关键词： 涡轮机械   动密封   动力特性系数   控制体方法   涡动频率  

摘要： 密封动力特性系数是评价透平机械转子稳定性的重要参数。对现有密封动力特性数值方法进行了综述,为了比较分析不同的密封动力特性数值方法,分别建立了基于控制体方法的双控体求解模型和基于CFD方法的稳态旋转坐标系与瞬态转子平动、转子单/多频椭圆涡动求解模型,综合比较分析了控制体法、CFD稳态旋转坐标系法和CFD瞬态平动法、单/多频法之间的差异以及适用范围,研究了转子涡动频率对密封气流力和动力特性系数的影响。研究结果表明:控制体法求解速度较快,但求解精度较低,稳态旋转坐标系法只适用于转子做小轨迹同心涡动的求解模型,瞬态平动法求解速度较慢;瞬态单/多频法考虑了转子的涡动频率,更符合密封的实际工作情况;应用瞬态单/多频法得到的密封气流力变化频率与转子涡动频率相同,但气流力的相位滞后于转子涡动位移的相位;密封的直接刚度系数随着转子涡动频率的增加而增大,交叉刚度系数和阻尼系数的绝对值随着转子涡动频率的增加而减小。

关键词： 机械系统动力学   多体系统动力学   教学改革   现代机械设计技术  

摘要： 机械系统动力学也称多体系统动力学。针对目前国内高等教育对现代机械设计技术中机械系统动力学的重视程度不足,以及课程讲授过程中的问题,根据多年的教学实践,从教材、教学内容、教学方法和教学手段等方面,归纳总结几点切实可行的教改措施,为机械系统动力学教学提供建设性意见和建议。

关键词： 机械臂   动力学方程   ADAMS仿真   AnsysWorkBench  

摘要： 针对井下巷道支护人工打锚索安全性差且效率低的问题,锚杆钻机设计新增了锚杆机械臂。根据机械臂的实际尺寸建立了三维模型,并且以机器臂设计运动学理论为基础,推导出锚杆钻机机械臂动力学方程。由于锚杆钻机机械臂在实际运动下受力相对复杂,并且对整机稳定性有较大的影响,故将机器臂的虚拟样机模型导入多体动力学软件ADAMS中进行动力学仿真,观察钻架在恒定外负载作用下,通过调整两个油缸之间的行程是否能实现钻架在整个工作空间的所有工作位置,找到整机在不同姿态打锚索时的最恶劣工况,查看该工况下油缸受力变化及伸缩臂受力状况。在仿真时刻3s时,机械臂升降油缸受力超过该油缸的许用极限抗压力1.9×10~5 N,需要运用Ansys Work Bench对伸缩臂承受扭转力较大处进行扭转静强度校核,以保证机械臂的动力学需求,为日后"工业机械臂"动力学分析提供一定的理论基础和实践经验。

关键词： Polyurethane   Crosslinking   Maturation   Moisture   FTIR-ATR spectroscopy   Dielectric spectroscopy   Tensile test  

摘要： This work addresses the impact of fabrication conditions (curing procedure, moisture) on the properties of a two-part polyurethane system. While Part I treats the chemical, dielectric and mechanical properties during cure and storage in dry conditions, this second part focuses on these properties, along with the caloric glass transition, in moist conditions, with moisture being introduced at different times during and right after cure. It is shown that the presence of water during fabrication severely affects final properties (increased polymer mobility, reduced mechanical stiffness) by reacting with isocyanate (forming primary amines and urea) and thereby lowering crosslink density, even for seemingly uncritical humidity conditions. Other chemical side reactions involving moisture are also identified. Water furthermore accelerates polymer dynamics substantially. Overall, moisture during cure and storage is clearly identified as a cause for property variations of PU. (C) 2018 Elsevier Ltd. All rights reserved.

关键词： 数据包络分析法   层次分析法   动力机械   选择  

摘要： 传统的动力机械评价与选择的方法易受到主观因素的影响。DEA-AHP模型能够有效避免主观因素的影响,并为备选的动力机械进行优选全排序。本文采用DEA-AHP模型为动力机械提出了评价与选择的指标体系,并运用DEA-AHP模型以算例的形式对农四师68团29.4kW拖拉机进行评价与选择,可为动力机械的评价与选择提供科学的理论依据。

关键词： 新工科建设   机械系统动力学   创新型人才   研究生教育  

摘要： 机械动力学系统是机械类研究生一门重要的学位课,用以提高学生建模和分析系统的能力。文章针对新工科建设以培养创新型人才为目标,基于学生在学习机械系统动力学课程中存在的问题,提出对应的教改思路,通过课堂教学和实验实践,使学生分析解决问题的能力大大提高。

关键词： 机械手   ADAMS   动力学分析  

摘要： 从机器人动力学参数设计角度出发,利用Solidworks对5自由度机械手建立运动仿真模型,运用ADAMS仿真软件对机械手进行动力学仿真,分析其关节处的位移、速度与加速度及其角速度与角加速度。通过对关节处的速度与加速度参数分析,观察机械手的运动姿态及其关节末端的速度与加速度特性,为实现机械手的运动精度控制提供参考。

关键词： NANOCOMPOSITE materials   MECHANICAL behavior of materials   CRYSTALLIZATION kinetics   POLYBUTENES   NANOPARTICLES  

摘要： In this article, the influence of type of carbon-based nanofillers such as carbon nanotubes (CNT), carbon-black (CB), and fullerene (F), on the non-isothermal crystallization kinetics of poly(butylene succinate) (PBS) was investigated. The particle geometry of the nanofillers was focused. The filler content was 0.5% and 1.0% by weight. The nanocomposites were processed in a laboratory twin screw compounder. The spherulitic morphology and crystal-growth were observed with a hot stage polarized optical microscopy. The structure of nanocomposites in the nanometer scale was characterized using transmission electron microscopy (TEM). The influence of nanofillers on the mechanical properties of PBS nanocomposites was measured by using dynamical mechanical analysis (DMA). The investigation of non-isothermal crystallization kinetics was performed by using differential scanning calorimetry (DSC) at four different cooling rates (5, 15, 25, and 50 degrees C/min). Analysis of melt crystallization data obtained was performed at a wide range of cooling rates with Avrami, Ozawa, and the combined Avrami-Ozawa models. The combined Avrami-Ozawa method could adequately describe the primary stage of non-isothermal crystallization of the neat PBS and PBS nanocomposites. It was shown that nanocomposites including CNT particles represented the highest nucleation ability in comparison to fullerene and carbon-black. Conversely, despite the fact that CNT exhibited a dominant heterogeneous nucleating efficiency, its performance in crystallization rate was contradicted. This interesting phenomenon indicated that the impeding effect of CNT particles during crystallization was stronger than its nucleation efficiency. The order of nucleation activity of each nanofiller from the lowest to the highest was as CNT>F>CB. POLYM. COMPOS., 39:2705-2721, 2018. (c) 2016 Society of Plastics Engineers

关键词： Plumbene   Two-dimensional nanomaterials   Molecular dynamics   Mechanical properties  

摘要： In recent years, two-dimensional (2D) nanomaterials have received tremendous attention due to their unique structure and extraordinary properties. Graphene, silicene, germanene and stanene- the 2D allotropes of carbon, silicon, germanium and fin have been reported in last few years and their properties have been studied elaborately. Plumbene, the 2D allotrope of lead, is recently reported. It is analogous to graphene, silicene, germanene and stanene in single-layered and hexagonal arrangement of atomic structure. In this paper, molecular dynamics simulations have been carried out to study the effect of sample size, temperature and strain velocity on the mechanical properties of plumbene. The results show superior mechanical properties of the single layer plumbene sheet which is several times higher than bulk lead making it a suitable candidate for using as reinforcement to develop high strength nanocomposites.

关键词： Planar mechanical system   Dynamic behavior   Joint clearance   Flexible links  

摘要： Joint clearance and flexible links are two important factors that affect the dynamic behaviors of planar mechanical system. Traditional dynamics studies of planar mechanism rarely take into account both influence of revolute clearance and flexible links, which results in lower accuracy. And many dynamics studies mainly focus on simple mechanism with clearance, the study of complex mechanism with clearance is a few. In order to study dynamic behaviors of two-degree-of-freedom (DOF) complex planar mechanical system more precisely, the dynamic analyses of the mechanical system with joint clearance and flexibility of links are studied in this work. Nonlinear dynamic model of the 2-DOF nine-bar mechanical system with revolute clearance and flexible links is built by Lagrange and finite element method (FEM). Normal and tangential force of the clearance joint is built by the Lankarani-Nikravesh and modified Coulomb's friction models. The influences of clearance value and driving velocity of the crank on the dynamic behaviors are researched, including motion responses of slider, contact force, driving torques of cranks, penetration depth, shaft center trajectory, Phase diagram, Lyapunov exponents and Poincar, map of clearance joint and slider are both analyzed, respectively. Bifurcation diagrams under different clearance values and different driving velocities of cranks are also investigated. The results show that clearance joint and flexibility of links have a certain impact on dynamic behavior of mechanism, and flexible links can partly decrease dynamic response of the mechanical system with clearance relative to rigid mechanical system with clearance.