曲线半径及轴重对轮轨摩擦功影响的研究

基于非Hertz滚动接触理论利用数值计算方法详细分析了静态接触情况下,轴重和曲线半径对轮轨接触质点间等效应力、接触斑粘滑区的分布、总滑动量和摩擦功的影响。分析计算表明,轴重增加引起轮轨接触质点间等效应力,接触质点间粘滑区的面积以及总滑动量的变化,同时对轮轨接触质点阍的摩擦功的变化有重要影响;小曲线半径处轮轨接触质点间的总滑动量,接触斑滑移区的面积以及摩擦功都明显增大,导致曲线上钢轨磨损加剧。因此曲线半径和轴重是影响轮轨滚动接触磨损的重要因素。     

Electrical contact resistance for a conductive Velcro system

Conductive Velcro fasteners are employed as a joining material for various types of connections due to their detachable properties, especially when an electrical connection is established. Due to microscopic actual contact spots between the fibers and hooks in the Velcro joints, a large electrical contact resistance is established that could cause heat and potentially short-circuit. In this study, the electrical contact resistance behaviors in the conductive Velcro are estimated according to the electrical load, frequency and load amplitude. The wear behavior due to these effects is investigated after long exposure to dynamic loading under a specific frequency at a low load amplitude.     

Investigation of high cycle and low cycle fatigue interaction on fretting behavior

Fretting-fatigue behavior and damage accumulation under a variable-amplitude cycling load is investigated in a configuration involving a cylindrical indenter in contact with finite width plate. Relative magnitudes of cyclic tangential and bulk loads not only affect the contact conditions, but also their relative positions with respect to each other. Several stick–slip conditions on the contact surface may develop during the application of variable-amplitude fatigue load, and these are secondary and tertiary slips as well as shake-down. Further, residual shear traction develops during the application of cyclic load. The appropriate characterization of fretting-fatigue behavior or life should, therefore, include the complete history of applied cyclic tangential and bulk loads. Furthermore, experiments from a previous study conducted under a variable-amplitude fatigue loading condition are analyzed to characterize the damage accrual from its individual components involving constant-amplitude fatigue load by incorporating the contact mechanics and a multi-axial fatigue critical plane parameter. This analysis shows that there is nonlinear damage accumulation during variable-amplitude fretting-fatigue load.     

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.     

渐开线直齿轮啮合过程中载荷及应力的计算机模拟

基于有限元法,提出了一种渐开线直齿轮在啮合过程中参数化模型的表达方法,建立了能随啮合过程变化自动调整接触区和整体模型的啮合轮齿有限元分析模型,采用有限元弹性接触分析方法对轮齿从啮入至啮出整个啮合周期的接触载荷分配及分布、应力分布的变化等进行了研究。采用该方法可实现用计算机模拟齿轮在啮合过程中载荷及应力的变化。     

微动疲劳参数对钢丝微动疲劳磨损演化的影响*

微动疲劳易引起钢丝表面磨损和横截面积损失,进而造成钢丝断裂失效并缩短钢丝绳使用寿命。不同微动疲劳参数（接触载荷、疲劳载荷、钢丝直径和交叉角度）引起差异的钢丝微动疲劳磨损特性,故研究微动疲劳参数对钢丝微动疲劳磨损演化规律影响至关重要。基于摩擦学理论和Marc仿真软件构建钢丝微动疲劳磨损模型,探究接触载荷、疲劳载荷、交叉角度和钢丝直径对钢丝微动疲劳磨损演化的影响规律。结果表明：钢丝微动疲劳磨损体积主要与接触载荷和疲劳载荷有关;疲劳钢丝的磨损深度、磨损率及磨损体积随着接触载荷的增加而增大,且不同接触载荷下疲劳钢丝磨损体积均随着循环次数的增加而呈线性增加;随疲劳载荷幅值的增加,疲劳钢丝的磨损深度、磨损率及磨损体积均呈增加趋势;在不同疲劳载荷范围下疲劳钢丝的磨损体积均随着循环次数的增加而呈线性增加;当接触载荷、疲劳载荷及钢丝间摩擦因数相同时,不同交叉角度和不同加载钢丝直径下疲劳钢丝的磨损体积相同。     

齿形参数对双圆弧齿轮接触跑合性能的影响

以三维弹性接触有限元法及双圆弧齿轮跑合仿真的数学模型为基础，提出了优化的双圆弧齿轮齿形参数，对ＧＢ１２７５９－９１齿形及优化齿形的双圆弧齿轮接触、跑合性能进行对比分析，得到齿形参数对双圆弧齿轮接触、跑合性能的影响规律。     

ELASTOPLASTIC CONTACT BEHAVIOR AND SHAPE OPTIMIZATION OF DRILLPIPE THREAD

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旋滑条件下弹流润滑数值分析

建立旋滑条件下椭圆接触弹流润滑的数学模型,用多重网格法求得该条件下的完全数值解,研究速度、载荷、偏心距和椭圆比对油膜厚度、形状和压力的影响。结果表明,偏心距较小时,油膜厚度和形状都与普通弹流有明显的不同;速度、载荷和椭圆比增加及偏心距减小,均会导致接触区两侧最小膜厚的差值增大,油膜形状的非对称性增强;速度、椭圆比增加,油膜厚度增加,接触区压力减小,载荷增加或偏心距减小,油膜厚度减小,接触区压力增加。     

Microtribology and Friction-Induced Material Transfer in Layered MoS2 Nanoparticles Sprayed on a Steel Surface

Frictional performance of molybdenum disulfide (MoS₂) particles sprayed on a substrate is investigated in a ball-on-disc tribometer. The ability of large (~2 μm) and small (~50 nm) particles to generate low-friction transfer film is investigated with a view to elucidate the requirement for film formation. Particle migration, particle stability in the contact region, oxidation potential, and particle adhesion to the substrate are explored within a span of operating parametersp; normal load, and sliding velocity. It is found that the larger particles are able to migrate to the contact to raise a homogeneous but nonuniform low-friction transfer film that flows plastically to yield large contact areas, which aid in wear protection. Within the present load and speed range, the inability of small particles to stay in the contact region and undergo basal slip militates against the formation of a low-friction transfer film.     

滑块运动和形位参数对颗粒流润滑特性的影响规律

针对机械工程中的典型摩擦学研究对象-斜面滑块摩擦副,基于非连续介质力学的离散单元法建立非流态颗粒流润滑的模型.通过研究颗粒膜厚度、斜面倾角、运动速度三个运动和形位变量,探讨它们对承载分布、摩擦因数等的影响.同时以颗粒和多体接触点为对象,考察颗粒流润滑中平均接触力、体功、接触摩擦功和颗粒总动能的微观特性.研究认为颗粒流润滑承载量与滑块的倾角、速度成正比关系,与颗粒膜厚度成反比.颗粒流润滑的压力峰出现在楔形的入口区.适度减小斜面倾角和颗粒膜厚度,有利于减少摩擦.相对而言,滑块速度对摩擦因数的影响不大.     

Characterization of fretting wear behavior of Cu–Al coating on Ti–6Al–4V substrate

Fretting wear and fretting fatigue are two commonly observed material damages when two contacting bodies with a clamping load are under the oscillatory motion. In this study, fretting wear damage of Cu–Al coating on titanium alloy, Ti–6Al–4V substrate was investigated using the dissipated energy approach. Fretting tests were conducted with either no fatigue load or the maximum fatigue load of 300 MPa and stress ratio of 0.1 on the substrate (specimen). In order to investigate the effect of contact load and contact size, different pad sizes and contact loads were used in the tests. Accumulated dissipated energy versus wear volume data showed a linear relationship regardless of fatigue loading condition on specimen with the smaller pad size. However, two separate linear relationships were observed based on the fatigue loading condition with the larger pad size, such that a relatively more dissipated energy was required for a certain amount of wear with fatigue load on the specimen. The linear relationship between the accumulated dissipated energy and wear volume for both pad sizes extended from partial to gross slip regimes and was not affected by the applied contact load. Further, fretting tests with and without fatigue load resulted in different shapes of fretting loops when the larger pad size was used.     

风力发电增速器齿轮齿廓修形有限元分析

简要介绍了齿廓修形的原理,详细介绍了齿廓修形参数的表达方式和选择原则.建立了多齿有限元接触分析模型,给出了计算齿廓修形量的方法,考察了修形前后齿间载荷分配和接触线上载荷分布情况.     

Influence of Nut Design on the Load Distribution over the Turns of a Screw

Screw turns are known to be deformed as a result of the contact pliability of the nut and screw surfaces. The load distribution over the turns is investigated when fine thread is employed, when compression nuts are replaced by tension–compression and tension nuts, when the roughness of the working surfaces is increased, and when the load on the screw is increased.     

The lubrication of roller bearings subjected to couples

Roller bearings are generally designed to carry mainly radial loads with additional bearings employed to carry any axial loads. Occasionally, limited space prevents the use of these additional bearings, so that the roller bearing itself must carry the axial load. This must be taken by flanges on both the inner and outer races. The effect on the rollers themselves is to modify the load distribution on their generator contact surfaces because the rollers are now subjected to a couple in addition to the radial load. A similar effect is produced if two flanged (or unflanged) roller bearings support a loaded shaft. As its ends are nominally direction fixed, the rollers must be subjected to terminal couples, thus causing a modification to the load distribution at the contact surfaces. The effect of such forces on the footprint shape and on the bearing lubrication is investigated.     

Load Distribution in Flexibly Supported Three-Row Roller Slew Bearings

In general, slew bearings are less firmly supported by their mounting structures than small bearings. Hence, the load distribution around the bearing may be vastly different than that predicted by classic bearing formulas. The finite element method has been employed to determine load distribution in a three-row roller slew bearing mounted between two flexible, ring shaped supporting structures composed of beam elements. A special bar element modeling the system of two opposite rollers in contact with its raceways has been elaborated. The influence of the nonlinear force displacement characteristics of each of the rollers, as well as the influence of gaps between the rollers and its raceways have been taken into account. The nonlinear contact problem has been solved utilizing the Newton-Raphson method with continually updated stiffness matrix due to both element nonlinearity and contact status. The effect of a number of design parameters on the load distribution has been investigated.     

配对角接触球轴承静态力学特性

根据角接触球轴承实际的工作状态,建立背靠背配对安装形式的角接触球轴承接触模型,计算配对轴承在径向、轴向和力矩载荷联合作用下的载荷分布以及接触应力,并将理论值与Ansys有限元法得出的结果进行对比,验证计算结果的准确性。详细研究了在联合载荷的作用下,配对轴承静态载荷分布以及接触特性的变化规律。研究结果表明:配对角接触球轴承的内圈在联合载荷作用下发生位移,使载荷分布不均匀,接触应力主要集中在载荷对称线附近的部分钢球上,改变联合载荷会使两列轴承载荷分布规律以及接触特性发生变化。     

Some Observations on Frictional Force During Fretting Fatigue

Frictional force behavior during fretting fatigue and its interdependence on other fretting variables are investigated. Both coefficient of static friction and the normalized frictional force (i.e., the ratio of frictional force and normal contact load) increase during the earlier part of a fretting fatigue test and then both reach to a stabilized value. The variation of temperature in the contact region and normalized frictional force with increasing cycle numbers and bulk stress show similar trend implying that normalized frictional force represents the average friction in the contact region during a fretting fatigue. An increase in bulk stress, relative slip, and hardness of pad material results in an increase of the normalized frictional force, while an increase in contact load, frequency and temperature decreases the normalized frictional force. The normalized frictional force is also affected by the contact geometry. On the other hand, coefficient of static friction increases with an increase in the hardness of mating material, temperature and roughness from shot-peening treatment, but is not affected by contact geometry and displacement rate. Further, the normalized frictional force is not affected by the contact geometry, roughness and applied bulk stress level when fretting fatigue test is conducted under slip controlled mode, however it increases with increasing applied relative slip and decreasing contact load in this case.     

Fractal prediction model of thermal contact conductance of rough surfaces

The thermal contact conductance problem is an important issue in studying the heat transfer of engineering surfaces,which has been widely studied since last few decades,and for predicting which many theoretical models have been established.However,the models which have been existed are lack of objectivity due to that they are mostly studied based on the statistical methodology characterization for rough surfaces and simple partition for the deformation formats of contact asperity.In this paper,a fractal prediction model is developed for the thermal contact conductance between two rough surfaces based on the rough surface being described by three-dimensional Weierstrass and Mandelbrot fractal function and assuming that there are three kinds of asperity deformation modes:elastic,elastoplastic and fully plastic.Influences of contact load and contact area as well as fractal parameters and material properties on the thermal contact conductance are investigated by using the presented model.The investigation results show that the thermal contact conductance increases with the increasing of the contact load and contact area.The larger the fractal dimension,or the smaller the fractal roughness,the larger the thermal contact conductance is.The thermal contact conductance increases with decreasing the ratio of Young’s elastic modulus to the microhardness.The results obtained indicate that the proposed model can effectively predict the thermal contact conductance at the interface,which provide certain reference to the further study on the issue of heat transfer between contact surfaces.     

Effect of Porosity and Normal Load on Dry Sliding Friction of Polymer Foam Blocks

In this study, the effect of porosity on the dry sliding fiction of ethylene-vinyl acetate (EVA) foams was investigated under different normal load conditions. EVA foam blocks with varying porosities were slid against a smooth stainless steel plate under dry conditions. The friction coefficient increased with increasing porosity under all of the normal load conditions. In addition, the contact area was estimated using a contact model considering elastic buckling of the cell walls (elastic collapse). The elastic collapse area in the anterior portion of the EVA foam block increased with increasing normal load and porosity, which resulted in an increased contact area. Furthermore, the friction coefficient was positively correlated with the estimated contact area divided by the normal load, indicating that adhesion friction increases with increasing porosity of polymer foams. These results may contribute to the design of high-friction, lightweight shoe sole tread blocks prepared using polymer foam blocks.