Elastohydrodynamic Lubrication Research Articles

Dynamics modeling and analysis of planetary gear mechanism under mixed elastohydrodynamic lubrication

Mixed elastohydrodynamic lubrication is the main lubrication style of planetary gear mechanism. In this paper, the dynamic characteristics of planetary gear mechanism under mixed elastohydrodynamic lubrication are investigated using a computational methodology. First, the mathematic model of mixed elastohydrodynamic lubrication is proposed, in which the scaling factors are introduced to depict the balance between the asperities and the oil film. A stiffness formula of the oil film is presented to describe the time varying stiffness of the teeth pair. Then, the stiffness of the oil film and the general stiffness are obtained. Finally, dynamic model of the planetary gear mechanisms is established and the dynamic responses of planetary gear mechanism are analyzed. The influences of operational parameters on stiffnesses and dynamic characteristics of the planetary gear mechanisms are investigated. The simulation results indicate that the nonlinear dynamics characteristics are significantly influenced by the lubrication conditions.

Multi-objective optimization of tribological properties of camshaft bearing pairs using DNN coupled with NSGA-II algorithm and TOPSIS

PurposeThis study aims to propose an optimization framework using deep neural networks (DNN) coupled with nondominated sorting genetic algorithm II and technique for order preference by similarity to an ideal solution method to improve the tribological properties of camshaft bearing pairs of internal combustion engine.Design/methodology/approachA lubrication model based on the theory of elastohydrodynamic lubrication and flexible multibody dynamics was developed for a V6 diesel engine. Setting DNN model as fitness function, the multi-objective optimization genetic algorithm and decision-making method were used to optimize the bearing pair structure with the goal of minimizing the total friction loss and the difference of the average values of minimum oil film thickness.FindingsThe results show that the lubrication state corresponding to the optimized bearing pair structure is elastohydrodynamic lubrication. Compared with the original structure, the optimized structure significantly reduces the total friction loss.Originality/valueThe optimized performance and corresponding structural parameters are obtained, and the optimization results were verified through multibody dynamics simulation.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-12-2023-0417/

Application of back propagation neural network in the analysis of isothermal elastohydrodynamic lubrication

Developing stable and high precision solution methods has always been the most important research topic in the field of elastohydrodynamic lubrication (EHL). The emergence of artificial neural network(ANN) provide new ideas for the research of EHL solving methods. In this article, a model based on back propagation neural network (BPNN) is proposed to obtain the film thickness and pressure distribution of EHL. The model is established to predict the EHL characteristics with the data obtained from numerical calculation, and more numerical data are used to validate the results predicted by the ANN model. The orthogonal experimental method is utilized to tune model’s parameters. In addition, the method of increasing hidden layers is used to enhance the predictive ability of model. Finally, a double layer BPNN is completed that could predict oil pressure and film thickness accurately. And the exploratory research presented in this paper will provide a simple method for predicting lubrication behavior in industrial applications.

Nonequilibrium molecular dynamics investigation on friction behavior of organic friction modifiers under dynamic load

The evolution of lubricating oil containing additives under dynamic conditions is very significant, since its oil film structure, velocity distribution, and adsorption characteristic have an obvious impact on friction and wear. In this work, a boundary lubrication (BL) and elastohydrodynamic lubrication (EHL) models under sinusoidal load were carried out by nonequilibrium molecular dynamics (NEMD), using iron oxide as the substrate, hexadecane, and stearic acid molecules as lubricants. It was discovered that the friction in the BL regime is positively correlated with the degree of additive’s solidification. The greater the solidification degree of additive layers, the less the interlacing between them, making the relative motion between them easier and reducing compression oscillation within them, thus diminishing friction in the BL regime. The degree of liquefaction of base oil and additive regions, as well as the velocity difference within their interlacing zones, are connected to the friction force in the EHL regime. At high frequencies as well as large amplitudes, the oil film has a very high degree of liquefaction, which facilitates internal shear in it, and there is essentially no relative motion between the base oil and additives, resulting in low friction.

EFFECT OF ENTRAINMENT SLIDING DIRECTION TO THE TRACTION COEFFICIENT FOR EHL ELLIPTICAL CONTACTS IN THE LINEAR ISOTHERMAL REGION

Abstract In many mechanical systems, elastohydrodynamic lubrication (EHL) is apparent in the interfaces be-tween rolling/sliding parts. Even at small slide-to-roll ratios, detailed knowledge about the oil film thickness and the traction exerted by the highly pressurized lubricant is crucial for the efficiency and reliability of such systems. In this study, we explore the impact of surface velocity direction and contact ellipticity on traction characteristics within EHL through numerical simulations using an elliptical EHL contact solver based on a fully-coupled finite-element approach. We present a novel master curve that correlates the traction coefficient slope with the lubricant entrainment direction and ellipticity, for el-liptical EHL contacts in the piezo-viscous elastic (PE) regime. We demonstrate that the master curve, shows a maximum relative difference of less than 3.2% to the numerical simulation results, proving its efficacy. This curve is particularly beneficial for multibody dynamics models, providing a reliable tool for predicting frictional forces in systems where elliptical EHL contacts, operating within the PE regime at low SRR, are prevalent.

Point Contact EHL and Wear in Intermittent Motion

Abstract Intermittent motion is a complex process that involves constant speed, deceleration, static stages and acceleration. Theoretical analysis suggests that shortening the period of intermittent motion can increase the film thickness during static stages, thereby extending the life of the part. Currently, an increasing number of studies are focusing on small oscillatory movements or vibrations. However, the impact of intermittent motion cycles on the film thickness and wear in the contact area still needs to be investigated. Optical interference and acoustic emission (AE) were employed as experimental methods to investigate simple sliding point contact intermittent motion. The lubrication state transition of full film-starvation-wear in the contact area was observed and the experimental results confirmed the correctness of the elastohydrodynamic lubrication (EHL) theoretical analysis. Additionally, the regularity of starvation and AE signal change with time during intermittent motion were summarized. An in-depth analysis of the reasons why intermittent motion with a short period generates less wear was performed. This analysis provides novel ideas to reduce wear of intermittent motion mechanisms. Overall, this research contributes to the understanding of the wear during intermittent motion and provides essential insights for wear reduction in this area.

Newton-GMRES Method for Thermal Elasto-hydrodynamic Lubrication of Line Contact Problems

Newton-GMRES Method for Thermal Elasto-hydrodynamic Lubrication of Line Contact Problems

Effect of Indentation at Rolling Body Surfaces on the Film Formation in Elastohydrodynamic Lubrication Contact under Dynamic Loads

Vibration is one of the phenomena that can occur during the operation of roller bearing which can lead to dynamic loads being subjected to the contact and thus, affect the film formation in elastohydrodynamic (EHD) contacts. On the other hand, surface textures such as artificial indentations and grooves greatly affect the film formation as well as the corresponding pressure distribution in the contact area, since the depths of the textures are usually much greater than the film thickness in the contact. This paper investigates the influence of an artificial indentation located on the rolling element, which passes through a point contact under dynamic loads by means of numerical simulations. Solutions to the Reynolds equation are performed and calculations of the elastic deformation equation, force balance equation and lubricant properties equations to show the effects of both indentation passing through the contact and a sinusoidal dynamic load on the film thickness as well as pressure profile of EHD point contact. Moreover, the effect of frequency and amplitude excitation of the dynamic load on the film formation was investigated. The results revealed that the artificial indentation under sinusoidal dynamic load of EHD point contact induced a significant effect on the thickness of the film formation and pressure distribution.

Understanding the Influences of Multiscale Waviness on the Elastohydrodynamic Lubrication Performance, Part II: The Partial-Film Condition

This paper is the second part of a two-part report studying the responses of a typical point-contact elastohydrodynamic lubrication (EHL) system to multiscale roughness mimicked by wavy surfaces. The wavy surfaces are defined by three key parameters: amplitudes, frequencies, and directions. The previous Part I paper focuses on the full film lubrication condition, while the current paper focuses on the partial film regime where asperity contacts occur. A transient thermal EHL model simulates lubrication problems with different waviness parameters, loads, and speeds. The total number of simulations is 1600. Performance parameters, including the asperity contact ratio, minimum film thickness, maximum pressure, central point film thickness, central point pressure, mean film thickness, coefficient of friction (COF), and the maximum temperature rise, are obtained for each simulation. These performance parameters are post-processed in the same manner as those in the previous Part I paper. The influences of the waviness parameters, load, and speed values on the eight performance parameters are discussed.

A Novel Non-Contact Method for Monitoring the Acoustic Emission From Mixed Elastohydrodynamic Lubrication Contacts

Abstract Lubricated non-conformal contacts, such as between gear teeth, operate with high levels of mixed lubrication, where the amount of direct asperity contact depends on operating parameters that influence the film thickness. Understanding of the levels of surface interaction is key to optimizing component life, and there is considerable interest in sensitive monitoring methods such as Acoustic Emission (AE). Researchers have shown that AE can detect subtle changes in lubrication conditions, using sensors mounted directly on the rotating gears. However, the use of such sensors is complex and unsuitable for implementation in real gearboxes. The alternative, of using sensors placed on housings, is hampered by signal attenuation and noise. This paper presents a novel, non-contact stationary sensor, coupled by an oil film to the rotating gear, which is shown to be capable of detecting important changes in lubrication conditions with significantly higher consistency and precision than housing-mounted sensors, whilst avoiding the complexities of gear-mounted sensors.

Numerical study on mixed lubrication performance of misaligned microgroove water-lubricated bearings considering cavitation and turbulence effects

This study presents a mixed lubrication model for misaligned microgroove water-lubricated bearings (WLBs). The model considers the effects of cavitation and turbulence to assess the mixed lubrication performance of WLBs with various microgroove morphologies. The equations are discretized using the control volume method (CVM) and solved by using the Fischer–Burmeister–Newton–Schur method for dealing with the constrained system. A comparison of the experimental data from the published literature demonstrates the model and methodology's validity. On this basis, the effects of rotational speed, load, and misalignment angle on the mixed lubrication performance of microgroove WLBs are investigated. The numerical results indicate that the left-triangular microgroove exhibits the best-mixed lubrication performance. Under elastohydrodynamic lubrication and mixed lubrication conditions, there are significant discrepancies in microgroove morphology. However, this discrepancy diminishes with increasing misalignment angles. The edge contact problem resulting from journal misalignment can be efficiently mitigated by selecting the proper microgroove morphology. This study provides useful guidance for the optimal design and mixed lubrication performance improvement of WLBs.

Non-Newtonian thermal elastohydrodynamic mixed lubrication in elliptical contact for nutation-driven double circular-arc spiral bevel gear pair

The new type of nutation reducer with double circular-arc bevel gears as the core is characterized by advantages such as high load-bearing capacity, large transmission ratio, and simple structure. The existing research focuses on the machining methods of gears and the analysis of tooth contact. This article first time establishes an elastohydrodynamic mixed lubrication model for nutation double circular-arc bevel gear. The model's accuracy is verified by comparing it with existing test results. On this basis, combined with the finite element method and tooth contact analysis, the meshing characteristics, tooth contact characteristics, and lubrication characteristics of the double circular-arc bevel gear tooth surface are analyzed. The results show that the entrainment velocity at the contact point of the tooth surface during transmission is much higher than the sliding velocity. Consequently, the meshing motion is close to pure rolling. In constant-speed transmission, the oil film thickness at the engaging-in and exit points near the edge of the tooth surface is smaller than that at other meshing points on the tooth surface. This phenomenon can easily lead to mixed lubrication and lubrication failure, as well as tooth surface wear. During deceleration, i.e., when the speed drops to 50 r/min, the oil film thickness approaches zero, indicating that the gear has entered a mixed lubrication state. The proposed model can be used for lubrication analysis and improving transmission accuracy of nutation double circular-arc bevel gears.

Implementation of a New Smart-Lubricant Model Using Generalized Newtonian Approach in Soft Elastohydrodynamic Lubrication

Abstract This is an inceptive attempt to replace the classical Bingham fluid model with a continuous double Newtonian power-law-based constitutive equation for smart lubricants like magneto-rheological, electro-rheological, and ferro-fluids. The implementation of Bingham model in hydrodynamic (HD) and elastohydrodynamic (EHD) lubrication analyses is highly challenging and inconvenient due to its inherent discontinuity. Therefore, the present work demonstrates the use of an already existing rheological model with an appropriate set of parameters to describe the flow behavior of smart lubricants in a soft-EHD lubrication algorithm based on the generalized Newtonian approach. The formation of both floating and adherent cores validates the proposed model. An extensive parametric study is also performed to explore the effects of operating speed, load, and slide-to-roll ratio on the soft-EHL characteristics. The results are very promising, showing that it's possible to customize smart lubricants to match specific operating conditions. This is achieved by adjusting the yield stress value accordingly, allowing for the desired variation in lubrication characteristics.

Coupling Study on Quasi-Static and Mixed Thermal Elastohydrodynamic Lubrication Behavior of Precision High-Speed Machine Spindle Bearing with Spinning

In this work, a modified numerical algorithm that couples the quasi-static theory with the mixed thermal elastohydrodynamic lubrication (mixed-TEHL) model is proposed to examine the mechanical properties and lubrication performance of the spindle bearing that is used in a high-speed machine tool with spinning. The non-Newtonian fluid characteristics of the lubricant and the non-Gaussian surface roughness are also considered. Moreover, the mechanical properties and lubrication state of the bearing are examined in various service environments. The results indicate that the temperature reduces the lubrication efficiency, which in turn exerts a significant impact on the mechanical properties. The lubrication that either behaves in the manner of Newtonian or non-Newtonian fluid has a relatively negligible influence on the bearing working state, while the non-Gaussian surface roughness significantly alters the oil film thickness and temperature. Calculations with different operating conditions demonstrate that the operating parameters (i.e., axial load, rotation speed) will directly affect the performance of the bearings via the changes in the oil film thickness and the temperature.

A stiffness model for EHL contact on smooth/rough surfaces and its application in mesh stiffness calculation of the planetary gear set

A method for calculating time-varying mesh stiffness (TVMS) is proposed by developing a stiffness model for elastohydrodynamic lubrication (EHL) contact and integrating it into three-dimensional loaded tooth contact analysis (3D-LTCA). To enhance the accuracy and efficiency of the calculation, three projects have been implemented: Firstly, the contact deformation is modified, and the average slope is employed for calculating contact stiffness. Secondly, the lubricant film and EHL contact deformation are introduced into the deformation compatibility equations. Finally, the polynomial chaos expansion (PCE) surrogate model is employed to enhance computational efficiency. The proposed method is used to calculate TVMS of the planetary gear set, and the influence of speed, torque, and rough surface are analyzed. The results show that the previous method underestimating the TVMS of planet-ring gear pair in double-teeth meshing area under fluid lubrication, while the proposed method effectively addresses the issue. The different effects of the torque and speed on TVMS under mixed and fluid lubrication also have been revealed, emphasizing the significance of surface topography in gear meshing characteristics.

Application of the Hybrid Model to elastohydrodynamic lubrication (EHL) Reference Liquids for Pressure Extrapolation

Reference liquids can be useful in the new quantitative approach to elastohydrodynamic lubrication calculations only if the properties of these liquids are accurately modeled. The pressures in the contact sometimes exceed the pressures of the available viscometer measurements, requiring extrapolation. The previtreous piezoviscous response is always faster-than-exponential with an apparent divergence pressure. Therefore, any model used for extrapolation must possess these characteristics and up to now published work in EHL, where extrapolation of the viscosity of squalane is performed, has not. To date, the only model that has been shown to accurately extrapolate to high pressure is the Hybrid model. This expression was applied to six reference liquids for use in quantitative investigations in EHL. Viscosity measurements for trioctyl trimellitate were extended to 1.1 GPa. The results obtained with high pressure viscometers are preferable over molecular dynamics results if they are different. Molecular dynamics calculations are not currently capable of reliable results for viscosity at very high pressures.

Quantitative Elastohydrodynamic Lubrication—Seventeen Years In

Abstract Seventeen years have passed since the first full elastohydrodynamic lubrication (EHL) simulation employed the real pressure and shear dependence of viscosity measured in viscometers to accurately predict film thickness and friction. This is the appropriate time to enumerate the advances in understanding brought on by the application of high-pressure rheology to the EHL problem. The pressure dependence of the low-shear viscosity, which has been measured in viscometers for nearly a century, differs from the narratives taught to tribology students and often used to justify inaccurate models. The central film thickness often depends on the shear-thinning at low pressure and time–temperature–pressure superposition demands that the same shear dependence be active at the high pressure where friction is generated. In this article, some of the revelations resulting from quantitative EHL are reviewed. For example, it has been discovered that the minimum film thickness in point contacts depends upon the viscosity at the highest pressures of the contact. This explains the errors in the classical formulas, which were based upon the fictional narratives concerning piezoviscous response, and the assumption of film thickness governed by inlet conditions. Quantitative EHL provides quantitative predictions of contact behavior.

Research on Prediction Methods for Eccentric Wear of the Slipper Pair in Axial Piston Pumps and Lubrication Performance Analysis

Abstract The axial piston pump (APP) is the core power component of hydraulic systems. Its friction pair wear can cause the degradation of piston pump performance until the function is completely lost or the service life is terminated. The slipper pair, prone to wear failure, is selected as the research object in this paper. The prediction method for the eccentric wear of the slipper pair is established, and the gradual change rules of the performance with wear accumulation are explored. The micro-surface rough peak contact and the stress state of the slipper pair are analyzed, and the mixed lubrication model of the slipper pair is established based on the elastohydrodynamic lubrication (EHL) model of the slipper pair. Grid discretization of a sealing belt of the slipper pair is carried out based on two-body abrasive wear and Archard adhesive wear models to improve the prediction method for eccentric wear of the slipper pair. The effectiveness of the proposed method is verified by a surface morphology analysis of the worn slipper. The results showed that the wear depth and wear width of the outer edge of the slipper bottom surface are positively correlated with the oil discharge pressure and the inclination angle of the swashplate, and negatively correlated with the rotational speed and the dynamic oil viscosity. The outer edge of the slipper is wedged after eccentric wear, and the hydrodynamic effect of the lubricating oil film of the slipper pair is enhanced. Hence, proper wear can improve the lubrication performance of the slipper pair.

ANALYSIS OF THE MIXED ELASTOHYDRODYNAMIC LUBRICATION CHARACTERISTICS OF DOUBLE CIRCULAR ARC GEARS CONSIDERING MICRO-MORPHOLOGY

Different machining methods result in diverse morphologies on the surface of gears. Under mixed lubrication, the friction and contact behavior of the real morphology interface becomes more complex. This study aims to analyze the influence of different process tooth surface micro-morphologies on the lubrication performance of the meshing area of double circular arc gears. Focusing on elastohydrodynamic lubrication (EHL) characteristics of the double circular arc gears as the research object, the study involved measuring the three-dimensional morphology of the meshing position of the double circular arc gears. Non-Gaussian simulation technology was then used to characterize the micromorphology of the tooth surface resulting from different processes (i.e. hobbing process and skiving process). The tooth surface suction speed was determined based on the meshing characteristics of double circular arc gears. Based on the theory of point contact EHL, Reynolds equation and film thickness equation considering micromorphology were derived, and numerical solutions were obtained using the multi-grid method. A point contact EHL model for double circular arc gears was established considering the micromorphology of tooth surfaces in different processes. Results show that, the oil film pressure is positively correlated with rotational speed and torque. The oil film thickness is positively correlated with rotational speed, and negatively correlated with input torque. The negative of micro-morphology on the tooth surface on the lubrication performance could lead to an increase in pressure fluctuations and the absence of the second pressure peak. The uneven distribution of oil film and the lager roughness value could result in obvious fluctuations. The lubrication performance of the contact area in the gear hobbing process is superior to that in the gear skiving process. This study lays the foundation for further exploring the coupling characteristics between the double circular arc gear transmission system and elastic fluid dynamic lubrication. Keywords: Double circular arc gears, Point contact, Mixed EHL, Lubrication characteristics, Micro- morphology.

Influence of cashew nut shell liquid on corrosion and tribocorrosion behavior of metallic alloys

Cashew Nut Shell Liquid (CNSL) has been explored in several applications within the sustainability principles and circular economy of agro-industrial product waste. To understand the anticorrosive and lubricant properties of CNSL solutions, a multi-faceted approach, incorporating electrochemical analyses, immersion test, tribological measurements, and tribochemical characterization was carried out for mild steel AISI 1012, stainless steel AISI 420 and Aluminium Alloy 6061. Electrochemical analyses reveal a positive impact of CNSL, inducing a shift in corrosion potential to more positive values and a decrease in corrosion current, indicating effective corrosion inhibition for stainless steel and aluminium alloy. In mild steel, CNSL exhibits a mixed-type inhibition with efficiency increasing in correlation with oil concentration. The lubricating properties of CNSL are evident according to the coefficients of friction (COF) obtained and the elastohydrodynamic lubrication regime observed during tribological tests. Tribochemical tests demonstrate a reduction in wear and an improvement in tribocorrosion behavior under sliding conditions. CNSL emerges as a promising tribocorrosion mitigator, demonstrating multifaceted benefits. The concentration-dependent effects highlight the need for optimization in specific applications, particularly for passive materials. CNSL not only inhibits corrosion through film formation but also provides effective lubrication, reducing friction, wear, and chemical-mechanical degradation. This research contributes valuable insights to corrosion science. It proposes practical applications for CNSL in diverse industrial contexts, showcasing its potential as an environmentally friendly and sustainable solution for tribocorrosion challenges.