Research Papers: Applications

J. Tribol. 2019;141(9):091101-091101-9. doi:10.1115/1.4044082.

A nonlinear dynamic model is proposed to investigate the dynamic interactions between the rolling element and cage under rotational speed fluctuation conditions. Discontinuous Hertz contact between the rolling element and the cage and lubrication and interactions between rolling elements and raceways are considered. The dynamic model is verified by comparing simulation result with the published experimental data. Based on this model, the interaction forces and the contact positions between the rolling element and the cage with and without the rotational speed fluctuation are analyzed. The effects of fluctuation amplitude, fluctuation frequency, and cage pocket clearance on the interaction forces between the rolling element and the cage are also investigated. The results show that the fluctuation of the rotational speed and the cage pocket clearance significantly affects the interaction forces between the rolling element and the cage.

Commentary by Dr. Valentin Fuster

Research Papers: Elastohydrodynamic Lubrication

J. Tribol. 2019;141(9):091501-091501-11. doi:10.1115/1.4043902.

An elastohydrodynamic lubrication model for a rigid ball in contact with a transversely isotropic half-space is constructed. Reynolds equation, film thickness equation, and load balance equation are solved using the finite difference method, where the surface vertical displacement or deformation of transversely isotropic half-space is considered through the film thickness equation. The numerical methods are verified by comparing the displacements and stresses with those from Hertzian analytical solutions. Furthermore, the effects of elastic moduli, entertainment velocities, and lubricants on fluid pressure, film thickness, and von Mises stress are analyzed and discussed under a constant load. Finally, the modified Hamrock–Dowson equations for transversely isotropic materials to calculate central film thickness and minimum film thickness are proposed and validated.

Commentary by Dr. Valentin Fuster

Research Papers: Friction and Wear

J. Tribol. 2019;141(9):091601-091601-9. doi:10.1115/1.4043906.

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

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(9):091602-091602-11. doi:10.1115/1.4043903.

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

Commentary by Dr. Valentin Fuster

Research Papers: Hydrodynamic Lubrication

J. Tribol. 2019;141(9):091701-091701-12. doi:10.1115/1.4043971.

This paper presents a numerical study for nonlinear rotordynamic response with bifurcations of tilting pad journal bearings when pad–pivot friction forces are taken into account. A Stribeck friction model is employed to determine the friction coefficient for the contacts between the pads and the spherical-type pivots. The boundary/mixed/hydrodynamic friction mode is determined for each pad surface based on the instantaneous angular motion of the pads. A Jeffcott type rotor supported on 5-pad tilting pad journal bearings is used for the structural model, and finite element fluid film models are utilized to calculate the reaction forces and moments on the pads. The simulation results show that pad–pivot friction plays an important role in determining the stability of the rotor system. For the autonomous condition, the friction induces a Hopf bifurcation and generates limit cycles at high rotor spin speed (>14 krpm), which were originally stable equilibrium states with a no friction condition. For the nonautonomous condition, the 1× synchronous response becomes subsynchronous/quasiperiodic responses in the high-speed range (>14 krpm) with the appearances of Neimark-Sacker bifurcations. It is shown that the outbreak points and corresponding response types are highly dependent on the state of disk imbalance. A comparison of the linear and nonlinear models clearly illustrates the importance of retaining nonlinear forces to determine potential deleterious vibration.

Commentary by Dr. Valentin Fuster

Research Papers: Lubricants

J. Tribol. 2019;141(9):091801-091801-7. doi:10.1115/1.4043904.

The antiwear properties of ionic liquids (ILs) as lubricant additives were studied with polyethylene glycol (PEG) used as the lubricant base oil. The quantum parameters of the ILs were calculated using a Hartree–Fock ab initio method. Correlation between the scale of the wear scar diameter and quantum parameters of the ILs was studied by multiple linear regression (MLR) analysis. A quantitative structure tribo-ability relationship (QSTR) model was built with a good fitting effect and predictive ability. The results show that the entropy of the ILs is the main descriptor affecting the antiwear performance of the lubricant system. To improve the antiwear performance of the lubricants, the entropy of the system should be decreased, reducing the system randomness and increasing the system regularity. A major influencing factor on the entropy of a system is the intra- and intermolecular hydrogen bonds present. Therefore, enhanced antiwear properties of lubricants could be achieved with a three-dimensional netlike structure of lubricant formed by hydrogen bonding.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(9):091802-091802-7. doi:10.1115/1.4043972.

Thermocapillary migration is an interfacial phenomenon that describes liquid flow on a nonisothermal surface from warm to cold regions in the absence of external forces. It is a typical lubricant loss mechanism in tribosystems. To ensure continued reliability of lubricated assemblies, knowledge of the migration capacity of different liquids and solids is needed. In the present work, migration experiments were conducted on various liquid lubricants on different solid surfaces. It was found that polar lubricants such as ionic liquids and polyethylene glycol hardly migrate on the tested surfaces, and the antimigration capacity of the polytetrafluoroethylene surface was discovered to be very high. Particular attention is paid to the migration mechanism associated with surface tension and contact angle. General guidelines for evaluating the migration capacities of different liquids on solids are proposed.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Tribol. 2019;141(9):094501-094501-7. doi:10.1115/1.4044084.

In this study, tribological characteristics of diethylene glycol dimethyl ether (DGM)/diethylene glycol dimethyl ether–diesel blend A (10 vol% DGM + 90 vol% diesel)/diesel were compared after being subjected to quill roller-ball friction and four-ball friction tests. The lubricity and antiwear property of blend A were worse than those of diesel, albeit considerably better than those of DGM. In addition, effects of diesel and blend A on the tribological characteristics of synthetic base oils were examined by using a four-ball friction machine and the fuzzy analytic hierarchy process: trimellitate TM128/TM320A, alkyl naphthalene AN5, and polyalphaolefin PAO6/mPAO65 exhibited good tribological characteristics, and 5 wt% blend A mixed with base oil maintained good tribological characteristics stability. Analysis results revealed that the excellent tribological characteristics of trimellitate, alkyl naphthalene, and polyalphaolefin are related to the rigid molecules of benzene rings and long-branched chains, naphthalene rings and long-branched chains, and long-branched alkanes. By the mixing of diesel or blend A, the stable lubricating oil film resisted the deterioration of the tribological characteristics.

Commentary by Dr. Valentin Fuster

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