機械外文翻譯---機構(gòu)介紹

1、<p>　　Introduction to mechanism</p><p>　　The 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

2、 mechanical devices that can be used as basic elements of a mechanism.</p><p>　　Gear system, in which toothed members in contact transmit motion between rotating shafts.</p><p>　　Cam system, whe

3、re a uniform motion of an input member is converted into a nonuniform motion of the output member.</p><p>　　Plane and spatial linkages are also useful in creating mechanical motions for a point or rigid body

4、.</p><p>　　A kinematic chain is a system of 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 f

5、ixed and the movement of any other link to a new position will cause each of the other links to move to definite predictable position, the system is a constrained kinematic chain. Otherwise, the system is an unconstraine

6、d kinematic chain.</p><p>　　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 con

7、sists of links (or 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 fixe

8、d, become (i) mechanisms if at least two other links remain mobility, or (ii) stuctures if no mobilit</p><p>　　Mechanisms are used in a great variety of machines and devices. The simplest closed-loop linkage

9、 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 is called the

10、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 out</p><p>　　The four-bar

11、linkage has some special configurations created by making one or more links infinite in length。The slider-crank （or crank and slider）mechanism is a four-bar chain with a slinder replacing an infinitely long output link。T

12、he 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 than on a fixed link. These are called inversions of the slider

13、-crank，produced when another link（the crank，cou</p><p>　　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 li

14、mited performance 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

15、the same diagram。The fist step in the motion analysis of more complicated mechanisms is to sketch the equivalent kinematic or skel</p><p>　　The next step in the kinematic analysis of mechanisms is to determi

16、ne 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 toground。There are hundreds

17、 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 follow</p&

18、gt;<p>　　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

19、 mechanism is investigated based on the mechanism geometry plus possibily other known characteristics。Kinematic synthesis，on the other hand，is the proess of designing a mechanism to accomplish a desired task。Here，both ch

20、oosing the type as well as the dimensions of the new mechanism can be part o</p><p>　　Movement analysis</p><p>　　One of the simplest and most useful mechanisms is the four-bar linkage。Most of th

21、e following description will concentrate on this linkage，but the procedures are also applicable to more complex linkages。</p><p>　　We already known that a four-bar linkage has one degree of freedom。Are there

22、 any more that are useful to know about four-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 feat

23、ure，including positions，velocities and accelerations。</p><p>　　The four-bar linkage may take form of a so-called crank-rocker or a double-rocker or a double-crank（drag-link）linkage，depending on the range of

24、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 li

25、nkage，where 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 </p><p>　　Notice that the same four-bar linkage can be a different type

26、，depending on which link is specified as the 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 mechan

27、ism does not change indifferent inversions。</p><p>　　Besides having knowledge of the extent of the links，it would be useful to have a measure of how well a mechanism might“run”before actually building it。Har

28、tenberg 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 a

29、n output member。The resulting output force or torque is not only a function of the geometry of the linkage，it is generally the</p><p>　　If a mechanism has one degree of freedom (e.g. a four-bar linkage), the

30、n prescribing one position parameter, such as 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 expressio

31、n relating the absolute angular positions of the links of a four-bar linkage. This will be much more useful than a graphical analysis procedure when analyzing a number of positions and /or a number of differen</p>

32、<p>　　The relative velocity or velocity polygon method of performing a velocity analysis of a mechanism is one of several methods available. The pole represents all points on the mechanism having zero velocity. Lin

33、es 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 ve

34、locity for the two corresponding points on the mechanism. </p><p>　　Another method is the instantaneous center or instant center method, which is a very useful and often quicker in complex linkage analysis.

35、An instantaneous center or instant center is a point at which there is no relative velocity between two links of a mechanism at that instant. In order to locate the locations of some instant centers of a given mechanism,

36、 the Kennedy’s theorem of three centers is very useful. It states that the three instantaneous centers of three bodies moving relative to one</p><p>　　The acceleration of links of a mechanism is of interest

37、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 machine

38、s, all acceleration components should be expressed in one and the same coordinate system : the inertia frame of reference of the fixed link of the mechanism. .</p><p>　　Notice that in general there are two c

39、omponents of acceleration of a point on a rigid body rotating about a ground pivot. One component has the direction tangent to the path of this point, pointed in the same sense of the angular acceleration of this body, a

40、nd 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 called the normal

41、 or centripetal accele</p><p><b>　　譯文：</b></p><p><b>　　機構(gòu)介紹</b></p><p>　　機構(gòu)是機械運動的一個部分，他的功能是把運動從一個剛體傳遞或轉(zhuǎn)換到另一個剛體。用作機構(gòu)基本零件的一般機械裝置有三種類型：</p><p>　　齒輪

42、系統(tǒng)，在回轉(zhuǎn)軸之間通過接觸傳遞運動的齒狀零件。</p><p>　　凸輪系統(tǒng)，把輸入零件的均勻運動轉(zhuǎn)換成輸出零件的非今年暈運動的裝置。</p><p>　　平面和空間連桿機構(gòu)，使點或剛體產(chǎn)生機械運動的使用裝置。</p><p>　　運動鏈是一個鏈接系統(tǒng)，它們或者彼此鉸接或者互相接觸，相互間能夠產(chǎn)生相對運動。如果鏈接中的某個連桿被固定，而其它任何一個連桿運動到新的位置

43、將導(dǎo)致其它各個連桿也運動到確定的預(yù)期位置，該系統(tǒng)就是一個可約束的運動鏈。否則，該系統(tǒng)是一個非約束運動鏈。</p><p>　　機構(gòu)或連桿就是一個可約束的傳動鏈，是一個從輸入到輸出以傳遞運動和力為目的的機械裝置。連桿機構(gòu)通常由被認為是剛體的構(gòu)件或桿組成，它們之間用銷軸鉸接，例如用柱銷（圓形的）或棱柱體的銷軸鉸接，形成開式或閉式（回環(huán)式）的運動鏈。如果這樣的運動鏈至少有一個構(gòu)件被固定并且：（i）如果至少有兩個構(gòu)件能保

44、持運動，就變?yōu)闄C構(gòu)；（ii）如果沒有一個構(gòu)件能夠運動，則成為結(jié)構(gòu)。換句話說，機構(gòu)內(nèi)部的剛性桿件之間能夠相對運動，而結(jié)構(gòu)則不能。由于連桿系統(tǒng)能組成簡單機構(gòu)并完成復(fù)雜的任務(wù)，例如非線性運動的傳遞和力的傳遞，因而它們在機構(gòu)研究中受到了更多的關(guān)注。</p><p>　　許多機器和裝置都使用機構(gòu)。最簡單的閉環(huán)連接是四連桿，它具有三個動桿（加上一個固定桿）和四個回轉(zhuǎn)副。連接動力源或原動件的桿叫輸入桿，有一個移動鉸和一個固定鉸

45、。輸出桿將另一個移動鉸和另一個固定鉸連接起來。連桿即浮動桿將兩個移動鉸鏈接起來，把輸入傳送到輸出桿。</p><p>　　把四連桿的一個或幾個桿無限延長就會產(chǎn)生一些特殊的機構(gòu)。曲柄滑塊機構(gòu)就是一個四連桿，只不過用滑塊替換了一個無限長的輸出桿。內(nèi)燃機也是類似的一種機構(gòu)。有些其它形式的四桿機構(gòu)，其滑塊在一個動桿上導(dǎo)移運動而不是在一個固定桿上。把另一個桿（曲柄，連桿或滑塊）固定，可以生成曲柄滑塊機構(gòu)的變異機構(gòu)。<

46、/p><p>　　雖然四連桿和曲柄滑塊機構(gòu)的應(yīng)用非常廣泛，但是我們可以發(fā)現(xiàn)這些連桿機構(gòu)的性能仍然有限。某些要求更高的環(huán)境使用的連桿機構(gòu)有更多的元件。然而，想象多回環(huán)的連桿機構(gòu)的運動常常非常困難。特別是當其它零件出現(xiàn)在同一圖中的時候。對于比較復(fù)雜的機構(gòu)的運動分析，第一步是繪制等效運動圖或示意圖這種示意圖類似于電路示意圖，僅僅表示出機構(gòu)的主要本質(zhì)，體現(xiàn)影響其運動的關(guān)鍵尺寸。運動圖可用兩種形式中的一種：一是草圖（按比例畫出

47、，但放大比例不精確）；二是比例運動圖（通常用于進一步分析其位置.位移.速度.加速度.力.和扭矩傳遞等）。為了便于參考，用數(shù)字對構(gòu)件進行編號（以機架為1開始編號），而用英文字母標注回轉(zhuǎn)副。</p><p>　　機構(gòu)運動分析的第二部是確定機構(gòu)的自由度數(shù)。我們所說的自由度，指的是使機構(gòu)所有構(gòu)件相對于機架具有確定位置所需要的獨立輸入的數(shù)目。人們可以發(fā)明有數(shù)以千計的不同類型的連桿機構(gòu)。就像一個裝有各種各樣連桿的袋子，里面有

48、：二桿組.三桿組.四桿組以及連桿.回轉(zhuǎn)副.滑動副.凸輪隨動件.齒輪.鏈條.鏈輪.皮帶.皮帶輪等。（球形運動副 .螺旋副.以及允許三維相對運動的其它連接尚未包括進去，因為這里僅僅討論的是平行平面內(nèi)的平面運動。）你還可以設(shè)想這些元件在一起形成各種類型連桿機構(gòu)的可能性。存在幫助人們形成這些的規(guī)律嗎？實際上，大多數(shù)機構(gòu)的任務(wù)是要求一個單一的輸入被傳遞到一個單一的輸出。因此單一自由度的機構(gòu)是使用最多的機構(gòu)類型。例如，由直覺很容易可以看出四連桿就是

49、一個單一自由度的連桿機構(gòu)。</p><p>　　畫運動圖和確定機構(gòu)自由度的過程，是機構(gòu)運動分析和綜合過程的第一個階段。具體機構(gòu)的運動可以根據(jù)機構(gòu)的幾何形狀和可能知道的其它特性來分析。另一方面，運動綜合是設(shè)計一個機構(gòu)以完成所要求的任務(wù)的過程。因此，選擇新機構(gòu)的類型和尺寸是運動綜合的一部分。想象相對運動的能力，推想出一個機構(gòu)按現(xiàn)在那樣的方式設(shè)計的原因并對一個具體設(shè)計進行改進的能力是一個成功的運動學家的標志。雖然這些能

50、力有些來自先天的創(chuàng)造性，然而更多的是因為在實踐中提高了技術(shù)水平。</p><p><b>　　運動分析</b></p><p>　　最簡單、最有用的機構(gòu)之一是四連桿。以下論述中的大部分內(nèi)容集中討論這種連桿機構(gòu)，其分析步驟也適用于更復(fù)雜的連桿機構(gòu)。</p><p>　　我們已經(jīng)知道四連桿具有一個自由度。關(guān)于四連桿，還有其它更有用的內(nèi)容需要了解嗎？

51、確實有！它們包括格拉肖夫準則、變異的概念、死點的位置(分歧點)、分支機構(gòu)、傳動角以及它們的運動特征，包括位置、速度和加速度。</p><p>　　四連桿的形式可能有所謂的曲柄搖桿、雙搖桿或者雙曲柄（拉桿）機構(gòu)，這主要取決于與固定桿相連接的兩根桿的運動范圍、曲柄搖桿機構(gòu)的輸入構(gòu)件曲柄可以持續(xù)旋轉(zhuǎn)360，而輸出構(gòu)件僅僅搖動或擺動。作為一個特例，某些平行四連桿的長度等于輸出 桿的長度，連桿的長度和固定桿的長度也相等，輸

52、入桿和輸出 桿都可以作整周轉(zhuǎn)動或者轉(zhuǎn)換成稱為反平行四邊形機構(gòu)的交叉結(jié)構(gòu)。格拉肖夫準則表明：如果任意兩桿之間能作連續(xù)的相對轉(zhuǎn)動，那么平面連桿中最長桿與最短桿的長度之和。</p><p>　　應(yīng)該注意：相同的四連桿可能 類型并不相同，這取決于哪一根桿被規(guī)定為機架（固定桿）。運動變異就是固定傳動鏈中不同的桿件產(chǎn)生不同機構(gòu)的過程。值得注意的是，不同變異機構(gòu) 連桿間的相對運動并沒有改變。</p><p&

53、gt;　　除了要具備構(gòu)件回轉(zhuǎn)范圍的知識，制造這前先建立一個檢驗機構(gòu)“運轉(zhuǎn)”效果的度量方法也是很有用的。哈登伯格（Hartenberg）說到：“運轉(zhuǎn)”這個術(shù)語是指運動傳給輸出構(gòu)件的有效性，這意味著運轉(zhuǎn)平穩(wěn)，用最大的分力產(chǎn)生驅(qū)動輸出構(gòu)件的力或扭矩。最終輸出的力或扭矩不僅與連桿的幾何形狀有關(guān)，通常還與動力或慣性力有關(guān)，并且常常是靜態(tài)力的幾倍。為了分析低速運動或者為了更方便地掌握任一機構(gòu) 如何運動的方法，傳動角的概念是非常有用的。在機構(gòu)運動期間

54、，傳動角在改變。零度傳動角可能發(fā)生在某些特殊位置上。這時無論 施加到輸入桿上的力有多大，輸出桿都無法運動。實際上，由于回轉(zhuǎn)副中存在摩擦，根據(jù)經(jīng)驗，實際設(shè)計的機構(gòu)的傳動角一般比給定值要大?，F(xiàn)在已經(jīng)研究出用矩陣來衡量連桿機構(gòu)運動傳遞的效果的定義。矩陣中行列式的值（它含有給定幾何形狀的連桿的輸出運動變量相對輸入變量的導(dǎo)數(shù)）是該連桿機構(gòu)在具體位置上可去性的一個尺度。</p><p>　　如果機構(gòu)具有一個自由度（例如四連桿

55、），則確定一個位置參數(shù)，如輸入桿的角度，就完全確定了該機構(gòu)其余桿件的位置（忽視分支機構(gòu)的可能性）。我們可以研究出一個關(guān)于四連桿各桿件絕對角位置的分析表達式。當分析各種位置或各種不同機構(gòu)的時候，這個分析表達式比幾何圖形分析程序要有用得多，因為這種表達式更易于編程進行自動化計算。</p><p>　　在機構(gòu)速度分析中相對速度 法或速度多邊形法是幾個經(jīng)常采用的方法之一。原點代表機構(gòu)上所有速度為零點。從原點到速度多邊形上

56、的各點所畫的線代表機構(gòu)上相應(yīng)各點的絕對速度。速度多邊形上的任意兩點間的連線代表了該機構(gòu) 對應(yīng)兩點的相對速度。</p><p>　　另一種方法是瞬時中心法或瞬心法，這是一種非常有用的、快速對復(fù)雜連桿機構(gòu)進行分析的方法。瞬心是一個點，該點在那一瞬間，機構(gòu)上的兩構(gòu)件之間不存在相對運動。為了找出已知機構(gòu)某些瞬心的位置，肯尼迪（Kennedy）的三心理論是非常有用的。該理論認為：彼此相對運動的三個瞬心必定在一條直線上。&l

57、t;/p><p>　　機構(gòu)各構(gòu)件的加速度也令人關(guān)注，因為實驗室影響慣性力，繼面影響機器零件的應(yīng)力、軸承載荷、振動和噪音。由于最終的目的是機器和機構(gòu)慣性力的分析，因而所有加速度的各分量都應(yīng)該同時畫在同一坐標系中，即畫在機構(gòu)的固定構(gòu)件的慣性坐標系中。</p><p>　　應(yīng)該注意的是：繞固定鉸旋轉(zhuǎn)的剛體上的點的加速度分量通常有兩個。一個分量方向與該點的軌跡相切，指向與該物體的角加速度的方向相同，叫