【正文】 of machines and devices. The simplest closedloop linkage is the fourbar 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 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 fourbar linkage has some special configurations created by making one or more links infinite in length。 Although the fourbar linkage and slidercrank mechanism are very useful and found in thousands of applications， we can see that these linkages have limited performance level。 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 黃石理工學院 畢業(yè)設計 (論文 ) 外文文獻翻譯 the mechanism， which， however， embodies the the key dimensions that affect its motion。 The next step in the kinematic analysis of mechanisms is to determine the number of degree of freedom of the mechanism。（ spherical and helical joints as well as other connections that allow threedimensional relative motion are not included， as only planar motion in parallel planes are discussed here）。 For example， it is easy to see intuitively that a fourbar linkage is a singledegreeoffreedom linkage。 Here， both choosing the type as well as the dimensions of the new mechanism can be part of kinematic synthesis。 Most of the following description will concentrate on this linkage， but the procedures are also applicable to more plex linkages。 The input crank of a crankrocher type can rotate continuously through 3600 ，while the output link just “ rocks”（ or oscillates）。 Kinematic inversion is the process of fixing different links of a chain to create different mechanisms。 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。 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。用作機構基本零件的一般機械裝置有三種類型： （ 1） 齒輪系統(tǒng)，在回轉軸之間通過接觸傳遞運動的齒狀零件。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將導 致其它各個連桿也運動到確定的預期位置，該系統(tǒng)就是一個可 約束 的運動鏈。如果這樣的運動鏈至少有一個構件被固定并且：（ i）如果至少有兩個構件能保持運動，就變?yōu)闄C構；（ ii）如果沒有一個構件能夠運動，則成為結構。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）和四個回轉副。 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。把另一個桿（曲柄，連桿或滑塊）固定，可以生成曲柄滑塊機構的變異機構。特別是當其它零件出現(xiàn)在同一圖中的時候。 機構運動分析的第二部是確定機構的自由度數(shù)。（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。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構。因此，選擇新機構的類型和尺寸是運動綜合的一部分。以下論述中的大部分內(nèi)容集中討論這種連桿機構，其分析步驟也適用于更復雜的連桿機構。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，輸入桿和輸出 桿都可以作整周轉動或者轉換成稱為反平行四邊形機構的交叉結構。值得注意的是，不同變異機構 連桿間的相對運動并沒有改變。為了分析低速運動或者為了更方便地掌握任一機構 如何運動的方法，傳動角的概念是非常有用的。實際上，由于回轉副中存在摩擦，根據(jù)經(jīng)驗，實際設計的機構的傳動角一般比給定值要大。我們可以研究出一個關于四連桿各桿件絕對角位置的分析表達式。從原點到速度多邊形上的各點所畫的線代表機構上相應各點的絕對速度。為了找出已知機構某些瞬心的位置，肯尼迪（ Kennedy）的三心理論是非常有用的。 應該注意的是：繞固定鉸旋轉的剛體上的點的加速度分量通常有兩個。