Showing papers in "Tribology Transactions in 2016"

Monitoring of Wind Turbine Gearbox Condition through Oil and Wear Debris Analysis: A Full-Scale Testing Perspective

Abstract: Despite the wind industry's dramatic development during the past decade, it is still challenged by premature turbine subsystem/component failures, especially for turbines rated above 1 MW. Because a crane is needed for each replacement, gearboxes have been a focal point for improvement in reliability and availability. Condition monitoring (CM) is a technique that can help improve these factors, leading to reduced turbine operation and maintenance costs and, subsequently, lower cost of energy for wind power. Although technical benefits of CM for the wind industry are normally recognized, there is a lack of published information on the advantages and limitations of each CM technique confirmed by objective data from full-scale tests. This article presents first-hand oil and wear debris analysis results obtained through tests that were based on full-scale wind turbine gearboxes rated at 750 kW. The tests were conducted at the 2.5-MW dynamometer test facility at the National Wind Technology Center at t...

Tribological Performance of Silver Nanoparticle–Enhanced Polyethylene Glycol Lubricants

Abstract: This article introduces a new type of nanoparticle additive for tribological purposes. A nanolubricant was synthesized and studied that consists of metallic silver nanoparticles suspended in polyethylene glycol (PEG). Silver nanoparticles were prepared directly in liquid PEG by introducing aqueous silver nitrate and subsequent reduction by PEG. The nanolubricant exhibits excellent stability due to poly(vinyl pyrollidone) used as the coating agent. Thorough tribological analysis was performed on the nanolubricant, including rheology, friction, wear, and Stribeck curve analysis. Results show that the nanoparticle additives are capable of reducing both friction and wear at low concentrations. Stribeck curve analysis also revealed that the particles are effective in reducing friction in both the boundary and mixed lubrication regimes. The possible friction and wear reduction mechanism of silver nanoparticles is also discussed in the current work.

Optimization and Effect of Weight Fraction of MoS2 on the Tribological Behavior of Mg-TiC-MoS2 Hybrid Composites

Abstract: This investigation studies the dry sliding wear behavior of magnesium (Mg) matrix composites reinforced with titanium carbide (TiC) and molybdenum disulfide (MoS2) fabricated using a powder metallurgy technique. The effects of both TiC (0–10%) and MoS2 (0–10%) content on the tribological properties are investigated. Wear tests are carried on magnesium reinforced with TiC and MoS2 individually and together in different proportions, using a pin-on-disc apparatus under dry sliding condition. The experiments were made using a Taguchi L27 orthogonal array with five factors at three levels. The wear resistance of the developed composites improved significantly compared to that of the magnesium matrix due to the effect offered by both reinforcements. Analysis of variance was used to verify the significance of factors influencing wear. In addition, the worn surfaces of the wear-tested specimens were examined using a scanning electron microscope coupled with energy-dispersive spectroscopy.

Texture Shape Optimization for Seal-Like Parallel Surfaces: Theory and Experiment

Abstract: The geometric shape of surface textures has a major influence on the tribological performance of textured surfaces. This study aims to improve the tribological performance of seal-like specimens with a circular ring configuration by introducing novel texture geometries that generate the highest load-carrying capacity (LCC). A numerical optimization approach is presented to determine the optimum texture shapes for a ring geometry with different constraints on the textured area. The results show that the optimum textures of different area ratios (ARs) have similar chevron shapes with flat fronts. Also presented are the tribological test results of the optimum texture shape (AR = 20%) and their comparison with three regular shapes. It is found that the optimum texture shape provides the lowest friction coefficient under different test conditions.

Influence of Currents from Electrostatic Charges on WEC Formation in Rolling Bearings

Abstract: White etching crack (WEC) early bearing failures occur when the rolling contact is subjected to a so-called additional load such as an electrical current flowing through the bearing, in addition to the pure rolling load (pHz). Tests on rolling bearings showed that a low electrical direct current flow, such as that resulting from electrostatic charges, can lead to WEC failures in oil-lubricated roller bearings and greased ball bearings.The WEC formation in the performed tests was dependent on the current, electrical polarity, load type (rotating or stationary ring load), and bearing load. A black oxidation of the WEC critical bearing ring led to a significant increase in lifetime. Based on the findings, the failure hypothesis “cathodic WEC fatigue” for electrical direct current-initiated WEC failures was established.

Tribological Behavior of Aluminum Hybrid Composites Studied by Application of Factorial Techniques

Abstract: This article analyzes the influence of graphite reinforcement, load, sliding speed, and sliding distance on tribological behavior of A356 aluminum matrix composites reinforced with silicon carbide and graphite using the full-factorial design. The wear rates of A356/10SiC composite material and A356/10SiC/1Gr and A356/10SiC/3Gr hybrid composites have been analyzed. The composites were obtained by a modified compocasting procedure. Tribological tests were performed on a block-on-disc tribometer without lubrication. The testing included sliding speeds of 0.25 and 1.0 m/s, normal loads of 10 and 20 N, and sliding distances of 300 and 900 m. The analysis of the obtained results was performed using the full-factorial method based on the signal-to-noise (S/N) ratio. The effects of load, sliding speed, weight percentage of graphite reinforcement, and sliding distance on the wear rate are 38.99, 17.87, 13.95, and 11.25%, respectively. The best tribological characteristics were exhibited by the A356/10SiC/1...

Three-dimensional simulation of rolling contact fatigue crack growth in UIC60 rails

Abstract: In this article, a UIC60 rail with accurate geometry is studied by employing the finite element method. For this purpose, a three-dimensional elastic-plastic finite element model is conducted using model. In addition, the stress distribution of wheel-rail operation is acquired, and its effects on fatigue life are specified by damage mechanic methods. In the finite element model, the displacements and stress intensity factors (SIFs) are computed on the crack near the leading edge to calculate crack propagation trajectories and crack growth rate. The modified Paris model is used to estimate fatigue crack growth rates.

Tribological Effects of Vegetable Oil as Alternative Lubricant: A Pin-on-Disk Tribometer and Wear Study

Abstract: The investigation of lubricated friction and wear is an extended study. The aim of this study is to investigate the friction and wear characteristics of double fractionated palm oil (DFPO) as a biolubricant using a pin-on-disk tribotester under loads of 50 and 100 N with rotating speeds of 1, 2, 3, 4, and 5 ms−1 in a 1-h operation time. In this study, hydraulic oil and engine oil (SAE 40) were used as reference base lubricants. The experiment was conducted using aluminum pins and an SKD 11(alloy tool steel) disc lubricated with test lubricants. To investigate the wear and friction behavior, images of the worn surface were taken by optical microscopy. From the experimental results, the coefficient of friction (COF) rose when the sliding speed and load were high. In addition, the wear rate for a load of 100 N for all lubricants was almost always higher compared to lubricant with a load of 50 N. The results of this experiment reveal that the palm oil lubricant can be used as a lubricating oil, which ...

Fast High-Resolution Simulation of the Gross Slip Wear of Axially Symmetric Contacts

Abstract: In the present article, we study the development of a wear profile in an axially symmetric contact under conditions of gross slip and assumption of the Reye-Archard wear criterion. Simulations are carried out using the method of dimensionality reduction and a full finite element method (FEM) formulation. The calculation time of the proposed model is several orders lower than that of FEM-based models and allows for much higher spatial resolution.

Evaluation on Effectiveness of CVD and PVD Coated Tools during Dry Machining of Incoloy 825

Abstract: With wide applications of nickel-based superalloys in strategic fields, it has become increasingly necessary to evaluate the performance of different advanced cutting tools for machining such alloys. With a view to recommend a suitable cutting tool, the present work investigated various machinability characteristics of Incoloy 825 using an uncoated tool, chemical vapor deposition (CVD) of a bilayer of TiCN/Al2O3, and physical vapor deposition (PVD) of alternate layers of TiAlN/TiN-coated tools under varying machining conditions. The influence of cutting speed (51, 84, and 124 m/min) as well as feed (0.08, 0.14, and 0.2 mm/rev) was comparatively evaluated on surface roughness, cutting temperature, cutting force, coefficient of friction, chip thickness, and tool wear using different cutting tools. Although the CVD-coated tool was not useful in decreasing surface roughness and temperature, a significant reduction in cutting force and tool wear could be achieved with the same coated tool under a high ...

Impact of Water on the Rheology of Lubricating Greases

Abstract: The operational life of bearings is often determined by the performance of the lubricating grease. The consistency of the grease prevents it from leaking out of the bearing and provides good sealing properties. The possible ingress of water into the bearing will have a considerable impact not only on this consistency but also on the lubricating ability of the grease. There are numerous applications where water ingress may occur, such as in the steel, food, pulp, and paper industries. Some greases are less sensitive to water than others. No specific guidelines are available to select the proper grease for bearings subjected to water ingress. The goal of the article is to contribute to the development of such guidelines for greases subjected to water ingress by studying the impact of water on grease rheology. Fully formulated, commercially available greases with the most common thickeners and base oils are used as model greases. It will be shown that water strongly influences rheological properties such as zero-shear viscosity, yield stress, and storage modulus. Calcium sulfonate greases were found to become stiffer after absorbing a considerable amount of water, leading to an increase in zero-shear viscosity and yield stress. However, lithium, lithium complex, and polyurea greases were found to soften, with appreciable changes in measured rheological properties

A Review of Microstructural Alterations around Nonmetallic Inclusions in Bearing Steel during Rolling Contact Fatigue

Abstract: Microstructural alterations in bearing steels during rolling contact cycling have been reported in the literature for more than 60 years. These changes appear in different shapes and locations. One class of such alterations is “butterfly wings”: regions of microstructurally transitioned material that appear diagonally around nonmetallic inclusions and may serve as fatigue crack initiation sites. Over the course of the past half a century numerous experimental and multiple analytical efforts have been made to understand and model this phenomenon, yet a lot is to be discovered and understood about root causes and mechanisms leading to butterfly formation. This article presents a comprehensive overview of the crack nucleation phenomena due to butterfly formation, its characteristics, and its negative impact on bearing service life. Significant attempts that have been made to solve the problem over the past half a century are mentioned, with a focus on recent work. Unanswered dilemmas are particularly...

Effect of Addition of Tertiary-Butyl Hydroquinone into Palm Oil to Reduce Wear and Friction Using Four-Ball Tribotester

Abstract: Replacing mineral oil with vegetable oils as lubricants continues to attract interest due to their environmentally friendly characteristics and ease of disposal. However, one disadvantage can be low thermal oxidative stability. The purpose of our research is to investigate the ways in which oxidation can be contained by combining palm oil with a phenolic antioxidant. A homogeneous mix of palm oil and tertiary-butyl hydroquinone was found to exhibit satisfactory antioxidant properties and reduce wear and friction. Superior mineral engine oil was used to benchmark the performance.

Effect of Al2O3 Particle Size on Electrical Wear Performance of Al2O3/Cu Composites

Abstract: Al2O3/Cu composites (1.0 vol%) reinforced with different size of α-Al2O3 particles were fabricated by a powder metallurgy method and electrical sliding wear tests were performed on a self-made pin-on-disk electrical wear tester. The effect of Al2O3 particle size on electrical wear performance of the Al2O3/Cu composite was studied, and the wear mechanism of the Al2O3/Cu composite was also discussed based on worn surface observations. The results show that the tribological properties of A12O3/Cu composite are closely related to the mechanical properties. With an increase in Al2O3 particle size, the wear rates of A12O3/Cu composites have a reverse variation with hardness of A12O3/Cu composites. In the range of 50–100 nm, Al2O3/Cu composites have the highest wear resistance and mechanical properties. Microstructural observation revealed that the wear mechanisms of Al2O3/Cu composites were mainly adhesive wear and plastic deformation accompanied by a small amount of arc damage. In addition, the plastic...

Effects of Zinc Borate and Fly Ash on the Mechanical and Tribological Characteristics of Brake Friction Materials

Abstract: Friction materials based on several combinations of zinc borate (ZB) and fly ash (FA) were fabricated and characterized for their mechanical and tribological performance. The triboperformance of the friction materials was evaluated on a Chase friction testing machine according to the brake lining quality test procedure as per SAE J661. The composites were manufactured based on a nonasbestos organic-based friction material for an automotive brake system and contained typical ingredients for commercial brake friction materials. The composites had a fixed composition of 15 wt% resin, 15 wt% fibers, and 5 wt% friction additives. ZB and FA as fillers were added to the the raw materials mixture at a total fraction of 65 wt%. The results showed that ZB and FA contents have a significant influence on the mechanical and tribological properties of the friction composites. In particular, the composites containing 0–5 wt% ZB and 65–60 wt% FA showed better friction stability and improved fade resistance compar...

Rotordynamic Analysis of a Shaft Using Magnetorheological and Nanomagnetorheological Fluid Journal Bearings

Abstract: The rotordynamic behavior of a system supported by journal bearings is critical to its reliability. A suitable method of control of the orbital motion of a shaft in a journal bearing is the use of smart lubricants, in effect fluids with controllable physical properties. There are various categories of smart lubricants. One class of smart lubricants, magnetorheological fluids, are produced as a dispersion of magnetic particles in a carrier fluid, which is usually a conventional lubricant. These particles form chains under the influence of a magnetic field, which hinder the lubricant flow, thus changing its apparent viscosity. Magnetorheological fluids (MRFs) exhibit high yield stress, low delay of response, and relatively low friction while not in their active state. A subcategory of MRFs, nanomagnetorheological fluids (NMRFs) with particle size on the nanometer scale, exhibits lower yield stress than MRFs but display high viscosity. The effect of the MRFs and NMRFs on the rotordynamic behavior of ...

Development of a Granular Cohesive Model for Rolling Contact Fatigue Analysis: Crystal Anisotropy Modeling

Abstract: In rolling contact fatigue (RCF), failure mechanisms are known to be very sensitive to material microstructure. Yet, among the different numerical models developed to predict RCF life, few models use a microstructure representation. A granular cohesive finite element model has been developed to simulate progressive damage of a structure subject to RCF and to investigate failure initiation mechanisms. This article focuses on the implementation of crystal elasticity in the model. The numerical analysis of a representative volume element (RVE) validates the use of cubic elasticity to represent crystal behavior. The influence of the RVE size and the influence of boundary conditions applied on the RVE are evaluated in the finite element approximation framework. With regard to the implementation of cubic elasticity in the RCF model, the generation of stress singularities at triple junctions is first highlighted. Then the average value of the intergranular shear stress is proved to be mesh size independe...

Film Forming Behavior of Greases Under Starved and Fully Flooded EHL Conditions

Abstract: Improving knowledge on the film forming behavior of greases is essential to be able to develop efficient greases. This article examines how operating conditions (e.g., temperature, lubrication condition [fully flooded/starved]) and base oil viscosity influence the film forming properties of greases by comparing the behavior of two lithium-based greases and their respective base oils in rolling point contact. It is found that the onset and degree of starvation is controlled by speed (u) × viscosity (ν)/load (W) factor (uν/W) and temperature and that low uν/W values promote entrainment of thickener into contact. Thus, grease with low base oil viscosity shows significant thickener entrainment in the low speed region compared to the one with high base oil viscosity, which leads to the formation of thickener-rich viscous material during extended running with the low base oil viscosity grease. The results suggest that the shape of the film thickness versus speed curve is viscosity and uν/W range depende...

Investigation on Wear Resistance and Fatigue Damage of Laser Cladding Coating on Wheel and Rail Materials under the Oil Lubrication Condition

Abstract: The aim of this study is to investigate the wear resistance and fatigue spalling damage of wheel and rail materials with and without laser cladding coating under oil lubrication using a rolling–sliding machine. It illustrates that the laser cladding Co-based alloy coating improves the wear resistance of wheel and rail rollers. Serious spalling is dominant for untreated wheel and rail rollers. The wheel or rail roller undergoing laser cladding treatment takes on slight abrasive wear and visible ploughing. Furthermore, there are no cracks on the contact surface and subsurface. The laser cladding Co-based alloy coating exhibits outstanding resistance to wear and fatigue spalling damage due to its microstructure in the coating under oil lubrication.

A Hybrid Method for the Segmentation of a Ferrograph Image Using Marker-Controlled Watershed and Grey Clustering

Abstract: Aimed at the correct segmentation of wear particles in ferrograph images, a new method combining marker-controlled watershed and an improved grey clustering algorithm is proposed in this article. First, the marker-controlled watershed is applied to ferrograph images to efficiently obtain the initial segmentation of wear particles. Then, an improved grey clustering algorithm utilizing color characteristics and relative position information is applied to merge the oversegmented regions after the watershed segmentation. This new algorithm is tested for ferrograph images and the results are compared with those of other algorithms. The experimental results show that the proposed method is effective for the segmentation of large wear particles and fine wear debris deposited as chains on the ferrograph, and it is proven to be a practical method for segmenting wear particles quickly and accurately.

Investigations on the Influence of Surface Texturing on a Couple Stress Fluid–Based Journal Bearing by Using JFO Boundary Conditions

Abstract: The present study was conducted to examine the effect of laser surface texturing combined with couple stress fluids on the hydrodynamic lubrication of finite journal bearing in this work. The Jakobsson-Floberg-Olsson (JFO) boundary conditions were engaged instead of Reynolds boundary conditions to achieve realistic results. Moreover, the results were computed and authenticated with the previous published work. It was observed that the load-carrying capacity is increased with couple stresses for smooth journal bearings at different eccentricity ratios. However, the increment in load-carrying capacity with texture affects only at low eccentricity ratios. The combined effects of texturing with couple stress fluids lower the performance of journal bearings at different eccentricity ratios.

Pressure Distribution of a Multidisc Clutch Suffering Frictionally Induced Thermal Load

Abstract: The transient thermal and thermoelastic analysis of an automotive multidisc clutch is performed to investigate the thermal deformation and the contact pressure distribution of the friction surfaces. An alternating finite volume method (FVM) and finite element method (FEM) for temperature distribution and contact pressure distribution, including the coupling between temperature, displacement, and contact pressure, is employed in MATLAB. A penalty function method is used to impose the constraints on the contact boundary. The computational results indicate that the contact pressure distributions on the surfaces near the pressure plate are more uniform than those near the opposite plate, and the nonuniformity is enlarged with the temperature rise. The contact pressure and temperature distributions on the two friction surfaces of the separate plate are dissimilar and asymmetric, which is in good agreement with the test results. The results further show that the contact pressure peaks diminish and the n...

Friction, Wear, and Scuffing Characteristics of Marine Engine Lubricants with Nanodiamond Particles

Abstract: Many kinds of additives are generally added to engine lubricants to improve performance. These chemical additives are harmful to both humans and the environment. For this reason, the research trend in the lubricant industry is to reduce the use of chemical additives in engine oils. Carbon materials like nanodiamonds are candidates among many physical additives. Nanodiamond particles are round, very hard, chemically stable, and highly heat conductible. In this research, nanodiamond particles were uniformly dispersed in marine engine lubricants. A matrix synthesis method was used for dispersion with various concentrations. Friction and wear tests were performed to measure the friction and wear amounts, and scuffing tests were performed. The friction coefficients were decreased with the addition of nanodiamond particles. Due to their octagonal and almost spherical shape, the particles could act as rolling contact elements between two lubricated sliding surfaces. In addition, it was found that there w...

Experimental Study of Roughness Effect in a Rolling–Sliding EHL Contact. Part I: Roughness Deformation

Abstract: This article presents experimental results of artificial roughness behavior inside elastohydrodynamic contacts. An optical tribometer with a high-speed camera was used to measure the film thickness distribution inside the contact. The results are compared to the amplitude attenuation theory including a description of harmonic components modification. Part I of the article deals with roughness deformation. The effects of central film thickness and slide-to-roll ratio on the roughness feature deformation were investigated for two lubricants. It was found that roughness is independent of sliding magnitude and position inside a highly loaded zone. The results showed a good correlation with Hooke's approximate amplitude attenuation model. However, it reaches a limiting deformation under thin film conditions. Part II of the article deals with complementary effect connected with surface roughness under rolling–sliding conditions.

Influence of Polyfluo-Wax on the Friction and Wear Behavior of Polyimide/Epoxy Resin–Molybdenum Disulfide Bonded Solid Lubricant Coating

Abstract: Polyimide/Epoxy resin–molybdenum disulfide bonded solid lubricant coatings (denoted as PI/EP-MoS2) were prepared. The influence of polyfluo-wax (denoted as PFW) on the microhardness and friction and wear behavior of as-prepared PI/EP-MoS2 lubricant coating was measured using a microhardness tester and a reciprocating ball-on-disc tribometer, respectively. The worn surfaces of the lubricant coatings were observed with a scanning electron microscope, and their wear rate was determined with a Micro XAM surface mapping microscope. Moreover, the transfer films formed on the counterpart steel ball surfaces were analyzed by X-ray photoelectron spectroscopy. Results indicate that the incorporation of a proper content of PFW filler is effective at improving the antifriction performance of the PI/EP-MoS2 lubricant coating while maintaining better wear resistance. Moreover, the friction coefficient of the lubricant coating decreases with increasing content of PFW from 2 to 10%, and the one with a filler cont...

Effect of Diamond-Like Carbon Coatings on Ball Bearing Performance in Normal, Oil-Starved, and Debris-Damaged Conditions

Abstract: In this study, a process was developed to deposit wear-resistant metal-doped diamond-like amorphous hydrocarbon and carbon coatings onto spherical rolling elements. Nanocomposite WC/a-C:H, TiC/a-C, and Ti/a-C:H coatings were applied in a closed-field unbalanced magnetron sputtering system and their mechanical, compositional, and microstructural properties were examined. The ability of the coated spherical rolling elements to affect bearing performance in normal, oil-starved, and debris-damaged conditions was evaluated in thrust bearing tests. WC/a-C:H-coated spherical rolling elements (balls) exhibit the ability to improve bearing performance in normal, oil-starved, and debris-damaged conditions.

Performance of Hydrostatic Textured Thrust Bearing with Supply Holes Operating with Non-Newtonian Lubricant

Abstract: The present article proposes a technique to improve the performance of textured hydrostatic thrust bearings with the use of oil supply holes. The article provides a comparison between smooth and surface-textured hydrostatic thrust bearings. To get an accurate prediction of the caviation regime, the Jakobsson-Floberg-Olsson (JFO) mass conservation algorithm has been used instead of the Reynolds boundary conditions. It has been observed that surface texture results in a significant decrease in friction power loss along with an improvement in other performance parameters. The study also reveals that the location of the supply hole for the lubricant system significantly affects the bearing performance.

A Parametric Study on Friction Instabilities in Mechanical Face Seals

Abstract: The present article aims to analyze and evaluate how dynamic parameter design influences the behavior of mechanical face seals and improves their performance by detecting undesirable phenomena like stick–slip and ringing. Those phenomena occur in situations of boundary lubrication.A new lumped parameter model is proposed to study mechanical face seals used in automotive cooling water pumps in order to examine how dynamic parameters such as mass, moment of inertia, and stiffness are involved in stick–slip phenomena. The aim of this work is to provide a practical design tool to predict stick–slip conditions and to choose the optimal mechanical specifications. Numerical simulations demonstrate how the critical service conditions vary with design parameters and permit tabulation of the results.

Measurements of Flow Rate and Force Coefficients in a Short-Length Annular Seal Supplied with a Liquid/Gas Mixture (Stationary Journal)

Abstract: Deep sea compression systems must work under strenuous conditions with either gas in liquid or liquid in gas mixtures, mostly inhomogeneous. Off-design operation affects the mechanical system's overall efficiency and reliability, with penalties in leakage and rotordynamic performance of secondary flow components, namely, seals. This article introduces a test rig to characterize the leakage and dynamic force coefficients of a short-length annular seal (L/D = 0.36, clearance = 0.127 mm) operating under various flow regimes ranging from pure gas, to bubbly (liquid in gas), to foamy (gas in liquid), to pure liquid. The test rig includes of rotating journal and a softy supported cartridge that make a clearance annular seal that is supplied with a liquid/gas mixture. Flowmeters record the fluid's passage, and with manual control of the streams, the mixture has a known liquid (or gas) volume fraction at the seal inlet plane. Two orthogonally mounted electromagnetic shakers excite the cartridge with perio...

Experimental Study on Corrugation of a Sliding Surface Caused by Frictional Self-Excited Vibration

Abstract: A series of tests is conducted on a pin-on-disc tester to study the effect of frictional self-excited vibration on evolution of the scar profiles. A laser displacement sensor is used to measure the profile size of the worn scars. An accelerometer is used to measure vibration of the pin specimen. The test results show that a sustained frictional self-excited vibration easily occurs under dry friction. When the vibration lasts for a long enough time, corrugation is generated on the sliding surface of the disc specimen. The wavelength of corrugation is approximately equal to the sliding speed multiplied by the period time of the friction-induced vibration.