機(jī)構(gòu)介紹外文翻譯

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

2、contact 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 o

3、f 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,

4、the 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 mobi

6、lity 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.m

7、echanisms 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 inf

9、inite 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 th

10、an 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 perform

11、ance 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 sk

12、etch 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 d

13、iagram 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。（spheric

16、al 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 r

17、ules 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 sin

18、gle-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 po

19、ssibily 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 reas

20、on 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 an

21、d 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 dou

23、ble-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，w

24、here 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

27、measure 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 outpu

28、t 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 m

29、echanism 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 cont

31、ains 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 a

32、s 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

34、available. 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 relativ

35、e 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 co

38、mponents 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 t

39、o 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 ca

40、lled the normal or centripetal acceleration. this component is present due to the changing direction of the velocity vector. 譯文：機(jī)構(gòu)介紹機(jī)構(gòu)是機(jī)械運(yùn)動的一個部分，他的功能是把運(yùn)動從一個剛體傳遞或轉(zhuǎn)換到另一個剛體。用作機(jī)構(gòu)基本零件的一般機(jī)械裝置有三種類型：（1） 齒輪系統(tǒng)，在回轉(zhuǎn)軸之間通過接觸傳遞運(yùn)動的齒狀零件。（2） 凸輪系統(tǒng)，把輸入零件的均勻運(yùn)動轉(zhuǎn)換成輸出零件的非今年暈運(yùn)動的裝置。（3） 平面和空間連桿機(jī)構(gòu)，使點(diǎn)或剛體產(chǎn)生機(jī)械運(yùn)動的使用裝置。 運(yùn)動鏈?zhǔn)且粋€鏈接系統(tǒng)，

41、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(yùn)動。如果鏈接中的某個連桿被固定，而其它任何一個連桿運(yùn)動到新的位置將導(dǎo)致其它各個連桿也運(yùn)動到確定的預(yù)期位置，該系統(tǒng)就是一個可約束的運(yùn)動鏈。否則，該系統(tǒng)是一個非約束運(yùn)動鏈。機(jī)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(yùn)動和力為目的的機(jī)械裝置。連桿機(jī)構(gòu)通常由被認(rèn)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(yùn)動鏈。如果這樣的運(yùn)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)件能保持運(yùn)動，就變?yōu)闄C(jī)構(gòu)；（ii）如果沒有一個構(gòu)件能夠運(yùn)動，則成為結(jié)構(gòu)。換句話說，機(jī)構(gòu)內(nèi)部的剛性桿

42、件之間能夠相對運(yùn)動，而結(jié)構(gòu)則不能。由于連桿系統(tǒng)能組成簡單機(jī)構(gòu)并完成復(fù)雜的任務(wù)，例如非線性運(yùn)動的傳遞和力的傳遞，因而它們在機(jī)構(gòu)研究中受到了更多的關(guān)注。許多機(jī)器和裝置都使用機(jī)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。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。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(jī)構(gòu)。曲柄滑塊機(jī)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(jī)也是類似的一種機(jī)構(gòu)。有些其它形式的四桿機(jī)構(gòu)，其滑塊在一個動桿上導(dǎo)移運(yùn)動而

43、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(jī)構(gòu)的變異機(jī)構(gòu)。雖然四連桿和曲柄滑塊機(jī)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(jī)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(jī)構(gòu)有更多的元件。然而，想象多回環(huán)的連桿機(jī)構(gòu)的運(yùn)動常常非常困難。特別是當(dāng)其它零件出現(xiàn)在同一圖中的時候。對于比較復(fù)雜的機(jī)構(gòu)的運(yùn)動分析，第一步是繪制等效運(yùn)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(jī)構(gòu)的主要本質(zhì)，體現(xiàn)影響其運(yùn)動的關(guān)鍵尺寸。運(yùn)動圖可用兩種形式中的一種：一是草圖（按比例畫出，但放大比例不精確）；二是比例運(yùn)動圖（通常用于進(jìn)一步分析其位置.位移.速度.加速度.力.和扭矩傳遞等）。為了便

44、于參考，用數(shù)字對構(gòu)件進(jìn)行編號（以機(jī)架為1開始編號），而用英文字母標(biāo)注回轉(zhuǎn)副。機(jī)構(gòu)運(yùn)動分析的第二部是確定機(jī)構(gòu)的自由度數(shù)。我們所說的自由度，指的是使機(jī)構(gòu)所有構(gòu)件相對于機(jī)架具有確定位置所需要的獨(dú)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(jī)構(gòu)。就像一個裝有各種各樣連桿的袋子，里面有：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(yùn)動副 .螺旋副.以及允許三維相對運(yùn)動的其它連接尚未包括進(jìn)去，因?yàn)檫@里僅僅討論的是平行平面內(nèi)的平面運(yùn)動。）你還可以設(shè)想這些元件在一起形成各種類型連桿機(jī)構(gòu)的可能性。存在幫助人們形成這些的規(guī)律嗎？實(shí)際上，大多數(shù)機(jī)構(gòu)的任

45、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(jī)構(gòu)是使用最多的機(jī)構(gòu)類型。例如，由直覺很容易可以看出四連桿就是一個單一自由度的連桿機(jī)構(gòu)。畫運(yùn)動圖和確定機(jī)構(gòu)自由度的過程，是機(jī)構(gòu)運(yùn)動分析和綜合過程的第一個階段。具體機(jī)構(gòu)的運(yùn)動可以根據(jù)機(jī)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(yùn)動綜合是設(shè)計一個機(jī)構(gòu)以完成所要求的任務(wù)的過程。因此，選擇新機(jī)構(gòu)的類型和尺寸是運(yùn)動綜合的一部分。想象相對運(yùn)動的能力，推想出一個機(jī)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(jìn)行改進(jìn)的能力是一個成功的運(yùn)動學(xué)家的標(biāo)志。雖然這些能力有些來自先天的創(chuàng)造性，然而更多的是因?yàn)樵趯?shí)踐中提高了技術(shù)水平。運(yùn)動分析最簡單、最有用的機(jī)構(gòu)之一是四連桿。以下論述中的大