Showing papers in "Journal of Tribology-transactions of The Asme in 2019"

Fabrication of Hybrid Surface Composites AA6061/(B4C + MoS2) via Friction Stir Processing

Abstract: Poor tribological properties restrict structural applications of aluminum alloys and surface composites of aluminum alloys have gained more attention in material processing. The addition of solid lubricant reinforcement particles along with abrasive ceramics contributes to the enhancement of tribological performance of surface composites. In the present study, the solid-state technique, friction stir processing (FSP) was used to develop mono (B4C) and hybrid (B4C + MoS2) surface composites in the AA6061-T651 aluminum alloy. The hybrid surface composites were produced by varying an amount of MoS2. Multipass FSP with different direction strategies was adopted for achieving uniform distribution of reinforcement powders in the aluminum matrix. Microstructure analysis showed a uniform dispersal of reinforcement particles without any clustering or agglomeration in the processing zone. Microhardness and wear performance of mono and hybrid composites improved in comparison with the base metal. The mono surface composite exhibited the highest hardness while the hybrid surface composite (75%B4C + 25%MoS2) achieved the highest wear resistance. This was attributed to the solid lubricant nature of MoS2. Furthermore, dissolution of the strengthening precipitate condition during multipass FSP without reinforcement particles resulted in the reduction of hardness and wear resistance.

Honed Surface Multiscale Prediction Based on Multi-Stage Honing Process of Engine Cylinder Bore

Abstract: Sequential honing process is usually implemented in engine cylinder bore processing to obtain the cross-hatched surface texture with excellent function to balance lubricant storage capacities and supporting performance. Many researches have devoted to correlating honed surface quality of cylinder bore with honing process parameters by means of experiments or simulations. Quite a few efforts have addressed the effect of sequential multiple steps on the surface texture in the honing of engine cylinder bore. However, these researches cannot provide an explicit and analytical methodology to predict honed surface texture efficiently and accurately. This paper presents an analytical and explicit methodology to incorporate a proposed microscale abrasive model into the analytical simulation process of sequential honing. The proposed abrasive model synthetically considers the shape, size, posture, and position of abrasives randomly distributed in honing stone, which is incorporated into honing head motions in terms of rotation, oscillation and feeding. The kinematics of honing head is calculated by space-time discretization to capture the interaction between honing stones and cylinder bore surface. The above procedure acts as each single step for the sequential honing processes. This study investigates the sequential honing of two stages including semi-finish honing and plateau honing at different feeding speeds by applying the abrasive model with different abrasive sizes. The formation of cross-hatched surface texture was successfully achieved sequentially by semi-finish honing and plateaus honing. Then the Abbott-Firestone Curve of the honed surface can be obtained to analyze the influences of abrasive size and honing time of two stages on the surface roughness. Correctness of surface roughness predicted by the model is verified by comparing with a group of experiment measurements in terms of Abbott-Firestone Curve. Most errors of all the predicted Rk roughness family roughness parameters in the two honing stages are less than 15%. Based on the model, simulations are done to analyze the influences of abrasive size and honing duration time of two stages on the surface roughness. The result shows that the larger abrasive used in finish honing leads to the decrease of the material portions Mr1, Mr2 and the increase of the reduced valley depth Rvk. The longer plateau honing duration time is preferred to produce the larger Mr1, Mr2 and the smaller Rvk.

A Review on Wear Between Railway Wheels and Rails Under Environmental Conditions

Abstract: The wheel-rail contact is an open system contact, which is subjected to various environmental conditions, such as temperature, humidity, water, and even leaves. All these environmental factors influence wheel-rail wear. Classical wheel-rail wear has been extensively studied under dry and clean conditions previously. However, with changes in environmental conditions, the wear rate and wear mechanism can change. This paper reviews recent contributions to wheel-rail wear with a special focus on the influence of environmental conditions. The main part includes the basics of wheel-rail wear, experimental methodology, wear and rolling contact fatigue (RCF), and some measures to counter these degradation mechanisms.

A Coupled Multibody Finite Element Model for Investigating Effects of Surface Defects on Rolling Contact Fatigue

Abstract: A coupled multibody elastic–plastic finite element (FE) model was developed to investigate the effects of surface defects, such as dents on rolling contact fatigue (RCF). The coupled Voronoi FE model was used to determine the contact pressure acting over the surface defect, internal stresses, damage, etc. In order to determine the shape of a dent and material pile up during the over rolling process, a rigid indenter was pressed against an elastic plastic semi-infinite domain. Continuum damage mechanics (CDM) was used to account for material degradation during RCF. Using CDM, spall initiation and propagation in a line contact was modeled and investigated. A parametric study using the model was performed to examine the effects of dent sharpness, pile up ratio, and applied load on the spall formation and fatigue life. The spall patterns were found to be consistent with experimental observations from the open literature. Moreover, the results demonstrated that the dent shape and sharpness had a significant effect on pressure and thus fatigue life. Higher dent sharpness ratios significantly reduced the fatigue life.

Tribological Properties of Nanoclay-Infused Banana Fiber Reinforced Epoxy Composites

Abstract: The objective of this work is to study the tribological properties of natural fiber based composites using nanotechnology. The naturally available banana plant fibers were treated with nanoclay particles, and these treated fibers were then reinforced in an epoxy polymer to form composites. The friction and wear properties of nanoclay-treated banana fiber (NC-BF) reinforced composites were compared with untreated banana fiber (UT-BF) reinforced composites. Short NC-BF- and UT-BF-reinforced composites with fiber concentration ranging from 20 wt % to 60 wt % were prepared by the vacuum resin infusion processing method. The result indicates that the NC-BF-reinforced composites have shown improved friction and wear properties. Microscopy examination revealed that NC-BF-reinforced composites were able to form a transfer layer between the wear test specimen wear surface and counter face, resulting in improved wear properties. The nanoclay particles also induce increased hardness and friction to the composites and improve braking properties.

A New Asperity-Scale Mechanistic Model of Tribocorrosive Wear: Synergistic Effects of Mechanical Wear and Corrosion

Abstract: A corrosive wear model is considered at the asperity-scale of a tribocorrosive wear system as well as the traditional Archard-type mechanical wear model. The geometry of the surface asperities are modified in a contact mechanics model with respect to both corrosive and mechanical wear calculations. This model was presented and validated for prediction of the electrochemistry in the first part of this work. The material used in the experimental part of this work was CoCrMo plate (working electrode) and Si3N4 ball as the counter body in a reciprocating configuration. Experiments were conducted at loads of 5, 7.5, and 10N and the contributions of total mechanical wear and corrosion were measured. The model is then tuned to predict the chemical and mechanical components of the total wear of the system. The synergistic effect of corrosion on mechanical wear and mechanical wear on corrosion are modelled numerically in this work. The values are then used to explain different components of mechanistic tribocorrosive wear models present in the literature. This deterministic model, for the first time, calculates the corrosion-enhanced wear in a tribocorrosive wear environment and proposes that changes in the topography are responsible for this synergistic effect. The results show a linear dependence of the corrosion enhanced wear, wear enhanced corrosion and the pure mechanical wear on the applied load. Results also suggest that the wear enhanced corrosion has a significant contribution in the overall degradation of the material.

Boundary Lubricating Properties of Black Phosphorus Nanosheets in Polyalphaolefin Oil

Abstract: As a novel layered material, black phosphorus (BP) shows unexpected characteristics in many aspects including tribological application. In this work, BP was prepared through ball milling from red phosphorus (RP). The boundary lubricating properties of the BP nanosheets were investigated on a ball-on-disk tribometer as lubricating additives in polyalphaolefin oil. The micromorphologies, concentration, and composition of the typical chemical elements on the worn surfaces were measured by the 3D laser scanning microscope, scanning electron microscope, and X-ray photoelectron spectrometer, respectively. The results show that bulk BP can be found after RP was milled at 500 rpm for 36 h. The Raman intensity of the BP increased initially and then decreased with the increase in milling time, and the maximum intensity can be obtained at 60 h. The BP nanosheets displayed excellent antifriction and anti-wear performances as lubricating additives in PAO6 oil for steel/steel contact in boundary lubrication regime. The boundary lubrication mechanism of the BP nanosheets is dominated by the physical slippery effect of the laminated nanosheets and the tribofilm on the rubbing surfaces.

Pristine and Alkylated MoS2 Nanosheets for Enhancement of Tribological Performance of Paraffin Grease Under Boundary Lubrication Regime

Abstract: In the present study, lubricating grease was developed with paraffin oil and 12-lithium hydroxy stearate metal soap as a thickening agent. MoS2 nanosheets were synthesized by hydrothermal method and functionalized with 1-octadecanethiol (i.e., MoS2-ODT). The MoS2 and MoS2-ODT nanosheets were dispersed in the grease with different concentrations to evaluate its tribological performance. Tribological results unveiled that the addition of MoS2 nanosheets in grease appreciably reduced the coefficient of friction and mean wear volume of tribo-interfaces as compared with pure grease. Energy dispersive spectroscopy (EDS) spectrum revealed the deposition of MoS2 on the worn surface and confirmed a thin tribo-film which protects steel tribo-pair against wear.

Prediction of Surface Wear of Involute Gears Based on a Modified Fractal Method

Abstract: A new wear prediction method of tooth surfaces of involute gears based on a real tooth surface model and a modified fractal method is developed. The real tooth surface model of an involute gear pair is introduced, and microgeometry feature detection of tooth surfaces is achieved by monitoring variations of normal vectors of each discrete data point of the real tooth surface model. To predict wear progression of tooth surfaces of a gear pair, an abrasive wear analysis model and the modified fractal method are used to analyze contact performance and its changes with accumulation of surface wear. The abrasive wear analysis model can analyze wear depths of gear tooth surfaces with sliding distances, local contact pressure, and directions of wear progression based on Archard's model. The modified fractal method is proposed to calculate instantaneous contact stiffness and estimate elastic and plastic deformation regions based on an asperity contact model. Microgeometry features of tooth surface asperities can be described as the basis of an asperity contact model and allow tooth contact analysis of real tooth surface models with their local microgeometry feature changes due to plastic deformations. Feasibility and effectiveness of this wear prediction method were verified by comparing predicted results of gear surface wear progression with gear wear test results.

Microstructure and Tribological Performance of Alumina–Aluminum Matrix Composites Manufactured by Enhanced Stir Casting Method

Abstract: Al–Zn–Mg–Cu matrix composites reinforced with (0–20 wt %) Al2O3 particles have been manufactured by enhanced stir casting technique. Microstructural characterization of cast composites by optical, field emission scanning electron microscope (FESEM), energy dispersive X-ray (EDS) and X-ray diffraction (XRD) reveals homogeneous distribution of reinforcements in Al-alloy matrix with MgZn2 plus Al2CuMg intermetallics. With increasing particle content, hardness of composite rises considerably in spite of marginal rise in porosity. Tribological performance under two-body abrasion has been studied considering central composite design (CCD) apart from identification of mechanisms of wear via characterizations of abraded surfaces and debris. Composites exhibit significantly reduced wear rate and coefficient of friction (COF) irrespective of test conditions, since mechanisms of abrasion are observed to change from microplowing and microcutting in unreinforced alloy to mainly delamination with limited microplowing in composites. Effects of four independent factors (reinforcement content, load, abrasive grit size, and sliding distance) on wear behavior have been evaluated using response surface-based analysis of variance (ANOVA) technique. Dominant factors on both wear rate and COF are identified as reinforcement content followed by grit size and load. Combined optimization of wear rate and COF employing multiresponse optimization technique with desirability approach as well as regression models of individual responses have been developed, and their adequacies are validated by confirmatory tests. The developed mathematical models provide further insight on the complex interactions among wear performances of the selected materials and variables of abrasive system. The optimum amount of reinforcement is identified at around 15 wt % for achieving the lowest values of both wear rate and COF.

Tribological and Wetting Properties of TiO2 Based Hydrophobic Coatings for Ceramics

Abstract: Hydrophobic and self-cleaning photocatalytic ceramics and concrete with potential for the superhydrophobicity are promising novel materials for civil engineering applications including buildings, bridges, road pavements, and airport runways. Due to embedded liquid-repellent properties, such materials have low water and salt absorption and, therefore, enhanced durability. However, in applications requiring high traction (e.g., tire and pavement), there is a concern that reduced adhesion may compromise the friction. This paper reports on wetting, dry friction, and roughness properties of TiO2 coated (hydrophilic) and polymethyl hydrogen siloxane (PMHS) coated (hydrophobic) self-cleaning ceramic tiles. The coefficient of friction values of the tile–rubber interface do not change significantly with the applications of the coatings up to 0.67 for hydrophilic TiO2 based and up to 0.46 for hydrophobic TiO2 + PMHS coatings versus 0.45 for uncoated reference. Friction has adhesion and roughness-related components and this response can be attributed to the roughness component of friction due to TiO2 coating. The challenges related to hydrophobic coatings, including the durability and future research, are also discussed.

Effect of Tool Pin Profile on the Microstructure and Tribological Properties of Friction Stir Processed Al-20 wt% Mg2Si Composite

Abstract: This study was undertaken with the aim of modifying the microstructure and improving the tribological properties of in situ Al-20Mg2Si composite. For this purpose, friction stir processing (FSP) was applied on the composite at constant travel and rotation speeds of 110 mm/min and 1500 rpm, respectively, using three different tool pin profiles: threaded tapered cylindrical, triangular tapered, and triangular threaded tapered. The sliding wear tests were conducted using a pin-on-disc apparatus under the applied load of 30 N and sliding distance of 1000 m at room temperature. The results showed that FSP substantially improved tribological properties of the as-cast composite. The best result was observed in the sample processed by the threaded triangular tapered tool, where compared with the as-cast composite, its wear rate and average friction coefficient decreased by almost 40% and 18%, respectively. This improvement can be attributed to the significant refinement and uniform redistribution of Mg2Si intermetallics, especially the coarse irregular-shaped primary crystals, the formation of ultrafine grains, and elimination of casting defects from the substrate microstructure of the processed sample, which improves its hardness and increased its potential in supporting the oxide tribolayer on the composite surface.

Parametric studies of mechanical power loss for helical gear pair using a Thermal EHL model

Abstract: In this study, a comprehensive mechanical efficiency model based on the thermal elastohydrodynamic lubrication (TEHL) is developed for a helical gear pair. The tribological performance of the helical gear pair is evaluated in terms of the average film thickness, friction coefficient, mechanical power loss, mechanical efficiency, etc. The influence of basic design parameters, working conditions, thermal effect, and surface roughness are studied under various transmission ratios. Results show that the contribution of thermal effect on the tribological performance is remarkable. Meanwhile, the rolling power loss constitutes an important portion of the total mechanical power loss, especially around the meshing position where the pitch point is located in the middle of contact line and the full elastohydrodynamic lubrication (EHL) state with the friction coefficient less than 0.005. The proper increase of normal pressure angle and number of tooth can improve the tribological performance. The influence of helix angle on the mechanical efficiency is less significant. A positive addendum modification coefficient for pinion and a negative addendum modification coefficient for wheel are good for improving the mechanical efficiency. The results provide the tribological guidance for design of a helical gear pair in engineering.

Design of Tilting-Pad Journal Bearings Considering Bearing Clearance Uncertainty and Reliability Analysis

Abstract: The dynamic characteristics of tilting-pad journal bearings (TPJBs) are strongly related to their geometric parameters, most importantly the bearing clearance. In turn, the bearing clearance in TPJBs is strongly dependent on the machining tolerances of the bearing parts and their assembling. Considering that, the machining tolerances of the pads can be of the same magnitude order of the oil film thickness in the bearing, it is uncertain that the TPJB will have the originally designed geometry after assembling. Therefore, the resultant dynamic characteristics of the TPJB also become uncertain. In this work, we present an investigation of tilting-pad bearings and their equivalent dynamic coefficients when subjected to dimensional variability. First, we perform a stochastic analysis of the system using a thermo-hydrodynamic (THD) model of the tilting-pad bearing and considering the bearing clearance in each pad as an independent random variable (varying between minimum and maximum values). We show that the scattering of the results of the dynamic coefficients is limited by the values obtained from TPJBs with all pads with maximum or minimum possible clearances. Second, we apply the concepts of reliability analysis to develop a design procedure for tilting-pad bearings. This design methodology considers the results obtained in the stochastic analysis and it allows the Engineer to appropriately design the bearing for a given probability of success or, inversely, a given probability of failure. Such approach assures a level of reliability to the dynamic coefficients of designed TPJBs in face of their dimensional variability.

Tribological Investigation of Multilayer Graphene Reinforced Alumina Ceramic Nanocomposites

Abstract: We investigated the wear resistance properties of high-frequency induction heat (HFIH) sintered alumina (Al2O3) ceramic nanocomposites containing various multilayer graphene (MLG) concentrations. The tribology of the monolithic Al2O3 and nanocomposites samples was assessed against spherical ceramic (Si3N4) counter sliding partner at sliding loads ranging from 6 to 40 N using ball-on-disk wear test configuration. Compared with the monolithic Al2O3, the incorporation of 1.0 vol % MLG reduced the friction coefficient by 25% and the wear rate by 65% in the MLG/Al2O3 nanocomposites tested under 40 N sliding load. Based on the mechanical properties, brittle index, and microstructure, the active wear mechanisms for the nanocomposites were analyzed. The MLG contributed in the nanocomposites tribology process, indirectly, by enhancing the mechanical properties and, directly, by reducing the friction between the counter sliding partners. The synergistic role of MLG thin triboflim and twirled MLG for improving the tribological performance of the nanocomposites is discussed.

Study of the Stiffness Matrix of Preloaded Duplex Angular Contact Ball Bearings

Abstract: This paper studies the stiffness characteristics of preloaded duplex angular contact ball bearings. First, a five degrees-of-freedom (5DOF) quasi-static model of the preloaded duplex angular contact ball bearing is established based on the Jones bearing model. Three bearing configurations (face-to-face, back-to-back, and tandem arrangements) and two preload mechanisms (constant pressure preload and fixed position preload) are included in the proposed model. Subsequently, the five-dimensional stiffness matrix of the preloaded duplex angular contact ball bearing is derived analytically. Then, an experimental setup is developed to measure the radial stiffness and the angular stiffness of duplex angular contact ball bearings. The simulated results match well with those from experiments, which prove the validity of the proposed model. Finally, the effects of bearing configuration, preload mechanism, and unloaded contact angle on the angular stiffness and the cross-coupling are studied systematically.

Tribological Influences of CuO Into 3Y-TZP Ceramic Composite in Conformal Contact

Abstract: The aim of the study was to investigate the friction and wear phenomena of 3 mol % yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP) ceramics with the inclusion of copper oxide (CuO) in large area conformal contact geometry. The pin-on-disk tribometer was used to conduct the dry sliding test using CuO/3Y-TZP as pin and alumina as counter surface. The coefficient of friction (μ) for CuO-added 3Y-TZP was decreased by ∼38% compared to pure 3Y-TZP due to formation of protective tribo film to the substrate. In addition, the experiments also showed that the specific wear rate (k) was reduced by ∼54% with the inclusion of CuO in to 3Y-TZP matrix. The different phases of the zirconia, copper, and yttria as well as the phase transformation before and after sliding test were identified by X-ray diffraction (XRD) analysis. Field emission scanning electron microscopy (FESEM) and energy dispersive X-ray (EDS) analysis revealed the existence of CuO in the patchy layers in the worn-out surface of the tested CuO/3Y-TZP sample leading to lower coefficient of friction and improve the wear resistance against alumina counterface in conformal contact geometry. Severe wear mechanism was the dominating factor due to the local plastic deformation of the large number of asperities since the pair of contact was conformal.

MoDTC Tribochemistry in Steel/Steel and Steel/Diamond-Like-Carbon Systems Lubricated With Model Lubricants and Fully Formulated Engine Oils

Abstract: This work focuses on the tribochemistry of molybdenum dithiocarbamate (MoDTC) oil additive to improve friction behavior of diamond-like-carbon (DLC) coated systems lubricated in boundary regime. Raman microscopy has been used to investigate surface tribolayers formed on coated (hydrogenated a-C:H and non-hydrogenated ta-C) and steel surfaces when lubricated with model lubricants and commercial engine oils. The effect of the additive package and the type of DLC played a crucial role in the development and composition of the tribolayer and the friction performance. The additive package contained in the fully formulated (FF) oils limited the friction reduction capabilities of MoDTC additive for every material pair. Accelerated a-C:H coating wear related to MoDTC tribochemistry was found. For the first time, it has been shown that a distinctive MoS₂-containing tribolayer can be formed on the ta-C surface, leading to a coefficient of friction lower than 0.04. The underlying mechanisms of MoDTC/surface interactions and their effect on friction and wear are discussed.

Experimental Investigation on Slurry Erosion Behavior of 304L Steel, Grey Cast Iron, and High Chromium White Cast Iron

Abstract: The materials used for the slurry transportation system experience erosion wear due to the impact of suspended solid particles. In the present experimental investigation, a large size slurry pot tester was used to investigate the slurry erosion behavior of steel 304L, grey cast iron, and high chromium white cast iron in the velocity range of 9.0–18.5 m/s. Experiments were conducted by rotating the wear specimens in the pot tester at 1% weight concentration of Indian standard sand. The erosion behavior of the three target materials was evaluated by varying the orientation angle from 15 to 90 deg and particle size from 256 to 655 µm. The erosion rate was found to increase with velocity having power index value varying between 2 and 3, which increases with an increase in impact angle and depends on the target material. The erosion rate of the material also increases with the increase in particle size with the power index varying between 0.8 and 1.4 depending on the target material. No significant change was noticed in the mechanism of erosion of the target materials with the variation in velocity in the present range of test conditions. Empirical correlations are proposed to estimate the total erosion rate of all the three materials as a contribution of cutting and deformation wear.

A Model for Oil Flow and Fluid Temperature Inlet Mixing in Hydrodynamic Journal Bearings

Abstract: The quality of predictions for the operating behavior of high-speed journal bearings strongly depends on realistic boundary conditions within the inlet region supplying a mixture of hot oil from the upstream pad and fresh lubricant from the inlet device to the downstream located pad. Therefore, an appropriate modeling of fundamental phenomena within the inlet region is essential for a reliable simulation of fluid and heat flow in the entire bearing. A theoretical model including hydraulic, mechanical, and energetic effects and the procedure of its numerical implementation in typical bearing codes for thermo-hydrodynamic lubrication is described and validated. Convective and conductive heat transfer as well as dissipation due to internal friction in the lubricant is considered for the space between pads or the pocket where the inlet is located. In contrast to most other models, the region between the physical inlet and the lubricant film is part of the solution domain and not only represented by boundary conditions. The model provides flow rate and temperature boundary conditions for extended Reynolds equation and a three-dimensional (3D) energy equation of film and inlet region, respectively. The impact of backflow from the inlet region to the outer supply channel possibly occurring in sealed pockets is taken into account. Moreover, the model considers the influence of turbulent flow in the inlet region.

Effect of Heat Treatment on Tribological Properties of Ni-B Coatings on Low Carbon Steel: Wear Maps and Wear Mechanisms

Abstract: Among the alternatives for using low-carbon steel in parts with heavy wear, as gears and bearing surfaces, Ni-B electroless coatings deposited on these steels are considered due to their wear resistance. Wear maps, elaborated from friction or wear results found for different evaluated conditions, are a very useful tool for the selection of materials based on tribological properties. However, wear maps for electroless Ni-B coatings are very scarce. In this work, dry sliding wear tests with different loads and sliding velocities were performed on Ni-B electroless coatings applied on AISI/SAE 1018 steel, with and without heat treatment at 450 °C for 1 h, with the aim of determining the effect of the heat treatment on the friction coefficients and wear rates. Contour and profile maps, and finally friction and wear maps, were constructed for each of the coatings evaluated. The coating properties before and after the heat treatment were studied by means of scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), scratch tests, nanoindentation, and differential scanning calorimetry (DSC). Sliding wear tracks were studied using SEM, energy-dispersive spectroscopy (EDS), and micro-Raman spectroscopy. Good agreement between experimental and predicted values was found in friction and wear maps. Wear mechanisms change from flattening in less severe conditions to abrasion in more severe conditions, besides spalling and adhesive wear in untreated coatings. Moreover, abrasive wear is lower in heat-treated coating than in untreated coating.

Experimental study of the leakage and rotordynamic coefficients of a long smooth seal with two-phase, mainly-oil mixtures

Abstract: This paper experimentally studies the leakage and rotordynamic performance of a long-smooth seal with air–oil mixtures. Tests are performed with inlet gas-volume-fraction gas volume fraction (GVF) = 0%, 2%, 4%, 6%, and 10%, rotor speed ω = 5, 7.5, 10, and 15 krpm, inlet temperature Ti = 39.4 °C, exit pressure Pe = 6.9 bars, and pressure drop (PD) = 31, 37.9, and 48.3 bars. Test results show that adding air into the oil flow does not change the seal's mass flow leakage m˙ discernibly but significantly impacts the seal's rotordynamic characteristics. For all PDs and speeds, K increases as inlet GVF increases from zero to 10% except for 6% ≤ inlet GVF ≤ 10% when PD = 48.3 bars, where K decreases as inlet GVF increases. The K increment will increase a pump rotor's natural frequency and critical speed. Increasing the rotor's natural frequency would also increase the onset speed of instability (OSI) and improve the stability of the rotor. Adding air into the oil flow has little impact on cross-coupled stiffness k, direct damping C, and effective damping Ceff. Ceff = C − k/ω + mqω, where mq is the cross-coupled virtual-mass. Test results are compared to predictions from San Andrés's (San Andrés, 2011, “Rotordynamic Force Coefficients of Bubbly Mixture Annular Pressure Seals,” ASME J. Eng. Gas Turbines Power, 134(2), p. 022503.) bulk-flow model, which assumes that the liquid–gas mixture is isothermal and homogenous. The model reasonably predicts m˙, C, and Ceff. All predicted K values are positive, while measured K values are negative for some test cases. Predicted k values are close to measurements when ω = 5 krpm and are larger than measurements when 7.5 ≤ ω ≤ 15 krpm.

A Thermal Elastohydrodynamic Lubrication Model for Crowned Rollers and Its Application on Apex Seal–Housing Interfaces

Abstract: This paper presents a thermal elastohydrodynamic lubrication (EHL) model for analyzing crowned roller lubrication performances under the influence of frictional heating. In this thermal EHL model, the Reynolds equation is solved to obtain the film thickness and pressure results while the energy equation and temperature integration equation are evaluated for the temperature rise in the lubricant and at the surfaces. The discrete convolution fast Fourier transform (DC-FFT) method is utilized to calculate the influence coefficients for both the elastic deformation and the temperature integration equations. The influences of the slide-to-roll ratio (SRR), load, crowning radius, and roller length on the roller lubrication and temperature rise are investigated. The results indicate that the thermal effect becomes significant for the cases with high SRRs or heavy loads. The proposed thermal EHL model is used to study the thermal-tribology behavior of an apex seal–housing interface in a rotary engine, and to assist the design of the apex seal crown geometry. A simplified crown design equation is obtained from the analysis results, validated through comparison with the optimal results calculated using the current crowned-roller thermo-EHL (TEHL) model.

Smoothed Particle Hydrodynamics Simulation of Oil-Jet Gear Interaction1

Abstract: In this paper, the complex two-phase flow during oil-jet impingement on a rotating spur gear is investigated using the meshless smoothed particle hydrodynamics (SPH) method. On the basis of a two-dimensional setup, a comparison of single-phase SPH to multiphase SPH simulations and the application of the volume of fluid method is drawn. The results of the different approaches are compared regarding the predicted flow phenomenology and computational effort. It is shown that the application of single-phase SPH is justified and that this approach is superior in computational time, enabling faster simulations. In the next step, a three-dimensional single-phase SPH setup is exploited to predict the flow phenomena during the impingement of an oil-jet on a spur gear for three different jet inclination angles. The oil’s flow phenomenology is described and the obtained resistance torque is presented. Thereby, a significant effect of the inclination angle on the oil spreading and splashing process as well as the resistance torque is identified.

Wide Range Measurement of Lubricant Film Thickness Based on Ultrasonic Reflection Coefficient Phase Spectrum

Abstract: The ultrasonic technique is very effective in measuring lubricant film thickness in a noninvasive manner. To estimate the film thickness with reflection signals, two main ultrasonic models are often applied in cases of different film thicknesses; they are the spring model for thin films and the resonant model for thick films. However, when measuring oil film thicknesses distributed in a wide range, there is an inherent blind zone between these two models. This problem is especially prominent in online monitoring because the abrupt variation of film thickness is highly correlated with the occurrence of abnormal conditions. To address this issue, we further proposed a method using the phase spectrum of reflection coefficient which can cover a wide range of film thicknesses. The slight variation of reflection signal in the blind zone can then be identified and bridged the measurement gap between those two traditional models. A calibration rig was used to verify the theoretical analysis and the results indicated that the developed model is capable of providing reliable ultrasonic measurement of lubricant film thicknesses in a wide range.

Wear and Mechanical Contact Behavior of Polymer Gears

Abstract: Extensive investigations have been carried out in the present paper to understand polymer gear performance, i.e., wear and contact behaviors. The experimental results and possible wear mechanisms for polymer gears run against themselves have been presented, especially the wear rate of the polymer gears under different running speeds and loads. The tested samples were made of three different materials (acetal, nylon, and polycarbonate (PC)) and the effects of two different manufacturing techniques were also investigated (i.e., machine-cut and injection-molded polymer gears). The polymer gear performances (wear and life) were recorded using a uniquely designed and built test rig for this purpose. The testing results have been compared with the existing literature for polymer fatigue and wear theory. Further extensive investigations have been carried out to understand the wear phenomena on tooth flank surface profile of these gears and the data obtained have been discussed.

Recursive Characteristics of a Running-in Attractor in a Ring-on-Disk Tribosystem

Abstract: To explore the recursive characteristics of a running-in attractor, recurrence plot (RP) and recursive parameters are used to investigate the dynamic features of the structure. The running-in attractor is constructed based on friction noise signals generated from the ring-on-disk wear experiments. The RPs of the running-in attractor are then reproduced in a two-dimensional space. Recursive parameters, recurrence rate (RR), entropy (ENTR), and trend of recurrence (RT) are calculated. Results show that the RP evolves from a disrupted pattern to a homogeneous pattern and then returns to a disrupted pattern in the entire wear process, corresponding to the “formation–stabilization–disappearance” stage of the running-in attractor. The RR and ENTR of the running-in attractor sharply increase at first, remain steady, and then sharply decrease. Moreover, the inclination of RT in the normal wear process is smaller than those in the other two processes. This observation reveals that the running-in attractor exhibits high stability and complexity. This finding may contribute to the running-in state identification, process prediction, and control.

Real-Time Measurement of Dynamic Wheel-Rail Contacts Using Ultrasonic Reflectometry

Abstract: The contact condition between the wheel and the rail is paramount to the lifespan, safety, and smooth operation of any rail network. The wheel/rail contact condition has been estimated, calculated, and simulated successfully for years, but accurate dynamic measurement has still not been achieved. Methods using pressure-sensitive films and controlled air flow have been employed, but both are limited. The work described in this paper has enabled, for the first time, the measurement of a dynamic wheel/rail contact patch using an array of 64 ultrasonic elements mounted in the rail. Previous work has successfully proved the effectiveness of ultrasonic reflectometry for static wheel/rail contact determination. The dynamic real-time measurement is based on previous work, but now each element of an array is individually pulsed in sequence to build up a linear measurement of the interface. These cross-sectional, line measurements are then processed and collated resulting in a two-dimensional contact patch. This approach is able to provide not only a contact patch, but more importantly, a detailed and relatively high-resolution pressure distribution plot of the contact. Predictions using finite element methods (FEM) have also been carried out for validation. Work is now underway to increase the speed of the measurement.

Effect of Rolling Strain on the Mechanical and Tribological Properties of 316 L Stainless Steel

Abstract: The mechanical and tribological performances of 316 L stainless steel subjected to different cold rolling (CR) strains were investigated. The microhardness and strength of 316 L stainless steel were improved attributed to the formation of high-density defects, such as dislocations and parallel lamellar structures. Furthermore, the tribology tests were conducted under dry sliding at room temperature. With the increase in rolling strain, the wear rate of 316 L stainless steel gradually decreased due to the improvements in microhardness and strength. For the as-received specimen, the strong adhesive wear leads to the maximum wear rate compared with the cold rolled specimens. Under higher rolling strain conditions, the grain boundary embrittlement caused by oxygen reaction leads to the formation of oxidative abrasive under dry sliding conditions, and then the oxidative abrasive could serve as the third body at the siding interface. Consequently, there is a transition phase where the wear mechanism gradually shifts from adhesive to abrasive wear.