Hydrodynamic Lubrication Regime Research Articles

Sliding-Induced Rehydration in Hydrogels for Restoring Lubrication and Anticreeping Capability.

Fluid exudation in cartilage under normal loading can be counteracted by a sliding-induced rehydration phenomenon, which has a hydrodynamic origin related to a wedge effect at the contact inlet. Similar to cartilage, hydrogels also exhibit tribological rehydration properties, and we mimic this phenomenon to restore hydration lubrication and overcome creeping. It occurs within a specific velocity range and is mainly dependent on the applied load and hydrogel network structures. Crucially, a certain velocity in the mixed lubrication regime can produce a hydrodynamic pressure peak at the wedge and drive the rehydration inflow to overcome the extrusion. At lower sliding velocities in the boundary lubrication regime, inflows are insufficient to counteract fluid exudation, whereas at higher velocities in the hydrodynamic lubrication regime, the inlet wedge effect would diminish. These results suggest that tribological rehydration offers a novel approach to enhancing load-bearing capacity and maintaining lubrication in the hydrogels.

Non-Polar Chain-Enabled Suspension of Carbon Nanoparticles in Base Oil

The transition to electric vehicles (EVs) has introduced new challenges in lubrication, demanding innovative solutions to ensure consistent performance. One promising approach is the use of nanoparticle additives, which have the potential to improve lubrication performance significantly. However, achieving a stable suspension of these nanoparticles in lubricating oils remains a critical challenge, as suspension stability is essential for maintaining consistent performance and maximizing the benefits of these advanced additives. In this study, carbon nanoparticles (CNPs) were modified with dodecylamine (DDA) to achieve stable suspension in nonpolar fluids. The successful functionalization was confirmed by the FTIR results, which showed characteristic peaks of various bonding. The suspension stability tests demonstrated that DDA-CNPs remained suspended for over 60 days in the Polyalphaolefin (PAO) oil, whereas unmodified CNPs were sedimented within 3–7 days. The rheological behavior was measured under different shear rates and temperatures. Viscosity measurements indicated that DDA-CNPs maintained a lower value compared to base PAO. The lubricants’ friction coefficient (COF) was also determined under various speeds and loads. The addition of DDA-CNPs at a concentration of 0.05 wt.% resulted in a significant reduction in COF, with values decreasing by 26% compared to base PAO oil under a load of 1 N. Additionally, the COF for DDA-CNPs was consistently lower than that of PAO, with reductions ranging from 15% to 18% across all tested speeds. The Stribeck curve further highlighted the improved performance of DDA-CNPs across boundary, mixed, and hydrodynamic lubrication regimes. These findings suggest that DDA-CNPs significantly improve the lubrication performance of PAO oil, making them suitable for advanced lubrication applications in automotive and industrial systems.

Controlling stick–slip in low-speed motion with a lifting force of magnetic fluid

Stick–slip is a standard friction-induced self-excited vibration that usually occurs in the boundary or mixed lubrication regimes. Broadening of the hydrodynamic lubrication regime is conducive to suppressing stick–slip motion. In this paper, the load carrying capacity of a magnetic fluid (MF) film in the presence of a magnetic field is derived based on the modified Reynolds equation. An additional lifting force produced by MF under the magnet was applied between the tribopairs to achieve the full fluid lubrication. Thus, the stick–slip is expected to be inhibited in a lower speed scope. The effect of magnet thickness on the lifting force is investigated experimentally and theoretically. Special attention is given to the influence of the lifting force on the friction and the critical transition speed of the hydrodynamic lubrication regime. Results demonstrate that the lifting force increases with the increment of the magnet thickness. The presence of the additional lifting force expands the hydrodynamic lubrication and makes the critical transition speed move left, as shown by the friction transitions on the Stribeck curve. Therefore, stick–slip motion can be suppressed at a lower sliding speed. Such beneficial effects are more pronounced in thicker magnets. It can be confirmed that, so long as the lifting force is higher than the normal load, the friction will invariably operate in the full film lubrication and the stick-slip motion may be eliminated theoretically.

ALTERNATE SLIDING FRICTION FORMULATIONS OF ELASTO - HYDRODYNAMIC OF SPUR GEARS

In this paper, several sliding friction formulations used in spur gear dynamics are examined and compared in terms of the predictions of interfacial friction forces and off-line-of-action. Computing friction formulations include Coulomb models with time-varying friction coefficients and empirical expressions based on elasto - hydrodynamic and/or boundary lubrication regime principles. Predicted results compare well with friction force measurements and appeared completely conformed with specific objectives of this paper are established as follows: (1) propose an improved MDOF spur gear pair model with time-varying coefficient of' friction, µ(t), given realistic mesh stiffness profiles.(2) comparatively evaluate alternate sliding friction models and predict the interfacial friction forces and motions in the off-line-of-action (OLOA) direction: and (3) validate one particular model (III) by comparing predictions to the benchmark gear friction force measurements

AI techniques for evaluating misaligned journal bearing performance: An approach beyond the Sommerfeld number

This paper presents two novel artificial intelligence-based approaches for evaluating the performance of heavily loaded marine journal bearings including shaft misalignment. Traditionally, the Sommerfeld number has been used as a key parameter to evaluate the performance similarity between different bearings. However, this method has limitations, particularly when dealing with complex elastic geometries, heavily loaded journal bearings and shaft misalignment. The first proposed approach leverages neural networks to analyze key bearing performance parameters and provide a more accurate and comprehensive assessment of bearing performance similarity, including additional parameters beyond the Sommerfeld number limitations. The second method utilizes artificial intelligence convolutional networks to assess the bearing similarity based on their simulated pressure profiles under isoviscous and isothermal hydrodynamic lubrication regime. The effectiveness of the proposed methods is demonstrated and compared to the traditional Sommerfeld number method, discussing various potential applications and extensions of this concept.

From mixed to hydrodynamic regime in lubricated sliding of carbon fiber tows

Carbon fiber tows are used as reinforcement material in composite elements. To ensure mechanical integrity of the composite structure, it is important to ensure a good positioning of the carbon fibers in the mold during the manufacturing process. The positioning of the carbon fiber tows depends greatly on the friction with the mold. In the present paper, the friction between a carbon tow placed on a cylindrical pin rubbing against a rotating glass disk is experimentally studied in both dry and resin-lubricated conditions on a dedicated tribometer. The tests are performed for different loading levels and sliding speeds. The results are compared to numerical simulations taking into account contact mechanics and lubrication. The tows are modeled using a simple elastic foundation approach coupled with the Reynolds equation when the resin flow is considered. It appears that the impact of the deformability of the tow on its frictional behavior is significant. It conditions the transition between mixed and hydrodynamic lubrication regimes.

Deterministic Simulation of Surface Textures for the Piston Ring/Cylinder Liner System in a Free Piston Linear Engine

The use of synthesis gas (SYNGAS) from waste gasification has been pointed out as a key strategy to help the energy transition. However, SYNGAS’ low calorific power is considered a difficult obstacle to its technological use in internal combustion engines. To overcome this, a novel free-piston linear motor has been proposed to pave the way for the use of SYNGAS in the mobility sector. Surface texturing has vast potential to reduce friction losses in this system. This study utilizes a deterministic numerical model to investigate the mixed lubrication performance of a textured piston ring/cylinder liner conjunction in a free piston engine. The model considers the simultaneous solution of the lubrication and asperity contact problems at the roughness scale, including texturing features on the cylinder surface. The numerical model employs the Reynolds equation with mass-conserving cavitation to calculate the inter-asperity fluid pressure. The rough contact model utilizes the Hertz theory for elastic contact to calculate the contact pressure at each asperity between the piston liner surface and the admitted smooth and rigid ring surface. Surface texturing demonstrated remarkable effectiveness, particularly in the hydrodynamic lubrication regime, with a maximum friction reduction of 38.5% observed for an area coverage of 50%. This was accompanied by a notable shift in the transition from the boundary to the mixed lubrication regime. The textured surfaces exhibited consistent efficiency in reducing fluid pressure and shear stress as the coverage of the textured areas increased. The incorporation of dimples on these surfaces played a crucial role by augmenting the lubricant storage capacity while concurrently reducing the real shear and contact areas. This study offers valuable insights into the nuanced friction-reducing mechanisms of surface textures, illuminating their influence on the coefficient of friction and the formation of lubricant films across various lubrication regimes.

DEVELOPMENT OF A MATHEMATICAL MODEL FOR ANALYZING THE ECONOMIC EFFECT OF THE MODIFIED DESIGN OF A RADIAL SLIDING BEARING AND ITS PREDICTION

In modern heavy-loaded friction units, various types of antifriction coatings are used on the surface of the bearing profile of the bearing sleeve to ensure high load-bearing capacity and increase the duration of the hydrodynamic regime, and consequently, the economic effect of sliding bearings.
 In the study, the creation and development of mathematical models of a radial sliding bearing with an antifriction polymer coating of the support surface containing a groove is carried out. The key role here is played by the method of analytical forecasting, which determines the mechanism of friction and the conditions for its transition from boundary to liquid.
 The aim of the study is to develop a mathematical model of a radial sliding bearing with a polymer coating of the bearing sleeve bearing surface containing an axial groove to predict the economic effect.
 The objective of the study is to develop new mathematical models and analyze the movement of lubricant in the working gap of a radial sliding bearing, taking into account design features to predict the economic effect.
 The difference from the existing models of tribosystems with antifriction coatings is the presence of a groove, which ensures stable ascent of the shaft on the hydrodynamic wedge and reduces wear during start-up and run-outs, hence increasing the service life of tribosystems.
 Using the lubricant motion equation and the continuity equation, new mathematical models are obtained that take into account the design features of the bearing sleeve surface – polymer coating, groove width, rheological properties of the lubricant used. Based on the results of an experimental study on the AI 5018 friction machine, the economic efficiency of net discounted income was calculated on samples in the form of partial inserts.
 As a result of the research, new mathematical models for engineering calculations have been developed that allow predicting the efficiency, reliability and duration of the hydrodynamic lubrication regime, as well as determining the main tribotechnical parameters. Net discounted income based on the results of theoretical and experimental research is estimated at 0.36 million rubles per year.

Experimental comparison of the transition speed of a hydrodynamic journal bearing lubricated with oil and magnetorheological fluid

A journal bearing test bench is used to find the transition speed between the hydrodynamic and mixed lubrication regimes for a modified magnetorheological (MR) fluid. It is shown that the transition speed of the bearing can be reduced by applying a local magnetic field near minimum film when it is lubricated with the MR fluid, and that this will only marginally increase friction. The lubricating performance of the MR fluid is compared to that of a reference oil, and all experimental results are compared with a Finite Element model based on the Reynolds equation.

Experimental Investigations to Study the Effectiveness of Cepstral Features to Detect Surface Fatigue Wear Development in a FZG Spur Geared System Subjected to Accelerated Tests

Gears are essential machine elements used to transmit power and motion from one unit to another under desired angular velocity ratio. Various types of gears have been developed to fulfill power transmission requirements in industrial applications. Under normal or fluctuating operating conditions, increase in fatigue load cycles, transition in lubrication regimes, fluctuating loads and speeds, etc., result in various surface fatigue wear modes which affect the performance of geared system. The severity of wear anomalies developed on gear tooth surfaces can be assessed by using vibration signals acquired from the gear box. On the other hand, reliable wear assessment is very important to perform maintenance action which depends on the sensors, data acquisition procedure, vibration signal analysis and interpretation. This paper presents results of the experimental investigations carried out to assess initiation and propagation of surface fatigue failure wear modes developed on gear tooth contact surfaces. A FZG back to back power recirculation type spur gearbox was used to conduct fatigue test experiments on spur gears under accelerated test conditions. Accelerated test conditions resulted in a rapid transition of lubrication regimes, i.e., hydrodynamic lubrication regime to boundary lubrication regime which triggered surface fatigue faults on gear tooth surfaces. A cepstral analysis method was used to assess fault severity in the geared system. The results obtained from the cepstral features were correlated to various surface fatigue faults and reduction in gear tooth stiffness. Results obtained from the experimental investigations highlighted the suitability of cepstral features to assess incipient faults developed on spur gear tooth surfaces.

Lubrication mechanisms of dispersed carbon microspheres in boundary through hydrodynamic lubrication regimes

HypothesisCarbon microspheres have been shown to reduce friction and surface wear at relatively low speeds and high applied loads (i.e., within the boundary lubrication regime). We hypothesize that in dilute colloidal lubricating systems there is an interplay between the size of the carbon microspheres and the lubrication gap size, which determines the dominant lubricating mechanism of the system. ExperimentsA 60 wt% aqueous glycerol solution was used as the base lubricant and compared to various carbon particle-based lubricant formulations ranging in particle concentrations from 0.05 to 0.30 vol%. The tribological properties of the various lubricant formulations were tested on a pin-on-disk tribometer. A simplified Stribeck plot was produced to understand the changing mechanism of lubrication over a wide range of conditions. FindingsThe Stribeck curves show that the carbon microspheres assist lubrication by a rolling mechanism primarily in the boundary lubrication regime. A 0.20 vol% carbon-based lubricant formulation showed the best friction reduction compared to the base lubricant. Increasing speed increases the lubricating gap between the friction pair beyond the size of the particles, thereby nullifying the rolling mechanism of the particles. We introduce a modified specific film thickness parameter to determine the lubrication regime in a particle-lubricant system.

Dynamic Behavior of a Tilting-Pad Thrust Bearing Operating Under a Hybrid Lubrication Regime

Abstract The present contribution aims to evaluate the effects of the oil pressure injection on the dynamic behavior of a hybrid-lubricated tilting-pad thrust bearing. First, a thermohydrodynamic model is applied to determine the equilibrium position as dictated by pressure and temperature distributions resulting from both hydrodynamic and hybrid lubrication regimes. Subsequently, the perturbation and frequency reduction methods are applied to determine the stiffness and damping coefficients of the bearing for various operational conditions. The results show that the dynamic force coefficients are strongly dependent on the operational conditions of the bearing, such as static load and rotating speed, as well as the perturbation frequency, for both lubrication regimes. The quantification of the injection pressure influence on the force coefficients of hybrid-lubricated thrust bearings is the main original contribution of this work.

Numerical simulation on startup performance of thrust bearing under different acceleration scenarios

PurposeThis paper aims to study the startup performance of thrust bearing. The effects of acceleration scenarios, roughness, the area ratio of texture and texture depth on the transient startup performance of the thrust bearing were analyzed.Design/methodology/approachThe lubrication model is solved by the Reynolds equation with the mass-conservation boundary condition. The Greenwood and Tripp contact model is used to predict asperity contact load. The finite volume method is used to discretize the governing equations.FindingsBy studying the bearing performance with different acceleration functions, it was found that the higher the acceleration at the beginning of the startup, the faster the thrust bearing operates under the hydrodynamic lubrication regime in the start stage. It appears that the friction and contact time of asperity increase with the increasing roughness. The optimal area ratio of texture is within 30%–50%. The depth of texture ranging from 1 to 2 is the best.Originality/valueThis paper proposes a transient mixed lubrication analysis model of the thrust bearing. This model can be used to analyze the variations of tribological performance and lubrication regime of the thrust bearing under different acceleration scenarios.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-09-2022-0268/

Synergic effect of TMD coating on textured steel using micro-EDM

Modifying the surface of materials through texturing and coating can have a significant impact on their tribological properties. In the present study, a hybrid surface modification process, texturing combined with coating, has been applied to a steel surface and the tribological behavior in lubricated conditions was studied. The surface of the steel was textured using micro-electric discharge machining (micro-EDM) to create two distinct patterns (rectangular and circular). Following this, transition metal dichalcogenides (TMD); tungsten-sulphur‑carbon (WSC) coating was applied to the surface using magnetron sputtering. To evaluate the specimens, they were subjected to tribological testing in various lubrication conditions using a block-on-ring configuration. The results were corroborated using a reciprocating sliding test and compared to those of a smooth uncoated specimen. The findings revealed that the tribological properties of the textured and coated specimens were enhanced, particularly in specimens with circular dimple patterns. The WSC coated circular dimple specimens showed a reduction of the COF by 7–12 %, in the boundary lubrication regime and by 9–25 %, in the mixed and hydrodynamic lubrication regimes as compared to the untreated specimen.

Thin films in hydrodynamic lubrication regime: The Osiris friction setup.

A model hydrodynamic lubrication tribometer consisting of two hydrodynamic journal bearings working under thin film conditions was developed to investigate the mechanisms of hydrodynamic friction with low-viscosity fluids and the role of surface effects. A small nominal radial clearance of about 5 µm was considered between the two surfaces. This fully instrumented setup provides in situ information on the sheared fluid film in terms of simultaneous measurements of film thickness; localization and extension of the cavitation zone, with a resolution of 30°; nominal friction torques up to 0.5 Nm, with an accuracy of 0.05 mNm; temperature; and the position of the shaft for velocities up to 12 000 rpm. To illustrate the capability of the Osiris tribometer, thin hydrodynamic film measurements were performed on smooth surfaces. The results are presented here, and the thermal effects, acceleration, and inertia contributions are discussed. Finally, the influence of the surface topography using textured surfaces was demonstrated and the role of adsorbed layers on the surface due to fluid formulation was highlighted.

Wear factors and mechanisms of L-80 steel casings

Casing wear in directional drilling is inevitable and may result in catastrophic failure of the casing column. It is thus essential to understand its mechanisms and quantify its extent by estimating the casing wear factor. In this research, actual field casing samples, drilling pipe joints and muds were considered. An in-house built testing facility was used to test several L-80 casing samples by considering three rotational speeds of the drill pipe tool joint (DP-TJ) (115, 154, and 207 rpm) and three side loads (1000 N, 1200 N and 1400 N). The influence of the water- and oil-based muds on the coefficient of friction, wear volume, wear factor and wear mechanisms were investigated. The results revealed that under water-based mud (WBM) lubrication casing wear volume and wear factors were more than twice that of oil-based mud (OBM) lubrication. Moreover, it was observed that as the side load increased under both OBM and WBM lubrication at a constant rpm, both wear volume and wear factor increased. However, increasing the rotational speed while maintaining the side load constant decreased the wear factor, owing to the localized softening effect caused by the high heat generation at the contact area and the possible hydrodynamic lubrication regime change at higher speeds. The analysis of the digital microscopic images taken at the wear region shows that abrasive mechanism was dominant in the case of OBM lubrication. On the other hand, both abrasive and adhesive wear mechanisms were present under WBM lubrication.

Simulation based analysis to study the effect of compression ring crown height in ring liner assembly on the performance of four stroke petrol engine

Among different piston rings of an Internal Combustion Engine (IC), the top compression ring-liner assembly shared a momentous amount of the total frictional losses, particularly at the Top Dead Center (TDC) and Bottom Dead Center (BDC) where boundary lubrication subsists. In the present study, the lubrication mechanism in piston ring cylinder liner conjunction for top compression ring using one dimensional Reynolds equation assuming axisymmetric contact was studied. The hydrodynamic and mixed lubrication regimes were taken during the study. Gumbel boundary conditions were used for Reynolds equation solution. For boundary pressure calculation, first, combustion chamber pressure variation was determined, then, interring pressure was calculated assuming orifice volume method by solving mass flow rate through crevice volumes using Runge-Kutta 4th order method. The Reynolds equation was solved using finite difference method. Whole solution process was repeated for different engine speed and crown height of top compression ring to study the effect of them in wear and power loss. First, the solution was found for 3000, 5000 and 7500 RPM keeping constant crown height of 10 µm. Average power loss was found to be increased with the increasing engine speed. There was increased in oil film thickness around mid-stroke with the increased engine speed, but there was no significant change in film thickness with the engine speed at the TDC and BDC positions. Then, the solution was again repeated for different RPM varying the crown height. The compression ring with lower crown height showed the better wear performance by increasing the minimum film thickness in critical zones of engine cycle for all RPM. The crown height of 5,7 and 9 µm gave the minimum power loss (77.89, 163.47 and 297.33 W) for 3000, 5000 and 7500 rpm. Moreover, the power loss in the range of 5-9 µm had no significant differences.

Fluid–Structure Interaction Analysis in Ball Bearings Subjected to Hydrodynamic and Mixed Lubrication

The mathematical and computational modeling of the lubricated contact between bearing surfaces is presented to analyze the sliding friction using a realistic 3D model on a microscopic scale. The fluid–structure interaction model evaluates the effects of lubricant film thickness on friction in hydrodynamic and mixed lubrication regimes. Higher contact pressures are seen at the peaks of asperities, especially during mixed lubrication, in which the fluid volume is smaller. Calculated friction coefficients from a homogenization procedure of shear and normal forces in the hydrodynamic and mixed lubrication, close to 0.0045 and 0.014, respectively, were accurate and within the range specified in the Stribeck curve. Results demonstrate the computational model allows examining the effects of lubrication on contact between rough surfaces.

Multiscale Wear Simulation in Textured, Lubricated Contacts

Specific surface textures may reduce the friction and increase the lifting forces in lubricated contacts. For the detrimental operating condition of mixed friction, wear is induced by the solid contact. In this study, a methodology for wear calculation in textured, lubricated contacts is presented that considers the wear-induced surface topography evolution. Based on the Reynolds differential equation, the mass-conserving cavitation model according to Jakobsson, Floberg, and Olsson (JFO), a wear-dependent asperity contact pressure curve and the wear equation according to Archard, wear in a wedge-shaped, textured lubrication gap was calculated. The results show the wear behavior of textured lubrication gaps. Based on the wear simulations, the tribological behavior of the textured surfaces compared to smooth surfaces is discussed. It is evident that textures, which improve the tribological performance in the hydrodynamic lubrication regime, are not necessarily associated with low wear values in a lubrication condition in the mixed friction regime. The analysis of the wear-dependent parameters initially showed a ‘recovery’ of the tribological system with increasing wear until the performance decreased again after a specific reversal point. This behavior is attributed to the relative position of the surface textures in the lubrication gap.

Tribological Performance of a Composite Cold Spray for Coated Bores

The tribological performance of a thermal sprayed, mirror-like surface with localized protuberances was investigated through tribotests and computational simulation. A composite coating with a 410L steel matrix and M2 tool steel hard particles was applied by the cold spray process as a bore coating for combustion engines. The presence of protuberances promoted the quick formation of an antifriction tribofilm when tested with an SAE 0W-16 containing ZDDP and MoDTC, which significantly reduced the asperity friction in comparison to the conventional engine coated bores in reciprocating tribological tests. An in-house computational model using deterministic numerical methods was used for the mixed and hydrodynamic lubrication regime. Lubricant film thickness and friction were simulated for a piston ring versus the proposed coating. The computer simulations showed that the protuberances reduced the hydrodynamic friction by increasing the otherwise very thin oil film thickness of mirror-like surfaces. Although not intuitive, this result was caused by the reducing of the oil film shear rate.