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Lubricants, Volume 13, Issue 10 (October 2025) – 5 articles

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19 pages, 3344 KB

Open AccessArticle

Grease Film Behavior in Ball Bearings

by Denis Cojocaru, Gelu Ianuș, Vlad Cârlescu, Bogdan Chiriac and Dumitru Olaru

Lubricants 2025, 13(10), 429; https://doi.org/10.3390/lubricants13100429 - 25 Sep 2025

Abstract

To the film thicknesses of grease-lubricated ball bearings, the viscosity of the base oil is considered in the Elastohydrodynamic Lubrication (EHL) equations. For very low speeds, the grease film thickness is much larger than the calculated base oil film thickness. Initially, the grease film thickness decreases with speed to a minimum value, followed by an increase, thus generating a “V-shape pattern”. To evidence this behavior of grease film in a ball bearing, the authors used the method of measuring electrical resistance. Using an oil with a viscosity close to a grease base oil viscosity, a relationship was obtained between the electrical resistance of the ball bearing and the average film thickness in the ball–race contacts. Based on this relationship, the variation in the grease film thickness was obtained by measuring electrical resistance at a bearing speed between 1 and 500 rpm for short running periods of 60 s. A “V-shape pattern” was evidenced with a minimum value of grease film thickness at around 10 rpm. Additionally, the electrical resistance methodology was considered, evidencing the good stability of the film thickness for long operation time at speeds between 200 rpm and 1500 rpm. After 8 running hours, minor fragmentation of the soap filaments was observed under the scanning electron microscope compared to the fresh grease structure, without affecting the thickness of the grease film. Full article

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16 pages, 10621 KB

Open AccessArticle

Effect of Graphite Content on Mechanical Properties and High-Temperature Tribological Behavior of Cu-Ni-Sn-Mo-Gr Self-Lubricating Composites

by Zhen Li, Jingde Liu, Songlin Lu, Fuyan Liu, Guirong Yang and Jingbo Wang

Lubricants 2025, 13(10), 428; https://doi.org/10.3390/lubricants13100428 - 24 Sep 2025

Abstract

Copper matrix self-lubricating composites are critical for high-temperature industrial applications. In this study, Cu-Ni-Sn-Mo-Gr composites with 3–7 wt.% graphite were fabricated via spark plasma sintering (SPS). The influence of graphite content on microstructure, mechanical properties, and tribological behavior from room temperature (RT) to 500 °C were systematically investigated. The results demonstrate that increasing graphite content progressively reduces density, hardness, and yield strength, whereas it significantly enhances high-temperature tribological performance. The composites with 7 wt.% graphite addition achieve outstanding self-lubricity and wear resistance across the RT-500 °C, achieving an average friction coefficient of 0.09 to 0.21 and a wear rate of 1.32 × 10⁻⁶ to 7.52 × 10⁻⁵ mm³/N·m. Crucially, temperature-dependent lubrication mechanisms govern performance: graphite-dominated films enable friction reduction at RT, while synergistic hybrid films of graphite and in situ-formed metal oxides (Cu₂O, CuO, NiO) sustain effective lubrication at 300–500 °C. Full article

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15 pages, 1726 KB

Open AccessArticle

Nano Oil Additive Improves Internal Combustion Engine Efficiency and Life Expectancy

by Ding Lou, Jordan Morrison, Greg Christensen, Craig Bailey, Rose Gerani, Aaron Nardi and Rob Hrabe

Lubricants 2025, 13(10), 427; https://doi.org/10.3390/lubricants13100427 - 24 Sep 2025

Abstract

Internal combustion engines remain a predominant source of global energy consumption, contributing substantially to both operational costs and greenhouse gas emissions. This work evaluates a nanomaterial-based engine oil additive that reduces friction and wear and increases torque, horsepower, and fuel efficiency. This novel nano oil additive contains functionalized carbon nanotubes and hexagonal boron nitride nanosheets that are dispersed in base oil using a proprietary ultrasonication process. Block-on-ring tests performed by multiple testing facilities demonstrated up to a 17% decrease in coefficient of friction and up to a 78% decrease in wear compared to the base oil after treating with the nano oil additive. Thermal properties enhancement by the nano oil additive was evaluated and increases up to 17 °C in thermal stability were obtained. Additionally, the nano oil additive increased torque and horsepower by an average of 7% in motorcycles and 2.4% in pickup trucks. Most importantly, the nano oil additive demonstrated improvements in fuel economy in both gasoline and diesel engines, with laboratory tests reporting 3–5% increases and practical field tests on a commercial truck fleet reporting an average of a 6% increase. The improved engine efficiency leads to reduced turbo temperature in heavy diesel engines and prolonged engine life expectancy and will significantly improve global environmental sustainability. Full article

(This article belongs to the Special Issue Recent Advances in Automotive Powertrain Lubrication)

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23 pages, 18904 KB

Open AccessArticle

Influence Factor Analysis and Uncertainty Quantification of the Static Characteristics of Organic Working Fluid Aerodynamic Journal Bearings Considering Microscale Effect

by Ming Liu, Qiuwan Du, Shanfang Huang, Xiao Yan, Xinan Chen, Shuaijie Shi and Cheng Zhang

Lubricants 2025, 13(10), 426; https://doi.org/10.3390/lubricants13100426 - 23 Sep 2025

Abstract

The organic working fluid journal bearing is expected to enhance organic Rankine cycle system compactness significantly. In order to serve the practical application of organic working fluid bearings, this study analyzes the influence of key design parameters on the static characteristics under microscale effects. Uncertainty quantification is performed using three methods to address operational deviations. The results reveal the correlations for static characteristic indicators with design parameters in detail. Rarefied gas effects cause negligible pressure deviations (<0.21%), whereas surface roughness significantly improves load capacity. Sensitivity analyses (Morris and Sobol methods) identify eccentricity ratio and gas film thickness as the most influential parameters. KDE results indicate near-normal probability distributions for load and attitude angle. This study provides valuable insights for the design optimization and operational control of organic fluid bearings. Full article

(This article belongs to the Special Issue Gas Lubrication and Dry Gas Seal, 2nd Edition)

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17 pages, 2309 KB

Open AccessArticle

A Real-Time Dynamic Temperature Prediction Method for Double-Steel Plates in Wet Clutches

by Zhigang Zhang, Yongle Liu and Xiaoxia Yu

Lubricants 2025, 13(10), 425; https://doi.org/10.3390/lubricants13100425 - 23 Sep 2025

Abstract

Wet clutches are extensively employed in automotive transmission systems due to their benefits of smooth shift and stable operation. However, existing methodologies have not yet thoroughly analyzed the real-time dynamic temperature distribution of wet clutches, and the heating and heat transfer mechanisms during the sliding friction process of friction pairs remain underexplored. To address these gaps, this study proposes a real-time dynamic temperature prediction model for wet clutches and investigates the heat generation and transfer mechanisms in the friction pair sliding process. Specifically, the heat production and exchange dynamics of the wet clutch friction pair are systematically analyzed, followed by an examination of the real-time temperature variation of the separator plate under both high-slip and low-slip speed conditions. In the numerical simulations, the predicted temperature values from the proposed model demonstrate excellent agreement with experimental measurements, with dynamic peak temperature discrepancies remaining within ±2 °C. Furthermore, the validated temperature evolution laws are corroborated by experimental results obtained from a dedicated wet clutch performance test rig, thereby providing comprehensive empirical verification of the proposed real-time dynamic temperature prediction framework for wet clutch separator plates. In summary, the model can accurately capture the temperature variation characteristics of wet clutches under different operating conditions, providing a reliable basis for real-time thermal management of transmission systems. It holds significant practical value for optimizing cooling system design, extending clutch service life, and ensuring shifting quality in vehicles. Full article

(This article belongs to the Special Issue Advanced Protective Composite Coatings and Films: Tribological Mechanisms and Applications)

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