On the influence of a pin type on the friction losses in pin bearings

The aim of this paper is to analyze lubricated revolute joints from the viewpoint of friction losses. The paper deals with the lubricated revolute joints composed of a pin and two more elements connected by means of the pin. It is considered here how different pin types affect friction losses in pin bearings. Three existing pin types are investigated: a pin press-fitted either in the first or in the second element or the one free to rotate in both elements. As a measure of friction losses, a dissipation function is determined in all three cases. Using the results obtained, the advantages of using the full-floating pin with respect to two other types are demonstrated by means of a numerical example.     

Effects of tool rotation and pin diameter on fatigue properties of friction stir welded lap joints

The objective of this study was to determine the effects of the tool pin diameter and tool rotation on the fatigue behaviour of friction stir welded (FSW) lap joints. FSW lap joints of AA 5754 aluminium alloy plates were produced by means of a conventional semiautomatic milling machine. Consequently, defect free FSW lap joints were produced on alloy plates at a constant traverse speed but with different tool pin diameter and tool rotation. Therefore, within this study, tool rotation and the tool pin diameter were accepted as variable parameters, while others held fixed. The results of the tests performed, indicate that an optimisation is required for the studied parameters, in order to obtain reasonable fatigue strength. An index, related to tool rotation, traverse speed, pin diameter, and pin height can be identified and used to select optimum parameters for FSW applications.     

Tribological behaviour of MoS2 and inorganic fullerene-like WS2 nanoparticles under boundary and mixed lubrication regimes

Abstract

For several years different types of nanoparticles have been considered and studied as potential friction modifying lubricant additives. Some nanoparticles can reduce the friction coefficient by 30–70%, depending on the base oil and the experimental conditions. In the present study, an experimental analysis on tribological properties of inorganic fullerene-like metal dichalcogenides was performed in comparison with MoS₂ 2H layered structures. Tribological tests were carried out on a pin on disc tribometer in ambient air. Several contact conditions are analysed in order to realise boundary and mixed lubrication regimes. The experimental study was performed on a mineral base oil, and particle concentration effects were analysed. Antifriction properties were evaluated by measuring the friction coefficient and are presented as generalised Stribeck diagrams. Inorganic fullerene-like WS₂ and MoS₂ nanoparticles present interesting friction reduction properties when tested in boundary and mixed lubrication.     

计及摩擦的摆线轮齿与针齿的分形接触模型

为综合体现摆线轮齿与针齿的宏观特征和微观特征对接触特性的影响,应用Weierstrass-Mandelbrot函数和矢量函数构建了摆线轮齿与针齿的表面形貌模型,应用MATLAB绘制了各向同性的粗糙针齿以及单个摆线轮齿的二维截面图,提出了摆线轮齿与针齿的接触比例系数,该接触比例系数始终小于1,且随啮合点的变化而变化,针齿与摆线轮齿的内凹部分接触时的接触比例系数远大于针齿与摆线轮齿外凸部分接触时的值。计及摩擦因素的影响,构建了单对摆线轮齿与针齿的分形接触模型,分析了摩擦因子、结合面的微观特征和宏观特征对接触特性的影响。研究结果表明,相同载荷下,接触面积随摩擦因子的增大而增大,随结合面粗糙度的增大先增大后减小,随针齿半径的增大而减小,随中心距的增大而减小,随针轮中心圆半径的增大而增大,随针轮齿数的增加而减小。     

Analysis of the friction and wear behavior of hot work tool scale: application to the hot rolling process

In hot forming processes, the interface tool/product is important for the quality of the finished product. In hot rolling, the scale formed on the roll material plays an essential role. As soon as the contact oxide–oxide is established between the roll and the slab, friction allows the process to start. However, the oxide scale will continue to grow under the cyclic action of hot contacts and water cooling and will be subjected to thermo-mechanical stresses. Beyond a certain critical thickness, the oxidized surface layer of the cylinders has not sufficient mechanical strength to withstand the shear stresses.

The wear and friction behavior of the oxides appearing on the surface of the hot working rolls is not well known. The influence of these oxides on the friction and, consequently, the quality of the products of finishing mills, seems very significant. So, in this study, we investigate the evolution of the friction coefficient and the wear, according to the growth, the nature and the thickness of the formed scale. We use a high temperature pin on disc tribometer. The pin consists of material “rolls” while the disc consists of the slab. The pin is instrumented with thermocouples in order to couple the friction coefficient measurements with the thermal gradient in the pin and the surface temperature and the formed oxides. Then, the characterization of the surfaces is done by scanning electronic microscopy (SEM) and EDS analyses. We use the method of sin² Ψ to evaluate residual stresses of oxide and correlate these data with shear stress behavior.     

A Biphasic Model for Hip-Joint Replacement

The ball-and-socket geometry of the hip joint makes kinematic analysis of the joint motion relatively straightforward in comparison to other joints. The load-carrying surfaces of both ball and socket are covered with tough viscoelastic material known as cartilage. A number of lubrication theories have been proposed in the literature to account for the low coefficient of friction and low wear observed in healthy joints. The actual mechanism by which joints are capable of sustaining large repetitive loads with virtually no wear and with very little friction has not been fully understood. Therefore, analytical studies are presented for the understanding of the lubrication mechanism occurring in hip-joint replacements under restricted motion during standing or in the supporting phase during walking. The viscoelastic fluid has been considered to represent the synovial fluid in the fluid-film region. The problem described here has been analyzed in two regions (the porous matrix and the fluid-film region) separately along with suitable matching and boundary conditions at the interface. It has been concluded that the effect of the viscoelastic parameter for a particular gap is to increase the load capacity, indicating positive effects of the increase in concentration of suspended particles in the lubricant region. It has been observed that the coefficient of friction decreases with increasing values of the viscoelestic parameter. This is due to the fact that as the viscoelastic parameter increases, the concentration of hyaluronic acid molecules increases. It may also be noted from the results that the coefficient of friction decreases with increasing values of slip parameter. This shows that the slip velocity occurring at the porous boundary helps in maintaining normal functioning of human joints.     

Identification of a friction model—Application to the context of dry cutting of an AISI 1045 annealed steel with a TiN-coated carbide tool

The characterization of friction coefficients at the tool-chip-workpiece interface remains an issue. This paper aims to identify a friction model able to describe the friction coefficient at this interface during the dry cutting of an AISI1045 with TiN coated carbide tools. A new tribometer has been designed in order to reach relevant values of pressures and sliding velocities. This set-up is based on a modified pin-on-ring system. Additionally a numerical model simulating the frictional test has been associated in order to quantify average friction coefficients around the spherical pin, from the standard macroscopic data provided by the experimental system. A range of cutting speeds has been investigated. It has been shown that the friction coefficient is very much dependant on the sliding velocity. A new friction model has been identified based on the average local sliding velocity.     

The effect of amplitude of vibration on the coefficient of friction for different materials

This work examines how friction coefficients are affected by amplitude of normal vibration at different frequencies. Variation of friction coefficient with the amplitude of normal vibration is investigated experimentally when mild steel pin slides on different types of material such as glass fiber reinforced plastic, cloth reinforced ebonite, polytetrafluoroethylene (PTFE), rubber and mild steel. For this, a pin-on-disc apparatus having facility of vibrating the test samples at different amplitudes and frequencies of vibration was designed and fabricated. During the experiments, the effects of sliding velocity, roughness, normal load and duration of rubbing were also investigated. Studies have shown that the friction coefficient decreases with the increase of amplitude of vibration within the observed range. The observed ranges of amplitude of vibration were 10–200 μm. In this study, it is also observed that the rate of reduction of friction coefficient has a particular relationship on the amplitude and frequency of vibration. The experimental results are compared with those available in the literature and simple physical explanations are provided.     

Tribological and nanoscale research on model friction couples intended for hip joint prostheses

Abstract

The paper presents the results of tribological and nanoscale research on model friction couples intended for hip joint prostheses. The tribological tests were performed by means of reciprocating pin on plate testing machine. The investigated friction pairs contained plates rubbing against polymer pins. The test plates were made from seven kinds of ceramics containing different concentrations of ZrO₂ and Al₂O₃, and two kinds of Co–Cr alloy. The test pins were made from UHMWPE. Tribological tests were performed in conditions of Ringer solution circulation. On the basis of friction force measurements, for each investigated friction couple, the average coefficient of friction was calculated. On the basis of total wear measurements, for each investigated couple, the wear intensity was calculated. Before and after every test, the plates and pins were analysed by means of atomic force microscopy. The difference in plate surface roughness was determined by the results of the atomic force microscopy analyses.

It was stated, that in the case of investigated friction joints, working under reciprocating motion, the wear and friction coefficient correlates with the surface roughness of plate specimens. For the plates with higher surface roughness, the lower friction coefficient and also lower UHMWPE pin wear intensity were observed. The friction coefficient and wear intensity were increasing with decreasing surface roughness. The correlation is confirmed by the differences in material transfer process. Considering investigated friction couples, the pin polymer material is smeared on the ceramic plates with the highest surface roughness creating a thin polymer film. In the case of ceramic surfaces with the lowest surface roughness, the strong adhesive bounds are created and some large particles of polymer are transferred to ceramic surface.     

A Computational Study of the Effects of Strain Hardening in Micro-asperity Friction Models

In this paper, we study the effects of plastic strain hardening and damage on the friction response of a surface at the microscopic and macroscopic scales. At the microscale, the role played by hardening and damage in the response of a single asperity is determined using three-dimensional finite element analysis. The sample materials for the asperity are Al 2024-T3 and Ti6Al4V, which are modeled as isotropic bilinear plastic and include Johnson–Cook damage. The friction responses for two different damage models (Johnson–Cook and Bao–Wierzbicki) are also compared for the perfectly plastic case. In the simulations the asperity is initially compressed in order to induce a normal preload and then sheared to study the friction response. Then a statistical homogenization approach is used to propagate these effects to the macroscale. Toward this end, the surface is modeled as an isotropic Gaussian random process. The computed microscale responses are parameterized, and the overall macroscopic response of the surface is determined. Results of this study show that, at the microscale, strain hardening increases the coefficient of friction, particularly at low interference values. Similarly, material response plays a significant role at the macroscale over a wide range of normal force values.     

基于销/盘试验机的摩擦噪声试验与仿真研究

为了研究摩擦噪声的产生机制和影响因素,利用销/盘试验机对以转子式压缩机曲轴-法兰材料制成的销/盘试样进行摩擦噪声试验研究;应用ABAQUS有限元软件,建立销/盘试验机有限元模型,利用复特征值分析方法预测该系统摩擦噪声的主频,并与试验结果进行对比;讨论相关参数对系统的稳定性和摩擦噪声的影响。结果表明：销与盘法向和横向振动的耦合是引起系统自激振动和摩擦噪声的重要因素;当系统摩擦界面处的摩擦因数大于系统临界摩擦因数时,系统开始出现不稳定振动,且摩擦因数越大,系统越不稳定,越易出现摩擦噪声;法向载荷对系统稳定性的影响不大;选择具有合适弹性模量的摩擦副材料可以抑制摩擦噪声的产生。     

基于盘／销装置的摩擦尖叫噪声研究

为了研究摩擦尖叫的产生机理,利用汽车制动盘、摩擦材料及铝合金圆销设计了一种长度可调的盘／销摩擦试验装置,进行了不同销长的摩擦尖叫试验。基于盘／销零件的约束模态试验结果,建立了装置的有限元模型。利用复特征值分析方法研究了销长、摩擦因数、载荷、速度和材料特性等因素对摩擦尖叫噪声的影响。结果表明：当圆销和制动盘间弯曲模态频率相近时会形成模态耦合,系统不稳定,产生摩擦尖叫噪声;通过改进系统结构、适当降低摩擦因数和调整材料特性可以减轻或消除摩擦尖叫。     

The effect of interface friction on the inplane flexibilities of machine tool joints

The effect of interface friction between contact surfaces on the flexibilities of machine-tool joints is studied. The joint flexibility is determined by applying unit load successively at one end of the joint while keeping the other end fixed. Only plane problems have been taken up for consideration. A finite element based numerical procedure is used for solving the problem. Results are presented for the care of a hypothetical machine tool joint and dovetail joint for various values of friction coefficient. It is observed that the decrease in flexibility is only 15% for values of friction coefficient μ = 0.2.     

不同活塞销孔衬套-活塞销摩擦副匹配性能研究

活塞销孔衬套-活塞销是柴油机重要的摩擦副之一,其匹配性能的优劣直接影响使用寿命和动力特性。为探讨不同制备工艺的活塞销孔衬套与柴油机在用活塞销的匹配性能,将分别采用挤压和旋压工艺制备的活塞销孔衬套,以及进口衬套分别与柴油机在用活塞销匹配,在摆动摩擦副摩擦磨损试验台进行承载、抗咬合和耐磨损性能试验。试验结果表明：制备工艺对摩擦副匹配性能具有一定的影响;从摩擦扭矩、摩擦温度、宏观形貌和磨损量4个方面分析,相同工况下挤压衬套承载极限、抗咬合性能和耐磨损性能均优于旋压衬套,具备对标进口衬套的能力,可为国内活塞销孔衬套的选配提供参考。     

Friction and Wear at High Sliding Speeds

Sliding tests have been carried out using a variety of soft metal and nonmetal pins on a rotating steel disk at speeds up to 150 m/s. A new high-speed friction apparatus in which the normal force, the friction force and the friction coefficient are recorded, was used. In general, the wear rate increased drastically, and the friction coefficient decreased moderately as the sliding speed was raised, these changes being especially pronounced when pin materials of low melting temperature were used. The friction data are in good agreement with those obtained by others using the pin-on-disk geometry. However, although in many cases the interface reached the melting temperature of the lower melting sliding material, the very low friction coefficient values of under 0.05 reported by some investigators were not reached.     

Modeling and simulation of joint clearance effects on mechanisms having rigid and flexible links

In this study, kinematic and dynamic characteristics of a planar four-bar mechanism having joint clearance and link flexibility are investigated. Assuming that joint clearance as a virtual massless link, artificial clearances are developed at crank-coupler and coupler-follower joints. Contact model in joints with clearance is established using the nonlinear spring-damper model and the friction effect is considered using the Coulomb friction model. Then the simulation is implemented and the kinematic and dynamic characteristics of model mechanism are investigated. The computational methodology can predict the effects of clearance on planar mechanism having rigid and flexible links. The results are evaluated for the case of stationary phase, and naturally show that extreme values occur in output of the mechanism with clearances. Due primarily to the suspension effect of the flexible link, values of these peaks in the flexible mechanism decrease relative to that of the rigid mechanism. In addition, the flexibility has a significant effect on the vibration response of the mechanism with joint clearance.     

The influence of aspect ratio on the interface pressure distribution of cylindrical wear specimens

The interface stress distributions arising in linearly elastic cylindrical pins sliding against a flat rigid counterface are considered in this paper. Coulomb friction is assumed, and the interface pressure distribution arises by virtue of a displacement applied to the far end of the pin. Various aspect ratios are studied in order to assess stiffness effects in pin-on-disc tribotesting.     

截面形状对疏水性微肋阵内减阻特性的影响*

在截面为圆形、菱形和椭圆形微肋阵内涂覆不同性能的疏水性涂层,形成疏水性微肋阵。去离子水在疏水性微肋阵内表面的接触角分别为99.5°、119.5°和151.5°。测试去离水以不同流速流经疏水性试验段流道内压力降和摩擦阻力系数。试验结果表明,在相同的接触角和流量下,圆形微肋阵内压力降最高而椭圆形最低;当接触角为99.5°,雷诺数Re低于600时,椭圆形微肋阵内减阻率要高于菱形和圆形微肋阵,当Re>600时前者要低于后两者。随着接触角的增大,圆形内减阻率要明显高于菱形和椭圆形微肋阵,菱形和椭圆形微肋阵在低Re下较为接近,在高Re下前者的减阻率要高于后者;表面疏水性处理能够显著降低摩擦阻力,同时推迟流动分离和尾流区转捩,因此对于分离较早、压差阻力较大的微肋阵的减阻效果更加明显。     

Tribological Performance of UHMWPE and PFPE Coated Films on Aluminium Surface

A thin layer of Ultra High Molecular Weight Polyethylene (UHMWPE) or UHMWPE + PFPE is coated onto cylindrical aluminium (Al) pin (4.6 mm diametre) surface with the aim of providing wear resistant coating on this soft and tribologically poor metal. The coefficient of friction and wear life of the coated samples are investigated on a pin-on-disk tribometre under different normal loads (394–622 g) and two sliding speeds (0.1 and 0.31 m/s) against uncoated Al disk as the counterface. Both coatings provide coefficient of friction values in the range of 0.02–0.2 as compared to 0.4–1.0 for uncoated Al. There is tremendous improvement in the wear life of the pin, with UHMWPE + PFPE film giving wear life approximately twice to thrice higher than that with only UHMWPE film. A thin polymer film is transferred to the disk surface during sliding providing very long-term wear life (continuous low coefficient of friction) despite visual removal of the film from the pin surface. The present films will have applications in gears and bearings as solid or boundary lubricants for automotive and aerospace component.     

在不同润滑油下齿轮锻造材料摩擦特性研究

采用销-盘摩擦副接触方式在不同流体润滑及载荷下,对齿轮锻造用SCr420H合金结构钢进行摩擦试验.采用齿轮油、石蜡油以及加工润滑油润滑.利用在不同润滑及载荷下随速度变化的摩擦因数变化曲线图分析摩擦材料表面摩擦特性.利用Stribeck曲线和摩擦表面形貌SEM照片分析在不同润滑油及载荷下的摩擦状态和摩擦行为.结果表明:SCr420H合金结构钢在最低动黏度的石蜡油润滑下摩擦因数最高,且随速度增大而减少;在齿轮油和加工润滑油润滑下,最低载荷时具有最高的摩擦因数,但摩擦因数随载荷增大而减少,速度对摩擦因数影响不大;在齿轮油和加工润滑油润滑下摩擦副处于流体润滑状态,在石蜡油润滑下显示临界润滑摩擦状态.