Comparison and analysis of two Coulomb friction models on the dynamic behavior of slider-crank mechanism with a revolute clearance joint

The objective of this study is to investigate the effects of the Coulomb dry friction model versus the modified Coulomb friction model on the dynamic behavior of the slider-crank mechanism with a revolute clearance joint. The normal and tangential forces acting on the contact points between the journal and the bearing are described by using a Hertzian-based contact force model and the Coulomb friction models, respectively.The dynamic equations of the mechanism are derived based on the Lagrange equations of the first kind and the Baumgarte stabilization method. The frictional force is solved via the linear complementarity problem(LCP) algorithm and the trial-and-error algorithm.Finally, three numerical examples are given to show the influence of the two Coulomb friction models on the dynamic behavior of the mechanism. Numerical results show that due to the stick friction, the slider-crank mechanism may exhibit stick-slip motion and can balance at some special positions, while the mechanism with ideal joints cannot.

Kinematic Analysis of Elliptic-Gear & Eccentric Slider-Crank & Uniform-Pitch Screw Weft-Insertion Mechanism

In order to analyze the kinematic performances of elliptic-gear & eccentric slider-crank & uniform-pitch screw weft-insertion mechanism proposed by us,kinematic mathematic models of the mechanism were established,and an aided analytic software was compiled.Influences of some important parameters on the kinematic characteristics of the mechanism were analyzed.A group of preferable parameters were obtained according to the software and some requirements.Comparisons of kinematic performances were made between the proposed mechanism and the variable-pitch screw insertion mechanism of C401 rapier loom manufactured by Vamatex Corp in Italy.The results show that the proposed mechanism can meet the requirements of inserting wefts.

A method to calculate and counterbalance the inertia force of slider-crank mechanisms in high-speed presses

A new method to calculate and counterbalance the inertia force of slider-crank mechanisms in high-speed mechanical presses was put forward. By analyzing the kinematic characteristics of a center-located slider-crank mechanism whose crank rotates at a constant angular velocity,the kinematic parameters of the slide,connecting rod and crank were formulated approximately. On the basis of the results above,three inertia forces and the input moment in the mechanism during its idle running were investigated and formulated by dynamic analysis. A verification experiment was performed on a slider-crank mechanism at a high-speed press machine. The forces derived from the established formulas were compared respectively with those obtained by the ADAMS software and the classical method of connecting rod mass substitution. It was experimentally found that the proposed formulas have an improved performance over related earlier techniques. By use of these results,a 1 000 kN 1 250 rpm four-point high-speed press machine was designed and manufactured. The slide of this press is driven by four sets of slider-crank mechanisms with symmetrical layout and opposite rotation directions to counterbalance the horizontal inertia forces. Four eccentric counterbalance blocks were designed to counterbalance the vertical force after their mass and equivalent eccentric radius were formulated. The high-speed press machine designed by the proposed counterbalance method has worked with satisfactory performance and good dynamic balance for more than four years in practical production.

Design and Analysis on Compression Mechanism of Small Square Bales of Sugarcane Leaf Baler

With more and more attention paid to comprehensive utilization of agricul-tural biomass resources, environment pol ution caused by large-scale burning of sug-arcane leaves can be effectively avoided, and it is urgent to resolve storage issue of sugarcane leaf. Through referring to the relevant domestic baler models and re-search findings, preliminary design and analysis were conducted on key components compression mechanism of smal sugarcane leaf baler, involving the open compres-sion principle for sugarcane leaf square baler, determination of the crank slider-type compression mechanism and the main parameters such as crank length, compres-sion frequency, feed rate and feed inlet length. Based on SolidWorks software, this paper established the 3-dimensional models of slider-crank type compression mech-anism, and imported it into ADMAMS software for the motion simulation analysis, to establish the foundation for the design and manufacture of subsequent smal sugar-cane leaf square baler prototype.

Crack deflection occurs by constrained microcracking in nacre

For decades, nacre has inspired researchers because of its sophisticated hierarchical structure and remarkable mechanical properties, especially its extreme fracture toughness compared with that of its predominant constituent,CaCO3, in the form of aragonite. Crack deflection has been extensively reported and regarded as the principal toughening mechanism for nacre. In this paper, our attention is focused on crack evolution in nacre under a quasi-static state. We use the notched three-point bending test of dehydrated nacre in situ in a scanning electron microscope(SEM) to monitor the evolution of damage mechanisms ahead of the crack tip. The observations show that the crack deflection actually occurs by constrained microcracking. On the basis of our findings, a crack propagation model is proposed, which will contribute to uncovering the underlying mechanisms of nacre’s fracture toughness and its damage evolution. These investigations would be of great value to the design and synthesis of novel biomimetic materials.

Variation Characteristics of Focal Mechanisms of Small Earthquakes before Four Strong Earthquakes in Xinjiang

With the point source dislocation model and the velocity structure of a layered medium,focal mechanisms of small earthquakes are calculated using the maximum amplitude of the direct P- and S-waves in the vertical component. By system clustering,and using the vector synthesis method,the average focal mechanism solution is obtained. Using the above method,this paper analyzes the variation characteristics of the source ruptures and the P-axis azimuths of small earthquakes around the seismic zones before four strong earthquakes occurring since 2003 in the western part of north Tianshan and the middle part of Tianshan. The result shows that 2 ~ 3 years before the strong earthquakes,the focal mechanism types of small earthquakes are distributed randomly, and obvious dominant distributions are observed one year before the strong earthquakes. There are obvious changes in the P-axis azimuth.

底部双排配筋单孔空心预制梁力学性能试验研究

为了研究新型装配式单孔空心预制梁的抗弯承载力和变形性能,并提高其工作性能。试验共设计并制作了3根单孔空心预制梁和1根实心预制梁,通过试验和理论计算相结合的分析手段,重点研究单孔空心预制梁和实心预制梁的破坏机理、裂缝发展、荷载-挠度曲线、变形性能及抗弯承载力等。结果表明:单孔空心预制梁与普通实心预制梁的破坏特征相似,且其裂缝分布较均匀、间距小,延性好;在相同荷载作用下,单孔空心预制梁挠度大于实心预制梁,其整体受力变形性能相对优越。提出了截面有效面积等效法和混凝土轴心抗压强度等效法2种计算方法,通过对比分析发现,混凝土轴心抗压强度等效法计算值误差仅为0.82%,满足工程要求,而且单孔空心预制梁与实心梁相比还可节约材料8.72%。因此,单孔空心预制梁具有良好的应用前景。

取秧侧向零偏移的空间轨迹再生稻分插机构设计与试验

为了解决现有空间行星轮系式分插机构在再生稻宽窄行机械化种植取秧过程中出现侧向偏移量和侧向偏转角的技术难题,本文提出一种具有局部平面轨迹特性的空间轨迹不等速行星轮系机构,并开展基于取秧口和机构回转中心位置约束下的宽窄行分插机构综合研究。构建了基于关键位姿点(取秧起始点、取秧结束点、推秧点)的空间轮系机构运动综合模型,利用关键位姿点求解机构杆长参数与空间交错轴信息,并通过优选二杆相对角位移参数实现机构传动比分配。将不完全非圆齿轮副引入空间行星轮系机构,利用间歇机构锁止弧约束行星轴,实现机构取秧过程侧向零偏移量、侧向零偏转角的平面轨迹段。通过仿真分析与机构样机试验验证了机构实际作业性能与理论设计相一致,结果表明:分插机构取秧侧向零偏移量,取秧侧向零偏转角,推秧侧向总偏移量为50.24 mm、取秧角为5.18°、推秧角为71.56°、推秧侧向角为16.26°、插秧穴口宽度为22.43 mm、轨迹高度为289.76 mm,满足预期设计要求。最后通过田间试验验证,分插机构可实现等行距取秧口和既定机构回转中心下宽行(40 cm)与窄行(20 cm)间隔机插,满足再生稻宽窄行种植要求。

引入注意力机制的偏转字符识别算法

由于传统人工抄表成本高且受恶劣环境限制,使得智能抄表成为以后的发展方向。常见的电表主要可分为机械字轮电表和液晶电表两类。其中,由于机械字轮电表存在的偏转问题,导致其在字符识别的过程中出现了字符特征信息缺失的情况,从而使该表型识别准确率较低。为了解决此问题,本文对YOLOv5识别算法的骨干网部分进行了修改,提升了算法对机械字轮电表偏转字符的识别效果。首先,在网络模型中引入了CBAM注意力机制,提升了网络模型对偏转字符的特征提取能力。其次,将切片操作(Focus)替换成一个6×6的卷积,并使用更快的SPPF池化结构替换了原先的SPP池化结构以提升算法的运算速度。为了测试模型的识别效果,采集了329张电表偏转字符样本进行实验,整体识别准确率可以达到99.4%。同时采集了1500张液晶电表样本对模型的泛化性进行测试,识别准确率达到了99.6%。实验结果表明,本文方法解决了偏转字符识别率低的问题,同时验证了识别模型具有很强的泛化性。

微流体装置力源机构的优化设计

为解决微流控系统中流体介质贮存装置输出压力不稳定等问题,设计了一种柔顺机构作为装置的力源机构。该柔顺机构基于正负刚度叠加理论设计,由四根固定导向梁和两根平面弹簧组成。首先,利用链式梁约束模型和广义打靶法分别求解出固定导向梁和平面弹簧的位移-载荷特性,建立优化模型,得到了满足设计要求的最优解。然后,为验证计算模型的准确性,使用ANSYS对柔顺梁进行大变形分析,结果显示该计算模型和有限元分析相比,误差小于1%。最后,通过实验发现该柔顺机构输出力的实验值和理论设计值存在误差,经过分析发现是加工精度、材料弹性模量误差以及只考虑了中性轴的变形等原因所导致的。

大型梁式承台模型试验及跨中挠度简化计算方法研究

为了解大型梁式承台的力学性能,以某快速化改造工程的大型梁式承台为背景,按照缩尺比1∶10制作梁式承台模型进行力学性能试验,结合有限元计算,分析梁式承台结构受力、破坏模式、开裂情况等,并研究不同跨高比条件下梁式承台的受力特性。结果表明:在跨中荷载作用下,梁式承台受力大致可以分为线弹性阶段、裂缝发展阶段和屈服阶段3个阶段;线弹性阶段截面应力分布已不满足平截面假定,剪切效应对结构变形的影响不可忽略;模型最终破坏模式为弯曲破坏;裂缝主要表现为跨中弯曲裂缝和斜裂缝;跨高比越小,梁式承台剪切刚度越大。为考虑剪切效应对梁式承台变形的影响,拟合得出梁式承台跨中挠度的简化计算方法,将该方法计算值及规范计算值与试验值进行对比,可得在裂缝发展阶段,规范计算值与试验值偏差较大,所提方法计算值与试验值偏差较小,该方法可用于线弹性阶段及裂缝发展阶段梁式承台跨中挠度的初步估算。

Closure Effect ofⅠ+ⅡMixed-mode Crack for EA4T Axle Steel

The crack-closure effect is a crucial factor that affects the crack growth rate and should be considered in simulation analysis and testing.A mixed-mode I+II loading fatigue crack growth test was performed using EA4T axle steel specimens.The variation of the plastic-induced crack closure(PICC)effect and the roughness-induced crack closure(RICC)effect during crack deflection in the mixed-mode is examined in this study.The results show that the load perpendicular to the crack propagation direction hinders the slip effect caused by the load parallel to the crack propagation direction under mixed-mode loading,and the crack deflection is an intuitive manifestation of the interaction between the PICC and RICC.The proportion of the RA value change on the crack side caused by contact friction was reduced by the interaction between PICC and RICC.The roughness of the crack surface before and after the crack deflection is different,and the spatial torsion crack surface is formed during the crack propagation process.With the increase of the crack length,the roughness of the fracture surface increases.During the crack deflection process,the PICC value fluctuates around 0.2,and the RICC value is increased to 0.15.

基于LC-OPA的空间激光通信技术进展

液晶相控阵既可以在空间激光通信技术建立多点间空间激光通信链路,又能代替压电偏摆镜的精瞄方式。阐述液晶相控阵应用趋势;通过对液晶相控阵中液晶分子电致双折射和多缝衍射光束偏转原理,提出替压电偏摆镜配合终端进行瞄准捕获跟踪的方案;以“一对二”空间激光通信终端为例,提出基于液晶相控阵多点间空间激光通信终端方案和链路工作模式,分析方案优势,并进行未来展望。

基于AutoCAD Mechanical的货架横梁设计

介绍了Autodesk AutoCAD Mechanical软件的转动惯量和挠度线的计算功能,并以实例介绍其在货架横梁设计中的应用,对横梁的截面进行惯性矩的计算,并用惯性矩的计算结果计算挠度线和安全系数,提高了计算精度并加快了设计进度。

基于AutoCAD Mechanical的轴类零件设计

介绍了Autodesk公司的AutoCAD Mechanical软件及其轴生成器和轴计算的基本功能,介绍了基于AutoCAD Mechanical的轴的设计的基本步骤,并以实例介绍了轴的绘制和设计计算的方法,最后讨论了各个设计步骤的注意事项和优缺点。实践表明,采用AutoCAD Mechanical可以快捷绘制出机械图样,直观、方便、准确实现轴的设计计算,减少了设计时间,大大提高工作效率,但存在一些缺陷,如强度校核点需要用户自己寻找,结果是否符合要求需要用户自己判断。

致密储层缝内暂堵转向压裂裂缝扩展规律数值模拟

基于损伤力学理论,建立了储层渗流-应力-损伤耦合裂缝扩展数值模型,将模型结果与室内真三轴水力压裂物理模拟实验结果进行了对比,验证了模型的准确性,并基于该模型探讨了压裂液黏度、排量、水平地应力差以及储层岩石非均质性对缝内暂堵转向压裂效果的影响。研究结果表明:(1)储层渗流-应力-损伤耦合裂缝扩展数值模型是联合流体流动控制方程与岩石变形方程形成整体控制方程,通过在初始裂缝扩展路径上某一区域人为设置高强度的岩石物理力学参数和较小的储层渗透率值,实现缝内暂堵的模拟。(2)缝内暂堵转向压裂裂缝扩展模型数值模拟的分支缝数量、主裂缝面积、主裂缝延伸方向等与室内真三轴水力压裂物理模拟实验的结果基本一致,该模型可实现缝内暂堵后基质的破裂与新裂缝扩展模拟,对缝内暂堵转向压裂裂缝扩展情况也具有较好的模拟效果。(3)压裂液的黏度和排量越大,缝内暂堵转向压裂裂缝长度、改造面积及偏转角度均明显增大,且逐渐由单一裂缝向复杂裂缝转变。当水平地应力差小于7.5 MPa时,缝内暂堵转向压裂的效果较好;当水平地应力差为10~15 MPa时,压裂效果变差;当水平地应力差大于15 MPa时,裂缝几乎不偏转;储层非均质性会影响裂缝局部的扩展路径,但对裂缝总体扩展趋势影响甚微。

建筑用不同取代率粉煤灰再生混凝土的力学性能及耐久性能研究

以废弃混凝土为再生骨料,采用湿法粉磨处理I级粉煤灰,制备出不同取代率粉煤灰再生混凝土,通过XRD、SEM、力学性能测试、干缩率测试等手段,研究了不同取代率的I级粉煤灰对再生混凝土的力学性能和耐久性能的影响。结果表明,湿法粉磨处理后的粉煤灰表面玻璃体被破坏,火山灰效应增强,从而提高了再生混凝土的水化速率,使水化产物C—S—H和AFt数量增加,混凝土密实度增大。粉煤灰再生混凝土的抗压强度随粉煤灰取代率的增大先增大后降低,粉煤灰取代率45%的混凝土的抗压强度最大为42.05 MPa,其水化产物Ca(OH)_(2)的含量最多为11.742%。挠度测试表明,粉煤灰取代率45%的混凝土屈服阶段对应的载荷为217 N,挠度最大为1.7 mm,在28 d时粉煤灰取代率45%的混凝土的干缩率最低为1.021×10^(-4)。综合可知,粉煤灰取代率45%的混凝土性能最优。

带式输送机跑偏机理及纠偏系统设计

基于煤矿采掘工作面带式输送机在运输物料过程中频繁出现跑偏现象,造成输送机运输效率低,甚至出现撕带、断带事故,严重影响着煤矿安全生产,基于此,分析了矿用带式输送机跑偏机理,并设计了一套纠偏系统,通过在店坪煤矿3022巷SSJ-100型带式输送机中安装使用效果来看,该系统有效阻止了输送机跑偏现象,跑偏事故率降低了85%,减少设备维修费用达40余万元,取得了显著应用成效。

共振碎石化路面弯沉响应特征及模量敏感性分析

共振碎石化路面的结构强度评估工作对于掌握其承载能力至关重要。为探究路面结构弯沉的变化特点,基于ABAQUS建立共振碎石化路面有限元模型,分析弯沉空间分布规律,进一步将碎石化层分为三层、变换其模量,分析弯沉对其敏感性变化。研究发现弯沉沿横断面呈现“M”形分布,纵断面呈现“山”形分布。随着各层模量的增加,弯沉出现不同程度的降低,在低模量区间结构层弯沉变化较快;在中高模量区间,弯沉随着碎石化上层和中层模量增大仍保持较高水平的降低速率,但弯沉对碎石化下层模量变化的敏感性逐渐降低。

分散剂对PP/POE/滑石粉复合材料综合性能的影响

利用双螺杆挤出机制备了聚丙烯(PP)/聚烯烃弹性体(POE)/滑石粉/分散剂复合材料,并对材料流动性、力学性能及热性能进行表征,探讨分散剂对不同熔融指数体系的影响,并优化复合材料体系中分散剂种类、含量及复配形式。结果表明,分散剂TAS-2A在提升流动性及力学性能的同时会降低材料的热变形温度;YY-5023力学性能提升效果不明显,但能显著提升材料热变形温度;TAS-2A与YY-5203的最适宜添加量分别为2%和0.25%,且二者复配具有协同效应。