基于平均流量理论的轧制润滑动力学数值分析

基于Patir-Cheng平均流量理论,建立了考虑轧制界面粗糙度和轧件表面波纹度影响的混合润滑动力学方程.并以实验室冷轧硬铝过程为例,通过数值分析方法研究了表面波纹度、表面粗糙度、润滑液粘度系数等因素对于轧制界面压力和摩擦力的影响.     

冷轧铝工作区的混合润滑特性研究

为研究冷轧铝工作区的混合润滑特性,基于平均流量理论建立考虑表面粗糙度的冷轧铝工作区混合润滑模型,并通过相关文献的数据验证模型的正确性.在不同轧制速度、润滑油黏度以及前后张应力条件下对整个工作区内的润滑特性进行分析,研究轧制工艺参数对油膜厚度、接触面积比以及应力分布的影响.仿真结果表明:随着轧制速度的提高,轧制压力有一定...     

Investigation of the strain distribution with lubrication during the deep drawing process

A lubrication/friction model can be implemented in FEM codes to predict the contact area ratio, friction coefficient and strain distribution in lubricated deep drawing process. In the lubrication analysis, the surface roughness effect on lubrication flow is included by using Wilson and Marsault's average Reynolds equation that is appropriated for mixed lubrication with severe asperity contact. With regard to the asperity contact theory, the well-known flattening effect is considered. Friction is expressed in terms of variables such as lubricant film thickness, sheet roughness, lubricant viscosity, interface pressure, sliding speed, and strain rate. The proposed lubrication/friction model combined with a finite element code of deep drawing process to predict the contact area ratio, friction coefficient and strain distribution. Numerical results showed that the present analysis provides a good agreement with the measured strain distributions.     

Research of oil film thickness model and surface quality in cold rolling copper foil

Rolled copper foil is widely used in high frequency and speed transmission of fine line printed circuit board, because of its high strength, good toughness and high density. In this paper, a theoretical model for copper foil rolling in mixed lubrication regime was developed on the basis of the average volume flow model and asperity flattening model. A more accurate relation for the variation of the lubricant viscosity with pressure and temperature was considered. The cold rolled copper foil experiment was carried on with different viscosity of rolling oil and pass reduction. The effects of rolling oil viscosity and pass reduction on lubricant pressure, contact area ratio and film thickness ratio were studied. The calculation results agree well with the measured data from copper foil rolling experiment. For obtaining higher surface quality, the rolling oil viscosity is about 10 mm²/s, and the pass reduction is about 30%. Copyright © 2013 John Wiley & Sons, Ltd.     

A Mixed Lubrication Model for Computer Simulation of Extrusion Processes

A mixed lubrication/friction model for extrusion process is developed in the present research. The model combines a rigid-plasticity finite element code to simulate the interface condition between the tooling and workpiece in the extrusion operation. The influence of surface roughness on lubricant flow is treated by using the average Reynolds equation. The active lubrication regime and appropriate friction factor were determined from the current local values of interface variables such as mean lubricant film thickness and workpiece and tooling roughness, in addition to the more traditional external variables such as interface pressure, node sliding velocity and strain rate of the workpiece. Numerical results using the coupled code include friction stress and normal pressure under different lubrication conditions are compared with experimental investigation. The discrepancy is very small and the proposed model proved to be very efficient in predicting interface friction condition in the extrusion processes.     

Effects of cold rolling process variables on final surface quality of stainless steel thin strip

In cold rolling some surface defects, known as pits, are due to lubricant that, entrapped in the deep valleys of the surface roughness, is nearly incompressible and acts like an inclusion avoiding microcavity elimination. During the rolling process, when specific favorable conditions can be set up, the lubricant may be expelled by the microplasto-hydrodynamic lubrication (MPHL) mechanism and pits may be recovered. In this paper the Λ_m parameter, index of the MPHL, is investigated together with the neutral point position to better understand the practical process recommendations for surface defect recovery. By means of finite element analysis of a Sendzimir’cold rolling process, the sensitivity of these objective functions are studied by means of a design of experiment analysis changing the major process variables like back tension, friction coefficient, reduction parameter, initial thickness, and roll diameter.     

热效应对粗糙表面倾斜轴颈轴承润滑性能的影响

联立基于平均流量模型的广义Reynolds方程、三维能量方程和固体热传导方程等,计算了计及热效应时,不同表面形貌和轴颈倾斜角下的轴承油膜压力、油膜温度、油膜反力、端泄流量、摩擦系数和保持轴承稳定工作的力矩.考虑和不考虑热效应的轴承润滑特性计算结果表明,计入热效应时,表面形貌和润滑油粘压效应对偏心率较大的轴颈倾斜轴承润滑性能影响较大.     

The investigation of contact ratio in mixed lubrication

In order to describe the mixed lubrication in nano-scale which is constituted from dry friction, boundary lubrication, and thin film lubrication, a contact ratio between surfaces of a glass disk and a steel ball in a pure rolling process has been measured by the technique of Relative Optical Interference Intensity (ROII) with a resolution of 0.5 nm in the vertical direction and 1 μm in the horizontal direction. The relationships between the contact ratio and its influence factors have been investigated. Experimental results indicate that the contact ratio in the static state is related to the combined surface roughness, maximum Hertz pressure, and the combined elastic module of tribo-pair in an exponential function. The decrease of rolling speed or lubricant viscosity, and the increase of the pressure will enhance the dynamic contact ratio which is the contact ratio measured in the rolling process. The addition of polar additives into basic oil will reduce the contact ratio. The contact ratio between rough surfaces is larger than that between smooth surfaces in the higher speed region. However, the former becomes smaller than the later after speed decreases below a critical value. A formula for calculating the dynamic contact ratio is given in the end of the paper.     

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 Two-Wavelength Model of Surface Flattening in Cold Metal Rolling with Mixed Lubrication

A new model of surface flattening is developed for cold metal rolling in the mixed regime. Longitudinal surface roughness is modeled by two separate wavelengths. The new model follows the asperity crushing analysis of Sutcliffe (1999) for unlubricated rolling but additionally includes a hydrodynamic model to account for the effect of the lubricant. The effect of various parameters including speed, reduction in strip thickness, roughness wavelength and lubricant properties is examined. The results show similar behavior to previous models of mixed lubrication, with a speed parameter A_s having the most influence, and confirm the results for unlubricated rolling that the short wavelength components of the surface roughness persist more than the long wavelength components. The predicted changes in roughness are in good agreement with experiments.     

表面横向波形点接触弹流动压性能的研究

物体的表面结构形态在有些场合下对润滑状况有重要的影响作用.本文研究了考虑表面粗糙度的点接触问题的特点,建立了点接触的分析模型和接触物体表面的横向波形数学模型,应用多网格重积分方法求解了在给定条件下其与时间相关的弹流动压性能的解.主要分析了表面横向波形的点接触的润滑性能,揭示了油膜压力、油膜厚度在油膜接触区上随参数变化的特征和规律.     

Thermoelastohydrodynamic Analysis of Spur Gears with Consideration of Surface Roughness

Thermoelastohydrodynamic lubrication (TEHL) analysis for spur gears with consideration of surface roughness is presented. The model is based on Johnson’s load sharing concept where a portion of load is carried by fluid film and the rest by asperities. The solution algorithm consists of two parts. In the first part, the scaling factors and film thickness with consideration of thermal effect are determined. Then, simplified energy equation is solved to predict the surfaces and film temperature. Once the film temperature is known, the viscosity of the lubricant and therefore friction coefficient are calculated. The predicted results for the friction coefficient based on this algorithm are in agreement with published experimental data as well as those of EHL simulations for rough line contact. First point of contact is the point where the asperities carry a large portion of load and the lubricant has the highest temperature and the lowest thickness. Also, according to experimental investigations, the largest amount of wear in spur gears happens in the first point of contact. Effect of speed on film temperature and friction coefficient has been studied. As speed increases, more heat is generated and therefore film temperature will rise. Film temperature rise will result in reduction of lubricant viscosity and consequently decrease in friction coefficient. Surface roughness effect on friction coefficient is also studied. An increase in surface roughness will increase the asperities interaction and therefore friction coefficient will rise.     

粗糙度各向异性对润滑接触面摩擦的影响

齿轮、轴承、凸轮等重载接触副的性能受表面粗糙度的显著影响。高负载情况下的摩擦因数与润滑接触面粗糙度的各向异性相关。测量的表面粗糙度可以分解为一系列具有不同波长、幅值的正弦表面粗糙度,因此,考虑各向异性正弦表面粗糙度,构建粗糙表面点接触瞬态弹性流体动力润滑(TEHL)模型,提出基于多重网格算法的粗网格构造新方法,提高粗糙表面润滑问题求解的稳健性。研究表面粗糙度各向异性对高负载情况下摩擦因数的影响规律。结果表明,粗糙度的各向异性影响接触面压力、油膜厚度分布、粗糙度形变量,从而影响摩擦因数。提出一个组合函数来量化粗糙度各向异性对摩擦因数的影响,表明全膜润滑到混合润滑的过渡不仅与载荷、速度等工况参数相关,还与粗糙度各向异性相关。     

Die-Workpiece Interfacial Behaviors in Axisymmetric Forging Processes with Flat Dies

A new piezo-viscous, average flow model combined with an advanced slab method is used to investigate the lubrication in axisymmetric forging processes with flat dies. Interactions between lubricant transport and surface roughness are studied. Influence of friction on the outward motion of the workpiece is also considered. It is shown that the lubricant is transported outward in a speed slower than half the surface speed, and the film thickness is smaller than the case of the smooth surface if asperity contact is not severe. However, when asperity contact is pronounced, the lubricant pressure distribution is quite different from the prior research. Mixed lubrication is observed in most of the region where asperity contact occurs. Boundary lubrication can only be found in a narrow area near the workpiece edge.     

On the hydrodynamic liquid lubrication analysis of slider/disk interface

A numerical solution for ultrathin hydrodynamic liquid lubrication of slider/disk interface is introduced. Both surface roughness effects and non-Newtonian behavior of the liquid lubricant are incorporated into the hydrodynamic lubrication analysis. A non-Newtonian liquid is used as the lubricant, and its behavior is described by a power-law rheological model. The contact pressure is calculated for a Gaussian surface roughness. The hydrodynamic load capacity is calculated by using an averaged form for the Reynolds equation. The finite difference scheme, with Gauss–Seidel iterative-relaxation method, is applied to solve the average Reynolds equation. The effects of surface roughness parameter, surface pattern parameter, and the power-law exponent on hydrodynamic pressure distribution, hydrodynamic load capacity are studied and discussed.     

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 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.     

The Effectiveness of Boundary and Hydrodynamic Lubrication when Cold Rolling Aluminum Metal

How hydrodynamic and boundary lubrication affect a lubricant's film strength when cold rolling aluminum was studied using a laboratory mill. The film strength of the lubricant was determined by increasing the amount of reduction until a rapid rise in load and temperature produced a herringbone pattern on the surface of the metal. The hydrodynamic lubrication was changed by increasing the viscosity of the base oil or by increasing the rolling speed. The boundary lubrication was changed by increasing the concentration of the additives or by changing the type of additives. The results of the test showed that either increasing the amount of the hydrodynamic lubrication or increasing the amount of the boundary lubrication were effective ways to increase the film strength of the lubricant; however, the effectiveness of each decreased as the calculated film thickness of the lubricant increased. It is proposed that this can be explained by the decrease in contact area between the work roll asperities and the surface of the sheet as the thickness of the lubricant film increases.     

水压摩擦副中的压力与流量特性

水的低粘度使得水压润滑间隙中的惯性力与粘性力几乎处于同一数量级,因此研究摩擦副的润滑特性时,不能忽略惯性力的影响。惯性力的作用可能改变间隙流场的局部甚至全局流态,从层流变为紊流,因此应用经典润滑理论所得的计算结果将产生太大的误差。采用CFD方法从N-S方程出发对水压摩擦副中压力与流量特性的惯性影响和表面粗糙度效应进行了研究,并与经典润滑理论结果进行了对比,发现表面粗糙度和惯性力对润滑间隙中的压力和流量特性均有一定的影响。其中,惯性力对端面摩擦副中的压力分布具有一定的影响,对端面副和径向副的泄漏流量均有影响,并且惯性力使得表面粗糙度对流场流态的影响得以体现,因此以N-S方程而不是雷诺方程为模型所得的计算结果更加准确。     

Elastohydrodynamic Lubrication of Rough Surfaces under Oscillatory Line Contact with Non-Newtonian Lubricant

This paper presents the results of a transient analysis of elastohydrodynamic lubrication (EHL) of two parallel cylinders in line contact with a non-Newtonian lubricant under oscillatory motion. Effects of the transverse harmonic surface roughness are also investigated in the numerical simulation. The time-dependent Reynolds equation uses a power law model for viscosity. The simultaneous system of modified Reynolds equation and elasticity equation with initial conditions was solved using the multigrid, multilevel method with full approximation technique. The film thickness and the pressure profiles were determined for smooth and rough surfaces in the oscillatory EHL conjunctions, and the film thickness predictions were verified experimentally.

For an increase in the applied load on the cylinders or a decrease in the lubricant viscosity, there is a reduction in the minimum film thickness, as expected. The predicted film thickness for smooth surfaces is slightly higher than the film thickness obtained experimentally, owing primarily to cavitation that occurred in the experiments. The lubricant film under oscillatory motion becomes very thin near the ends of the contact when the velocity goes to zero as the motion direction changes, but a squeeze film effect keeps the fluid film thickness from decreasing to zero. This is especially true for surfaces of low elastic modulus. Harmonic surface roughness and the viscosity and power law index of the non-Newtonian lubricant all have significant effects on the film thickness and pressure profile between the cylinders under oscillatory motion.