Friction of hydraulic rod seals at high velocities

Hydraulic cylinders are preferred if a large power density and high dynamics are required because of the low mass which has to be accelerated. Maybe the most critical machine element in fluid power devices is the rod seal. A failure leads to a contamination of the environment and the friction is dominated by the seals. A test rig was developed to measure the seal friction at high accelerations and velocities. In an experimental study the influence of acceleration, sealed pressure and temperature was captured for a polytetrafluoroethylene (PTFE) step seal. Furthermore, the friction of different seal types was measured and the principle suitability of compact seals made from a polyurethane (PU) compound is highlighted.     

Experimental study of air squeeze effect on high-frequency friction contact

A special tribometer was developed which was used to test sliding friction force between PTFE-based composites and bronze with normal (out-of-plane) or transverse (in-plane) high-frequency vibrations under three different environmental pressures. The influences of environmental pressure, vibration amplitude and sliding velocity on sliding friction coefficient were studied. The results show that the effect of environmental pressure on reduction of sliding friction is outstanding. With the increase of vacuum, the reduction of sliding friction by high-frequency vibrations decreased, especially the reduction of sliding friction by normal vibration. The sliding friction coefficient with high-frequency vibrations slowed down as the vibration amplitude increased. With increase of sliding velocity, the time-averaged friction coefficient with transverse vibration increased.     

减振器活塞杆摩擦热分析及温度与硬度的关系

针对汽车减振器活塞杆发生偏磨以及偏磨面与未偏磨面交界处淬硬层减薄的情况,用有限元方法分析了活塞杆瞬态温度场,获得达到稳态状况下活杆偏磨磨段平均温度为65℃。通过闪点温度的计算得到活塞杆偏磨表面的特征温度为261℃。本文分析了偏磨面边缘硬度降低与活塞杆偏磨表面湿度的关系,认为高频淬火的集肤效应使得活塞杆亚表层组织未能完全转变为马氏体,回火稳定性差,偏磨面边缘因受摩擦热的影响引起了回火软化现象,导致了偏磨面与未偏磨面的边缘淬硬层减薄。而最表层由于获得完全细化的马氏体组织,拥有较好的回火稳定性,与未偏磨面磨面与未偏磨面的边缘淬硬层减薄。而最表层由于获得完全细化的马氏体组织,拥有较好的回火稳定性,与未偏磨面表层硬度相当。     

The influence of humidity on the fretting behaviour of PVD TiN coatings

The influence of the relative humidity (RH) in ambient air on the friction and wear behaviour of PVD TiN coatings subjected to contact vibrations against corundum and bearing steel (100Cr6) counterbodies has been investigated. The fretting experiments were performed in the gross-slip regime on TiN coatings produced by three different PVD processes. The results indicate two basic friction characteristics. At low relative humidity (RH < 10%), the friction force is in the range of the normal force whereas it is less than one third of the normal force in atmospheres of high relative humidity (RH > 80%). A transition from high to low friction was observed during the course of experiments performed in atmospheres of medium relative humidity. The duration of the high friction phase in such transitions was found to depend on fretting parameters such as the normal force and the vibration frequency. This humidity dependence of the friction force was found for both counterbody materials. The size of the damaged surface area as well as the volumetric wear on the TiN coatings were found to be largest at low relative humidity. Fretting damage occurs over a smaller area but extends more into the depth at high relative humidity. The size of the fretted surface area induced on TiN is larger for Cr-steel than for corundum counterbodies.     

考虑结合面法向刚度的拉杆转子轴向振动特性

针对接触刚度解析模型参数确定困难、精度难以保证等问题,提出了根据弹塑性粗糙表面微体单元受力变形的有限元分析结果确定轮盘结合面法向刚度的方法;为准确获取拉杆转子的轴向振动特性,建立了考虑轮盘结合面法向刚度的集中质量动力学模型;运用上述方法和模型计算了某型实验转子轴向振动的固有频率,并将结果与实测数据进行对比,误差低于5%,证明了该方法的有效性;改变拉杆预紧力,进一步研究预紧力对拉杆转子动力学行为的影响,结果表明：拉杆预紧力对转子的作用效果存在一个饱和区域,可为拉杆预紧力数值的确定提供重要的设计依据。     

汽车减振器油封摩擦力变化规律研究*

采用数值模拟与实验验证相结合的方法开展对汽车减振器油封摩擦特性的研究,得到不同工况下的摩擦力变化规律;在往复油封摩擦力实验台上研究了摩擦力随润滑油温度、润滑油压力和活塞杆速度的变化规律。结果表明:随活塞杆速度的增加油封摩擦力呈先增加再减小后趋于稳定的变化规律;随润滑油温度的升高油封摩擦力逐渐减小;随润滑油压力的增加油封摩擦力逐渐增大。根据油封摩擦力变化规律,可以推断出活塞杆速度较低、润滑油温度较高、润滑油压力较低时,应使得减振器油封有较大的过盈量或抱紧力,以提高汽车减振器的性能和使用寿命。实验结果与数值仿真结果基本一致,验证了混合润滑数值模型的正确性。     

梁杆碰撞激发瞬态波传播的动态子结构模型

针对梁杆碰撞问题,通过推导物理坐标和模态坐标下的动力学控制方程,采用黏结接触模型和单轴压缩局部接触模型处理碰撞产生的接触约束,提出一种适用于梁杆碰撞激发瞬态波传播研究的动态子结构模型。与理论解的比较表明,该动态子结构模型具有良好的数值收敛性和较高的计算精度。数值计算结果显示,该模型能够有效地分析碰撞激发的杆中轴向波和梁中弯曲波的传播、反射和相互干涉。借助该模型,分别研究轴向波和弯曲波通过碰撞接触约束的传播效应,研究结果表明,柔性构件碰撞的一个显著特征是波传播效应直接影响碰撞力响应,并使碰撞力响应形式变得异常复杂。     

圆锥安全离合器的理论分析

对圆锥安全离合器的主要设计参数轴向压紧力进行了理论研究,指出母线向摩擦力只是摩擦合力的分量,远比起传动作用的周向摩擦力小得多,计算轴向压紧力时应忽略不计。从理论上解决了当前存在的不同设计手册采用不同计算公式的问题。     

轴向超声振动辅助磨削的磨削力建模

以单颗磨粒为对象,分析轴向超声振动下磨粒的运动特性。在此基础上,将磨削力分为切屑变形力和摩擦力两部分,分别分析了轴向超声振动对切屑变形力和摩擦力的影响。在切屑变形力方面,轴向超声振动改变了磨粒运动方向与主切削方向间的夹角;在摩擦力方面,轴向超声振动降低了磨粒与工件间的摩擦因数。基于此建立了轴向超声振动辅助磨削的磨削力模型。通过对21Ni Cr Mo5H进行了轴向超声振动辅助磨削的磨削力试验,确定了模型中的常数,并验证了所建模型的正确性。建立的磨削力模型是轴向超声振动辅助磨削的磨削力预测的一种有效方法,对轴向超声振动辅助磨削机理的认识具有较大意义。     

附加轴向力对离心风机自由端滚动轴承温升的影响

摩擦力矩的大小决定了轴承的功率消耗和发热量的大小,发热量的大小直接影响轴承的温升,严重时直接导致轴承的失效。结合算例对SKF推出的滚动轴承摩擦力矩的计算模型及其影响因素进行了分析,重点探讨了附加轴向力对离心风机自由端轴承摩擦力矩的影响。在离心风机自由端滚动轴承设计选型中,通常不考虑轴向力的影响,但结果表明,附加轴向力对自由端轴承摩擦力矩影响显著。本文为离心风机自由端滚动轴承设计选型提供工程实用价值。     

Study of ultrasonic vibrations’ effect on friction stir welding

In this paper, effect of ultrasonic vibrations on friction stir welding (FSW) is studied. Ultrasonic vibrations were employed on the tool in pin direction (perpendicular to the welding direction). To do this study, a vibration tool was designed by Abaqus software in a way to have a longitudinal frequency about 20 kHz and was then manufactured and assembled with an ultrasonic transducer and was controlled using an ultrasonic generator to oscillate ultrasonically with a peak-to-peak amplitude of 10 μm. After preparation of experimental setup, some experiments were performed on AA6061-T6 as a work material, and the effect of ultrasonic vibrations on force, temperature, tensile strength, and hardness was investigated in FSW. Based on the achieved results, ultrasonic vibrations can decrease force and increase temperature in FSW.     

受轧件水平振动影响的板带轧机非线性振动特性

基于Roberts摩擦因数公式和Hill轧制力公式,建立能够表征不同摩擦状态下的动态轧制力模型;在此基础上进一步考虑轧机结构的振动和轧件振动之间的相互影响,提出轧件-轧辊耦合振动模型。根据广义耗散的Lagrange原理,分别沿轧制方向和垂直于轧制方向建立动力学平衡方程;采用多尺度法求解出考虑系统内共振的幅频特性方程,并仿真分析不同外部激励和非线性参数作用下的轧机振动规律。研究结果表明：滑动摩擦状态下耦合系统对内部非线性参数变化和外部扰动变化的敏感程度远远高于静摩擦状态下的情况;适当选取耦合三次项非线性参数,可以将系统不稳定振动的出现控制在一个较小的频率区间,削弱轧件-轧辊耦合振动对板带轧机振动的影响。     

力反馈主操作手附加力的补偿策略研究

针对力反馈主操作手的连杆重力、关节摩擦力以及惯性力影响医生真实感知反馈力信息的问题,建立了一套完整的动力学模型。基于力反馈主操作手动力学模型,分别建立了主操作手的重力补偿模型、摩擦力补偿模型以及惯性力补偿模型。通过力补偿控制策略来消除力反馈主操作手本身所具有的重力、关节摩擦力以及惯性力伴随关节运动对反馈作用力造成的干扰,实现了医生的真实力觉感知,从而提高了微创手术的精度。     

超低温微铣削芳纶纤维复合材料表面质量

在常温与液氮超低温环境下对芳纶纤维增强复合材料进行微铣削对比加工试验，获取不同参数环境下的加工表面质量和铣削力数据。结果表明：超低温铣削加工环境能够对加工材料进行改性，降低强度、韧性等属性，使其易于切削，减少抽丝拉毛缺陷的产生，纤维断屑彻底，有助于获得更加优异的表面形貌。超低温环境充分抑制了烧蚀缺陷的产生，提高了加工表面质量，铣削表面粗糙度可达到2 μm以下。与常温环境相比，超低温加工环境具有以下优势：在较高的进给速度下仍能保持良好的铣削表面质量，可以在保证加工质量的同时，一定程度上提高加工效率；铣削力减小明显，刀具表面磨损较小；改善了芳纶纤维复合材料的铣削加工性能，提高了铣削加工表面质量。     

Tribological Properties of Self-Assembled Monolayers and Their Substrates Under Various Humid Environments

Using friction force microscopy (FFM) under controlled environments, we have systematically investigated the humidity effect on the frictional properties of two important classes of self-assembled monolayers (SAMs), i.e., N-octadecyltrimethoxysilane (OTE, CH₃(CH₂)₁₇Si(OCH₃)₃) on SiO₂(OTE/SiO₂), and N-alkanethiols on Au(111), together with their respective substrates. Experimental results show that both OTE and alkylthiol SAMs can decrease the friction force between a Si₃N₄ atomic force microscope (AFM) tip and substrates. The nearly humidity-independent friction of the two kinds of SAMs indicates that these SAMs are ideal lubricants in applications of micro-electro-mechanical systems (MEMS) under different environments. The humidity dependence—as the humidity increases, the friction first increases and then decreases—of the two substrates, SiO₂ and Au(111), can be explained by the adsorption of water. The decrease in the friction at high humidity is attributed to the low viscosity in the multilayers of water, while the increase in the friction at low humidity can be explained by the high viscosity between the water monolayer and the surfaces (AFM tip and sample), possibly due to the confinement effects. The effect of modification of the AFM tip with alkanethiol molecules on the humidity dependence of Au(111) friction has also been investigated.     

ABRASIVE FLOW MACHINING WITH ADDITIONAL CENTRIFUGAL FORCE APPLIED TO THE MEDIA

Abrasive flow machining (AFM) is one of the important non-traditional metal finishing technologies which was introduced during the late 1960s. The process has found applications in a wide range of fields such as aerospace, defence, surgical and tool manufacturing industries. Recently, an effort has been made towards the performance improvement of this process by applying centrifugal force on the abrasive media with the use of a rotating centrifugal force generating (CFG) rod introduced in the workpiece passage. The results have been encouraging. The present paper discusses the results of changing the parameters like shape and rotational speed of CFG rod, extrusion pressure, number of process cycles and abrasive grit size. The results indicate that all the input variables have significant effect on the response parameters, which for the present study were taken as material removal and surface roughness. An analytical model is proposed for the velocity and the angle at which abrasive particles attack the workpiece surface in the process.     

ABRASIVE FLOW MACHINING WITH ADDITIONAL CENTRIFUGAL FORCE APPLIED TO THE MEDIA

Abrasive flow machining (AFM) is one of the important non-traditional metal finishing technologies which was introduced during the late 1960s. The process has found applications in a wide range of fields such as aerospace, defence, surgical and tool manufacturing industries. Recently, an effort has been made towards the performance improvement of this process by applying centrifugal force on the abrasive media with the use of a rotating centrifugal force generating (CFG) rod introduced in the workpiece passage. The results have been encouraging. The present paper discusses the results of changing the parameters like shape and rotational speed of CFG rod, extrusion pressure, number of process cycles and abrasive grit size. The results indicate that all the input variables have significant effect on the response parameters, which for the present study were taken as material removal and surface roughness. An analytical model is proposed for the velocity and the angle at which abrasive particles attack the workpiece surface in the process.     

Novel operation mode for eliminating influence of inclination angle and friction in atomic force microscopy

The accuracy of topography imaging in contact force mode of atomic force microscopy (AFM) depends on the one-to-one corresponding relationship between the cantilever deflection and the tip–sample distance, whereas such a relationship cannot be always achieved in the presence of friction and incline angle of sample surface. Recently, we have developed a novel operation mode in which we keep the van der Waals force as constant instead of the applied normal force, to eliminate the effect of inclination angle and friction on topography imaging in the contact force mode. We have improved our AFM to enable the new operation mode for validation. Comparative experiments have been performed and the results have shown that the effect of friction and inclination angle on topography imaging in contact mode of AFM can be eliminated or at least decreased effectively by working in the new operation mode we present.     

汽车钛合金连杆切削加工中的刀具磨损研究

以汽车钛合金连杆为研究对象,分析了钛合金连杆在切削加工工艺过程中的刀具磨损量、切削力和刀具寿命随着切削速度、润滑压力的变化规律。试验分析结果表明:刀具磨损量随着切削速度的增加而增加,随着水射流润滑压力的增加先减小后增加;刀具切削力和轴向力均随着水射流压力的增大先减小后增加,但轴向力的变化较切削力更加敏感,变化速率更快;刀具寿命随着切削速度的增加而减小,随着水射流压力的增加先增大后减小,钛合金连杆的最佳工况为切削速度75m/min,润滑压力10MPa。