The Experts below are selected from a list of 96 Experts worldwide ranked by ideXlab platform
Q. Jane Wang - One of the best experts on this subject based on the ideXlab platform.
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Experimental and Numerical Investigations of the Stribeck Curves for Lubricated Counterformal Contacts
Journal of Tribology-transactions of The Asme, 2016Co-Authors: Tao He, Jiaxu Wang, Q. Jane WangAbstract:The Stribeck curve is an important means to demonstrate the frictional behavior of a lubricated interface during the entire transition from boundary and mixed to full-film lubrication. In the present study, a new test apparatus has been built that can operate under rolling–sliding conditions at a continuously variable speed in an extremely wide range, approximately from 0.00006 to 60 m/s, covering six orders of magnitude. Hence, a complete Stribeck curve can be measured to reveal its basic characteristics for lubricated Counterformal contacts. The measured curves are compared with numerical simulation results obtained from an available unified mixed elastohydrodynamic lubrication (EHL) model that is also capable of handling cases during the entire transition. A modified empirical model for the limiting shear stress of lubricant is obtained, and a good agreement between the measured and calculated Stribeck curves is achieved for the tested base oils in all the three lubrication regimes, which thus well validates the simulation methods employed. Both the experimental and numerical results indicate that the Stribeck curves for Counterformal contact interfaces behave differently from those for conformal contacts. When the rolling speed increases at a fixed slide-to-roll ratio, the friction continuously decreases even in the full-film lubrication regime due to the reduction of the lubricant limiting shear stress caused mainly by the rise of the surface flash temperature. In addition, the test results indicate that the boundary additives in a commodity lubricant may have considerable influence on the boundary lubrication friction but that on the friction in the mixed and full-film lubrication appears to be limited.
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On the Stribeck Curves for Lubricated Counterformal Contacts of Rough Surfaces
Journal of Tribology-transactions of The Asme, 2015Co-Authors: Jiaxu Wang, Q. Jane WangAbstract:The “Stribeck curve” is a well-known concept, describing the frictional behavior of a lubricated interface during the transition from boundary and mixed lubrication up to full-film hydrodynamic/elastohydrodynamic lubrication. It can be found in nearly every tribology textbook/handbook and many articles and technical papers. However, the majority of the published Stribeck curves are only conceptual without real data from either experiments or numerical solutions. The limited number of published ones with real data is often incomplete, covering only a portion of the entire transition. This is because generating a complete Stribeck curve requires experimental or numerical results in an extremely wide range of operating conditions, which has been a great challenge. Also, numerically calculating a Stribeck curve requires a unified model with robust algorithms that is capable of handling the entire spectrum of lubrication status. In the present study, numerical solutions in Counterformal contacts of rough surfaces are obtained by using the unified deterministic mixed elastohydrodynamic lubrication (EHL) model recently developed. Stribeck curves are plotted in a wide range of speed and lubricant film thickness based on the simulation results with various types of contact geometry using machined rough surfaces of different orientations. Surface flash temperature is also analyzed during the friction calculation considering the mutual dependence between friction and interfacial temperature. Obtained results show that in lubricated concentrated contacts, friction continuously decreases as speed and film thickness increase even in the full-film regime until extremely high speeds are reached. This is mainly due to the reduction of lubricant limiting shear stress caused by flash temperature rise. The results also reveal that contact ellipticity and roughness orientation have limited influence on frictional behaviors, especially in the full-film and boundary lubrication regimes.
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Transient Thermomechanical Analysis of Sliding Electrical Contacts of Elastoplastic Bodies, Thermal Softening, and Melting Inception
Journal of Tribology-transactions of The Asme, 2009Co-Authors: Wei Chen, Q. Jane WangAbstract:Sliding electrical contacts are found in many electromechanical devices, such as relays, switches, and resistance spot welding. Temperature rise due to sliding friction and electrical current may be the major source of sliding electrical contact deterioration. This paper reports the development of a three-dimensional thermo-elasto-plastic contact model of Counterformal bodies, which takes into account transient heat flux, temperature-dependent strain hardening behavior, and a realistic heat partition between surfaces. Transient contact simulations induce a significant increase in computational burden. The discrete convolution and fast Fourier transform and the conjugate gradient method are utilized to improve the computation efficiency. The present model is used to study the case of a half-space sliding over a stationary sphere, and both are made of 7075 aluminum alloy; the contact resistance is considered mainly due to the surface oxide film. The simulation results indicate that the transient contact model is able to capture the history of plastic deformation accumulation and the material melting inception.
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Thermomechanical Analysis of Elastoplastic Bodies in a Sliding Spherical Contact and the Effects of Sliding Speed, Heat Partition, and Thermal Softening
Journal of Tribology-transactions of The Asme, 2008Co-Authors: Wei Chen, Q. Jane WangAbstract:A thermomechanical analysis of elasto-plastic bodies is a necessary step toward the understanding of tribological behaviors of machine components subjected to both mechanical loading and frictional heating. A three-dimensional thermoelastoplastic contact model for Counterformal bodies has been developed, which takes into account steady state heat flux, temperature-dependent strain hardening behavior, and interaction of mechanical and thermal loads. The fast Fourier transform and conjugate gradient. method are the underlying numerical algorithms used in this model. Sliding of a half-space over a stationary sphere is simulated with this model. The friction-induced heat is partitioned into two bodies based on surface temperature distributions. In the simulation, the sphere is considered to be fully thermoelastoplastic, while the half-space is treated to be thermoelastic. Simulation results include surface pressure, temperature rise, and subsurface stress and plastic strain fields. The paper also studies the influences of sliding speed and thermal softening on contact behaviors for sliding speed ranging three orders of magnitude.
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Transient thermo-elasto-plastic spherical contact analyses considering effects of thermal softening and heat partition
STLE ASME 2008 International Joint Tribology Conference, 2008Co-Authors: Wei Chen, Q. Jane WangAbstract:Frictional heating leads to the temperature rise, thermal expansion, and the thermo-elasto-plastic deformation, which may be responsible for the failure of components under contact and relative motion. This paper reports the development of a three-dimensional thermo-elasto-plastic contact model of Counterformal bodies, which account for the transient heat transfer, temperature-dependent strain hardening behavior of materials, and realistic heat partition between surfaces. An extensive study on the contact of a sliding half-space over a stationary ball is conducted using this model.Copyright © 2008 by ASME
Homer Rahnejat - One of the best experts on this subject based on the ideXlab platform.
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Physics of ultra-thin surface films on molecularly smooth surfaces
Proceedings of the Institution of Mechanical Engineers Part N: Journal of Nanoengineering and Nanosystems, 2006Co-Authors: Mircea Teodorescu, Sashi Balakrishnan, Homer RahnejatAbstract:This paper investigates the physics of ultra-thin surface films in nano-scale Counterformal conjunctions. A novel approach is proposed in which the transient behaviour of the fluid film is integrated with the contact mechanics of the approaching bodies. The method predicts film thickness and pressure distribution, as well as local elastic deflection and resulting sub-surface stress tensor. It is found that inertial dynamics of bodies, as determined by local squeeze film effect, prevents diminution of film below certain thickness and reduces the solvation effect. This approach has direct applications for micro-scale mechanisms, where prediction of thin surface adhered film thickness is required for tribological purposes, as well as structural in-service integrity of contacts of vanishing dimensions.
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harmonic analysis to determine contact characteristics of concentrated Counterformal contacts
Volume!, 2004Co-Authors: Mircea Teodorescu, Homer Rahnejat, R GoharAbstract:Contact mechanics of solids of revolution is characterised by their deformation behaviour under load. This is strongly influenced by their geometry and elastic properties. These parameters and the applied load determine the deformation of the contiguous solids, giving rise to contact pressure distribution and sub-surface stress fields, which are necessary to determine fatigue spalling performance. Load bearing surfaces are usually lubricated and the deformation of contiguous solids is often crucial in providing a gap for lubricant film formation and avoidance of asperity interactions on adjacent surfaces and the ensuing wear. Therefore, determination of contact deformation is essential in prediction of contact conditions. This usually requires the solution of the elasticity integral in the form of elliptic functions, which are discretised and achieved through time intensive numerical methods. In lubricated Counterformal contacts under high loads and with materials of high elastic moduli, this amounts for the major computing resource requirement within any form of analysis, such as the usual elastohydrodynamic lubrication. The paper shows that any arbitrary pressure distribution over a given contact area may be represented by a harmonic series. The response of the elastic solids to the application of such a harmonic series leads to the evaluation of their contact deformation and sub-surface stress field of also a harmonic nature. The repercussion of this approach is that for a given applied contact load, harmonic analysis may be employed in order to analytically obtain the same predictions as those with much more time consuming numerical analysis. The paper proves the analytical approach by comparison with the case of an infinite line contact, or a one-dimensional contact, for which analytic solution based on the Hertzian theory exists as a classical case. Then, the conformance of the methodology to deviations of surface friction. An advantage of the method over those reported in literature is the simultaneous evaluation of the local contact deformation, as well as the sub-surface stress field. This approach can be extended to the case of rough surfaces, where the harmonic analysis may be used as an approximation.© 2004 ASME
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Nano-lubricant film formation due to combined elastohydrodynamic and surface force action under isothermal conditions
Proceedings of the Institution of Mechanical Engineers Part C: Journal of Mechanical Engineering Science, 2001Co-Authors: M. F. Abd-alsamieh, Homer RahnejatAbstract:This paper presents the results of numerical prediction of the lubricant film thickness and pressure distribution in concentrated Counterformal point contact under isothermal conditions. The operating conditions, which include load and speed of entraining motion, promote the formation of ultra-thin films; these are formed under the combined action of elastohydrodynamic lubrication (EHL), the surface contact force of solvation and molecular interactions due to the presence of Van der Waals forces. A numerical solution has been carried out, using the low-relaxation Newton-Raphson iteration technique, applied to the convergence of the hydrodynamic pressure. The paper shows that the effect of surface forces become significant as the elastic film (i.e. the gap) is reduced to a few nanometres. The numerical predictions have been shown to conform well to the numerical work and experimental findings of other research workers.
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Ultra-thin lubricating films under transient conditions
Journal of Physics D, 2001Co-Authors: M. F. Al-samieh, Homer RahnejatAbstract:The mechanism of fluid film lubrication in ultra-thin conjunctions under Counterformal concentrated circular point contacts is discussed in this paper. The significance of squeeze film action in increased load carrying capacity is highlighted. The numerical predictions made in acceleration-deceleration motion of a ball against a flat race is found to conform with the experimental findings of other research workers for polar branched lubricants down to a film thickness in the region of 20 nm. This conformance with the experimental findings has enabled a fundamental understanding of the multi-physics of film formation when conditions promoting the formation of ultra-thin films of the order of few nanometres have been employed, with the use of a non-polar lubricant. This paper provides the first ever solution of the combined effect of viscous, surface and molecular forces under transient conditions.
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Valve-train dynamics: a simplified tribo-elasto-multi-body analysis
Proceedings of the Institution of Mechanical Engineers Part K: Journal of Multi-body Dynamics, 2000Co-Authors: M. Kushwaha, Homer RahnejatAbstract:AbstractThis paper presents a model of a cycloidal cam-flat follower pair. The model incorporates the inertial elements, the assembly constraint functions and the sources of compliance in the valve train. The sources of compliance include the valve spring characteristics, including the spring surge effect under dynamic conditions, as well as the contact compliance between the cam and the flat follower. The contact domain is treated as a Counterformal concentrated lubricated region subjected to an elasto-hydrodynamic regime of lubrication (EHL). The prevailing contact geometry is one of finite line contact.The paper presents the results of simultaneous solution of the Lagrangian dynamics for the non-linear constrained system, together with an approximate quasi-static elastohydrodynamic solution of the lubricated contact conjunction at each time step by an extrapolated oil-film thickness formula for combined entraining and squeeze film action. The effect of spring surge on the contact separation and residua...
Wei Chen - One of the best experts on this subject based on the ideXlab platform.
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Transient Thermomechanical Analysis of Sliding Electrical Contacts of Elastoplastic Bodies, Thermal Softening, and Melting Inception
Journal of Tribology-transactions of The Asme, 2009Co-Authors: Wei Chen, Q. Jane WangAbstract:Sliding electrical contacts are found in many electromechanical devices, such as relays, switches, and resistance spot welding. Temperature rise due to sliding friction and electrical current may be the major source of sliding electrical contact deterioration. This paper reports the development of a three-dimensional thermo-elasto-plastic contact model of Counterformal bodies, which takes into account transient heat flux, temperature-dependent strain hardening behavior, and a realistic heat partition between surfaces. Transient contact simulations induce a significant increase in computational burden. The discrete convolution and fast Fourier transform and the conjugate gradient method are utilized to improve the computation efficiency. The present model is used to study the case of a half-space sliding over a stationary sphere, and both are made of 7075 aluminum alloy; the contact resistance is considered mainly due to the surface oxide film. The simulation results indicate that the transient contact model is able to capture the history of plastic deformation accumulation and the material melting inception.
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Thermomechanical Analysis of Elastoplastic Bodies in a Sliding Spherical Contact and the Effects of Sliding Speed, Heat Partition, and Thermal Softening
Journal of Tribology-transactions of The Asme, 2008Co-Authors: Wei Chen, Q. Jane WangAbstract:A thermomechanical analysis of elasto-plastic bodies is a necessary step toward the understanding of tribological behaviors of machine components subjected to both mechanical loading and frictional heating. A three-dimensional thermoelastoplastic contact model for Counterformal bodies has been developed, which takes into account steady state heat flux, temperature-dependent strain hardening behavior, and interaction of mechanical and thermal loads. The fast Fourier transform and conjugate gradient. method are the underlying numerical algorithms used in this model. Sliding of a half-space over a stationary sphere is simulated with this model. The friction-induced heat is partitioned into two bodies based on surface temperature distributions. In the simulation, the sphere is considered to be fully thermoelastoplastic, while the half-space is treated to be thermoelastic. Simulation results include surface pressure, temperature rise, and subsurface stress and plastic strain fields. The paper also studies the influences of sliding speed and thermal softening on contact behaviors for sliding speed ranging three orders of magnitude.
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Transient thermo-elasto-plastic spherical contact analyses considering effects of thermal softening and heat partition
STLE ASME 2008 International Joint Tribology Conference, 2008Co-Authors: Wei Chen, Q. Jane WangAbstract:Frictional heating leads to the temperature rise, thermal expansion, and the thermo-elasto-plastic deformation, which may be responsible for the failure of components under contact and relative motion. This paper reports the development of a three-dimensional thermo-elasto-plastic contact model of Counterformal bodies, which account for the transient heat transfer, temperature-dependent strain hardening behavior of materials, and realistic heat partition between surfaces. An extensive study on the contact of a sliding half-space over a stationary ball is conducted using this model.Copyright © 2008 by ASME
J.m. Wang - One of the best experts on this subject based on the ideXlab platform.
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Non-Hertzian conformal contact at wheel/rail interface
Proceedings of the 1995 IEEE ASME Joint Railroad Conference, 1995Co-Authors: Huimin Wu, J.m. WangAbstract:The Transportation Technology Center conducted an investigation to study non-Hertzian conformal contact, roller conformal contact, and Hertzian Counterformal contact at the wheel/rail interface. The computational results show that a significant error (up to 72 percent) in both stress distribution and contact area may be produced by using the approximate Hertzian conformal solution to solve non-Hertzian conformal contact, which is the current practice in most vehicle dynamic simulation models. An even greater error (up to 400 percent) may be produced by using the Hertzian Counterformal solution to solve non-Hertzian conformal contact. The study also shows that, in elastic contact, the separation function of two contact bodies plays a dominant role in the determination of contact stresses and area. It is not proper to present the separation function by a quadratic function in non-Hertzian conformal contacts.
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non hertzian conformal contact at wheel rail interface
IEEE ASME Joint Railroad Conference, 1995Co-Authors: Huimin Wu, J.m. WangAbstract:The Transportation Technology Center conducted an investigation to study non-Hertzian conformal contact, roller conformal contact, and Hertzian Counterformal contact at the wheel/rail interface. The computational results show that a significant error (up to 72 percent) in both stress distribution and contact area may be produced by using the approximate Hertzian conformal solution to solve non-Hertzian conformal contact, which is the current practice in most vehicle dynamic simulation models. An even greater error (up to 400 percent) may be produced by using the Hertzian Counterformal solution to solve non-Hertzian conformal contact. The study also shows that, in elastic contact, the separation function of two contact bodies plays a dominant role in the determination of contact stresses and area. It is not proper to present the separation function by a quadratic function in non-Hertzian conformal contacts. >
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A Deterministic Conformal/Counterformal Microcontact Model
Journal of Tribology-transactions of The Asme, 1994Co-Authors: V. Aronov, S. Nair, J.m. WangAbstract:Microcontact models, describing contact of two rough surfaces, are fundamental for the modeling of friction and wear. This paper presents a critical analysis of existing models and discusses their limitations. A new deterministic microcontact model based on conformal/Counterformal contact of asperities, as opposed to the combined surface statistics and Counterformal asperity contact, is presented. Based on this model, a computer program has been developed. The input data are the digitized 3-D topographies, separately measured from the two contacting surfaces. The program first determines the original mating position and then calculates the surface contact parameters: contact radius, contact pressure, real contact area and the number of elastic, plastic, conformal, and Counterformal contacts.
Jiaxu Wang - One of the best experts on this subject based on the ideXlab platform.
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Effect of surface topography associated with arbitrary velocity direction on the lubrication film thickness in elliptical contacts
Industrial Lubrication and Tribology, 2018Co-Authors: Wei Pu, Jiaxu Wang, Guangwu Zhou, Ke Xiao, Junyang LiAbstract:Purpose The purpose of this study is to describe and observe the effect of surface topography associated with arbitrary directions of rolling and sliding velocities on the performance of lubricating films in elliptical contacts. Design/methodology/approach The most recently published mixed elastohydrodynamic (EHL) model by Pu and Zhu is used. Three different machined rough surfaces are discussed and the correlated inclined angle of surface velocity varies from 0° to 90° in the analyzed cases. These cases are carried out in a wide range of speeds (five orders of magnitude) while the simulated lubrication condition covers full-film and mixed EHL down to the boundary lubrication. Findings The results indicate that the variation of the average film thickness corresponding to different entrainment angles is distinct from those without considering surface roughness. In addition, the surface topography appears to have an immense effect on the lubrication film thickness in the exceptive situation. Originality/value This paper has not been published previously. Surface roughness has attracted much attention for many years owing to the significant influence on lubricating property. However, previous studies mainly focus on the Counterformal contact with the same direction between surface velocity and principal axis of the contact zone. Little attention has been paid to the specific condition with the arbitrary direction of rolling and sliding velocities found in hypoid gears and worm, and some other components. The purpose of this study is to describe and observe the effect of surface topography associated with arbitrary directions of rolling and sliding velocities on the performance of lubricating films in elliptical contacts based on the most recently published mixed EHL model by Pu and Zhu.
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Experimental and Numerical Investigations of the Stribeck Curves for Lubricated Counterformal Contacts
Journal of Tribology-transactions of The Asme, 2016Co-Authors: Tao He, Jiaxu Wang, Q. Jane WangAbstract:The Stribeck curve is an important means to demonstrate the frictional behavior of a lubricated interface during the entire transition from boundary and mixed to full-film lubrication. In the present study, a new test apparatus has been built that can operate under rolling–sliding conditions at a continuously variable speed in an extremely wide range, approximately from 0.00006 to 60 m/s, covering six orders of magnitude. Hence, a complete Stribeck curve can be measured to reveal its basic characteristics for lubricated Counterformal contacts. The measured curves are compared with numerical simulation results obtained from an available unified mixed elastohydrodynamic lubrication (EHL) model that is also capable of handling cases during the entire transition. A modified empirical model for the limiting shear stress of lubricant is obtained, and a good agreement between the measured and calculated Stribeck curves is achieved for the tested base oils in all the three lubrication regimes, which thus well validates the simulation methods employed. Both the experimental and numerical results indicate that the Stribeck curves for Counterformal contact interfaces behave differently from those for conformal contacts. When the rolling speed increases at a fixed slide-to-roll ratio, the friction continuously decreases even in the full-film lubrication regime due to the reduction of the lubricant limiting shear stress caused mainly by the rise of the surface flash temperature. In addition, the test results indicate that the boundary additives in a commodity lubricant may have considerable influence on the boundary lubrication friction but that on the friction in the mixed and full-film lubrication appears to be limited.
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On the Stribeck Curves for Lubricated Counterformal Contacts of Rough Surfaces
Journal of Tribology-transactions of The Asme, 2015Co-Authors: Jiaxu Wang, Q. Jane WangAbstract:The “Stribeck curve” is a well-known concept, describing the frictional behavior of a lubricated interface during the transition from boundary and mixed lubrication up to full-film hydrodynamic/elastohydrodynamic lubrication. It can be found in nearly every tribology textbook/handbook and many articles and technical papers. However, the majority of the published Stribeck curves are only conceptual without real data from either experiments or numerical solutions. The limited number of published ones with real data is often incomplete, covering only a portion of the entire transition. This is because generating a complete Stribeck curve requires experimental or numerical results in an extremely wide range of operating conditions, which has been a great challenge. Also, numerically calculating a Stribeck curve requires a unified model with robust algorithms that is capable of handling the entire spectrum of lubrication status. In the present study, numerical solutions in Counterformal contacts of rough surfaces are obtained by using the unified deterministic mixed elastohydrodynamic lubrication (EHL) model recently developed. Stribeck curves are plotted in a wide range of speed and lubricant film thickness based on the simulation results with various types of contact geometry using machined rough surfaces of different orientations. Surface flash temperature is also analyzed during the friction calculation considering the mutual dependence between friction and interfacial temperature. Obtained results show that in lubricated concentrated contacts, friction continuously decreases as speed and film thickness increase even in the full-film regime until extremely high speeds are reached. This is mainly due to the reduction of lubricant limiting shear stress caused by flash temperature rise. The results also reveal that contact ellipticity and roughness orientation have limited influence on frictional behaviors, especially in the full-film and boundary lubrication regimes.
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A Theoretical Analysis of the Mixed Elastohydrodynamic Lubrication in Elliptical Contacts With an Arbitrary Entrainment Angle
Journal of Tribology-transactions of The Asme, 2014Co-Authors: Wei Pu, Jiaxu Wang, Ying ZhangAbstract:Numerical simulations of the elastohydrodynamic lubrication (EHL) have been conducted by many researchers, in which the entrainment velocity is usually parallel to one of the axes of Hertzian contact ellipse. However, in some engineering applications, such as the Counterformal contacts in spiral bevel and hypoid gears, entraining velocity vector may have an oblique angle that could possibly influence the lubrication characteristics significantly. Also, a vast majority of gears operate in mixed EHL mode in which the rough surface asperity contacts and lubricant films coexist. These gears are key elements widely used for transmitting significant power in various types of vehicles and engineering machinery. Therefore, model development for the mixed EHL in elliptical contacts with an arbitrary entrainment angle is of great importance. In the present paper, a recently developed mixed EHL model is modified to consider the effect of arbitrary entraining velocity angle, and the model is validated by comparing its results with available experimental data and previous numerical analyses found in literature. Based on this, numerical simulations are conducted to systematically study the influence of entrainment angle on lubricant film thickness in wide ranges of speed, load, and contact ellipticity. The obtained results cover the entire lubrication spectrum from thick-film and thin-film lubrication all the way down to mixed and boundary lubrication. In addition, minimum film thickness prediction formula is also developed through curve-fitting of the numerical results.
