The influence of surface roughness and the contact pressure distribution on friction in rolling/sliding contacts

A numerical contact model is used to study the influence of surface roughness and the pressure distribution on the frictional behaviour in rolling/sliding contacts. Double-crowned roller surfaces are measured and used as input for the contact analysis. The contact pressure distribution is calculated for dry static contacts and the results are compared with friction measurements in a lubricated rolling/sliding contact made with a rough friction test rig. The mean pressure is suggested as a parameter that can be used to predict the influence of surface roughness on the friction coefficient in such contacts. The results show two important properties of the friction coefficient for the friction regime studied in this paper: (1) there is a linear decrease in friction coefficient as a function of the slide-to-roll ratio, and (2) the friction coefficient increases linearly with increasing mean contact pressure up to a maximum limit above which the friction coefficient is constant. The absolute deviation of experimental results from the derived theory is for most cases within 0.005.     

考虑摩擦因数与滑动速度相关时的轮轨滚动接触有限元分析

使用与滑动速度相关的摩擦因数替代库伦摩擦定律中的常系数,结合mixed Lagrangian/Eulerian方法建立轮轨滚动接触有限元模型,分析牵引力主导的蠕滑工况下的干燥状态的轮轨滚动接触特性。通过与摩擦因数取值为常数的轮轨滚动接触分析结果对比发现:与滑动速度相关的摩擦因数对轮轨滚动接触最大接触应力和接触斑面积影响不大,均在1%以内;但是对轮轨接触斑内最大Mises应力、最大纵向切应力、最大横向切应力和最大等效塑性应变影响较大,特别是对最大纵向切应力影响幅度近20%;更需要引起注意的是对轮轨滚动接触摩擦力矢量分布和切向塑性应变分布影响明显,这对轮轨滚动接触疲劳损伤分析非常重要。     

Static friction coefficient of some plastics against steel and aluminum under different contact conditions

The static friction behavior was investigated in the case of POM-based polymers containing additives, designated DE20076, DE100KM and DE12017 as well as Nylon-66 designated ZY103HSL, when in contact with AISI 1045 steel and 6061 aluminum alloy. A designed apparatus that detects the exact moment corresponding to the transition from rest to slip was used to measure the coefficient of static friction. The impact of normal load, dwell time before slip, apparent area of contact and lubrication by water and paraffin oil on the static friction characteristics was studied. A decrease of the static coefficient of friction with low normal load followed by a levelling off governed by its effect on the real area of contact and stiffness of the counterface was observed. This also yielded a linear decrease of slip acceleration as well as stick time. A polynomial equation, which increases monotonically, was found to express well the effect of stick time on static friction, independently of the materials in contact. Adhesive effects were generated by the presence of both lubricants with the result of increase of the static coefficient of friction.     

Rolling friction and adhesion between smooth solids

Rolling friction has been explained in terms of crack propagation through an adhesive joint. The contact between a smooth cylinder and flat has been regarded as an adhesive junction bounded by two cracks moving in the same direction at the same speed, one crack continually opening and one closing. Propagation of these cracks requires a force which is calculated from crack theory and shown to be equal to the friction.The theory has been verified experimentally using glass cylinders rolling on smooth rubber. Results show that rolling friction is closely connected with peel adhesion. Moreover, this adhesion interpretation of rolling friction between smooth surfaces explains several observations: (a) the existence of a static rolling friction, (b) the unusually high value of friction and its independence from load and roller radius, (c) the marked effect of lubricant or dust.     

不同真空度气体对粗糙表面接触摩擦的影响

在大气压作用下,通过对不同真空度下接触物体表面摩擦力的测量,研究气体对粗糙接触物体表面摩擦的影响,该实验采用的摩擦副材料是铝合金/有机玻璃、45钢/有机玻璃。实验结果显示:随着真空度的增大,2种摩擦副的摩擦力近似呈现线性变化,静摩擦因数则先减小而后趋于稳定,再有回增的趋势;45钢/有机玻璃摩擦副静摩擦因数取得最小值所对应的真空度比铝合金/有机玻璃摩擦副的更大。通过分析得出静摩擦因数变化的原因:随着真空度的增大,摩擦副接触间隙间的气体密度减小,当固体刚开始运动时,产生摩擦的气体分子与固体表面的分子减少;随着真空度的增大,正压力增大,材料的变形程度增大。     

Elastic–plastic spherical contact under combined normal and tangential loading in full stick

The behavior of an elastic–plastic contact between a deformable sphere and a rigid flat under combined normal and tangential loading with full stick contact condition is investigated theoretically. Sliding inception is treated as a plastic yield failure mechanism, which allows static friction modeling under highly adhesive conditions. Several contact parameters such as: junction tangential stiffness, static friction force and static friction coefficient are extensively investigated. The phenomenon of junction growth and the evolution of the plastic zone in the contact region are briefly described. It is found that at low normal dimensionless loads the static friction coefficient decreases sharply with increasing normal load, in breach with the classical laws of friction. As the normal load further increases the static friction coefficient approaches a constant value that is about 0.3 for many material properties combinations.     

Static friction at high contact temperatures and low contact pressure

The problem of measuring static friction at high temperatures and low contact pressures is theoretically analyzed, as well as the instruments for the accurate determination of the coefficient of static friction at high temperatures for contact pairs made of steel and bronze. The results of experimental investigation show that at low contact pressure and temperature above 120°C coefficient of static friction dramatically increases. This increase in the coefficient of friction can be of great practical value with regard to the load capacity of contacts where external forces are balanced with friction forces.     

Modeling the effect of skewness and kurtosis on the static friction coefficient of rough surfaces

Engineering surfaces possess roughnesses that exhibit asymmetrical height distributions. However, the Gaussian distribution is most often used to characterize the topography of surfaces, and is also used in models to predict contact and friction parameters. In this paper, the effects of kurtosis and skewness on different levels of surface roughness are investigated independently. This is accomplished by adopting the Pearson system of frequency curves and used in conjunction with a static friction model for rough surfaces to calculate the friction force and friction coefficient. This study is the first attempt to independently model the effect of kurtosis and skewness on the static friction and friction coefficient. It is predicted that surfaces with high kurtosis and positive skewness exhibit lower static friction coefficient compared to the Gaussian case. More importantly, it is predicted that, for high kurtosis values, the static friction coefficient decreases with decreasing external force rather than increasing as seen with increasing skewness. This is a very promising result for applications involving smooth lightly loaded contacts such as magnetic storage devices and microelectromechanical systems. The practical significance of the present model is specifically demonstrated on static friction predictions in magnetic storage head–disk interfaces. Such predictions can be used to determine the optimal characteristics of such devices prior to fabrication to achieve lower friction in terms of surface roughness, mechanical properties, apparent contact area, and operational environment.     

大滑滚比工况下弹流摩擦试验研究

针对大滑滚比工况,在自行改造的双盘摩擦磨损试验机上,完成了一系列线接触弹流摩擦试验;测量了不同工况下的摩擦因数,对试验结果进行了全面的分析,得到了一系列摩擦曲线,反映了多种具体工况下,各工况参数对摩擦因数的影响。结果表明：载荷比较小时,摩擦因数对载荷或转速的变化非常敏感;随着载荷的增大,摩擦因数对载荷与转速的敏感程度急剧下降;大滑滚比工况下,摩擦因数随滑滚比的变化幅度不大;摩擦因数随载荷增大而减小,且减小幅度随载荷增大而减小;摩擦因数随转速增大而减小,且减小幅度随转速增大而减小。     

聚四氟乙烯涂层在高载荷条件下的摩擦学性能研究

制备了以45#钢为基体的聚四氟乙烯（PTFE）涂层,利用自制的高载荷条件下摩擦因数测试装置研究了PTFE涂层的摩擦学性能,结果表明：PTFE涂层在4~90MPa范围内具有优良的减摩性能,最低摩擦因数为0.032。正压力与摩擦因数服从负指数衰减模型（Fit Exponential Decay）,随着载荷的增加,静摩擦因数和滑动摩擦因数先迅速降低,然后趋于定值。     

基于分形接触的静摩擦系数预测

根据Ｍ－Ｂ分形接触模型对静摩擦系数进行预测。对分形参数Ｄ、Ｇ及材料参数φ对静摩擦系数的影响进行研究。结果表明静摩擦系数随载荷的增加而增加。这与在极低载荷下静摩擦系数很小的试验事实吻合,因此,Ｍ－Ｂ分形接触模型可能仅适用于载荷极小的情况。     

圆弧接触线回转体摩擦传动运动学研究

针对摩擦传动中的滑动问题，在对圆弧接触线回转体摩擦传动运动学研究的基础上，以曲轴圆角滚压加工为实例，分析了圆弧接触线回转体摩擦传动中减小滑动的方法和思路，避免了求解滑动率这一难点问题。还为曲轴滚压加工中滚刀参数及滚压工艺参数的合理设计提供了优化方法。     

A Study of the Static/Kinetic Friction Behavior of PTFE-Based Fabric Composites

Three different polytetrafluoroethylene (PTFE)-based fabric composites were prepared. The static/kinetic friction behaviors of these composites under different loads and speeds were studied. A 3D laser microscope and profile measurement apparatus were used for analysis of the morphology and weave structure of the composites, and the contact temperature of these composites under different loads and speeds was monitored continuously using a high-precision thermal resistor. In addition, a dynamic mechanical analysis (DMA) apparatus was used to explore the thermal and mechanical properties of PTFE-based fabric composites. The results demonstrated that speed/load, weave structure, and fiber form have an important influence on static and kinetic friction behavior of the fabric composites. Generally, the static friction coefficient is greater than the kinetic friction coefficient, except when considering light load conditions. Under light load conditions, the static friction coefficient is equal to the kinetic friction coefficient. In addition, the kinetic friction coefficient first increased and then decreased with increased speed, but the static coefficient increased first and then remained at an almost constant value. At all sliding speeds, the static and kinetic friction coefficients of tape yarn composites are better than those of the multifilament yarn composites. Weave structure has no effect on the static friction coefficient, but it has a significant influence on the kinetic friction coefficient.     

水润滑条件下转速对车轮钢滚动接触疲劳和磨损性能的影响

为探究水润滑条件下转速对车轮钢滚动接触疲劳和磨损性能的影响,利用滚动接触摩擦磨损试验测试不同转速下车轮试样的剥离寿命、摩擦因数和磨损率,并结合磨损形貌和裂纹扩展形貌观察,对比分析不同转速下摩擦磨损和剥离寿命的影响因素。结果表明:随转速提高,车轮材料氧化程度加剧,导致摩擦因数逐渐增加;当转速由250 r/min增至500 r/min时,摩擦因数增幅较小,应变速率增加导致磨损率下降,当转速由500 r/min增至1000 r/min时,摩擦因数急剧增加,导致材料磨损率增加;随转速提高,剖面塑性流动层厚度、裂纹扩展角度、裂纹分叉深度和最大扩展深度均呈现减小趋势。转速增加带来的摩擦因数的增加,一方面缩短裂纹萌生寿命,另一方面减小了裂纹发生向上转折的深度,最终导致滚动接触疲劳寿命随转速的增加而减小。     

Surface friction under time-dependent loads

A review is presented of the behaviour of static friction between metal surfaces under dynamic loading. It is found that the static friction coefficient is usually represented as a function of stationary contact time, although it has been suggested that the rate of tangential loading may be an important factor in the friction process. Much of the available experimental data cannot separate the effects of contact time and loading rate and thus is of little use in establishing fundamental friction behaviour. Experiments aimed at clarifying this aspect of the friction problem are described. The results show that at high loading rates the coefficient of static friction is independent of contact time and suggest that the rate of tangential force application is controlling friction behaviour in situations in which contact time has previously been considered to be the relevant parameter.     

轮胎稳态运动学与六分力预报Ⅰ:理论与方法

提出一种新的轮胎运动学描述和六分力预报理论。滚动接触是汽车轮胎力学、轮轨动力学的核心问题,由于涉及刚体转动与有限变形,滚动接触运动学与动力学的描述与求解非常困难。用拉格朗日—欧拉混合描述法分析大变形滚动接触结构的速度场、加速度场和接触变形。以车轮定位角为卡尔丹角,用拉格朗日描述,得到了包含刚体转动和弹性变形的轮胎速度场。而接触区域的变形和受力用欧拉描述,通过欧拉网格和拉格朗日网格的信息传递,完成滚动结构动力学分析。所提出的理论可以退化到Fiala模型,并可以从理论上解释子午线轮胎的伪侧偏和伪侧倾现象。基于所建立的运动学理论和非线性有限元,建立轮胎六分力预报方法。针对某轿车子午线轮胎,分析轮胎接地面滑移速度、接地面积、接地压力、侧向剪力分布等随着侧偏角的变化规律,并研究该轮胎侧偏力和回正力矩随着胎面刚度和摩擦因数的参数敏感性。结果表明轮胎侧偏刚度和回正刚度主要受结构刚度控制,而峰值侧偏力和峰值回正力矩主要受摩擦因数控制。将利用所建立的方法和试验,探讨带束层结构对大规格子午线轮胎侧偏特性的影响规律,进一步验证所提出的理论和方法的正确性。所提出的理论和方法开辟了直接从轮胎设计预报轮胎六分力的新途径。     

Study on the oxidation of stainless steels 304 and 304L in humid air and the friction during hot rolling

In rolling process, the contact friction is of crucial importance for accurate modeling, optimum design and control of industrial rolling processes. It is important to characterize the features of the oxide scale of stainless steel in hot strip rolling because the scale on the strip surface affects friction coefficient and thermal conductivity coefficient. To some extent, the rolling force and friction condition depend on the thickness and the microstructure of the oxide scale. Oxidation tests of stainless steels 304 and 304L were carried out in a high temperature electric resistance furnace. The humid air in which the water vapour content can be controlled was generated and remained to flow into the chamber of the furnace in 2.5 × 10⁻⁴ m³/s to study the effect of humidity on the oxidation of stainless steels. The microstructure and thickness of oxide scale layer of stainless steels were obtained and two or three oxide layers can be found. The humid air has a significant effect on the growth of oxide scale. Hot rolling tests were carried out on Hille 100 rolling mill. The friction condition at the roll–strip interface during hot rolling of stainless steel was determined and the transfer of surface roughness was discussed.     

Static friction processes under dynamic loads and vibration

The influence of external vibration on friction phenomena was investigated. The influence of vibration on the primary displacement was assessed. The influence of the normal pressure and the amplitude of external vibration on the static friction was also investigated. Non-linear relations between the coefficient of static friction and the external vibration and between the coefficient of static friction and the normal pressure where vibration occurs were established. Static and dynamic loads on the same friction pair were studied. Sinusoidal vibration in the contact area caused a decrease in the coefficient of static friction of about 60% which is important practically, especially in this design of machine connections where vibration occurs.     

MT-2缓冲器初压力理论计算

通过分析MT-2缓冲器在装车以后缓冲器各主要元件位移量间的几何关系,并推导此缓冲器阻抗力计算公式和结合落锤实验数据,得出其初压力的理论计算公式,并反推静摩擦系数、分析几何接触角对初压力的影响。研究发现,考虑复原弹簧力与否所得出的初压力是有一定差距的。在精确计算初压力时,最好把复原弹簧力考虑在内。不论是否考虑复原弹簧力,初压力都分别随静摩擦系数、各几何接触角的增加而增加。另外,各几何接触角对初压力的影响是不同的。     

Analysis of interface temperature,forward slip and lubricant influence on friction and wear in cold rolling

To improve cold rolling processes, it is necessary to understand and to optimise contact at roll–strip interface. Thus a simulation test has been developed in our laboratory. The upsetting rolling test (URT) enables to study friction and iron fines pollution by the reproduction of the main industrial contact conditions such as plastic strain, normal and tangential stresses and forward slip. On the basis of the URT, a new experimental protocol has been developed to reproduce industrial lubrication regime. Moreover, a new heating system has been designed to simulate interface temperature which has a decisive effect on lubricant behaviour. These optimisations permit to analyse contact temperature, forward slip and lubricant influence on friction, iron fine pollution and surface aspects. A great influence of temperature and lubricant on friction and wear has been put forward. Actually an increase of the Coulomb friction coefficient associated with a decrease of the iron fines quantity have been shown with an increase of temperature. These results seem to indicate more adhesive wear when temperature increases.