Lubrication Characteristics Research Articles

Studies on newly developed hBN/graphene-based nano-fluids supported by cryogenic cooling conditions in improving the tribological performance of Ti6Al4V alloy

Hexagonal Boron Nitride (hBN) is often referred to as a “soft” material due to its layered structure and properties that distinguish it from conventional hard materials like ceramics. The layered structure of hBN imparts lamellar lubrication characteristics and the weak van der Waals forces between adjacent layers allow for easy sliding, leading to low frictional resistance. The softness of hBN allows for ease of processing into various forms, facilitating its incorporation into lubricants, coatings, and composite materials. Therefore, the aim of this work is to enhance the lubricating capabilities of the nano-fluids and optimize the frictional behavior of Ti6Al4V alloy against tungsten carbide (WC) abrasive ball for potential biomedical applications, especially for combination of Ti6Al4V femoral stem and carbide femoral head. The newly formulated nano-fluids combine the unique properties of hBN and graphene to create a synergistic lubricating environment. The tribology and advanced characterization techniques were used to analyze the wear behavior of Ti6Al4V alloy surfaces. The results demonstrated that the applciaiton of cryogenically cooled lubricants with nanoparticles exhibited the lowest wear depth and friction forces, as a result of their increased viscosity.

Modeling and analysis of elastohydrodynamic lubrication of orthogonal face gear with installation errors

Installation error is one of the most prominent factors affecting the lubrication of face gear. To analyze the lubrication performance of the face gear with installation errors, the installation error model of the orthogonal face gear transmission system is established. Then, the contact path, equivalent curvature radius, load distribution and entrainment velocity are studied using the face gear loaded tooth contact analysis (LTCA). On this basis, the lubrication governing equations of the face gear are established according to the elastohydrodynamic lubrication (EHL) theory. In addition, the oil film thickness, oil film pressure and coefficient of friction in the process of face gear drives are calculated using the method of progressive mesh densification (PMD). Finally, the effects of axis intersection angle error, axis intersection error and axial displacement error on lubrication characteristics are analyzed. The results reveal that when the axial intersection angle error and axial displacement error are negative and the axial intersection error is positive, the minimum oil film thickness of the face gear tooth surface is less than that without error. When the error values are the same, the minimum oil film thickness and coefficient of friction have larger changes with the axial displacement error than with the axis intersection error, and the contact path has a bigger movement range.

Study of the effect of macrogeometry of rubbing steel bodies at their point contact on the tribological characteristics of lubricants under boundary lubrication conditions

The effect of the shape of contacting nonconformal elements of friction units that form an initial point contact on the tribological characteristics of lubricants during friction in the boundary lubrication mode. Point contact of rubbing bodies is realized on four-ball friction machines when testing tribological characteristics of lubricants in accordance with Russian GOST 9490–75 and GOST 23.221–84 Standards.However, these tests are carried out with the friction of standard bearing balls and does not allow taking into account the impact of materials of rubbing bodies on the results of the experiment. Therefore, two alternative designs of mandrels were developed, which allow implementation of two friction schemes: «rotating ball clamped in the spindle of the machine – three roller face planes» and «rotating ball — three cylindrical rollers». The load on the friction unit for both friction schemes, as well as in parallel on the mandrel under testing, in which three balls are installed, is the same (108 N), the spindle rotation speed is equal to 1 rpm, duration of testing is 60 min. The diameter of balls used in these experiments was 12.7 mm, and diameters of rollers were 5 and 8 mm, respectively. The material of samples was ShKh-15 tool steel or its 100Cr6 analogue. The designs of mandrels and the dimensions of samples were chosen in such a way that in all cases the contact areas of the experimental samples with the wearing (upper) ball would lie on a circle with a diameter of 7.32 mm. The tests were carried out under dry friction, and in boundary lubricated conditions in a PAO-4 polyalphaolefin oil medium, as well as in the same oil medium with the additives of oleic acid and DF-11 (a solution of zinc dialkyldithiophosphate in a low-viscosity oil). It is shown that the composition of a lubricating medium has a decisive effect on the test results (i.e., on the values of the friction coefficients and on the values of the areas of wear scars), while the geometry of the rubbing bodies, as well as the maximum Hertz stresses in the tribological contact, had a little effect on the antifriction and anti-wear characteristics of oils.

Improving Drill-String Extended Reach Efficiency in High-Temperature and High-Pressure Conditions: An Experimental Investigation

Abstract Drill string buckling behavior is of interest to the oil and gas industries. Buckling severity increases as drilling continues and causes a lock-up condition beyond which drilling cannot commence. The friction force between the drill string and the wellbore is key in influencing drill string buckling initiation and progression. Herein, the effects of adding nano-sepiolite (NSP) as an additive to water-based drilling fluids on the friction between the drill string and wellbore wall were investigated. Various samples with different sizes and compositions of NSP were tested for their ability to reduce the coefficient of friction (COF) that delays the onset of buckling to facilitate deep drilling by providing improved lubrication characteristics. The drill string sinusoidal and helical buckling and lock-up condition and the axial force transfer (AFT) were experimentally investigated. The water-based drilling fluids’ lubricity, COF between the drill string and the wellbore wall, and the stability of the rheological properties of the NSP-modified water-based drilling fluids containing 1–6 wt% NSP were tested in a high-temperature and high-pressure (HTHP) environment utilizing an in-house experimental setup. Sepiolite in nano-form at 4 wt% and with a size distribution of 30–60 nm improved the stability of the water-based drilling fluids rheology, significantly decreased the COF of the water-based drilling fluids, and improved the drill string axial force transfer. NSP additives significantly improved the COF and the drill string AFT in the HTHP environment compared with that of other commercial drilling fluids.

Transient lubrication analysis of journal-thrust coupled bearing considering time-varying loads and thermal-pressure coupled effect

The journal-thrust coupled bearing (abbreviated as JTC bearing) works at the end of the crankshaft in diesel engine, with the function of carrying radial load and prevent axial movement of the crankshaft. The journal part and the thrust part of this special bearing both form the oil film lubrication when subjected to the time-varying loads in axial and radial directions during the working condition. In this work, the JTC bearing in marine diesel engines was analyzed using a transient thermal elastohydrodynamic (TEHD) lubrication model. The thermal and pressure (thermal-pressure) coupled effects were taken into consideration based on the flow, pressure, and thermal continuity conditions between the journal and thrust parts. Moreover, the transient time-varying load was considered in the lubrication analysis to explore the coupled effect under the actual operating conditions. A test rig was built to validate the established transient TEHD lubrication model for the JTC bearing. The thermal-pressure coupled effect between the journal and thrust parts was found to significantly affect the transient lubrication behaviors of the JTC bearing, which cannot be ignored. Also, different boundary conditions were investigated for the influence of the coupled effect on transient lubrication characteristics, including axial clearances, radial clearances, waveforms of radial load, incline angles and inlet oil temperatures. This work could theoretically support the lubrication design of the JTC bearings used in the diesel engine crankshafts.

The addition of SiO2-GO hybrid material enhanced the tribological properties of fluorocarbon resin in marine corrosive environments

Fluorocarbon resin (FC), as a commonly used topcoat material for coating systems, is susceptible to the combined effects of seawater corrosion and ocean current erosion when working in marine environments for a long time, often causing hard particle wear and damage. In order to improve the tribological properties of FC resin, graphene oxide (GO) was modified by sol-gel method, and SiO2-GO hybrid nanomaterials and SiO2-GO/FC composite coatings were innovatively prepared. Through a 50 days long-term immersion experiment, the friction coefficient and wear rate of the composite coating were characterized. The study showed that the addition of SiO2-GO effectively reduced the friction coefficient and wear rate of the coating before corrosion immersion. The average friction coefficient of 0.4 wt% SiO2-GO/FC under a 10 N load for 15 minutes was 0.172, and the wear rate was 1.19×10−4 mm3/Nm, a decrease of 69.9% compared to pure FC coating. After long-term immersion, an oxide layer generally appeared between the coating and the substrate. On the 50th day of immersion, FC, GO/FC, 0.2 wt% SiO2-GO/FC, and 0.6 wt% SiO2-GO/FC all lost their protective ability, while 0.4 wt% SiO2-GO/FC maintained basic tribological properties due to its excellent corrosion resistance. This was mainly due to the lubrication characteristics and good dispersibility of SiO2-GO, which endowed the coating with stronger wear resistance. In addition, the maze effect formed by SiO2-GO within the coating reduced corrosion damage, indirectly improving the tribological performance of the coating.

Performance analysis of columnar convex-concave compound microtexture thrust bearing

PurposeThis study aims to enhance the lubrication performance of thrust bearings. The influence of columnar convex–concave compound microtexture on bearing performance is investigatedDesign/methodology/approachBased on the compound microtexture model of thrust bearings, considering surface roughness and turbulent effect, the variation of lubrication characteristics with the change in the compound microtexture parameters is studied.FindingsThe results indicate that, compared with circular microtexture, the maximum pressure of compound microtexture of thrust bearings increases by 7.42%. Optimal bearing performance is achieved when the internal microtexture depth is 0.02 mm. Turbulent flow states and surface roughness lead to a reduction in the optimal depth. The maximum pressure and load-carrying capacity of the bearing decrease as the initial angle increases, whereas the friction coefficient increases with the increase in the initial angle. The lubrication performance is best for bearings with a circumferential parallel arrangement of microtexture.Originality/valueThe novel composite microtexture with columnar convex-concave is proposed, and the computational model of thrust bearings is set. The influence of surface roughness and turbulent flow on the bearing performance should be considered for better conforming with engineering practice. The effect of microtexture depth, arrangement method and distribution position on the lubrication performance of the compound microtexture thrust bearing is investigated, which is of great significance for improving tribology, thrust bearings and surface microtexture theory.

Effect of elastic deformation on lubrication characteristics in different clearances and head size of metallic artificial hip joint

The issue of lubrication in metallic artificial hip joints is discussed to prevent the two bearing surfaces from contacting each other. This study aims to analyse the characteristics of elastohydrodynamic lubrication (EHL) and investigate the effect of radial clearance and head size on the tribological performance of metallic artificial hip joints under steady-state conditions using the Fluid-Structure-Interaction (FSI) method. The average load in the z-direction and flexion-extension motion around the y-axis were used to represent normal walking. This study demonstrates that the film thickness profile from the validation study aligns with the findings of the previous study. The contour of the fluid pressure in the central contact zone changes with different time steps – the direction of fluid movement corresponds to the solid-fluid reaction. Additionally, the deformation of the cup surfaces contributes to the continuous fluid film that separates the contact surfaces. Furthermore, design parameters also influence EHL performance. It has been discovered that less radial clearance and a larger head size led to an increase in fluid film thickness.

Polyvinylpyrrolidone hydrogel coating for ureteral stent: Safety and performance evaluation.

Ureteral stents are commonly used in urology. However, complications such as encrustation and infection on the surface of the stent, and injury to the ureteral mucosa can occur after implantation, causing discomfort for patients. We intend to confirm the biosafety of polyvinylpyrrolidone (PVP) hydrophilic coating and its lubrication properties for surface modification of ureteral stents to reduce friction and improve patient comfort. Based on our previous studies, we have developed a PVP hydrophilic coating for surface modification of ureteral stents. We firstly investigated the cytotoxicity, intradermal irritation, delayed type hypersensitivity, and acute systemic reactions of stent coating extracts. We further characterized the break strength, retention strength, and dynamic friction of the stent. The cell survival rate of all experimental groups was greater than 70%. No hypersensitivity reaction, systemic toxicity reaction, or obvious intradermal reaction were observed. The above results indicate that the test results of the modified stent meet the requirements of ISO 10993-5: 2009 (Cytotoxicity); ISO 10993-10:2021 (Sensitization and Irritation); ISO 10993-11:2017 (Acute Systemic Toxicity). After soaking in artificial urine for an extended period, there was no obvious change in its super-slip performance. Our results confirm the safety and lubrication characteristics of PVP hydrophilic coating for ureteral stent surface modification. The performance of this coating has the potential to reduce complications after stent implantation, thereby improving patient comfort, reducing medical burden, and has a good clinical application prospect.

Experimental and numerical study of the mixed lubrication under the action of magnetic ionic liquid additives

In this paper, the tribological characteristics of an oil-soluble magnetic fluid additive under mixed lubrication are studied by experiments and numerical simulation. [bmim][FeCl4] is dissolved in CF10W-40 lubricating oil as a magnetic liquid additive, and its friction coefficient is tested by a point contact friction tester at different temperatures, rotational speeds and magnetic field intensities. The transition condition of lubrication state is obtained through analyzing the Stribeck curves based on the experiments, and the strength model of boundary film is established accordingly. A mixed lubrication model is established by substituting the boundary film strength model and the surface roughness model into the hydrodynamic lubrication model based on Reynolds equation. The results show that the magnetic solution as an additive can obviously reduce friction and wear, and the effect is more obvious under the condition of magnetic field. The boundary film strength model can accurately reflect the transition characteristics of lubrication state in the presence of boundary film, and the mixed lubrication model based on boundary film strength model can more precisely reflect the tribological characteristics of friction pairs, so this study provides a new theoretical method for the related research on the influence of boundary film on lubrication characteristics.

Synthetic oils based on pentaerythritol esters. Kinematic viscosity

One of the main characteristics of lubricants is the viscosity index, which shows the dependence of oil viscosity on temperature. The higher the viscosity index, the better the lubricating properties of the oil. Esters of various polyols, in particular pentaerythritol, are very effective in this direction. This work examines a wide range of high-purity pentaerythritol esters of 14 compounds of varying degrees of branching and studies the temperature dependences of kinematic viscosity in the temperature range from melting temperatures to 150 °C. Based on the obtained data, the values of viscosity indexes were calculated, and the contribution of the carboxylic acid structure to the viscosity properties was also considered.

Highly Interfacial Active Gemini Surfactants as Simple and Versatile Emulsifiers for Stabilizing, Lubricating and Structuring Liquids

AbstractTo date, locking the shape of liquids into non‐equilibrium states usually relies on jamming nanoparticle surfactants at an oil/water interface. Here we show that a synthetic water‐soluble zwitterionic Gemini surfactant can serve as an alternative to nanoparticle surfactants for stabilizing, structuring and additionally lubricating liquids. By having a high binding energy comparable to amphiphilic nanoparticles at the paraffin oil/water interface, the surfactant can attain near‐zero interfacial tensions and ultrahigh surface coverages after spontaneous adsorption. Owing to the strong association between neighboring surfactant molecules, closely packed monolayers with high mechanical elasticity can be generated at the oil/water interface, thus allowing the surfactant to produce not only ultra‐stable emulsions but also structured liquids with various geometries by using extrusion printing and 3D printing techniques. By undergoing tribochemical reactions at its sulfonic terminus, the surfactant can endow the resultant emulsions with favorable lubricity even under high load‐bearing conditions. Our study may provide new insights into creating complex liquid devices and new‐generation lubricants capable of combining the characteristics of both liquid and solid lubricants.

Highly Interfacial Active Gemini Surfactants as Simple and Versatile Emulsifiers for Stabilizing, Lubricating and Structuring Liquids.

To date, locking the shape of liquids into non-equilibrium states usually relies on jamming nanoparticle surfactants at an oil/water interface. Here we show that a synthetic water-soluble zwitterionic Gemini surfactant can serve as an alternative to nanoparticle surfactants for stabilizing, structuring and additionally lubricating liquids. By having a high binding energy comparable to amphiphilic nanoparticles at the paraffin oil/water interface, the surfactant can attain near-zero interfacial tensions and ultrahigh surface coverages after spontaneous adsorption. Owing to the strong association between neighboring surfactant molecules, closely packed monolayers with high mechanical elasticity can be generated at the oil/water interface, thus allowing the surfactant to produce not only ultra-stable emulsions but also structured liquids with various geometries by using extrusion printing and 3D printing techniques. By undergoing tribochemical reactions at its sulfonic terminus, the surfactant can endow the resultant emulsions with favorable lubricity even under high load-bearing conditions. Our study may provide new insights into creating complex liquid devices and new-generation lubricants capable of combining the characteristics of both liquid and solid lubricants.

Study on Grease Lubrication and Electric Erosion Characteristics in AC Electric Fields

Protecting motor bearings from electric erosion is crucial as electric vehicles evolve. To better understand how lubrication interacts with electric discharge within motor bearings during varying speeds of vehicle operation, an optical ball-on-disk tribometer was modified to investigate the influence of alternating current (AC) electric fields on film thickness, friction force under various lubrication regions, and discharge characteristics. The study revealed that in AC electric fields, as the lubrication state shifts from mixed lubrication to fluid lubrication region, the electrical characteristic of the lubricating oil film changes from resistive to capacitive, accompanied by an increase in discharge frequency. Under the elastohydrodynamic lubrication (EHL) region, an electrical potential difference between the surfaces separated by the lubrication film leads to a reduction in film thickness, which can be attributed to the generation of Joule heating. If the potential difference across the oil film increases to the threshold voltage, destructive discharge occurs with the emission of a significant amount of purple light. Joule heating generated by the AC electric fields also results in a reduction in the friction coefficient under the fluid lubrication region. However, due to the reduction in film thickness, the lubrication state eventually moves to mixed lubrication, leading to a substantial increase in the friction coefficient. In addition, the study also investigated the use of grease with a nanographite conductive additive. It was found that inappropriate additive amounts can lead to discharge phenomena occurring outside the contact region.

Transient mixed-lubrication and contact behavior analysis of metal liquid film under magneto-thermal effect

To investigate the contact state and interface behavior of the armature-rail (A/R) interface under magneto-thermal shock, this study employs the lubrication theory and considers the coupling effect of electromagnetic and temperature fields. A transient mixed hydrodynamic lubrication model is established using the finite difference method. The study investigates the evolution mechanism of rough surface metal liquid film, electrical contact behavior, and transient mixed lubrication characteristics at the interface under complex magneto-thermal excitation. The study suggests that the electrical contact behavior of the metal liquid film during electromagnetic launch process exhibits significant fluctuation characteristics, which have a crucial influence on the temperature distribution of the metal liquid film. Moreover, due to the fluctuation behavior of electrical contact and factors such as surface roughness, the temperature of the metal liquid film exhibits a complex spatial distribution pattern. The temperature peak continuously increases as the electromagnetic launch progresses. Furthermore, the initially uniform distribution of the metal liquid film gradually transforms into a non-uniform, island morphology due to the joint action of the magneto-thermal effect. However, the liquid metal film still exhibits excellent lubricating properties, which play a crucial role in reducing friction and wear in the electromagnetic launcher system (EMLs).

Analysis of lubrication characteristics of dynamic-static pressure hybrid thrust bearing considering key factors under eccentric loads

The lubrication characteristics of the dynamic-static pressure hybrid thrust bearing under eccentric load determined the machining accuracy and the thermal stability of machine tools. To improve the bearing's performance, the lubrication characteristics were simulated by theoretical establishments and numerical simulations with variable viscosity under different eccentric loads and rotational speeds. Finally, the simulation methods were validated by experiments. Results indicated that an appropriate offset distance enhanced load capacity. The load and rotational speed significantly influenced the oil film pressure. The impact of the load on temperature was minimal, whereas the maximum temperature increased significantly with the increase in rotational speed. The runway chamber had a high load carrying capacity, the sector chamber had a low temperature rise, and the double rectangular chamber had high dynamic pressure under eccentric load.

Tribological characterization of SAE20W40 lubricant added with surface activated nanoparticles (Al2O3 and SiO2)

This research work focuses on the performance characteristics of the standard SAE20W40 lubricant with the addition of nanoparticles (aluminum oxide (Al2O3) and silicon dioxide (SiO2) in equal proportion). The nanoparticles were surface-activated using oleic acid to homogenize the lubricant dispersion. The lubricant was added with 0.1, 0.3, and 0.5 wt% of nanoparticles and subjected to a mechano-thermal process to synthesize nanolubricant. The physio-chemical properties (flash point, fire point, thermal stability, kinematic viscosity, acid value, and iodine value) and tribological characteristics (specific wear rate, friction coefficient, and wear mechanism) of the nanolubricant were determined and correlated with dispersed nanoparticles. Instrumental characterization of SEM, EDS, TEM, FT-IR, and UV-Vis tests were performed to validate the surface-activated nanolubricant. The base lubricant demonstrated favorable tribological characteristics when enhanced with 0.1 wt% additives.

Analysis of the thermal lubrication characteristics of cycloid pinwheel transmission mechanisms in RV reducers

In this paper, the cycloidal pin wheel drive mechanism of the secondary drive component of the rotary vector (RV) reducer is studied. Under the condition of grease lubrication, considering tooth profile modifications and machining errors, we established a three-dimensional load-bearing contact analysis model of a cycloid pinwheel transmission mechanism. We studied the time-varying contact characteristics of the gear teeth and dynamic joint surfaces of the inner and outer races of a crankshaft bearing. Considering the thermal effects, we proposed an analysis model of the lubrication characteristics of cycloid pinwheel transmission mechanisms. We also studied the thermal lubrication properties, such as load de-formation, lubrication film thickness, contact pressure and temperature increase at the lubrication contact area. The accuracy of the model was verified by temperature experiments on the inner raceway of an RV reducer bearing and the evolution law of the dynamic thermal lubrication characteristics of the inner and outer races of the crankshaft bearing, the meshing gear tooth pair and the whole process of single pin tooth meshing were revealed. This study provides an effective method for analyzing the temperature increases of the key joints of RV reducers.

Lubrication and wear reduction characteristics of graphene oxide/ionic liquid composite coating

Graphene oxide (GO) exhibits good hydrophilicity and chemical harmonizability attributed to its extensive oxygen-containing functional groups, thereby conferring superior lubrication and wear reduction characteristics in water lubrication systems. In this study, a composite coating of graphene oxide/ionic liquids (GO/ILs) was prepared using impregnation and ultrasonic spraying techniques. The influence of GO, ILs on the lubrication and wear reduction characteristics of silicon carbide (SiC) was studied. Additionally, the coefficient of friction (COF), wear characteristics, and triboelectric characteristics of water-lubricated SiC interface were studied through alterations in speed and load. It was found that the GO/ILs composite coatings effectively enhanced friction lubrication properties by increasing the open-circuit voltage at the liquid-solid interface, thereby strengthening the mutual repulsive polarization electric field. Notably, the GO/ILs composite coating consistently delivered continuous and stable lubrication across a broad speed range under the combined support of ILs and the friction voltage effect. When the load exceeded 80 N, the COF for SiC with the GO/ILs composite coating consistently remained below 0.02 throughout the friction process. Remarkably, the COF of GO/ILs composite coating exhibited a substantial reduction of 92.36 % in comparison to uncoated SiC under conditions of 120 N and 1200 r/min. It was further revealed that the improved tribological properties of the GO/ILs composite coatings stemmed from the combined effect of the in-situ GO friction film, self-lubricating properties of GO, and coupled effect of the polarized electric field at the interface.

Experimental and numerical study on the dynamic and lubrication characteristics of the rotor-bearing-frame system in a variable-speed scroll compressor

The variable-speed scroll compressors can adapt to the various demands of heating and cooling capacity, and the rotational speeds significantly influence the rotor dynamic and bearings lubrication, which might cause performance and reliability issues. In this paper, numerical models for the rotor-bearing-frame system are developed by coupling the Reynolds equation, rotor dynamic equation, and the flexibility equation of the frame. The experimental apparatus is also established to directly measure the journal displacement response in the hermetic high-pressure shell of the compressor. The predicted journal center trajectories are compared with the experimental results and they show good agreement. Results show that the whirl radius decreases with the increase of rotational speed at the measuring point, while the opposite tendency can be observed at the bottom of the main bearing. Transient dynamic analysis of the rotor shows that the tilting and deformation of the rotor vary with respect to different rotational speeds. The displacement of the crank part could lead to the separating tendency between orbiting and fixed scrolls through the coupled orbiting bearing, especially at the lower rotational speed, due to the pull of main balance weight. The numerical results also reveal that the distribution of oil film pressure is dominated by the squeeze effect due to the relatively stable magnitude but the variable direction of loads on the shaft. The influence of the deformation of the cantilevered bearing frame on the oil film thickness is significant but the maximum deformation moves downward at the higher rotational speed.