Search

This paper addresses the lubrication issues experienced by rolling element bearings operating under limited lubricant supply (LLS) or starvation conditions. A novel three-dimensional comb-tooth-groove (CTG) surface pattern is introduced to promote oil replenishment through unidirectional force. The transport of oil droplets on the CTG pattern is examined to evaluate this force, while groove length and depth are characterized as key factors influencing the surface force. More practically, the impact of CTG patterns on bearing lubrication under LLS conditions is assessed through experiments on bearings modified with CTG surface arrays. These modified bearings exhibit reduced torque and temperature rise, highlighting the significant advantages of CTG surfaces in enhancing bearing performance. [ABSTRACT FROM AUTHOR]

The mechanical and thermo-physical properties of a coating material affect the lubrication performance. The present study aims at to investigate the effect of mechanical and thermo-physical properties of coating material on the lubrication performance of rolling contact operating under Thermal Elasto-Hydrodynamic Lubrication (TEHL) regime. The mechanical and thermo-physical properties of the coating material considered in the present analysis are elastic modulus, density, thermal conductivity and specific heat. Furthermore, the effect of coating thickness is also studied. The Commercial Multiphysics software ANSYS is used in the present analysis to model and analysis of the finite line contact obtained in the rolling contact. It is observed that the pressure intensity increases with the elasticity of the coating. The maximum fluid pressure developed at the contact region 1.2GPa with the application of material possessing higher modulus of elasticity i.e. 420 GPa. With increase in coating thickness the contacts width reduces. The material having lower density, thermal conductivity and heat capacity exhibits as an insulating material which obstructs the transfer of heat from the fluid to the contacting surfaces. This leads to increase the temperature of the lubricant. With these properties the maximum developed temperature observed to be 351K. [ABSTRACT FROM AUTHOR]

A better understanding about the rolling contact fatigue and micropitting performance of machine component surfaces lubricated with environmentally friendly lubricants is critical to designing and further formulating new lubricants intended to be used in rolling–sliding contacts such as those found in gear and bearing applications. In this work, the frictional behaviour and rolling contact fatigue (RCF) performance of DLC, Cr/a-WC:H/a-C:H and a-C:Cr coatings under glycerol-based lubrication in rolling sliding contact conditions have been investigated. Traction maps, Stribeck curves, and fatigue plots have been generated by using a micropitting test rig (MPR). The initiation and progression of micropitting was monitored by means of white light optical interferometry and scanning electron microscopy (SEM). Results indicated that glycerol-based lubricants exhibited a significant friction reduction as the hydrodynamic effect is enhanced at higher rolling-speeds. Under boundary lubrication the friction coefficient was significantly higher compared to the values obtained with a commercial mineral-based transmission oil. Compared to uncoated steel surfaces, DLC coatings effectively reduced the volume loss and micropitting progression. Irrespective of the coating thickness, DLC showed an excellent tribological behaviour when the base lubricant favours the onset of mild-wear, over micropitting. When the lubricant formulation favoured the onset of micropitting, the coatings tended to prematurely fail due to debonding from the substrate, and local micro-spallation. The experiments demonstrated that friction reduction does not necessarily correspond with a reduction of micropitting. • The tribological properties of DLC coatings under glycerol–based lubrication were investigated. • Glycerol–water lubrication triggered superlubricity in a DLC–coated contact. • Micropitting and mild–wear equilibrium was influenced by the lubricant formulation. • DLC coatings inhibited micropitting progression under glycerol–based lubrication. [ABSTRACT FROM AUTHOR]

Purpose: Rolling bearing operation under mixed and boundary lubrication conditions may lead to heavy adhesive or abrasive wear, which may lead to wear-induced rolling bearing failure. The purpose of this paper is to investigate the wear protection capabilities of different grease compositions at varying temperatures. It is considered that the temperature influences the lubrication conditions, the behaviour of grease components, namely, bleed oil and thickener, as well as the tribofilm formation due to tribo-chemical interactions between additives and surfaces. Design/methodology/approach: In this study, four different greases were produced on the basis of a mineral base oil by varying the thickener and the addition of ZDDP. Various grease-lubricated rolling bearing experiments were conducted in a wide temperature range from 0°C to 120°C. Subsequently, the wear pattern, tribofilm formation and grease structures were analysed. Thereby, the influence of the different grease thickeners and the performance of ZDDP as a common antiwear and extreme pressure additive was evaluated. Findings: The results show a strong temperature-dependency and allow a classification of temperature ranges concerning wear protection. At low temperatures, all greases provide a very good wear protection without the evidence of additive-based tribofilm formation. In the experiments at elevated temperatures, ZDDP tribofilms were formed. The formation depends on the thickener type: in comparison to lithium thickener, polyurea thickener favours more protective tribofilms at the same temperature. The experimental results show that medium temperatures in the range of 40°C–60°C are critical concerning wear due to the insufficient tribolayer formation and limited load carrying capacity of the grease. Originality/value: Temperature is a key operating parameter for grease lubrication in roller bearings. The experimental work enables consideration of different impact pathways of temperature by combining roller bearing tests and microanalysis. [ABSTRACT FROM AUTHOR]

This paper proposes a method for detecting insufficient lubrication in rolling bearings, by utilising a low cost MEMS ultrasonic microphone. We run a set of grease lubricated bearings under load, alternating between good and insufficient lubrication conditions by adjusting speed and temperature, over an 18 day experiment, followed by a short validation experiment on a different set of bearings on a secondary test rig. Vibration signals are recorded from an ultrasonic microphone, using a purpose built prototype sensor, then band-pass and high-pass filtered and subsequently analysed using statistical time-domain features. The viscosity ratio Kappa of the lubricant is used as a metric for quality of oil-film formation, and is calculated throughout the experiment from rotating speed and temperature. The results are compared with the predictions from a linear regression model and a simple Neural Network trained on ensembles of three and ten selected features. The three Hjorth's Parameters activity , mobility and complexity calculated on high-pass filtered microphone signals from the experiments show a good correlation with the viscosity ratio Kappa. When combining these, using a Linear Regression model or a simple two layer Neural Network model, we are able to predict the viscosity ratio with good precision. The proposed method may be used to assess the quality of oil film formation in bearings during operation under varying speed and temperature conditions. [ABSTRACT FROM AUTHOR]

Rolling element bearings and gears are critical components of mechanical systems such as wind turbines and automotive engines and transmissions that use oil-based lubricants. Oil-based lubricants used in these applications many times need periodic replacement, which not only adds more cost but also affects overall productivity. More importantly, the used oil generates hazardous waste creating huge environmental problems. In this study, we demonstrated that nanomaterials can be employed as solid lubricants in combination with diamond like carbon (DLC) films in a dry nitrogen environment under rolling/sliding contacts. Using a micropitting rig which is generally used to test and qualify materials and lubricants for bearings and gear systems for industrial applications, we have tested diamond like carbon (DLC) material pairs in an oil-free, dry nitrogen environment along with two-dimensional MoS2 combined with nanodiamond as a solid lubricant. We show that superlubricity (traction coefficient of friction is 0.003) was achieved through the formation of a carbon rich superlubricious tribolayer at the interface reducing the overall friction by a minimum of 20 times, and no surface damage was observed as compared to Steel-Steel contacts lubricated with oil (0.06–0.07). The current work paves the way for developing oil-free solid lubricants in a variety of applications involving rolling/sliding contacts. [ABSTRACT FROM AUTHOR]

In the process of post-production testing of rolling bearings, quality classification and/or qualification of finished products as good and defective is performed. In view of mass production, bearing testing should be carried out within a short period of time. Vibroacoustic (VA) processes are widely used for post-production testing. Parameterization of bearing vibrations is largely determined by stabilization of lubrication conditions and VA processes, which to some extent depend on temperature. The article presents the results of temperature measurements carried out during the testing of a set of new tapered roller bearings. On their basis, an attempt was made to estimate the value of rms values of bearing vibration accelerations. This is based on the known relationships between the temperature and viscosity of the oil as well as the relationship between viscosity and acceleration of the bearing vibrations. The estimated acceleration values were verified experimentally. The time after which the temperature of a bearing is stabilized was also determined. The paper also shows the influence of a lubricant on these processes, mainly its kinematic viscosity. [ABSTRACT FROM AUTHOR]

This study aimed to investigate the formation and alteration of tribofilm during pure rolling contact under sufficient lubrication. The surface state changes were also analyzed. Rolling contact experiments with varying stress cycles were conducted using a ball-rod testing machine. Characterizations on surface state including morphology, roughness and plastic deformation were executed and chemical composition and valence state of tribofilm were detected for different rolling periods. The results indicated tribofilm was formed and evolved due to the effect of Hertz Contact stress. Surface morphology changes and plastic deformation were caused by Hertz Contact, simultaneously. The micro-sliding destroyed the tribofilm to a certain degree, resulting in the change of surface state and tribofilm composition compared to the pure rolling region. [ABSTRACT FROM AUTHOR]

Abstract The film forming and friction properties of nine greases covering different thickener types, viscosities and NLGI grades have been investigated. Film thickness results are in agreement with previous work that all greases form a thicker film than the base oil counterpart at low speeds, suggesting that lambda ratio and kappa of greases can be much higher than their corresponding oils. The base oil viscosity appears to govern the grease film thickness at low speeds by a direct relationship for polyurea greases whereas by an inverse relationship for lithium and calcium complex greases. Grease friction results indicate that the shape of grease friction curves can differ quite significantly from that of oils. Possible underlying mechanisms that govern friction properties are discussed. Graphical abstract Image 1 Highlights • Nine greases covering different thickener types, viscosities and NLGI grades. • Low speed grease film thickness depends on the thickener type & base oil viscosity. • Polyurea greases former a thicker low speed film followed by Ca and Li greases. • Lambda ratio and kappa of greases are much higher than those of their base oils. • Grease friction curve shapes differ from that of conventional oil Stribeck curves. [ABSTRACT FROM AUTHOR]

Purpose This aim of this paper has been to investigate the squeeze effect of a water-lubricated tilting-pad thrust bearing during start-up and shut-down periods.Design/methodology/approach In this paper a numerical model with a squeeze and slippage effect was adopted to analyse the asymmetry characteristic of a tilting-pad thrust bearing during start-up and shut-down periods. A test rig was built to verify numerical results, which were a combined measurement method in which acceleration sensor and torque sensor were used simultaneously to determine the angle change of the thrust pad.Findings It was found that as the velocity gradient increased, the difference of the minimum dimensionless film Hmin could be ignored in the start-up process. But in the shut-down process, as the velocity gradient increased, the value of Hmin also increased, which showed that there was an asymmetry characteristic of the tilting bearing in two processes. This phenomenon was verified by measuring the friction torque curve in the test.Originality/value The results of the studies demonstrated that the velocity gradient could be designed to reduce the friction of the thrust bearing, which would be beneficial to the working life of the tilting-pad thrust bearing. [ABSTRACT FROM AUTHOR]

Experimental and numerical analyses of the lifetime and crack propagation behavior of silicon nitrides under rolling contact fatigue (RCF) loading are surveyed. Intercomparison of four major test methods for RCF, namely balls-on-flat (BOF), balls-on-rod (BOR), two-roller, and four-ball configurations, are performed. The differences in the implications of the test results between the first three methods, are explored by using a test specimen made of material from the same production batch. With an understanding of the differences, the influences of variables such as the state of lubrication and surface roughness on RCF behavior is discussed. The influence of mechanical properties of test materials is also discussed in chronological order to trace the history of trends in the material development of silicon nitrides. Crack growth models based on observation results of propagation from an artificially induced crack, semi-elliptical or partial ring crack, are comparatively discussed along with the results of numerical analysis for more simplified initial crack models. An observational study of propagation behavior from a natural flaw is also addressed. Finally, future work to improve the reliability of ceramic bearings is discussed. [ABSTRACT FROM AUTHOR]

A hierarchical multiscale method is proposed in this paper to study the rolling contact fatigue with the consideration of lubricant effects. This multiscale model consists of a molecular model of lubricant and a continuum model of rolling contact components. At the nanoscale, molecular dynamics is employed to model the lubricant and to calculate the friction coefficient at the rolling contact surface. At the macroscale, the finite element method is used to conduct stress analysis of the rolling contact component so that the fatigue life can be predicted. The calculated friction coefficient is passed from the molecular model to the continuum model in the proposed multiscale model. The effect of fluctuating load on the rolling contact fatigue life is also studied in this paper. The objective of this paper is to develop a new multiscale framework which connects the nanoscale and the mascroscale. It shall be noted that a commonly used fatigue life model is employed in the continuum model. [ABSTRACT FROM AUTHOR]

Purpose The stability characteristics of an oil film directly influence the safety and service life of mill oil-film bearings. However, very limited work has been done to address the stability characteristics of mill oil-film bearings. To this end, this paper aims to investigate the stability characteristics of mill oil-film bearings through theoretical and experimental analysis.Design/methodology/approach For the first time, a special designed experiment platform was developed to investigate the stability characteristics of mill oil-film bearings. In addition, a theoretical model of lubricating film of the tested bearings was established to analyze the oil-film stability. The theoretical results were compared with the experimental results.Findings The comparison results demonstrate that the critical influential factors on the bearing stability were the eccentricity ratio and the ratio of bearing length to diameter. The mill bearing was likely to be unstable under a small load and at a high rotational speed.Practical implications The paper includes implications for suitable operation conditions in practical use of mill oil-film bearings.Originality/value This paper fulfills an identified need to investigate oil-film stability of mill bearings for practical applications. [ABSTRACT FROM AUTHOR]

Rolling element bearings are the most critical parts of any rotary mechanism. One of the challenges in any bearing design is to minimize the friction and maximize life for a particular service environment. For critical aerospace systems such as jet/liquid rocket engines, bearings are the key to improving performance benchmarks. Though bearings are a common machine element, literature related to criticality of bearings used in aerospace industries is very limited. The selection of precision class of bearings, surface finish and overall quality of critical contact surfaces in bearings, lubricant selection, lubrication scheme, processing of bearings, integration into mechanisms have been discussed in this review. A glimpse of emerging technologies in active bearing systems and smart bearing solutions has been included. [ABSTRACT FROM AUTHOR]

Grease plays an important role in reducing friction and wear of rolling bearings and improving their service life and reliability. However, increasing attention is being paid to the environmental pollution caused by discarded grease. In this paper, calcium sulfonate complex grease (CSCG) was prepared by using green raw materials, namely poly-alpha-olefin oil as the base oil and complex calcium sulfonate soap as thickener. In order to improve the lubrication and damping performance of the grease, hexagonal boron nitride (hBN) and nano-Al 2 O 3 , which have the potential to replace toxic additives, are mixed into the grease in different mass proportions. The thickener structure of each grease was characterized and analyzed. According to the bearing vibration and life of the two key performance indicators, four-ball tests and bearing vibration tests under limited lubricant supply (LLS) were performed. The results show that CSCG with 0.75 wt% hBN and 0.25 wt% Al 2 O 3 (BN75AL25) has the best tribological and vibration suppression performance. The mean friction coefficient decreased by 11.3%, the wear scar diameter decreased by 6.1%, and the worn surface morphology improved significantly compared with CSCG. The frequency domain analysis showed that high, medium and low frequency vibrations were suppressed by 19%, 11.7% and 16.4%, respectively. In conclusion, BN75AL25 has the potential to become a green high-performance grease under LLS, which is worth further research and exploration. [Display omitted] • The nano-hBN can promote the formation of calcium carbonate protective layer. • The nano-Al 2 O 3 can improve the tribological performance of the grease with hBN. • The nano-hBN and nano-Al 2 O 3 can reduce the vibration by momentum exchange. [ABSTRACT FROM AUTHOR]

Heavy load contacts are less presented in the literature because of small area of industry utilization. Large dimensions of the bodies in contact is an additional limit of the study presented furthermore. The paper presents a particular case of an example in wood industry and subjects like loads, materials, defects or contact conditions will be touched. Observations regarding insufficient lubrication and change of contact conditions in use were made. [ABSTRACT FROM AUTHOR]

By considering real wheel and track geometry and structures, as well as their coupled interaction, an explicit finite element method is applied to simulate the transition of wheel–rail rolling from single-point tread contact to two-point tread-flange contact. The evolutions of the contact position, stress magnitude and direction, adhesion–slip distribution, and wheel–rail relative velocities in the two contact patches are considered to investigate the transient dynamic effects during the contact transition. Important findings include that the transition to two-point contact can result in friction saturation and excite waves in the contact, causing local intensification and relaxation of compression, as well as turbulence of the micro-slip; the local relative velocity in the contact patch is a good measure of the dynamic effects. [ABSTRACT FROM AUTHOR]

This study focuses on the damage mechanisms in silicon nitride rolling elements used in hybrid (ceramic–metal) bearings that operate under non-conformal contact. To get an insight into the prevailing damage modes compared to the real application, a rolling contact experiment was designed to mimic the contact conditions. Hertzian contact pressures ranged from 3.0 to 5.9 GPa (500–4150 N). In order to approach pure rolling, the experiments were run without inducing any gross slip. Extensive surface and subsurface damage analysis was performed using conventional ceramography as well as FIB cross-sectioning. Finite element simulations were carried out to illustrate the stress state prevailing under different loading conditions. Surface damage to rollers subjected to contact pressures up to 5.1 GPa (2500 N) was mainly dominated by micro-spalling, which was induced due to the presence of snowflake structures. At the highest applied loads, damage appeared as a combination of macro-cracking and micro-spalling. Crack propagation was attributed to different mechanisms: (a) fatigue-induced fracture and (b) lubricant-driven crack propagation in the subsurface. [ABSTRACT FROM AUTHOR]

This study investigated friction behavior of a bearing system with two interfaces involved: a roller component experiencing rolling–sliding interaction against twin cylinders under point contacts while simultaneously undergoing pure sliding interaction against a socket under a conformal contact. Lubrication modeling predicted a strong correlation between the roller׳s rolling condition and the system׳s friction behavior. Experimental observations first validated the analytical predictions using steel and iron components. Diamond-like-carbon (DLC) coating and AlMgB 14 –TiB 2 coating with a carbon topcoat (BAMC) were then applied to the roller and twin cylinders, respectively. Testing and analysis results suggest that the coatings effectively decreased the slip ratio for the roller–cylinder contact and the sliding friction at both bearing interfaces and, as a result, significantly reduced the system frictional torque. [ABSTRACT FROM AUTHOR]

Actual volume of the supplied oil that functions as a lubricant in the contact regions of rolling bearings is intriguing and difficult to determine. In this study, we propose a novel experimental method that combines synchronized dual-camera imaging with laser-induced fluorescence (LIF) using extra illumination to observe the oil flow in a ball bearing. In addition, we perform simulations, which reveal that most of the oil layer adhering to the component surfaces is not directly involved in lubrication of the contact regions. The thickness of the oil layer along the rolling contact trace decreases markedly as the decayed area appears. The oil layer decay severely weakens the oil replenishment, and its degree increases with the capillary number Ca. [ABSTRACT FROM AUTHOR]

The wear fatigue of slewing bearings at the contact areas of raceways (AISI 4140 steel (AS)) and rollers (AISI 52100 steel) seriously reduces the service life of equipment. AS-SnAgCu-TiC composites (AS-SACT) were prepared by filling multi-solid lubricants into surface biomimetic textures of AS to improve the wear resistance. Tribological properties were tested using a block-on-ring tribometer. The materials of block and ring were AS and AISI 52100 steel, respectively. Compared with that of AS, coefficient of friction of AS-SACT was decreased by 67.93%, and wear height was reduced by 42.19%. Wear triggered the diffusion of SnAgCu from the bionic textures to worn surface. The hard phase of nano-TiC was wrapped by a cluster of multi-layer lubricants to form spherical particles, which not only converted sliding to rolling contact, but also repaired pits and furrows on worn surface. Hence, AS-SACT with self-repairing behavior had excellent properties of antifriction and wear resistance. [Display omitted] •Tribological properties of AISI 4140 steel were improved by texture and SnAgCu–TiC. •Average COF of AS-SnAgCu-TiC was 67.93% lower than that of AS (AISI 4140 steel). •Wear defects were filled with solid lubricants to produce self-repairing behavior. •Spherical particles transformed the frictional mode from sliding to rolling. •Nano-TiC particles wrapped by SnAgCu can promote wear defects to be filled. [ABSTRACT FROM AUTHOR]

Diamond-like carbon (DLC) and Boron Carbide (B 4 C) coatings are promising candidates for sliding–rolling contact parts, especially gears to increase scuffing load capacity and prolong lifetime. Generally, commercially available fully formulated oils are compatible with ferrous surfaces, but normally have not been optimised for coatings. In this study, the tribological properties of tungsten carbide containing DLC (WC-DLC), hydrogenated DLC (H-DLC) and B 4 C coatings against steel ball with two fully formulated oils in boundary lubrication conditions have been evaluated. They were compared with that of a Steel/Steel tribocouple. Interactions between lubricants and sliding surfaces were also investigated using surface characterisation techniques such as XPS and Raman Spectroscopy. It was found that Steel/H-DLC system provided the lowest friction coefficient for both oils. [ABSTRACT FROM AUTHOR]

A novel method for in situ profile measurements on rolling contact test specimens has been demonstrated. A chromatic confocal probe was integrated into a twin-disc machine and used for disc profile measurements during periodically interrupted wear tests. The arrangement allowed detailed studies on the progress of the wear during a test without the removal of discs from the tribometer. Data from the interrupted tests was compared with the results of continuous tests. Both types of tests were run with two grades of cast iron dicsc in lubricated contact with steel. The results of this new method were in good agreement with those of a conventional diamond stylus probe method applied before and after the tests. [ABSTRACT FROM AUTHOR]

Elastohydrodynamically lubricated (EHL) contacts rarely exist as single contacts. Multiple contacts or single contacts subjected to the repeated over-rolling represent more often the case in practical applications. A typical example is the rolling element bearing. A lubricant rupture mechanism at each contact outlet determines the lubricant availability to the succeeding contact. This work presents a quantitative description of the lubricant film thickness rupture in EHL contact outlet with the use of the fluorescent microscopy. A rupture ratio of the film thickness between two diverging surfaces exiting the contact was measured for both pure rolling and rolling-sliding conditions. The influence of variation of several parameters such as lubricant properties, rolling speed or rolling element ellipticity to the lubricant rupture ratio was investigated. Understanding of the physical phenomena of the lubricant rupture extends further possibilities in both experimental and theoretical researches of the starved EHL. [ABSTRACT FROM AUTHOR]

Three types of differently shaped lasered microtextures (microdimples) were produced on the surface of a U75V hot-rolled rail roller with a TY-FM-20S laser marking machine. Wheel flange/rail gauge lubrication starvation tests of four kinds of rail samples running against CL60 wheel samples were carried out, and the friction properties of the wheel/rail steels were studied. The results indicate that the injection of lubricating grease dominates the friction coefficient reductions and that the lubricating effects are closely related to the shapes of the lasered microtextures; for instance, the rectangular and rhombic textures show a better lubricating effect. Furthermore, the wear rates depend on the wear mechanism of the wheel/rail steel; for example, delamination and spalling always result in severe wear while peeling results in mild wear. The materials from the rolling-sliding surface to the matrix region of the wheel/rail materials can be divided into three parts: the plastic deformation region, transition region and undeformed matrix region. The average crack length and angle values of the textured rail and coupled wheel steels are much smaller than those of the untextured rail and coupled wheel steels; thus, the textured rail and coupled wheel steels exhibit better rolling contact fatigue (RCF) resistance. The effects of the lasered microtextures shapes on the wear and fatigue damage are determined based on the fluid dynamic pressure on the microtexture. Notably, rhombic lasered microtextures can be adopted to improve the grease storage capacity, thereby reducing wear and fatigue damage. • Three types of differently shaped lasered microtextures were produced on the rail surface. • Lubrication starvation tests of four kinds of rail samples were carried out. • Lubricating effect, wear and rolling contact fatigue behaviours were proposed and studied. • The textured rail and coupled wheel steels demonstrated better RCF resistance. • The lubricating and damage mechanism of textured rail were analysed and revealed. [ABSTRACT FROM AUTHOR]

ABSTRACT In the paper, the previously developed asymptotic approach to solution of the steady isothermal problem of elastohydrodynamic lubrication (EHL) for heavily loaded point contacts is applied to a lubricated point contact with rolling and spinning. It is shown that the whole contact region can be subdivided into three subregions. The central region can be subdivided into the Hertzian region and two adjacent boundary layers - the inlet and exit zones. The main results of the paper are threefold: (i) it is shown that in the central parts of the inlet and exit zones, the mechanisms and the equations controlling the behaviour of the lubrication contact parameters in heavily loaded point and line EHL contacts are identical, (ii) asymptotically precise formulas for the central and exit lubrication film thickness for pre-critical and over-critical starved and fully flooded lubrication regimes are analytically derived, and (iii) the inlet and exit zone asymptotically valid equations are uniform across all steady heavily loaded line and point EHL contacts for lubricants with the same rheology. These asymptotically valid equations were analysed and numerically solved in previously published work based on the stable methods utilising the specific regularisation approach developed for lubricated line contacts. Cases of pre-critical and over-critical lubrication regimes are considered. The formulas for the lubrication film thickness for pre-critical and over-critical starved and fully flooded lubrication regimes allow for simple analysis of the film thickness as a function of spinning angular speed, angle of the entrained lubricant and other pertinent contact characteristics. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

A remarkable feature in the study of scuffing is that confusions and controversies have occupied this research area for many years. Occurrence of scuffing is attributed to different sources. Until now, the most important researches don't establish a unit criterion to be use as safe criteria against scuffing failure. Under particular combinations including contact pressure, lubrication, speed and friction, a critical temperature is reached in the vicinity of the contact. At this temperature, the breakdown film with local welding or adhesion of the contacting surfaces can appear. The paper develops a temperature distribution model for rolling contacts related to the bearings scuffing risk. The observation of the scuffed contact is conducted. Samples from the scuffed region of the inner race are examined by scanning electron microscopy and elemental chemical analysis of EDS type is made. The scuffing phenomenon is discussed after examining and analyzing these photographs. [ABSTRACT FROM AUTHOR]

Abstract: Grease is commonly used to lubricate various machine components such as rolling bearings and seals. In this paper the flow of lubricating grease passing restrictions is described. Such flow occurs in rolling bearings during relubrication events where the grease is flowing in the transverse (axial) direction through the bearing and is hindered by guide rings, flanges etc, as well as in seals where transverse flow occurs, for example during so-called breathing caused by temperature fluctuations in the bearing. This study uses a 2D flow model geometry consisting of a wide channel with rectangular cross-section and two different types of restrictions to measure the grease velocity vector field, using the method of Micro Particle Image Velocimetry. In the case of a single restriction, the horizontal distance required for the velocity profile to fully develop is approximately the same as the height of the channel. In the corner before and after the restriction, the velocities are very low and part of the grease is stationary. For the channel with two flow restrictions, this effect is even more pronounced in the “pocket” between the restrictions. Clearly, a large part of the grease is not moving. This condition particularly applies to the cases with a low-pressure drop and where high consistency grease is used. In practice this means that grease is not replaced in such “corners” and that some aged/contaminated grease will remain in seal pockets. [Copyright &y& Elsevier]

Abstract: Rolling tools made of advanced ceramics have some attractive properties with respect to their wear resistance and high temperature performance but are critical due to the brittle failure mode. Lifetime predictions must be based on a probabilistic approach considering sub-critical and cyclic propagation of the inherent flaws. We will present a comprehensive reliability analysis for a rolling contact fatigue test with a silicon nitride roll using the program STAU. Based on a cyclic crack growth law, we have extended the capabilities of STAU in order to account for a complex load history typically found in the case of rolling applications. A scheme is presented for relating the failure probability to the crack density observed on the roll which allows for a quantitative comparison of predictions and experimental observations. It turns out that the water-based lubrication most critically affects the failure behaviour since water strongly promotes cyclic crack propagation. [Copyright &y& Elsevier]

Additives in lubrication technology perform by physically or chemically adsorbed films on tribological surfaces. In water lubricated tribosystems when the environment is very corrosive the additives effect is especially important. Typically applied corrosion inhibitors neutralize pH of contaminants in the fluid. Ethanolamines and ethylamines are known as ligands which can form chelate bonds with metals via their amino, hydroxyl and deprotonated hydroxyl groups. In tribology they are widely accepted, as corrosion inhibitors and detergents especially for water-based lubricants. This study inquires static corrosion behaviour of the solution as well as tribological performance in two types of tribotesters which represent different contact situations. Understanding the performance of the first chemisorbed layers of additives on the work piece yields important information for the optimization of lubrication in aqueous solutions. The structure of the adsorbed compounds based on amines on the tribological surfaces was studied with usage of angle-resolved X-ray photoelectron spectroscopy (ARXPS). Analysis of the ARXPS results was performed using stratification method, which provides information of the depth and relative amount of each element in each layer. The experimental results were compared with simulations performed with simulation of electron spectra for surface analysis software. The objective was to confirm the accuracy of the performed analysis. [ABSTRACT FROM AUTHOR]

Abstract: Shelling is one of the typical rolling contact fatigue (RCF) failures of railway wheels. Creepage between wheels and rails due to rolling radius difference in curves is inevitable on railway tracks. Therefore, there are tangential stresses and slip phenomena on wheel treads. Eventually, initiation of RCF cracks being the origin of shelling is accelerated by creepage. The objective of the present paper is to evaluate RCF property of railway wheel steel. The effect of the slip ratio on the RCF strength of the railway wheel steel was evaluated. RCF tests were conducted using two cylindrical contact specimens under water lubrication at a slip ratio of 0.0–1.0%. The range of slip ratio is determined to assume a creepage between wheel and rail during curving. As a result, it was found that the traction coefficient increased with the increase in the slip ratio, and the fatigue strength decreased simultaneously. The results were evaluated by a shakedown map and Hirakawa’s RCF map. Experimental fatigue limits could be expressed more precisely by the criterion of Hirakawa’s RCF map than that expressed by the well-known shakedown map. Stress intensity factors (SIFs) of the cracks produced by the RCF test were calculated by finite element (FE) analysis where the effect of water pressure due to the penetration of water into the cracks is taken into account. Two peaks of the maximum tangential SIF occurred during one cycle of rolling contact. The direction of the crack propagation was estimated by the maximum tangential stress criteria. The results of the RCF test showed that the cracks were initiated at the surface, propagated obliquely in the depth direction and then branched into two directions. One was towards the surface and the other was towards the depth. These two crack directions were inspected experimentally, corresponding to the estimated crack directions from the two peaks of SIF by FE analysis. The effect of slip ratio on RCF crack propagation was discussed by using the SIFs of the RCF cracks where water penetration into the cracks was considered. [Copyright &y& Elsevier]

Abstract: This paper addresses rolling contact fatigue (RCF) testing in ultra high vacuum (UHV) under high speed conditions. A ball–rod RCF test platform has been adapted for testing in UHV conditions that allows rapid accumulation of stress cycles, over 10 million cycles within 5h of testing at 130Hz rotation. The UHV environment and solid lubrication enables good vibration detection for the onset of spall. In this paper, approximately 0.2μm of silver is applied to the balls and provides sufficient lubrication for up to 25h of testing, or 50 million stress cycles in high vacuum at 130Hz. Seventy-nine RCF tests using thin-film silver lubrication have been completed covering two ball sizes, and two rod and ball materials. 9.53mm diameter Rex 20 steel and silicon-nitride (Si3N4) rods were tested against 7.94mm diameter Rex 20 and 12.7mm diameter M50 steel balls. It was found that ball size and material hardness did not affect the stress cycle life over a Hertzian contact stress range of 2.1–4.2GPa and Rockwell C hardness range of 62–77. Rather, the key limiter to test length is lubrication depletion based on 79 tests and an average silver thickness of 0.2μm. One of the two failure modes were observed for all tests: (i) early life spall of the silver coating, and (ii) depletion of silver lubrication followed by spall failure of both the ball and rod surfaces. A third-body storage model along with the Control Volume Fraction Coverage (CVFC) assumption and analysis was used to predict lubrication availability between asperities on the third body. There is good agreement between calculated and measured post-test lubrication thickness using the third-body storage model. [Copyright &y& Elsevier]

Abstract: Rolling contact fatigue tests were carried out on ring specimens made of quenched and tempered SAE 5135 gear steel with three different steel-production processes, through a bi-disc machine under pure rolling condition and water lubrication. Early formation of micro-pits then coalescing into macro-pits was observed on the rolling surface, while the final failure was caused by subsurface originated spalling phenomena. Microscope analysis of specimens section highlighted the complex surface and subsurface crack layout, and permitted to recognise sulphides as preferential sites for cracks initiation. The inclusion content was analysed throughout the extreme value statistics and the maximum expected inclusion in the Hertzian contact zone was introduced in a failure assessment diagram recently proposed, which resulted effective in predicting the specimen failures. [Copyright &y& Elsevier]

Abstract: The effect of shot peening on rolling contact fatigue (RCF) and lubricant film thickness within non-conformal rolling/sliding contacts operated under mixed lubrication conditions was observed in this study. Rolling contact fatigue tests and film thickness measurements were carried out using specimens with modified surface topography by shot peening process using glass beads having diameter between 0.07 and 0.11mm. It has been shown that the effect of shot peening on RCF has no positive effect even if shot peened surface of the roller exhibited somewhat higher hardness in contrast to the grounded surface. The reduction of RCF may be caused due to asperities interactions because after shot peening the surface roughness of the roller was increased. Film thickness measurements confirmed that the contact is realized actually only between asperity peaks of shot peened ball and smooth disc. Conversely, no negative effect on RCF was observed when the shot peened surface of the roller was polished. The polish of asperity peaks causes the creation of lands and micro-cavities, which may be employed as lubricant micro-reservoirs. From film thickness measurements it has been observed that lubricant emitted by shallow micro-cavities can provide the local increase in lubrication film thickness, which thereby reduces asperities interactions. Similar results were obtained for start-up conditions where the squeeze lubricant enlarges film thickness and reduces surface interactions. From the obtained results, it can be suggested that properly designed surface topography modification could help to increase the efficiency of lubrication films leading to the enhancement of contact fatigue life of non-conformal mixed lubricated rolling/sliding contacts. [Copyright &y& Elsevier]

Abstract: In this study we investigated the sliding velocity dependency of the coefficient of friction for a Si-containing diamond-like carbon (DLC-Si) film in an automatic transmission fluid (ATF) under a wide range of contact pressures. The DLC-Si film and a nitrided steel with a surface roughness, RzJIS, of around 3.0μm were used as disk specimens. A high-carbon chromium steel (JIS-SUJ2) bearing ball was used as a ball specimen. Friction tests were conducted using a ball-on-disk friction apparatus under a wide range of sliding velocites (0.1–2.0m/s) and contact pressures (P max: 0.42–3.61GPa) in ATF. The friction coefficients for the nitrided steel had a tendency to decrease with an increase in sliding veloicity under all the contact pressure conditions; however, the friction coefficients for the DLC-Si film were stable with respect to sliding velocities under all the contatct pressures. These results indicate that the DLC-Si film suppresses the stick-slip motion during sliding againt steel in ATF, which is a desired frictional characteristic for the electromagnetic clutch disks used under lubrication. Furthermore, the DLC-Si film showed a higher wear resistance and lower aggression on the steel ball specimen than the nitrided steel. There were less hydrodynamic effects on the friction coefficient for the DLC-Si film possibly due to maintenance of the initial surface roughness and its poorer wettability with the fluid. X-ray photoelectron spectroscopy (XPS) analysis of the sliding surfaces revealed that the adsorption film derived from the succinimide on the sliding surfaces of the DLC-Si film and the mating steel ball also contributed to the sufficient and less sliding-velocity-dependant friction coefficients. [Copyright &y& Elsevier]

Abstract: In order to know the relationship between the occurrence of flaking leading to the rail breakage and wear of the rail, on-site investigations and analysis of the broken rail were carried out. As a result, it was found out that the occurrence rate of the flaking damage was high at gauge corner side of high rail in moderate curves with radius from 800m to 1800m. It was also found that the rail wear in the section where lubrication was applied was about a half of that in the section without lubrication. Moreover, it was found that flaking damage was generated on the running surface or in the close vicinity of the surface of rail head under the condition of repeated loading of shear and tangential forces caused by the rolling contact with wheels. It could be assumed that the balance between the wear and the accumulation of micro-strain or fatigue was a key to occur the flaking damage. [Copyright &y& Elsevier]

Abstract: A profound description of friction in wheel–rail contact plays an essential role for optimization of traction control strategies, as input quantity for railway simulations in general and for the estimation of wear and rolling contact fatigue. A multitude of wheel–rail contact models exists, however, traction–creepage curves obtained from measurements show quantitative and qualitative deviations. There are several phenomena which influence the traction–creepage characteristics: Mechanisms resulting from surface roughness, frictional heating or the presence of interfacial fluids can have a dominating influence on friction. In this paper, a new wheel–rail contact model, accounting for these influential parameters, will be presented. The presented model accounts for the interaction of an interfacial fluid model for combined boundary and mixed lubrication of rough surfaces with a wheel–rail contact model that additionally accounts for frictional heating. A quantitative comparison with measurements found in the literature is not conducted, since the exact conditions of the measurements are mostly unknown and parameters can easily be adjusted to fit the measurements. Emphasis is placed on the qualitative behavior of the model with respect to the measurements and good agreement is found. The dependence of the maximum traction coefficient on rolling velocity, surface roughness and normal load is studied under dry and water lubricated conditions. [Copyright &y& Elsevier]

Lubrication in cold rolling plays an important part for process feasibility and process quality. The hydrodynamic process of lubrication is very complicated and affected by many material and process parameters. This paper examined partial lubrication in the cold rolling process. The average flow Reynolds equation for rolling lubrication was 2set, which considered the pressure-viscosity and average flow effects. Lubricating factors such as sidling, surface waviness, lubricant viscosity, surface roughness, and reduction ratio were investigated. The results of the lubrication equation show that sliding, lubricant viscosity, and surfaces roughness affect the values of rolling friction. Surface waviness and reduction ratio also influence both rolling pressure and rolling friction. [ABSTRACT FROM AUTHOR]

Abstract: Surface micro dents may act as lubricant reservoirs to reduce friction and wear in sliding and rolling contact applications. Surface patterning has become a valuable technique for fabricating micro dents. Alternative methods such as micromachining present obvious limitations in comparison with laser shock peening (LSP). In this paper, the use of micro LSP along with an automatic XY table proves to be an attractive and reliable method for producing micro dent arrays with enhanced surface integrity. Surface topography, residual stress, and microhardness of the fabricated micro dent arrays on polished Ti–6Al–4V surfaces have been characterized. It has been shown that LSP is capable of efficiently fabricating mass micro dent arrays with controllable size via adjusting laser power. The center area of the peened dents has the highest hardness. In addition, high compressive residual stress can also be produced. [Copyright &y& Elsevier]

Abstract: The detection of improperly lubricated bearings from vibration patterns is a difficult task especially when records from short operating periods are available. This is exactly the case of end-quality assessment systems. This problem has been addressed by applying both cyclostationary analysis and spectral kurtosis for the selection of a frequency band in which variations in vibration patterns are most expressed. The approach was evaluated on a test-set comprising 63 electrical motors with fault-free and 21 with improperly lubricated bearings. The results reveal that improper lubrication is expressed as increase in the spectral components at bearing cage and ball spin frequency. [Copyright &y& Elsevier]

Abstract: The rolling contact fatigue (RCF) life of highly loaded machine components is significantly influenced by the surface roughness features so that there is a continuous effort to design the topography of rubbing surfaces to enhance lubrication efficiency and prolong the operation of machine components. It can be suggested from the recent experimental results that lubricant emitted from shallow micro-dents could effectively lift off the real roughness features and reduce the asperities interactions within rolling/sliding mixed lubricated contacts. Thereby the additional supply of lubricant from surface features could help to reduce the risk of surface damage through the reduction of the interaction of rubbing surfaces during start-up or starvation. However, the introduction of such roughness features into the rubbing surfaces of highly loaded non-conformal contacts should consider not only the effects on lubrication film thickness but also on RCF. That is why this study is focused on the effects of surface texturing on RCF within non-conformal rolling/sliding contacts operated under mixed lubrication conditions. The principal task has been whether possible beneficial effect on film thickness is not accompanied by the reduction in RCF life. Textures with various sizes of micro-dents and their arrangement within the contacts have been considered. It has been found that results obtained with textured surfaces have exhibited no obvious reduction in RCF. Conversely, some increase in RCF using textured surfaces was observed that could be attributed to the positive contribution of micro-dents working as lubricant micro-reservoirs that reduce asperities interactions. Nevertheless, further experiments are necessary to confirm this possible beneficial contribution of surface texturing on RCF. [Copyright &y& Elsevier]

Abstract: The dent initiated flaking life of a bearing is sometimes shorter than the calculated life. Therefore, it is important to develop an enhanced-life bearing for contaminated lubrication conditions. In this paper, the authors indicated that the dent initiated flaking life of a raceway is significantly reduced in bearings where the general surface roughness of the rolling elements is increased by dent generation due to operating in contaminated lubrication. Additionally, we confirmed that bearings with rolling elements made of material with fine silicon and manganese nitride precipitates, for the purpose of reducing the degradation of surface roughness, are effective in enhancing bearing life. [Copyright &y& Elsevier]

Abstract: The paper presents an experimental and numerical investigation of non-conformal lubricated contacts in which anomalous film shapes occur. The experiments were concerned with the contact between a steel ball and the plane surface of a glass disc at various slide-roll ratios. A paraffin base mineral oil was used as a lubricant and friction coefficients and film thicknesses were measured. It was found that for slide-roll ratios with the disk moving faster anomalous elastohydrodynamic lubrication (EHL) films were obtained characterized by a “dimple” in the central region of the contact. Numerical thermal–elastohydrodynamic analyses were carried out to simulate both film thickness and friction corresponding to the experimental conditions using Newtonian and Ree-Eyring rheological models. Initial results from this study suggest that neither of these lubricant models predict the correct detailed film shape and the experimental friction at the same time. An alternative lubricant model including both thermal and limiting shear stress effects (wall slippage) is currently under development. [Copyright &y& Elsevier]

Abstract: The effect of different slide-to-roll ratios has been experimentally investigated under steady state and transient conditions using a steel ball in contact with the plane surface of a glass disc. Under transient conditions the entraining velocity has been varied with a sinusoidal law at two different frequencies. Measurements of the friction force and film thickness using optical interferometry have been made. For the same working conditions, different friction coefficient trends found for positive and negative slide-to-roll ratios can be related to different film thickness values and shapes. The combination of different thermal effects could be a possible explanation for the obtained results. [Copyright &y& Elsevier]

Abstract: In this study the tribological and hydrothermal behaviour of silicon carbide was examined. No significant reaction layer was found after tribochemical experiments conducted on sintered silicon carbide (SSiC) and after hydrothermal treatment using a hydrothermal diamond anvil cell (HDAC). Mechanical contact of sliding parts induced superficial amorphisation and chemical analysis showed a slight increase in oxygen content within the topmost part of the surface. Static experiments using single-crystal silicon carbide (SCSiC) and a hydrothermal diamond anvil cell revealed an active oxidation mechanism and silica as the result of solution and precipitation. [Copyright &y& Elsevier]

Abstract: The compatibility of trans-1,4,5,8-tetranitro-1,4,5,8-tetraazadecalin (TNAD) with some energetic components and inert materials of solid propellants was studied by using the pressure DSC method where, cyclotetramethylenetetranitroamine (HMX), cyclotrimethylenetrinitramine (RDX), 1,4-dinitropiperazine (DNP), 1.25/1-NC/NG mixture, lead 3-nitro-1,2,4-triazol-5-onate (NTO–Pb), aluminum powder (Al, particle size=13.6μm) and N-nitrodihydroxyethylaminedinitrate (DINA) were used as energetic components and polyethylene glycol (PEG), polyoxytetramethylene-co-oxyethylene (PET), addition product of hexamethylene diisocyanate and water (N-100), 2-nitrodianiline (2-NDPA), 1,3-dimethyl-1,3-diphenyl urea (C2), carbon black (C.B.), aluminum oxide (Al2O3), cupric 2,4-dihydroxy-benzoate (β-Cu), cupric adipate (AD-Cu) and lead phthalate (φ-Pb) were used as inert materials. It was concluded that the binary systems of TNAD with NTO–Pb, RDX, PET and Al powder are compatible, and systems of TNAD with DINA and HMX are slightly sensitive, and with 2-NDPA, φ-Pb, β-Cu, AD-Cu and Al2O3 are sensitive, and with PEG, N-100, C2 and C.B. are incompatible. The impact and friction sensitivity data of the TNAD and TNAD in combination with the other energetic materials under present study was also obtained, and there was no consequential affiliation between sensitivity and compatibility. [Copyright &y& Elsevier]

Abstract: An analysis was performed to evaluate the stress distribution in composite pin loaded joints. The analysis involves specification of displacement expressions in the form of a trigonometric series that satisfy the boundary conditions for the contact region in terms of a set of undetermined coefficients. Based on this assumed distribution, the Lekhnitskii complex variable approach is used to obtain the stress functions needed to evaluate the contact stresses within the joint. Unknown coefficients in the displacement expression were obtained by assuming coulomb friction within the contact region and evaluating the displacement at discrete points within this region. Material properties of carbon fiber reinforced plastic laminates were used for this study and the stress distribution for different values of coefficient of friction analyzed. The analysis revealed that friction affects the stress distribution around the hole boundary with, in general, the peak stresses varying with increasing values of frictional coefficient. [Copyright &y& Elsevier]