Point Contact Elastohydrodynamic Lubrication Research Articles

Numerical study on the interaction of transversely oriented ridges in thermal elastohydrodynamic lubrication point contacts using the Eyring shear-thinning model

Transient behaviour of tribo-characteristics caused by transversely oriented ridges on point contact surfaces was investigated based on a thermal elastohydrodynamic lubrication analysis. The ridges were assumed to exist on both the contact surfaces with different velocities. Results show that the interaction of ridges gives a large influence on the local film thickness, pressure, friction coefficient, temperatures on both the solid surfaces and temperature in the oil film. It is also pointed out that the size of the contact bodies brings strong effect on the temperature distribution and shear rate as well as on the friction coefficient. Furthermore, it is revealed that under rolling-sliding conditions, the shear-thinning property of the lubricant is negligible when the size of the contact body is large enough. However, shear-thinning effect plays an important role when the size is extremely small.

Experimental and Numerical Investigation of Inlet Dimple in Point and Elliptical Contact Elastohydrodynamic Lubrication Films

The experimental observation of the inlet dimple of the elastohydrodynamic lubrication (EHL) film was carried out in point and elliptical contacts, respectively, and then, the numerical analysis under point contacts was conducted. The inlet dimple appears and then moves upstream toward the inlet and finally disappears with the increase of the entrainment speed. In the meantime, the dimple depth increases at the beginning and then decreases. The high load leads to a wide entrainment speed range where dimple exists. The varying range of the entrainment speed corresponding to the dimple appearance is smaller at a smaller included angle between the minor axis of elliptical contact and the entrainment direction.

Wavelet Based Numerical Solution of Elasto-hydrodynamic Lubrication Problems via Lifting Scheme

Wavelet theory is a relatively new and an emerging area in mathematical research, particularly wavelets are successfully used in fast algorithms for easy implementation. Elasto-hydrodynamic lubrication model involves coupled system as well as highly nonlinearity, is very difficult to solve numerically by standard methods to obtain better accuracy with low CPU time. The wavelet based schemes are appropriate to get a sufficient accuracy in low CPU time. In this paper, we present the wavelet based numerical solution of elastohydrodynamic lubrication line and point contact problems using lifting scheme. The proposed scheme gives higher accuracy in terms of super convergence in less CPU time, which has been illustrated through the test problems.

Lubrication characteristics of ball–cylinder pair in eccentric ball piston pumps

Ball–cylinder pair is the key friction pair of eccentric ball piston pumps. The reliability of ball–cylinder pair is significant to the performance of ball piston pump. The amended point contact elastohydrodynamic lubrication (EHL) model of ball–cylinder pair is developed in this paper. Non-Newtonian behavior of lubricants and slide-roll feature of ball–cylinder pair are considered. Pressure distribution and film profile of ball–cylinder pair are calculated. The characteristics of friction and lubrication are analyzed. The result shows that a film dimple takes place at the inlet of the contact, corresponding to a pressure mutation. For friction coefficient, with the increase of entrainment velocity of ball–cylinder pair, the curve goes down first and then goes up to the peak. After that, the curve decreases slightly to stability. Also, the curve decreases first and increases slowly to stability with respect to the normal load of ball–cylinder pair. Tendency of the curves representing friction coefficient is different from the classical Stribeck curve. The simulating results are validated by test.

Film Thickness and Rolling Resistance in Starved Elastohydrodynamic Lubrication of Point Contacts With Reflow

Elastohydrodynamic lubrication (EHL) film thickness and rolling resistance play a critical role in determining friction, wear, life, and other tribological characteristics of rolling bearings. Although film thickness formulas are widely used and experimentally verified, accurate prediction of the film thickness is still difficult under starved conditions. This paper presents a numerical study of starved EHL point contacts using a nonuniform inlet film thickness obtained from a modified Coyne–Elrod boundary condition. An experimental verification of the numerical results is also presented. Based on the results of a parametric study, inlet distance formulas are obtained as a function of the initial film thickness, the fully flooded central film thickness, and the capillary number. By using the inlet distance formulas and the Hamrock–Dowson formulas, the central film thickness, the minimum film thickness, and the viscous rolling resistance can be calculated.

The effect of an oscillatory entrainment velocity on the film thickness in thermal EHL point contact

In this study, the mathematical model for transient thermal elastohydrodynamic lubrication (EHL) was established and the effect of tangential vibration on thermal EHL was investigated. The pressure calculation was carried out by a multi-grid technique, the elastic deformation was evaluated by a multilevel multi-intergration method, and the temperature was calculated by a single-direction sequential column sweeping process. The present study aims to explain the multi-dimple phenomenon observed experimentally by Ehret and Bauget [Proc Inst Mech Eng Part J: J Eng Tribol 2001; 215: 289–300]. The results show that a tangential vibration produces significant change of pressure and film thickness and it seems to be the most possible mechanism of the movement of multi-dimples observed by Ehret and Bauget [Proc Inst Mech Eng Part J: J Eng Tribol 2001; 215: 289–300]. Furthermore, the effect of the curvature radius, viscosity–pressure coefficient, load and the parameters of vibration was also investigated.

An adaptive finite element procedure for fully-coupled point contact elastohydrodynamic lubrication problems

This paper presents an automatic locally adaptive finite element solver for the fully-coupled EHL point contact problems. The proposed algorithm uses a posteriori error estimation in the stress in order to control adaptivity in both the elasticity and lubrication domains. The implementation is based on the fact that the solution of the linear elasticity equation exhibits large variations close to the fluid domain on which the Reynolds equation is solved. Thus the local refinement in such region not only improves the accuracy of the elastic deformation solution significantly but also yield an improved accuracy in the pressure profile due to increase in the spatial resolution of fluid domain. Thus, the improved traction boundary conditions lead to even better approximation of the elastic deformation. Hence, a simple and an effective way to develop an adaptive procedure for the fully-coupled EHL problem is to apply the local refinement to the linear elasticity mesh. The proposed algorithm also seeks to improve the quality of refined meshes to ensure the best overall accuracy. It is shown that the adaptive procedure effectively refines the elements in the region(s) showing the largest local error in their solution, and reduces the overall error with optimal computational cost for a variety of EHL cases. Specifically, the computational cost of proposed adaptive algorithm is shown to be linear with respect to problem size as the number of refinement levels grows.

A numerical study of fatigue life in non-Newtonian thermal EHL rolling–sliding contacts with spinning

This paper presents a study on fatigue life in non-Newtonian thermal elastohydrodynamic lubrication (TEHL) point contacts with spinning. A numerical procedure is developed and extended to rolling contact fatigue (RCF) life. The results show that the effect of entraining velocity on the RCF life is closely related to ellipticity. The RCF life first decreases steeply and then gradually with increase in slide–roll ratio. However, the RCF life may increase slightly at a large slide–roll ratio. Spinning is beneficial for reduction of longitudinal friction coefficient; however, even for smooth surface contact, the RCF life can be slightly reduced by spinning.

Influence of thermal effects on elastohydrodynamic (EHD) lubrication behavior at high speeds

This paper describes a study of point contact elastohydrodynamic (EHD) lubrication behavior at high speeds (up to 20 m s−1). Central film thicknesses were measured by optical interferometry device. The influence of slide-roll ratio and operating temperature on the central film thickness was determined. The influence of thermal effects on the reduction of film thickness was discussed via the analysis of numerical simulation method considering thermal effects. Subsequently, the experimental data was used to amend a set of unified parameters for the thermal corrections for different types of oil at high speeds.

Effect of Nanotexturing on Increase in Elastohydrodynamic Lubrication Oil Film Thickness

The effects of nanotexturing on oil film thickness and shape under pointcontact elasto-hydrodynamic lubrication (EHL) conditions were experimentally investigated. A disk-onball friction tester with an optical interferometer was used to measure oil film thickness and to observe the oil film shape. Periodic groove structures with a spiral, perpendicular, or parallel shape and with various groove depths and distances were formed by irradiation of a femtosecond laser onto the surface of steel balls. These nanotextured balls were tested under a load of 20 N and at rotational speeds from 1.0 to 3.0 m/s. Most of the balls with nanotexturing had a thicker oil film than those without texturing. The groove depth and angle were the key parameters determining the thickness of the oil film as they controlled the amount of side leakage of oil from the contact point. Optimization of these parameters resulted in an oil film that was almost twice as thick as that on the ball without texturing.

Point Contact Elastohydrodynamic Lubrication in Confined Gap

Point Contact Elastohydrodynamic Lubrication in Confined Gap

Theoretical and Experimental Investigations on EHL Point Contacts with Different Entrainment Velocity Directions

Film thickness prediction plays an important role in evaluating the performance and durability of machine elements under elastohydrodynamic lubrication (EHL). However, current formulae may not be appropriate for general conditions occurring in real contacts. This study investigates the effect of different lubricant entrainment velocity angles on film thickness distribution. For this purpose, a steady-state isothermal EHL model is used under a wide range of parameter sets including varying sum velocity, contact pressure, and sum velocity angle. Considerable differences in the trend of the central film thickness with respect to the lubricant entrainment velocity angle for low and high loaded contacts are shown. The results are compared with experimental measurements by means of an optical ball-on-disc tribometer and a twin-disc machine using capacitance method. Good agreement between numerical results and experimental measurements was found.

On friction regimes in quantitative elastohydrodynamics

This paper identifies the different friction regimes encountered in elastohydrodynamic lubricated point contacts using a quantitative, physics-based approach. The idea is to link different traction regimes to dimensionless numbers and identify ranges of those numbers where a given regime is encountered or dominates the friction response of the contact. A numerical investigation of traction in point contacts lubricated with a typical mineral oil is employed. The measured thermo-physical properties of the oil are used without any modification to force agreement with experiments. The authors propose four friction regimes delimited by the combined values of three dimensionless parameters.

On the linear finite element analysis of fully coupled point contact elastohydrodynamic lubrication problems

The fully coupled approach for the solution of elastohydrodynamic lubrication point contact problems requires the numerical solution of the elasticity problem on a large 3D domain. A sufficiently fine computational mesh is required to obtain the elastic deformation solution to the necessary accuracy, and hence, a sufficiently accurate elastohydrodynamic lubrication point contact solution. This article discusses the accuracy of an elastohydrodynamic lubrication point contact solution over different finite element meshes. We present a family of efficient 3D meshes which can be used to calculate accurate elastohydrodynamic lubrication point contact solutions at a minimal computational cost. This article also illustrates the fact that the unstructured hierarchical meshes can lead to a poor quality elastohydrodynamic lubrication solution unless an appropriate post-processing, smoothing technique is applied.

An efficient preconditioned iterative solution of fully-coupled elastohydrodynamic lubrication problems

This paper presents the fast preconditioned iterative solution to large sparse linear systems arising from the application of Newton and quasi-Newton methods to fully coupled elastohydrodynamic lubrication line and point contact problems. The new blockwise preconditioner that is presented combines the use of multigrid for the linear elasticity block and a separate approximation to precondition the Reynolds block. Two variants of the solver are considered, based upon the use of algebraic and geometric multigrid respectively. Numerical results are presented in order to validate the discretization and solution method and then to contrast the performance and efficiency of the proposed solution strategies compared to the use of a state-of-the-art sparse direct solver. These results demonstrate that, unlike the sparse direct solver, the preconditioned iterative approach is able to perform at computational and memory costs that both grow linearly with the number of unknowns.

Adaptive high-order Discontinuous Galerkin solution of elastohydrodynamic lubrication point contact problems

This paper describes an adaptive implementation of a high order Discontinuous Galerkin (DG) method for the solution of Elastohydrodynamic Lubrication (EHL) point contact problems. These problems arise when modelling the thin lubricating film between contacts which are under sufficiently high pressure that the elastic deformation of the contacting elements cannot be neglected. The governing equations are highly non-linear and include a second order partial differential equation that is derived via the thin-film approximation. Furthermore, the problem features a free boundary, which models where cavitation occurs, and this is automatically captured as part of the solution process. The need for spatial adaptivity stems from the highly variable length scales that are present in typical solutions. Results are presented which demonstrate both the effectiveness and the limitations of the proposed adaptive algorithm.

Numerical investigation of the use of machinery low-viscosity working fluids as lubricants in elastohydrodynamic lubricated point contacts

This study proposes a numerical investigation of the potential use of machinery working fluids as lubricants in contacts operating under an elastohydrodynamic regime. These fluids are usually of very low viscosity and pressure–viscosity dependence. This is why, unmixed with oil, they have been of little interest for the tribological community. Hence, their rheological properties are poorly known. In fact, these are restricted to a narrow range of conditions compared to the range of interest in EHL applications. This is why some measurements are carried out in order to determine both the viscosity and density of these uncommon lubricants. Besides, their viscosity being low, high-mean entrainment speeds are required for a sufficiently thick lubricant film to build up. This leads to an important thermal dissipation within the contact. Thermal effects are included in the analysis in order to make the estimation of film thicknesses and friction coefficients in these contacts as accurate as possible. Results are discussed in the light of the peculiar properties of machinery low-viscosity working fluids.

On the non-linear dynamic behavior of elastohydrodynamic lubricated point contact

The complex dynamic concepts of mechanical systems are regarded each day as new barriers to be overcome. One of the most complex systems, despite its common construction design, is the rolling element bearings. The interactive dynamic interfaces of such bearings are normally disregarded by engineering analysis on the day to day basis due to its complexities. This paper intends to propose a new approach to the characterization of the elastohydrodynamic lubricated point contacts on such components, in order to fully depict its non-linear dynamic behavior, avoiding the use of rough hypothesis on a systemic procedure. A multi-level method was used to solve the coupled lubrication–deformation problem, alongside a Newmark-ß integrator of the motion equation for the contact system. A range of dynamically similar contacts were evaluated, so as to characterize its nonlinear dynamic behavior. A least-squares method was applied to the multi-level algorithm results, fitting the displacements–force relation to a linear and also to a third order polynomial stiffness. The fitting results were compared, clearly showing the nonlinear behavior of such contacts. Also, the oil film damping was regarded as viscous, leading to good overall response. Some peculiarities of the proposed adjust method are also considered.

Lubrication mechanisms with laser-surface-textured surfaces in elastohydrodynamic regime

Micro-texturing is very efficient to make surfaces super-hydrophobic according to a physical process known as ‘lotus effect’. In the recent past, tribologists have also investigated surface texturing because it could be a promising way to increase the lifetime of contacts or to make its working conditions more severe. Moreover, significant friction coefficient reductions have been obtained with textured surfaces in different lubrication regimes. In this presentation, transient lubrication phenomena induced by laser-made circular dimples passing through an elastohydrodynamic lubrication point contact are analysed both experimentally and numerically. Under rolling—sliding conditions, the passage of an isolated dimple modifies the film thickness distribution. Deep dimples induce an oil film failure. On the contrary, a shallow dimple locally generates a large increase in the film thickness. Dimples geometry and the slide/roll ratio modify the pressure distribution and then control the level and extension of the local film reinforcement. Experiments are simulated numerically with a two-dimensional multi-grid solver. This allows one to have a better understanding of the lubrication mechanism with a textured surface as a function of dimple geometry, entrainment speed, and contact pressure. The key point is the control of tribological conditions to maintain the viscosity of the lubricant high enough inside the dimple in order that the shearing can expel it to locally enhance the film thickness.

Effects of temperature on pure squeeze EHL motion using grayscale interferometric method

Purpose – The purpose of this paper is to present a novel method to investigate the microscopic mechanism of the oil film under the pure squeeze elastohydrodynamic lubrication (EHL) motion. An optical EHL squeeze tester is used to explore the effects of squeeze velocity, load, temperature, and lubricant viscosity on the dimple film thickness that occurs when a ball approaches a flat plate covered by a thin layer of oil.Design/methodology/approach – The grayscale interferometric technique was used to study the thickness of the lubricating film in an EHL point contact. The light source was a He‐Ne laser. Through the transparent optical glass and by means of optical interference, the interference fringe patterns of the contact region were observed by a charge‐coupled device camera recording. The two elastic bodies were a sapphire disk and a steel ball. The contact was lubricated with paraffin‐based oil.Findings – Results show that increasing the squeeze speed, load, viscosity, and decreasing the temperature,...