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Lubricants, Volume 6, Issue 1 (March 2018)

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Cover Story (view full-size image) As most real surfaces are rough, friction is a multiscale phenomenon that involves contact between [...] Read more.
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Editorial

Jump to: Research, Review

Open AccessEditorial Acknowledgement to Reviewers of Lubricants in 2017
Lubricants 2018, 6(1), 6; doi:10.3390/lubricants6010006
Received: 11 January 2018 / Revised: 11 January 2018 / Accepted: 11 January 2018 / Published: 11 January 2018
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Abstract
Peer review is an essential part in the publication process, ensuring that Lubricants maintains high quality standards for its published papers.[...] Full article

Research

Jump to: Editorial, Review

Open AccessArticle Acetabular Cups in 60 mm Metal-on-Metal Bearings Subjected to Dynamic Edge-Loading with 70° Peak-Inclination in 10-Million Cycle Simulator Study
Lubricants 2018, 6(1), 1; doi:10.3390/lubricants6010001
Received: 25 October 2017 / Revised: 5 December 2017 / Accepted: 8 December 2017 / Published: 21 December 2017
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Abstract
Wear simulation of total-hip arthroplasty (THA) involves hip biomechanics, tribology, bearing designs and cup wear-patterns. This is the first demonstration of cup edge-loading using the “Inverted-cup” test mode. Benefits included, (i) clinically relevant wear-patterns, and (ii) cup inclinations varying from ideal to edge-loaded
[...] Read more.
Wear simulation of total-hip arthroplasty (THA) involves hip biomechanics, tribology, bearing designs and cup wear-patterns. This is the first demonstration of cup edge-loading using the “Inverted-cup” test mode. Benefits included, (i) clinically relevant wear-patterns, and (ii) cup inclinations varying from ideal to edge-loaded during each 1-s simulator cycle. The 60 mm head and cup bearings in metal-on-metal (MOM) hip joints showed run-in and steady-state wear phases to 10-million cycles (Mc). MOM edge-wear was not unduly high at 1.7 mm3/Mc overall, this 3-fold higher than 60 mm MOM study without edge-loading. One MOM outlier averaged 2.7 mm3/Mc, this representing the break-away wear (BAW) phenomena. A surprising result was that cups contributed 75–93% of total wear. The most disturbing conclusion from review of laboratory studies was that MOM wear-rates varied 1 to >30 mm3/Mc for reasons not understood. These data suggested a new hypothesis, that MOM bearings were very sensitive to external stimuli, be they simulator artifact or patient related. Full article
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Open AccessArticle Effect of Surface Texturing on Cast Iron Reciprocating against Steel under Cyclic Loading in Boundary and Mixed Lubrication Conditions
Lubricants 2018, 6(1), 2; doi:10.3390/lubricants6010002
Received: 21 September 2017 / Revised: 20 November 2017 / Accepted: 25 December 2017 / Published: 2 January 2018
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Abstract
This study investigates the effect of laser surface texturing on the friction behavior of grey cast iron reciprocating under boundary, mixed lubrication conditions and cyclic loading. Four geometrical parameters of micro-textures (feature depth, feature diameter, area fraction, and sliding direction) were studied using
[...] Read more.
This study investigates the effect of laser surface texturing on the friction behavior of grey cast iron reciprocating under boundary, mixed lubrication conditions and cyclic loading. Four geometrical parameters of micro-textures (feature depth, feature diameter, area fraction, and sliding direction) were studied using a design of experiments (DoE) approach. The results showed that depending on the geometry, micro-textures can have either a positive or a negative effect on the friction behavior. The DoE analysis revealed that the coefficient of friction is mainly affected by the interaction of the feature depth and its diameter. It was found that this can be related to the aspect ratio of the dimples, and the best results were obtained for an aspect ratio of 0.1 and 0.17. Full article
(This article belongs to the Special Issue Improvement of Friction and Wear by Laser Surface Texturing)
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Open AccessArticle Testing of WS2 Nanoparticles Functionalized by a Humin-Like Shell as Lubricant Additives
Lubricants 2018, 6(1), 3; doi:10.3390/lubricants6010003
Received: 2 November 2017 / Revised: 1 January 2018 / Accepted: 3 January 2018 / Published: 4 January 2018
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Abstract
Nanoparticles of transition metal dichalcogenides (TMDC) have been known to reduce friction and wear when added to oil-type liquid lubricants. Aggregation limits the ability of the nanoparticles to penetrate into the interface between the two rubbing surfaces—an important factor in friction reduction mechanisms.
[...] Read more.
Nanoparticles of transition metal dichalcogenides (TMDC) have been known to reduce friction and wear when added to oil-type liquid lubricants. Aggregation limits the ability of the nanoparticles to penetrate into the interface between the two rubbing surfaces—an important factor in friction reduction mechanisms. Doping has been successfully used to reduce agglomeration, but it must be done in the production process of the nanoparticles. The use of surface-functionalized nanoparticles is less common than doping. Nonetheless, it has the potential to reduce agglomeration and thereby improve the reduction of friction and wear. In this study, we present the results of preliminary tribological ball-on-flat tests performed with WS2 nanoparticles functionalized by a humin-like conformal shell, as additives to polyalphaolefin-4 (PAO-4) oil. We tested WS2 inorganic nanotubes (INTs) and two grades of inorganic fullerene-like nanoparticles (IFs). The shell/coating was found to improve friction reduction for IFs but not for INTs through better dispersion in the oil. The thicker the coating on the IFs, the less agglomerated they were. Coated industrial-grade IFs were found, by far, to be the best additive for friction reduction. We suggest the combination between reduced agglomeration and poor crystallinity as the reason for this result. Full article
(This article belongs to the Special Issue Novel Lubricant Additives)
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Open AccessArticle Characterization of High-Power Turbomachinery Tilting Pad Journal Bearings: First Results Obtained on a Novel Test Bench
Lubricants 2018, 6(1), 4; doi:10.3390/lubricants6010004
Received: 27 November 2017 / Revised: 29 December 2017 / Accepted: 3 January 2018 / Published: 6 January 2018
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Abstract
Tilting pad journal bearings are usually employed in turbomachines for their stable behavior at high rotational speeds. Devoted test rigs have been realized to validate the predictions of theoretical models. However, the design of new high-performance and large-size bearings needs to be supported
[...] Read more.
Tilting pad journal bearings are usually employed in turbomachines for their stable behavior at high rotational speeds. Devoted test rigs have been realized to validate the predictions of theoretical models. However, the design of new high-performance and large-size bearings needs to be supported by experimental investigations on high-performance large test rigs. The main characteristics of a recently built facility for testing large tilting pad journal bearings with diameters from 150 to 300 mm are described in this work. The test rig is versatile and can be used to test bearings of different size, configurations and to investigate the influence of many parameters, even the effect of misalignment. Sample results of the static characterization of a four-pad high-performance tilting pad journal bearing are reported evidencing some transient effects. A few sample dynamic results are also reported. The presented experimental results demonstrated the capabilities of the rig for investigating the static and the dynamic characteristics of the bearings accurately measuring slow and fast variables. Full article
(This article belongs to the Special Issue Friction and Lubrication of Sliding Bearings)
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Open AccessArticle Comparison of Perturbed Reynolds Equation and CFD Models for the Prediction of Dynamic Coefficients of Sliding Bearings
Lubricants 2018, 6(1), 5; doi:10.3390/lubricants6010005
Received: 10 November 2017 / Revised: 10 December 2017 / Accepted: 6 January 2018 / Published: 10 January 2018
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Abstract
The accuracy and utility of rotordynamic models for machinery systems are greatly affected by the accuracy of the constituent dynamic bearing models. Primarily, the dynamic behavior of bearings is modeled as linear combination of mass, damping, and stiffness coefficients that are predicted from
[...] Read more.
The accuracy and utility of rotordynamic models for machinery systems are greatly affected by the accuracy of the constituent dynamic bearing models. Primarily, the dynamic behavior of bearings is modeled as linear combination of mass, damping, and stiffness coefficients that are predicted from a perturbed Reynolds equation. In the present paper, an alternative method using Computational Fluid Dynamics (CFD) with a moving boundary is used to predict the dynamic coefficients of slider bearings and the results are compared with the more commonly employed perturbed Reynolds equation model. A linear slider bearing geometry is investigated and the results serve as precursors to similar investigations involving the more complex journal bearing geometries. Time and frequency domain methods for the estimation of dynamic coefficients are shown to give comparable results. For CFD with a moving boundary, temporal inertia is found to have a significant effect for a reduced, squeeze Reynolds number less than one. The temporal inertia effect is captured through an added mass coefficient within the dynamic model of the bearing. Full article
(This article belongs to the Special Issue Friction and Lubrication of Sliding Bearings)
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Open AccessArticle Application of a Thermodynamic Concept for the Analysis of Structural Degradation of Soap Thickened Lubricating Greases
Lubricants 2018, 6(1), 7; doi:10.3390/lubricants6010007
Received: 8 November 2017 / Revised: 2 January 2018 / Accepted: 9 January 2018 / Published: 11 January 2018
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Abstract
Lubricating greases are special lubricants with a wide range of application. The tribologically stressed grease is used as tribological system and finally modeled as an open thermodynamic system. This study investigated the phenomenon of self-optimization and applied to the process of shearing a
[...] Read more.
Lubricating greases are special lubricants with a wide range of application. The tribologically stressed grease is used as tribological system and finally modeled as an open thermodynamic system. This study investigated the phenomenon of self-optimization and applied to the process of shearing a grease. The conditions for self-optimization and the consequences of created dissipative structures are investigated according to the interpreted literature. Full article
(This article belongs to the Special Issue Lubricating Greases 2017)
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Open AccessCommunication Influence of Slip and Lubrication Regime on the Formation of White Etching Cracks on a Two-Disc Test Rig
Lubricants 2018, 6(1), 8; doi:10.3390/lubricants6010008
Received: 4 December 2017 / Revised: 2 January 2018 / Accepted: 10 January 2018 / Published: 12 January 2018
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Abstract
A common cause for maintenance and downtime in multiple fields of the mechanical transmission industries are premature rolling bearing failures due to white etching cracks (WEC). Within this work, WEC have been successfully recreated on a two-disc test rig under rolling contact loading
[...] Read more.
A common cause for maintenance and downtime in multiple fields of the mechanical transmission industries are premature rolling bearing failures due to white etching cracks (WEC). Within this work, WEC have been successfully recreated on a two-disc test rig under rolling contact loading without additional loading such as hydrogen pre-charging. This paper summarizes the state of the investigations regarding the influence of the slip type and the lubrication regime on the WEC formation on the two-disc test rig. Full article
(This article belongs to the Special Issue Tribology of Machine Elements--Smart Lubricants)
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Open AccessArticle Contaminant Particle Motion in Lubricating Grease Flow: A Computational Fluid Dynamics Approach
Lubricants 2018, 6(1), 10; doi:10.3390/lubricants6010010
Received: 21 November 2017 / Revised: 29 December 2017 / Accepted: 9 January 2018 / Published: 17 January 2018
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Abstract
In this paper, numerical simulations of particle migration in lubricating grease flow are presented. The rheology of three lithium greases with NLGI (National Lubricating Grease Institute) grades 00, 1 and 2 respectively are considered. The grease is modeled as a single-phase Herschel–Bulkley fluid,
[...] Read more.
In this paper, numerical simulations of particle migration in lubricating grease flow are presented. The rheology of three lithium greases with NLGI (National Lubricating Grease Institute) grades 00, 1 and 2 respectively are considered. The grease is modeled as a single-phase Herschel–Bulkley fluid, and the particle migration has been considered in two different grease pockets formed between two concentric cylinders where the inner cylinder is rotating and driving the flow. In the wide grease pocket, the width of the gap is much smaller compared to the axial length scale, enabling a one-dimensional flow. In the narrow pocket, the axial and radial length is of the same order, yielding a three-dimensional flow. It was found that the change in flow characteristics due to the influence of the pocket lateral boundaries when going from the wide to the narrow pocket leads to a significantly shorter migration time. Comparing the results with an existing migration model treating the radial component contribution, it was concluded that a solution to the flow in the whole domain is needed together with a higher order numerical scheme to obtain a full solution to the particle migration. This result is more pronounced in the narrow pocket due to gradients in the flow induced by the lateral boundaries. Full article
(This article belongs to the Special Issue Lubricating Greases 2017)
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Open AccessArticle Influence of Laser Pulse Number on the Ablation of Cemented Tungsten Carbides (WC-CoNi) with Different Grain Size
Lubricants 2018, 6(1), 11; doi:10.3390/lubricants6010011
Received: 6 December 2017 / Revised: 15 January 2018 / Accepted: 16 January 2018 / Published: 20 January 2018
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Abstract
The ultra-short pulse laser has attracted attention as an advanced tool for functionalizing surface topography, since it has high accuracy and results in little damage. In a previous study, some innovative patterns were introduced on cemented carbide surfaces, such as dimples, which are
[...] Read more.
The ultra-short pulse laser has attracted attention as an advanced tool for functionalizing surface topography, since it has high accuracy and results in little damage. In a previous study, some innovative patterns were introduced on cemented carbide surfaces, such as dimples, which are commonly used as oil reservoirs for bearings. The accuracy is not only related to the inherent features of the laser, but also to the machining processes. Within this context, this study aims to investigate the influence of machining parameters (i.e., pulse number in this study) on the ablation mechanism and resulting surface integrity. Two cemented carbide grades, possessing similar chemical composition but different grain size (small and large), are machined using a femtosond laser set-up with variant pulse number (1–20). The geometrical properties of the produced structure and surface integrity are statistically investigated using different microscopy techniques. It is found that the dimple depth is approximately proportional to the pulse number for both grades, and the coarse grade leads to a higher rate of depth increase. Damage is found in the form of melting and cracking for the binder and the grains, respectively; but this is more pronounced for large-grain grade in terms of the scale and depth. However, these observations are only found at a very superficial position. Full article
(This article belongs to the Special Issue Selected Papers in the Sixth World Tribology Congress (WTC 2017))
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Open AccessArticle EHD Effects in Lubricated Journal Bearing
Lubricants 2018, 6(1), 12; doi:10.3390/lubricants6010012
Received: 30 October 2017 / Revised: 16 January 2018 / Accepted: 16 January 2018 / Published: 23 January 2018
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Abstract
This paper presents a numerical analysis of the influence of deformation of infinite parallel cylindrical solids in partial journal bearing on the oil film characteristics. The stationary elastohydrodynamic EHD problems for three design models of bearings are considered: (1) The bearing in which
[...] Read more.
This paper presents a numerical analysis of the influence of deformation of infinite parallel cylindrical solids in partial journal bearing on the oil film characteristics. The stationary elastohydrodynamic EHD problems for three design models of bearings are considered: (1) The bearing in which the basic contribution to the elastic displacement of the surface brings the thin elastic liner; (2) The elastic cylinder and the elastic bushing which is modeled by the elastic space with a cylindrical cut; (3) The elastic cylinder and the elastic bushing in the presence of the thin elastic liner with a small module of elasticity. It is shown that when the minimum film thickness is fixed and deformations of the elastic solids increase, then the load capacity increases, reaches a maximum, and then decreases. The deformation of solids can raise load capacity many times over. When the deformation of solids increases from zero, the pressure distribution changes from the distribution of pressure in the case of rigid bodies to the distribution of pressure which takes place with the dry contact of elastic bodies. Full article
(This article belongs to the Special Issue Friction and Lubrication of Sliding Bearings)
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Open AccessArticle An Arbitrary Lagrangian–Eulerian Formulation for Modelling Cavitation in the Elastohydrodynamic Lubrication of Line Contacts
Lubricants 2018, 6(1), 13; doi:10.3390/lubricants6010013
Received: 21 December 2017 / Revised: 20 January 2018 / Accepted: 22 January 2018 / Published: 24 January 2018
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Abstract
In this article an arbitrary Lagrangian–Eulerian (ALE) formulation for modelling cavitation in elastohydrodynamic lubrication (EHL) is derived and applied to line contact geometry. The method is developed in order to locate the position of cavitation onset along the length of the contacting region
[...] Read more.
In this article an arbitrary Lagrangian–Eulerian (ALE) formulation for modelling cavitation in elastohydrodynamic lubrication (EHL) is derived and applied to line contact geometry. The method is developed in order to locate the position of cavitation onset along the length of the contacting region which gives the transition from liquid to vapour in the fluid. The ALE is implemented by introducing a spatial frame of reference in which the solution is required and a material frame of reference in which the governing equations are solved. The spatial frame is moved from the material frame according to the error in the Neumann pressure gradient constraint required at the cavitation location when Dirichlet constraints are imposed for pressure in the liquid phase. Results are calculated under both steady-state and transient operating conditions using a multigrid solver. The solutions obtained are compared to established literature and conventional approaches to modelling cavitation which show that the ALE formulation is an alternative, straightforward and accurate means of implementing such conditions in EHL. This is achieved without the penalties associated with the numerical modelling of Heaviside functions or free boundaries. Full article
(This article belongs to the Special Issue Computer Simulation in Tribology and Friction)
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Open AccessArticle Experimental and Numerical Analysis on the Seizure of a Carbon-Filled PTFE Central Groove Journal Bearing during Start-Up Period
Lubricants 2018, 6(1), 14; doi:10.3390/lubricants6010014
Received: 18 December 2017 / Revised: 17 January 2018 / Accepted: 24 January 2018 / Published: 26 January 2018
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Abstract
During the start-up period, if the journals bearing bushing and shaft materials have different heat conductivities, the thermal expansion of the shaft might be more rapid than that of the bushing. This reduces the radial clearance between the bearing components and generates higher
[...] Read more.
During the start-up period, if the journals bearing bushing and shaft materials have different heat conductivities, the thermal expansion of the shaft might be more rapid than that of the bushing. This reduces the radial clearance between the bearing components and generates higher shear stresses in the lubricating fluid, which can lead to bearing seizure, resulting in the deterioration of the journal bearing. (1) First, we present an experimental case study of the seizure of a central groove journal bearing with a carbon-filled polytetrafluoroethylene (PTFE) bushing and a steel shaft; (2) Then, a static thermoelastohydrodynamic (TEHD) simulation is compared to Cristea’s experiments (Cristea, A.-F., 2012); (3) Then, a pseudo-transient TEHD simulation is performed in order to numerically predict the observed phenomena on the test rig; (4) Finally, the evolution of the oil flow rate and the bushing temperatures during the start-up period determined with the pseudo-transient TEHD modelling are compared to the experimental data of the test rig. The pressure field is determined with the thermohydrodynamic (THD) Elrod’s modelling, with Vijayaraghvan’s robust solver algorithm. The pseudo-transient temperature field in the bearing components as well as the thermal and mechanical deformations are calculated with a commercial software. Full article
(This article belongs to the Special Issue Friction and Lubrication of Sliding Bearings)
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Open AccessArticle CFD Modeling of the Effect of Different Surface Texturing Geometries on the Frictional Behavior
Lubricants 2018, 6(1), 15; doi:10.3390/lubricants6010015
Received: 5 December 2017 / Revised: 29 December 2017 / Accepted: 22 January 2018 / Published: 26 January 2018
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Abstract
In order to understand the effect of surface texturing parameters on the frictional behavior of textured surfaces and to correlate results of different lubrication regimes, Computational Fluid Dynamics (CFD) numerical analysis of the fluid flow was performed for four different textured surface geometries.
[...] Read more.
In order to understand the effect of surface texturing parameters on the frictional behavior of textured surfaces and to correlate results of different lubrication regimes, Computational Fluid Dynamics (CFD) numerical analysis of the fluid flow was performed for four different textured surface geometries. The aim of the present research paper is to get theoretical background for the frictional behavior of textured surfaces under hydrodynamic lubrication. Since it is unrealistic to make a direct analysis of a real problem that can possess more than several thousand micro-dimples, the purpose is then to investigate the flow in single cells of periodical micro-dimple patterns and to extract useful conclusions for the lubrication’s framework. Among all geometries studied, optimum geometry shapes in terms of hydrodynamic performance were reported. It was found that the best hydrodynamic performance was achieved with the rectangular geometry (lowest shear force). Full article
(This article belongs to the Special Issue Tribological Performance of Textured Surfaces)
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Open AccessArticle TEHL Simulation on the Influence of Lubricants on the Frictional Losses of DLC Coated Gears
Lubricants 2018, 6(1), 17; doi:10.3390/lubricants6010017
Received: 6 December 2017 / Revised: 17 January 2018 / Accepted: 31 January 2018 / Published: 10 February 2018
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Abstract
Diamond-Like Carbon (DLC) coatings can reduce fluid friction in TEHL contacts (thermo-elastohydrodynamic lubrication) of meshing gears. This study investigates the influence of different base oils i.e., mineral, polyalphaolefin and polyglycol oil on the friction of DLC coated spur gears. Thereby, a transient TEHL
[...] Read more.
Diamond-Like Carbon (DLC) coatings can reduce fluid friction in TEHL contacts (thermo-elastohydrodynamic lubrication) of meshing gears. This study investigates the influence of different base oils i.e., mineral, polyalphaolefin and polyglycol oil on the friction of DLC coated spur gears. Thereby, a transient TEHL simulation model based on the finite element based full-system approach coupled iteratively with the thermal equations is applied, considering mechanical and thermal properties of the DLC coatings. Results show a clear reduction of fluid friction in DLC coated gears for all considered lubricants. This can be traced back to higher TEHL temperatures for DLC coated gears, which is due to its low thermal inertia resulting in a thermal insulation effect. Full article
(This article belongs to the Special Issue Selected Papers in the Sixth World Tribology Congress (WTC 2017))
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Open AccessCommunication Facilitating the Study of the Texturing Effect on Hydrodynamic Lubrication
Lubricants 2018, 6(1), 18; doi:10.3390/lubricants6010018
Received: 13 December 2017 / Revised: 17 January 2018 / Accepted: 29 January 2018 / Published: 11 February 2018
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Abstract
To facilitate fundamental study of the surface texturing effect on hydrodynamic lubrication, analytical and experimental tools are required. While there is an extensive amount of theoretical and analytical analyses in the literature, relevant experimental studies are much rarer. A detailed study requires techniques
[...] Read more.
To facilitate fundamental study of the surface texturing effect on hydrodynamic lubrication, analytical and experimental tools are required. While there is an extensive amount of theoretical and analytical analyses in the literature, relevant experimental studies are much rarer. A detailed study requires techniques by which one can (a) produce micron-sized textures on a millimeter-scale area on a specimen surface and (b) accurately measure the lubricating film thickness and load-carrying capacity of a bearing. The paper introduces the use of an efficient laser technique (direct laser interference patterning) and a custom-designed fixed-incline slider tester to address these points. A steel slider was textured with the laser technique to produce a surface pattern in the inlet region of the bearing contact. The characterization of the load-carrying capacity for different convergence ratios K is presented. Full article
(This article belongs to the Special Issue Improvement of Friction and Wear by Laser Surface Texturing)
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Open AccessArticle Phospholipid Vesicles in Media for Tribological Studies against Live Cartilage
Lubricants 2018, 6(1), 19; doi:10.3390/lubricants6010019
Received: 10 December 2017 / Revised: 29 January 2018 / Accepted: 7 February 2018 / Published: 11 February 2018
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Abstract
Introduction: Pre-clinical testing of hemiarthroplasty devices requires that the tribological conditions present in vivo with live cartilage be closely duplicated. A current limitation in the tribological testing of live cartilage involves the use of cell-culture media as lubricant. Study Aim: to develop and
[...] Read more.
Introduction: Pre-clinical testing of hemiarthroplasty devices requires that the tribological conditions present in vivo with live cartilage be closely duplicated. A current limitation in the tribological testing of live cartilage involves the use of cell-culture media as lubricant. Study Aim: to develop and test a new hyaluronan-phospholipid based medium (HA–phospholipid medium) that combines the rheological and frictional properties of synovial fluid with the nourishing properties of culture media to keep cells alive. Materials and Methods: The HA–phospholipid medium consisted of culture medium with added phospholipid dipalmitoylphosphatidylcholine (0.3 mg/mL), and hyaluronic acid (2.42 mg/mL). A standard cell culture medium was used as the control. The rheology of each medium was determined using a flat plate configuration. Bovine calf cartilage was used to assess cell viability and friction in each medium. For friction measurements, a cobalt-chrome alloy ball was articulated against cartilage disks immersed in medium. Results: Lipid vesicles 0.1 to 50 μm in diameter were identified in the HA–phospholipid medium. Cartilage cell viability was significantly higher in the HA–phospholipid medium (62% ± 8%, 95% CI) than in control medium (49.5% ± 5%) (p = 0.009). The HA–phospholipid medium exhibited strong shear-thinning behavior, similar to synovial fluid, with viscosities ~100-fold higher at 10 s−1 and 5-fold higher at 20,000 s−1 than the approximately Newtonian control medium. The HA–phospholipid medium also yielded 20% lower friction values than the control medium after one hour of testing. Conclusions: The rheological and friction results indicate that the HA–phospholipid medium is superior to the control cell culture medium in emulating the shear thinning and lubricative properties of natural synovial fluid, making it more clinically relevant for in vitro wear and friction testing with live cartilage. Full article
(This article belongs to the Special Issue Biolubrication and Biomimetic Lubrication)
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Open AccessArticle Tribological Behavior of HNBR in Oil and Gas Field Applications
Lubricants 2018, 6(1), 20; doi:10.3390/lubricants6010020
Received: 10 January 2018 / Revised: 8 February 2018 / Accepted: 11 February 2018 / Published: 13 February 2018
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Abstract
The common usages of elastomeric components in oil and gas field applications are in dynamic atmospheres; especially sealing appliances that are in relative motion when interacting with surfaces. Therefore, their performance and service life mainly depend on the wear and friction characteristics in
[...] Read more.
The common usages of elastomeric components in oil and gas field applications are in dynamic atmospheres; especially sealing appliances that are in relative motion when interacting with surfaces. Therefore, their performance and service life mainly depend on the wear and friction characteristics in use. The objective of this scientific work is to identify the effect of swelling-induced ageing on the tribological properties and surface damage mechanisms of hydrogenated nitrile butadiene rubber (HNBR) in contact with different liquids. Furthermore, the investigation of the co-relation between mechanical properties and surface properties in the tested conditions is indispensable. In the swollen state, deteriorated mechanical properties were observed; however, in de-swollen conditions, the mechanical properties were restored. As far as the surface characterization is concerned, when the HNBR was swollen by a standard IRM 903 solvent, its wear was greater compared with the un-swollen specimen (1.1 times) despite the lower coefficient of friction (COF) (reduced by ~25%) and surface temperature (reduced by ~2.4 °C). In the de-swollen condition, wear was even greater (6 times), but the COF and surface temperature were situated in between those recorded in the swollen and un-swollen conditions. With swelling, greater wear damage and lower COF were observed; higher surface ageing (softness), which eases crack growth, created bigger debris. Under the conditions used, in the de-swollen states, the bulk mechanical properties were almost recovered, in contrast to the surface properties, which were still significantly impaired. Full article
(This article belongs to the Special Issue Selected Papers in the Sixth World Tribology Congress (WTC 2017))
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Open AccessArticle Design Optimization of an Automotive Turbocharger Thrust Bearing Using a CFD-Based THD Computational Approach
Lubricants 2018, 6(1), 21; doi:10.3390/lubricants6010021
Received: 24 December 2017 / Revised: 12 February 2018 / Accepted: 13 February 2018 / Published: 22 February 2018
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Abstract
In a quest to reduce fuel consumption and emissions of automotive combustion engines, friction losses from many different sources need to be minimized. For modern designs of turbochargers commonly used in the automotive industry, reduction of friction losses results in better efficiency and
[...] Read more.
In a quest to reduce fuel consumption and emissions of automotive combustion engines, friction losses from many different sources need to be minimized. For modern designs of turbochargers commonly used in the automotive industry, reduction of friction losses results in better efficiency and also contributes to a faster transient response. The thrust bearing is one of the main contributors to the mechanical losses of a turbocharger. Therefore, it is crucial to optimize the design of the thrust bearing so that it has minimum friction losses while keeping sufficient thrust carrying capacity. One of the main challenges of turbocharger thrust bearing design, is that rotation speed is not fixed: the turbocharger may have a rotation speed which varies between 0 to as much as 250 kRPM. Moreover, the thrust bearing generates considerable heat, which changes the temperature of the oil film and its surroundings. In the present work, the design of the thrust bearing of an automotive turbocharger has been optimized. A CFD-based Thermohydrodynamic (THD) computational approach has been developed, taking into consideration heat dissipation, conjugate heat transfer throughout the bearing domain including the surrounding parts, as well as shear thinning and cavitation in the lubricant domain. An optimizer has been coupled to the CFD solver, with the aim of identifying bearing designs with reduced friction losses. Two bearing concepts have been evaluated: a taper-land design—which is a commonly applied thrust bearing concept—as well as a pocket bearing design. The resulting optimum pocket designs exhibit improved performance, in comparison to the optimum taper-land design. The present results indicate that (a) the pocket design concept can substantially contribute to further reducing the friction losses of a turbocharger, and (b) optimal design parameters of pocket bearings depend on the specific application (size, operating conditions), therefore detailed calculations should be performed to verify optimum performance. Full article
(This article belongs to the Special Issue Friction and Lubrication of Sliding Bearings)
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Open AccessArticle Improvement of Thrust Bearing Calculation Considering the Convectional Heating within the Space between the Pads
Lubricants 2018, 6(1), 22; doi:10.3390/lubricants6010022
Received: 14 August 2017 / Revised: 29 January 2018 / Accepted: 10 February 2018 / Published: 26 February 2018
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Abstract
A modern thrust bearing tool is used to estimate the behavior of tilting pad thrust bearings not only in the oil film between pad and rotating collar, but also in the space between the pads. The oil flow in the space significantly influences
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A modern thrust bearing tool is used to estimate the behavior of tilting pad thrust bearings not only in the oil film between pad and rotating collar, but also in the space between the pads. The oil flow in the space significantly influences the oil film inlet temperature and the heating of pad and collar. For that reason, it is necessary to define an oil mixing model for the space between the pads. In the bearing tool, the solutions of the Reynolds equation including a cavitation model, the energy equation and the heat transfer equation are done iteratively with the finite volume method by considering a constant flow rate. Both effects—laminar/turbulent flow and centrifugal force—are considered. The calculation results are compared with measurements done for a flooded thrust bearing with nominal eight tilting pads with an outer diameter of 180 mm. The heat convection coefficients for the pad surfaces mainly influence the pad temperature field and are adjusted to the measurement results. In the following paper, the calculation results for variable space distances, influence of different parameters on the bearing behavior and operating condition at high load are presented. Full article
(This article belongs to the Special Issue Friction and Lubrication of Sliding Bearings)
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Open AccessArticle Tribological Performance of Phosphonium Ionic Liquids as Additives in Lithium Lubricating Grease
Lubricants 2018, 6(1), 23; doi:10.3390/lubricants6010023
Received: 8 January 2018 / Revised: 14 February 2018 / Accepted: 20 February 2018 / Published: 27 February 2018
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Abstract
Three oil–miscible phosphonium ionic liquids with different structures were synthesized and used as additives for a Poly Alpha Olefin 10 (PAO 10) lithium lubricating grease. Different from other additives, the ionic liquids participated in saponification and were trapped in the entanglement network formed
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Three oil–miscible phosphonium ionic liquids with different structures were synthesized and used as additives for a Poly Alpha Olefin 10 (PAO 10) lithium lubricating grease. Different from other additives, the ionic liquids participated in saponification and were trapped in the entanglement network formed by the thickener. The effects of the ionic liquids on the rheological properties of lithium lubricating grease were characterized through a small-amplitude oscillatory shear (SAOS) measurement. The tribological performances of the lubricating greases were determined on a four-ball friction tester. The addition of phosphonium ionic liquids caused a decrease in the storage (G′) and loss (Gʺ) modulus of lubricating grease. However, the ionic liquids were found to be effective lubricant additives for improving the friction-reducing and antiwear properties of PAO 10 lithium lubricating grease. Full article
(This article belongs to the Special Issue Novel Lubricant Additives)
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Open AccessArticle A Preliminary Study to Enhance the Tribological Performance of CoCrMo Alloy by Fibre Laser Remelting for Articular Joint Implant Applications
Lubricants 2018, 6(1), 24; doi:10.3390/lubricants6010024
Received: 31 January 2018 / Revised: 26 February 2018 / Accepted: 27 February 2018 / Published: 2 March 2018
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Abstract
CoCrMo alloy has long been used as a pairing femoral head material for articular joint implant applications because of its biocompatibility and reliable tribological performance. However, friction and wear issues are still present for CoCrMo (metal)/CoCrMo (metal) or CoCrMo (metal)/ultrahigh molecular weight polyethylene
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CoCrMo alloy has long been used as a pairing femoral head material for articular joint implant applications because of its biocompatibility and reliable tribological performance. However, friction and wear issues are still present for CoCrMo (metal)/CoCrMo (metal) or CoCrMo (metal)/ultrahigh molecular weight polyethylene (UHMWPE) (plastic) pairs in clinical observations. The particulate wear debris generated from the worn surfaces of CoCrMo or UHMWPE can pose a severe threat to human tissues, eventually resulting in the failure of implants and the need for revision surgeries. As a result, a further improvement in tribological properties of this alloy is still needed, and it is of great interest to both the implant manufacturers and clinical surgeons. In this study, the surface of CoCrMo alloy was laser-treated by a fibre laser system in an open-air condition (i.e., no gas chamber required). The CoCrMo surfaces before and after laser remelting were analysed and characterised by a range of mechanical tests (i.e., surface roughness measurement and Vickers micro-hardness test) and microstructural analysis (i.e., XRD phase detection). The tribological properties were assessed by pin-on-disk tribometry and dynamic light scattering (DLS). Our results indicate that the laser-treated surfaces demonstrated a friction-reducing effect for all the tribopairs (i.e., CoCrMo against CoCrMo and CoCrMo against UHHMWPE) and enhanced wear resistance for the CoCrMo/CoCrMo pair. Such beneficial effects are chiefly attributable to the presence of the laser-formed hard coating on the surface. Laser remelting possesses several competitive advantages of being a clean, non-contact, fast, highly accurate and automated process compared to other surface coating methods. The promising results of this study point to the possibility that laser remelting can be a practical and effective surface modification technique to further improve the tribological performance of CoCr-based orthopaedic implants. Full article
(This article belongs to the Special Issue Biolubrication and Biomimetic Lubrication)
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Open AccessArticle Reducing Friction with a Liquid Film on the Body Surface
Lubricants 2018, 6(1), 25; doi:10.3390/lubricants6010025
Received: 23 January 2018 / Revised: 22 February 2018 / Accepted: 27 February 2018 / Published: 7 March 2018
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Abstract
A flow of a thin layer of liquid is simulated on a flat surface of a body located in a stream of air. Liquid film on the surface of the body reduces frictional resistance and can be used as a boundary layer control
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A flow of a thin layer of liquid is simulated on a flat surface of a body located in a stream of air. Liquid film on the surface of the body reduces frictional resistance and can be used as a boundary layer control element. The paper presents a mathematical model of the film flow on a half-plane, located at an angle to the horizon. The fluid flow is determined by the force of gravity and friction from the external air current. A model of an incompressible viscous fluid is used in the boundary-layer approximation. The terms of the motion equation are averaged over the film thickness according to the Leibniz rule. In the cross section of the film, a quadratic law is adopted for the distribution of the longitudinal velocity, taking into account friction on the film surface. An analytical solution of the problem is obtained in the form of series in powers of the small parameter for determining the film thickness and the average longitudinal velocity along the length of the plate. It is shown that the friction decreases with flow around a half-plane with a film of liquid on the surface. Full article
(This article belongs to the Special Issue Advanced Lubrication for Energy Efficiency)
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Open AccessArticle High-Speed Measurements of Steel–Ice Friction: Experiment vs. Calculation
Lubricants 2018, 6(1), 26; doi:10.3390/lubricants6010026
Received: 6 February 2018 / Revised: 2 March 2018 / Accepted: 4 March 2018 / Published: 9 March 2018
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Abstract
An ultra-thin water film plays the decisive role in steel–ice friction in bobsleighing. The water film has a thickness on the order of nanometers and results from the superposition of an existing quasi-liquid layer and additional surface water generated by frictional heat. When
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An ultra-thin water film plays the decisive role in steel–ice friction in bobsleighing. The water film has a thickness on the order of nanometers and results from the superposition of an existing quasi-liquid layer and additional surface water generated by frictional heat. When friction is measured as function of sliding velocity, the coefficients decrease according to the typical Stribeck behavior. However, for highest sliding velocities, it is still unknown whether friction decreases further or shows an increase due to viscous drag. Both tendencies are essential for the construction of safe bobsleighs and bobsleigh tracks. This contribution presents results of high-speed experiments up to 240 km/h for a steel slider on a disk of ice at different ice temperatures. In addition, using the friction model of Makkonen, friction coefficients were calculated as function of sliding velocity and ice temperature. The significant correlation between experimental results and model calculation supports the model conception of frictional melting and viscous shearing. Full article
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Open AccessArticle A Novel Approach for Modeling Surface Effects in Hydrodynamic Lubrication
Lubricants 2018, 6(1), 27; doi:10.3390/lubricants6010027
Received: 18 December 2017 / Revised: 5 March 2018 / Accepted: 8 March 2018 / Published: 12 March 2018
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Abstract
The common approach for the flow factor calculation is based on using the Reynolds equation to simulate the micro-level flow. However, for structured surfaces the fluid flow cannot be represented correctly, due to the assumptions made when deriving the Reynolds equation. In this
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The common approach for the flow factor calculation is based on using the Reynolds equation to simulate the micro-level flow. However, for structured surfaces the fluid flow cannot be represented correctly, due to the assumptions made when deriving the Reynolds equation. In this work, a novel method using the Navier-Stokes equations for the calculation of the micro-level flow is presented and validated against results from Patir and Cheng. The three-dimensional lubrication gap was generated by a rough Gaussian random surface and a perfectly smooth moving counter surface, in order to be available for different numerical methods. The presented results illustrate similar trends for both the approaches. Additionally, the use of the Navier-Stokes equations allows for the observance of surface induced effects which cannot be resolved by the approach of Patir and Cheng. Furthermore, a numerical approach for a shear flow factor calculation with a rough moving surface is presented and validated against other simulation methods. While the validation is maintained with pressure- and temperature-independent density and viscosity, these effects will be taken into account for later research activities of textured surfaces. Full article
(This article belongs to the Special Issue Friction and Lubrication of Sliding Bearings)
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Open AccessArticle Wear Analysis of a Heterogeneous Annular Cylinder
Lubricants 2018, 6(1), 28; doi:10.3390/lubricants6010028
Received: 28 February 2018 / Revised: 14 March 2018 / Accepted: 15 March 2018 / Published: 18 March 2018
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Abstract
Wear of a cylindrical punch composed by two different materials alternatively distributed in annular forms is studied with the method of dimensionality reduction (MDR). The changes in surface topography and pressure distribution during the wear process is obtained and validated by the boundary
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Wear of a cylindrical punch composed by two different materials alternatively distributed in annular forms is studied with the method of dimensionality reduction (MDR). The changes in surface topography and pressure distribution during the wear process is obtained and validated by the boundary element method (BEM). The pressure in each annular ring approaches a constant in a stationary state where the surface topography does not change any more. Furthermore, in an easier manner, using direct integration, the limiting profile in a steady wear state is theoretically calculated, as well as the root mean square (RMS) of its surface gradient, which is closely related to the coefficient of friction between this kind of surface and an elastomer. The dependence on the wear coefficients and the width of the annular areas of two phases is obtained. Full article
(This article belongs to the Special Issue Computer Simulation in Tribology and Friction)
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Open AccessArticle An Alternative Approach to Simulating an Entire Particle Erosion Experiment
Lubricants 2018, 6(1), 29; doi:10.3390/lubricants6010029
Received: 1 February 2018 / Revised: 16 March 2018 / Accepted: 17 March 2018 / Published: 20 March 2018
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Abstract
Solid particle erosion affects many areas, such as dust or volcanic ash in areo-engines. The development of protective materials and surface engineering is costly and time consuming. A lot of effort has been placed into the advancement of models to speed up this
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Solid particle erosion affects many areas, such as dust or volcanic ash in areo-engines. The development of protective materials and surface engineering is costly and time consuming. A lot of effort has been placed into the advancement of models to speed up this process. Finite element or discrete element-based models are quite successful in predicting single or multiple impacts. However, they reach their limit if an entire erosion experiment is to be simulated. Therefore, in the present work, an approach is presented which combines various aspects of the former models with probability considerations. It is used to simulate the impact of more than one billion Alumina particles onto a steel substrate. This approach permits the simulation of an entire erosion experiment on an average PC (i5-2520M CPU@2.5 GHz processor, 4 GB main memory) within about six hours. The respective predictions of wear scar and impact-mass/mass-loss curve are compared to the real experiment. Full article
(This article belongs to the Special Issue Selected Papers in the Sixth World Tribology Congress (WTC 2017))
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Open AccessReview Are Ionic Liquids Good Boundary Lubricants? A Molecular Perspective
Lubricants 2018, 6(1), 9; doi:10.3390/lubricants6010009
Received: 18 December 2017 / Revised: 4 January 2018 / Accepted: 10 January 2018 / Published: 16 January 2018
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Abstract
The application of ionic liquids as lubricants has attracted substantial interest over the past decade and this has produced a rich literature. The aim of this review is to summarize the main findings about frictional behavior of ionic liquids in the boundary lubrication
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The application of ionic liquids as lubricants has attracted substantial interest over the past decade and this has produced a rich literature. The aim of this review is to summarize the main findings about frictional behavior of ionic liquids in the boundary lubrication regime. We first recall why the unusual properties of ionic liquids make them very promising lubricants, and the molecular mechanisms at the origin of their lubricating behavior. We then point out the main challenges to be overcome in order to optimise ionic liquid lubricant performance for common applications. We finally discuss their use in the context of electroactive lubrication. Full article
(This article belongs to the Special Issue Ionic Liquids: Friction and Lubrication Mechanisms)
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Open AccessReview Controllable Sliding Bearings and Controllable Lubrication Principles—An Overview
Lubricants 2018, 6(1), 16; doi:10.3390/lubricants6010016
Received: 22 November 2017 / Revised: 3 January 2018 / Accepted: 15 January 2018 / Published: 7 February 2018
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Abstract
Hydrodynamic and aerodynamic lubrication regimes in their controllable forms have been intensively investigated over the last two decades. With the aim of reducing friction and improving thermal, static, and dynamic characteristics of radial sliding bearings, different types of electro-mechanical actuators have been coupled
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Hydrodynamic and aerodynamic lubrication regimes in their controllable forms have been intensively investigated over the last two decades. With the aim of reducing friction and improving thermal, static, and dynamic characteristics of radial sliding bearings, different types of electro-mechanical actuators have been coupled to such bearings. Depending on (i) the actuator type; (ii) the actuation principle, i.e., hydraulic, pneumatic, piezoelectric or magnetic among others; and (iii) how such an actuator is coupled to the sliding bearings, different regulation and control actions of fluid film pressure and lubricant flow can be obtained. The most common actions are: (a) the control of the injection pressure to modify the fluid film pressure statically as well as dynamically; (b) the adjustment of the angle and direction of injection flow (mostly passive action); (c) the control of the sliding bearing gap and its preload via moveable and compliant sliding surfaces; and (d) the control of the lubricant viscosity. All four parameters, i.e., pressure, flow (velocity profiles), gap and viscosity, are explicit parameters in the modified form of Reynolds’ equations for active lubrication. In this framework, this paper gives one main original contribution to the state-of-the-art of radial sliding bearings and controllable lubrication: a comprehensive overview about the different types of controllable sliding bearings and principles used by several authors. The paper ends with some conclusive remarks about advantages and drawbacks of the different design solutions for controllable sliding bearings and the main challenges to be overcome towards industrial applications. Full article
(This article belongs to the Special Issue Friction and Lubrication of Sliding Bearings)
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