Thermoelastic Analysis of Grooved Friction Clutches Using Finite Element Method

High temperatures appear on friction clutch surfaces due to the heat generated between the contacting surfaces during the slipping period. In some cases the maximum temperatures will exceed the allowable working temperature; friction clutches will fail rapidly when working under these conditions for a long time. Therefore, it is necessary to study the effect of design parameters (e.g., grooves) on the thermoelastic behavior of friction clutches to avoid these kinds of failures or at least increase the lifetime of friction clutch. A finite element method will be used to investigate the effect of the circumferential groove on the thermoelastic behavior of a single-disc clutch during a sliding phase. Axisymmetric models have been developed in this analysis to simulate the dry clutch systems during the sliding period. The effect of the groove area ratio G.R (defined as the groove area divided by the nominal contact area) on thermal and elastic behaviors is investigated. The results showed that the groove size has a significant effect on the magnitude and distribution of contact pressure, temperature field and heat generated along the frictional surfaces.     

The performance of friction materials in overload clutches

The performance of seven friction and mating material combinations has been measured in overload clutch conditions. Significant differences in performance were observed between the material combinations in different conditions indicating that choice of material and contact pressure are important when designing an overload clutch. Using consistency of the static and dynamic coefficients of friction as performance indices, the sintered metal material was superior to the organic materials.The performance of these materials did not correlate with the performance expected in other applications, such as automotive brakes and clutches. For example, the mean values of the static and dynamic coefficients of friction were in some cases over 100% higher than the manufacturers' quoted figures. It is suggested that these differences are due to the unique conditions imposed by overload clutches in that firstly the material is permanently clamped up, which prevents the escape of wear debris, and secondly when slipping it always operates at maximum work capacity.It is suggested that overload clutch performance could be improved by facilitating the escape of wear debris, and grooved friction material may be a simple method of achieving this.     

Evaluating lifetime performance of limited slip differentials

Extensive research has been performed regarding wet clutch function and performance. Although wet clutches are used in both automatic transmissions and limited slip differentials in cars, most research has been performed for wet clutches incorporated in automatic transmissions. The operating conditions of wet clutches in automatic transmissions differ from the operating conditions of the wet clutches used in limited slip differentials. Therefore, a method and a test bench to use in the investigation of the degradation of limited slip differentials were developed in this work. The typical operating conditions of the limited slip differential and the differences compared with wet clutches incorporated in automatic transmissions were also addressed. Tests performed showed that the developed test bench and method can be used to address differences in frictional response over time for different types of operating conditions. Copyright © 2013 John Wiley & Sons, Ltd.     

单向离合器接触摩擦及承载性能的研究

探讨了多体接触型单向离合器的摩擦及承载特性;采用弹性力学有限元方法分析了该离合器摩擦自锁性能和摩擦锁紧中的接触体与内外圈的接触强度特性;建立了该离合器的摩擦及承载性能分析的数学模型。通过数值分析,给出了内外圈和接触体的应力分布曲线以及接触产生的最大应力位置。结果表明,该离合器的摩擦自锁特性与接触构件的几何形状、材料特性、载荷以及接触状态有关。摩擦啮合过程是几何学与摩擦学的耦合。研究中不仅计人了静摩擦和摩擦锁紧时刚体位移的影响,而且还考虑到接触摩擦锁紧过程中存在的滑动摩擦,通过确定滑动区域,正确判定了离合器的摩擦锁紧状态。最后通过实例计算分析验证了本方法的有效性。     

Influence of surface topography on friction characteristics in wet clutch applications

In heavily loaded wet clutches, such as in limited slip differentials, sintered friction materials are sometimes used due to their resilience at high loads and high temperatures as well as their competitive cost in comparison to alternative friction materials.During the lifetime of the clutch, changes in the friction materials’ topography occur. These changes will influence the friction characteristics of the clutch, and therefore affect the anti-shudder performance of the transmission system. This paper investigates the influence of, and classifies, changes in the topography of the sintered friction material.The topography is measured by utilizing vertical scanning interferometry. Different parameters are investigated in order to find relevant parameters correlating to the wear of the material.Results show that changes in the topography of the friction material do indeed influence the friction characteristics of the clutch and that it is possible to calculate relevant topography parameters that describe the amount of wear the material has been subjected to.     

Influence of operating conditions on friction and temperature characteristics of a wet clutch engagement

To optimise the gear change in automatic transmissions, more knowledge is needed of the engagement behaviour of wet clutches. A factorial design investigation of the engagement of a wet clutch has been carried out. The friction and temperature characteristics have been studied. The experiments were carried out in an apparatus that can vary sliding velocity, drive torque, inertia, force rate, and lubricant flow. The results show how these parameters affect the response data: engagement time, developed energy, temperature rise, maximum torque, maximum power, static friction, dynamic friction, and initial friction. There are interaction effects between some of the input parameters, but they are relatively small. The friction coefficient varies over time but is independent of the input parameters, except for dynamic friction, which decreases with increasing sliding velocity, drive torque, and inertia. The temperature rise was found to be proportional to developed energy and both these were most affected by sliding velocity. The drive torque and force rate have the greatest effect on the engagement time.     

Contact properties of a wet clutch friction material

Wet clutches are required to transmit torque and also prevent motion in automatic transmissions. Their performance is critically dependent on a friction material which comprises one of the contacting surfaces. Friction materials are usually a composite of fibres, naturally occurring minerals and particles of silicon and graphite, which are all bonded together with a resin. The material formed has very rough surfaces with much steeper slopes than normally-finished steel surfaces. When the friction material is loaded against a relatively flat counterface the real area of contact is only a small percentage of the nominal area and consists of many small, independent “contact units”. It is important to know the conditions present in the contact units (spatial dimensions and pressure) in order to understand and model wet clutch lubrication.In this study, the contact units formed between a paper based friction material and a glass counterface have been investigated under different pressures and during rubbing. A contact visualisation technique is used to directly view and capture images of the contact. The real area of contact and the number of individual units is subsequently determined by image analysis. It is found that the real area of contact increases approximately linearly with applied load, and increases rapidly with rubbing, due to wear. As the load is increased, the number of individual contact units increases up to a critical pressure, suggesting more parts of the material support the load. Above the critical pressure the contact units may be deforming elastically and/or plastically to form larger units. After rubbing, large contact units are formed by flat areas on the tops of the contacting fibres, which are formed during wear. The topography of individual fibres is studied before and after the wearing process using atomic force microscopy, and the results support the truncating wear mechanism.     

湿式换挡离合器摩擦片磨损量计算方法的研究

介绍了综合传动湿式换挡离合器的工作原理,通过分析离合器结合分离过程的油压和滑摩转速变化规律,拟合出离合器工作油压与转速的特性方程,建立了离合器摩擦片磨损量计算模型。依据离合器摩擦副磨损试验结果,确定了油压、滑摩转速影响指数与结合次数的关系。利用组建的综合传动湿式换挡离合器磨损量测试试验系统,进行了18 000次离合器换挡试验。试验结果表明,湿式换挡离合器磨损量可以表示为工作油压与滑摩转速的函数。     

Measurement and Characterization of “Resonance Friction” at High Sliding Speeds in a Model Automotive Wet Clutch

The friction forces between various lubricated “friction materials” and sapphire disks were measured using a new “high-speed” rotating disk attachment to the surface forces apparatus (SFA). Two different clutch lubricants and two different friction materials were tested at sliding speeds and normal loads from 5 to 25 m/s, and 0.2 to 1 N (nominal pressures ~1 MPa), respectively. The results show that “resonance friction”—characterized by large amplitude oscillatory (i.e., sinusoidal) vibrations, also known as shudder or chatter—dominates dynamical considerations at high sliding speed, replacing the smooth sliding or low-amplitude stick–slip that is characteristic of low speed/low load sliding. The characteristic (rotational) speeds or frequencies at which resonance friction occurs depend only on the coupled/uncoupled mechanical resonance frequencies of the loading and friction-sensing mechanisms. In contrast, the intensity of and time to enter/exit shudder depends strongly on the lubricating oil and, to a lesser extent, on the friction material. Physical–chemical analyses of the friction materials before and after testing showed that the samples undergo primarily structural rather than chemical changes. Our results provide new fundamental insights into the resonance friction phenomenon and suggest means for its control.     

湿式双离合器自动变速器起步研究及仿真

以搭载湿式双离合器自动变速器某轿车为例,介绍了双离合器自动变速器的结构和工作原理。基于Matlab/Simulink模块仿真平台建立了装有该自动变速器车辆起步模型,模型采用两个离合器同时起步。根据起步条件确定离合器的起步控制策略,通过控制两个离合器最终油压参数来保证起步过程的快速、平顺。对双离合器自动变速器起步过程进行了仿真分析。仿真计算结果表明:加速度、冲击度和离合器滑摩功均在合理的范围内。     

Microscopic stick–slip in friction force microscopy

Friction force measurements were performed on 2-hydroxy stearic acid (2-HSA) and 12-hydroxy stearic acid (12-HSA) coated silica surfaces in air using an atomic force microscope. The 2-HSA displayed viscoelastic behaviour with a yield point as the static–dynamic friction transition. Steady sliding motion was replaced by microscopic stick–slip at lower velocities and higher loads. Stick–slip motion was successfully described and fitted to a phenomenological model ascribed to interfacial material melting and freezing in periodic cycles. The stick–slip periodicity is of the same order as the contact diameter. The 12-HSA did not experience a yield point and exhibited steady sliding over the entire load and velocity regime. We attribute these observations to the difference in molecular configuration, shear strength and adsorption density of the stearic acid layers. This revised version was published online in August 2006 with corrections to the Cover Date.     

湿滑状态下轮胎路面摩擦特性的数值分析方法

为研究湿滑状态下轮胎路面的摩擦特性,以胎面橡胶和沥青路面作为研究对象,利用谐波叠加法建立三维粗糙路面模型,采用“伪”流体动力轴承作用等效反映路面水膜“密封”作用,综合使用有限元软件ABAQUS和计算流体动力学软件Fluent得到湿滑状态下橡胶与路面滑动接触时的橡胶接触压力、滞后摩擦力及路面水膜承载力,由此形成了综合兼顾橡胶材料、接触压力、滑动速度、路面形貌和路面水膜等多因素的轮胎与湿路面摩擦特性的仿真方法。通过橡胶与干-湿路面摩擦特性的变化与公开的试验对比,证明本方法的合理性和可行性,并进一步分析滑动速度、接触压力和路面特征对湿滑状态下轮胎路面摩擦特性的影响规律。研究结果为轮胎和路面的抗湿滑性设计及优化提供理论依据。     

Impact of lubricant formulation on the friction properties of carbon fiber clutch plates

Since their introduction over ten years ago, carbon fiber based friction materials have been employed by transmission builders in a wide variety of applications, including torque converter clutches, synchronizers, limited slip devices and shifting clutches. This new generation of materials gives improved durability relative to cellulose; carbon fiber materials offer inherently greater wear resistance and improved resistance to thermal degradation. However, carbon fiber based materials also bring inherently different friction characteristics than their cellulose based counterparts. As a result, a different approach to lubricant formulation is required to provide optimized friction control in applications where they are used. It is well known that in order to achieve and maintain the required friction in a clutch, the correct combination of surface properties and additive chemistry is required. In this paper the impact of different additive chemistries on the friction of carbon fiber clutch plates has been investigated. It will be shown that with the appropriate choice of additive system, carbon fiber based friction plates can offer a number of performance improvements over more conventional materials. Copyright © 2006 John Wiley & Sons, Ltd.     

干式DCT双离合器联合起步最优协调控制

基于自主开发的6速干式双离合器自动变速器(Dual clutch transmission, DCT),提出基于极小值原理的双离合器联合起步过程发动机和双离合器转矩协调最优控制算法。在起步滑摩阶段,以滑摩功和冲击度为性能指标,在反映驾驶员意图的终端约束下,采用极小值原理和改进的发动机恒转速控制,研究并确定起步过程中双离合器,发动机转矩及其转速。根据起步控制目标,确定分离离合器分离条件和离合器的转矩分配关系。在需求转矩切换阶段,将发动机输出转矩平滑切换到驾驶员需求转矩。在Matlab/Simulink软件平台上,搭建DCT车辆双离合器联合起步控制仿真模型,对不同驾驶意图和起步档位下DCT车辆的起步过程进行仿真。结果表明,所提出的基于极小值原理的发动机和双离合器转矩协调控制策略,有效地保证双离合器联合起步品质,反映起步意图,延长离合器使用寿命。将所得的离合器最优转矩控制律转化为离合器位置控制律,在离合器伺服控制试验台架上进行离合器位置闭环控制,得到较好的跟踪效果。     

湿式摩擦副滑摩过程摩擦热计算

针对湿式离合器摩擦副的结构特点,研究离合器摩擦副表面粗糙接触情况,改进平均流量模型,建立修正的雷诺方程用于计算滑摩过程中油膜压力和油膜厚度的变化规律。采用Greenwood-Tripp接触模型,建立摩擦副摩擦热方程,模拟湿式摩擦副在滑摩过程中油膜厚度、相对滑摩转速、接合油压以及摩擦转矩变化规律,对摩擦副滑摩过程中微凸体和油膜剪切作用产生的摩擦热进行分析,得到它们径向呈线性和抛物线的分布规律,讨论接合油压和相对滑摩转速对微凸体和油膜剪切作用产生摩擦热的影响,并通过钢片的温度场实验对模拟结果加以验证。研究表明:接合油压越大,单位时间内微凸体和油膜剪切作用产生的摩擦热越大,单位时间产生摩擦热峰值的时间越提前;相对转速差越大,微凸体在滑摩过程中单位时间产生的摩擦热越大,油膜则与之相反,且相对转速的变化对单位时间产生摩擦热峰值的时间无影响。     

Low-Speed Atomistic Simulation of Stick–Slip Friction using Parallel Replica Dynamics

Atomic stick–slip friction has been predicted by molecular dynamics simulation and observed in experiments. However, direct quantitative comparison of the two has thus far not been possible because of the large difference between scanning velocities accessible to simulations and experiments. In general, the slowest sliding speeds in MD simulations are at least five orders of magnitude larger than the upper limit available to experimentalists. To take a step toward bridging this gap, we have applied parallel replica dynamics, an accelerated molecular dynamics method, to the simulation of atomic stick–slip. The method allows molecular simulations to run parallel in time in order to extend their duration, thereby enabling lower scanning velocities. We show here that this method is able to predict atomic stick–slip friction accurately and efficiently at scanning speeds several orders of magnitude slower than standard molecular dynamics simulations. The accuracy and usefulness of this method is illustrated by correct prediction of the logarithmic dependence of friction on velocity.     

Wet clutch transmission fluid for AWD differentials: base fluid influence on friction characteristics

During the past few years several electronically controllable automotive transmission systems, where wet clutches are used as intelligent differentials, have emerged in the market. In this type of application the anti‐shudder properties of the lubricants are of vital importance. This paper investigates the influence of base fluids on the anti‐shudder properties of transmission fluids for wet clutches in all‐wheel‐drive systems. The investigated all‐wheel‐drive system featuring a wet multi‐plate clutch with a sintered brass‐based friction material is described. The test equipment used to determine the frictional characteristics of the transmission fluid is described. The parameters studied include base fluid type and base fluid viscosity. It is shown that the choice of base fluid has no impact on torque capacity, but that the base fluid influences the temperature dependence of the dynamic friction and the anti‐shudder properties. It is also shown that the major effect on the friction characteristics is caused by additive effects rather than base fluid effects. Copyright © 2006 John Wiley & Sons, Ltd.     

考虑摩擦副接触应力场和冷却流场的湿式离合器温度场分析

温度对湿式离合器摩擦副的摩擦特性和热失效具有重要影响。为了获取湿式离合器温度场的分布规律,建立摩擦副接触应力分布有限元模型和摩擦片沟槽内冷却流场数值计算模型,获得了摩擦副接触应力随离合器接合油压的变化规律和冷却流场对流换热随离合器转速的变化规律。在此基础上,提出考虑离合器摩擦副接触应力分布时变特性和冷却流场分布时变特性的离合器温度场数值计算模型。将所建温度场模型的仿真结果与试验结果作对比,验证了所建温度场模型的正确性。通过计算获得了湿式离合器接合过程中不同钢片在半径和厚度方向的温度分布规律,揭示了摩擦副接触应力场和摩擦片沟槽内冷却流场对离合器温度场的影响规律。结果表明,在离合器摩擦副半径方向上,摩擦副的温度分布规律与接触应力分布规律相一致。而摩擦片沟槽内冷却流场的对流换热主要影响离合器同步阶段的温度分布。     

Friction materials for overload clutches

To improve the performance of overload protection devices on agricultural machinery, investigations on the friction interface of a plate clutch were carried out on a simulator. Organic, sintered, bearing, plastic and oil-immersed paper friction materials were tried: oil-immersed paper gave the best overall performance, which was significantly different to that expected in automotive applications. This could well be due to the unique conditions imposed by overload clutches where the material is permanently clamped and, when slipping, operates at a constant high surface pressure     

The Frictional Properties of Newtonian Fluids in Rolling–Sliding soft-EHL Contact

A combined experimental and numerical study has been carried out to explore friction in rolling–sliding, soft-EHL contact. Experimental work has employed corn syrup solutions of different concentrations in water to provide a range of lubricant viscosities and has measured Couette friction in mixed rolling–sliding conditions over a wide range of entrainment speeds. A Stribeck curve has been generated, ranging from the boundary to full film, isoviscous-elastic lubrication regime. In the latter regime, friction coefficient is approximately proportional to the product of (entrainment speed × viscosity) raised to the power 0.55. Numerical solution of the isoviscous-elastic lubrication regime has been used to derive predictive equations for both Couette and Poiseuille friction in circular, soft-EHL contacts. This shows that in soft-EHL the Poiseuille or “rolling” friction can have magnitude comparable to the Couette friction. The calculated Poiseuille friction coefficient can be predicted from non-dimensional load and speed using a simple power law expression similar to that used for film thickness. However accurate prediction of calculated Couette friction coefficient requires a two-term power law expression. Comparison of experimental and numerical Couette friction coefficients shows quite good agreement between the two, with a similar non-dimensional speed dependence, but slightly lower predicted than measured values.