Transient Lubrication Research Articles

Variable load failure mechanism for high-speed load sensing electro-hydrostatic actuator pump of aircraft

This paper presents a novel transient lubrication model for the analysis of the variable load failure mechanism of high-speed pump used in Load Sensing Electro-Hydrostatic Actuator (LS-EHA). Focusing on the slipper/swashplate pair partial abrasion, which is considered as the dominant failure mode in the high-speed condition, slipper dynamic models are established. A forth sliding motion of the slipper on the swashplate surface is presented under the fact that the slipper center of mass will rotate around the center of piston ball when the swashplate angle is dynamically adjusted. Besides, extra inertial tilting moments will be produced for the slipper based on the theorem on translation of force, which will increase rapidly when LS-EHA pump operates under high-speed condition. Then, a dynamic lubricating model coupling with fluid film thickness field, temperature field and pressure field is proposed. The deformation effects caused by thermal deflection and hydrostatic pressure are considered. A numerical simulation model is established to validate the effectiveness and accuracy of the proposed model. Finally, based on the load spectrum of aircraft flight profile, the variable load conditions and the oil film characteristics are analyzed, and series of variable load rules of oil film thickness with variable speed/variable pressure/variable displacement are concluded.

Investigation of surface texture influence on hydrodynamic performance of parallel slider bearing under transient condition

Although surface texture has been becoming an attractive technique for improving hydrodynamic performance in lubrication system, the study of surface texture mainly focus on the steady-state load conditions. This investigation evaluates the effect of surface texture on hydrodynamic performance under transient lubrication in parallel bearing. In the case, considering cavitation phenomena in liquid lubricating film, a mass conserving formulation based on Elrod–Adams model with JFO conditions is then employed. The implementation of numerical simulation is discretized in spatial and time domain for calculating the pressure distribution. The effect of different geometrical configurations of lubricated contact on hydrodynamic performance is evaluated. The results show that surface texture in parallel bearing yields a great improvement in terms of friction force and friction coefficient.

Influence of surface texturing on the hydrodynamic performance of a thrust bearing operating in steady-state and transient lubrication regime

This paper presents a finite element modeling of hydrodynamic thrust bearings operating both in steady-state and transient lubrication regime. The presented numerical algorithm is conservative and makes it possible to determine, with a high degree of accuracy, the operating characteristics such as the friction torque, the film pressure, the film thickness and the oil flow. For different operating conditions, it was shown that the behavior of the thrust bearing is not the same when textures are applied on the rotor or on the stator. Thus, the difference in behavior between the stationary and non-stationary case was revealed. Lastly, it was predicted that placing the textures on the rotor could, under certain conditions, improve the hydrodynamic performance of the thrust bearing.

The Effect of Oil Droplet on the Lubrication Performance

Oil–air lubrication is widely used in the high-speed rotary machines; the oil is usually in the form of discrete droplets. The lubrication behavior of the oil droplet is rarely investigated. This paper investigates the effect of the oil droplet on lubrication performance based on the developed transient lubrication model with consideration of starvation conditions. The oil droplet is modeled as spherical segment with different heights, base radius, and positions, which will change the inlet oil supply conditions. The results show that the oil droplet with large size can generate thick oil film which is close to the one in fully flooded condition and can remain long time; the position of the oil droplet entering the contact region also has significant effect on the lubrication performance.

Transient lubrication of piston compression ring during cold start-up of SI engine

Cold start-up conditions differ from the normal operating conditions for an engine, and this discrepancy adversely affects the tribological performance of interfaces. The majority of health-hazardous engine exhaust emissions occur during the cold start-up of an engine; consequently this condition generates a major environmental concern. This study numerically investigates the transient hydrodynamic lubrication of the first compression ring during initial start-up of a cold engine by combining lubrication theory with the realistic oil rheology of multigrade oils. Specifically, in this study transient speed, variation in cylinder pressure, and high lubricant viscosity during start-up are considered to determine developing pattern of the lubricating film and of other tribological performance parameters. The cycle-by-cycle developments of lubricating film, friction force, power loss, and oil transport are analyzed as well. The start-ups of a cold and a warm engine are also compared in terms of frictional losses and oil transport. Different monograde and multigrade engine oils are used to evaluate oil-dependent performance. Results show that in all cases, performance during cold start-up is of greater significance than that of a warm engine. Low lubricant temperature and stated start-up conditions substantially increase amount of energy loss and net oil transport. Temperature-dependent variations in performance are also more significant in high viscosity-grade oils.

Effect of surface roughness pattern on transient mixed elastohydrodynamic lubrication

Besides the surface roughness of two contacting surfaces, the surface roughness pattern i.e. longitudinal, transverse and isotropic significantly influences the tribological performance of the mechanical element. Their impression is more pronounced under the mixed elasto-hydrodynamic lubrication condition. The cam and flat follower mechanism is a typical sample in which adverse tribological conditions, including direct boundary interactions occurs. In this paper, the effect of surface roughness pattern on the film thickness and friction coefficient in a cam follower mechanism is investigated. Asperity interaction and friction coefficient analysis is conducted based on a novel elasto-plastic model. The lubrication model is qualitatively compared with the experimental results obtained from the pin on disk experiments for various surface roughness orientations. The results of transient lubrication analysis for a cam and follower lubrication problem are presented. It is shown that the longitudinal surface roughness pattern has a more desirable tribological performance than transverse surface pattern.

Model and experiments of the transient evolution of a thin, evaporating liquid film

Experimental thickness profiles of evaporating thin films of n-octane and n-heptane on silicon were recorded using reflectometry. These films ranged from approximately ten nanometers thick in the adsorbed region to several micrometers thick in the meniscus region. For steady-state measurements, the reflectometer was traversed along the film to give thickness versus horizontal distance. For transient measurements, the reflectometer was fixed at several points as thickness was measured over time. While film thickness and substrate temperature measurements are practical, those of the evaporative flux profile and total heat transfer are not. Hence, a numerical model was formulated based on transient lubrication theory and modified to include kinetic evaporation and disjoining pressure. The model uses an ordinary differential equation solver to advance the governing equations in time with staggered finite differences for the spatial derivatives. Good correlation of the transient film thickness was achieved between the simulations and experiments. Thus, we have confidence in the inferred evaporative flux profiles and liquid–vapor interface temperatures. With this information, instantaneous heat transfer coefficients were derived for the entirety of the thin film region.

A Transient Thermoelastohydrodynamic Lubrication Model for the Slipper/Swashplate in Axial Piston Machines

A transient lubrication model has been developed for the sliding interface between the slipper and swashplate in axial piston hydraulic pumps and motors. The model considers a nonisothermal fluid model, microdynamic motion of the slipper, as well as pressure and thermal deformations of the bounding solid bodies through a partitioned solution scheme. The separate contributions of elastohydrostatic and elastohydrodynamic lubrication are studied. Although hydrostatic deformation dominates, hydrodynamic effects are crucial for actual operation. Finally, the impact of transient deformation on lubricant pressure is explored, with its consideration necessary for accurate analysis.

Effects of surface texturing in steady-state and transient flow conditions: Two-dimensional numerical simulation using a mass-conserving cavitation model

This paper presents a numerical investigation on the influence of surface texturing on the hydrodynamic performance of a two-dimensional parallel slider bearing operating in both steady-state and transient lubrication. The tribological behavior of the bearing is evaluated by means of a mass-conserving cavitation model based on a finite element discretization of a modified version of the unstationary Reynolds equation. While considering the impact of the operating parameters, the influence of various geometrical features such as the texture ratio, the texture density, or the dimple depth is determined by means of an extended parametric study. Furthermore, particular attention is given to the lubrication mechanisms describing the potential effects produced by texturing the moving pad of the bearing. Thus, it is shown that a textured moving surface can contribute to the generation of hydrodynamic lift on the basis of a squeeze effect being produced at the leading edge of the textured region. Finally, the study reveals how texturing the stationary pad and/or the moving pad of the bearing affects hydrodynamic performance. The analyses show that, for an applied load, texturing the moving pad of the bearing instead of the stationary one leads to a decrease in film thickness, while texturing both pads produces optimum bearing performance.

Numerical Study on the Transient Adjustment of Wafer’s Posture in Chemical Mechanical Planarization

Chemical mechanical planarization(CMP) is one of the key techniques in integrated circuit manufacture. Wafer carrier is a very important component of CMP system. For studying the effect of position of ball joint on the ability of transient adjustment of wafer carrier orientation in planarization process, mixed soft elastohydrodynamic lubrication model is adopted to analyze the flow and contact behaviors over the pad surface, and the transient change of wafer orientation is simulated combining the transient motion equations of carrier. The simulation results indicate that the higher the height of the ball joint, the faster the orientation adjustment of wafer. The disadvantage of higher ball joint height is that the maximum nominal contact pressure is increased. The increasing of friction moment is of benefit to maintain the wafer in a balance posture, but non-uniformity of contact pressure distribution is increased. Therefore, the reasonable design of carrier structure is very important for the performance of CMP. The analysis of dynamic characteristics of carrier using the transient mixed lubrication model proposed can offer helpful theoretical guidance to the design of carrier.

Three-Dimensional Hydrodynamic Lubrication Analysis of Cylinder Liner-Piston Ring

This paper constructs a three-dimensional transient hydrodynamic lubrication model for cylinder liner-piston ring based on the three-dimensional transient average Reynolds equation and asperity contact model. A computer program was written with FORTRAN to calculate hydrodynamic lubrication, in which the surface roughness, the variable viscosity effect and the deformation of the circumferential direction of the cylinder liner are taken into account. The film pressure distribution in different crank angle during the stroke, minimum film thickness and friction are computed and analyzed with this program. This three-dimensional transient hydrodynamic lubrication model provides a design basis for the friction analysis of cylinder liner-piston ring.

Theoretical Investigation of Transient Lubrication in Spur Gear

This paper presents performance characteristics of transient thermo-elastohydrodynamic lubrication (TEHL) in line contact with Newtonian fluids. The time-dependent modified Reynolds equation, energy equation and elasticity equation with initial conditions were solved numerically using finite different method, Newton-Raphson method and multigrid multilevel method for an involutes spur gear to obtain the film pressure profiles, film thickness profiles, film temperature and friction coefficient in the contact region. In this analysis, the load is applied on either two pairs or one pair of gear teeth. The simulation results show that at approach point, the film thickness is minimized and film temperature rapidly increases. Film temperature and friction coefficient were suddenly increase when the transition from two pairs to one pair and vice versa are modeled as a step variation of load. The friction coefficient and film temperature were occurrence at pitch point. Film temperature and friction coefficient increase but film thickness decreases when applied load increases. For increasing of speeds, film thickness and film temperature increase but friction coefficient decreses.

Insights into the dynamics of oral lubrication and mouthfeel using soft tribology: Differentiating semi-fluid foods with similar rheology

In-mouth lubrication or ‘oral tribology’ is believed to be a major contributor to the perception of surface-related mouthfeel attributes such as roughness and astringency. In this work, commercial soft-food systems – custards, yogurts and thickened creams, each formulated at varying fat levels with the aim of maintaining consumer acceptability – are characterized using rheology and soft tribology to gain insight into the physical origins of mouthfeel and the dynamics of oral lubrication, including the role of saliva. It is shown that, despite generating similar bulk rheological profiles (oscillatory and steady shear flow properties), foods within each product series exhibit unique tribological properties, attributed to the preferential entrainment of different phases and components at narrow gap. Food-saliva interactions are also shown to play a crucial role in determining transient lubrication properties, which may reflect dynamic oral processes. We explore the use of tribology to differentiate rheologically similar products as a starting point to develop improved approaches for investigating tribological processes occurring in the mouth during food consumption. Challenges associated with the application of tribology in this context are also discussed.

Analysis on the lubrication performances of journal bearing system using computational fluid dynamics and fluid–structure interaction considering thermal influence and cavitation

Since it's difficult to study the fluid/structure interactions in a rotor-bearing system using the conventional method, a new transient analysis method combining computational fluid dynamics and fluid–structure interaction was applied based on actual physical model. Both thermal influence and cavitation were studied. A comparison with the published experimental results was presented and discussed, and theoretical predictions agreed well with the experimental results. Four rotor-bearing systems with different grooves were studied. This developed method can be a very useful tool for the study on the bearing lubrication problem, and can effectively and accurately predict the transient lubrication process.

Existence and numerical results of a transient lubrication problem with cavitation

This work is devoted to an existence result of a transient lubrication problem with a time dependent velocity and in the presence of cavitation which is modeled by the Elrod–Adams method. The existence of a solution is based on the time semi-discretization method which yields to a sequence of a second kind variational inequalities. We also give some numerical results for a journal bearing system.

Modeling and simulation of high speed sliding

We analyze the processes of thermal softening, melting and subsequent melt lubrication that occur during the high speed sliding of a metal piece on another metal block. The temperature rise in the slider arising from both high speed interface friction and from the energy dissipated during plastic deformations is computed using simple analysis and the finite element method. Subsequently, we propose a mathematical model for the transient lubrication problem that describes the behavior of the molten film at the slider–rail interface. This model successfully predicts the evolution process of the melt thickness and the melt front velocity of the liquid film; these predictions agree with the experimentally observed dynamics of molten film better than those from other existing models.

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.

Investigations on the Transient Mould Friction Force in Slab Continuous Casting based on Fast Fourier Transformation

In continuous casting, the friction forces in the mould reflect the lubrication between the mould and slab, moreover, they affect the slab surface quality and caster productivity. In the present research the friction force is calculated by measuring the difference of working forces during HOT and COLD tests, which are carried out on the slab continuous caster equipped with hydraulic oscillators. Through analysing the mould friction using the Fast Fourier Transformation (FFT) method, the effects of mould oscillation mode and casting speed on the mould friction force are evaluated. The characteristics of the friction signal in the frequency domain are also investigated. Especially, the characteristics of friction force fluctuations before breakouts are discussed. The experimental and analytical results may contribute to more clearly understand the transient lubrication behaviour not only in the time domain but also in the frequency domain, which is also useful for the application of FFT in data analysis of mould processes.

Effect of sinusoidal roughened surfaces on pressure in lubricating film

The paper deals with theoretical solution and further calculation of hydrodynamic pressures in a thin lubrication film that divides two roughened opposite surfaces. A spatial two-dimensional transient lubrication model of such sliding surfaces with a sinusoidal profile was developed and applied in this context. Each of the surfaces was described by sinusoids, which may have different wavelengths in the x- and y-directions, but they have the same wavelength characteristics in identical directions. The travelling wave solution was used. Parametric calculations show a significant positive effect of roughness with a small number of peak-hollow waves, the effect vanishes when the wave number exceeds 450.

Transient Elastohydrodynamic Lubrication of Hip Joint Implants

A general transient elastohydrodynamic lubrication model was developed for artificial hip joint implants, particularly in which the three-dimensional time-dependent physiological load and motion components experienced during walking conditions were considered in the theoretical formulation, although only a predominantly vertical load combined with a flexion-extension motion was actually solved. A nominal ball-in-socket configuration was adopted to represent the articulation between the femoral head and the acetabular cup in both simplified and anatomical positions. An appropriate spherical coordinate system and the corresponding mesh grids were used in the general transient lubrication model. Additionally, an equivalent discrete spherical convolution model and the corresponding spherical fast Fourier transform technique were employed to facilitate the evaluation of elastic deformation of spherical bearing surfaces in hip joint implants. The general lubrication model was subsequently applied to investigate the transient lubrication performance of a typical metal-on-metal hip joint implant. The effects of both cup inclination angles in either anatomical or horizontally simplified positions and different lubricant viscosities on the transient elastohydrodynamic lubrication were analyzed under the predominant components of vertically dynamic loading and flexion-extension motion. It was found that the general lubrication model and the numerical methodology were efficient for the transient elastohydrodynamic lubrication analysis during walking condition in hip joint implants. Furthermore, the significant effect of squeeze-film action on maintaining and enhancing the total thin film thickness formation was discussed for the transient lubrication study of the typical hip joint implant.