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Research Papers: Hydrodynamic Lubrication

J. Tribol. 2019;141(7):071701-071701-15. doi:10.1115/1.4043506.

Misalignment between the shaft and the bearing of a journal bearing set may be inevitable and can negatively impact journal bearing performance metrics in many industrial applications. This work proposes a convex profile design of the journal surface to help counteract the negative effects caused by such a misalignment. A transient mass-conserving hydrodynamic Reynolds equation model with the Patir–Cheng flow factors and the Greenwood–Tripp pressure–gap relationship is developed to conduct the design and analysis. The results reveal that under transient impulse loading, a properly designed journal profile can help enhance the minimum film thickness, reduce mean and peak bearing frictions, and increase bearing durability by reducing the asperity-related wear load. The mechanism for the minimum film thickness improvement due to the profile design is traced to the more even distribution of the hydrodynamic pressure toward the axial center of the bearing. The reason for the reductions of the friction and wear load is identified to be the decreased asperity contact by changing the lubrication regime from mixed lubrication to nearly hydrodynamic lubrication. Parametric studies and a case study are reported to highlight the key points of the profile design and recommendations for profile height selection are made according to misalignment parameters.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(7):071702-071702-13. doi:10.1115/1.4043507.

To harness higher axial load capacity, a larger cone angle is used in conical bearings, resulting in an increase in the surface area which in turn increases the frictional power loss. The use of microgrooves in journal bearing helps in controlling this loss. Therefore, the present work is aimed to analyze conical hybrid journal bearing (i.e., combination of hydrostatic and hydrodynamic modes of operation) consisting of microgrooves along with shear thinning and piezo-viscous behavior of the lubricant. In this study, the microgroove attributes have been optimized by obtaining the solution of a Reynolds equation using finite element method and generalized minimum residual scheme (GMRES). These optimized groove attributes are used for numerically simulating the performance of the conical bearings. It has been observed that the best features of microgrooves and shear thinning behavior of the lubricant can be extracted to achieve better performance of the bearings. The results presented in this study are believed to be beneficial to the bearing designers and practising lubrication engineers.

Commentary by Dr. Valentin Fuster

Research Papers: Mixed and Boundary Lubrication

J. Tribol. 2019;141(7):072101-072101-10. doi:10.1115/1.4043598.

As a novel layered material, black phosphorus (BP) shows unexpected characteristics in many aspects including tribological application. In this work, BP was prepared through ball milling from red phosphorus (RP). The boundary lubricating properties of the BP nanosheets were investigated on a ball-on-disk tribometer as lubricating additives in polyalphaolefin oil. The micromorphologies, concentration, and composition of the typical chemical elements on the worn surfaces were measured by the 3D laser scanning microscope, scanning electron microscope, and X-ray photoelectron spectrometer, respectively. The results show that bulk BP can be found after RP was milled at 500 rpm for 36 h. The Raman intensity of the BP increased initially and then decreased with the increase in milling time, and the maximum intensity can be obtained at 60 h. The BP nanosheets displayed excellent antifriction and anti-wear performances as lubricating additives in PAO6 oil for steel/steel contact in boundary lubrication regime. The boundary lubrication mechanism of the BP nanosheets is dominated by the physical slippery effect of the laminated nanosheets and the tribofilm on the rubbing surfaces.

Commentary by Dr. Valentin Fuster
J. Tribol. 2019;141(7):072102-072102-12. doi:10.1115/1.4043606.

In the present study, lubricating grease was developed with paraffin oil and 12-lithium hydroxy stearate metal soap as a thickening agent. MoS2 nanosheets were synthesized by hydrothermal method and functionalized with 1-octadecanethiol (i.e., MoS2-ODT). The MoS2 and MoS2-ODT nanosheets were dispersed in the grease with different concentrations to evaluate its tribological performance. Tribological results unveiled that the addition of MoS2 nanosheets in grease appreciably reduced the coefficient of friction and mean wear volume of tribo-interfaces as compared with pure grease. Energy dispersive spectroscopy (EDS) spectrum revealed the deposition of MoS2 on the worn surface and confirmed a thin tribo-film which protects steel tribo-pair against wear.

Commentary by Dr. Valentin Fuster

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