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Lubricants, Volume 13, Issue 2 (February 2025) – 34 articles

Cover Story (view full-size image): This article presents an experimental investigation on the machining behavior of the additively manufactured Carbon-Fiber-Reinforced-Polymer (CFRP) under dry and lubricated conditions. The investigation was conducted by considering cutting speed, feed and depth-of-cut as the continuous variables. A comparison between the generated surface roughness of the dry and the lubricated cuts revealed that the presence of coolant contributed towards reducing surface roughness by more than 20% in most cases. In addition, the process was modeled by utilizing a multiple regression analysis. The Non-Dominated Sorting Genetic Algorithm 2 (NSGA-II) was employed to identify the optimal solutions that consider both minimizing surface roughness and maximizing the Material Removal Rate (MRR). View this paper
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20 pages, 5823 KiB  
Article
Oil Film Thickness and Meshing Efficiency for a Novel Closed Movable Tooth Gear Reducer
by Shuo Yang and Lizhong Xu
Lubricants 2025, 13(2), 74; https://doi.org/10.3390/lubricants13020074 - 7 Feb 2025
Viewed by 226
Abstract
Equations are developed for the relative sliding velocities, entrainment velocities, forces, friction coefficients, effective radius of curvature, oil film thickness, friction loss, and meshing efficiency for a novel closed movable tooth gear reducer. Using these equations, the relative sliding velocities and entrainment velocities [...] Read more.
Equations are developed for the relative sliding velocities, entrainment velocities, forces, friction coefficients, effective radius of curvature, oil film thickness, friction loss, and meshing efficiency for a novel closed movable tooth gear reducer. Using these equations, the relative sliding velocities and entrainment velocities of the reducer are studied. The meshing efficiencies and their changes along with meshing positions and main parameters are analyzed. Changes of the meshing efficiencies along with load torque are also studied. An efficiency experiment of the reducer prototype is carried out. Results indicate that the meshing efficiency of the novel closed reducer increases with increasing eccentric distance. When an error in the eccentric distance occurs, the meshing efficiency is significantly reduced. The measured efficiency is close to the calculated one, supporting the theoretical studies. Full article
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21 pages, 9473 KiB  
Article
Tribology and Hot Corrosion Behavior of MCrAlY-Based Multicomponent Coatings Containing Copper
by Bruno C. N. M. de Castilho, Navid Sharifi, Mary Makowiec, Pantcho Stoyanov, Christian Moreau and Richard R. Chromik
Lubricants 2025, 13(2), 73; https://doi.org/10.3390/lubricants13020073 - 7 Feb 2025
Viewed by 217
Abstract
The use of composite coatings containing solid lubricants is widely reported in the literature, in particular thermally sprayed coatings containing silver. However, these coatings are often limited in their maximum operating temperature due to the melting point of silver and due to reactions [...] Read more.
The use of composite coatings containing solid lubricants is widely reported in the literature, in particular thermally sprayed coatings containing silver. However, these coatings are often limited in their maximum operating temperature due to the melting point of silver and due to reactions between the components at temperatures above 500 °C. In this study, a novel coating is proposed, which consists of an MCrAlY-based matrix and the addition of components (Cu, Mo, and BaF2) to improve the wear resistance at elevated temperatures. The coatings were sprayed by high-velocity oxy-fuel, heat-treated at 1040 °C, and tribologically tested at room and elevated temperatures. Raman spectroscopy and scanning electron microscopy were used on worn and unworn regions of the coating to characterize the changes in microstructure caused by wear. The coatings were also exposed to oxidation and hot corrosion conditions to evaluate the resistance to high-temperature environments. The results have shown an improvement in wear rates of the coatings upon heat treatment and the formation of a smooth tribolayer at 300 °C. The as-sprayed coating was able to withstand the attack by molten salts without exposing the substrate, and minor weight gain was observed, indicating that the MCrAlY matrix was effective to protect the coating and the substrate against damages induced by salt penetration. Full article
(This article belongs to the Special Issue Coatings and Lubrication in Extreme Environments)
14 pages, 1899 KiB  
Article
A Novel Tool Wear Identification Method Based on a Semi-Supervised LSTM
by Xin He, Meipeng Zhong, Chengcheng He, Jinhao Wu, Haiyang Yang, Zhigao Zhao, Wei Yang, Cong Jing, Yanlin Li and Chen Gao
Lubricants 2025, 13(2), 72; https://doi.org/10.3390/lubricants13020072 - 7 Feb 2025
Viewed by 252
Abstract
Machine learning models have been widely used in the field of cutting tool wear identification, achieving favorable results. However, in actual industrial scenarios, obtaining sufficient labeled samples is time consuming and costly, while unlabeled samples are abundant and easy to collect. This situation [...] Read more.
Machine learning models have been widely used in the field of cutting tool wear identification, achieving favorable results. However, in actual industrial scenarios, obtaining sufficient labeled samples is time consuming and costly, while unlabeled samples are abundant and easy to collect. This situation significantly affects the model’s performance. To address this challenge, a novel semi-supervised method, based on long short-term memory (LSTM) networks, is provided. The proposed method leverages both small labeled and abundant unlabeled data to improve tool wear identification performance. The proposed method trains an initial tool wear regression model using LSTM, using a small amount of labeled samples. It then uses manifold regularization to generate pseudo-labels for the unlabeled samples. These pseudo-labeled samples are combined with the original labeled samples to retrain the MR–LSTM model iteratively to improve its performance. This process continues until a termination condition is met. The method considers the correlation between sample labels and feature structures, as well as the correlation between global and local sample labels. Experiments involving milling tool wear identification demonstrate that the proposed method significantly outperforms support vector regression (SVR) and recurrent neural network (RNN)-based methods, when a small amount of labeled samples and abundant unlabeled samples are available. The average R2 values in terms of the proposed method’s predicted results can reach above 0.95. The proposed method is a potential technique for low-cost tool wear identification, without the need to collect a large number of labeled samples. Full article
(This article belongs to the Special Issue Advanced Computational Studies in Frictional Contact)
20 pages, 4934 KiB  
Article
Curbing Petrochemical Lubricants by Plant-Based Chemicals: A Reliable Opportunity to Align with Sustainable Development Goals
by Jaime Nácher-Mestre, David Leal Cano, Kudama A. Habib, Clemente M. Branchadell and Jaume Pérez-Sánchez
Lubricants 2025, 13(2), 71; https://doi.org/10.3390/lubricants13020071 (registering DOI) - 6 Feb 2025
Viewed by 392
Abstract
The use of plant-based lubricants is a sustainable alternative to petrochemical lubricants. Their main advantages include proven tribological performance, higher biodegradability in the environment, and the absence of health hazards when inhaled, ingested, or in contact with the skin. In this study, nine [...] Read more.
The use of plant-based lubricants is a sustainable alternative to petrochemical lubricants. Their main advantages include proven tribological performance, higher biodegradability in the environment, and the absence of health hazards when inhaled, ingested, or in contact with the skin. In this study, nine best-selling commercial lubricants were purchased and compared with two proposed sustainable alternatives derived from plant-based resources. The alternative wet lubricant that was developed demonstrated superior tribological performance compared to the selected commercial lubricants. Meanwhile, the alternative dry lubricant exhibited a greater load capacity and high wear resistance in the presence of micrometric Al2O3 particles at 20 N. The results indicated that a proper formulation of plant-based resources in lubricants can achieve the same or even better functional performance than conventional lubricants, which are currently classified as hazardous under European regulations. Full article
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21 pages, 8879 KiB  
Article
Theoretical and Experimental Investigation of the Thermal Stability of a Cycloid Speed Reducer
by Milan Vasić, Mirko Blagojević, Milan Banić, Lorenzo Maccioni and Franco Concli
Lubricants 2025, 13(2), 70; https://doi.org/10.3390/lubricants13020070 - 6 Feb 2025
Viewed by 273
Abstract
High-precision drives are essential for ensuring accuracy and repeatability in positioning systems within robotics and industrial automation. Among these, cycloidal reducers are widely utilized due to their ability to deliver a high transmission ratio alongside high power density. However, compact designs often face [...] Read more.
High-precision drives are essential for ensuring accuracy and repeatability in positioning systems within robotics and industrial automation. Among these, cycloidal reducers are widely utilized due to their ability to deliver a high transmission ratio alongside high power density. However, compact designs often face challenges such as elevated operating temperatures caused by limited heat dissipation areas, making it crucial to assess thermal stability within the design process. While engineering practice typically determines the thermal stability of gear drives using ISO/TR 14179-2:2001, no specific methodologies have yet been developed for cycloidal reducers. To address this gap, this paper presents a novel mathematical model fine-tuned to quantify power dissipation and predict lubricant stabilization temperatures under varying operating conditions. The model employs a global energy balance approach, correlating total power losses with the heat dissipated from the reducer to the environment. Moreover, in this study, an experimental campaign was carried out to monitor the thermal behaviour of a cycloidal reducer under various operating conditions in terms of speed and transmitted torque. This was achieved through the analysis of images collected with an infrared thermal camera, both during the transient phase and under steady-state thermal conditions. The results demonstrate good alignment with experimental findings, although further refinements are required to develop specialized tools for cycloidal drives. Additional contributions of the present paper include the understanding of the time required to achieve thermal stability, as well as insights into heat generation and propagation. Beyond advancing scientific knowledge, this work also provides valuable practical guidance for engineering applications. Full article
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15 pages, 2406 KiB  
Article
Effect of Split Doublet Mode Pair on Brake Squeal of the Slightly Asymmetric Disc
by Dongwoo Seo and Jaeyoung Kang
Lubricants 2025, 13(2), 69; https://doi.org/10.3390/lubricants13020069 - 5 Feb 2025
Viewed by 305
Abstract
In this paper, the stability of the split doublet mode pair in a disc brake squeal is analytically investigated. A reduced-order doublet mode model is derived based on the formulation of annular plate vibration, sliding contact kinematics, and the assumed mode method. The [...] Read more.
In this paper, the stability of the split doublet mode pair in a disc brake squeal is analytically investigated. A reduced-order doublet mode model is derived based on the formulation of annular plate vibration, sliding contact kinematics, and the assumed mode method. The solution is approximated using a two-mode expansion for the doublet mode pair. The time-varying motion of the slightly asymmetric rotating disc is described through the moving mode shape function method. A stability analysis of the frequency split between the doublet mode pair is performed by calculating the characteristic multipliers using the Floquet theory. The stability boundary is determined within the domain of frequency split variations and other parameters. Finally, the influence of mode splitting on the squeal behaviour of the asymmetric disc is analysed and discussed. Full article
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17 pages, 15072 KiB  
Article
Effect of Pipe Wall Wear Defects on the Flow Characteristics of Slurry Shield Discharge Pipe
by Yingran Fang, Xinggao Li, Xingchun Li, Yidong Guo and Hongzhi Liu
Lubricants 2025, 13(2), 68; https://doi.org/10.3390/lubricants13020068 - 4 Feb 2025
Viewed by 338
Abstract
During slurry shield tunneling in hard rock or cobble strata, the discharge pipes suffer serve wear and damage. However, the effect mechanism of pipe wall wear defects on the flow characteristics of two-phase flow is unclear. In this study, a three-dimensional slurry particle [...] Read more.
During slurry shield tunneling in hard rock or cobble strata, the discharge pipes suffer serve wear and damage. However, the effect mechanism of pipe wall wear defects on the flow characteristics of two-phase flow is unclear. In this study, a three-dimensional slurry particle model of pipeline transport was established using the coupled computational fluid dynamics–discrete element method (CFD-DEM) considering the pipe wall wear defect, and the typical pipeline forms of straight pipe and 90° elbow pipe were selected as the research targets. The results indicated that the localized wear defect of pipes can lead to increased inhomogeneity in the velocity distribution, generating localized low-flow zones and resulting in a reduced flow rate or stagnancy in parts of the pipe. Meanwhile, the wear defect of the pipe results in local shape changes, so that the fluid flow path through the pipe is no longer smooth, causing more vortex/turbulence and secondary flow, where an increased vortex promotes localized kinetic energy reduction and creates larger pressure losses at the elbow. In addition, for the elbow pipe without wear defect, the pressure drop of the elbow increases quadratically from an increase of 6.5% to an increase of 16.9%, with the maximum wear depth increasing from 4 mm to 19 mm. For the straight pipe without wear defect, the pressure drop of the elbow increases linearly, from an increase of 2.2% to an increase of 10.2% with the maximum wear depth increasing from 4 mm to 19 mm. The paper investigates the potential mechanism of pipe flow characteristics influenced by wear defect and provides practical guidelines for the efficient operation of a slurry shield circulating system. Full article
(This article belongs to the Special Issue Recent Advances in Lubricated Tribological Contacts)
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14 pages, 4464 KiB  
Article
Effects of Graphene Nanoplatelets and Nanosized Al4C3 Formation on the Wear Properties of Hot Extruded Al-Based Nanocomposites
by Mihail Kolev, Rumyana Lazarova, Veselin Petkov and Rositza Dimitrova
Lubricants 2025, 13(2), 67; https://doi.org/10.3390/lubricants13020067 - 4 Feb 2025
Viewed by 391
Abstract
This study investigates the influence of graphene nanoplatelets (GNPs) and the formation of nanosized Al4C3 on the tribological performance of hot extruded aluminum-based nanocomposites. Al/GNP nanocomposites with varying GNP contents (0, 0.1, 0.5, and 1.1 wt.%) were fabricated through powder [...] Read more.
This study investigates the influence of graphene nanoplatelets (GNPs) and the formation of nanosized Al4C3 on the tribological performance of hot extruded aluminum-based nanocomposites. Al/GNP nanocomposites with varying GNP contents (0, 0.1, 0.5, and 1.1 wt.%) were fabricated through powder metallurgy, including ball milling, compaction, and hot extrusion at 500 °C, which was designed to facilitate the formation of nanosized carbides during the extrusion process. The effect of GNPs and nanosized carbides on the tribological properties of the composites was evaluated using dry friction pin-on-disk tests to assess wear resistance and the coefficient of friction (COF). Microstructural analyses using scanning electron microscopy and energy-dispersive X-ray spectroscopy confirmed the uniform distribution of GNPs and the formation of nanosized Al4C3 in the samples. Incorporating 0.1 wt.% GNPs resulted in the lowest wear mass loss (1.40 mg) while maintaining a stable COF (0.52), attributed to enhanced lubrication and load transfer. Although a higher GNP content (1.1 wt.%) resulted in increased wear due to agglomeration, the nanocomposite still demonstrated superior wear resistance compared to the unreinforced aluminum matrix. These findings underscore the potential of combining nanotechnology with precise processing techniques to enhance the wear and friction properties of aluminum-based composites. Full article
(This article belongs to the Special Issue Tribology in Manufacturing Engineering)
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17 pages, 5487 KiB  
Article
Friction in Cylindrical Joints
by Andrei Marius Mihalache, Vasile Merticaru, Vasile Ermolai, Oana Dodun, Gheorghe Nagiț, Adelina Hrițuc, Marius Ionuț Rîpanu and Laurențiu Slătineanu
Lubricants 2025, 13(2), 66; https://doi.org/10.3390/lubricants13020066 - 4 Feb 2025
Viewed by 284
Abstract
Cylindrical clearance joints are commonly employed in mechanisms that involve the rotation of a shaft spindle within a cylindrical sliding bearing. The intensity of the friction process in such joints is governed by several factors, including the clearance size between components, the materials [...] Read more.
Cylindrical clearance joints are commonly employed in mechanisms that involve the rotation of a shaft spindle within a cylindrical sliding bearing. The intensity of the friction process in such joints is governed by several factors, including the clearance size between components, the materials of the interacting surfaces, the properties and characteristics of the lubricant, the surface roughness (asperities), and the magnitude of the relative velocity between the joint’s components. To experimentally determine the friction coefficient in cylindrical clearance joints, a custom device was designed and implemented. This device is adaptable to a universal lathe and enables the measurement of the friction coefficient under varying normal forces and relative movement speeds between the joint components. The experimental data were subjected to mathematical analysis, leading to the development of an empirical model. This model effectively characterizes the direction and intensity of the influence of various factors on the friction coefficient, accounting for the use of different lubricants. The findings provide valuable insights into optimizing cylindrical clearance joints for improved performance in practical applications. Full article
(This article belongs to the Special Issue Recent Advances in Lubricated Tribological Contacts)
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28 pages, 2575 KiB  
Article
Improved Calculation of Dynamic Load Capacity for Cylindrical Roller Thrust Bearings: Numerical Update of the Lifetime Reduction Factor  for Bearings with Small and Medium Spin-to-Roll Ratios
by Paul Sauvage, Torben Terwey, Benjamin Lehmann and Georg Jacobs
Lubricants 2025, 13(2), 65; https://doi.org/10.3390/lubricants13020065 - 3 Feb 2025
Viewed by 443
Abstract
The standard procedures for calculating the lifetime of rolling bearings, defined by DIN ISO 281 and ISO/TS 16281, have been revisited in this work with a specific focus on redefining the η factor for cylindrical roller thrust bearings (CRTBs). The new η factor [...] Read more.
The standard procedures for calculating the lifetime of rolling bearings, defined by DIN ISO 281 and ISO/TS 16281, have been revisited in this work with a specific focus on redefining the η factor for cylindrical roller thrust bearings (CRTBs). The new η factor proposed in this study accounts for the additional spinning motion of the rolling elements on the raceway, which affects the lifetime of thrust roller bearings. By considering different spin-to-roll ratios (SRRs), the revised η factor results in a smaller lifetime reduction, improving from a 42% reduction with η = 0.85 to a 27% reduction with η = 0.91. This modification opens industrial opportunities for bearings that can handle higher loads or feature fewer or smaller rolling elements while maintaining the same lifespan target as bearings sized with the original η factor. An analytical and numerical methodology was developed to calculate the η factor for various bearing configurations. Two bearing geometries were selected to assess the influence of the SRR on bearing life. The methodology integrates calculations of the total friction coefficient, 2D and 3D stress distributions, and lifetime predictions based on subsurface-initiated fatigue failure modes. The numerical results demonstrate the impact of contact stresses and bearing kinematics on η. Although this study was based on numerical simulations, it sets the groundwork for experimental validation. Future work includes experimental testing to validate these findings, with a focus on subscale CRTBs subjected to varying γ values. Accelerated testing strategies, including higher rotational speeds and optimized lubrication, are proposed to enhance the accuracy of the results. These experiments would provide further insights into the life expectancy differences between various configurations, contributing to more precise lifetime calculations for CRTBs. Full article
(This article belongs to the Special Issue Recent Advances in Lubricated Tribological Contacts)
20 pages, 12877 KiB  
Article
Viable Use of Tire Pyrolysis Oil as an Additive to Conventional Motor Oil: A Tribological and Physical Study
by Abdullah A. Alazemi, Abdullah F. Alajmi and Sultan M. Al-Salem
Lubricants 2025, 13(2), 64; https://doi.org/10.3390/lubricants13020064 - 3 Feb 2025
Viewed by 674
Abstract
Stockpiled end-of-life tires (ELTs) pose a serious environmental concern. In the current investigation, ELT pyrolysis oil (i.e., pyro-oil) was studied as a potential additive to conventional motor oil. The pyro-oil samples were mixed in different concentrations of 10 to 50 wt.% with commercial [...] Read more.
Stockpiled end-of-life tires (ELTs) pose a serious environmental concern. In the current investigation, ELT pyrolysis oil (i.e., pyro-oil) was studied as a potential additive to conventional motor oil. The pyro-oil samples were mixed in different concentrations of 10 to 50 wt.% with commercial virgin motor oil to obtain a lubricant mixture. Chemical analyses were performed for the tire-recycled derivative material, as a potential route to utilize pyro-oils, valorize ELT waste, and reduce production costs of motor oil lubricants. Rheological examinations were performed to explore the impact of the pyro-oil on the rheological properties of the motor oil under several shearing rates and temperatures. Tribological analyses of the lubricant mixtures and the pure motor oil were accomplished to study the influence of the pyro-oil additive on the tribological behavior of motor oils. Lastly, thermal stability and wettability examinations were executed to assess the thermal and wetting properties of lubricant mixtures. The obtained results showed that adding a low concentration of the pyro-oil (≤10%) will sustain the motor oil’s chemical, wettability, thermal stability, rheological, and tribological properties, signifying a viable application of recycled ELTs and helping to reduce their environmental and economic impact. These findings offer a feasible route of use in the future to obtain low-cost oils with market specifications, utilizing pyro-oil as a sustainable and environmental oil additive. Full article
(This article belongs to the Special Issue Advances in Molecular Rheology and Tribology)
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19 pages, 9952 KiB  
Article
Multiple Regression Analysis and Non-Dominated Sorting Genetic Algorithm II Optimization of Machining Carbon-Fiber-Reinforced Polyethylene Terephthalate Glycol Parts Fabricated via Additive Manufacturing Under Dry and Lubricated Conditions
by Anastasios Tzotzis, Nikolaos Efkolidis, Kai Cheng and Panagiotis Kyratsis
Lubricants 2025, 13(2), 63; https://doi.org/10.3390/lubricants13020063 - 2 Feb 2025
Viewed by 591
Abstract
The present research deals with the processing of the additively manufactured Carbon-Fiber-Reinforced Polymer (CFRP) under dry and lubricated cutting conditions, focusing on the generated surface roughness. The cutting speed, feed, and depth of cut were selected as the continuous variables. A comparison between [...] Read more.
The present research deals with the processing of the additively manufactured Carbon-Fiber-Reinforced Polymer (CFRP) under dry and lubricated cutting conditions, focusing on the generated surface roughness. The cutting speed, feed, and depth of cut were selected as the continuous variables. A comparison between the generated surface roughness of the dry and the lubricated cuts revealed that the presence of coolant contributed towards reducing surface roughness by more than 20% in most cases. Next, a regression analysis was performed with the obtained measurements, yielding a robust prediction model, with the determination coefficient R2 being equal to 94.65%. It was determined that feed and the corresponding interactions contributed more than 45% to the model’s R2, followed by the depth of cut and the machining condition. In addition, the cutting speed was the variable with the least effect on the response. The Non-Dominated Sorting Genetic Algorithm 2 (NSGA-II) was employed to identify the front of optimal solutions that consider both minimizing surface roughness and maximizing Material Removal Rate (MRR). Finally, a set of extra experiments proved the validity of the model by exhibiting relative error values, between the measured and predicted roughness, below 10%. Full article
(This article belongs to the Special Issue Tribology in Manufacturing Engineering)
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19 pages, 8080 KiB  
Article
Rheological Properties and Lubricating Film Formation Performance of Very Low-Viscosity and Biodegradable Polyalphaolefins
by Yingjun Chen, Zhaorong He, Haiquan Wang, Yueming Li and Hui Wang
Lubricants 2025, 13(2), 62; https://doi.org/10.3390/lubricants13020062 - 2 Feb 2025
Viewed by 402
Abstract
Polyalphaolefins (PAOs) are regarded as superior lubricants, but the biodegradability of the very low-viscosity PAO2/PAO4 has been ignored over a long history, despite being inherently biodegradable (PAO2/PAO4 biodegradation rate >20% by OECD guidelines). Previous studies typically concentrated on a single viscosity grade of [...] Read more.
Polyalphaolefins (PAOs) are regarded as superior lubricants, but the biodegradability of the very low-viscosity PAO2/PAO4 has been ignored over a long history, despite being inherently biodegradable (PAO2/PAO4 biodegradation rate >20% by OECD guidelines). Previous studies typically concentrated on a single viscosity grade of PAO with additives, seldom engaging in comparative research efforts involving multiple low-viscosity grades of neat PAO concurrently. This study compares PAO2/PAO4 with non-biodegradable PAO6 regarding rheology and lubricating film formation. PAO2/PAO4 are Newtonian fluids with ≤10% viscosity fluctuation at high shear rates, while PAO6 shows a viscosity fluctuation of ≥15% at high shear rates. Viscosity–temperature equations are derived. An optical interference method measures lubricating film thickness. PAO2/PAO4 films are less sensitive to speed/load changes. PAO2 mainly works in boundary lubrication. Interference images show possible unique EHL characteristics of PAOs. The Hamrock–Dowson formula overestimates PAO6 film thickness at high speeds. Full article
(This article belongs to the Special Issue Tribological Properties of Biolubricants)
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17 pages, 13348 KiB  
Article
Structure Modulation and Self-Lubricating Properties of Porous TiN–MoS2 Composite Coating Under Humidity–Fluctuating Conditions
by Tiancheng Ye, Kai Le, Ganggang Wang, Zhenghao Ren, Yuzhen Liu, Liwei Zheng, Hui Tian and Shusheng Xu
Lubricants 2025, 13(2), 61; https://doi.org/10.3390/lubricants13020061 - 1 Feb 2025
Viewed by 650
Abstract
To improve the friction performance and service life of protective coatings in humidity-fluctuating environments, porous hard titanium nitride (TiN)–molybdenum disulfide (MoS2) composite coatings were prepared by using direct current magnetron sputtering (DCMS) with the mode of oblique angle deposition (OAD) and [...] Read more.
To improve the friction performance and service life of protective coatings in humidity-fluctuating environments, porous hard titanium nitride (TiN)–molybdenum disulfide (MoS2) composite coatings were prepared by using direct current magnetron sputtering (DCMS) with the mode of oblique angle deposition (OAD) and chemical vapor deposition (CVD) technologies. The structure and chemical component were characterized by field emission scanning electron microscopy (FESEM), energy dispersive spectrometer (EDS), grazing incidence X-ray diffraction (GIXRD), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The tribological properties of these TiN–MoS2 composite coatings were investigated. The results indicate that the porous TiN–MoS2 composite coating exhibited outstanding friction performance and long service life under humidity-fluctuating environments. At the initial 20% relative humidity (RH) stage, the MoS2 on the porous TiN–MoS2 composite coating surface worked as an effective lubricant; thus, the coating demonstrated excellent lubrication performance, and the friction coefficient (COF) was about 0.05. As the humidity was alternated to 70% RH, the lubrication effect diminished due to the production of molybdenum oxide (MoO3), and the COF was about 0.2, which was attributed to the degradation of MoS2 on the wear track and the release of fresh MoS2 from the porous TiN matrix. After the environmental conditions shifted from 70% to 20% RH, the MoO3 was removed, and the lubrication effect was restored. In summary, TiN–MoS2 porous composite coating offers a promising approach for lubrication in humidity-fluctuating environments. Full article
(This article belongs to the Special Issue Coatings and Lubrication in Extreme Environments)
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13 pages, 3360 KiB  
Article
Experimental Analysis on the Hysteresis Phenomenon in the Range of Subsynchronous Frequency as a Function of Oil Temperature with Regard to Turbochargers
by Márk Pesthy, Gusztáv Fekete, Máté Boros and Csaba Tóth-Nagy
Lubricants 2025, 13(2), 60; https://doi.org/10.3390/lubricants13020060 - 30 Jan 2025
Viewed by 401
Abstract
This study presents an experimental analysis of a turbocharger with semi-floating ring bearings, focusing on hysteresis in subsynchronous vibrations. Four automotive oils (SAE 0W-20, SAE 0W-30, SAE 5W-30, SAE 5W-40) were tested across six oil inlet temperatures from 20 °C to 120 °C [...] Read more.
This study presents an experimental analysis of a turbocharger with semi-floating ring bearings, focusing on hysteresis in subsynchronous vibrations. Four automotive oils (SAE 0W-20, SAE 0W-30, SAE 5W-30, SAE 5W-40) were tested across six oil inlet temperatures from 20 °C to 120 °C during ramp-up and ramp-down cycles to examine the effects of lubricant viscosity and temperature on rotor dynamics. Hysteresis and bifurcation points were observed at distinct rotational speeds in both directions, with subsynchronous components providing insights into rotor–lubrication interactions. This study applies the concept of hysteresis loop width for turbocharger rotors, highlighting its nonlinear dependence on oil temperature, an unexpected and unexplained phenomenon. Additionally, the results suggest that vibration sensors could provide real-time feedback on oil supply conditions, offering potential enhancements for turbochargers and other rotating machinery. Full article
(This article belongs to the Special Issue Recent Advances in Automotive Powertrain Lubrication)
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22 pages, 16700 KiB  
Article
Mechanical Property Degradation of Transmission Wire Rope Caused by Different Wear Evolution
by Xiangdong Chang, Fahui Shi, Xiao Chen, Yuxing Peng, Yu Tang, Wenjie Xiao and Ran Hu
Lubricants 2025, 13(2), 59; https://doi.org/10.3390/lubricants13020059 - 30 Jan 2025
Viewed by 378
Abstract
Steel wire rope serves as a critical load-bearing and transmission component in the transportation equipment utilized in coal mines. It exhibits various forms of damage during prolonged service, which significantly jeopardizes the safety reliability of the transportation equipment. To investigate the formation process [...] Read more.
Steel wire rope serves as a critical load-bearing and transmission component in the transportation equipment utilized in coal mines. It exhibits various forms of damage during prolonged service, which significantly jeopardizes the safety reliability of the transportation equipment. To investigate the formation process of the surface wear of steel wire rope and its influence on mechanical properties, the tribological evolution and performance degradation of the transmission wire rope under rope–sheave contact and rope–rope contact were studied in this paper. The wire rope’s friction coefficient (COF) is stable between 0.7 and 0.8 under two contact conditions. It is more likely to lead to the accumulation of frictional heat under the condition of rope–sheave contact. The primary wear mechanisms of transmission wire ropes are adhesive wear and fatigue wear. Additionally, the wear evolution leads to the nonlinear degradation of the tensile strength and bending fatigue life of the wire rope, and the wear damage caused by rope–sheave contact is more harmful. As the sliding distance between the wire rope and the sheave increases, the breaking force decreases from approximately 48 kN to 23 kN, and the number of bending fatigues of the wire rope before scrapping is reduced from approximately 6200 times to 200 times. Full article
21 pages, 7358 KiB  
Article
Dynamic Response Analysis of Ballastless Tracks Considering the Temperature-Dependent Viscoelasticity of Cement-Emulsified Asphalt Mortar Based on a Vehicle–Track–Subgrade Coupled Model
by Yunqing Chen, Bing Wu, Linquan Yao and Xianglong Su
Lubricants 2025, 13(2), 58; https://doi.org/10.3390/lubricants13020058 - 30 Jan 2025
Viewed by 420
Abstract
This study aims to explore the dynamic response of ballastless tracks under various temperatures of the cement-emulsified asphalt (CA) mortar layer and other environmental factors. CA mortar is the key material in the ballastless track structure, exhibiting notably temperature-dependent viscoelastic properties. It can [...] Read more.
This study aims to explore the dynamic response of ballastless tracks under various temperatures of the cement-emulsified asphalt (CA) mortar layer and other environmental factors. CA mortar is the key material in the ballastless track structure, exhibiting notably temperature-dependent viscoelastic properties. It can be damaged or even fail due to the continuous loads from trains. However, the dynamic behaviors of ballastless tracks considering the temperature-dependent viscoelasticity of CA mortar have been insufficiently studied. This paper captures the temperature-dependent viscoelastic characteristics of CA mortar by employing the fractional Maxwell model and applying it to finite element simulations through a Prony series. A vehicle–track–subgrade (VTS) coupled CRTS I ballastless track model, encompassing Hertz nonlinear contact and track irregularity, is established. The model is constrained symmetrically on both of the longitudinal sides, and the bottom is fixed on the infinite element boundary, which can reduce the effects of reflected waves. After the simulation outcomes in this study are validated, variations in the dynamic responses under different environmental factors are analyzed, offering a theoretical foundation for maintaining the ballastless tracks. The results show that the responses in the track subsystem will undergo significant changes as the temperature rises; a notable effect is caused by the increase in speed and fastener stiffness on the entire system; the CA mortar layer experiences the maximum stress at its edge, which makes it highly susceptible to damage in this area. The original contribution of this work is the establishment of a temperature-dependent vehicle–track–subgrade coupled model that incorporates the viscoelasticity of the CA mortar, enabling the investigation of dynamic responses in ballastless tracks. Full article
(This article belongs to the Special Issue Recent Advances in Lubricated Tribological Contacts)
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25 pages, 8306 KiB  
Article
Investigation of the Contact Characteristics of a Single-Nut Ball Screw Considering Geometric Errors
by Jun Liu, Huaxi Zhou, Xiaoyi Wang and Changguang Zhou
Lubricants 2025, 13(2), 57; https://doi.org/10.3390/lubricants13020057 - 29 Jan 2025
Viewed by 521
Abstract
As the critical performance index of ball screws, the contact characteristics have a significant influence on the lubricant properties, tribological properties, and wear properties of ball screws, which further directly affect the service life of ball screws. The non-uniform load distribution induced by [...] Read more.
As the critical performance index of ball screws, the contact characteristics have a significant influence on the lubricant properties, tribological properties, and wear properties of ball screws, which further directly affect the service life of ball screws. The non-uniform load distribution induced by geometric errors results in imbalances among balls along the nut, negatively impacting the service life of ball screws. This study focuses on the load distribution of single-nut ball screws under low-speed working conditions. This paper proposes a self-adjustable model of load distribution that considers the flexibility of the screw and nut with respect to the determination of the non-bearing ball. A refined model for axial stiffness is proposed to systematically analyze the influence of geometric errors on stiffness variations under various loading conditions. The results confirm the ability of the proposed model to reveal the static load distribution in view of geometric errors. The greatest discrepancy observed between the theoretical predictions and the experimental data was 9.22%. The numerical simulations demonstrate variation trends in the normal contact load, the loaded-ball number, and the axial deformation of a nut with geometric errors. Furthermore, the relationship between the axial stiffness of a single-nut ball screw and the geometric error is obtained. The self-adjustable model of load distribution is helpful for studying the carrying capacity of a single-nut ball screw. The findings of the study provide a definite reference for optimization of structural design and wear life prediction. Full article
(This article belongs to the Special Issue High Performance Machining and Surface Tribology)
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38 pages, 11631 KiB  
Review
Synthesis, Stability, and Tribological Performance of TiO2 Nanomaterials for Advanced Applications
by Kai Zeng, Liang Cheng, Wenjing Hu and Jiusheng Li
Lubricants 2025, 13(2), 56; https://doi.org/10.3390/lubricants13020056 - 29 Jan 2025
Viewed by 834
Abstract
The enhancement of tribological properties represents a pivotal strategy for achieving energy efficiency and environmental protection. Titanium dioxide (TiO2) nanomaterials have been garnering significant attention due to their exemplary tribological properties and due to the abundance of titanium reserves. The present [...] Read more.
The enhancement of tribological properties represents a pivotal strategy for achieving energy efficiency and environmental protection. Titanium dioxide (TiO2) nanomaterials have been garnering significant attention due to their exemplary tribological properties and due to the abundance of titanium reserves. The present review is concerned with the study of TiO2 nanomaterials in lubricants. The properties and various synthesis methods of TiO2 nanomaterials are presented. The dispersion stability of these TiO2 nanomaterials in lubricating oils is discussed in depth, as well as strategies to improve their dispersion stability, such as enhancing compatibility with base oils, reducing the dynamic light scattering (DLS) particle size, modulating the zeta potential, and optimizing the drying step. Aggregation and dispersion instability remain key challenges for TiO2 nanomaterials, especially bare TiO2 nanoparticles (NPs). In contrast, in situ surface-modified TiO2 NPs show improved stability and tribological performance, offering promise for further research. The tribological performance of lubricants has been demonstrated to be enhanced by TiO2 nanomaterials, with the observed enhancement attributed to the synergistic effect of multiple mechanisms, including rolling, patching, polishing, and the formation of a protective film. Furthermore, future research suggestions are proposed to provide a reference for the design and synthesis of high-performance TiO2 nano-lubricants and promote their wide application. Full article
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14 pages, 4258 KiB  
Article
Synergistic Tribological Performance of Phosphorus- and Sulfur-Based Extreme Pressure and Anti-Wear Additives
by Jingyu Wang, Jinhua Zheng, Jun Wang, Xiao Yao, Xing Xiong and Haipeng Huang
Lubricants 2025, 13(2), 55; https://doi.org/10.3390/lubricants13020055 - 28 Jan 2025
Viewed by 463
Abstract
Higher demands on extreme pressure lubrication performance are posed by stringent working conditions. In this study, the synergistic tribological properties of phosphate ammonium salt in combination with active sulfurized olefin (S1) and non-active sulfurized fatty acids (S2) were investigated to meet the needs [...] Read more.
Higher demands on extreme pressure lubrication performance are posed by stringent working conditions. In this study, the synergistic tribological properties of phosphate ammonium salt in combination with active sulfurized olefin (S1) and non-active sulfurized fatty acids (S2) were investigated to meet the needs under stringent working conditions. The anti-wear mechanisms were further explored using scanning electron microscopy (SEM) with EDS, X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure (XANES), and focused ion beam microscopy. The experimental results indicate that P-S2 demonstrates superior friction reduction and wear resistance under low loads, potentially attributable to its higher polarity, whereas P-S1 exhibits better wear resistance under high loads. P-S1 also shows superior extreme pressure performance attributed to its higher active sulfur content and stronger film-forming ability, evidenced by a thicker friction film (82.62 nm vs. 24.28 nm for P-S2). The study highlights that the variations in the synergistic tribological performance of phosphorus- and sulfur-based additives may link to differences in molecular structure, active sulfur content, polarity, and corrosiveness, with P-S1 demonstrating enhanced extreme pressure performance possibly through the formation of a multi-layered friction film of polyphosphate, sulfide, oligophosphate, and sulfate layers. Full article
(This article belongs to the Special Issue Friction and Wear Mechanism Under Extreme Environments)
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25 pages, 9118 KiB  
Article
Intelligent Analysis and Optimization of Lubrication Status Factor Based on Dynamically Loaded Roll Gap in Cold Strip Rolling
by Shuren Jin, Xu Li, Pengfei Wang, Feng Luan, Fangsheng Chen, Dianhua Zhang and Haidong Zhang
Lubricants 2025, 13(2), 54; https://doi.org/10.3390/lubricants13020054 - 28 Jan 2025
Viewed by 414
Abstract
Lubrication is a critical process in cold strip rolling, and the accurate characterization of lubrication characteristics is an essential factor affecting the strip quality. The roll bending and tilting roll in the flatness actuators change the loaded roll gap profile and affect the [...] Read more.
Lubrication is a critical process in cold strip rolling, and the accurate characterization of lubrication characteristics is an essential factor affecting the strip quality. The roll bending and tilting roll in the flatness actuators change the loaded roll gap profile and affect the lubrication characteristics by flatness dynamic correction, thus the mismatch between the actual and setting values of the lubrication status factor. Firstly, the flatness deviation correction model of roll bending and tilting roll based on the key information of the rolling process is established according to the high-order flatness target. Secondly, the characterization of the instantaneous oil film thickness in the work zone based on the loaded roll gap profile is derived from Reynolds’ equation. Finally, the explicit characterization method of the lubrication status factor in the rolling force model of the final stand is established with the work roll bending, tilting roll, and instantaneous oil film thickness of the work zone as variables, relying on the UCM five-stand, six-roll tandem cold rolling mill. The statistical evaluation and application results show that the mentioned optimization method can improve the setting accuracy of the rolling force by about 60% and the after-rolling gauge accuracy by about 50%. Full article
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11 pages, 2457 KiB  
Article
Low-Foaming/Aeration and Low-Traction Electric Drivetrain Fluid (EDF) Solutions for High-Speed E-Mobility
by Philip Ma, Donna Mosher and Chad Steele
Lubricants 2025, 13(2), 53; https://doi.org/10.3390/lubricants13020053 - 28 Jan 2025
Viewed by 608
Abstract
The use of electrically driven drivetrains is increasing for passenger cars and light-, medium-, and heavy-duty trucks. Off-the-shelf automatic transmission fluids (ATFs) are still being used as electric drivetrain fluids (EDFs). EDFs are trending toward lower viscosity for better energy efficiency and better [...] Read more.
The use of electrically driven drivetrains is increasing for passenger cars and light-, medium-, and heavy-duty trucks. Off-the-shelf automatic transmission fluids (ATFs) are still being used as electric drivetrain fluids (EDFs). EDFs are trending toward lower viscosity for better energy efficiency and better heat transfer capacity, while satisfying all the other challenging requirements, such as gear/bearing scuffing/wear protection, oxidative stability, copper corrosion, and coating/seal material compatibility. In this paper, we will highlight the importance of low foaming, low aeration, and low traction coefficient which are critical for the performance of the EDF during high-speed applications, measured using metrics such as energy efficiency, heat transfer capacity, and longer oil drain interval. Full article
(This article belongs to the Special Issue Tribology of Electric Vehicles)
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11 pages, 5586 KiB  
Article
The Tribological Reduction Mechanism of the Rubber Hexagonal Surface Texture of the Screw Pump Stator
by Zhongxian Hao, Songbo Wei, Deli Jia, Qinghai Yang, Xinglong Niu, Gang Zheng, Shijia Zhu and Xinfu Liu
Lubricants 2025, 13(2), 52; https://doi.org/10.3390/lubricants13020052 - 27 Jan 2025
Viewed by 554
Abstract
This paper develops a composite weaving structure, combining hexagonal micro-bumps and hexagonal grooves, in the design of the rubber surface of the screw pump. This allows us to solve the problem of high torque and fast wear of the rubber stator during the [...] Read more.
This paper develops a composite weaving structure, combining hexagonal micro-bumps and hexagonal grooves, in the design of the rubber surface of the screw pump. This allows us to solve the problem of high torque and fast wear of the rubber stator during the operation of screw pump lifting oil recovery, based on the bionic hexagonal surface structure, traditional surface damping principle, and fluid dynamic pressure lubrication theory. Finite element analysis is first conducted to quantitatively analyze the impacts of the parallel side distance, groove width, and groove depth on the surface flow field and wall pressure field of the composite hexagonal structure. Based on the simulation law, the rubber surface laser structure is then designed and prepared by nanosecond laser processing. Afterward, tribological experiments are conducted under the condition of long-term immersion in the actual extraction fluid of shale oil wells. This aims at simulating the actual downhole oil production conditions and quantitatively studying the impact of the size of the composite hexagonal structure on the lubrication characteristics of the friction part of the stationary rotor, as well as the effect of abrasion reduction. The results show that, within the simulation range, the smaller the parallel side distance, the higher the load-carrying capacity. In addition, the hexagonal weave with a parallel side distance of 3 mm has a higher wall load carrying capacity than that with distances of 4 mm and 5 mm. When the groove width is equal to 0.4 mm, the oil film load carrying capacity is higher than that in the case of 0.2 mm. When the groove depth increases, the oil film pressure first increases and then stabilizes or decreases after reaching 0.3 mm. In the hexagonal weave, the friction ratio of the rotor is equal to 0.4 mm. In the tribological experiment of hexagonal weave, the smaller the parallel side distance, the smaller the friction coefficient, and the 0.5 mm weave has the highest performance. Full article
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18 pages, 8358 KiB  
Article
Research on Rotor Dynamic Characteristics of High Speed Aviation Piston Pump
by Lijun Chen, Rushen Deng, Jun Zha, Jianning Gu, Tianxiang Xia and Runlin Chen
Lubricants 2025, 13(2), 51; https://doi.org/10.3390/lubricants13020051 - 25 Jan 2025
Viewed by 502
Abstract
The high-speed aviation piston pump plays a vital role in hydraulic systems in the aviation field. Extremely complex force situations happen during running operations due to the coupling between multiple components, as a result of the overall dynamic characteristics being complex and changeable, [...] Read more.
The high-speed aviation piston pump plays a vital role in hydraulic systems in the aviation field. Extremely complex force situations happen during running operations due to the coupling between multiple components, as a result of the overall dynamic characteristics being complex and changeable, which brings great difficulties and challenges to its performance optimization. Taking the high-speed aviation piston pump as the research object, a mechanical balance equation of the piston based on the dynamic balance method was proposed. The reaction force of the swashplate and the influence of rotational speed and outlet pressure on it were modeled. Through the balance of the system and the component subsystem, the load of the support bearing of the piston pump under different working conditions is analyzed, as well as the influence of the rotational speed and the outlet pressure on the bearing stiffness by the quasi-static method. In addition, the discrete model of the piston pump spindle and the discrete model of the rotor system are established. The accuracy of the model is verified by the finite element method. The maximum error of the spindle discrete model is 6.13%, and the maximum error of the rotor system discrete model is 15.28%. The transfer matrix analysis shows that the working condition parameters have little effect on the critical speed of the spindle and rotor system, and the outlet pressure has a more significant effect than the speed. The research results provide a theoretical basis and analysis method for the dynamic analysis and structural optimization of the high-speed aviation piston pump. Full article
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22 pages, 3892 KiB  
Article
Targeted Minimum Quantity Fluid Application in Machining
by Chandra Sekhar Rakurty, Patricio Ivan Varela and Alagar Krishnan Balaji
Lubricants 2025, 13(2), 50; https://doi.org/10.3390/lubricants13020050 - 25 Jan 2025
Viewed by 621
Abstract
The surface integrity of a machined component is crucial for its service life part. One of the main final specifications that a machined part is inspected for is the surface integrity metrics, including surface residual stresses, surface microhardness, surface roughness, and microstructure. In [...] Read more.
The surface integrity of a machined component is crucial for its service life part. One of the main final specifications that a machined part is inspected for is the surface integrity metrics, including surface residual stresses, surface microhardness, surface roughness, and microstructure. In this paper, the cutting fluid is strategically targeted to utilize heat energy effectively in the primary, secondary, and tertiary shear zones to positively affect the surface integrity metrics and machining mechanics. In this study, a lower quantity of the cutting fluids is targeted at the high-temperature zones to reduce the machining temperatures, thereby effectively simulating the effect of a ‘flood coolant’. The cutting fluid is applied simultaneously as a targeted Minimum Quantity Fluid (MQF) on the cutting tool’s flank and rake faces to improve the surface integrity metrics and chip formation. Also, this study analyzes the effect of the cutting fluid composition, the type of cutting fluid, and the amount of fluid quantities. The machining-induced surface integrity metrics are analyzed to understand the effects of targeted minimum quantity fluid application. The impact of the targeted application of cutting fluid on machining mechanics metrics, such as cutting forces and chip formation, is analyzed. Applying a targeted MQF application at the flank face of the cutting tool leads to higher compressive subsurface principal residual stresses. The results indicate that using MQF on both the flank and rake faces simultaneously enhances the surface integrity. The effect of a cutting fluid jet on the flank face is modeled to highlight the thermophysical properties that are crucial for selecting the appropriate cutting fluid to lower the machining-induced temperatures. With targeted MQF application, the fluid jet acts as a dynamic and external chip control mechanism. Overall, effectively managing temperatures in machining could enhance subsurface residual stresses and surface roughness using various cutting fluid combinations. Also, this paper presents a targeted cutting fluid application that improves the microstructural formation, enhancing chip control and producing machined surfaces and components with better surface integrity. Full article
(This article belongs to the Special Issue Recent Advances in Tribological Properties of Machine Tools)
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18 pages, 3930 KiB  
Article
Lubricant Viscosity Impact in Fuel Economy: Experimental Uncertainties Compensation
by Fernando Fusco Rovai and Eduardo Tomanik
Lubricants 2025, 13(2), 49; https://doi.org/10.3390/lubricants13020049 - 24 Jan 2025
Viewed by 504
Abstract
Climate constraints impose greenhouse gas emissions mitigation, and passenger cars have considerable contributions to contribute to this. To improve the engine efficiency of vehicles equipped with conventional powertrains, many technologies are available but with limited individual contribution. The experimental assessment of some technology [...] Read more.
Climate constraints impose greenhouse gas emissions mitigation, and passenger cars have considerable contributions to contribute to this. To improve the engine efficiency of vehicles equipped with conventional powertrains, many technologies are available but with limited individual contribution. The experimental assessment of some technology regarding fuel economy measurement results is sometimes lower than test uncertainties. This study proposes a methodology to compensate the fuel economy for two test uncertainties: vehicle speed variations and battery recharging. The proposed method can be applied when investigating the effects of different vehicle design changes, including engine power cell design. In this work, the proposed method is demonstrated on the test of two oils: one 5W40, the other 5W20, both without FM. Applying the proposed methodology to experimental results, the expected higher influence of oil viscosity on urban conditions could be observed, and the experimental results presented a much better correlation with the vehicle numerical simulation. Applying the proposed compensation, fuel savings of using the 5W20 in comparison to the 5W40 oil was 3.5% under urban conditions and 2.0% on highways. Full article
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23 pages, 3482 KiB  
Article
Comparative Study of Squalane Products as Sustainable Alternative to Polyalphaolefin: Oxidation Degradation Products and Impact on Physicochemical Properties
by Jessica Pichler, Adam Agocs, Lucia Pisarova, Ichiro Minami, Marcella Frauscher and Nicole Dörr
Lubricants 2025, 13(2), 48; https://doi.org/10.3390/lubricants13020048 - 24 Jan 2025
Viewed by 590
Abstract
The growing demand for sustainable lubricant solutions is driving the exploration of bio-based materials that deliver comparable performance to conventional, primarily fossil-based lubricant chemistries. This study focuses on squalane as a sustainable base oil, which can be derived from different renewable sources. A [...] Read more.
The growing demand for sustainable lubricant solutions is driving the exploration of bio-based materials that deliver comparable performance to conventional, primarily fossil-based lubricant chemistries. This study focuses on squalane as a sustainable base oil, which can be derived from different renewable sources. A total of two squalane products were evaluated for thermal-oxidative stability and benchmarked against a polyalphaolefin, PAO 4, of the same total carbon number. Oils artificially altered in a closed reactor were sampled and subjected to conventional lubricant analyses, including infrared spectroscopy, to determine the changes due to autoxidation over time. For in-depth information, direct-infusion high-resolution mass spectrometry and gas chromatography coupled with triple quadrupole mass spectrometry were employed to identify degradation products from thermo-oxidative stress. The results revealed substantial variability in the stability of squalane products, suggesting that differences in raw materials and production processes have a major impact on their performance, including rheological properties. The degradation products of polyalphaolefin and squalane, identified through detailed mass spectrometry, were analyzed to understand their impact on conventional physicochemical properties. While polyalphaolefin predominantly generated carboxylic acids with short to medium chain lengths as degradation products, squalane oxidation produced carboxylic acids with medium to long chain lengths as well as several alcohols and ketones. Despite these differences, squalane demonstrates its potential as a non-fossil hydrocarbon base oil, as squalane products matched and even exceeded PAO 4 stability. Full article
(This article belongs to the Special Issue Progress and Challenges in Lubrication: Green Tribology)
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19 pages, 10767 KiB  
Article
Surface Integrity and Machining Mechanism of Al 7050 Induced by Multi-Physical Field Coupling in High-Speed Machining
by Wei Lu, Chenbing Ni, Youqiang Wang, Chengguo Zong, Dejian Liu and Xingbao Huang
Lubricants 2025, 13(2), 47; https://doi.org/10.3390/lubricants13020047 - 22 Jan 2025
Viewed by 550
Abstract
Improving the surface quality and controlling the microstructure evolution of difficult-to-cut materials are always challenges in high-speed machining (HSM). In this paper, surface topography, defects and roughness are assessed to characterize the surface features of 7050 aluminum alloy (Al 7050) under HSM conditions [...] Read more.
Improving the surface quality and controlling the microstructure evolution of difficult-to-cut materials are always challenges in high-speed machining (HSM). In this paper, surface topography, defects and roughness are assessed to characterize the surface features of 7050 aluminum alloy (Al 7050) under HSM conditions characterized by high temperature, strain and strain rate. Based on multi-physical field coupling, the mechanism of microstructure evolution of Al 7050 is investigated in HSM. The results indicate that the surface morphology and roughness of Al7050 during HSM are optimal at fz = 0.025 mm/z, and the formation of surface defects (adherent chips, cavities, microcracks, material compression and tearing) in HSM is mainly affected by thermo-mechanical coupling. Significant differences are observed in the microstructure of different machined subsurfaces by electron backscatter diffraction (EBSD) technology, and high cutting speeds and high feed rates contributed to recrystallization. The crystallographic texture types on machined subsurface are mainly {110}<112> Brass texture, {001}<100> Cube texture, {123}<634> S texture and {124}<112> R texture, and the crystallographic texture type and intensity are significantly affected by multi-physical field coupling. The elastic–plastic deformation and microstructural evolution of Al7050 alloy during the HSM process are mainly influenced by the coupling effects of multiple physical fields (stress–strain field and thermo-mechanical coupling field). This study reveals the internal mechanism of multi-physical field coupling in HSM and provides valuable enlightenment for the control of microstructure evolution of difficult-to-cut materials in HSM. Full article
(This article belongs to the Special Issue Friction and Wear of Alloys)
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14 pages, 15081 KiB  
Article
CFD-Based Investigation of Static and Dynamic Pressure Effect in Aerostatic Bearings with Annular Grooves at High Speed
by Wentao Song, Minggui Li, Peng Cheng, Decheng Wang, Chenxi Shao and Junying Zhou
Lubricants 2025, 13(2), 46; https://doi.org/10.3390/lubricants13020046 - 22 Jan 2025
Viewed by 489
Abstract
Based on the SST k-ω turbulence model, this study investigated the flow fields of annular groove and non-groove small-hole throttling aerostatic bearings (AGSTABs and STABs). It examined the formation mechanisms of static and dynamic pressure effects in both flow fields at high speed, [...] Read more.
Based on the SST k-ω turbulence model, this study investigated the flow fields of annular groove and non-groove small-hole throttling aerostatic bearings (AGSTABs and STABs). It examined the formation mechanisms of static and dynamic pressure effects in both flow fields at high speed, evaluating how parameters such as eccentricity, groove width ratio, and depth ratio influence the average load capacity and static and dynamic pressure effects. The findings show that STABs combine static and dynamic pressure effects at high speeds, while AGSTABs decouple them to enhance load capacity, simultaneously reducing vortex and backflow intensity. At low eccentricities, AGSTABs exhibit superior performance over STABs, achieving 20% higher average load capacity at 0.1 eccentricity. Additionally, increasing eccentricity enhances static and dynamic pressure effects in both bearings. A larger groove width ratio decreases the throttling efficiency and dynamic pressure, with pressure dropping from 3.5 MPa (static) to 1.6 MPa, and 6.3 MPa (dynamic) to 1.7 MPa respectively, at 30,000 RPM. In contrast, the depth ratio of annular groove has only a minor impact on static and dynamic pressure effects. Full article
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14 pages, 4236 KiB  
Communication
Catalytic and Tribological Performances of a Novel Bi-Functional Ionic Liquid in Lubricating Ester Oil
by Yanan Wang, Huaigang Su, Jun Yin, Cheng Jiang, Qilong Zhao, Wenjing Lou and Qian Jia
Lubricants 2025, 13(2), 45; https://doi.org/10.3390/lubricants13020045 - 22 Jan 2025
Viewed by 613
Abstract
To address the detrimental effects of the residue of catalysts on the tribological performances of ester lubricants, a novel and efficient bi-functional ionic liquid 1-(3,5-di-tert-butyl-4-hydroxybenzyl)-3-methylimidazole di(2-ethylhexyl) phosphate ([(BHT-1)MIM][DEHP]) was prepared. The catalyst not only facilitates the synthesis of pentaerythritol tetra-hexanoate (PETH) through the [...] Read more.
To address the detrimental effects of the residue of catalysts on the tribological performances of ester lubricants, a novel and efficient bi-functional ionic liquid 1-(3,5-di-tert-butyl-4-hydroxybenzyl)-3-methylimidazole di(2-ethylhexyl) phosphate ([(BHT-1)MIM][DEHP]) was prepared. The catalyst not only facilitates the synthesis of pentaerythritol tetra-hexanoate (PETH) through the catalytic esterification reaction—achieving up to 96% conversion with a 94% yield—but also enhances the tribological performance of ester oil PETH when used as a lubricant additive. The tribological property has been improved remarkably: the mean friction coefficient for PETH + [(BHT-1)MIM][DEHP] is notably lower, at 0.110, compared to the PETH, which has a coefficient of 0.180. Meanwhile, the wear scar diameter of the steel ball, when lubricated with PETH + [(BHT-1)MIM][DEHP], is notably smaller than that of a steel ball lubricated solely with PETH. Especially, the reduction in the wear volume at 100 °C is up to 81.46% compared with the base oil PETH. [(BHT-1)MIM][DEHP], PETH + [(BHT-1)MIM][DEHP], and the worn track of the upper running ball and lower disc were systematically characterized by using Nuclear Magnetic Resonance (NMR) spectra, a Fourier Transform Infrared Spectrometer (FT-IR), a field emission scanning electron microscope (FESEM), Thermal gravity analysis (TG), X-ray photoelectron spectroscopy (XPS), and an optical microscope (OM). The wear mechanism of the tailored lubricant oil was discussed in terms of the chemical composition of the worn surface. Full article
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