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Lubricants, Volume 7, Issue 7 (July 2019)

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Cover Story (view full-size image) Molybdenum disulfide (MoS2) is one of the most broadly utilized solid lubricants, with prominent [...] Read more.
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Open AccessArticle
The Effect of the Pivot Stiffness on the Performances of Five-Pad Tilting Pad Bearings
Received: 27 May 2019 / Revised: 15 July 2019 / Accepted: 17 July 2019 / Published: 22 July 2019
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Abstract
The role of the pivot flexibility in tilting-pad journal bearings (TPJBs) has become essential, particularly for bearings working at high applied load and relatively high rotor speeds. Predictions from a simple bearing model with rigid pivots show incorrect estimation of the dynamic coefficients [...] Read more.
The role of the pivot flexibility in tilting-pad journal bearings (TPJBs) has become essential, particularly for bearings working at high applied load and relatively high rotor speeds. Predictions from a simple bearing model with rigid pivots show incorrect estimation of the dynamic coefficients in comparison with the experimental results. Normally, the more flexible the pad pivot, the lower the dynamic coefficients because the stiffness of the pivot takes in series with the stiffness and damping of the oil film. This paper investigates the influence of pivot stiffness on the dynamic force coefficients of two different five-pad TPJBs as a function of the applied static load and excitation frequency: rocker-backed pivot and spherical pivot bearings. In order to highlight the effect of the pivot stiffness in the spherical pivot bearing, displacement restriction components and elastic copper made shims have been used. Firstly, a thermo-elasto-hydrodynamic model for the static and dynamic characteristics of the two bearings is described. This model takes into account the flexibility of both pad and pivot. The pivot stiffnesses calculated by means of the Hertz theory and those obtained by experiments have been introduced and compared in the model. The clearance profiles of two tested bearing and the shaft center loci obtained by measurement and prediction are also shown. The dynamic coefficients of the two bearings obtained from the numerical simulation were compared with the experimental results. By the analysis it can be concluded that the effect of the pivot flexibility on the clearance profile, the shaft locus and on the dynamic coefficients is very significant. More important, it is important to estimate the pivot stiffness of each single pad using experimental measurements. Full article
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Open AccessArticle
Correlation of Tribological Behavior and Fatigue Properties of Filled and Unfilled TPUs
Received: 6 June 2019 / Revised: 12 July 2019 / Accepted: 17 July 2019 / Published: 20 July 2019
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Abstract
For a long service time, fatigue has been a typical problem that mechanical sealing materials face. How does it relate to tribological performance? In this study, filled and unfilled thermoplastic polyurethanes (TPUs) were investigated. Dumbbell and faint wait pure shear (FWPS) specimens were [...] Read more.
For a long service time, fatigue has been a typical problem that mechanical sealing materials face. How does it relate to tribological performance? In this study, filled and unfilled thermoplastic polyurethanes (TPUs) were investigated. Dumbbell and faint wait pure shear (FWPS) specimens were used to characterize the fatigue properties and crack growth rate of TPUs, respectively. Additionally, to identify the impact of temperature on fatigue tests, the tests were conducted at room temperature and 80 °C. Different tribological tests were conducted to investigate their tribological properties. Fracture surfaces from fatigue tests and worn surfaces from tribological tests were analyzed using a scanning electron microscope (SEM). Two wear models were verified to correlate between fatigue and tribological properties; one of the models is better for rough counter surfaces, while the other is advantageous if the counter surface is smooth. Full article
(This article belongs to the Special Issue Tribology of Seals)
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Open AccessArticle
Non Smooth Contact Dynamics Approach for Mechanical Systems Subjected to Friction-Induced Vibration
Received: 11 June 2019 / Revised: 9 July 2019 / Accepted: 10 July 2019 / Published: 15 July 2019
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Abstract
The modeling of contact is one of the main features of contact dynamics in the context of friction-induced vibrations. It can have a strong impact on the numerical results and consequently on the design choices during the optimization or specification of industrial mechanical [...] Read more.
The modeling of contact is one of the main features of contact dynamics in the context of friction-induced vibrations. It can have a strong impact on the numerical results and consequently on the design choices during the optimization or specification of industrial mechanical systems. This is particularly the case for scientific studies interested in brake squeal. The objective of the paper is to recall and to promote developments concerning the use of non smooth contact dynamics approach for numerical simulations based on finite element method. The specific problem of the prediction of self-excited vibration in the context of brake squeal is discussed. In order to illustrate the potential benefit for the mechanical community of using formulations and theoretical developments from the mathematical community, the stability analysis and the estimation of nonlinear vibrations of a brake system with multiple frictional interface is investigated. Full article
(This article belongs to the Special Issue Tribology and Contact Dynamics)
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Open AccessArticle
The Indentation Rolling Resistance in Belt Conveyors: A Model for the Viscoelastic Friction
Received: 26 May 2019 / Revised: 28 June 2019 / Accepted: 2 July 2019 / Published: 5 July 2019
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Abstract
In this paper, we study the steady-state rolling contact of a linear viscoelastic layer of finite thickness and a rigid indenter made of a periodic array of equally spaced rigid cylinders. The viscoelastic contact model is derived by means of Green’s function approach, [...] Read more.
In this paper, we study the steady-state rolling contact of a linear viscoelastic layer of finite thickness and a rigid indenter made of a periodic array of equally spaced rigid cylinders. The viscoelastic contact model is derived by means of Green’s function approach, which allows solving the contact problem with the sliding velocity as a control parameter. The contact problem is solved by means of an accurate numerical procedure developed for general two-dimensional contact geometries. The effect of geometrical quantities (layer thickness, cylinders radii, and cylinders spacing), material properties (viscoelastic moduli, relaxation time) and operative conditions (load, velocity) are all investigated. Physical quantities typical of contact problems (contact areas, deformed profiles, etc.) are calculated and discussed. Special emphasis is dedicated to the viscoelastic friction force coefficient and to the energy dissipated per unit time. The discussion is focused on the role played by the deformation localized at the contact spots and the one in the bulk of the thin layer, due to layer bending. The model is proposed as an accurate solution for engineering applications such as belt conveyors, in which the energy dissipated on the rolling contact of idle rollers can, in some cases, be by far the most important contribution to their energy consumption. Full article
(This article belongs to the Special Issue Adhesion, Friction and Lubrication of Viscoelastic Materials)
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Open AccessReview
Solid Lubrication with MoS2: A Review
Received: 13 June 2019 / Revised: 28 June 2019 / Accepted: 28 June 2019 / Published: 2 July 2019
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Abstract
Molybdenum disulfide (MoS2) is one of the most broadly utilized solid lubricants with a wide range of applications, including but not limited to those in the aerospace/space industry. Here we present a focused review of solid lubrication with MoS2 by [...] Read more.
Molybdenum disulfide (MoS2) is one of the most broadly utilized solid lubricants with a wide range of applications, including but not limited to those in the aerospace/space industry. Here we present a focused review of solid lubrication with MoS2 by highlighting its structure, synthesis, applications and the fundamental mechanisms underlying its lubricative properties, together with a discussion of their environmental and temperature dependence. The review also includes an extensive overview of the structure and tribological properties of doped MoS2, followed by a discussion of potential future research directions. Full article
(This article belongs to the Special Issue Friction Mechanisms)
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Open AccessArticle
Estimation of the Normal Contact Stiffness for Frictional Interface in Sticking and Sliding Conditions
Received: 12 June 2019 / Revised: 21 June 2019 / Accepted: 23 June 2019 / Published: 2 July 2019
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Abstract
Modeling of frictional contact systems with high accuracy needs the knowledge of several contact parameters, which are mainly related to the local phenomena at the contact interfaces and affect the complex dynamics of mechanical systems in a prominent way. This work presents a [...] Read more.
Modeling of frictional contact systems with high accuracy needs the knowledge of several contact parameters, which are mainly related to the local phenomena at the contact interfaces and affect the complex dynamics of mechanical systems in a prominent way. This work presents a newer approach for identifying reliable values of the normal contact stiffness between surfaces in contact, in both sliding and sticking conditions. The combination of experimental tests, on a dedicated set-up, with finite element modeling, allowed for an indirect determination of the normal contact stiffness. The stiffness was found to increase with increasing contact pressure and decreasing roughness, while the evolution of surface topography and third-body rheology affected the contact stiffness when sliding. Full article
(This article belongs to the Special Issue Tribology and Contact Dynamics)
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Open AccessArticle
Numerical Simulation of a Slipper Model with Multi-Lands and Grooves for Hydraulic Piston Pumps and Motors in Mixed Lubrication
Received: 29 April 2019 / Revised: 22 June 2019 / Accepted: 24 June 2019 / Published: 27 June 2019
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Abstract
A theoretical model of a slipper with multi-lands and multi-grooves for swashplate type axial piston pumps and motors was established, including surface interactions. Further, a numerical simulation was conducted under an unsteady state and mixed lubrication conditions. Four model configurations were considered: A [...] Read more.
A theoretical model of a slipper with multi-lands and multi-grooves for swashplate type axial piston pumps and motors was established, including surface interactions. Further, a numerical simulation was conducted under an unsteady state and mixed lubrication conditions. Four model configurations were considered: A slipper with a single main land; a slipper with inner and main lands and a groove; a slipper with outer and main lands and a groove; and a slipper with inner, main, and outer lands with two grooves. Numerical solutions were obtained across a wide range of operating conditions in terms of center clearance, pad attitude, contact pressure, flow rate, friction torque, power loss, and stiffness. The motion and characteristics were differentiated into two groups: Slippers with a single-land and an annex inner-land; and slippers with an annex outer-land and a triple-land. The single-land and annex inner-land slippers exhibited smaller pad swing, whereas the triple-land and annex outer-land slippers reduced contact pressure and power loss. Full article
(This article belongs to the Special Issue Industrial Tribo-Systems and Future Development Trends)
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Lubricants EISSN 2075-4442 Published by MDPI AG, Basel, Switzerland RSS E-Mail Table of Contents Alert
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