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Lubricants, Volume 9, Issue 10 (October 2021) – 10 articles

Cover Story (view full-size image): This study investigates mineral and synthetic lubricating oils' crystallization, melting, and low-temperature flow behavior to evaluate the relevance of pour point according to ASTM D97. The classification of three groups emerged from this process. The crystallization temperature of mineral oils (I) determined in thermal analysis and rheology correlates well with the pour point. For glass-like solidifying synthetic oils (II), the viscosity of 1000 Pas matches the pour point. The pour point provides no information for synthetic oils, especially esters, with complex crystallization behavior (III). They exhibit supercooling depending on the shear rate and cooling conditions. View this paper
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Article
Collaborative Optimization of CNN and GAN for Bearing Fault Diagnosis under Unbalanced Datasets
Lubricants 2021, 9(10), 105; https://doi.org/10.3390/lubricants9100105 - 15 Oct 2021
Viewed by 273
Abstract
Convolutional Neural Network (CNN) has been widely used in bearing fault diagnosis in recent years, and many satisfying results have been reported. However, when the training dataset provided is unbalanced, such as the samples in some fault labels are very limited, the CNN’s [...] Read more.
Convolutional Neural Network (CNN) has been widely used in bearing fault diagnosis in recent years, and many satisfying results have been reported. However, when the training dataset provided is unbalanced, such as the samples in some fault labels are very limited, the CNN’s performance reduces inevitably. To solve the dataset imbalance problem, a Generative Adversarial Network (GAN) has been preferably adopted for the data generation. In published research studies, GAN only focuses on the overall similarity of generated data to the original measurement. The similarity in the fault characteristics is ignored, which carries more information for the fault diagnosis. To bridge this gap, this paper proposes two modifications for the general GAN. Firstly, a CNN, together with a GAN, and two networks are optimized collaboratively. The GAN provides a more balanced dataset for the CNN, and the CNN outputs the fault diagnosis result as a correction term in the GAN generator’s loss function to improve the GAN’s performance. Secondly, the similarity of the envelope spectrum between the generated data and the original measurement is considered. The envelope spectrum error from the 1st to 5th order of the Fault Characteristic Frequencies (FCF) is taken as another correction in the GAN generator’s loss function. Experimental results show that the bearing fault samples generated by the optimized GAN contain more fault information than the samples produced by the general GAN. Furthermore, after the data augmentation for the unbalanced training sets, the CNN’s accuracy in the fault classification has been significantly improved. Full article
(This article belongs to the Special Issue Machine Learning in Tribology)
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Editorial
Interfacial Dissipative Phenomena in Tribomechanical Systems
Lubricants 2021, 9(10), 104; https://doi.org/10.3390/lubricants9100104 - 15 Oct 2021
Viewed by 295
Abstract
The last decade has experienced a tremendous development of several technologies that are likely to shape our future [...] Full article
(This article belongs to the Special Issue Interfacial Dissipative Phenomena in Tribomechanical Systems)
Article
Surface Hierarchy: Macroscopic and Microscopic Design Elements for Improved Sliding on Ice
Lubricants 2021, 9(10), 103; https://doi.org/10.3390/lubricants9100103 - 12 Oct 2021
Viewed by 417
Abstract
Frictional interaction with a surface will depend on the features and topography within the contact zone. Describing this interaction is particularly complex when considering ice friction, which needs to look at both the macroscopic and microscopic levels. Since Leonardo da Vinci shared his [...] Read more.
Frictional interaction with a surface will depend on the features and topography within the contact zone. Describing this interaction is particularly complex when considering ice friction, which needs to look at both the macroscopic and microscopic levels. Since Leonardo da Vinci shared his findings that roughness increases friction, emphasis has been placed on measuring surface coarseness, neglecting the contact area. Here, a profilometer was used to measure the contact area at different slicing depths and identify contact points. Metal blocks were polished to a curved surface to reduce the contact area; further reduced by milling 400 µm grooves or laser-micromachining grooves with widths of 50 µm, 100 µm, and 150 µm. Sliding speed was measured on an inclined ice track. Asperities from pileup reduced sliding speed, but a smaller contact area from grooves and a curved sliding surface increased sliding speed. An analysis of sliding speed versus contact area from incremental slicing depths showed that a larger asperity contact surface pointed to faster sliding, but an increase in the polished surface area reduced sliding. As such, analysis of the surface at different length scales has revealed different design elements—asperities, grooves, curved zones—to alter the sliding speed on ice. Full article
(This article belongs to the Special Issue Laser Technology in Tribology)
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Article
Experimental Investigation on the Effect of Different Micro-Geometries on Cutting Edge and Wiper Edge on Surface Roughness and Forces in Face Milling
Lubricants 2021, 9(10), 102; https://doi.org/10.3390/lubricants9100102 - 12 Oct 2021
Viewed by 298
Abstract
The significance of the micro-geometries on the cutting edge is known from numerous studies conducted in the past. However, the effect of micro-geometry on the wiper facet (also called the wiper edge) is not known. Hence, this paper investigates the effect of different [...] Read more.
The significance of the micro-geometries on the cutting edge is known from numerous studies conducted in the past. However, the effect of micro-geometry on the wiper facet (also called the wiper edge) is not known. Hence, this paper investigates the effect of different micro-geometries with a focus on geometry variation on the wiper edge of a milling insert on surface roughness and forces in face milling of SAE1070 high-carbon steel. Milling inserts with sharp, rounded, chamfered edges and their combinations were manufactured on the cutting edge and wiper edge for the study. Critical surface quality parameters such as the average surface roughness (Ra), mean depth of surface roughness (Rz), and force components such as radial force (Fx), cutting force (Fy), and axial force (Fz) were evaluated. Metal cutting tests were performed at three different cutting speeds and three different feed rates to study the influence of cutting parameters and the effect of edge geometries on surface roughness. The results were correlated with the force values to understand the machining dynamics. Finite element analysis was performed to evaluate the high and low-stress zones on the insert, workpiece, and chip to understand the metal cutting mechanism of different micro-geometries. The novel finding from the study is that having identical micro-geometries on the cutting and wiper edge is the preferred combination, whereas dissimilar micro-geometries result in reduced surface quality, increased forces, and high stress on the workpiece and chip. Full article
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Article
New Methodologies Indicating Adhesive Wear in Load Step Tests on the Translatory Oscillation Tribometer
Lubricants 2021, 9(10), 101; https://doi.org/10.3390/lubricants9100101 - 11 Oct 2021
Viewed by 370
Abstract
When looking in detail at analyses of the tribological load-carrying capacity of lubricants, it becomes apparent that an exclusive evaluation of the evolution of the coefficient of friction alone cannot provide any sufficient criteria for determining the occurrence of adhesive failure. For this [...] Read more.
When looking in detail at analyses of the tribological load-carrying capacity of lubricants, it becomes apparent that an exclusive evaluation of the evolution of the coefficient of friction alone cannot provide any sufficient criteria for determining the occurrence of adhesive failure. For this reason, extending the knowledge base by combining several criteria in order to draw a clearer picture of adhesive wear mechanisms is urgently required. This can be achieved by combining the evolution of coefficient of friction with stroke signals and/or the electrical contact resistance and/or contact temperature and/or acoustic emission and/or stroke zero position, frictional power input and further derived parameters. Full article
(This article belongs to the Special Issue Recent Advances in Experimental Tribology: Devices and Methods)
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Article
Modelling of Frictional Conditions in the Wheel–Rail Interface Due to Application of Top-of-Rail Products
Lubricants 2021, 9(10), 100; https://doi.org/10.3390/lubricants9100100 - 08 Oct 2021
Cited by 1 | Viewed by 487
Abstract
The coefficient of friction between a wheel tread and the top of the rail should be maintained at intermediate levels to limit frictional tangential contact forces. This can be achieved by applying top-of-rail products. Reducing the coefficient of friction to intermediate levels reduces [...] Read more.
The coefficient of friction between a wheel tread and the top of the rail should be maintained at intermediate levels to limit frictional tangential contact forces. This can be achieved by applying top-of-rail products. Reducing the coefficient of friction to intermediate levels reduces energy consumption and fuel costs, as well as damage to the wheel and rail surfaces, such as, e.g., wear, rolling contact fatigue, and corrugation. This work describes a simulation model that predicts the evolution of the coefficient of friction as a function of the number of wheel passes and the distance from the application site for wayside application of top-of-rail products. The model considers the interplay of three mechanisms, namely the pick-up of product by the wheel at the application site, the repeated transfer of the product between the wheel and rail surfaces, and the product consumption. The model has been parameterized with data from small-scale twin disc rig experiments and full-scale wheel–rail rig experiments. Systematic investigations of the model behaviour for a railway operating scenario show that all three mechanisms may limit the achievable carry-on distance of the product. The developed simulation model assists in understanding the interplay of the mechanisms that govern the evolution of the coefficient of friction in the field. It may aid in finding optimal product application strategies with respect to application position, application amount, and application pattern depending on specific railway operating conditions. Full article
(This article belongs to the Special Issue Tribology in Mobility)
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Article
Low-Temperature Rheology and Thermoanalytical Investigation of Lubricating Oils: Comparison of Phase Transition, Viscosity, and Pour Point
Lubricants 2021, 9(10), 99; https://doi.org/10.3390/lubricants9100099 - 06 Oct 2021
Viewed by 609
Abstract
According to the ASTM D97, the pour point is the temperature below which petroleum products cease to flow. To evaluate the relevance of pour point measurements for synthetic lubricating oils, we investigated the crystallization, melting temperature and low-temperature flow behavior of one mineral [...] Read more.
According to the ASTM D97, the pour point is the temperature below which petroleum products cease to flow. To evaluate the relevance of pour point measurements for synthetic lubricating oils, we investigated the crystallization, melting temperature and low-temperature flow behavior of one mineral and five synthetic lubricating oils. The classification of three groups emerged from this process. The formation of paraffin crystals in mineral oils (I) below the crystallization temperature causes shear-thinning behavior and a yield point. The crystallization temperature determined in the thermal analysis and rheology correlates well with the pour point. Synthetic lubricating oils, which solidify glass-like (II), exhibit a steady viscosity increase with falling temperature. The temperature at which viscosity reaches 1000 Pas corresponds well to the pour point. Synthetic oils, especially esters, with complex crystallization behavior (III), exhibit supercooling depending on the shear rate and cooling conditions. For these lubricating oils, the pour point provides no information for low-temperature applicability. Full article
(This article belongs to the Special Issue Green Tribology: New Insights toward a Sustainable World)
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Article
Operating Behavior of Sliding Planet Gear Bearings for Wind Turbine Gearbox Applications—Part II: Impact of Structure Deformation
Lubricants 2021, 9(10), 98; https://doi.org/10.3390/lubricants9100098 - 01 Oct 2021
Cited by 1 | Viewed by 453
Abstract
The use of planetary gear stages intends to increase power density in drive trains of rotating machinery. Due to lightweight requirements on this type of machine elements, structures are comparably flexible while mechanical loads are high. This study investigates the impact of structure [...] Read more.
The use of planetary gear stages intends to increase power density in drive trains of rotating machinery. Due to lightweight requirements on this type of machine elements, structures are comparably flexible while mechanical loads are high. This study investigates the impact of structure deformation on sliding planet gear bearings applied in the planetary stages of wind turbine gearboxes with helical gears. It focuses on three main objectives: (i) development of a procedure for the time-efficient thermo-elasto-hydrodynamic (TEHD) analysis of sliding planet gear bearing; (ii) understanding of the specific deformation characteristics of this application; (iii) investigation of the planet gear bearing’s modified operating behavior, caused by the deformation of the sliding surfaces. Generally, results indicate an improvement of predicted operating conditions by consideration of structure deformation in the bearing analysis for this application. Peak load in the bearing decreases because the loaded proportion of the sliding surface increases. Moreover, tendencies of single design measures, determined for rigid geometries, keep valid but exhibit significantly different magnitudes under consideration of structure deformation. Results show that consideration of structure flexibility is essential for sliding planet gear bearing analysis if quantitative assertions on load distributions, wear phenomena, and interaction of the bearing with other components are required. Full article
(This article belongs to the Special Issue Advances in Lubricated Bearings)
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Article
Operating Behavior of Sliding Planet Gear Bearings for Wind Turbine Gearbox Applications—Part I: Basic Relations
Lubricants 2021, 9(10), 97; https://doi.org/10.3390/lubricants9100097 - 01 Oct 2021
Cited by 1 | Viewed by 463
Abstract
The application of sliding planet gear bearings in wind turbine gearboxes has become more common in recent years. Assuming practically applied helix angles, the gear mesh of the planet stage causes high force and moment loads for these bearings involving high local loads [...] Read more.
The application of sliding planet gear bearings in wind turbine gearboxes has become more common in recent years. Assuming practically applied helix angles, the gear mesh of the planet stage causes high force and moment loads for these bearings involving high local loads at the bearing edges. Specific operating behavior and suitable design measures to cope with these challenging conditions are studied in detail based on a thermo-hydrodynamic (THD) bearing model. Radial clearance and axial crowning are identified as important design parameters to reduce maximum pressures occurring at the bearing edges. Furthermore, results indicate that a distinct analysis of the gear mesh load distribution is required to characterize bearing operating behavior at part-load. Here, operating conditions as critical as the ones reached at nominal load might occur. Wear phenomena can improve the shape of the gap in the circumferential as well as in axial direction incorporating a significant reduction of local maximum pressures. The complexity of the combination of these aspects and the additionally expected impact of structure deformation gives an insight into the challenges in the design processes of sliding planet gear bearings for wind turbine gearbox applications. Full article
(This article belongs to the Special Issue Advances in Lubricated Bearings)
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Article
Strengthening Properties and Wear Resistance of the Cu-xNi-yCo-Cr-Si Alloy by Varying Ni/Co and Zr Addition
Lubricants 2021, 9(10), 96; https://doi.org/10.3390/lubricants9100096 - 27 Sep 2021
Viewed by 380
Abstract
This work examines the effect of nickel (Ni) and cobalt (Co) concentrations on wear resistance behavior of Cu-xNi-yCo-Cr-Si (x = 8−9 wt%, y = 1−2 wt%) and the zirconium (Zr) addition, in order to obtain an alloy with similar or better properties than [...] Read more.
This work examines the effect of nickel (Ni) and cobalt (Co) concentrations on wear resistance behavior of Cu-xNi-yCo-Cr-Si (x = 8−9 wt%, y = 1−2 wt%) and the zirconium (Zr) addition, in order to obtain an alloy with similar or better properties than the C17510 alloy. In this study, hardness and wear resistance mechanism behavior were associated with the microstructure evolution of these alloys with different aging strengthening conditions. Different heat treatments (HT) were applied to three alloys Cu-8Ni-2Co-1.6Cr-2Si wt% (L1), Cu-8.5Ni-1.5Co-1.6Cr-2Si wt% (L2) and Cu-9Ni-1Co-1.6Cr-2Si-0.18Fe-0.3Zr wt% (L3), the properties obtained were compared with C17510 alloy. Specimens were characterized by different metallographic techniques; the microstructural characterization was made with scanning electron microscopy (SEM) and revealed regions of dendritic morphologies. The solidification sequence is primary Cu dendrites and some phases in the interdendritic regions. The solution heat treatment decreased (ST) the Hardness Rockwell B (HRB) and wear rate resistance, and there was an increase with the first aging heat treatment (HT1) for each alloy. The most outstanding combination of properties was found for Cu-8.5Ni-1.5Co-1.6Cr-2Si wt% alloy (L2C3) with 100 ± 1 HRB and specific wear rate resistance of 1.2 × 10−4 mm3/Nm, after the first aging heat treatment. Full article
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