Showing papers in "Tribology Transactions in 2008"

Application of Nanofluids in Minimum Quantity Lubrication Grinding

Abstract: This research investigated the wheel wear and tribological characteristics in wet, dry, and minimum quantity lubrication (MQL) grinding of cast iron. Water-based Al2O3 and diamond nanofluids were applied in the MQL grinding process and the grinding results were compared with those of pure water. During the nanofluid MQL grinding, a dense and hard slurry layer was formed on the wheel surface and could benefit the grinding performance. Experimental results showed that G-ratio, defined as the volume of material removed per unit volume of grinding wheel wear, could be improved with high-concentration nanofluids. Nanofluids showed the benefits of reducing grinding forces, improving surface roughness, and preventing workpiece burning. Compared to dry grinding, MQL grinding could significantly reduce the grinding temperature.

Effect of Particle Size on the Wear Resistance of Alumina-Filled PTFE Micro- and Nanocomposites

Abstract: It was long supposed that the ability of hard particle fillers to reduce the wear rate of unfilled PTFE (typically ∼ 10− 3 mm 3 /Nm) by an order of magnitude or more was limited to fillers of microscale or greater, as nano-fillers would likely be encapsulated within the large microscale PTFE wear debris rather than disrupting the wear mechanism. Recent studies have demonstrated that nano-fillers can be more effective than microscale fillers in reducing wear rate while maintaining a low coefficient of friction. This study attempts to further elucidate the mechanisms leading to improved wear resistance via a thorough study of the effects of particle size. When filled to a 5% mass fraction, 40- and 80-nm alumina particles reduced the PTFE wear rate to a ∼ 10−7 mm 3 /Nm level, two orders of magnitude better than the ∼ 10−5 mm 3 /Nm level with alumina micro-fillers at sizes ranging from 0.5 to 20 μm. Composites with alumina filler in the form of nanoparticles were less abrasive to the mating steel (stainless 3...

Design, Fabrication, and Performance of Open Source Generation I and II Compliant Hydrodynamic Gas Foil Bearings

Abstract: Foil gas bearings are self-acting hydrodynamic bearings made from sheet metal foils comprised of at least two layers. The innermost “top foil” layer traps a gas pressure film that supports a load while a layer or layers underneath provide an elastic foundation. Foil bearings are used in many lightly loaded, high-speed turbomachines such as compressors used for aircraft pressurization and small microturbines. Foil gas bearings provide a means to eliminate the oil system leading to reduced weight and enhanced temperature capability. The general lack of familiarity of the foil bearing design and manufacturing process has hindered their widespread dissemination. This paper reviews the publicly available literature to demonstrate the design, fabrication, and performance testing of both first- and second-generation bump-style foil bearings. It is anticipated that this paper may serve as an effective starting point for new development activities employing foil bearing technology.

Tribological Behavior of Deagglomerated Active Inorganic Nanoparticles for Advanced Lubrication

Abstract: The tribological behavior of novel, deagglomerated, and active molybdenum disulfide (MoS2) nanoparticles as additives in paraffin oil is presented. In a novel approach, the MoS2 nanoparticles were activated by their intercalation with organic molecules, particularly triglycerides (canola oil) and lecithin (source of phosphorus). A four-ball tribological test setup was used to measure the wear scar diameter, the coefficient of friction, and the extreme pressure properties of such formulated paraffin oils. The results showed significant influence of this uniquely designed MoS2 nanostructured additive on the coefficient of friction (0.07), the wear scar diameter, and the extreme pressure (315 kg) properties of the paraffin oil. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive x-ray spectroscopy (EDS) were also used for investigating size, the surface morphology, and the elemental composition of the nanoengineered lubricant. The characterization revealed a part...

Lubricant Replenishment in Rolling Bearing Contacts

Abstract: It is generally assumed that starved lubricated contacts in rolling element bearings are replenished by side flow. The rolling elements partly push the lubricant to the side of the contacts after which the re-flow will provide fresh lubricant for the next rolling element. This re-flow is driven by surface tension and restricted by the viscosity. Hence, thick oils or greases may yield problems here, which is generally observed in practice. This paper quantifies this re-flow by means of a numerical model using a so-called thin layer/film assumptions. The results here show that this form of replenishment may happen to some extent in single contacts but for sure not in rolling element bearings. There are two reasons for this: the time between successive overrollings is too short and secondly, the centrifugal effects on the inner ring will drive the flow in a vertical direction rather than transversely. This applies to cylindrical roller bearings (CRB)-type of surfaces. This may be different for tapered or sph...

Acoustic Emission and Its Relationship with Friction and Wear for Sliding Contact

Abstract: Further investigation of the relationships between friction and wear properties and the characteristics of acoustic emission was conducted in the case of dry and grease-lubricated sliding contact using a ball-on-cylinder testing apparatus. The effect of contamination simulated by the inclusion of glass bead particles was also explored. Experiments were performed at sliding speeds ranging from 0.09 m/s to 1.47 m/s, while maintaining a fixed load and duration. As a first observation and contrary to what could be expected, the higher speed did not contribute to the decrease in friction interpreted by a worsening of the starved regime that had a consequence of increasing wear. However, the results revealed a good correlation between the friction coefficient and acoustic emission (AE) rms voltage for dry sliding. Such a relationship may allow the prediction of a reasonable friction coefficient μ from an AE signal. It was also determined that the friction work correlated well with the corresponding integrated A...

Experimental and Molecular Dynamics Simulations of Tribochemical Reactions with ZDDP : Zinc Phosphate-Iron Oxide Reaction

Abstract: Zinc phosphate glass is considered to be the main constituent of tribofilms generated under boundary lubrication with zinc dialkyldithiophosphate (ZDDP), a well-known antiwear additive. The reaction occurring during friction between zinc phosphate glasses and steel native iron oxide layer is investigated by both an experimental approach and by Molecular Dynamics simulations (MD). The importance of this “tribochemical” reaction in the general ZDDP antiwear process is discussed.

Effects of Ceramic Fiber on the Friction Performance of Automotive Brake Lining Materials

Abstract: Friction material containing aluminum-silicon fiber was prepared. The effects of the aluminum-silicon fiber content on fade, recovery, and wear properties of the friction material were studied using a friction tester with a constant speed. Morphologies of the wear surfaces were observed by scanning electron microscopy (SEM). It was found that the heat fading resistance property of friction material was clearly improved when the content of aluminum-silicon wool was more than 5 wt.%, but the property of recovery declined and the wear rate increased slightly at the same time. The wear mechanisms were adhesive and abrasive, caused by the zircon sand, for the semi-metal friction material, while the abrasive wear of hybrid fiber reinforced composites was caused by cracked ceramic fibers and zircon sand.

Influence of Tip Relief Modification on the Wear of Spur Gears with Asymmetric Teeth

Abstract: Recently, spur gears with asymmetric teeth have been considered a way of increasing performance while maintaining the gearbox dimensions. Asymmetric teeth have different pressure angles on drive and coast sides. They provide, among other advantages, a high bending strength and low vibration. In spur gears with asymmetric teeth, wear has been observed to be a major failure mode. In this study, the impact of tip relief modification and pressure angle on the wear of spur gears with asymmetric teeth is numerically investigated. Here, the focus is on sliding wear. A wear model based on Archard's equation is employed to predict wear depth. The pressure angle and the tip relief are parameterized. In the analysis, instantaneous contact loads and Hertz pressures are used in wear depth calculations. It is shown that as the amount of the tip relief increases, the wear depth, particularly at the beginning and end of the mesh, decreases. As the number of wear cycles increases, the effect of the tip relief modification...

Traction of Lubricated Rolling Contacts between Thin-Film Coatings and Steel

Abstract: Tribological thin-film coatings can enhance the performance of mechanical components such as bearings and gears. Although a lubricant is present in most applications, the interactions of the lubricant with the coated surfaces are not always well understood. In the present study, Stribeck curves (i.e., traction coefficient vs. dimensionless film thickness λ) were generated for lubricated rolling contact between coated and uncoated surfaces. Chromium nitride, tungsten carbide–reinforced amorphous hydrocarbon, and silicon-incorporated diamond-like carbon coatings were evaluated. Compositions, hydrogen concentrations, Raman spectra, and surface energies are reported for the films. A ball-on-flat test configuration was used in 5%, 50%, and 100% slide-to-roll conditions. The test lubricant was a polyalphaolefin containing rust and oxidation inhibitor additives only. Differences in traction performance were observed for different coating types. Traction coefficients decreased at high λ with increased hydrocarbon...

A Numerical Model of a Reciprocating Rod Seal With a Secondary Lip

Abstract: A numerical model of an elastomeric reciprocating hydraulic rod seal, with both a primary and a secondary lip, has been developed. It is applicable to cases in which the stroke length is significantly larger than the seal width. Coupled fluid mechanics, contact mechanics, and deformation analyses are applied to each lip. Mass conservation and the pressure of the intervening fluid couple the two lips to each other. The leakage, the film thickness distribution, the fluid and contact pressure distributions, and the friction force are predicted for both the outstroke and the instroke for a typical seal, operating with mixed lubrication.

A New Approach for the Measurement of Film Thickness in Liquid Face Seals

Abstract: Face seals operate by allowing a small volume of the sealed fluid to escape and form a thin film between the contacting parts. The thickness of this film must be optimized to ensure that the faces are separated, yet the leakage is minimized. In this work the liquid film is measured using a novel ultrasonic approach with a view to developing a condition monitoring tool. The trials were performed in two stages. Initially tests were based on a lab simulation, where it was possible to compare the ultrasonic film thickness measurements with optical interference methods and capacitance methods. A direct correlation was seen between ultrasonic measurements and capacitance. Where ultrasonic and optical methods overlap, good correlation is observed; however, the optical method will not record film thickness above ∼ 0.72 μ m. A second set of trials was carried out, where the film thickness was monitored inside a seal test apparatus. Film thickness was successfully recorded as speed and load were varied. The results...

Experimental Test Results for Vibration of a High Speed Diesel Engine Turbocharger

Abstract: Diesel engine turbochargers are known to have operation into the self-excited unstable region. The operation in the nonlinear limit cycle has been tolerated on most applications to date. The need for a quieter and a smoother operation and in addition reduced emission levels have prompted new evaluations of the rotor bearing design for these systems. In this research, a commercial rotor-bearing dynamics computer program was used to evaluate the stability and transient response of a high-speed diesel engine turbocharger. Various models with varying bearing designs and properties were solved to obtain the linear stability threshold speeds and also the nonlinear transient response. The predicted whirl speed map shows two modes of instability and is very similar to recent on-engine test results conducted at Virginia Tech as part of a recent senior capstone design project. The development of the diesel engine test stand and the associated data acquisition system will be discussed. The results from the unloaded ...

Investigating the Effect of Surface Finish on Mixed EHL in Rolling and Rolling-Sliding Contacts

Abstract: Surface finish may significantly affect the lubrication performance of a tribological interface through the influence of topography on micro/nanoscale fluid flows around localized contacts at surface asperities. This paper aims to study the mixed lubrication performance of a group of engineered surfaces, including turned, isotropically finished, ground, and dimpled surfaces, under different operation conditions by means of a deterministic mixed elastohydrodynamic lubrication (EHL) model. The honed surface was used to mate with other surfaces. The results indicate that a longitudinal contact ellipse favors longitudinally oriented mating surface roughness and that a transverse contact ellipse, as well as a line contact, prefers a transversely orientated mating surface roughness for lubrication enhancement.

Investigation of the Tribological Behavior of Polytetrafluoroethylene at Cryogenic Temperatures

Abstract: Space applications are very demanding and require that lubricants provide low friction and predictable operation over a wide range of temperatures, environments, and contact conditions. Polytetrafluoroethylene (PTFE) is an attractive candidate solid lubricant due to its notably low friction coefficient, wide thermal range, and chemical inertness, but its tribology at space-relevant conditions has not been adequately investigated. This study seeks to gain insight into the cryogenic tribological behavior of PTFE using a well-studied linear reciprocating tribometer. The tribometer was contained within a nitrogen backfilled glove-box and tests were conducted at a constant background temperature of 296 K. Sliding experiments were conducted at a sliding speed of 50 mm/s and a normal pressure of 6.9 MPa, and the temperature of the lapped 304 stainless steel counterface was varied at 2% and 6% RH. Wear rate decreased monotonically with decreased interface temperature below 273 K in the absence and in the presence...

A Dynamic Model for Dissipative Structures used in Bump-Type Foil Bearings

Abstract: A gas bearing of bump-type foil comprises an underlying structure made of one or several strips of corrugated sheet metal covered by a top foil surface. The fluid film pressure thus needs to be coupled with the behavior of this structure for obtaining the equivalent bearing characteristics. The dynamic model of the bump-type foil structure is then of capital importance. The approach presented in this article is an extension of a previously presented static model considering the structure as a multidegree of freedom system. The continuous corrugated sheet is considered as a discrete one with two nodes and three degrees of freedom for each bump that are linked by linear springs. The hereby presented dynamic model is based on the time-domain integration of the elastic structure under a given load variation. Its efficiency is conditioned by the ability to deal with the discontinuities contained in the nonlinear Coulomb friction model. In order to avoid these numerical difficulties, the Coulomb friction forces...

The Flow Factors Considering the Elastic Deformation for the Rough Surface with a Non-Gaussian Height Distribution

Abstract: In the mixed lubrication regime, the roughness effects are very important due to the presence of interacting asperities. Patir and Cheng derived a method that rewrites the Reynolds equation in terms of the averaged flow factors. The flow factor method has been used in many applications to demonstrate the effects of surface roughness on the mixed lubrication. In this study, the flow factors considering the elastic deformation of the rough surface due to a direct contact of asperities are calculated. The pressure flow factor and the shear flow factor are evaluated for the rough surfaces with both Gaussian and non-Gaussian height distributions. The results show that there is a large difference of flow factors between the rough surface considering the elastic deformation and that assuming the rigid body due to an increase of the film thickness and a decrease of the slope of asperities after the elastic deformation of asperities.

Influence of Mixed-Lubrication and Rough Elastic-Plastic Contact on the Performance of Small Fluid Film Bearings

Abstract: A previously developed deterministic elastohydrodynamic (EHD) numerical model for small fluid film bearings functioning in the mixed lubrication regime is extended in this work by considering the rough contact. Several simplifying hypotheses are made: the shaft is considered rigid and smooth, turning at low speeds (isothermal regime), and the pad is assumed to have an elastic-perfectly-plastic behavior. The Reynolds equation is solved on a very fine mesh and the elasto-plastic pad deformation caused by the hydrodynamic pressure is taken into account. A deterministic active set-based method is used for determining the contact pressure, the contact extent, and the corresponding deformation. The results are presented for a partial journal bearing configuration, with a linear shaft velocity of 0.47 m/s under specific pressures varying up to 50 MPa. Two pad materials are considered, and the lubricant is either isoviscous or piezoviscous oil. The simulation results, presented as a function of the bearing specif...

The Thermoelastic Behavior of Thrust Washer Bearings Considering Mixed Lubrication, Asperity Contact, and Thermoviscous Effects

Abstract: A comprehensive numerical model is developed to investigate the behavior of a tilted thrust washer bearing. The goal of this work is to investigate the physical phenomena that distress the bearing system. This work includes thermoelastic deformations, which have been neglected in previous studies. The thrust washer bearing is subjected to non-axisymmetric loads within the planetary gear sets of automatic transmissions and consists of flat-faced washers placed between an idle helical gear and an adjacent carrier face. Various coupled numerical schemes model sliding friction, boundary lubrication, asperity contact, thermo-mechanical deformation, thermoviscous effects, and full-film lubrication. The model provides predictions of frictional torque, bearing temperature, hydrodynamic lift, and other indicators of bearing performance. The numerical model and theoretical predictions confirm the experimental results, showing that the bearing under consideration is very susceptible to the mechanisms of thermoelasti...

Influence of Critical Micelle Concentration (CMC) on Tribological Properties of Aqueous Solutions of Alkyl Polyglucosides

Abstract: Here, the results of studies on the use of water-alkyl polyglucosides (APGs) compositions as lubricants carried out with steel ball-polyamide disc apparatus are presented. The range of values of CMC in aqueous solutions of two different mixtures of APG was determined on the basis of physicochemical properties. The results obtained showed that aqueous solutions of APGs were effective lubricants lowering the friction coefficient and wear. Increase in activity of APGs was observed above CMC in bulk phase. This was in compliance with our earlier results that, above CMC, the formation of the adsorption layer on the interface led to significant lowering of the friction coefficient and increase in the ability of friction nodes to transfer a load.

Leakage and Groove Pressure of an Axial Piston Pump Slipper with Multiple Lands

Abstract: This article presents a new analytical model, based on Reynolds equation of lubrication, to evaluate the leakage and pressure distribution for an axial piston pump slipper taking into account the effect of nonvented grooves. The equations consider slipper spin and tilt and are extended to be used for a generic slipper with any number of grooves. A test rig has been designed and used to check experimentally the applicability of the theoretical equations and comparisons between theoretical and experimental results show a good agreement. The new theory can predict slipper leakage and pressure inside the groove with a high level of accuracy, especially at the very low slipper tilts that exist in practice. Experimentally, it is demonstrated that although a groove maintains a constant pressure along its path, under abnormal conditions the pressure differential can exist inside the groove. The effect of tangential velocity on groove pressure and slipper leakage is then studied experimentally, showing that as the...

Real-Time Viscosity Measurement for Condition-Based Monitoring Using Solid-State Viscosity Sensor

Abstract: For optimal condition-based monitoring the measurement of viscosity is essential. Consideration of the manufacturing sector renders laboratory approaches as unsuitable, due to the lag-time between sampling and results reporting. Therefore, the continuous measurement approach has been considered the most ideal, yielding instantaneous results reporting and quality control feedback. The historical design of viscometers has resulted in a complex, highly mechanical device associated with a high cost and selling price. Highly sensitive solid-state viscosity sensors in an online configuration will provide the benefits of fast results reporting, simple fluidics management, and advantageous cost/benefit relation. The sensor measures viscous damping of a surface shear wave on a quartz crystal to obtain the viscosity-density product of the sample. The sensor offers solid-state reliability, real-time response time, and is immune to flow conditions and vibration while designed for the challenging process control environments and for OEM applications.

Observations of Film Thickness Profile and Cavitation Around a Journal Bearing Circumference

Abstract: The film formation in a journal bearing and the extent of the cavitated region is an important contribution to load carrying capacity. In the present work, an ultrasonic method has been used to measure the film thickness profile around the circumference of an operating journal bearing. The reflection of an ultrasonic pulse at an oil layer can be used to determine the film thickness. A transducer was mounted inside a hollow shaft and the signal passed through slip rings. The journal bearing was run at a series of loads and rotational speeds. The transducer continuously pulsed and received as it swept around the bush circumference. This enabled the full film thickness profile to be established. In the converging region the predicted film shape agrees well with predictions from classical lubrication theory. When cavitation occurs, the presence of oil-air and aluminum-air interfaces disrupts the ultrasonic signal. Evidence of cavitation appearance is very clear from the measurement results. The minimum film thickness, the attitude angle, and the onset of cavitation are compared with theoretical solutions.

Particle Damage in Hertzian Contacts and Life Ratings of Rolling Bearings

Abstract: Particle denting, and contamination marks found on bearing raceways, can induce stress concentrations and facilitate surface initiated fatigue. The lubricant film developed at the dent and related local surface stresses are also significant to the crack initiation mechanism. In this article, a new methodology is presented to link the micro-EHL film and related local stresses to the fatigue life of rolling bearings. The applied methodology is based on Fourier analysis of the harmonic components of the surface microgeometry to predict stresses and induced lubricant film. The application of this method to actual bearing surfaces is discussed and analyzed in relation to some existing microcontact EHL solutions. A global evaluation of the use of the method to rolling bearings dynamic load ratings is also carried out. A comparison between experimentally obtained rolling bearing life and lives predicted using the present theory indicate the global ability of the model to describe the effect of the lubrication qu...

Design and Development of a Test Setup for Online Wear Monitoring of Mechanical Face Seals Using a Torque Sensor

Abstract: Online condition monitoring technique helps to detect the root cause of failure of any machine. The present article describes a design of the online failure monitoring facility for mechanical face seals. The experimental test facility is to operate the seal (i.e., carbon-graphite) in real conditions of fluid pressure, temperature, and misalignment, which occur in an industrial environment. The developed test setup consists of two proximity displacement sensors (accuracy 2 μm), one fiber-optic sensor (accuracy 10 μm), one accelerometer (3.97 mV/ms-2), and one non-contact torque sensor (accuracy 0.05 N.m). To validate the test facility, a typical conical carbon graphite (C = 59.195%, O = 4.625%, and Sb = 36.18%.) mechanical face seal (outer dia = 82 mm and inner dia = 63 mm) for a rotary joint used in steam/hot water was selected. The root cause of failure of such seals has been identified. Finally, recommendations have been made that provide some assistance to design the mechanical face seal.

A Study on P-S-N Curve for Rotating Bending Fatigue Test for Bearing Steel

Abstract: A study on the fatigue behavior of bearing steel by using rotating bending fatigue test rigs is carried out for bearing steel (JIS SUJ2 = AISI 52100) heat-treated to HRC58-62. Several P-S-N curves and fatigue life distributions, such as Weibull and log-normal, have been used for the discussion. As a result, the best fit for a life distribution of six lots each with a sample size of around 30 specimens, at stress levels from 0.94 GPa to 1.27 GPa, is obtained by the three-parameter Weibull distribution, followed by the lognormal distribution as second, and the two-parameter Weibull distribution as the third. The observation of the broken section of the test piece reveals that the initiation point of the failure is associated almost always with subsurface non-metallic inclusions. The fatigue limit could not be observed in the experimental results. It is also proposed that the relationship between the statistical life distributions of the test series and the P-S-N curve can be expressed by the same model as t...

Effect of Aramid Fiber and ZnO Nanoparticles on Friction and Wear of PTFE Composites in Dry and LN2 Conditions

Abstract: The tribological performance of PTFE and PTFE-based composites filled with aramid fiber and ZnO nanoparticles in dry sliding and liquid nitrogen (LN 2 ) conditions was studied comparitively. The wear scars on the test specimen were observed by scanning electron microscopy (SEM). The transfer films of the composite material formed on the sliding interfaces were studied by X-ray photoelectron spectroscopy (XPS). The tribological test results of the tribo-pairs with two ANSI 440C steel pins sliding on a composite disk showed that, compared with the pure PTFE, the friction coefficient of the PTFE-based composites filled with both aramid fiber and ZnO nanoparticles is reduced in dry sliding in room temperature (RT) air, whereas its friction coefficient is increased in LN 2 . The friction coefficient of the composite filled with 5 wt% ZnO nanoparticles alone increases compared to unfilled PTFE in both RT air or in LN 2 conditions. While in LN 2 , the friction coefficient of the same 5% ZnO material is lower, an...

Parametric Optimization of Periodic Textured Surfaces for Friction Reduction in Combustion Engines

Abstract: The aim of this paper is to analyze the tribological performance of different periodic textures of liner surfaces under hydrodynamic lubrication conditions, for an engine application. The impact of the different parameters of the periodic texture on its tribological performance is studied. First, a brief description of the numerical model is given. Then, the friction prediction tool is applied to the simulated periodic textures. Finally, optimal shapes and values of periodic textures are drawn out for the considered application.

Effect of Surface Texturing on Very Thin Film EHD Lubricated Contacts

Abstract: Micro-dents of various depths and shapes have been produced on rubbing surfaces to find out whether there could be some operational conditions suitable for surface texturing applications in highly loaded non-conformal contacts. The significant effect of the size and shape of the micro-dents has been observed as a function of the slide-to-roll ratio. Shallow micro-features within concentrated contact were found to increase the film thickness by supplying more lubricant to the contact under rolling/sliding conditions. However, in many cases this effect was accompanied with local film thickness reduction that even caused lubrication film breakdown under thin film lubrication conditions. Longitudinally and transversely oriented short grooves were found not to be completely suitable for the surface texturing applications from this point of view. Conversely, small circular micro-dents seem to be more promising as to their use to help to separate rubbing surfaces when a micro-textured surface is moving faster th...

Prediction of Roller Skewing in Tapered Roller Bearings

Abstract: The paper proposes a comprehensive model to predict the skew movement of a roller in tapered roller bearings. Numerical solutions are given on a LM48548/LM48510 bearing. The effect of the shaft speed, ring misalignment, and lubricant temperature on the roller skew angle is presented and compared to the experimental data available in the literature. The results indicate that the skew angle depends first on the misalignment angle between the cup and the cone. The roller skew is also greatly influenced by the traction at the roller-flange contact, the skew angle increasing with the traction at this contact, and by the geometry of the roller end.