关键词： 轮胎   机械手   模块化设计   虚拟样机   动力学特性  

摘要： 为了提高桁架机器人传送轮胎的效率和通用性,提出了一种可以抓取内径为304.8mm和内径为609.6mm的2种规格轮胎的机械手。通过分析轮胎的结构、放置形式以及上下料轮胎数量,运用Solidworks构建了轮胎上下料机械手的虚拟样机。并结合ANSYS仿真软件和实验,验证了机械手的动力学特性。结果表明,该机械手可以抓取7个内径为304.8mm的轮胎和4个内径为609.6mm的轮胎,并能准确完成轮胎上下料过程。

关键词： 农业劳动力   机械化   剩余劳动力转移  

摘要： 随着现代科技的不断发展,农业机械化成为了农业生产的必然趋势,将会推动农业劳动力向城市转移。基于此,文章就农业劳动力转移与农业机械化的相关性展开研究,首先分析农业劳动力转移与农业机械化的相互影响作用,提出相关协调发展策略。

关键词： 中国液压气动密封件工业协会   拉斯维加斯   电子化   专家委员会   流体动力   机械液压   副主任   整理  

摘要： 我这题目的英文名字叫Electronification,这是什么含义呢?比如我们以前都说Electronic,叫电子,电子就是3.0,现在提法叫Electronification,这就叫电子化了,这就是工业4.0的概念。协会领导让我到拉斯维加斯去看看,我确实看到了一些东西,给大家汇报一下,分四个问题。中国已经是液压大国了。大家看图1最底下那个红线,我们中国液压气动密封工业的发展和我们国家的GDP是相符的,和第二产业比呢,第二产业几乎是平的,但是我们却是上升的,所以在机械行业当中看出它的重要性了。我国目前液压规模大约是513亿。

关键词： 结构动力学   李群分析法   对称性   守恒量   不变解  

摘要： 为给复杂机械系统动力学特性定量和定性分析提供一个强有力的新途径,笔者将现代李群分析方法引入到机械结构动力学中。首先,通过矩阵解耦技术得到有限自由度机械结构振动系统在无限小群变换下导致的守恒量形式;其次,利用机械连续结构体三维弹性动力学方程允许的李点对称构造出方程组的系列群不变解。结果表明借助李群分析可以得到机械系统结构动力学更深层次的力学规律和运动现象。本研究不仅可以得到结构振动响应的精确解,同时也为数值计算提供了有效的验证手段。

关键词： Bileaflet mechanical heart valve   computational fluid dynamics   blood flow shear stresses   damage to blood cells  

摘要： Bileaflet mechanical heart valves (BMHVs) are widely used to replace diseased heart valves. However, patients may suffer from implant complications, such as platelet aggregation and damage to blood cells, which could lead to BMHV failure. These complications are related to the blood flow patterns in the BMHV. A three-dimensional computational fluid dynamic (CFD) model was developed to investigate blood hydrodynamics and shear stresses at different cardiac cycles. A user-defined function (UDF) code was developed to model the valve leaflet motion. This UDF updates the tetrahedral mesh according to the location of the valve leaflet, which enables modeling of complicated moving geometries and achieves solution convergence with ease without the need to adjust the relaxation factor values. The agreement between the experimental and numerical results indicates that the developed model could be used with confidence to simulate BMHV motion and blood flow. Furthermore, valve leaflet and valve pivot were found to be continuously exposed to shear stresses higher than 52.3 Pa which according to previous research findings may cause damage to blood platelets.

关键词： gamma-TiAl alloy   molecular dynamics   vacancy   temperature   mechanical properties  

摘要： Molecular dynamics simulation is used to analyze tensile strength and elastic modulus under different temperatures and vacancy concentrations. The effects of temperature and vacancy concentration on the mechanical properties of gamma-TiAl alloy are investigated. The results show that the ultimate stress, ultimate strain and elastic modulus decrease nonlinearly with increasing temperature and vacancy concentration. As the temperature increases, the plastic of material is reinforced. The influence of temperature on strength and elastic modulus is larger than that of vacancy concentration. The evolution process of vacancy could be observed clearly. Furthermore, vacancies with different concentrations develop into voids first as a function of external forces or other factors, micro cracks evolve from those voids, those micro cracks then converge to a macro crack, and fracture will finally occur. The vacancy evolution process cannot be observed clearly owing to the thermal motion of atoms at high temperature. In addition, potential energy is affected by both temperature and vacancy concentration.

关键词： MOLECULAR dynamics   NANOPOROUS materials   COPPER   NANOPARTICLES   SINTERING  

摘要： A molecular dynamics (MD) simulation is performed on the coalescence kinetics and mechanical behavior of a thermally sintered nanoporous copper (Cu) nanoparticulate system. To investigate the effect of particle size and sintering temperature on the coalescence of the nanoparticulate system, particles with sizes of 4, 5, and 6nm are sintered at temperatures of 300, 500, and 700K. To determine the thermal sintering process at elevated temperatures and ambient pressure, bulk periodic nanoparticle unit cells consisting of a finite number of nanoparticles are equilibrated through isothermal-Isobaric ensemble simulations. En thermally sintered configurations, uniaxlai tension/compression and shearing simulations are applied at a constant strain rate to derive stress-strain curves. It is found that stacking faults are actively generated in smaller nanoparticles even at a low sintering temperature, while local amorphization and surface and grain boundary diffusion are rather prominent in larger nanoparticles. Even at the same sintering temperature, the density of the sintered nanoparticle Increases as the size of the nanoparticle decreases. In elastic moduli, the same particle size dependency is observed, while no obvious difference Is observed in tension and compression. On the other hand, the yield strengths of the sintered nanoparticles in tension are larger than those in compression. The asymmetric yield strength of the sintered systems Is clarified by addressing the surface stress and surface equilibrium strain of atoms on the surface of nanopores by the evolution of atomic virial stress in tension and compression. (C) 2017 Elsevier Ltd. All rights reserved.

关键词： Notch   Strength   Deformation mode   Stress state   Plastic deformation  

摘要： Metallic glasses (MGs) with various geometrical notches have been one of the most attractive research topics because of their interesting mechanical behavior. In this work, molecular dynamics simulations were employed to examine the effect of internal notch depth on the mechanical behavior of Cu50Zr50 MGs. It is shown that both the strength and the deformation mode are sensitive to the notch depth, and in particular to the inter-void ligament distance. With increasing the notch depth, the notch intensification gets enhanced, while the deformation mode is seen to change from shear banding dominated to mixed mode and then to necking governed. Detailed stress distribution analysis reveals that the notch intensification and the deformation mode transition result from the stress state change induced by the presence of notches. The critical effective shear stress corresponding to local/atomic yielding in the MGs is identified to be around 1.9 GPa. In addition, the intrinsic size confinement effect also influences the deformation process. The findings are expected to provide useful guidance for materials design and investigation of yielding in MGs based on atomistic simulations.

关键词： Cas2   nuclease activity   structure and dynamics  

摘要： Cas2 protein in the CRISPR-Cas system functions as a scaffold for the acquisition of foreign DNA fragments, and as a nuclease against DNA and RNA substrates. Crystal structures of Cas2 have shown catalytically inactive conformational states that do not explain the mechanism of Cas2 nuclease activity. Here, we report that Xanthomonas albilineans Cas2 (XaCas2) assumes an inactive conformation in solution. Residual dipolar couplings and NMR relaxation, however, provide direct evidence on conformational dynamics at the predicted hinge region. Furthermore, XaCas2 transiently associates with metal ions for nuclease activity via highly mobile Asp8. Taken together, the dual function of Cas2 can be explained by a dynamic equilibrium of conformational states that serve as a scaffold or as a nuclease on demand.

关键词： Low-complexity tracking   High-order nonlinear systems   Unknown nonlinearities   Underactuated mechanical systems  

摘要： This paper presents a low-complexity design approach with predefined transient and steady-state tracking performance for global practical tracking of uncertain high-order nonlinear systems. It is assumed that all nonlinearities and their bounding functions are unknown and the reference signal is time varying. A simple output tracking scheme consisting of nonlinearly transformed errors and positive design parameters is presented in the presence of virtual and actual control variables with high powers where the error transformation technique using time-varying performance functions is employed. Contrary to the existing results using known nonlinear bounding functions of model nonlinearities, the proposed tracking scheme can be implemented without using nonlinear bounding functions (i.e., the feedback domination design), any adaptive and function approximation techniques for estimating unknown nonlinearities. It is shown that the tracking performance of the proposed control system is ensured within preassigned bounds, regardless of high-power virtual and actual control variables. The motion tracking problem of an underactuated unstable mechanical system with unknown model parameters and nonlinearities is considered as a practical application, and simulation results are provided to show the effectiveness of the proposed theoretical result.