Finger Friction Measurements on Coated and Uncoated Printing Papers

A macroscopic finger friction device consisting of a piezoelectric force sensor was evaluated on 21 printing papers of different paper grades and grammage. Friction between a human finger and the 21 papers was measured and showed that measurements with the device can be used to discriminate a set of similar surfaces in terms of finger friction. When comparing the friction coefficients, the papers group according to paper grade and the emerging trend is that the rougher papers have a lower friction coefficient than smoother papers. This is interpreted in terms of a larger contact area in the latter case. Furthermore, a decrease in friction coefficient is noted for all papers on repeated stroking (15 cycles back and forth with the finger). Complementary experiments indicate that both mechanical and chemical modifications of the surface are responsible for this decrease: (1) X-ray photoelectron spectroscopy measurements show that lipid material is transferred from the finger to the paper surface, (2) repeated finger friction measurements on the same paper sample reveal that only partial recovery of the frictional behaviour occurs and (3) profilometry measurements before and after stroking indicate small topographical changes associated with repeated frictional contacts.     

The Effect of DLC Coating Thickness on Elstohydrodynamic Friction

The application of surface coatings has been shown to reduce friction in elastohydrodynamic lubrication (EHL), not only in the mixed and boundary regime when asperity interactions occur, but also in the full film regime. Several studies suggest that the full film friction reduction is due to a violation of the no-slip boundary condition and thus slip is taking place between the solid and the liquid. Another hypothesis proposes that the full film friction reduction is due to the low thermal conductivity of diamond-like carbon (DLC) coatings. In this work, two DLC coatings with the same composition, but different thicknesses, are investigated with uncoated steel specimens as a reference, all with the same surface roughness. Friction tests in a ball-on-disk machine show that both coatings reduce friction compared to the uncoated reference case in full film EHL. The thicker coating is significantly more effective at reducing friction than the thinner one at a maximum friction reduction of 41 % compared to 29 % for the thinner coating. Moreover, contact angle measurements, surface energy measurements, and spreading parameter calculations show no statistically significant differences between the two coatings, suggesting that the friction reduction capabilities of coatings in full film EHL cannot be described by solid–liquid interactions alone. The difference in friction reduction between the specimens in this work is mainly attributed to different thermal properties.     

摩擦角在机械设计中的运用

几何法求解含摩擦的平衡问题,关键在于正确作出摩擦角,根据约束力的方向作出自锁域。三力平衡必汇交,交点H0若在边界上,就可顺利断定极、值性质,是极大还是极小,无须求解不等式。     

The Scale Effect of Roughness on Hydrodynamic Contact Friction

Multistage abrasive finishing processes (grinding, polishing, honing, etc.) are commonly used to produce the geometrical properties of a surface to meet its technical functionalities in the operating characteristics of contacting parts in friction, relating to their durability and reliability (running-in performance, wear resistance, load-carrying capacity, etc.). Coarse abrasive grits followed progressively finer ones are used, which leads to a multiscale stratified surface texture.

In this article, a numerical model of elastohydrodynamic (EHD) contact coupled to a multiscale surface texture model was developed that allows tracking the scale effect of surface features and their interactions on friction performance and lubricant flow under hydrodynamic lubrication conditions. Because the simulation model has as an input the surface topography and to overcome the variability in surface finish formation, textured surfaces at different stages of the finishing process were simulated (virtual texturing method). Surface topography can be decomposed into two principal components: superficial roughness and valleys. Superficial roughness was modeled using a fractal model and a scaling factor was introduced to model valley patterns. The results show the relationship between friction and surface scales.     

摩擦对橡胶胀管的影响及减摩研究

1　前言橡胶胀管机是一种新型的胀管机械 ,它以液压为动力 ,以橡胶为胀管介质 ,通过挤压置于换热管内橡胶弹性体 ,完成换热器、锅炉等的胀管作业 ,其胀头结构如图 1所示[1] 。图 1　胀头结构图胀头的芯轴上套装一个橡胶弹性体 ,两端依次为密封圈和挡圈 ,工作时将胀头伸入欲胀接的管端内 ,胀接时液压活塞抽动芯轴 ,使得橡胶弹性体轴向受压 ,径向发生膨胀 ,迫使管子端部与管板发生弹塑性变形而胀合 ,液压活塞动力撤除后 ,橡胶弹性体立即恢复原状 ,管子与管板仍留有残余接触压力 ,管端与管板结合在一起 ,完成胀接。胀接过程中 ,橡胶弹性体在轴…     

Contact of a Finger on Rigid Surfaces and Textiles: Friction Coefficient and Induced Vibrations

The tactile information about object surfaces is obtained through perceived contact stresses and friction-induced vibrations generated by the relative motion between the fingertip and the touched object. The friction forces affect the skin stress-state distribution during surface scanning, while the sliding contact generates vibrations that propagate in the finger skin activating the receptors (mechanoreceptors) and allowing the brain to identify objects and perceive information about their properties. In this article, the friction coefficient between a real human finger and both rigid surfaces and fabrics is retrieved as a function of the contact parameters (load and scanning speed). Then, the analysis of the vibration spectra is carried out to investigate the features of the induced vibrations, measured on the fingernail, as a function of surface textures and contact parameters. While the friction coefficient measurements on rigid surfaces agree with empirical laws found in literature, the behaviour of the friction coefficient when touching a fabric is more complex, and is mainly the function of the textile constructional properties. Results show that frequency spectrum distribution, when touching a rigid surface, is mainly determined by the relative geometry of the two contact surfaces and by the contact parameters. On the contrary, when scanning a fabric, the structure and the deformation of the textile itself largely affect the spectrum of the induced vibration. Finally, some major features of the measured vibrations (frequency distribution and amplitude) are found to be representative of tactile perception compared to psychophysical and neurophysiologic works in literature.     

Friction of the Human Finger Pad: Influence of Moisture,Occlusion and Velocity

The current paper describes an experimental study of the friction of the human finger pad. The data highlight the role of sweat secretion and contact occlusion in producing wide-ranging values for the coefficient of friction that are particularly sensitive to the tribological configuration, sliding velocity, surface roughness and porosity of the counterbody. In particular, the large coefficients of friction typically observed on dry smooth surfaces are associated with a relatively damp interface, and can be considerably reduced by either decreasing or increasing the interfacial moisture content or by surface roughening. It is concluded that the very large range in the values of the coefficient of friction reported in the literature mainly result from differences in occlusion time associated with different tribological configurations, as well as from variations in surface roughness and finger pad sweat rates.     

纳米坡缕石的分散工艺对润滑油摩擦性能的影响

研究了纳米坡缕石的分散工艺对润滑油摩擦性能的影响。结果表明,三种分散工艺均可将纳米坡缕石均匀分散到基础油中。在室温下放置24h后,方案二和方案三出现少量沉淀,方案一无沉淀出现。在摩擦试验条件下,三种方案对应的平均摩擦系数分别为0.06518、0.06688、0.07011。     

烧结工艺对铜基粉末冶金摩擦材料的影响

采用粉末冶金的方法烧结铜基摩擦材料,研究烧结温度、烧结压力、保温时间和成型压力对摩擦材料性能的影响。结果表明：随着烧结温度的升高,材料的密度、强度和硬度都降低,孔隙度增大;随着烧结压力的增大,硬度先增大后减小,压力达到20 MPa 时,硬度最大;随着保温时间的延长,抗压强度增大;随着成型压力的增大,抗压强度先增大后较小,成型压力为42 MPa 时,抗压强度最大。当材料的物理力学性能处于最佳范围时,材料的摩擦因数才达到最大,物理力学性能过高或过低都不利于摩擦因数的提高;对偶磨损量主要取决于摩擦材料的摩擦因数,随着摩擦因数的增大而增大。     

离合器摩擦副表面温度对摩擦因数的影响

通过对某型离合器摩擦副的摩擦学小样试验,研究了离合器在结合的滑动摩擦过程中,摩擦面温度对离合器摩擦材料摩擦因数的影响.采用扫描电子显微镜(SEM),分析了样件的摩擦表面形貌,探讨了产生影响的机制,并从摩擦因数角度探讨了微车离合器起步发抖和烧蚀的主要原因.微车离合器摩擦材料摩擦因数随着摩擦面温度先升高,然后趋于稳定,最后再降低,其稳定工作的温度区间为130～220℃;在摩擦面温度较低的工况下,摩擦因数较低,微车起步时,离合器传递的扭矩不足以克服道路阻力,引起微车起步发抖的现象;而在摩擦面温度过高的工况下,离合过程中,摩擦因数较低,传递扭矩效率低,导致离合器滑磨时间过长,引起烧蚀现象.     

摩擦力对工件夹紧误差的影响

夹具依靠与工件的接触来约束工件 ,必须考虑二者之间的摩擦力。摩擦力的非线性是分析中的难点 ,为了解决这个问题 ,本文提出了一个迭代算法 ,该算法充分考虑摩擦力的非线性 ,可用于复杂工件的分析。使用该方法计算了在夹紧力作用下由于夹具元件的变形造成的工件刚体位移。     

工况条件对微型汽车离合器摩擦副摩擦因数的影响

分析了影响摩擦因数的因素,设计了一套试验方案,在高温摩擦磨损试验机上研究了工况条件对微型汽车离合器摩擦副摩擦因数的影响规律.结果表明:微型汽车离合器摩擦副的摩擦因数随工作载荷和滑动速度的增加先减小,然后逐渐趋于稳定;随着温度的上升,摩擦副的摩擦因数先下降,当温度上升到摩擦片的软化温度时,摩擦因数开始回升,当温度继续上升到摩擦片的分解温度时,摩擦因数又开始下降.     

Effect of Surface Velocity Directions on Elastohydrodynamic Film Shape

This article is focused on the effects of the angle between lubricant entrainment velocity and sliding velocity on elastohydrodynamic film thickness distribution. Thin-film colorimetric interferometry was used to evaluate the film thickness distribution in smooth glass–steel contacts to provide basic data on the effects of the slide–roll ratio and the direction of sliding with respect to entrainment velocity. It was observed that as the sliding perpendicular to the entrainment velocity increased, the overall film thickness was reduced and asymmetry of the film profile with respect to the direction of the entrainment velocity increased. The asymmetry of the film profile with respect to the direction of the entrainment velocity increased with the entrainment speed or the overall film thickness. When the speed of the glass disk was larger than that of the steel ball, a dimple was formed even if there was a difference in direction between the entrainment and sliding velocities. A part of the dimple was exhausted from the elastohydrodynamic lubrication (EHL) conjunction as the angle between the entrainment and sliding velocities approached 90°.     

材料匹配性对盘式制动器摩擦温度场的影响

采用有限元法模拟盘式制动器制动过程瞬态温度场,研究了材料匹配性对制动器温度场的影响.研究表明:匹配性对摩擦表面温度场的影响主要反映在材料密度、比热容和热导率方面,即使同类铸铁制动盘,其材料密度、比热容和热导率不同,摩擦表面温度也不同;铸铁材料密度越低,则摩擦表面温度越低,且温度分布较均匀;就配对副而言,制动片性能参数对摩擦表面温度的贡献要比制动盘的大.     

The Effect of Grain Size on Stribeck Curve and Microstructure of Copper Under Friction in the Steady Friction State

In the present work, the effect of grain size on the friction and wear behavior of a copper (Cu) samples under different lubricant conditions was studied. The structural evolution of Cu subsurface layers under friction in different lubricant conditions was considered. All friction tests were conducted under laboratory conditions using a block-on-ring rig. The effects of sliding velocity and load on the friction coefficient and wear rate of Cu with different grain size (1, 30, and 60 μm) were analyzed. The Cu samples with the average grain size of 1 μm were obtained due to severe plastic deformation (SPD) by equal channel angular pressing (ECAP). The Stribeck curves for Cu samples with different virgin grain sizes were considered. Elasto-hydrodynamic lubrication (EHL) and boundary lubrication (BL) regions were mainly studied in the present work. Similar Stribeck curves were found out for Cu samples with different virgin grain size. A load of the transition from the EHL to BL region was increased with a decrease of the grain size. While the friction coefficients were similar in the EHL and BL regions for the samples with different grain sizes, the wear rate was increased remarkably with an increase the virgin grain size. Flow localization during friction in the BL region led to formation of the vortex structure in subsurface layers. Based on the dependence of the microhardness upon the depth, the degree of hardening (H) was evaluated. A correlation between the coefficient of wear and the deformation hardening of Cu samples with different virgin grain sizes was revealed. In order to take into account the effect of the grain size and to predict the Stribeck curve, a parameter, K, as the ratio between hardness of tested and annealed samples, was incorporated into the lubricant number. The theoretical values of the Stribeck curve calculated for preliminary deformed Cu samples (d = 1 μm) and annealed samples (d = 60 μm) were well coincided with the experimental results.     

减摩填料对重型汽车摩擦材料摩擦磨损性能的影响

研究石墨、三硫化二锑(Sb2S3)、焦炭3种减摩填料对树脂基多纤维增强摩擦材料摩擦磨损性能的影响。结果表明:加入3种减摩填料,摩擦材料均能够在较宽的范围内保持摩擦因数的稳定性。其中石墨在低温(100～150℃)下能保持较好的稳定性;Sb2S3在高温(250～300℃)下能保持较好的稳定性;焦炭在中低温(150～200℃)下能保持较好的稳定性;通过正交试验和极差分析,得出摩擦磨损性能较好的摩擦材料配方。     

Effect of Base Oil Structure on Elastohydrodynamic Friction

The EHD friction properties of a wide range of base fluids have been measured and compared in mixed sliding–rolling conditions at three temperatures and two pressures. The use of tungsten carbide ball and disc specimens enabled high mean contact pressures of 1.5 and 2.0 GPa to be obtained, comparable to those present in many rolling bearings. The measurements confirm the importance of molecular structure of the base fluid in determining EHD friction. Liquids having linear-shaped molecules with flexible bonds give considerably lower friction than liquids based on molecules with bulky side groups or rings. EHD friction also increases with viscosity for liquids having similar molecular structures. Using pure ester fluids, it is shown that quite small differences in molecular structure can have considerable effects on EHD friction. The importance of temperature rise in reducing EHD friction at slide–roll ratios above about 5% has been shown. By measuring EHD friction at several temperatures and pressures as well as EHD film thickness, approximate corrections to measured EHD friction data have been made to obtain isothermal shear stress and thus EHD friction curves. These show that under the conditions tested most low molecular weight base fluids do not reach a limiting friction coefficient and thus shear stress. However, two high traction base fluids appear to reach limiting values, while three linear polymeric base fluids may also do so. Constants of best fit to a linear/logarithmic isothermal shear stress/strain rate relationship have been provided to enable reconstruction of isothermal EHD friction behaviour for most of the fluids tested.     

树脂含量对湿式纸基摩擦材料性能的影响

就树脂含量对湿式纸基摩擦材料的摩擦磨损性能和表面形貌的影响进行了研究。结果表明:在试验条件下,材料的气孔率随树脂含量的增加而降低,当含量为25％时,材料综合性能较好,气孔率为32．43％,磨损率为1．32×10-8 cm3／J,且摩擦因数的压力稳定性和转速稳定性较高,材料的摩擦力矩曲线较为平稳,未出现明显的“公鸡尾”现象;另外,材料在摩擦前后的表面形貌也较好。     

预热时间对搅拌摩擦焊接的影响

基于固体力学的有限元方法建立搅拌摩擦焊接过程的热力耦合分析模型,并以此为基础,研究预热时间对搅拌摩擦焊接过程的影响,与试验测量的温度和试验观测到的材料流动现象的对比证明了该模型的有效性和正确性.数值模拟结果显示预热时间需要控制在1.000～3.281 s之间,在采用压紧力控制搅拌头轴向自由度的情况下,预热时间过短会导致孔洞缺陷的形成,而预热时间过长会导致飞边现象变得非常严重.搅拌摩擦焊接所需的有效功率不会随预热时间的变化发生明显改变.随着预热时间的延长,搅拌摩擦焊接中的最高温度略有增加,同时,焊接材料温度的升高使材料的流动应力下降,导致随搅拌头一起流动的物质点略有增加,说明搅拌头的搅拌效应随着预热时间的延长而变得明显.     

Understanding the Friction Mechanisms Between the Human Finger and Flat Contacting Surfaces in Moist Conditions

Human hands sweat in different circumstances and the presence of sweat can alter the friction between the hand and contacting surface. It is, therefore, important to understand how hand moisture varies between people, during different activities and the effect of this on friction. In this study, a survey of fingertip moisture was done. Friction tests were then carried out to investigate the effect of moisture. Moisture was added to the surface of the finger, the finger was soaked in water, and water was added to the counter-surface; the friction of the contact was then measured. It was found that the friction increased, up until a certain level of moisture and then decreased. The increase in friction has previously been explained by viscous shearing, water absorption and capillary adhesion. The results from the experiments enabled the mechanisms to be investigated analytically. This study found that water absorption is the principle mechanism responsible for the increase in friction, followed by capillary adhesion, although it was not conclusively proved that this contributes significantly. Both these mechanisms increase friction by increasing the area of contact and therefore adhesion. Viscous shearing in the liquid bridges has negligible effect. There are, however, many limitations in the modelling that need further exploration.