Elastohydrodynamic Effects Research Articles

Simplified Wear Model for Power Cylinder under Engine Operating Conditions

A simple numerical approach is developed to predict the wear and friction of piston ring pack and cylinder liner during the break-in period and under transient speed conditions. In the model, a solution of mixed lubrications using the load-shearing concept is presented based on the selected elastic-plastic equations of asperity contacts in conjunction with well-established elastohydrodynamic equations. The model takes shear-thinning, thermal effects in elastohydrodynamic lubrication (EHL) and lubricant starvation effects into account. Modified Archard’s wear coefficient to describe the piston ring against cylinder liner wear is introduced. The approach is valid for the calculations of the asperity pressure at low values of the ratio of oil film thickness to combined roughness of the interacting surfaces, that is, less than 0.5. Applications are discussed, including association of piston side thrust force on wear and friction characteristics of piston ring/cylinder bore, gas blowby through the ring pack on the cylinder liner wear, a strong coating dependence for the piston ring wear, and wear distribution across the stroke under engine operating conditions. A good agreement between the experimental and analytical data indicates that the proposed model can be used to predict the wear and friction of the piston ring pack sliding against cylinder liner.

Long range signature of liquid's inertia in nanoscale drainage flows.

In confinement, liquid flows are governed by a complex interplay of molecular, viscous and elastic forces. When a fluid is confined between two approaching surfaces, a transition is generally observed from a long range dynamical response dominated by viscous forces in the fluid to a short range elasto-hydrodynamic response due to the elastic deformation of the solid materials. This study investigates the behavior of fluids driven between oscillating solid surfaces using a dynamic Surface Force Apparatus. Our findings reveal that the dominant influence on fluid behavior arises from long-range inertial effects, superseding conventional elasto-hydrodynamic effects. Through systematic experimentation involving fluids of varied viscosities, diverse substrates, we identify key parameters and develop a comprehensive model which explains our measurements. Our findings not only provide insights into confined fluid dynamics but also offer practical implications for various applications in microfluidics, nanotechnology and liquid lubrication.

Photoinduced superlubricity on TiO2 surfaces

Superlubricity control is of great interest in both industry and scientific research, and several methods have been proposed to achieve this goal. In this work, ultraviolet (UV) light was introduced into titanium dioxide (TiO2) and silicon nitride (Si3N4) tribosystems to accomplish photoinduced superlubricity. The friction coefficients (COFs) between Si3N4 balls and TiO2 plates in the mixtures of sulfuric acid (H2SO4) solution and glycerol solution were obviously reduced, and the system entered the superlubricity region (COF < 0.01) after UV illumination at a speed of 56 mm/s. However, the COF was much larger without UV treatment than that with UV treatment. The formation of silica (SiO2) layers on the surfaces of Si3N4 balls and the elastohydrodynamic effects were determined to be fundamental to the low friction in this experiment, and the enhancement of the combination between the TiO2 surface and the hydroxy group of glycerol by UV illumination was the key to the photoinduced superlubricity in this system. These findings showed one method for achieving superlubricity by introducing a light field that could be further applied to special working conditions.

Investigation of the running-in process in photoinduced superlubricity

Photoinduced superlubricity on TiO2 surfaces is a newfound phenomenon which draws researchers’ attention. This study provides a new method to achieve superlubricity (COF&amp;lt;0.01) with an external light field. However, photoinduced superlubricity can only be realized under specific conditions. Improper running-in conditions, such as speed, load, and pH value, will lead to superlubricity failure even after ultraviolet illumination on the TiO2 surface. In this paper, different running-in loads, speeds, or pH values were used in the experiment of photoinduced superlubricity, and the worn surfaces after running-in and testing in 70% v/v glycerol aqueous solution were investigated thoroughly. Results reveal that the morphology of worn scars differs under different running-in conditions. While the running-in speeds and loads are too low (&amp;lt;0.03 m/s and &amp;lt;2 N) or too large (&amp;gt;0.1 m/s and &amp;gt;9 N), the photoinduced superlubricity will fail because of wrong lubrication state. When the pH value of running-in solution is less than 4.5, photoinduced superlubricity is easier to achieve. In discuss, mixed lubrication is believed to be the key to success of photoinduced superlubricity, because the elastohydrodynamic effect, doublelayer effect and adsorption of glycerol molecules works at the same time. In addition, due to the formation of the SiO2 layer on the Si3N4 ball and better attraction to lubricant molecules with hydroxyl radicals on the TiO2 surface, running-in in solutions with low pH values contributes to the success of photoinduced superlubricity. In any event, the ultraviolet illumination can reduce the friction coefficient of the TiO2/Si3N4 tribological system and can realize photoinduced superlubricity under appropriate running-in conditions.

A New Elastic Non Contacting Sealing Concept for Valves

The hydraulic binary counter concept was proposed as a means to realize compact and lightweight digital hydraulic cylinder drives for exoskeleton actuation. This counter principle is based on hydraulically actuated switching valves which have a hysteretic response with respect to the pilot pressure. Manufacturing tolerances of the tiny components caused unexpected high leakage which, in turn, led to a malfunction of the system.&#x0D; In this paper a new sealing concept, is introduced to solve this problem. It is based on a non-contact seal (sealing gap), which exploits a self-regulating, elasto-hydrodynamic effect to reduce a rather large initial gap h0 – to allow a rough tolerance – into a very small sealing gap h(x) to avoid dry friction of valve spool movement on the one hand but have very small leakage on the other hand.&#x0D; As sealing, an annular flexible ring made, e.g. of PTFE, is used which combines sufficient flexibility to allow this self adapting mechanism to a sufficient extent and to stay the high pressure loads.&#x0D; For the mathematical analysis of the concept, an approximate elasto-hydrodynamic analytical model is used. It sets the gap pressure, obtained from the Reynold’s equation for the sealing gap, the elastic restoring forces of the sealing ring, and the imparting pressure in equilibrium.&#x0D; The sealing concept is then simulated with a numerical model built with the finite element code ABAQUS, this results are compared with the one of the analytical model.&#x0D; It solves the Reynolds equation by a user defined subroutine. The simulation results indicate that this sealing principle delivers way better results than standard gap seals despite its rough manufacturing tolerances with one order of magnitude higher tolerance ranges. The application of this concept is not limited to small valves as needed for exoskeleton hydraulics but can be transferred to conventional type of hydraulics as well.

Development of a Machine Learning Model for Elastohydrodynamic Pressure Prediction in Journal Bearings

Abstract This paper presents a machine learning neural network capable of approximating pressure as the distributive result of elastohydrodynamic (EHD) effects for a journal bearing at steady state. Design of efficient, reliable fluid power pumps and motors requires accurate models of lubricating interfaces; however, many state-of-the-art simulation models are structured around numerical solutions to the Reynolds equation which involve nested iterative loops, leading to long simulation durations and limiting the ability to use such models in optimization studies. This study presents a machine learning model capable of approximating the pressure solution of the Reynolds equation for a journal bearing with given distributive geometric boundary conditions and considering cavitation and elastic deformation at steady-state operating conditions. A 1024-sample training set was generated using an in-house multiphysics simulator. A hyperparameter optimization study was conducted, leading to the six-layer U-Net convolutional neural network architecture proposed. After training, the neural network accurately predicted pressure distributions for test samples with different geometric inputs from the training data, and accurately estimated resultant journal bearing loads, showing the feasibility of post-processing the machine learning output for integration into other fluid power models. Additionally, the neural network showed promise in analyzing geometric inputs outside the space of the training data, approximating the pressure in a grooved journal bearing with reasonable accuracy. These results demonstrate the potential of a machine learning model to be integrated into fluid power pump and motor simulations for faster performance evaluation and optimization.

Characterizations of the oil film considering the elastohydrodynamic lubrication effect of the piston–cylinder interface

To study the oil film performance, a multi-physical coupling model considering the elastohydrodynamic lubrication effect of the piston–cylinder interface is established and verified. The oil film thickness is obtained by solving geometric formulae, and the dynamic pressure model is derived by solving the Reynolds equation. Considering the oil film thickness and pressure distribution of the piston–cylinder interface, the elastic deformation model is solved. Both theoretical and experimental results show that the oil film morphology is influenced by the elastic deformation of the piston surface. The impact of the minimum fluid film thickness and its influence on the piston surface are well analyzed and discussed. When taking elastic deformation into account, the viscous friction reduces greatly, especially during the oil discharging process. Based on the theoretical research, a unique test rig was designed and established for testing and measuring the oil film thickness and the viscous friction of the piston.

In silico total hip replacement wear testing in the framework of ISO 14242-3 accounting for mixed elasto-hydrodynamic lubrication effects

Currently, preclinical wear tests for total hip replacements is carried out by using mechanical simulators under a simplified normal gait cycle as recommended by the international guidelines ISO-14242. Nevertheless, the wear tests have a long duration and are expensive so that the aim of this paper was to propose a novel in silico approach for wear calculation in lubricated total hip replacements.The proposed in silico simulation is based on the accurate modelling of the tribological phenomena acting in the joint during certain kinematical and dynamical conditions under non-Newtonian unsteady mixed elasto-hydrodynamic lubrication conditions (MEHL). The analyzed hip joint is modeled as soft polymeric acetabular cup, PE GUR1020 (EtO), against hard metallic CoCrMo femoral head. The calculation of the fluid film pressure field within the joint gap is achieved during the in silico simulations by imposing the ISO 14242-3 kinematics and dynamics, to evaluate the cumulated wear volume over a single gait cycle by using a modified Archard’s model.The main purpose of this study is to propose an in-silico wear calculation approach reproducing the classical in vitro wear testing of total hip replacements accounting for unsteady synovial lubrication effects between the femoral head and the acetabular cup. The predicted wear, extrapolated from a novel proposed approach based on the wear slopes, was compared with the value from in vitro simulations showing a satisfactory agreement, allowing the authors to conclude that the proposed model is suitable to be used as in silico hip joint wear simulator.

Compliant Surfaces under Shear: Elastohydrodynamic Lift Force.

In this work, we have investigated the behavior under shear and compression of mica surfaces coated with poly(N-isopropylacrylamide) cationic microgels. We have observed the emergence of velocity dependent, shear-induced normal forces, which can be large enough to entrain a fluid film that separates the surfaces out of contact, driving the dynamic system from conditions of boundary to hydrodynamic lubrication. By implementing a feedback-loop control on the surface separation, we were able to quantify the magnitude of the lift force as a function of the surface separation and driving speed. Our results illustrate how elastohydrodynamic effects can play an important role in the lubrication of compliant surfaces, providing pathways for control of friction and wear.

Study on the load-carrying capacity of surface textured slipper bearing of axial piston pump

To introduce the elastohydrodynamic lubrication effect caused by the pressure distribution under a surface-textured slipper, the slipper's load-carrying capacity is studied. An elastohydrodynamic model was established to analyse the load-carrying capacity at different rotational speeds and load pressures. The comprehensive effects of tilting angle, area density, dimple depth to diameter ratio, and operating conditions are investigated. The results indicate that the optimum film thickness with the maximum stiffness coefficient can be obtained by adjusting the rotational speed, but the load-carrying capacity is slightly lower than that of a rigid surface considering the textured-surface deformation. The load-carrying capacity enhancement of a textured-surface slipper bearing is obvious at the optimum tilting angle, but the maximal load-carrying capacity becomes lower at a higher tilting angle. A textured slipper with an area density and a dimple depth to diameter ratio of 24% and 0.3, respectively, can generate a high load-carrying capacity under an elastohydrodynamic lubrication condition.

Scaling Criteria for Axial Piston Machines Based on Thermo-Elastohydrodynamic Effects in the Tribological Interfaces

In lieu of reliable scaling rules, hydraulic pump and motor manufacturers pay a high monetary and temporal price for attempting to expand their production lines by scaling their existing units to other sizes. The challenge is that the lubricating interfaces, which are the key elements in determining the performance of a positive displacement machine, are not easily scalable. This article includes an analysis of the size-dependence of these units with regard to the significant physical phenomena describing the behavior of their three most critical lubricating interfaces. These phenomena include the non-isothermal elastohydrodynamic effects in the fluid domain, and the heat transfer and thermal elastic deflection in the solid domain. The performance change due to size variation is found to be unavoidable and explained through fundamental physics. The results are demonstrated using a numerical fluid–structure–thermal interaction model over a wide range of unit sizes. Based on the findings, a guide to scaling swashplate-type axial piston machines such as to uphold their efficiency is proposed.

Modeling of radial piston machines considering elastohydrodynamic effects in both cam–piston and piston–cylinder lubricating interfaces

This study illustrates a novel numerical approach for investigating the complex physics involved in radial piston pump (rotating cam type) operation. This approach is based on a fully-coupled model that combines the evaluation of the main flow through the unit, realized by the pistons’ displacement, with the simulation of the internal lubricating interfaces, given by the piston–cylinder interface and the cam–piston interface. These interfaces represent the main source of power dissipation due to leakages and shear. The comprehensive multi-domain simulation tool presented in this paper incorporates a robust fluid–structure interaction based numerical model for the piston–cylinder lubricating interface as well as a model for the lubricant flow in the cam–piston interface. Since the approach used for piston–cylinder interface was previously presented by the authors, in this work particular emphasis is given to the description of the elastohydrodynamic lubrication model for the cam–piston interface. The overall coupling between these models enables an accurate estimation of the piston micro-motion that is critical to analyzing fluid flow in the lubricating gaps during pump operation. Results are shown with reference to a pump with four pistons designed to reach pressures up to 2500 bar. For this unit, the outer race of the cam—which is in contact with multiple pistons during pump operation—is unconstrained in its rotation, and it undergoes a complex motion due to which experimental measurements of its angular velocity were performed. The results from the study confirm the utility of the numerical model as an effective tool for modeling radial piston machines.

Contact fatigue initiation and tensile surface stresses at a point asperity which passes an elastohydrodynamic contact

Abstract Contact mechanics and tribology was combined with fundamental fatigue and fracture mechanics to form a new mechanism for surface initiated rolling contact fatigue. Following, fatigue was investigated numerically for single asperities and craters in lubricated rolling contact surfaces. The hypothesis suggests that asperity point contacts can create sufficiently large tensile stresses for fatigue. The investigated case corresponded to a heavily loaded truck gear with ground surfaces. Reynolds equation resolved the elastohydrodynamic effect of the asperity in the transient three dimensional contacts. The Findley critical plane criterion was used for multiaxial and non-proportional fatigue evaluation. The simulations confirmed the new mechanism for rolling contact fatigue and showed how asperities can create contact fatigue in the lubricated contacts even without slip.

Dynamic analysis of torus involute gear including transient elastohydrodynamic effects

The torus involute gear can compensate large axial misalignments and may possess good meshing characteristics without lead correction. In order to study its dynamic characteristics and verify its feasibility of the practical application, a new efficient rigid-elastic coupling dynamic model of multi-tooth is established which includes effects of lubrication oil film and tooth deformations directly in contact simulation of gears. In this model, each tooth is connected with the gearwheel by a rotatable spring-damper element whose stiffness is calculated through analysis of tooth deformation. The normal tooth contact force is determined via Lankarani and Nikravesh model. Variation of contact stiffness and rotatable spring stiffness with contact points are both taken into account. Combined with tooth contact analysis, the computation of friction coefficient is implemented with high efficiency by introducing the average lubrication oil film height. A three-dimension multi-body model of a torus involute gear pair is employed and verified by an impact experiment. The simulated results provide useful information about tooth impacts, dynamic transmission error and lubrication conditions like oil film heights and friction coefficients, and show that this type of gear can work with good meshing characteristics. The contributions in this paper lay theoretical basis for the application of the torus involute gear.

The application potential of SiO2, TiO2 or Ag nanoparticles as fillers in machining process fluids

Process fluids are important as they create the appropriate conditions for machine cutting by improving heat transfer, whereby extending the life of the cutting tool and increasing the surface quality of the product. The use of nanoadditives in the form of nanoparticles is highly efficient due to their high chemical and biological activity. Nanoparticles dispersed in a volume of process fluid can easily penetrate between friction surfaces and thereby greatly influence the elastohydrodynamic effect of the lubrication. In our experiment, the addition of nanoparticles has a positive effect on reducing the average of friction coefficient during the tribological process by 8.9% and reducing the variance of friction coefficient by 20.4%, as they lead to faster stabilization resulting in a significant reduction in the depth of material being removed by up to 52.1%. In this paper, the antibacterial properties of process fluids with nanoparticles of TiO2, SiO2 or Ag were also evaluated (respiratory rates and bacterial viability). The results indicated that the nanoparticles reduced bacterial respiratory rates and viability by up to 90.4%.

THE INFLUENCE OF THE LOAD AND JOINT CARTILAGE ELASTICITY MODULES ON SYNOVIAL FLUID VISCOSITY

During classical journal bearing lubrication the lubricant viscosity is independent of physical properties of cooperating bodies, which is well known by virtue of Hersey-Stribeck (H-S) curve presenting friction coefficient vs. Hersey number = viscosity´velocity/pressure. The result obtained by the H-S is valid for two cooperating bodies with homogeneous, isotropic properties, and for Newtonian oils omitting the elastohydrodynamic effects. In the presented paper, we take into account the two cooperating human joint cartilage surfaces, which, after new AFM measurements, have non-homogeneous hypo- or hyper-elastic properties, and the synovial fluid has non-Newtonian features. Moreover, the cartilage surface during human limb motion and during the squeezing and boosted squeezing effects gains important small deformations. From the abovementioned description, it follows that the H-S result cannot be acceptable in human joint lubrication [L. 1–6]. During human joint hydrodynamic lubrication, we observe the influence of the material coefficients of the hypo- and hyper-elastic cartilage tissue on the apparent viscosity of non-Newtonian synovial fluid occupying the thin joint gap limited by the two cartilage superficial layers. This problem has not been considered in scientific papers describing the hydrodynamic lubrication of the human joint. This problem attains significant meaning because, after numerous AFM laboratory measurements confirmed by the literature achievements, it follows that the joint cartilage tissue with a thin polar membrane made of two lipid molecules has no isotropic but anisotropic properties in general. These membranes are flat sheets that form a continuous barrier around the cartilage cells. Non-homogeneous, anisotropic biological bodies as distinct from classical isotropic materials have the various values of elasticity, hypoelasticity, or hyper-elasticity modules on individual places and directions. These places, loaded by the same forces, tend to various displacements and strains. In consequence, mutually connected physical implications caused by virtue of synovial fluid flow velocity and shear rates changes, indicate to us the conclusion that the dynamic viscosity of synovial fluid gains value variations caused by the cartilage’s physical properties during human joint lubrication.

Effect of elastohydrodynamic lubrication on the dynamic analysis of ball bearing

A numerical dynamic analysis of ball bearing is presented. The normal contact forces between balls and races are calculated, respectively, based on Hertz contact theory and elastohydrodynamic lubrication theory, and the results are compared to study the effect of elastohydrodynamic lubrication on the simulation. The dynamic model of ball bearing is developed with the assumptions that outer race is fixed in space and other elements have six degrees of freedoms. The lubricant film between balls and races is equivalent as the spring–damper system when elastohydrodynamic lubrication effect is considered. The equations of motion are solved by the fourth-order Runge-Kutta integration method with variable steps. The results indicate that the effect of elastohydrodynamic lubrication has great influence on the dynamic analysis result when the load is slight. As the load increases, the effect of elastohydrodynamic lubrication becomes small.

Analysis of thermoelastohydrodynamic performance of journal misaligned engine main bearings

To understand the engine main bearings’ working condition is important in order to improve the performance of engine. However, thermal effects and thermal effect deformations of engine main bearings are rarely considered simultaneously in most studies. A typical finite element model is selected and the effect of thermoelastohydrodynamic(TEHD) reaction on engine main bearings is investigated. The calculated method of main bearing’s thermal hydrodynamic reaction and journal misalignment effect is finite difference method, and its deformation reaction is calculated by using finite element method. The oil film pressure is solved numerically with Reynolds boundary conditions when various bearing characteristics are calculated. The whole model considers a temperature-pressure-viscosity relationship for the lubricant, surface roughness effect, and also an angular misalignment between the journal and the bearing. Numerical simulations of operation of a typical I6 diesel engine main bearing is conducted and importance of several contributing factors in mixed lubrication is discussed. The performance characteristics of journal misaligned main bearings under elastohydrodynamic(EHD) and TEHD loads of an I6 diesel engine are received, and then the journal center orbit movement, minimum oil film thickness and maximum oil film pressure of main bearings are estimated over a wide range of engine operation. The model is verified through the comparison with other present models. The TEHD performance of engine main bearings with various effects under the influences of journal misalignment is revealed, this is helpful to understand EHD and TEHD effect of misaligned engine main bearings.

Influence of charge density and calcium salt on stiffness of polysaccharides multilayer film

Abstract Polysaccharide multilayer film consisting of negatively charged pectin (PEC) and positively charged chitosan (CHI) is built by electrostatic self-assembly onto ellipsoidal iron oxide particles. The thickness growth and electrical properties of the PEC/CHI multilayer film are followed using electric light scattering method and electrophoresis. The main focus is on how the PEC and CHI charge density and addition of divalent calcium salt affect the stiffness of the PEC/CHI multilayer film. We found an excess of positive charges within the PEC/CHI film when it is constructed from highly charged CHI and less charged PEC (at pH 4). The excess of positive charge within the film causes a decrease in the amplitude of the electro-optical effect at high electric fields, more expressive for the films ending in CHI. This is related to operation of an elasto-hydrodynamic effect (dominating at strong fields), which favors perpendicular orientation of “soft” particles with respect to the external electric field. Addition of Ca2+ ions to the films ending in PEC causes small increase in the amplitude of the electro-optical effect at high fields, corresponding to an increase in the film stiffness.

Port and case flow temperature prediction for axial piston machines

Researchers at Purdue’s Maha Fluid Power Research Center have developed models that will enable computational design of piston machines. The core of the in-house developed program forms multi-domain models capturing the fluid–structure interaction phenomena taking place in the main lubricating interfaces (piston/cylinder, cylinder block/valve plate, and slipper/swash plate) of axial piston machines. The model allows studying the influence of a given pump or motor design on machine performance, power loss, and energy dissipation in those main lubricating interfaces. The behavior of the fluid film in these lubricating interfaces as well as the shape of the solid parts is temperature and pressure dependent. In order to solve for non-isothermal flow and to consider elasto-hydrodynamic effects, port and case temperatures are needed as boundary condition for the model. In case of analysis and optimization of existing pumps and motors, those boundary conditions can be taken from steady-state measurements; howeve...