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Research Papers: Applications

J. Tribol. 2019;141(5):051101-051101-13. doi:10.1115/1.4042656.

The effects of the race surface waviness on the cage dynamics, including cage slip ratios, cage instabilities, and time-averaged cage wear rates, in high-speed ball bearings are investigated. A dynamic model of high-speed ball bearings considering the cage effect and the race surface waviness is proposed. Based on the proposed dynamic model, the effects of the maximum wave amplitude (MWA) and the wave order (WO) of race surface waviness on cage slip ratio, cage instability, and time-averaged cage wear rate are investigated. The results show that the race surface waviness has a great effect on the cage dynamics. The waviness would increase the random impacts between balls and cage pockets and thus cause more instable motion of the cage. Although the ball skidding and the cage slip ratio decrease with the increase of MWA, the cage instability and the cage wear rate become severe when MWA increases. In addition, the effect of WO on cage dynamics is nonlinear. The current investigation could provide a theoretical tool for an in-depth understanding of the dynamics in a high-speed ball bearing.

Commentary by Dr. Valentin Fuster

Research Papers: Contact Mechanics

J. Tribol. 2019;141(5):051401-051401-13. doi:10.1115/1.4042676.

Rolling contact fatigue (RCF) is one of the major problems observed in gear mechanisms, which leads to high friction, ultimately resulting in high energy consumption. This paper demonstrates the evolution of surface topography during running-in and subsequent RCF tests under boundary or mixed-elastohydrodynamic lubrication regimes. The case-hardened disks of equal surface finish and hardness are used in the experiments, and the evolution of surface topography is investigated using a white light interferometer. Surface topography at different load stages is measured at three distinct points, on the disks and average roughness and topography parameters are reported. Semi-quantitative techniques are used to determine the asperity-level parameters at different load stages. From the running-in experiment, it is found that running-in is a fast process where substantial change in surface topography occurs due to plastic deformation of most prominent asperity. From the RCF test, it is concluded that within range of the fatigue cycles, the root-mean-square (RMS) roughness (Sq) is negatively correlated with the summit radius (R) and the autocorrelation length (Sal) and positively correlated with the summit density (Sds) and the RMS slope (Sdq). Scanning electron microscope (SEM) analysis reveals the disappearance of grinding ridges, the formation of micropits at a very small scale, and pit growth in the sliding direction.

Commentary by Dr. Valentin Fuster

Research Papers: Friction and Wear

J. Tribol. 2019;141(5):051601-051601-7. doi:10.1115/1.4042678.

In our research, we have focused on the estimation of tribological and mechanical characteristics of self-organized copper film, formed through a friction of brass-steel pair in aqueous solutions of carbolic acid. We have found out that self-organized copper film formed through a friction interaction of pair brass-steel is nanostructural. The data obtained through the indentation of self-organized copper films indicated size effect. With an increasing load and contact area of interacting bodies, the coefficient of friction first drops sharply with an increasing normal load and then begins to grow. We have found out that the adhesion component of friction contributes to the friction coefficient at small loads. We have shown that the hardness of self-organized copper films formed at friction in aqueous solutions of acids increases upon shifting from acetic to caproic acid.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(5):051602-051602-6. doi:10.1115/1.4042679.

SrSO4 ceramic was prepared by hot-pressed sintering and its friction behavior was investigated against the Al2O3 ball under the dry sliding condition from room temperature to 800 °C. From room temperature to 400 °C, the tribological properties of SrSO4 ceramic are quite poor with the friction coefficients of 0.65–0.83 and the wear rates of about 10−3 mm3/Nm. With the testing temperature increasing to 600 °C and 800 °C, a brittle to ductile transition of SrSO4 takes place because of the activated slip systems. The friction coefficient and wear rate of SrSO4 ceramic also obviously decrease to 0.37 and about 10−4 mm3/Nm at 800 °C. The significant improvement of the tribological properties is ascribed to the formation of a smooth and continuous SrSO4 lubricating film with excellent ductility and low shear strength at elevated temperature. SrSO4 is considered to be a potential candidate for high-temperature solid lubricant with excellent lubricity.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(5):051603-051603-13. doi:10.1115/1.4042658.

In this investigation, computational multibody system (MBS) algorithms are used to develop detailed railroad vehicle models for the prediction of the wear resulting from the pantograph/catenary dynamic interaction. The wear is predicted using MBS algorithms for different motion scenarios that include constant-speed curve negotiation and acceleration and deceleration on a tangent (straight) track. The effect of the vehicle vibration in these different motion scenarios on the contact force is further used to study the wear rates of the contact wire. The wear model used in this investigation accounts for the electrical and the mechanical effects. The nonlinear finite element (FE) absolute nodal coordinate formulation (ANCF), which is suitable for implementation in MBS algorithms, is used to model the flexible catenary system, thereby eliminating the need for using incremental-rotation procedures and co-simulation techniques. In order to obtain efficient solutions, both the overhead contact line and the messenger wire are modeled using the gradient-deficient ANCF cable element. The pantograph/catenary elastic contact formulation employed in this study allows for separation between the pantograph panhead and the contact wire, and accounts for the effect of friction due to the sliding between the pantograph panhead and the catenary cable. The approach proposed in this investigation can be used to evaluate the electrical contact resistance, contribution of the arcing resulting from the panhead/catenary separation, mechanical and electrical wear contributions, and the effect of the pantograph mechanism uplift force on the wear rate. Numerical results are presented and analyzed to examine the wear rates for different motion scenarios.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(5):051604-051604-11. doi:10.1115/1.4042872.

The sustainability of biolubricants as green alternatives for industrial and machinery lubrication is questionable due to their unreliable oxidative stability, high pour point, and easy accumulation of contaminants that affect their tribological performance. Bio-based ionic liquid (IL) lubricants, which are environmentally friendly liquid state salts, have overcome these concerns related to conventional biolubricants. The present study investigates the effect of varying cation–anion moieties in ILs to understand their tribological performance and industrial viability. The industrial viability was analyzed by scaling their friction and wear behaviors against conventional biolubricants, and petroleum-based oils. The study investigated both bio- and nonbio-based ILs. Among the ILs examined, P666,14Saccharinate, P666,14Salicyate, and P666,14Benzoate were found to have superior tribological properties. The presence of large alkyl cation chain length and large aromatic anion ring size in ILs can effectively reduce friction and wear. This study details the mechanism by which the structural combinations of anion and cation in ILs define the tribological behavior of the bulk IL. Additionally, this study also highlights the environmentally benign nature of IL lubricants for possible industrial applications.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(5):051605-051605-7. doi:10.1115/1.4043065.

Titanium is a highly interesting material in engineering because of its unique combination of high strength to weight ratio, excellent resistance to corrosion, and biocompatibility. However, the material’s low wear resistance, which is its inherent nature, limits its application in highly erosive conditions. In order to enhance the wear resistance of biomedical grade titanium with the help of a WC-Co coating, an electrospark deposition method was used in this work. The goal of this work is to investigate the effect of frequency and current upper limit in the electrospark deposition process on substrate properties. Hardness of the layers was measured by a microhardness tester. In order to study the morphology and microstructure of surface layers, scanning electron microscope was used. Tribological tests were conducted under technically dry friction conditions at a load of 12.5 N by a pin-on-disk tribometer. Titanium was observed in coating and metallurgical bonding between the coating and the substrate. The optimized sample's hardness was about 930 HV 0.1. Results showed that the presence of a carbide layer on the surface of titanium leads to a great enhancement of wear resistance of about 68% in the pin-on-disk test.

Commentary by Dr. Valentin Fuster

Research Papers: Hydrodynamic Lubrication

J. Tribol. 2019;141(5):051701-051701-14. doi:10.1115/1.4042657.

The performances of aerostatic bearings have an important impact on machining accuracy in the ultraprecision machine tools. In this paper, numerical simulation is performed to calculate the static and dynamic performances of a double-pad annular inherently compensated aerostatic thrust bearing, while considering the effects of the upper bearing and lower bearing. The static results calculated by the computational fluid dynamics (CFD) method are compared with the finite difference method (FDM) for the specific model. By using polynomial fitting, the load-carrying capacity (LCC) of the bearing is calculated and the relationship between eccentricity ratio, design parameters, and static stiffness is analyzed. The active dynamic mesh method (ADMM) is applied to obtain the dynamic performance of the double-pad aerostatic thrust bearing based on the perturbation theory. Meanwhile, the effects of supply pressure, orifice diameter, squeeze number, and eccentricity ratio are comprehensively considered. Moreover, the step response of the double-pad thrust bearing is analyzed by using the passive dynamic mesh method (PDMM) based on dynamic equation. Related dynamic parameters including natural frequency are obtained through a system identification toolbox with Matlab, which can be used to avoid resonance. It is found that the dynamic calculation results computed by the ADMM and the PDMM are very close. The proposed method can be used to provide guidance for the design and optimization of the double-pad aerostatic thrust bearings.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(5):051702-051702-8. doi:10.1115/1.4042773.

Electric submersible pumps (ESPs) provide artificial lift within oil wells. ESPs commonly fail from mechanical vibrations that increase as bearing clearances increase from debris, gas, and liquid pumped through the ESP. In order to understand journal bearing wear within an ESP, three stages of a mixed flow electric submersible pump were subjected to hydraulic fracture sand slurry in water. One hundred seventeen hours were conducted with sand and water, followed by 68 h with air added at 15% inlet gas volume fraction. The journal bearings were severely worn by the end of testing, and pump vibrations increased with increased bearing clearances. Bearing vibrations and clearances became significantly larger than the impeller labyrinth seal vibrations and clearances, indicating that the labyrinth seals became the dominant rotor support once the bearings were worn. Adding air increased the wear and rotor vibration orbit variability. Rotor vibration orbits were entirely independent of gas void fraction by the end of testing, indicating that the lubricant composition no longer directly impacted vibrations. Fine axial cracks from heat checking were observed on the journal of the bearings. Results indicate that controlling journal bearing wear is a critical factor for increasing operating lifetimes. Alternative bearing geometry and materials should be investigated to prevent the occurrence of three-body abrasion, limit the resultant wear rate from three-body abrasion, and limit the damage from heat checking.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(5):051703-051703-13. doi:10.1115/1.4042760.

The purpose of this study is to investigate the evolution of cavitation bubbles for the high-speed water-lubricated spiral groove thrust bearing. A theoretical model of cavitation bubble evolution considering multiple effects (interface, breakage, and coalescence of bubbles) was established for the bearing. A high-speed experimental setup was developed to measure the distribution of bubbles. The theoretical model is verified by the experimental data. The results show that the Boltzmann-type bubble transport equation can be used to describe the bubble evolution of the bearing under the breakup and coalescence at high-speed conditions; the volume of the bubble group presents a skewed distribution in equilibrium; the number of small-sized bubbles is greater than that of large-sized bubbles at high rotational speed; the bubbles are mainly distributed at the inlets and outlets of spiral grooves; the bubble number density increases with the groove depth and spiral angle; more bubbles are generated near the outer diameter of the bearing. The study provides a theoretical and experimental basis for the bubble evolution of the water-lubricated spiral groove bearing under high speeds.

Commentary by Dr. Valentin Fuster

Research Papers: Micro-Nano Tribology

J. Tribol. 2019;141(5):052001-052001-5. doi:10.1115/1.4042768.

This study described the synthesis and the tribological properties of surface-modified Field’s alloy nanoparticles, which were prepared by a facile one-step nanoemulsion method and using ethyl carbamate as a surfactant, as additives in liquid poly-alfa-olefin (PAO) oil. The size and morphology of nanoparticles were investigated by transmission electron microscopy (TEM). The zeta potential, viscosity, and stability properties of the surface-modified nanoparticles suspended in PAO oil (called nanofluid) with different mass concentrations were measured by a viscometer and Zeta potential analyzer, respectively. The tribological properties of the nanofluid were tested by a ball to disk wear and friction machine. Compared with pure PAO oil, the results showed that the nanofluids had better lubricating behaviors. When the mass concentration of modified nanoparticles was 0.08 wt. %, both the friction coefficient and the wear scar diameter were the lowest.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(5):052002-052002-9. doi:10.1115/1.4042677.

This study describes a facile synthesis of calcium carbonate (CaCO3) monodispersed fine particles from an abundant indigenous and economical source (quicklime) and its enhanced tribological performance as a green additive in commercial lithium grease (CLG). The effects of various experimental parameters on particle morphology were thoroughly examined, and the conditions were optimized. The synthesized uniform particles were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffractometry, and thermogravimetric (TG) /differential thermal analysis (DTA), and their results confirmed the calcite structure of the synthesized particles. The friction and wear studies were carried out under the applied load of 0.863 N at an ambient temperature for 5 min. The tribological performance of various amounts (1–7%) of cubic-CaCO3 (CCC) particles in CLG showed that 5 wt. % of CCC was the optimum concentration as additive in the present case. For comparison purposes, a commercial CaCO3 powder was used and a decrease in the friction coefficient of CLG was observed to be 33.4% and 16.4% for 5 wt. % CCC and commercial CaCO3 additives, respectively. The significantly enhanced antiwear and antifriction performance of the optimum CCC-CLG in comparison with the blank and commercial CaCO3-additized CLG was quite encouraging, and extensive studies in a real machine-operating environment are in progress for evaluation of the CCC-CLG blend to be used as an economical, green, and high-performance lubricant in mechanical components.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(5):052003-052003-9. doi:10.1115/1.4042873.

The objective of this work is to study the tribological properties of natural fiber based composites using nanotechnology. The naturally available banana plant fibers were treated with nanoclay particles, and these treated fibers were then reinforced in an epoxy polymer to form composites. The friction and wear properties of nanoclay-treated banana fiber (NC-BF) reinforced composites were compared with untreated banana fiber (UT-BF) reinforced composites. Short NC-BF- and UT-BF-reinforced composites with fiber concentration ranging from 20 wt % to 60 wt % were prepared by the vacuum resin infusion processing method. The result indicates that the NC-BF-reinforced composites have shown improved friction and wear properties. Microscopy examination revealed that NC-BF-reinforced composites were able to form a transfer layer between the wear test specimen wear surface and counter face, resulting in improved wear properties. The nanoclay particles also induce increased hardness and friction to the composites and improve braking properties.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(5):052004-052004-6. doi:10.1115/1.4043066.

Chromium (Cr)-based coatings have been widely used to strengthen the friction reduction and wear resistance on various kinds of surface. Here, the stable aqueous dispersion of oxidized multi-walled carbon nanotube (MWCNT) and graphene oxide nanosheets (GOS) was obtained by ultrasonic oxidation treatment. Then, MWCNT-Cr and GOS-Cr composite coatings were prepared using the direct current electrochemical co-deposition process on 420 stainless steel in the electrolyte with the addition of MWCNT and GOS under different current density and temperature. The morphology, structure, hardness and tribological properties of MWCNT-Cr and GOS-Cr composite coating are comparatively studied using pure Cr coating as a baseline. The friction reduction performance of MWCNT-Cr and GOS-Cr composite coatings was improved at optimum current density and temperature. The anti-wear properties of MWCNT-Cr and GOS-Cr composite coatings were enhanced by uniform embedment of MWCNT and GOS in coatings increasing the hardness and lubricity. This study suggests that the introduction of oxidized MWCNT and GOS with good dispersion could enhance the wear resistance and friction reduction of pure Cr coating due to their excellent dispersion, mechanical, and lubricant properties.

Commentary by Dr. Valentin Fuster

Research Papers: Other (Seals, Manufacturing)

J. Tribol. 2019;141(5):052201-052201-10. doi:10.1115/1.4043067.

Poor tribological properties restrict structural applications of aluminum alloys and surface composites of aluminum alloys have gained more attention in material processing. The addition of solid lubricant reinforcement particles along with abrasive ceramics contributes to the enhancement of tribological performance of surface composites. In the present study, the solid-state technique, friction stir processing (FSP) was used to develop mono (B4C) and hybrid (B4C + MoS2) surface composites in the AA6061-T651 aluminum alloy. The hybrid surface composites were produced by varying an amount of MoS2. Multipass FSP with different direction strategies was adopted for achieving uniform distribution of reinforcement powders in the aluminum matrix. Microstructure analysis showed a uniform dispersal of reinforcement particles without any clustering or agglomeration in the processing zone. Microhardness and wear performance of mono and hybrid composites improved in comparison with the base metal. The mono surface composite exhibited the highest hardness while the hybrid surface composite (75%B4C + 25%MoS2) achieved the highest wear resistance. This was attributed to the solid lubricant nature of MoS2. Furthermore, dissolution of the strengthening precipitate condition during multipass FSP without reinforcement particles resulted in the reduction of hardness and wear resistance.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Tribol. 2019;141(5):054501-054501-5. doi:10.1115/1.4042659.

This tech brief presents some basic theory and calculations to help assess the feasibility of surface-film lubricated dry skin pass of metal strips to enhance surface finish after cold-rolling operations. Results are presented of the required rate of heat removal from the rolling apparatus and the maximum rise of roll surface temperature for steel and aluminum strips under various parametric conditions of practical interest. The theory and the calculation tool may be used to perform analyses with other material, geometry, and operating parameters and to assist the design and development of surface-film lubricated dry skin-pass processes.

Commentary by Dr. Valentin Fuster

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