Lubricated Polymer Bearings: Experimental Research

Abstract: Surface smoothness of journal and bushing has significant influence on properties of sliding bearings. Relevant literature contains a number of theoretical works connected with the subject of surface roughness pattern and height. Calculations based on various models indicate that the structure of sliding surface influences bearing's hydrodynamic properties. One of the main conclusions is that roughness transversal to the sliding direction can increase hydrodynamic capacity of a bearing (H. Christensen and K. Tonder, 1973, Journal of Lubrication Technology, Vol. 4, pp 166–172). In bearings lubricated by a low-viscosity liquid such as water, the influence of roughness is of particular importance. This is due to low hydrodynamic film thickness and the resulting limited hydrodynamic capacity of these types of bearings. In fact, these are the reasons for deciding to conduct experimental research aimed at verifying the influence of roughness pattern on bearing properties. Research conducted for a selected group...

Current Status of and Future Innovations in Rolling Bearing Modeling

Abstract: Current state-of-the-art in modeling the performance of rolling bearings is reviewed in terms of fundamental analytical formulations and the development of computer codes for performance simulations. Some of the basic equations, which constitute the foundation of the various types of models, are reviewed before presenting a schematic approach for the development of rolling bearing models. Some of the key developments over the last several decades that have led to the current status of rolling bearing modeling are presented. Though some of the models are restricted to the developing organizations, and their use is only available in terms of application support, others have been packaged in the form of commercially available software products. These models provide immediate practical implementation of several tribological disciplines in their most up-to-date and advanced form. With the advancements in high-speed computing technologies, solutions to the most sophisticated analytical formulations have become ...

Wear and Friction Characteristics of Magnetorheological Fluid under Magnetic Field Activation

Abstract: In this article, wear and friction characteristics of a magnetorheological (MR) fluid were studied under different magnetic fields. Using a pin-on-disc tribometer, the wear loss and friction coefficient were obtained with and without a magnetic field. The friction and wear of three typical materials under magnetic field were investigated at various normal loads and rotating speeds. After the wear tests, the worn surfaces of specimens were observed using a scanning electron microscope (SEM) in order to investigate the wear mechanisms, and the contacting surfaces were analyzed by energy-dispersive X-ray spectroscopy (EDS) to investigate the variation of elemental composition on the worn surface. Test results showed that the MR fluid exhibits better tribological characteristics under a magnetic field compared to the case without a magnetic field. The general morphology of the MR particles and wear debris was observed to assist with the analysis of friction and wear. The SEM micrographs and EDS spectra of the...

Engine Wear Monitoring with OLVF

Abstract: In this article, an on-line visual ferrograph (OLVF) was used to monitor the wear condition of a vehicle engine during a bench test. A quantitative index of particle coverage area (IPCA) and wear debris ferro-image were obtained via OLVF to characterize wear degrees. On-line ferrograph analysis indicated some problems with the engine from about the 17th hour. A linear threshold value method to identify running condition of engines with the OLVF data was proposed. The OLVF data were in a fault zone after running for 25 h, when a base bolt broke and the engine stopped working during the test. The engine was disassembled and inspected after the test. Several wear scars were found on the cylinder walls as well as on the bearing surfaces. The oil was added at the 26th and 55th hours, which was also reflected by the variation of IPCA. Moreover, several off-line analyses were carried out in parallel. Off-line ferrograph data did not give any information about the abnormality and only showed stable wear debris co...

Hydrodynamic Lubrication of Surfaces with Asymmetric Microdimple

Abstract: Surface texturing has been recognized as a feasible method of surface engineering to improve the lubrication properties of mechanical components. This work focuses on the hydrodynamic lubrication properties of the textured surface with asymmetric microdimples. The validity of the Reynolds equation in asymmetric microdimples is discussed based on theoretical analysis and numerical simulation. The effects of modified Reynolds number, motion directions, and parameters of the microdimple profile are investigated. The following results were obtained: Firstly, the Reynolds equation is invalid under almost all of the general conditions used in experiments and industries. Secondly, the ultimate load-carrying capacity can be taken as a new characteristic parameter to assess the lubrication property of a textured surface. Thirdly, the motion directions have a great effect on the lubrication properties. The positive motion can generate more stable load capacity under all of the modified Reynolds numbers employed, ev...

Tribocorrosion Behaviors of Inconel 625 Alloy Sliding against 316 Steel in Seawater

Abstract: Passive alloys require not only high resistance to corrosion but also excellent tribological properties for seawater equipment applications. In this research, the tribocorrosion behaviors of Inconel 625 alloy sliding against 316 stainless steel were investigated using a ring-on-block test rig in artificial seawater and distilled water. The results showed that the friction coefficients were larger for sliding in distilled water as compared to seawater. However, the wear loss of Inconel 625 and 316 steel alloys was lower for sliding in distilled water than that in seawater. Interestingly, the corrosion of seawater obviously accelerated the wear. Moreover, with the increase of normal load, the wear loss increased but the average friction coefficient decreased in both aqueous media. The synergistic effect between wear and corrosion was significant, due to the mechanical removal of passive film. Seawater salinity does significantly influence the friction coefficient and wear loss. The Inconel 625 alloy has a m...

On the Steady Performance of Hydrostatic Thrust Bearing: Rabinowitsch Fluid Model

Abstract: The present theoretical analysis is to investigate the effect of non-Newtonian pseudoplastic lubricant (lubricant blended with viscosity index improver and viscosity thickener)–Rabinowitsch fluid model on the performance of externally pressurized hydrostatic thrust bearing. The expression for pressure has been derived using an energy integral approach. The load capacity and frictional torque of the bearing have also been numerically calculated for various values of viscosity index improver along with the centrifugal effects. The results so obtained are compared and found to be in good agreement with the earlier theoretical and practical results of Dowson, and the effect of viscosity index improver is also analyzed.

Influence of aerated lubricants on gear churning loss - An engineering model

Abstract: It is currently assumed that churning losses can be described by using only two physical parameters representative of the lubricant properties, i.e., density and viscosity. To verify this hypothesis, a number of transient measurements were carried out on a specific gear test rig over a range of oil temperatures. It appears that, for high temperatures and/or high rotational speeds, the drag torque can suddenly increase with an increasing Reynolds number. Extensive on-line lubricant aeration measurements were conducted showing that this particular behaviour can be related to churning losses when the fraction of air in the lubricant reaches a certain threshold. In order to quantify the influence of oil sump aeration on churning losses, a simplified original model, based on surface tension and lubricant aeration is proposed. This study shows clearly that density and viscosity are not sufficient to estimate churning losses under some specific operating conditions and emphasises the need to account for other physical properties of the lubricant.

Tribological Characteristics of Sustainable Fiber-Reinforced Thermoplastic Composites under Wet Adhesive Wear

Abstract: The friction and specific wear rate of sustainable kenaf fiber–reinforced polyurethane composites were investigated against stainless steel counterface and under wet contact conditions. The new composites were evaluated at different applied loads (50–80 N), sliding distances (up to 2.7 km), and fiber mat orientations. Scanning electron microscopy (SEM) was used to observe the damage features on the worn surfaces. The results revealed that sustainable kenaf fibers assisted in enhancing the wear and frictional performance of the polyurethane thermoplastic composite by about 59 and 90%, respectively. Operating parameters and mat orientation controlled the wear and the frictional behavior of the composite. Better wear performance was exhibited at high loads and when the fiber mats were oriented perpendicularly to the sliding direction. Observations of the worn surfaces revealed different features of damage such as microcracks, fiber tearing, fiber detachment, and delamination. However, there was no trace of f...

Material Inclusion Factors for Lundberg-Palmgren–Based RCF Life Equations

Abstract: Material inclusions in the form of hydrogen embrittlement, carbides, etc., are by-products of manufacturing processes and commonly present in bearing steel. The objective of this study was to develop a life equation for rolling contact fatigue phenomenon that accounts for the effects of inclusions. The life equation was developed using the fatigue results previously obtained using the damage model (Jalalahmadi and Sadeghi, Journal of Tribology vol. 132, 2010). Four modifying factors counting for effects of the stiffness, size, depth, and number of the inclusions are used to modify the life equation. These modifying coefficients are extracted from the simulations obtained from the Voronoi Finite element (FE) model and the Lundberg-Palmgren-based fatigue criterion (Jalalahmadi and Sadeghi, Journal of Tribology vol. 131, 2009). These simulations predict Weibull slopes and L 10 lives that are in good agreement with the previous theoretical and experimental results. It is seen that as inclusions become larger ...

Effects of B100 Biodiesel on Injector and Pump Piston

Abstract: In this study, the effects of biodiesel use in a diesel engine on an injector and fuel injection pump piston were experimentally analyzed. To this end, two engines with the same technical specifications were used; petroleum diesel was used in one of the engines and 100% (B100) biodiesel was used in the other engine. After the engines were run for 200 h, their injectors and pump pistons were examined and compared by performing scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis measurements. SEM and EDX analyses showed significant structural changes on the surfaces of the injector nozzle and pump piston in the event that B100 is used.

A Series Solution of the Long Porous Slider

Abstract: A series solution of the long porous slider problem where fluid is injected through the porous bottom is obtained using the homotopy perturbation method (HPM). Similarity solutions of coupled nonlinear ordinary differential equations resulting from the momentum equation are obtained. Numerical results are obtained and discussed for different values of Reynolds number of the velocity field. The numerical results demonstrate the validity and applicability of the method.

On the Effect of Isotropic Hardening on the Coefficient of Restitution for Single or Repeated Impacts Using a Semi-Analytical Method

Abstract: A semi-analytical method is proposed for the simulation of moderately high-speed impacts of a relatively hard particle on an elastic–plastic substrate. Potential applications of the current modeling approach include shot-peening and ultrasonic peening as well as impacts on functional surfaces such as mechanical seals or bearings. The coefficients of restitution for single impacts on an elastic–perfectly plastic semi-infinite body or one with isotropic hardening have been computed for various impact velocities. The current approach is three dimensional and numerically efficient in terms of CPU time and memory space required. Numerical results are in good agreement with those obtained by other authors for elastic–perfectly plastic behavior. Coefficients of restitution resulting from repeated impacts at a constant initial velocity are also studied. Finally, equations fitting these numerical results are proposed.

Effects of Lubrication Mode on Friction and Heat Partition Coefficients at the Tool–Work Material Interface in Machining

Abstract: The quantification of friction coefficient along the tool–work material interface in machining remains an issue in tribology. This article aims at identifying the evolution of friction coefficient for a large range of sliding velocity during the machining of an AISI 4140 steel (290 HB) with a TiN-coated carbide tool. The influence of various lubricants (straight oil or emulsion) and lubrication modes (flow or mist) is investigated and compared to a dry sliding situation (dry machining). It has been shown that, in dry machining, the friction coefficient decreases with the sliding velocity until reaching a lower limit around 0.2. On the contrary, the presence of a straight oil significantly decreases friction coefficients to a value around 0.1. Emulsion enables a significant decrease of friction coefficient to around 0.2 for low sliding velocities, whereas its action is absent for higher sliding velocities. Oil mist exhibits an intermediate behavior. Finally, it has been shown that all kinds of lubrication ...

A New Method to Visualize Grease Flow in a Double Restriction Seal Using Microparticle Image Velocimetry

Abstract: A new method to visualize and quantify grease flow in between two sealing lips or, in general, a double restriction seal is presented. Two setups were designed to mimic different types of seals; that is, a radial and an axial shaft seal. The flow of the grease inside and in between the sealing restrictions was measured using microparticle image velocimetry. The results show that grease flow due to a pressure difference mainly takes place close to the rotating shaft surface with an exponentially decaying velocity profile in the radial direction. Consequently, contaminants may be captured in the stationary grease at the outer radius, which explains the sealing function of the grease.

Thrust Bump Air Foil Bearings with Variable Axial Load: Theoretical Predictions and Experiments

Abstract: Thrust air foil bearings are critical components in high-efficiency turbomachinery, such as two-stage compressors subjected to large and irregular axial forces. In this article, a model of thrust bump foil bearings that predicts deflection with variable axial load is developed assuming no tilting effect of the thrust collar. To predict the air clearance, deflection of the elastic foundation was used in the air film height equation. Combined Dirichlet and Neumann-type boundary conditions were used for static load performance predictions. To verify the theoretical model, tests were performed with three different thrust air foil bearings with outer radii of 45, 50, and 55 mm. The rotating speed ranged from 10,000 to 25,000 rpm. From the test results, the model using nonlinear stiffness was in better agreement with the experimental results than the model using linear stiffness.

Stiffness and Damping Coefficient Estimation of Compliant Surface Gas Bearings for Oil-Free Turbomachinery

Abstract: Foil gas bearings are a key technology in many commercial and emerging Oil-Free turbomachinery systems. These bearings are non-linear and have been difficult to analytically model in terms of performance characteristics such as load capacity, power loss, stiffness and damping. Previous investigations led to an empirically derived method, a rule-of-thumb, to estimate load capacity. This method has been a valuable tool in system development. The current paper extends this tool concept to include rules for stiffness and damping coefficient estimation. It is expected that these rules will further accelerate the development and deployment of advanced Oil-Free machines operating on foil gas bearings

Comparison of the Effects of Biodiesel and Mineral Diesel Fuel Dilution on Aged Engine Oil Properties

Abstract: Dilution of engine oil occurs when fuel is injected late in the combustion cycle to regenerate the diesel particulate filter used for trapping particulate emissions. Fuel dilution reduces oil viscosity and the concentration of engine oil additives, potentially compromising lubricant performance. Biodiesel usage may compound these issues due to its oxidative instability, and its higher boiling point compared to mineral diesel potentially causes it to concentrate more in the oil sump. In this work, different amounts of mineral diesel and biodiesel (soy methyl ester, SME) were combined with 15W-40 CJ-4 diesel engine oil in laboratory oil aging experiments. Fuel was added and oil samples were withdrawn at periodic intervals. The oils were analyzed using typical oil analysis procedures to determine their condition, and wear evaluations under boundary lubricating conditions were determined using a high-frequency reciprocating rig (HFRR). Results showed that fuel dilution accelerated engine oil degradation, with...

A Comparison Study Between Acoustic Sensors for Bearing Fault Detection Under Different Speed and Load Using a Variety of Signal Processing Techniques

Abstract: The use of ultrasonic sensor technology to detect incipient and evolving defects in rotating components such as bearings and gears is more desirable due to their high resolution. In a previous study, the sensitivity of a variety of sensors including an air-coupled ultrasound transducer to bearing faults was analyzed and thoroughly discussed. This article investigates the effectiveness of two ultrasonic sensors, namely, air-coupled and piezoelectric ultrasound transducers for rolling element bearings damage diagnostics. The former is a noncontact sensor and the latter is a contact sensor. An accelerometer was also used as the baseline sensor for comparison purposes. A series of tests was carried out on a laboratory test rig running with defective and undamaged healthy bearings under variable shaft speeds and several radial loads. The data were analyzed using selected signal processing techniques covering time, frequency, and advanced joint time–frequency domains. The results showed that certain acoustic fe...

Effect of Cryogenic Treatment on the Microstructure and Wear Resistance of X45Cr9Si3 and X53Cr22Mn9Ni4N Valve Steels

Abstract: The effect of cryogenic treatment on the microstructure and wear resistance of X45Cr9Si3 and X53Cr22Mn9Ni4N valve steels is investigated by conducting an optical and scanning electron microscopy (SEM) study and reciprocating wear test (ASTM standard G-133). The materials are subjected to shallow (193 K) and deep cryogenic treatment (85 K), and the microstructure and wear resistance are compared with those of conventional heat treatment. The wear test data show that for a maximum load of 30 N and 5 Hz frequency, the wear resistance of the X45Cr9Si3 and X53Cr22Mn9Ni4N improved by 15.2 and 10.3%, respectively, due to shallow cryogenic treatment (SCT) and 42.39 and 22.08%, respectively, due to deep cryogenic treatment (DCT) compared to that of the conventional heat treatment. The enhancement in the wear resistance of the X45Cr9Si3 and X53Cr22Mn9Ni4N valve steels observed through the optimized DCT was 46.51 and 27.8%, respectively, compared to the samples without cryogenic treatment. The microstructural study ...

A Combined Molecular Dynamics and Finite Element Analysis of Contact and Adhesion of a Rough Sphere and a Flat Surface

Abstract: A combined molecular dynamics and finite element model and simulation of contact and adhesion between a rough sphere and a flat surface has been developed. This model uses the results of molecular dynamics (MD) simulations, obtained using an embedded atom potential, of a nanoscale Ru-Ru asperity contact. A continuum finite element model of an elastic–plastic microscale Ru-Ru contact bump is then created. In this model, the surface roughness is represented by a system of nanoscale asperities, each of which is represented by a nonlinear hysteretic force vs. distance relationship. The nonlinear hysteretic character of these relations is determined from curve-fits of the MD results. Load vs. interference and contact area vs. interference are determined using this two-scale model for loading and unloading. Comparisons with a single-scale continuum model show that the effect of the nanoscale asperities is to reduce both the adhesion and the real area of contact. The choice of Ru as the material for this work is...

Rolling-Element Fatigue Testing and Data Analysis - A Tutorial

Abstract: In order to rank bearing materials, lubricants and other design variables using rolling-element bench type fatigue testing of bearing components and full-scale rolling-element bearing tests, the investigator needs to be cognizant of the variables that affect rolling-element fatigue life and be able to maintain and control them within an acceptable experimental tolerance. Once these variables are controlled, the number of tests and the test conditions must be specified to assure reasonable statistical certainty of the final results. There is a reasonable correlation between the results from elemental test rigs with those results obtained with full-scale bearings. Using the statistical methods of W. Weibull and L. Johnson, the minimum number of tests required can be determined. This paper brings together and discusses the technical aspects of rolling-element fatigue testing and data analysis as well as making recommendations to assure quality and reliable testing of rolling-element specimens and full-scale rolling-element bearings.

A Theoretical and Experimental Study on Characteristics of Water-Lubricated Double Spiral–Grooved Seals

Abstract: The spiral-grooved seal is a prime candidate for application to liquid oxygen (LOX) turbopumps. A theoretical model of double spiral–grooved seals dealing with the viscosity–temperature relation is presented. The effect of operating parameters (rotational speed, supply pressure) and configurative parameters (depth of spiral groove) on the basic static characteristics (opening force, leakage, static friction torque, face temperature, and power loss) of double spiral–grooved seals are examined. Comparisons are presented between measurements and theoretical predications for a narrow spiral-grooved face seal with an average diameter of 100 mm, operating at high speed and using water as a test fluid. The theoretical and experimental results indicate that the temperature on seal faces rises and friction torque increases with the speed and supply pressure, and these increases vary slightly with the depth of the spiral groove. These findings lend great theoretical and experimental insights to the design of face s...