机构与机器机械类外文翻译、外文文献翻译、中英文翻译

外文原文： Mechanism and Machines A system that transmits forces in a predetermined manner to accomplish specific objectives may be considered a machine. A mechanism may be defined in a similar manner, but the term mechanism is usually applied to a system where the principal function is to transmit motion. Kinematics is the study of motion in mechanism, while the analysis of force and torques in machined is called dynamics. Once the need for a machine or mechanism with given characteristics is identified, the design process begins. Detailed analysis of displacements, velocities, and accelerations is usually required. This part of the design process is then followed by analysis of force and torques. The design process may continue long after first model have been produce and include redesigns of component that affect velocities, accelerations, force, and torques. In order to successfully compete form year to year, most manufacturers must continuously modify their product and their methods of production. Increases in production rate, upgrading of product performance, redesign for cost and weight reduction, and motion analysis of new product lines are frequently required. Success may hinge on the correct kinematic and dynamic analysis of the problem. Many of the basic linkage configurations have been incorporate into machines designed centuries ago, and the term we use to describe then have change over the year. Thus, definitions and terminology will not be consistent throughout the technical literature. In most cases, however, meanings will be clear form the context of the descriptive matter. A few terms of particular interest to the study of kinematic and dynamics of machines are define below. Link A link is one of the rigid bodies or members joined together to form a kinematic chain. The term rigid link or sometimes simply link is an idealization used in the study of that does not consider small deflections due to strains in machine members. A perfectly rigid or inextensible link can exist only as a textbook type of model of a real machine member. For typical machine part, maximum dimension changes are of only a one-thousandth of the part length. We are justified in neglecting this small motion when considering the much greater motion characteristic of most mechanisms. The word link is used in a general sense to include cams, gears, and other machine members in addition to cranks, connecting rods and other pin-connected components. Degrees-of-freedom The number of degrees-of-freedom of a linkage is the number of independent parameters required to position of every link relative to the frame or fixed link. If the instantaneous configuration of a system may be completely defined by specifying one independent variable, that system has one degree-of-freedom. Most practical mechanisms have one degree-of-freedom. An unconstrained rigid body has six degrees-of-freedom: translation in three coordinates and rotation about three coordinate axes. If the body is restricted to motion in a plane, there are three degrees-of-freedom: translation in two coordinate directions and rotation within the plane. Lower and Higher Pairs Connections between rigid bodies consist of lower and higher pairs of elements. The two elements of a lower pair have theoretical surface contact with one another, while the two elements of a higher pair have theoretical point or line contact (if we disregard deflections). Lower pairs are desirable from a design standpoint since the load at the joint and the resultant wear is spread over the contact surface. Thus, geometric changes or failure due to high contact stresses and excessive wear may be prevented. Mechanism A mechanism is a kinematic chain in which one link is considered fixed for the purpose of analysis, but motion is possible in other links. As noted above, the link designated as the fixed link need not actually be stationary relative to the surface of the earth. A kinematic chain is usually identified as a mechanism if its primary purpose is the modification or transmission of motion. Machine A mechanism designed for the purpose of transmitting forces or torques is usually called a machine. Engine A machine that involves conversion of energy to produce mechanical power is commonly called an engine. Thus, the crankshaft, connecting rod, piston, and cylinder of an automotive engine would be an engine by the above definitions, while other drive train components such as the transmission, differential, and universal joint would be considered machines. Machines and engines may have the same configuration as other mechanisms that do not convert energy and are not intended to transmit significant levels of force or torque. Thus, for the purpose of kinematic analysis, the above distinction between mechanism, machine, and engine may be of only academic importance. A Mechanism has been defined as “a combination of rigid or resistant bodies so formed and connected that they move upon each other with definite relative motion.” Mechanisms form the basic geometrical elements of many mechanical devices including automatic packaging machinery, typewriters, mechanical toys, textile machinery, and others. A mechanism typically is designed to create a desired motion of a rigid body relative to a reference member. Kinematic design of mechanisms is often the first step in the design of a complete machine. When forces are considered, the additional problems of dynamics, bearing loads, stresses, lubrication, and the like are introduced, and the larger problem becomes one of machine design. The function of a mechanism is to transmit or transform motion from one rigid body to another as part of the action of a machine. There are three types of common mechanical devices that can be used as basic elements of a mechanism. Gear Systems Gear systems, in which toothed members in contact transmit motion between rotating shafts. Gears normally are used for the transmission of motion with a constant angular velocity ratio, although noncircular gears can be used for nonuniform transmission of motion. Cam Systems Cam systems, where a uniform motion of an input member is converted into a nonuniform motion of the output member. The output motion may be either shaft rotation, slider translation, or other follower motions created by direct contact between the input cam shape and the follower. The kinematic design of cams involves the analytical or graphical specification of the cam surface shape required to drive the follower with a motion that is a prescribed function of the input motion. Plane and Spatial Linkages They are also useful in creating mechanical motions for a point or rigid body. Linkages can be used for three basic tasks. (1) Rigid body guidance. A rigid body guidance mechanism is used to guide a rigid body through a series of prescribed positions in space. (2) Path generation mechanism will guide a point on a rigid body through a series of points on a specified path in space. (3) Function generation. A mechanism that creates an output motion that is a specified function of the input motion. Mechanisms may be categorized in several different ways to emphasize their similarities and differences. One such grouping divides mechanisms into planar, spherical, and spatial categories. All three groups have many things in common； the criterion which distinguishes the groups, however, is to be found in the characteristics of the motions of the links. A planar mechanism is one in which all particles describe plane curves in space and all these curves lie in parallel planes； i.e. the loci of all points are plane curves parallel to a single common planar mechanism in its true size and shape on a single drawing or figure. The plane four-bar linkage, the plate cam and follower, and the slider-crank mechanism are familiar examples of planar mechanisms. The vast majority of mechanisms in use today are planar. A spherical mechanism is one in which each link has some point which remains stationary as the linkage moves and in which the stationary points of all links lie at a common location； i.e., the locus of each point is a curve contained in a spherical surface, and the spherical surfaces defined by several arbitrarily chosen points are all concentric. The motions of all particles can therefore be completely described by their radial projections, or “shadows,” on the surface of a sphere with properly chosen center. Hookes universal joint is perhaps the most familiar example of a spherical mechanism. Spatial mechanisms, on the other hand, include no restrictions on the relative motions of the particles. The motion transformation is not necessarily coplanar, nor must it be concentric. A spatial mechanism may have particles with loci of double curvature. Any linkage which contains a screw pair, for example, is a spatial mechanism, since the relative motion within a screw pair is helical. 译文： 机构与机器 一 个系统，它按预先确定的方式来传输动力完成的具体的目标也许可以被认为是机器。一种机构也可以以类似的方式定义，但长期的机构通常是适用于一个系统的主要职能是传递运动。运动学是研究机构运动，而分析力和力矩的机械称为动力学。 一旦需要给出识别一个机构或机械装置的特点，设计过程就开始了。通常需要仔细地分析位移，速度和加速度。这部分的设计过程后，其次是分析力和力矩。设计过程中可能会继续很长时间后产生第一种模式，其中包括重新设计的组成部分，影响速度，加速度，力和力矩。年复一年的为了竞争成功，大部分的制造商必须不断 地修改他们的产品及其生产方法。提高生产速度，提高产品性能，重新设计的成本和减轻体重，运动分析和新的生产线往往是需要的。成功或许取决于正确的运动学和动力学的分析的问题。 许多基本的连接装置构造世纪以前已经成为机器设计的组成部分，和我们使用这个术语形容当时的变化超过一年。因此，定义和专门的术语将不符合整个技术的文献。在大多数情况下，但是，含义将是明确的背景下形成的重要性的描述。有几个方面特别感兴趣的研究机器运动学和动力学的定义如下。 杆件 一个杆件是一个严格的机构或其共同组成一个运动链。长期严格的杆 件或有时只是使用一个理想化的杆件研究，由于机件拉紧不考虑微小挠度。一个完全不弯曲或不可拉长的杆件可能存在不仅是一种教科书式的模型，一个真正的机器的构件。对于典型的机械部分，最大尺寸的变化是只有长度部分的千分之一。当我们考虑多数机械装置的运动特性时我们有理由忽视这个小小的运动。这个杆件定理中使用的一般意义上包括凸轮，齿轮，和其他构件除了曲柄、连杆和其他引脚连接组件。 自由度 自由度的数量的联系是一些独立的参数必须立场的每一个环节相对内或固定杆件。如果即可改造的系统可以完全确定指定一个独立的变量，该系统有一个 自由度。多数实用的机械装置就有一个自由度。 一个无约束刚体有 6 个自由度：直线移动在三个坐标和旋转运动三个坐标轴。如果该机构是限制于在一个平面运动，那有三个自由度：直线运动在两个坐标方向和在平面内的旋转。 高副和低副 链接的刚体之间包括高副和低副两个要素。这两个因素中的低副是两个理论表面之间的接触，而这两个因素中的高副是理论的点或线接触（如果我们忽视了挠度）。 低副是从设计的角度来看是可取的，由于联合负荷以及由此产生的磨损分布在整个接触面。因此，几何变化或失败而高接触应力和过度磨 损或许是可以避免的。 机械装置 机械装置是一个运动链系中的一环被认为是特定的目的是为了分析，但运动可能是其他的环节。如上所述，特定的杆件为指定的杆件不需要与实际相对固定在地球表面。如果运动学链主要目的是缓和或传输动力，其就通常被作为一种机械装置， 机器 这种机构设计是为达到转递动力或力矩的目的通常是所谓的机器。 发动机 一个机器需要能量转换而产生的机械动力通常称为发动机。因此，曲轴，连杆，活塞和气缸的自动的发动机由上面所述的发动机的定义，而其他的传动部件，例如变速箱，差速器，和万向联 轴器都被称为为机械装置。机器和发动机或许有相同装置，其他的机械装置不能转换动力，而是为了传输大的动力或者是扭矩。因此，为了运动学的分析，上述机械装置、机器、发动机之间的区别，可能仅仅在学术上有重要性。 机构就是：由刚体或者是有承载能力的物体连接而组成的组合体，他们在运动时候彼此间具有确定的相互运动。 机构是由构成这些机械设备的基本的几何单元，这些机械设备包括自动包装机、打字机、机械的玩具、纺织机等等。机构设计的目的是使一个刚体相对某一个参考的构件产生所需要的相对运动。机构的运动设计通常是设计一个完整的机器的 第一步。在考