Fluid-solid interaction model for hydraulic reciprocating O-ring seals

Elastohydrodynamic lubrication characteristics of hydraulic reciprocating seals have significant effects on sealing and tribology performances of hydraulic actuators,especially in high parameter hydraulic systems.Only elastic deformations of hydraulic reciprocating seals were discussed,and hydrodynamic effects were neglected in many studies.The physical process of the fluid-solid interaction effect did not be clearly presented in the existing fluid-solid interaction models for hydraulic reciprocating O-ring seals,and few of these models had been simultaneously validated through experiments.By exploring the physical process of the fluid-solid interaction effect of the hydraulic reciprocating O-ring seal,a numerical fluid-solid interaction model consisting of fluid lubrication,contact mechanics,asperity contact and elastic deformation analyses is constructed with an iterative procedure.With the SRV friction and wear tester,the experiments are performed to investigate the elastohydrodynamic lubrication characteristics of the O-ring seal.The regularity of the friction coefficient varying with the speed of reciprocating motion is obtained in the mixed lubrication condition.The experimental result is used to validate the fluid-solid interaction model.Based on the model,The elastohydrodynamic lubrication characteristics of the hydraulic reciprocating O-ring seal are presented respectively in the dry friction,mixed lubrication and full film lubrication conditions,including of the contact pressure,film thickness,friction coefficient,liquid film pressure and viscous shear stress in the sealing zone.The proposed numerical fluid-solid interaction model can be effectively used to analyze the operation characteristics of the hydraulic reciprocating O-ring seal,and can also be widely used to study other hydraulic reciprocating seals.     

点接触混合润滑中两种求解模型的比较

混合润滑是典型零部件主要的润滑状态,根据表面形貌表征方式的不同,混合润滑模型一般分为统计学模型和确定性模型两类.为研究2种模型求解粗糙表面点接触混合润滑性能的差异,通过基于平均流量模型和GW模型的统计模型、基于统一Reynolds方程的确定性模型,分析并比较不同表面粗糙度、卷吸速度、载荷以及润滑油环境黏度时2种模型预测...     

A multi-scale model for friction in cold rolling of aluminium alloy

A simple and robust friction model is proposed for cold metal rolling in the mixed lubrication regime, based on physical phenomena across two length scales. At the primary roughness scale, the evolution of asperity contact area is associated with the asperity flattening process and hydrodynamic entrainment between the roll and strip surfaces. The friction coefficient on the asperity contacts is related to a theoretical oil film thickness and secondary-scale roll surface roughness. The boundary friction coefficient at the “true” asperity contacts is associated with tribo-chemical reactions between fresh metal, metal oxide, boundary additives, the tool and any transfer layer on the tool. The asperity friction model is verified by strip drawing simulations under thin film lubrication conditions with a polished tool, taking the fitting parameter of the boundary lubrication friction factor on the true contact areas equal to 0.1. Predicted values of average friction coefficient, using a boundary friction factor in the range 0.07–0.1, are in good agreement with measurements from laboratory and industrial rolling mill trials.     

A numerical method for the calculation of lubricant pressures in bearings with mixed lubrication

A model for mixed lubrication, assuming that the total normal load applied to the plane of the lubricated surfaces is carried partly by the hydrodynamic action of the lubrication film and partly by asperity contacts and that the total friction force between the lubricated surfaces is partly due to viscous friction and partly to asperity contacts, was used to develop a numerical solution for pressure distribution in a bearing experiencing mixed lubrication. The geometry treated and the pressure distribution obtained were for a simple slider bearing, but the method could easily be extended to other shapes. The model is based on measured roughness of a real surface. Real load carrying capacity and drag can therefore be determined since they are related directly to bearing pressure distribution     

Experimental and Numerical Study of Micropitting Initiation in Real Rough Surfaces in a Micro-elastohydrodynamic Lubrication Regime

Micropitting is a form of surface fatigue damage that happens at the surface roughness scale in lubricated contacts in commonly used machine elements, such as gears and bearings. It occurs where the specific film thickness (ratio of smooth surface film thickness to composite surface roughness) is sufficiently low for the contacts to operate in the mixed lubrication regime, where the load is in part carried by direct asperity contacts. Micropitting is currently seen as a greater issue for gear designers than is regular pitting fatigue failure as the latter can be avoided by control of steel cleanliness. This paper describes the results of both theoretical and experimental studies of the onset of micropitting in test disks operated in the mixed lubrication regime. A series of twin disk mixed-lubrication experiments were performed in order to examine the evolution of micropitting damage during repeated cyclic loading of surface roughness asperities as they pass through the contact. Representative measurements of the surfaces used in the experimental work were then evaluated using a numerical model which combines a transient line contact micro-elastohydrodynamic lubrication (micro-EHL) simulation with a calculation of elastic sub-surface stresses. This model generated time-history of stresses within a block of material as it passes through the contact, based on the instantaneous surface contact pressure and traction at each point in the computing mesh at each timestep. This stress time-history was then used within a shear-strain-based fatigue model to calculate the cumulative damage experienced by the surface due to the loading sequence experienced during the experiments. The proposed micro-EHL model results and the experimental study were shown to agree well in terms of predicting the number of loading cycles that are required for the initial micropitting to occur.     

Calculation of mixed lubrication at piston ring and cylinder liner interface

This paper reports on the theoretical analysis of mixed lubrication for the piston ring. The analytical model is presented by using the average flow and asperity contact model. The cyclic variations of the nominal minimum oil film thickness are obtained by numerical iterative method. The total friction is calculated by using the hydrodynamic and asperity contact theory. The effects of the roughness height, pattern, and engine speed on the nominal minimum film thickness, friction force, and frictional power losses are investigated. As the roughness height increases, the nominal oil film thickness and total friction force increase. Also, the effect of the surface roughness on the boundary friction is dominant at low engine speed and high asperity height. The longitudinal roughness pattern shows lower mean oil film pressure and thinner oil film thickness compared to the case of the isotropic and transverse roughness patterns.     

On the shear stress of elastohydrodynamic lubrication in elliptical contacts

Results of mathematical modelling of elastohydrodynamic lubrication of rolling contacts are presented. Effects of dimensionless parameters such as speed, normal load, elliptical parameters and coefficient of limiting shear stress on shear stress distributions have been studied. Moreover, profiles on hydrodynamic pressure and film thickness in EHD contacts have been studied. It has been found that shear stress profiles on two contact surfaces in entraining direction are similar with each other in some way. Shear stresses of fluid film on contact surfaces vary with many factors, which reveals the mechanism of traction in elastohydrodynamically lubricated contacts.     

A deterministic model for line-contact partial elastohydrodynamic lubrication

A deterministic model for partial elastohydrodynamic lubrication (EHL) is presented in this paper. The modelling methodology adopts some of the concepts used in the stochastic modelling of partial EHL and some of the procedures for deterministic calculation of asperity pressures. The model is shown to be capable of simulating the basic process of asperity interaction and solid-to-solid contact within an EHL conjunction of rough surfaces. Deterministic results of transient partial EHL in line contacts are obtained when one pair or multiple pairs of asperities collide. The model may help to gain a fundamental understanding of the transient behaviour of asperity interactions in lubricated concentrated contacts of rough surfaces. Asperity pressures may be calculated more accurately than the conventional analyses under dry and static contact conditions. The work represents a first attempt in deterministic modelling of tribo-contacts operating in the mixed regime of micro-EHL and boundary lubrication. Future work will aim at developing more realistic models incorporating factors such as three-dimensional asperity contacts, asperity plastic deformation, thermal effects and the effect of tribo-chemistry.     

基于改进型混合膜润滑模型的金属轧制过程的数值仿真

根据 Wilson和 Chang的粗糙度补偿模型和 Von Mises均匀变形模型 ,在轧制过程中 ,一种新的混合膜润滑模型被建立。一个更精确的平均雷诺方程被用来计算液体动压力。随变形而变化的屈服应力也在模型中被考虑。并编制了程序用来计算接触面积 ,膜厚和流体动压力。该模型使用较少的假设 ,且更切合实际 ,收敛快。它能应用于较宽速度范围的轧制状态     

A computer model of mixed lubrication in point contacts

A numerical model of mixed lubrication is presented in this paper. The idea introduced here is that asperity contact may be viewed as a result of a continuous decrease in film thickness, so that the transition between contact and non-contact is continuous and the same mathematical model should work for both regions. The pressure over the thin films is assumed to obey the Reynolds equation, and the solution of the equation, under the condition of h→0, is expected to be the same as that predicted by the theory of elasticity. To achieve convergent and stable solutions, the left-hand side terms of the Reynolds equation are switched off when the local film thickness approaches zero, leading to a reduced Reynolds equation. Pressure distributions over the entire computation domain are thus obtained through solving a unified equation system without identifying hydrodynamic or asperity contact regions. Computations were conducted for several example cases and results show that convergent solutions are achievable on different types of roughness, over a wide range of λ ratios (0.01 to infinity), and for different slide-to-roll ratios (0.0–2.0).     

Effect of shot peening on rolling contact fatigue and lubricant film thickness within mixed lubricated non-conformal rolling/sliding contacts

The effect of shot peening on rolling contact fatigue (RCF) and lubricant film thickness within non-conformal rolling/sliding contacts operated under mixed lubrication conditions was observed in this study. Rolling contact fatigue tests and film thickness measurements were carried out using specimens with modified surface topography by shot peening process using glass beads having diameter between 0.07 and 0.11 mm. It has been shown that the effect of shot peening on RCF has no positive effect even if shot peened surface of the roller exhibited somewhat higher hardness in contrast to the grounded surface. The reduction of RCF may be caused due to asperities interactions because after shot peening the surface roughness of the roller was increased. Film thickness measurements confirmed that the contact is realized actually only between asperity peaks of shot peened ball and smooth disc.Conversely, no negative effect on RCF was observed when the shot peened surface of the roller was polished. The polish of asperity peaks causes the creation of lands and micro-cavities, which may be employed as lubricant micro-reservoirs. From film thickness measurements it has been observed that lubricant emitted by shallow micro-cavities can provide the local increase in lubrication film thickness, which thereby reduces asperities interactions. Similar results were obtained for start-up conditions where the squeeze lubricant enlarges film thickness and reduces surface interactions.From the obtained results, it can be suggested that properly designed surface topography modification could help to increase the efficiency of lubrication films leading to the enhancement of contact fatigue life of non-conformal mixed lubricated rolling/sliding contacts.     

Numerical and Experimental Studies on Tribological Behaviors of Cu-Based Friction Pairs from Hydrodynamic to Boundary Lubrication

When studying the tribological behaviors of a Cu-based friction pair in different lubrication regimes, calculation of the real contact area of asperity contacts is crucial but difficult. In this work, a mixed lubrication model in plane contacts is developed, and pin-on-disc tests are carried out. The real contact area ratio, load sharing ratio, and friction coefficient are investigated. Effects of sliding velocity, temperature, and pressure are considered. The results show that when the maximum contact area ratio is about 14.6%, the load sharing ratio of asperity contacts is about 95%. The friction coefficient obviously increases from less than 0.04 to about 0.15 as the regime changes from hydrodynamic to boundary lubrication. Asperities have a significant influence on the local lubrication of a Cu-based friction pair, and the action of hydrodynamic pressure cannot be ignored.     

Deterministic mixed lubrication modelling using roughness measurements in gear applications

The presence of surface roughness on the teeth of hardened and ground power transmission gears is an unavoidable consequence of their manufacture. The paper discusses the effect of surface roughness when the elastohydrodynamic lubricant film thickness developed between the gear tooth surfaces is small compared to the heights of the roughness features. The ratio of these quantities, called the Λ value, may be well below unity in typical applications. For such thin film conditions the moving roughness features cause the elastohydrodynamic contact between the gears to be highly transient in nature. Surface roughness features on the working surfaces of the gears move past each other during meshing and these asperity encounters are associated with extreme pressure perturbations, or with film breakdown and isolated asperity boundary lubrication events. The paper reviews approaches used to study this problem and describes a coupled approach to solving the elastic and hydrodynamic equations. This allows numerical solutions to be obtained for these extreme conditions so that transient contact events associated with mixed lubrication can be predicted in a unified numerical solution scheme. Typical results obtained from such an analysis are presented including surface fatigue modelling and contact strain energy calculations.     

滑动条件下弹流润滑的屈服膜厚与屈服边界

本文讨论了弹流润滑的屈服问题，从润滑剂的极限剪应力可以得到屈服膜厚的表达式。屈服膜厚是弹流润滑膜的下降 ，当利用拟合公式得到的膜厚小于屈服膜厚时，由于润滑已经处在非牛顿区，所拟全公式不再适用。本文还给出了用膜厚形式表示的屈服准则，并详细讨论了最大压力，平均速度和滑滚比对屈服膜厚的影响。     

A thermal analysis of strip-rolling in mixed-film lubrication with O/W emulsions

Increase of both roll and strip surface temperatures can significantly affect a rolling process, roll conditions and strip mechanical properties. A comprehensive thermal analysis in cold rolling, especially in a mixed film regime, is needed to understand how thermal fields develop in roll and strip during rolling. It requires a simultaneous solution of the mixed film model for friction in the roll bite and the thermal model for roll and strip thermal fields. This paper presents a numerical procedure to analyse strip rolling process using lubrication with oil-in-water (O/W) emulsions. The thermal model includes the effect of heat generation due to the strip deformation and frictional shear stress at the asperity contacts. The numerical analysis employs a coupled thermal model and a mixed film lubrication model for calculating the friction and the asperity deformation in the bite. The thermal model considers the initial temperatures of the roll and strip, temperature rise due to the strip plastic deformation and friction. While the O/W mixed-film lubrication model takes into account the effect of surface roughness and oil concentration (%vol) of the emulsion. The thermal effect is analysed in terms of strip surface temperature and roll temperature, which are influenced by rolling parameters such as reduction, rolling speed, oil concentration in the emulsion. The results of the parametric study indicate that the effect of oil concentration on the thermal field is relatively small compared to that of reduction ratio and rolling speed. The reduction ratio increases the maximum interface temperature in the roll bite. In the mixed film regime, rolling speed also increases the maximum interface temperature and alters the temperature field of the strip. The numerical procedure was validated against known experimental data and can readily be extended to hot rolling or used to analyse roll strip temperature subjected to different cooling system.     

Sixty years of EHL

It is now 60 years since Ertel produced the first solution to the elastohydrodynamic lubrication (EHL) problem. There has been enormous progress since then, both in numerical modelling and in experimental research on EHL. The moving, rough surface EHL problem can now be solved on laptop‐level computers, while maps of film thickness, pressure and temperature can be obtained experimentally from within rolling/sliding contacts. However, there remain some important questions that have not been fully resolved. One of the most contentious is how to describe the rheological properties of lubricants under the very severe conditions present in thin film EHL contacts. A second is how to model mixed lubricated contact, where the fluid film can break down at asperity conjunctions. But perhaps the greatest challenge to researchers in EHL is to produce useful design equations for predicting the performance of machine components operating in EHL and thereby ensure that EHL theory becomes an integral part of the design process. Copyright © 2006 John Wiley & Sons, Ltd.     

Lubrication of Strip Rolling in the Low-Speed Mixed Regime

An analytical model for strip rolling in the low-speed mixed lubrication regime is developed. An average Reynolds equation for longitudinal saw-tooth surfaces under conditions of high fractional contact area, is combined with an analysis for asperity flattening under conditions of bulk plastic flow, to treat lubrication in the mixed regime. Analyses for the inlet zone and work zone and the influence of pressure on viscosity are included in the model. The model indicates that hydrodynamic lubrication effects are important at much lower speeds than previously considered possible. The film thickness predicted by the model is somewhat smaller than that measured using the oil drop method in rolling aluminum alloy with a mineral oil.     

Influence of Mixed-Lubrication and Rough Elastic-Plastic Contact on the Performance of Small Fluid Film Bearings

A previously developed deterministic elastohydrodynamic (EHD) numerical model for small fluid film bearings functioning in the mixed lubrication regime is extended in this work by considering the rough contact. Several simplifying hypotheses are made: the shaft is considered rigid and smooth, turning at low speeds (isothermal regime), and the pad is assumed to have an elastic-perfectly-plastic behavior. The Reynolds equation is solved on a very fine mesh and the elasto-plastic pad deformation caused by the hydrodynamic pressure is taken into account. A deterministic active set-based method is used for determining the contact pressure, the contact extent, and the corresponding deformation. The results are presented for a partial journal bearing configuration, with a linear shaft velocity of 0.47 m/s under specific pressures varying up to 50 MPa. Two pad materials are considered, and the lubricant is either isoviscous or piezoviscous oil. The simulation results, presented as a function of the bearing specific pressure, include eccentricity, the film thickness, the friction torques, the contact extent, etc. Stribeck curves showing the evolution of the friction coefficient in the hydrodynamic and mixed lubrication regimes are also discussed.     

Effect of surface texturing on mixed lubricated non-conformal contacts

The behaviour of surface texturing based on shallow micro-dents was observed within mixed lubricated non-conformal contacts and compared with results obtained under thin film elastohydrodynamic conditions. Thin film colorimetric interferometry was used to observe the changes in lubrication film thickness. It was found that lubricant emitted by micro-dents could effectively lift off the real roughness features that provided an increase in average but also the local minimum film thicknesses. On the contrary to smooth contact conditions no film thickness reduction is obvious either downstream or upstream the micro-dent. The possible beneficial effect of surface texturing on mixed lubricated contact was checked through the qualitative wear test. It confirmed that an array of shallow micro-dents reduced asperity interactions of rubbing surfaces. Moreover, the effect of micro-dents on rolling contact fatigue was also considered in this study. It has been shown that individual dents would have to be much deeper compared to those used in surface texturing experiments to cause reduction in contact fatigue life. It can be suggested from the obtained results that properly designed surface texturing could help to increase the separation of rubbing surfaces under mixed lubrication conditions.