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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. Hooke’s 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.
機構(gòu)與機器
一個系統(tǒng)，它按預(yù)先確定的方式來傳輸動力完成的具體的目標也許可以被認為是機器。一種機構(gòu)也可以以類似的方式定義，但長期的機構(gòu)通常是適用于一個系統(tǒng)的主要職能是傳遞運動。運動學(xué)是研究機構(gòu)運動，而分析力和力矩的機械稱為動力學(xué)。
一旦需要給出識別一個機構(gòu)或機械裝置的特點，設(shè)計過程就開始了。通常需要仔細地分析位移，速度和加速度。這部分的設(shè)計過程后，其次是分析力和力矩。設(shè)計過程中可能會繼續(xù)很長時間后產(chǎn)生第一種模式，其中包括重新設(shè)計的組成部分，影響速度，加速度，力和力矩。年復(fù)一年的為了競爭成功，大部分的制造商必須不斷地修改他們的產(chǎn)品及其生產(chǎn)方法。提高生產(chǎn)速度，提高產(chǎn)品性能，重新設(shè)計的成本和減輕體重，運動分析和新的生產(chǎn)線往往是需要的。成功或許取決于正確的運動學(xué)和動力學(xué)的分析的問題。
許多基本的連接裝置構(gòu)造世紀以前已經(jīng)成為機器設(shè)計的組成部分，和我們使用這個術(shù)語形容當時的變化超過一年。因此，定義和專門的術(shù)語將不符合整個技術(shù)的文獻。在大多數(shù)情況下，但是，含義將是明確的背景下形成的重要性的描述。有幾個方面特別感興趣的研究機器運動學(xué)和動力學(xué)的定義如下。
桿件 一個桿件是一個嚴格的機構(gòu)或其共同組成一個運動鏈。長期嚴格的桿件或有時只是使用一個理想化的桿件研究，由于機件拉緊不考慮微小撓度。一個完全不彎曲或不可拉長的桿件可能存在不僅是一種教科書式的模型，一個真正的機器的構(gòu)件。對于典型的機械部分，最大尺寸的變化是只有長度部分的千分之一。當我們考慮多數(shù)機械裝置的運動特性時我們有理由忽視這個小小的運動。這個桿件定理中使用的一般意義上包括凸輪，齒輪，和其他構(gòu)件除了曲柄、連桿和其他引腳連接組件。
自由度 自由度的數(shù)量的聯(lián)系是一些獨立的參數(shù)必須立場的每一個環(huán)節(jié)相對內(nèi)或固定桿件。如果即可改造的系統(tǒng)可以完全確定指定一個獨立的變量，該系統(tǒng)有一個自由度。多數(shù)實用的機械裝置就有一個自由度。
一個無約束剛體有6個自由度：直線移動在三個坐標和旋轉(zhuǎn)運動三個坐標軸。如果該機構(gòu)是限制于在一個平面運動，那有三個自由度：直線運動在兩個坐標方向和在平面內(nèi)的旋轉(zhuǎn)。
高副和低副 鏈接的剛體之間包括高副和低副兩個要素。這兩個因素中的低副是兩個理論表面之間的接觸，而這兩個因素中的高副是理論的點或線接觸（如果我們忽視了撓度）。
低副是從設(shè)計的角度來看是可取的，由于聯(lián)合負荷以及由此產(chǎn)生的磨損分布在整個接觸面。因此，幾何變化或失敗而高接觸應(yīng)力和過度磨損或許是可以避免的。
機械裝置 機械裝置是一個運動鏈系中的一環(huán)被認為是特定的目的是為了分析，但運動可能是其他的環(huán)節(jié)。如上所述，特定的桿件為指定的桿件不需要與實際相對固定在地球表面。如果運動學(xué)鏈主要目的是緩和或傳輸動力，其就通常被作為一種機械裝置，
機器 這種機構(gòu)設(shè)計是為達到轉(zhuǎn)遞動力或力矩的目的通常是所謂的機器。
發(fā)動機 一個機器需要能量轉(zhuǎn)換而產(chǎn)生的機械動力通常稱為發(fā)動機。因此，曲軸，連桿，活塞和氣缸的自動的發(fā)動機由上面所述的發(fā)動機的定義，而其他的傳動部件，例如變速箱，差速器，和萬向聯(lián)軸器都被稱為為機械裝置。機器和發(fā)動機或許有相同裝置，其他的機械裝置不能轉(zhuǎn)換動力，而是為了傳輸大的動力或者是扭矩。因此，為了運動學(xué)的分析，上述機械裝置、機器、發(fā)動機之間的區(qū)別，可能僅僅在學(xué)術(shù)上有重要性。
機構(gòu)就是：由剛體或者是有承載能力的物體連接而組成的組合體，他們在運動時候彼此間具有確定的相互運動。
機構(gòu)是由構(gòu)成這些機械設(shè)備的基本的幾何單元，這些機械設(shè)備包括自動包裝機、打字機、機械的玩具、紡織機等等。機構(gòu)設(shè)計的目的是使一個剛體相對某一個參考的構(gòu)件產(chǎn)生所需要的相對運動。機構(gòu)的運動設(shè)計通常是設(shè)計一個完整的機器的第一步。在考慮力的作用時應(yīng)該考慮動力學(xué)、軸承的載荷、應(yīng)力、潤滑等一系列問題。在所考慮的問題的范圍擴大之后，機構(gòu)設(shè)計就變成了機器設(shè)計。
作為機器的一個組成部分，機構(gòu)的作用是在剛體之間相互傳遞或轉(zhuǎn)換運動。經(jīng)常用到的基本機構(gòu)有以下三種：應(yīng)該考慮動力學(xué)、軸承的載荷、應(yīng)力、潤滑等一系列問題。在所考慮的問題的范圍擴大之后，機構(gòu)設(shè)計就變成了機器設(shè)計。
作為機器的一個組成部分，機構(gòu)的作用是在剛體之間相互傳遞或轉(zhuǎn)換運動。經(jīng)常用到的基本機構(gòu)有以下三種：
齒輪機構(gòu) 在這種機構(gòu)中，各各轉(zhuǎn)軸之間的運動是由相互嚙合的齒輪來傳遞。齒輪通常用來傳遞角速度比常值的運動，但是非圓輪可以用來傳遞角速度比為變數(shù)的運動。
凸輪機構(gòu) 在這種機構(gòu)中，輸入件的等速連續(xù)運動被轉(zhuǎn)換成輸出件的不等速運動。輸出的運動可以是軸的轉(zhuǎn)動、滑塊的移動、或者其他從動件的運動。這些運動都是從動件與作為輸入件的凸輪的輪廓的直接接觸而產(chǎn)生的。凸輪的運動設(shè)計就是采用解析法或者是圖解法來確定凸輪的輪廓形狀，使其能夠帶動從動件實現(xiàn)輸出運動的制定函數(shù)這個功能。
平面和空間連桿機構(gòu) 此類機構(gòu)也是用來使機構(gòu)上的某一點或者是剛體實現(xiàn)機械運動的，兩岸的基本作用有三種：
（1）剛體導(dǎo)向 剛體導(dǎo)向機構(gòu)是用來引導(dǎo)一個剛體，使其通過空間的一系列預(yù)訂的位置;
（2）實現(xiàn)軌跡 實現(xiàn)軌跡機構(gòu)將引導(dǎo)剛體上的一個點，使其通過空間指定的空間軌跡上的一系列點；
（3）實現(xiàn)函數(shù) 此類機構(gòu)所產(chǎn)生的輸出運動是輸出運動的指定函數(shù)。
為了強調(diào)各種機構(gòu)之間的相同之處和不同之處，可以把它們按照幾種不同的方式進行分類。一種分類方式就是將機構(gòu)分成平面、球面和空間等三類。這三類機構(gòu)有很多共同之處，然而，可以根據(jù)其構(gòu)件的運動特點來確定分類準則。
在平面機構(gòu)中，所有的質(zhì)點在空間所走過的軌跡都是平面曲線，所有這些平面曲線都位于相互平行平面上，也就是說所有的軌跡都是平行于一個共同平面的平面曲線。這一特性使平面機構(gòu)上任意選定的一個點都可以按其真實尺寸和形狀在一個視圖上表示出運動軌跡。平面四桿機構(gòu)、平板凸輪機構(gòu)和其他的從動件、曲柄滑塊機構(gòu)是大家都比較熟悉的平面機構(gòu)的例子?，F(xiàn)在使用的機構(gòu)大多數(shù)機構(gòu)是平面機構(gòu)。
在球面機構(gòu)中，當機構(gòu)運動時，每一個構(gòu)件上都有一個點是靜止的所有構(gòu)件的靜止點都處于同一個位置，也就是說每一點的軌跡都是球面曲線。所有各點運動時所在的球面都是同心的。因而，所有質(zhì)點的運動都能用它們在以適當選取的點為中心的球面上的徑向投影來完整的進行描述。虎克萬向聯(lián)軸器或許會是人們最熟悉的一個球面機構(gòu)的例子。
從另一方面講，在空間機構(gòu)中質(zhì)點的相對運動不受約束。運動的變換不要求共面，不要求同心。空間機構(gòu)上許多質(zhì)點的運動軌跡可能具有雙重曲率。舉例子，任何含有螺旋副的連桿機構(gòu)，由于它的相對運動是螺旋線形的，因此是空間的機構(gòu)。