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Research Papers: Applications

J. Tribol. 2018;141(1):011101-011101-10. doi:10.1115/1.4041020.
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Extensive investigations have been carried out in the present paper to understand polymer gear performance, i.e., wear and contact behaviors. The experimental results and possible wear mechanisms for polymer gears run against themselves have been presented, especially the wear rate of the polymer gears under different running speeds and loads. The tested samples were made of three different materials (acetal, nylon, and polycarbonate (PC)) and the effects of two different manufacturing techniques were also investigated (i.e., machine-cut and injection-molded polymer gears). The polymer gear performances (wear and life) were recorded using a uniquely designed and built test rig for this purpose. The testing results have been compared with the existing literature for polymer fatigue and wear theory. Further extensive investigations have been carried out to understand the wear phenomena on tooth flank surface profile of these gears and the data obtained have been discussed.

Commentary by Dr. Valentin Fuster

Research Papers: Elastohydrodynamic Lubrication

J. Tribol. 2018;141(1):011501-011501-11. doi:10.1115/1.4040474.

Predicting the mixed thermal lubrication performance and fatigue life of point contact components becomes more and more important with the increasing demand for the load capacity of machinery. To achieve this, a deterministic mixed thermal elastohydrodynamic lubrication (TEHL) model in point contacts considering surface roughness is developed in this study. This model is capable of determining the pressure and temperature under different lubrication regimes from mixed to full-film lubrication. Then, the established model is extended to the subsurface stress and fatigue life predictions. Numerical simulations are conducted to analyze the lubrication characteristics and fatigue life for the three-dimensional (3D) sinusoidal surfaces with variable directions. Results show that increasing entraining velocity contributes to the reduction of pressure fluctuation and prolongation of fatigue life. However, the resulting temperature increases with the entraining velocity. As for the influence of lubricant viscosity, increasing it prolongs the fatigue life, especially under mixed TEHL conditions. What's more, the effect of rough surface texture feature on fatigue life has a close relationship with the lubrication regime.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(1):011502-011502-14. doi:10.1115/1.4040723.
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In this study, a comprehensive mechanical efficiency model based on the thermal elastohydrodynamic lubrication (TEHL) is developed for a helical gear pair. The tribological performance of the helical gear pair is evaluated in terms of the average film thickness, friction coefficient, mechanical power loss, mechanical efficiency, etc. The influence of basic design parameters, working conditions, thermal effect, and surface roughness are studied under various transmission ratios. Results show that the contribution of thermal effect on the tribological performance is remarkable. Meanwhile, the rolling power loss constitutes an important portion of the total mechanical power loss, especially around the meshing position where the pitch point is located in the middle of contact line and the full elastohydrodynamic lubrication (EHL) state with the friction coefficient less than 0.005. The proper increase of normal pressure angle and number of tooth can improve the tribological performance. The influence of helix angle on the mechanical efficiency is less significant. A positive addendum modification coefficient for pinion and a negative addendum modification coefficient for wheel are good for improving the mechanical efficiency. The results provide the tribological guidance for design of a helical gear pair in engineering.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(1):011503-011503-15. doi:10.1115/1.4040979.

In this work, a mixed lubrication model, applicable to cam-roller contacts, is presented. The model takes into account non-Newtonian, thermal effects, and variable roller angular velocity. Mixed lubrication is analyzed using the load sharing concept, using measured surface roughness. Using the model, a quasi-static analysis for a heavily loaded cam-roller follower contact is carried out. The results show that when the lubrication conditions in the roller-pin contact are satisfactory, i.e., low friction levels, then the nearly “pure rolling” condition at the cam-roller contact is maintained and lubrication performance is also satisfactory. Moreover, non-Newtonian and thermal effects are then negligible. Furthermore, the influence of roller-pin friction coefficient on the overall tribological behavior of the cam-roller contact is investigated. In this part, a parametric study is carried out in which the friction coefficient in the roller-pin contact is varied from values corresponding to full film lubrication to values corresponding to boundary lubrication. Main findings are that at increasing friction levels in the roller-pin contact, there is a sudden increase in the slide-to-roll ratio (SRR) in the cam-roller contact. The value of the roller-pin friction coefficient at which this sudden increase in SRR is noticed depends on the contact force, the non-Newtonian characteristics, and viscosity–pressure dependence. For roller-pin friction coefficient values higher than this critical value, inclusion of non-Newtonian and thermal effects becomes highly important. Furthermore, after this critical level of roller-pin friction, the lubrication regime rapidly shifts from full film to mixed lubrication. Based on the findings in this work, the importance of ensuring adequate lubrication in the roller-pin contact is highlighted as this appears to be the critical contact in the cam-follower unit.

Commentary by Dr. Valentin Fuster

Research Papers: Friction and Wear

J. Tribol. 2018;141(1):011601-011601-9. doi:10.1115/1.4040511.

In multipoint operations like drilling, cutting velocities and cutting-edge geometries vary along cutting lips so is the rate of progression of flank wear. Analytical evaluation of flank wear land width in the case of complex tools has received a limited attention so far. This work evaluates progression of flank wear in orthogonal machining and adopts it to drilling. An abrasive flank wear has been modeled, wherein, cutting speed determines the rate of abrasion, and the feed rate determines the chip load. The model considers stress distribution along rake surfaces, and temperature-dependent properties of tool and work materials. Assuming that the flank wear follows a typical wear progression as in a pin-on-disk test, the model evaluates cutting forces and the consequent abrasive wear rate for an orthogonal cutting. To adopt it to drilling, variation in cutting velocity and dynamic variation in rake, shear, and friction angles along the length of the cutting lips have been considered. Knowing the wear rate, the length of the worn-out flank (vb) has been evaluated. The model captures progression of flank wear in zones (i), (ii), and (iii) of a typical tool-life plot. It marginally underestimates the wear in the rapid wear region and marginally overestimates it in the steady-state region.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(1):011602-011602-12. doi:10.1115/1.4040805.

Polyether ether ketone (PEEK) and its composites are recognized as alternative bearing materials for use in arthroplasty because of their excellent mechanical properties. In this paper, torsional friction tests of PEEK against the CoCrMo alloy, simulating the contact mode between the prosthesis tibia and femur, were carried out under a 25% calf serum solution in a Leeds Prosim knee simulator. The torsional friction behavior of PEEK against the CoCrMo alloy was investigated under various normal loads (1000 N, 1600 N and 2200 N), torsional angular displacement amplitudes (±1 deg, ±3 deg, and ±5 deg), and the number of cycles (7500, 15,000, and 30,000). The torsional friction characteristics and damage mechanism are discussed. The results show that PEEK exhibited low friction coefficient under the different conditions. With increases in the torsional angle and normal load, three types of torque/angular displacement amplitude (Tθ) curves (i.e., linear, parallelogram, and elliptical loops) were observed and analyzed during the process of torsional friction. With the increase of the torsional angle, the coefficient of friction decreases. And the contact states change from the partial slip regime to the slip regime. The greater the torsional angle displacement, the more severe the damage to the PEEK surface. With an increase in the normal load, the wear scars increased. The wear depth is deepened and the width is widened, and the wear gradually becomes serious with an increase in the load. The small load is more likely to cause damage to the central area of PEEK, and the larger load causes more serious damage to the marginal region. The central and marginal regions of the PEEK sample have different wear characteristics. The worn surfaces of the central regions were characterized by convex ridges resulting from plastic deformation, while curved ploughs and fatigue peeling appeared in the marginal region. The wear mechanism of PEEK in the central region is plastic deformation, while fatigue wear and abrasive wear mainly appeared in the marginal region.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(1):011603-011603-10. doi:10.1115/1.4041073.

A five-level four-factor central composite design multivariable model was constructed for the evaluation of the combined effect of operating parameters such as percentage reinforcement (0–10%), load (5–25 N), sliding speed (1–5 m/s), sliding distance (500–2500 m) on the wear rate of mica reinforced metal matrix composites. The microwave-assisted powder metallurgy technique was used to fabricate the composites. The wear tests were performed according to statistical designs to develop an empirical predictive regression model. The interaction of percentage reinforcement and sliding distance indicated the significant impact on wear rate. The statistical analysis confirms the optimum composition of mica blends leading to the best possible wear rate. No rapid wear region was identifiable in the morphology of worn composite surfaces.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(1):011604-011604-10. doi:10.1115/1.4041018.

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

Commentary by Dr. Valentin Fuster

Research Papers: Hydrodynamic Lubrication

J. Tribol. 2018;141(1):011701-011701-16. doi:10.1115/1.4040510.

The tribological performance of a compression ring-cylinder liner system (CRCL) is numerically studied. A thermal-mixed lubrication model is developed for the lubrication analysis of the CRCL with consideration of the cylinder liner deformation. An oil transport model coupled with a mass conservation cavitation algorithm is employed to predict the oil consumption and the transition between the fully flooded lubrication condition and starved lubrication condition. On this basis, the effects of the oil supply and cylinder liner deformation on the frictional characteristics are investigated under cold and warm engine conditions. The results show that the cylinder liner deformation and oil supply have great influence on the tribological performance of the CRCL. Better tribological performance and lower oil consumption can be obtained by reasonably controlling the oil supply.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(1):011702-011702-13. doi:10.1115/1.4040806.

To simulate the change rate of the friction coefficient μ with respect to the sliding speed V, that is, the μ-V slope, a model combining macroscale and microscale phenomena is proposed. The macroscale model obtains distributions of the fluid pressure and fiber contact pressure over the whole engagement face, and the microscale model obtains the friction coefficient of each fiber contact through a detailed model for single-protuberance fiber contact. An experiment was conducted to obtain the μ-V slope by changing the wave height of separator faces, and the simulation and experimental results were compared. The combined model is advantageous for representing experimental μ-V relationships at small and large wave heights in comparison with models using only the macroscale behavior. Both experimental and simulation results showed the μ-V slope becoming more negative with increasing wave height. The simulation results revealed possible causes for the negative slope. In the wavy separator, the fluid friction that contributes to the positive slope is difficult to achieve due to the large film thickness, and the load-sharing ratio of the fiber contact tends to decrease due to wedge action of the fluid film. These phenomena shift the μ-V slope to the negative.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(1):011703-011703-11. doi:10.1115/1.4041021.

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

Commentary by Dr. Valentin Fuster

Research Papers: Lubricants

J. Tribol. 2018;141(1):011801-011801-14. doi:10.1115/1.4040836.

The principle of isosterism was employed to design low- or zero-sulfur anti-wear lubricant additives. Thiobenzothiazole compounds and 2-benzothiazole-S-carboxylic acid esters were employed as templates. Sulfur in the thiazole ring or in the branched chain was exchanged with oxygen, CH2, or an NH group. Similarly, the template's benzimidazole ring was replaced with a quinazolinone group. Quantitative structure tribo-ability relationship (QSTR) models by back propagation neural network (BPNN) method were used to study correlations between additive structures and their anti-wear performance. The features of rubbing pairs with different additives were identified by energy dispersive spectrometer-scanning electron microscope analysis. A wide range of samples showed that sulfur substitution in additive molecules was found to be reasonable and feasible. Combined effects of the anti-wear additive and the base oil were able to improve anti-wear performance.

Topics: Wear , Lubricants , Sulfur
Commentary by Dr. Valentin Fuster

Research Papers: Other (Seals, Manufacturing)

J. Tribol. 2018;141(1):012201-012201-10. doi:10.1115/1.4040596.

The paper presents the experimental results obtained for brush seals of 38 mm diameter operating with air at pressure differences up to 7 bars and rotation frequencies up to 500 Hz. The seals had bristles of 70 μm diameter, made of Haynes 25. Seals with two radial interferences (0 and 100 μm) between the brush and the rotor were tested. The presented running in procedure underlines the influence of the initial wear on the brush temperatures. The test results consisted of leakage mass flow rates. The temperatures of a limited number of points on the brush and on the rotor were also recorded. The results confirmed the important impact of the radial interference on the leakage. The test data were further confronted with theoretical predictions obtained with an original model. The model considers the brush as a deformable porous medium. Its local porosity and permeability are obtained from a fluid–structure interaction between the bristle pack and the leakage flow. The comparisons showed nearly close values of the mass flow rates. The differences between experimental and theoretical predictions are considered to be due to an underestimation of the porosity because the model neglects the friction forces between bristles and between the bristles and the rotor.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(1):012202-012202-10. doi:10.1115/1.4040595.

This paper aims to establish a theoretical feasibility of metal cold rolling with only surface-film boundary lubrication. To this end, a mathematical model for surface-film lubricated cold rolling is developed. It is formulated to factor in the interdependence of mechanics, heat transfer, and surface-film lubrication with three submodels: the lubrication-friction model, the stress-deformation model, and the thermal model. Governing equations are obtained based on fundamental physics of the rolling process and tribochemistry of the surface-film lubrication. The equations are solved simultaneously with full numerical methods of solutions. Sample results are presented to evaluate the model and to show the theoretical potential of the surface-film lubrication for cold rolling. The model may be used as a theoretical tool to aid the research and development of surface-film lubrication technology for cold rolling. It may be further developed in conjunction with precision experiments.

Commentary by Dr. Valentin Fuster
J. Tribol. 2018;141(1):012203-012203-13. doi:10.1115/1.4040925.

The piston ring pack and the ports on the cylinder linear wall have a great impact on the performance of the two-stroke opposed-piston engine. In this work, a piston ring pack model for this type of engine was generated to incorporate the exhaust ports. The effect of the exhaust ports was considered by modifying the existing friction force equation and the gas flow continuity equations. The developed model was implemented in an opposed-piston opposed-cylinder engine (a specific type of opposed-piston engine) to investigate the backpressure and the associated axial movement of all the rings of the piston ring pack under various working conditions. The results show that the gas pressure in all the regions of the piston ring pack and the axial movement of the rings are strongly affected by the exhaust ports. The gas pressure in some regions of the ring pack declines with the increase of the engine speed, while the effect of the combustion pressure (CP) on the axial movement of the ring pack can be neglected.

Commentary by Dr. Valentin Fuster

Research Papers: Tribochemistry and Tribofilms

J. Tribol. 2018;141(1):012301-012301-12. doi:10.1115/1.4041017.

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

Commentary by Dr. Valentin Fuster

Discussion

J. Tribol. 2018;141(1):011704-011704-1. doi:10.1115/1.4040386.

It should be noted that the literature survey in this paper is lacking [1]. The author had neglected to discuss previous work on gas/liquid seals [2], as well as the literature on numerical formulations (in particular the finite element method (FEM)) with upwinding mass conserving algorithms [3]. Ruan et al. [2] included the effects of rough surface contact, and viscous and frictional heating, while Miller and Green [3] included dynamics and transient phenomena. Combining both works [2] and [3], they offered more realistic analyses for face seals operation. Miller and Green [3] also offered a finite volume method that has significant advantages over the FEM: (a) the finite volume method is an explicit formulation (as opposed to an implicit formulation of the FEM), (b) the computational resources needed (memory, etc.) are smaller and execution times are at least twice as fast, as experience shows, and (c) ease of coding. The literature is also rich with cavitation algorithms that have not been mentioned, some of the most efficient are Refs. [4] and [5]. Hence, this work falls short of comparing its results with previous work or stressing its unique contributions.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Tribol. 2018;141(1):014501-014501-6. doi:10.1115/1.4040512.

In this paper, triethanolamine modified graphene oxide (TMGO) has been synthesized by filtering and drying the high-temperature reaction solution of graphene oxide (GO) and triethanolamine. The tribological performance of TMGO and GO in de-ionized water were investigated using a four-ball tribometer. The microscopic morphology of the worn surface was analyzed by optical microscope and scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). The results showed that the average friction coefficient (AFC) and wear scar diameter (WSD) of 0.1 wt % TMGO decreased by 21.9% and 6.2% compared with the two values of 0.1 wt % GO, and no corrosion occurred on metal surface. The minimum of the AFC and WSD occurred at 0.3 wt % TMGO. This study provides a new reference for the application of graphene oxide in lubrication.

Commentary by Dr. Valentin Fuster

Closure

J. Tribol. 2018;141(1):016001-016001-1. doi:10.1115/1.4040387.

The author of paper [1] would like to thank Professor Green for the relevant discussion. He highlights some oversights in the state of the art and comparison to previous work, first on face seal analysis and second on cavitation modeling. As the paper deals with the definition of a general fluid rheology model for hydrodynamic lubrication with gases and liquids, only the cavitation is addressed in this closure.

Commentary by Dr. Valentin Fuster

Errata

Commentary by Dr. Valentin Fuster

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