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Research Papers: Applications

J. Tribol. 2017;140(1):011101-011101-12. doi:10.1115/1.4036451.

Cage motions in ball bearings are investigated using a dynamic analysis program. Increases in the cage friction coefficient induce unstable motions of the cage. The instability is more likely to occur under high load and low‐speed conditions due to less ball-race sliding. A simple theory of cage instability is developed, and a critical cage friction coefficient formula is proposed, which is a function of the cage mass, ball-race traction, ball-cage contact stiffness, cage rotational speed, and number of balls. The prediction of this formula agrees with the results of the dynamic analysis. With a nonuniform separation between the balls, a high-speed whirl is superimposed on the normal whirl with the ball group speed. The direction of the high-speed whirl is the same as the cage rotational direction in inner race rotation (IR), but they are opposite in outer race rotation (OR). These results agree with some experimental results in the literature and validate the dynamic analysis.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(1):011102-011102-9. doi:10.1115/1.4036835.

The transient simulation of the journal bearing temperature in the internal gear pumps is hard due to the complicated shaft motion caused by the complicated loads. In this paper, a thermohydrodynamic analysis method, based on dynamic mesh techniques, is presented with the application of the general computational fluid dynamics (CFD) code fluent. This method can simulate the complex whirling orbit induced temperature variation in internal gear pumps and has taken into account the conduction in the rotating and orbiting rotor of a hydrodynamic bearing. A test rig has been built according to the structure of an internal gear pump to carry out the validation. The results show that the model is reliable. The relationship between bearing temperature, leakage, and axial clearance in the internal gear pump has been studied. It is found that the bearing temperature will decrease slightly, while the leakage increases heavily with larger axial clearance. A thermohydrodynamic analysis of the self-lubricating bearing in the internal gear pump has been done based on this method. The results show that the pressure profile changes regularly with the whirling motion of the journal, while the whirling motion has little effect on the distribution of the temperature. Besides, the increase of the whirling radius will result in the decrease of the pressure profile and the increase of the temperature profile.

Commentary by Dr. Valentin Fuster

Research Papers: Coatings and Solid Lubricants

J. Tribol. 2017;140(1):011301-011301-9. doi:10.1115/1.4036628.

Ternary iron–tungsten–phosphorus (Fe–W–P) coatings were electrodeposited with different sodium tungstate (NaWO4·2H2O) concentration on stainless steel 316 L substrate. These coatings were characterized by energy dispersive X-ray spectrometer (EDX), scanning electron microscope (SEM), and X-ray diffraction (XRD). The friction and wear behavior of these coatings were investigated using ball-on-disk tribometer under dry conditions. This study reveals a nanocrystalline and nodular structure with nanometric grain size of the deposited alloy. The maximum level of incorporation of tungsten (W) is about 29.54 at %. It was obtained with 0.5 M of sodium tungstate concentration, and it increases the microhardness of the coatings. Moreover, it was found that Fe–W–P coatings had significantly improved the tribological properties of the substrate due to their higher wear resistance and lower friction coefficient.

Commentary by Dr. Valentin Fuster

Research Papers: Contact Mechanics

J. Tribol. 2017;140(1):011401-011401-9. doi:10.1115/1.4036720.

This work is devoted to investigate the effects of thermal buckling on the tribological characteristics of a Cu-based wet clutch by artificially modifying friction pairs into different contact ratios. A thermal lubrication model is provided, and corresponding experiments are conducted on the wet clutch comprehensive test bench. The friction results from measurements and simulations for such modified friction pairs are analyzed. The results show that, as the contact ratio reduces, surface temperature rises obviously, and friction coefficient increases dramatically, so that local friction torque and total output torque grow significantly. In addition, the vibration of the output torque becomes more severe as the contact ratio reduces. Therefore, the nonuniform contact after thermal buckling exacerbates the friction characteristics of friction pairs severely and accelerates the failure of wet clutches.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(1):011402-011402-6. doi:10.1115/1.4036915.

Recently, generalizing the solution of the adhesiveless random rough contact proposed by Xu, Jackson, and Marghitu (XJM model), the first author has obtained a model for adhesive contact near full contact, under the Johnson, Kendall, and Roberts (JKR) assumptions, which leads to quite strong effect of the fractal dimension. We extend here the results with closed-form equations, including both loading and unloading which were not previously discussed, showing that the conclusions are confirmed. A large effect of hysteresis is found, as was expected. The solution is therefore competitive with Persson's JKR solution, at least in the range of nearly full contact, with an enormous advantage in terms of simplicity. Two examples of real surfaces are discussed.

Commentary by Dr. Valentin Fuster

Research Papers: Friction and Wear

J. Tribol. 2017;140(1):011601-011601-7. doi:10.1115/1.4036912.

The dynamic evolutionary law and tribological behavior of tribopair AISI 52100-AISI 1045 were studied via the multifractal method. Friction experiment was performed on a ring-on-disk tribometer under lubrication, and the multifractal detrended fluctuation analysis (MF-DFA) method was adapted to characterize the multifractality of the friction coefficient. The multifractal spectra first exhibited a left-hook, then right-hook, and left-hook, respectively, during the friction stages. The multifractal spectrum width W decreases in running-in friction process, maintains at small values in steady friction process, and increases rapidly in increasing friction process. Corresponding shuffled series was analyzed to distinguish that the multifractality of friction coefficient is due to the long-range correlation of the fluctuations. The results inform quantitative interpretations of friction system's tribological behavior and friction process identification.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(1):011602-011602-6. doi:10.1115/1.4036721.

This paper deals with the improvement in surface properties and microscratch resistance of graphites by means of an ultrasonic nanocrystalline surface modification (UNSM) technique. The surface roughness and surface hardness of the untreated and UNSM-treated graphites were investigated using an atomic force microscopy (AFM) and a microhardness tester, respectively. The scratch resistance was assessed using a microscratch tester at a progressive load. Moreover, a Raman spectroscopy was employed to characterize the microstructure of graphites before and after UNSM treatment. The scratch test results revealed that the resistance to scratch of both UNSM-treated graphites was found to be better in comparison with the untreated graphites. The increase in scratch resistance of both UNSM-treated graphites may be mainly attributed to the reduced surface roughness and increased surface hardness by UNSM treatment. The graphite produced by Poco exhibited a higher resistance to scratch compared to that of the graphite produced by Mersen. The objective of this study is to extend the service life of three-dimensional (3D) cover glass moldings made of graphite by the application of UNSM treatment through the understanding the effects of surface roughness and surface hardness on the scratch defect generation behavior during glass molding process.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(1):011603-011603-19. doi:10.1115/1.4036918.

Friction and wear properties of hypereutectic Al–25Si alloy were studied under dry and lubricated sliding conditions. Hypereutectic Al–25Si alloys were prepared by rapid solidification process (RSP) under the T6 condition. Experimental studies were conducted using a ball on disk type tribometer. The effect of the sliding distance and normal load on the friction and wear were investigated. The coefficient of friction (COF) remained stable with an increase in the sliding distance (250–1500 m) and decreased with an increase in the normal load (10–50 N), whereas the wear rate decreased with an increase in the sliding distance, and increased with the increase in the normal load up to 40 N and then attained a steady-state value under dry and lubricated sliding conditions. The improvements in COF and wear rate were mainly attributed to the morphology, size, and distribution of hypereutectic Si particles due to its fabrication process. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), optical microscopy, and three-dimensional (3D)-surface profilometer were used for characterization of the wear tracks. The dominant wear mechanisms for a hypereutectic Al–25Si alloy were adhesive wear, abrasive wear, and plastic deformation.

Commentary by Dr. Valentin Fuster
J. Tribol. 2017;140(1):011604-011604-21. doi:10.1115/1.4036922.

Fretting wear studies were conducted on AZ91 and AE42 magnesium alloys against steel ball. Average coefficient of friction (COF) remains steady with the increase in running time. However, average coefficient of friction decreases with the increase in normal load and frequency and marginally increases with the increase in amplitude. A constant average coefficient of friction of 0.06 was observed for both AZ91 and AE42 under dry sliding conditions at normal load of 50 N. Wear volume increases linearly with increasing running time. Wear volume first decreases sharply, attains minima, and then increases marginally with the increase in normal load. However, wear volume increases with increasing amplitude and frequency. Higher specific wear rate of 10.5 × 10−6 mm3 N−1·m−1 was observed for AE42, as compared to 4.5 × 10−6 mm3 N−1·m−1 for AZ91. The wear in magnesium alloy was caused by a combination of adhesion, abrasion, oxidation, delamination, and plastic deformation under different fretting conditions.

Commentary by Dr. Valentin Fuster

Research Papers: Hydrodynamic Lubrication

J. Tribol. 2017;140(1):011701-011701-19. doi:10.1115/1.4036770.

The form of the Reynolds-type equation which governs the macroscopic mechanics of hydrodynamic lubrication interfaces with a microscopic texture is well-accepted. The central role of the ratio of the mean film thickness to the texture period in determining the flow factor tensors that appear in this equation had been highlighted in a pioneering theoretical study through a rigorous two-scale derivation (Bayada and Chambat, 1988, “New Models in the Theory of the Hydrodynamic Lubrication of Rough Surfaces,” ASME J. Tribol., 110, pp. 402–407). However, the resulting homogenization theory still remains to be numerically investigated. For this purpose, after a comprehensive review of the literature, three microscopic regimes of lubrication will be outlined, and the transition between these three regimes for different texture types will be extensively demonstrated. In addition to conventional textures, representative re-entrant textures will also be addressed.

Commentary by Dr. Valentin Fuster

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