机构介绍外文翻译

1、introduction to mechanismthe function of 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.(1) gear system, in which toothed members in c

2、ontact transmit motion between rotating shafts.(2) cam system, where a uniform motion of an input member is converted into a nonuniform motion of the output member.(3) plane and spatial linkages are also useful in creating mechanical motions for a point or rigid body.a kinematic chain is a system of

3、 links, which are either jointed together or are in contact with one another in a manner that permits them to move relative to one another. if one of the links is fixed and the movement of any other link to a new position will cause each of the other links to move to definite predictable position, t

4、he system is a constrained kinematic chain. otherwise, the system is an unconstrained kinematic chain.a mechanism or linkage is a constrained kinematic chain, and is a mechanical divice that has the purpose of transferring motion and force from a source to an output. a linkage consists of links (or

5、bar),generally considered rigid,which are connected by joints, such as pin (or revolute) or prismatic joints, to form open or closed chains (or loops). such kinematic chains,with at least one link fixed, become (i) mechanisms if at least two other links remain mobility, or (ii) stuctures if no mobil

6、ity remains. in other words, a mechanism permits relative motion between its “rigid links”; a structure does not. since linkages make simple mechanisms and can be designed to perform complex tasks, such as nonlinear motion and force ransmission, they will receive much attention in mechanism study.me

7、chanisms are used in a great variety of machines and devices. the simplest closed-loop linkage is the four-bar linkage, which has three moving links (plus one fixed link) and four pin joints. the link that is connected to the power source or prime mover and has one moving pivot and one ground pivot

8、is called the input link. the output link connects another moving povit to another ground povit. the coupler or floating link connected the two moving pivots, thereby “coupling” the input to the output link.the four-bar linkage has some special configurations created by making one or more links infi

9、nite in length。the slider-crank （or crank and slider）mechanism is a four-bar chain with a slinder replacing an infinitely long output link。the internal combustion engine is built based on this mechanism。other forms of four- bar mechanisms exist in which a slider is guided on a moving link rather tha

10、n on a fixed link. these are called inversions of the slider-crank，produced when another link（the crank，coupler，or slider）is fixed link。although the four-bar linkage and slider-crank mechanism are very useful and found in thousands of applications，we can see that these linkages have limited performa

11、nce level。linkages with more members are often used in more demanding circumstances。however it is often difficult to visualize the movement of a multiloop linkage，especially when other components appear in the same diagram。the fist step in the motion analysis of more complicated mechanisms is to ske

12、tch the equivalent kinematic or skeleton diagram。the skeleton diagram serves a purpose similar to that of the electrical schematic or circuit diagram in that it displays only the essential skeleton of the mechanism，which，however，embodies the the key dimensions that affect its motion。the kinematic di

13、agram takes one of two forms：a sketch（proportional but not exactly to scale），and the scaled kinematic diagram（usually used to further analysis：position，displacement，velocity，acceleration，force and torque transmission，etc。）。for convenient reference，the links are numbered（starting with ground link as

14、number 1），while the joints are lettered。the next step in the kinematic analysis of mechanisms is to determine the number of degree of freedom of the mechanism。by degree of freedom we mean the number of independent inputs required to determine the positions of all links of the mechanism with respect

15、toground。there are hundreds of thousands of different linkage types that one could invent。envision a bag containing a large variety of linkage components：binary，ternary，quaternary，and so on，links ；pin joints，slide joints；cams and cam followers；gears，chains，sprockets，belts，pulleys，and so on。（spherica

16、l and helical joints as well as other connections that allow three-dimensional relative motion are not included，as only planar motion in parallel planes are discussed here）。furthermore，imagine the possibility of forming all sorts of linkage types by putting these components together。are there any ru

17、les that help govern how these mechanisms are formed？ actually most mechanism tasks require a single input to be transferred to a single output。therefore，single-degree-of-freedom mechanisms are the forms used most frequently。for example，it is easy to see intuitively that a four-bar linkage is a sing

18、le-degree-of-freedom linkage。the process of drawing kinematic diagrams and determining degrees of freedom of mechanisms are the first steps in both the kinematic analysis and synthesis process。in kinematic analysis，a particular given mechanism is investigated based on the mechanism geometry plus pos

19、sibily other known characteristics。kinematic synthesis，on the other hand，is the proess of designing a mechanism to accomplish a desired task。here，both choosing the type as well as the dimensions of the new mechanism can be part of kinematic synthesis。the ability to visualize relative motion，to reaso

20、n why a mechanism is designed the way it is，and the ability to improve on a particular design are marks of a successful kinematician。although some of this ability comes in the form of innate creativity，much of it is a learned skill that improves with practice。movement analysisone of the simplest and

21、 most useful mechanisms is the four-bar linkage。most of the following description will concentrate on this linkage，but the procedures are also applicable to more complex linkages。we already known that a four-bar linkage has one degree of freedom。are there any more that are useful to know about four-

22、bar linkage？indeed there are！these include the grashof criteria，the concept of inversion，dead-center position（branch points），branching，transmission angle and their motion feature，including positions，velocities and accelerations。the four-bar linkage may take form of a so-called crank-rocker or a doub

23、le-rocker or a double-crank（drag-link）linkage，depending on the range of motion of the two links connected to the ground link。the input crank of a crank-rocher type can rotate continuously through 360，while the output link just “rocks”（or oscillates）。as a particular case，in a parallelogram linkage，wh

24、ere the length of the input link equals that of the output link and the lengths of the coupler and the ground link are also the same，both the input and output link may rotate entirely around or switch into a crossed configuration called an antiparallelogram linkage。grashofs criteria states that the

25、sum of the shortest and longest links of a planar four-bar linkage cannot be greater than the sum of the remaining two links if there is to be continuous relative rotation between any two links。notice that the same four-bar linkage can be a different type，depending on which link is specified as the

26、frame（or ground）。kinematic inversion is the process of fixing different links of a chain to create different mechanisms。note that the relative motion between links of a mechanism does not change indifferent inversions。besides having knowledge of the extent of the links，it would be useful to have a m

27、easure of how well a mechanism might“run”before actually building it。hartenberg mentions that“run”is a term that means effectiveness with which motion is imparted to the output link ；it implies smooth operation，in which a maximum force component is available to produce a force or torque in an output

28、 member。the resulting output force or torque is not only a function of the geometry of the linkage，it is generally the result of dynamic or inertia force which is often several times as large as the static force。for the analysis of low-speed operations or for an easily obtainable index of how any me

29、chanism might run，the concept of the transmission angle is extremely useful。during the motion of a mechanism，the transmission angle changes in value。a transmission angle of 0 degree may occur at a specific position，on which the output link will not move regardless of how large a force is applied to

30、the input link。in fact，due to friction in the joints，the general rule of thumb is to design mechanisms with transmission angle of large than a specified value。matrix-based definitions have been developed which measure the ability of a linkage to transmit motion。the value of a determinant（which conta

31、ins derivatives of output motion variables with respect to an input motion variable for a given linkage geometry）is a measure of the movability of the linkage in a particular position。if a mechanism has one degree of freedom (e.g. a four-bar linkage), then prescribing one position parameter, such as

32、 the angle of the input link, will completely specify the position of the rest of the mechanism (discounting the branching possibility). we can develop an analytical expression relating the absolute angular positions of the links of a four-bar linkage. this will be much more useful than a graphical

33、analysis procedure when analyzing a number of positions and /or a number of different mechanisms, because the expressions will be easily programmed for automatic computation. the relative velocity or velocity polygon method of performing a velocity analysis of a mechanism is one of several methods a

34、vailable. the pole represents all points on the mechanism having zero velocity. lines drawn from the pole to points on the velocity polygon represent the absolute velocities of the corresponding points on the mechanism. a line connecting any two points on the velocity polygon represents the relative

35、 velocity for the two corresponding points on the mechanism. another method is the instantaneous center or instant center method, which is a very useful and often quicker in complex linkage analysis. an instantaneous center or instant center is a point at which there is no relative velocity between

36、two links of a mechanism at that instant. in order to locate the locations of some instant centers of a given mechanism, the kennedys theorem of three centers is very useful. it states that the three instantaneous centers of three bodies moving relative to one another must lie along a straight line.

37、 the acceleration of links of a mechanism is of interest because of its effect on inertia force, which in turn influences the stress in the parts of a machine, bearing loads, vibration, and noise. since the ultimate objective is inertia-force analysis of mechanisms and machines, all acceleration com

38、ponents should be expressed in one and the same coordinate system : the inertia frame of reference of the fixed link of the mechanism. .notice that in general there are two components of acceleration of a point on a rigid body rotating about a ground pivot. one component has the direction tangent to

39、 the path of this point, pointed in the same sense of the angular acceleration of this body, and is called the tangential acceleration .its presence is due solely to the rate of change of the angular velocity. the other component, which always points toward the center of rotation of the body, is cal

40、led the normal or centripetal acceleration. this component is present due to the changing direction of the velocity vector. 译文：机构介绍机构是机械运动的一个部分，他的功能是把运动从一个刚体传递或转换到另一个刚体。用作机构基本零件的一般机械装置有三种类型：（1） 齿轮系统，在回转轴之间通过接触传递运动的齿状零件。（2） 凸轮系统，把输入零件的均匀运动转换成输出零件的非今年晕运动的装置。（3） 平面和空间连杆机构，使点或刚体产生机械运动的使用装置。 运动链是一个链接系统，它

41、们或者彼此铰接或者互相接触，相互间能够产生相对运动。如果链接中的某个连杆被固定，而其它任何一个连杆运动到新的位置将导致其它各个连杆也运动到确定的预期位置，该系统就是一个可约束的运动链。否则，该系统是一个非约束运动链。机构或连杆就是一个可约束的传动链，是一个从输入到输出以传递运动和力为目的的机械装置。连杆机构通常由被认为是刚体的构件或杆组成，它们之间用销轴铰接，例如用柱销（圆形的）或棱柱体的销轴铰接，形成开式或闭式（回环式）的运动链。如果这样的运动链至少有一个构件被固定并且：（i）如果至少有两个构件能保持运动，就变为机构；（ii）如果没有一个构件能够运动，则成为结构。换句话说，机构内部的刚性杆件

42、之间能够相对运动，而结构则不能。由于连杆系统能组成简单机构并完成复杂的任务，例如非线性运动的传递和力的传递，因而它们在机构研究中受到了更多的关注。许多机器和装置都使用机构。最简单的闭环连接是四连杆，它具有三个动杆（加上一个固定杆）和四个回转副。连接动力源或原动件的杆叫输入杆，有一个移动铰和一个固定铰。输出杆将另一个移动铰和另一个固定铰连接起来。连杆即浮动杆将两个移动铰链接起来，把输入传送到输出杆。把四连杆的一个或几个杆无限延长就会产生一些特殊的机构。曲柄滑块机构就是一个四连杆，只不过用滑块替换了一个无限长的输出杆。内燃机也是类似的一种机构。有些其它形式的四杆机构，其滑块在一个动杆上导移运动而不

43、是在一个固定杆上。把另一个杆（曲柄，连杆或滑块）固定，可以生成曲柄滑块机构的变异机构。待添加的隐藏文字内容2虽然四连杆和曲柄滑块机构的应用非常广泛，但是我们可以发现这些连杆机构的性能仍然有限。某些要求更高的环境使用的连杆机构有更多的元件。然而，想象多回环的连杆机构的运动常常非常困难。特别是当其它零件出现在同一图中的时候。对于比较复杂的机构的运动分析，第一步是绘制等效运动图或示意图这种示意图类似于电路示意图，仅仅表示出机构的主要本质，体现影响其运动的关键尺寸。运动图可用两种形式中的一种：一是草图（按比例画出，但放大比例不精确）；二是比例运动图（通常用于进一步分析其位置.位移.速度.加速度.力.和

44、扭矩传递等）。为了便于参考，用数字对构件进行编号（以机架为1开始编号），而用英文字母标注回转副。机构运动分析的第二部是确定机构的自由度数。我们所说的自由度，指的是使机构所有构件相对于机架具有确定位置所需要的独立输入的数目。人们可以发明有数以千计的不同类型的连杆机构。就像一个装有各种各样连杆的袋子，里面有：二杆组.三杆组.四杆组以及连杆.回转副.滑动副.凸轮随动件.齿轮.链条.链轮.皮带.皮带轮等。（球形运动副 .螺旋副.以及允许三维相对运动的其它连接尚未包括进去，因为这里仅仅讨论的是平行平面内的平面运动。）你还可以设想这些元件在一起形成各种类型连杆机构的可能性。存在帮助人们形成这些的规律吗？实

45、际上，大多数机构的任务是要求一个单一的输入被传递到一个单一的输出。因此单一自由度的机构是使用最多的机构类型。例如，由直觉很容易可以看出四连杆就是一个单一自由度的连杆机构。画运动图和确定机构自由度的过程，是机构运动分析和综合过程的第一个阶段。具体机构的运动可以根据机构的几何形状和可能知道的其它特性来分析。另一方面，运动综合是设计一个机构以完成所要求的任务的过程。因此，选择新机构的类型和尺寸是运动综合的一部分。想象相对运动的能力，推想出一个机构按现在那样的方式设计的原因并对一个具体设计进行改进的能力是一个成功的运动学家的标志。虽然这些能力有些来自先天的创造性，然而更多的是因为在实践中提高了技术水平。运动分析最简单、最有用的机构之一是四连杆。以下论述中的