Empirical Investigation of Electricity Self-Generation in a Lubricated Sliding–Rolling Contact

The paper reports the empirical observations of voltage generation in a lubricated tribocontact with different oils altering load, sliding and temperature. The investigation is done in the context of research of the root cause of white etching cracks (WEC) failure in bearings. Tested oils of different additive packages found completely different electrical behavior. The oil, which is known to produce WECs in laboratory tests, demonstrated non-zero voltage generation.     

Tribological Performance of PTFE–Metal Binary Coatings in Rolling/Sliding Contact

Tribological performance of surface coatings with embedded PTFE reservoirs in rolling/sliding contact is reported. Using two different coating materials and two shapes and patterns of PTFE reservoirs test samples in the form of discs were prepared and tested in a four-ball contact configuration under loads corresponding to nominal contact pressure of 0.5 and 1.0 GPa. It was found that one coating, namely aluminium–bronze with embedded PTFE reservoirs is suitable for applications where rolling is also associated with a degree of sliding and there is no external lubrication.     

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

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

Experimental Depiction of the Hypothesis of Flake-Like Wear Debris Generation in Rolling Contact Fatigue of the Rail–Wheel Contact Interface

Flake-like particles represent a common type of wear debris generated during the rolling contact fatigue wear test using a twin-disc test rig. It is argued that these flake-like particles are generated during the delamination process due to plastic shear strain accumulation at the wearing surfaces. This hypothesis has been developed in the last decades to explain the particle generation mechanism, yet it has not been proven conclusively. This research provides strong experimental evidence of the creation processes of wear debris propagation, aggregation, transfer, and compaction, therefore confirming the existing hypothesis and enhancing the understanding of wear mechanisms in the rolling contact interface.     

Effect of Carbides on Wear Characterization of High-Alloy Steels under High-Stress Rolling–Sliding Condition

This article describes the wear characterizations of high-speed steel composed of vanadium carbide and high-chromium cast iron composed of chromium carbide. These metals were studied under rolling–sliding conditions with a sliding ratio of 10% using a self-made ring–ring wear testing machine. The fine microstructure of carbides and failure behaviors were analyzed by scanning electron microscopy and high-resolution electron microscopy. The results showed that carbide significantly affected the wear properties and failure behaviors of metals. The relative wear resistance of high-speed steel reinforced by vanadium carbides was twice that of high chromium cast iron composed of chromium carbides. Chromium carbide was characterized by a stacking fault substructure, and slips occurred in chromium carbide under high-stress contact, resulting in crack formation. Vanadium carbide was reinforced and pinned by large amounts of nanoparticles, which prevented its dislocation under high-stress rolling–sliding conditions, thereby effectively resisting crack initiation. Furthermore, the (200) lattice plane of vanadium carbide is coherent with the (111) lattice plane of austenite, preventing cracks from forming at the interface of the vanadium–carbide matrix. The morphology and hardness of vanadium carbide also contributed to the excellent wear property of high-speed steel.     

Analysis of Thermal Stability of High-Performance Rolling–Sliding Contacts in Mixed Lubrication

Thermal instability has been considered by pioneer researchers to be one of the most promising lines for a fundamental investigation into the failure mechanisms of rolling–sliding contacts. This article uses a recently developed mixed lubrication model that integrates interrelated topographical, mechanical, thermal, and tribochemical aspects to study the thermal instability of high-performance rolling–sliding contacts. The effects of various system parameters on the relation between the system bulk temperature and the heat generation in the contact are analyzed. The parameters include surface roughness; contact component size; surface and lubricant mechanical, thermal, and tribochemical properties; and operating conditions. Key results and their implications to system design and operation considerations are summarized in the Conclusion section of the article in relation to enhancing the thermal stability of the contact, particularly under adverse lubrication conditions.     

Rolling Contact Testing of Vapor Phase Lubricants—Part II: System Performance Evaluation

The relative effects of several vapor lubrication parameters on bearing performance were examined using a ball-on-rod tester. Lubricants included in the evaluation were a tertiary-butylphenyl phosphate (TBPP), a 2 cSt polyalphaolefin blended with 15 percent TBPP (PAO+), the TBPP blended with 33 percent tributyl phosphate (TBPP+), a cyclophosphazine (X-1P), a polyphenylether (5P4E), and a perfluoroalkylether (Z). Parameters included in the study were bearing temperature, vapor concentration, and vapor temperature. Additionally, a solid lubricant coating was included to improve the bearing performance under cold-start conditions. The lubricants containing phosphorus demonstrated the best high temperature performance. The TBPP lubricant failed shortly after test at 650°C., while the X-1P lubricant performed satisfactorily over an eight-hour period at 650°C. The TBPP+ lubricant demonstrated the widest temperature range capability, with 600°C operation and a projected pump-ability point of–-45°C. Lubricant concentration was the most significant system parameter affecting bearing friction and wear.     

Solid Lubrication of Silicon Nitride with Cesium-Based Compounds: Part I — Rolling Contact Endurance,Friction and Wear

The high temperature rolling contact endurance, friction, and wear of 16 cesium-based compounds with solid lubricating properties were investigated on silicon nitride (Si₃N₄). Some were also investigated on bearing tool steels and several state-of-the-art high temperature solid lubricants were investigated for comparison. Experiments were conducted in air at temperatures up to 650°C, contact stresses up to 4.34 GPa, and a pure rolling surface speed of 1.8 m/s. Although all of the cesium-based compounds exhibited self-lubricating properties, the best overall performance was achieved with a cesium silicate reaction film formed in-situ (Cs₂O·xSiO₂) and a hydrated cesium silicate bonded coating (Cs₂O·3SiO₂·nH₂O). Bonded coatings of cesium oxythiotungstate + tungsten disulfide mixture (Cs₂WOS₃ + WS₂) and cesium hydroxide (CsOH) also performed well. It is hypothesized that high temperature chemical reactions between the cesium-containing compounds and the silicon nitride surface form a lubricious cesium silicate film.     

Estimation of Generated Axial Force Considering Rolling–Sliding Friction in Tripod-Type Constant Velocity Joint

ABSTRACT

This study proposes a new generated axial force (GAF) estimation model of tripod-type constant velocity (CV) joints. For development of the model, kinematic analysis was performed to derive the relative coordinates of components and contact points. Through the analysis, the normal load that acts on contact points was also obtained. This study employs two friction models—pure sliding and rolling–sliding—to obtain the friction coefficients on the contact points. Particularly for the rolling–sliding model, this study used the experimental analysis on rolling–sliding ratio and friction coefficients were studied using a tribometer. By introducing two models, this study considers not only the pure sliding friction but also the rolling–sliding friction that occurs between spherical rollers and tracks.

This study verifies the GAF estimation model by comparing the simulation results with the experimental results. A tripod-type CV joint was set as a target and its GAF was derived by the model. Then, its actual GAF was measured and the results were compared with each other. A GAF measurement system was set up for the measurement in this study. The estimated results show similar trends with the measured results under low-resistance torque condition and the GAF model provides very accurate estimation under high-resistance torque conditions.     

A Theoretical Analysis of Mixed Lubrication by Macro Micro Approach: Part I—Results in a Gear Surface Contact

Mixed lubrication is a key to bring the performance analysis to the failure analysis in most tribological components. A macro-micro approach to mixed lubrication has been developed in the present model. The relation between the average contact pressure and the average gap for a typical rough contact patch is first determined numerically in micro scale. Using this relation, the average gap, average oil-film pressure, and average contact pressure in a mixed-lubricated elastohydrodynamic contact can be solved simultaneously in macro scale by treating the contact to be smooth. The total pressure is assumed a superposition of average asperity contact pressure and lubricant pressure. The new approach is simple, efficient and robust, and covers entire range of the load ratio, from unity (dry contact) to zero (full-film EHL). In addition, it can be used for a wide range of operating conditions and on a much larger contact area with a much less computing time than deterministic simulation of mixed lubrication. Implementation of the Fast Fourier Transform (FFT) allows for a rapid calculation of the elastic deformation and asperity con/act pressure. As a demonstration to this new approach, a parametric study of dimensionless speed, load and contact shape on the load ratio and gap ratio was conducted for a gear rough surface of the load ratio, from unity (dry contact) to zero (full-film EHL). In addition, it can be used for a wide range of operating conditions and on a much larger contact area with a much less computing time than deterministic simulation of mixed lubrication. Impletmentation of the Fast Fourier Transform (FFT) allows for a rapid calculation of the elastic deformation and asperity contact pressure. As a demonstration to this new approach, a parametric study of dimensionless speed, load and contact shape on the load ratio and gap ratio was conducted for a gear rough surface     

Contact Fatigue Analysis of a Dented Surface in a Dry Elastic–Plastic Circular Point Contact

The present article proposes a methodology for the computational analysis of damage induced in the vicinity of dents in a dry circular point contact under repeated rolling. The failure risk is evaluated through the use of the Dang Van multiaxial fatigue criterion. The dent is a typical surface defect encountered in rolling element bearings when operating in contaminated environments. It is usually created by a solid particle not removed by seals or filters when passing through an EHL conjunction. Since local plasticity occurs when the debris is first entrapped between the contacting surfaces, and later when the resulting dents are subjected to moving contact load, the elastic–plastic behavior of the material should be captured by the model. First, the dent shape and the subsurface stress and strain fields produced by the presence of a spherical particle are obtained by the finite element method. Second, the rolling of the load over the surface defect is simulated using a semi-analytical elastic–plastic code. The simulations are carried out for two different debris materials, both ductile but one significantly softer than the contacting surfaces, i.e., made of stainless steel 316L, the other one being made of bearing steel AISI 52100 similar to the contacting surfaces. The dent shape and initial stress and strain states are first presented. Subsequent stress and strain states after a few rolling cycles are then presented. Finally the effects of the coefficient of friction, presence of residual stress, and contact load magnitude are highlighted.     

The Prediction of Contact Pressure–Induced Film Thickness Decay in Starved Lubricated Rolling Bearings

Under starved conditions the thickness and distribution of the lubricant film in an elastohydrodynamically lubricated (EHL) contact is directly related to the distribution of lubricant on the track in the inlet to the contact. In starved lubricated rolling bearings this lubricant distribution is determined by many effects. The authors have developed a model to predict the oil lost from the track induced by EHL pressure with no replenishment. A complete bearing is modeled with multiple rolling element EHL contacts and with the applied load to the rolling elements varying along the circumference of the bearing. Results of the oil layer thickness on the track are presented for a ball bearing and a spherical roller bearing for different bearing loads and rotational speeds. The predicted layer thickness decay rate for a ball bearing is significantly larger than for a spherical roller bearing and the predicted effect of the bearing load on the decay rate is small compared to the effect of the rotational speed. The predicted decay periods due to the contact pressure effect are small compared to the observed (grease) life of bearings. The results show that a bearing cannot sustain an adequate layer of oil on the running track unless significant replenishment takes place.     

Tribological performance of PFPE and X-1P lubricants at head–disk interface. Part I. Experimental results

X-1P, a cyclic phosphazene lubricant, is studied and compared with polar perfluoropolyether (PFPE) lubricant Z-Dol. Contact angles of lubricants are measured on different solid surfaces. Contact start-stop (CSS), drag, and ball-on-flat tests are performed and the results are discussed. Drag tests under high vacuum are also performed and discussed. Experimental results show that lubricant X-1P exhibits lower static friction and higher durability than lubricant Z-Dol, especially at high humidity. Higher durability is also observed for X-1P under the high vacuum condition compared with lubricant Z-Dol. Diamond-like carbon (DLC) overcoat on the Al₂O₃–TiC slider surface lowers friction and prolongs durability, especially for lubricant Z-Dol at high humidity, whereas for X-1P, there is no benefit of DLC. X-1P as an additive shows some improvement in durability at high humidity as compared to lubricant Z-Dol. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermohydrodynamic Analysis of a Journal Bearing in a Turbulent Flow Regime—Part I: Theory

An attempt is made to formulate a thermohydrodynamic model of fluid-film lubrication that is valid in turbulent flow regimes. The model considers the flow to be a small perturbation of turbulent Couette flow. The modified Reynolds equation is obtained for the turbulent flow and the integro-differential energy equation makes the turbulence analysis easier by replacing the conductive terms in terms of the convective boundary conditions at two solid surfaces. Sample results applying the perturbation method agree well with available experimental data and theoretical methods.     

EHL Performance of the Lubricant With Shear Strength: Part I — Boundary Slippage and Film Failure

This paper analytically investigates the isothermal line contact elastohydrodynamic lubrication of three lubricants with much different shear strengths under the nondimensional operating parameters of w = 2.15e-4 and U = 2.53e-10 applying the lubricant ideal viscoplastic rheological model. The boundary slippage of the low-shear-strength lubricant occurring in the EHL inlet zone was found and results in a much thinner film compared to the classical EHL theory prediction. The film boundary slippage and its growth with the slide/roll ratio variation of tile low-shear\- strength lubricant exhibit special phenomena, which are much different from those of the high-shear-strength lubricant. The easy occurrence of film failure in concentrated contact in the case of high sliding speed, heavy load, large slide/roll ratio, and low-shear-strength lubricant was concluded due to the severe friction heating on the surface conjunction and the lubricant thermal desorption on tile lubricant/surface boundary. The EHL film failure mechanism was further recognized.     

Experimental and Numerical Study of Multibody Contact System with Roller Bearing–-Part II: Semi-Finite Element Analysis and Optimal Design of Housing

A typical roller bearing system consists of five contact parts: the housing, outer ring, inner ring, roller set, and the shaft. A finite element calculation procedure is described to analyze a five-body contact roller bearing system. If an analytical solution is used to calculate the deformations of the roller and the ring/shaft combination, a semi-finite element governing equation can be derived by simplifying the five-body contact bearing system into a three-body contact system. The semi-finite element calculation results correlate closely with the test results obtained in Part I of this paper (1). The analysis indicates that the initial gap between the housing and the outer ring and the loading positions have significant influence on the load distribution in the bearing. By optimal design of the housing, the load distribution becomes more uniform and the fatigue life of the bearing can be increased.     

An Experimental Study on the Concentration and Shape of Dents Caused by Spherical Metallic Particles in EHL Contacts©

An experimental study of metallic contaminant effects on surface indentation in EHL contacts is presented. Particles are initially spherical and are composed of M50 high-carbon steel powder. Their diameter ranges from 32 to 40 μm. An original lubrication system with a controlled level of contamination was built. The contaminant distribution and concentration are measured on-line by an automatic particle counter. Tests are conducted on a two-disk machine with different operating conditions. Particles may travel through the EHL contact only one time, the lubricant flow being used only once. The oil is a synthetic one qualified under the MIL-L-23699 specification. An optical profilometer is used to describe the indent topography and a CCD video camera to count the number of dents.

The test bench is described and the experimental procedure is presented. Specific tests were performed to quality the contamination bench. The combined effects of particles concentration and test duration on dent distribution were studied. Some results on the shape and concentration of indents versus operating conditions are presented. It is shown that over the range of test conditions considered, the number of indents on the raceways can be estimated from the particle concentration in the oil bulk. This leads to the conclusion that the particle entry ratio is close to one, i.e., the concentration of particles inside the EHL contact is close to those in the bulk.     

Ionic Liquids as Lubricants of Titanium–Steel Contact. Part 2: Friction, Wear and Surface Interactions at High Temperature

The tribological behaviour and surface interactions of titanium sliding against AISI 52100 steel have been studied at 200 and 300 °C in the presence of two commercial imidazolium room temperature ionic liquid (ILs): 1-octyl-3-methylimidazolium tetrafluoroborate (L108) and 1-hexyl-3-methylimidazolium hexafluorophosphate (LP106). L108 presents the higher thermal stability but gives higher friction coefficients and wear rates than LP106, with long running-in periods and high friction values, both at 200 and 300 °C. Friction and wear rates for LP106 are lower and decrease as the temperature increases from 25 to 200 °C. At 200 °C, LP106 shows a constant friction coefficient, without running-in, produces a mild wear on titanium and no surface damage on steel. LP106 fails at 300 °C, close to its degradation temperature, due to tribochemical decomposition through partial dissociation of the hexafluorophosphate anion, with formation of a phosphorus-rich layer on the steel ball, while the titanium wear track surface is heterogeneous, showing regions with the presence of fluoride and others with the presence of phosphate. When the steel ball is substituted for a ruby sphere under the same conditions at 300 °C, a low friction coefficient and mild wear is observed, due to the higher stability of the LP106 lubricant at the ruby–titanium interface. The friction coefficients, wear mechanisms and surface interactions have been studied by means of friction-distance records, SEM, EDX and XPS.     

The Effect of Dwell Time on the Static Friction in Creeping Elastic–Plastic Polymer Spherical Contact

A theoretical model is developed to study the effect of dwell time on the junction growth and static friction of a creeping polymer sphere in contact with a rigid flat under full stick contact condition. A rapid normal loading into the elastic–plastic contact regime is followed by a rest period during which creep takes place causing contact area growth, and stress relaxation that can completely eliminate the plastic zone in the sphere. At the end of this rest time, an increasing tangential loading is applied to the flat till sliding inception occurs. During this loading step, further increase of the contact area and reappearing of a plastic zone in the sphere take place. An increase in static friction resulting from the dwell time during the creep stage is clearly demonstrated and explained.