機(jī)器機(jī)械設(shè)備外文文獻(xiàn)翻譯、中英文翻譯、外文翻譯
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翻譯資料名稱(外文) Machine
翻譯資料名稱(中文) 機(jī)器
機(jī) 器
人們給機(jī)器這個(gè)詞下了各種各樣的定義，但對(duì)本文來(lái)說(shuō)，機(jī)器是具有某種獨(dú)特用途的設(shè)備，用來(lái)簡(jiǎn)少或代替人力或畜力，以完成各種體力工作。工具可以是看作最簡(jiǎn)單的機(jī)器。機(jī)器的作用可以包括把化學(xué)能、熱能、電能或核能轉(zhuǎn)換成機(jī)械能，或者反過(guò)來(lái)把機(jī)械能轉(zhuǎn)換成其它幾種能，或者其作用僅僅是用來(lái)改變和傳遞力和運(yùn)動(dòng)。所有的機(jī)器都有一個(gè)輸入端，一個(gè)輸出端，一個(gè)轉(zhuǎn)換或變換裝置和傳遞裝置。
從天然能源（例如氣流、水流、煤、石油或鈾）獲得其輸入能量并將其轉(zhuǎn)換成機(jī)械能的機(jī)器稱作原動(dòng)機(jī)。風(fēng)車、水輪、汽輪機(jī)、蒸汽機(jī)和內(nèi)燃機(jī)都是原動(dòng)機(jī)。這些機(jī)器的輸入端是各不相同的，而輸出端則通常都是一根旋轉(zhuǎn)軸，它能用作其它機(jī)器（例如發(fā)電機(jī)、液壓泵、或空氣壓縮機(jī)）的輸入端，所有這后三種裝置都?xì)w類為發(fā)動(dòng)機(jī)，它們產(chǎn)生的電能、液能或氣能能夠分別用作電動(dòng)機(jī)、液動(dòng)機(jī)或氣動(dòng)機(jī)的輸入端。這些發(fā)電機(jī)能夠用來(lái)驅(qū)動(dòng)帶有各種輸出端的機(jī)器，諸如材料加工機(jī)、包裝機(jī)或輸送機(jī)。所有既不是原動(dòng)機(jī)、發(fā)電機(jī)也不是電動(dòng)機(jī)的機(jī)器都?xì)w類為工作機(jī)。這類機(jī)器也包括各種手動(dòng)機(jī)具，例如計(jì)算機(jī)和打字機(jī)。
如果工作機(jī)是一臺(tái)由電動(dòng)機(jī)驅(qū)動(dòng)的泵，來(lái)自發(fā)電站的原動(dòng)機(jī)的能經(jīng)過(guò)發(fā)電機(jī)和電動(dòng)機(jī)傳到工作機(jī)，其情況如圖1所示。工作機(jī)也能直接由一個(gè)小型的直接連接的原動(dòng)機(jī)（例如汽油發(fā)電機(jī)）驅(qū)動(dòng)，如圖1虛線所示；然而，對(duì)于大多數(shù)由動(dòng)力傳動(dòng)的工作機(jī)來(lái)說(shuō)，來(lái)自原動(dòng)機(jī)的能是沿圖中實(shí)線所示的順序傳輸?shù)摹?
在某些情況下，上述各種機(jī)器都組裝成一個(gè)機(jī)組。例如在柴油發(fā)電機(jī)車中，柴油機(jī)是原動(dòng)機(jī)，它驅(qū)動(dòng)發(fā)電機(jī)，發(fā)電機(jī)再把電供給電動(dòng)機(jī)以驅(qū)動(dòng)車輪。
下面舉汽車中的幾個(gè)例子。
在汽車中，基本問(wèn)題是利用汽油的爆發(fā)力來(lái)提供動(dòng)力使后輪轉(zhuǎn)動(dòng)。幾個(gè)汽缸里汽油的爆發(fā)推動(dòng)活塞向下，這種平移運(yùn)動(dòng)（線性運(yùn)動(dòng)）的傳遞和轉(zhuǎn)換成曲軸的旋轉(zhuǎn)運(yùn)動(dòng)是靠連桿來(lái)實(shí)現(xiàn)的，連桿把每一個(gè)活塞同曲軸（圖21）連接起來(lái)，曲柄是曲軸的一部分?；钊⑵?、曲柄和連桿組合稱作滑塊曲柄機(jī)構(gòu)（也叫曲柄連桿機(jī)構(gòu)）；這是將平移轉(zhuǎn)換成旋轉(zhuǎn)（例如發(fā)電機(jī)）或?qū)⑿D(zhuǎn)轉(zhuǎn)換成平移（例如泵）通常采用的方法。
閥門作用在于允許汽油空氣混合物進(jìn)入圓筒中并且耗盡燃燒的氣體；這些閥門由凸輪的楔入作用打開(kāi)和關(guān)閉突出部分，通過(guò)齒輪或者鏈傳動(dòng)由曲軸驅(qū)動(dòng)旋轉(zhuǎn)的凸輪軸。
在八汽缸四沖程發(fā)電機(jī)中，曲軸每轉(zhuǎn)1/4圈在沿其長(zhǎng)度方向某一點(diǎn)就獲得一個(gè)推力。為了消除這種斷續(xù)的推力對(duì)曲軸轉(zhuǎn)速的影響以使轉(zhuǎn)速平穩(wěn)，采用了一個(gè)飛輪。飛輪是一個(gè)連在曲軸上的重型輪子，它借助其慣性來(lái)抵消和減輕轉(zhuǎn)速的任何波動(dòng)。
由于內(nèi)燃機(jī)所傳遞的扭轉(zhuǎn)（旋轉(zhuǎn)力）取決于它的轉(zhuǎn)速，所以內(nèi)燃機(jī)不能在有負(fù)載的情況下起動(dòng)。為了使汽車發(fā)電機(jī)能夠在無(wú)負(fù)載狀態(tài)下起動(dòng)然后再連到車輪上而又不致使發(fā)電機(jī)減速、停車或滅火，必須有離合器和變速箱。離合器用來(lái)使曲軸和變速箱接合或分離，而變速箱則分幾級(jí)改變輸入軸和輸出軸之間的速比和變速箱的扭矩。在低速檔，輸出轉(zhuǎn)速很低，而輸出軸扭矩較發(fā)電機(jī)的扭矩要大，所以汽車能夠起動(dòng)；在高速檔，汽車正在高速前進(jìn)，因此輸入和輸出的扭矩和轉(zhuǎn)速是相等的。
安裝車輪的軸都裝在固定在后彈簧上的后橋殼里，并且由變速箱借助主動(dòng)軸來(lái)起動(dòng)，當(dāng)汽車行駛時(shí)，彈簧隨公路上的凸凹不平而上下彎曲，使橋殼同變速箱產(chǎn)生相對(duì)運(yùn)動(dòng)；要允許運(yùn)動(dòng)而不干涉扭矩傳輸，主動(dòng)軸的每個(gè)末端都有一個(gè)萬(wàn)向節(jié)。
主動(dòng)軸是垂直于對(duì)后橋。直角連接通常用錐齒輪來(lái)實(shí)現(xiàn)，其傳動(dòng)比使輪軸的轉(zhuǎn)速為主動(dòng)軸轉(zhuǎn)速的1/3-1/4。后橋殼里還裝有差動(dòng)齒輪，差動(dòng)齒輪使兩個(gè)后輪既能由同一根主動(dòng)軸來(lái)驅(qū)動(dòng)，又能在轉(zhuǎn)彎時(shí)具有不同的轉(zhuǎn)速。
像所有的運(yùn)動(dòng)機(jī)械裝置一樣，汽車也不能避免摩擦力的作用。在發(fā)電機(jī)、變速箱、后橋殼和所有的軸承里，摩擦作用都是有害的，因?yàn)槟Σ猎黾恿藢?duì)發(fā)電機(jī)要求的功率，潤(rùn)滑可以減少但不能消除這種摩擦作用。另一方面，由于輪胎同路面之間以及同制動(dòng)器之間的摩擦作用，才能實(shí)現(xiàn)牽引和制動(dòng)。驅(qū)動(dòng)風(fēng)扇、發(fā)電機(jī)和其它輔助設(shè)備的皮帶，是一種依靠摩擦作用的裝置。摩擦對(duì)離合器的工作也是很有用的。上述的某些裝置以及下面要談到的其它一些裝置在以各種方式裝配起來(lái)的用來(lái)完成各種體力工作的各種機(jī)器里都能找到。由于各種工作機(jī)的功能不同，并缺乏共同的特性，所以本文不論述專用的工作機(jī)。本文既不討論原動(dòng)機(jī)的工作性能，也不論述液壓裝置、氣動(dòng)裝置或電器裝置的作用。本文只研究作為機(jī)器的各組成部分的基本機(jī)械裝置的作用和結(jié)構(gòu)。大多數(shù)這些裝置的功能是用來(lái)傳遞和轉(zhuǎn)換力和運(yùn)動(dòng)。而另一些裝置，例如彈簧、飛輪、軸和緊固件，則是用來(lái)完成各種輔助功能。
從這篇文章的目的來(lái)說(shuō)機(jī)器可以進(jìn)一步被定義為：機(jī)器是由兩個(gè)或兩個(gè)以上耐用的、相對(duì)約束的部件組成的裝置，這些部件可以傳遞和轉(zhuǎn)換力和運(yùn)動(dòng)以便于做功。機(jī)械零件應(yīng)該經(jīng)久耐用這一要求意味著它們應(yīng)能承受外加負(fù)載而不致?lián)p壞和失靈。盡管大多數(shù)機(jī)械零件是具有適當(dāng)尺寸的固態(tài)金屬零件，但也采用非金屬材料、彈簧、液壓元件以及像皮帶這樣的拉力零件。
機(jī)器最顯著的特性在于其零件都是相互連接或?qū)颍瑥亩鼈儽舜说南鄬?duì)運(yùn)動(dòng)都是受約束的。例如，相對(duì)于汽缸體來(lái)說(shuō)，往復(fù)式發(fā)電機(jī)的活塞受汽缸的約束從而做直線運(yùn)動(dòng)；曲軸上的各點(diǎn)受主承軸的約束作圓周運(yùn)動(dòng)；要作其它形式的相對(duì)運(yùn)動(dòng)是不可能的。
在某些機(jī)器中，有些零件只部分受約束。如果零件用彈簧或摩擦件相互連接，零件彼此之間的相對(duì)行程可以是固定不變的，但零件的運(yùn)動(dòng)可能會(huì)受到彈簧的剛度、摩擦力和零件的質(zhì)量的影響。
如果機(jī)器的所有零件都是剛度比較大的元件，在負(fù)載作用下?lián)隙瓤梢院雎圆挥?jì)，那就可以看成完全約束并且零件的相對(duì)運(yùn)動(dòng)可以不必考慮引起相對(duì)運(yùn)動(dòng)的力。例如，對(duì)一臺(tái)往復(fù)式發(fā)動(dòng)機(jī)的曲軸的某一規(guī)定的轉(zhuǎn)速來(lái)說(shuō)，連桿和活塞上各點(diǎn)相應(yīng)的速度是能夠計(jì)算出來(lái)的。對(duì)于規(guī)定的輸入運(yùn)動(dòng)來(lái)說(shuō)，機(jī)器各零件的位移、速度和加速度的確定是機(jī)械運(yùn)動(dòng)學(xué)討論的主題。進(jìn)行這樣的計(jì)算時(shí)不必考慮各種有關(guān)的力，因?yàn)檫\(yùn)動(dòng)都是受約的。
根據(jù)這一定義，力和運(yùn)動(dòng)都在機(jī)器中被傳遞和轉(zhuǎn)換。機(jī)器各零件相互連接和導(dǎo)向以便從一定的輸入運(yùn)動(dòng)產(chǎn)生所需的輸出運(yùn)動(dòng)的方式稱作機(jī)器的結(jié)構(gòu)。往復(fù)式發(fā)電機(jī)的活塞、連桿和曲軸組成一個(gè)機(jī)構(gòu)以便把活塞的直線運(yùn)動(dòng)變成曲軸的旋轉(zhuǎn)運(yùn)動(dòng)。
雖然機(jī)器在工作中既包括力又包括運(yùn)動(dòng)，但是機(jī)器的主要作用是為了省力或轉(zhuǎn)換運(yùn)動(dòng)。而變速箱常作為減速器，杠桿本質(zhì)上是增力裝置。然而，在機(jī)器中運(yùn)動(dòng)和力是分不開(kāi)的，并且總是反比。杠桿輸出的力大于輸入的力，但是輸出地運(yùn)動(dòng)小于輸入的運(yùn)動(dòng)。同樣，齒輪減速器的輸出轉(zhuǎn)速小于輸入轉(zhuǎn)速，但是輸出扭矩大于輸入扭矩。在第一種情況下，力的增加伴隨著運(yùn)動(dòng)損失，而在第二種情況下，運(yùn)動(dòng)的損失伴隨著扭矩的增大。
盡管某些機(jī)器的主要作用能夠辨認(rèn)出來(lái)，還是難以把所有的機(jī)器都?xì)w類為力的改變裝置或者運(yùn)動(dòng)的轉(zhuǎn)換裝置；某些機(jī)器應(yīng)同時(shí)歸入這兩類裝置。然而，所有機(jī)器都必須能夠起到改變運(yùn)動(dòng)的作用，因?yàn)槿绻粋€(gè)機(jī)械裝置的零件不會(huì)運(yùn)動(dòng)，則該機(jī)械裝置只是一個(gè)結(jié)構(gòu)，而不是機(jī)器。在研究零件的運(yùn)動(dòng)時(shí)，機(jī)械設(shè)計(jì)者習(xí)慣于討論機(jī)器的機(jī)構(gòu)。
雖然所有的機(jī)器都有機(jī)構(gòu)，因此都能起到改變運(yùn)動(dòng)的作用，而某些機(jī)器在設(shè)計(jì)時(shí)并沒(méi)有改變力的要求；所產(chǎn)生的各個(gè)力是由運(yùn)動(dòng)部件的摩擦作用和慣性作用所引起的，并不表現(xiàn)為有用的輸出作用力。這一類通常包括量測(cè)儀表和時(shí)鐘。
在定義中提到的“功”要按照它的科學(xué)含義來(lái)解釋。在機(jī)械學(xué)中，功是力沿著力的作用方向做出的結(jié)果，它等于平均力和運(yùn)動(dòng)距離的乘積。如果一個(gè)人扛著一個(gè)重物沿水平路線運(yùn)動(dòng)，根據(jù)這個(gè)定義他沒(méi)有做功，因?yàn)榱瓦\(yùn)動(dòng)相互垂直，即力是垂直的，而運(yùn)動(dòng)是水平的。如果他扛著重物登上一段樓梯或者梯子，他就做了功，因?yàn)樗茄刂饔昧Φ耐环较蜻\(yùn)動(dòng)的。從數(shù)學(xué)上來(lái)講，如果用F表示力（單位為磅或者公斤），S表示距離（單位為英尺或者米），則功等于作用力F乘以力所運(yùn)動(dòng)的距離S；即功=F*S。
如果一個(gè)力使一個(gè)物體繞一固定軸或樞軸旋轉(zhuǎn)，則所做的功等于扭矩（T）乘以旋轉(zhuǎn)的角度（以弧度表示的角度）。
在用力和運(yùn)動(dòng)來(lái)給機(jī)器的機(jī)械功的作用下定義時(shí)，上述有關(guān)功的概念是一些基本概念，這些概念表明機(jī)器中力和運(yùn)動(dòng)的不可分性。由于摩擦的關(guān)系，從機(jī)器中輸出的功永遠(yuǎn)小于輸入的功，而效率，即輸出的功和輸入的功，兩者的比值永遠(yuǎn)小于100%。
輸出的力和輸入的力之比稱作機(jī)械利益（MA），它表示改變力的功能，而輸入的運(yùn)動(dòng)和輸出的運(yùn)動(dòng)之比稱作速度比（VR），它表示改變運(yùn)動(dòng)的功能。如果效率很高，這兩個(gè)比值幾乎相等；如果輸出的力是輸入的力的十倍，則輸入的運(yùn)動(dòng)必須是輸入運(yùn)動(dòng)的十倍；即力之所得等于運(yùn)動(dòng)之所失。摩擦只影響機(jī)械利益而不影響速比。
為了從輸出功和輸入功的比值計(jì)算效率，就必須知道輸出的力和輸入的力通過(guò)規(guī)定的距離所作的功。由于這需要確定通過(guò)這一距離的平均力，所以它是很不方便的。比較方便的方法是從負(fù)載的瞬時(shí)值和負(fù)載的運(yùn)動(dòng)速率來(lái)測(cè)定機(jī)械的效率。為此，功率公式是非常有用的。
功率是做功的速率的單位在說(shuō)英語(yǔ)的國(guó)家力是馬力（hp）——馬力等于每分鐘33，000英尺磅，因此每分鐘240英尺磅等于240/33,000=0.00727 hp。
在討論諸如杠桿和輪與軸這一類簡(jiǎn)單的機(jī)械時(shí)，最方便是把輸入的力叫做“作用力”，而把輸出的力叫做“負(fù)載”。因此機(jī)械利益是負(fù)載和作用力之比，速比是作用力的運(yùn)動(dòng)（位移或速度）除以負(fù)載的相應(yīng)運(yùn)動(dòng)。
MACHINE
The word machine has been given a wide variety of definitions, but for the purpose of this article it is a device, having a unique purpose, that augments or replaces human effort for the accomplishments of physical tasks. Tools may be regarded as the simplest class of machines. The operation of a machine may involve the transformation of chemical, thermal, electrical, or nuclear energy into mechanical energy, or vice versa, or its function may simply be to modify and transmit forces and motions. All machines have an input, an output, and a transforming or modifying and transmitting device.
Machines that receive their input energy form a natural source, such as air currents, moving water, coal, petroleum, or uranium, and transform it into mechanical energy are known as prime movers. Windmills, waterwheels, turbines, steam engines, and internal-combustion engines are prime movers. In these machines the inputs vary; the outputs are usually rotating shafts capable of being used as inputs to other machines, such as electric generators, hydraulic pumps, or air compressor. All three of the latter devices may be classified as generators; their outputs of electrical, hydraulic, and pneumatic energy can be used as input to electric, hydraulic, or air motors. These motors can be used to drive machines with a variety of outputs, such as materials processing, packaging, or conveying machinery. All machines that are neither prime movers, generators, nor motors may be classified as operators. This category also includes manually operated instruments of all kinds, such as calculating machines and typewriters.
If the operator is a pump driven by an electric motor, the flow of energy from the prime mover at the power plant through the generator and motor to the operator is as shown in Figure 1. The operator can also be driven directly by a small, direct-connected prime mover, such as a gasoline engine, as shown by the dotted line Figure 1; for most power-driven operators, however, the flow of energy form the prime mover follows the solid lines.
In some cases, machines in all categories are combined in one unit. In a diesel-electric locomotive, for example, the diesel engine is the prime mover, which drives the electric generator, which, in turn, supplies electric current to the motors that drive the wheels.
The following are some examples supplied by an automobile.
In an automobile, the basic problem is harnessing the explosive effect of gasoline to provide power to rotate the rear wheels. The explosion of the gasoline in the cylinders pushes the pistons down, and the transmission and modification of the crankshaft is effected by the connecting rods that join each piston to the cranks (Figure 21) that are part of the crankshaft. The piston, cylinder, crank, and connecting rod combination is known as a slider-crank mechanism; it is a commonly used method of converting translation to rotation (as in an engine) or rotation to translation (as in a pump).
To admit the gasoline-air mixture to the cylinders and exhaust the burned gases, valves are used; these are opened and closed by wedging action of cams (projections) on a rotating camshaft that is driven from the crankshaft by gears or a chain.
In a four-stroke-cycle engine with eight cylinders, the crankshaft receives an impulse at some point along its length every quarter revolution. To smooth out the effect of these intermittent impulses on the speed of the crank-shaft, a flywheel is used. This is a heavy wheel, attached to the crankshaft, that by its inertia opposes and moderates any speed fluctuations.
Since the torque (turning force) that it delivers depends on its speed, an internal-combustion engine cannot be started under load. To enable an automobile engine to be started in an unloaded state and then connected to the wheels without stalling, a clutch and a transmission are necessary. The former makes and breaks the connection between the crankshaft and the transmission, while the latter changes, in finite steps, the ratio between the input and output speeds and torques of the transmission. In low gear, the output speed is low and the output torque higher than the engine torque, so that the car can be started moving; the car is moving at a substantial speed and the torques and speeds are equal.
The axles to which the wheels are attached ate contained in the rear axle housing, which is clamped to je rear springs, and are driven from the transmission by the drive shaft, as the car moves and the springs flex in response to bumps in the road, the housing moves relative to the transmission; to permit this movement without interfering with the transmission of torque, a universal join is attached o each end of the drive shaft.
The drive shaft is perpendicular to the rear axles. The right-angled connection is usually made with bevel gears having a ratio such that the axles rotate at from one-third to one-fourth the speed of the drive shaft. The rear axle housing also holds the differential gears that permit both rear wheels to be driven from the same source and to rotate at different speeds when turning a corner.
Like all moving mechanical devices, automobiles cannot escape from the effects of friction. In the engine, transmission, rear axle housing, and all bearings, friction is undesirable, since it increases the power required from the engine; lubrication reduces bit does not eliminate this friction. On the other hand, friction between the tires and the road and in the brake shoes makes traction and braking possible. The belts that drive the fan, generator, and other accessories are friction-dependent devices. Friction is also useful in the operation of the clutch. Some of the devices cited above, and others that are described below, are found in machines of all kinds of physical tasks. Because of this diversity of function and the lack of common characteristics, this article will not be concerned with specific operators. Neither will it deal with the overall performance of prime movers, nor with the operation of hydraulic, pneumatic, or electrical devices. It will consider only the operation and structure of the basic mechanical devices that are the constituent part of machines. The function of most of these devices is to transmit and modify force and motion. Other devices, such as spring, flywheels, shafts, and fasteners, perform supplementary functions.
For the purpose of this article a machine may be further defined as a device consisting of two or more resistant, relatively constrained parts that may serve to transmit and modify force and motion in order to do work. The requirement that the part of a machine be resistant implies that they be capable of carrying imposed loads without failure or loss of function. Although most machine part are solid materials bodies of suitable proportions, nonmetallic materials, springs, fluid pressure organs, and tension organs such as belts are also employed.
The most distinctive characteristic of a machine is that the parts are interconnected and guided in such as way that their motions relative to one another are constrained. Relative to the block, for example, the piston of a reciprocating engine is constrained by the cylinder to move on a straight path; points on the crankshaft are constrained by the main bearings to move on circular paths; no other forms of relative motion are possible.
On some machines the parts are only partially constrained. If the parts are interconnected by springs or friction members, the paths of the parts relative to one an other may be the stiffness of the parts may be affected by the stiffness of the springs, friction, and the masses of the parts.
If all the parts of a machine are comparatively rigid member whose deflections under load are negligible, then the containment may be considered complete and the relative motions of the parts can be studied without considering the forces that produce them. For a specified rotational speed of the crankshaft of a reciprocating engine, for example, the corresponding speeds of points on the connecting rod and the piston can be calculated. The determination of the displacements, velocities, and accelerations of the parts of a machine for a prescribed input motion is the subject matter of kinematics of machines. Such calculations can be made without considering the forces involved, because the motions are constrained.
According to the definition, both forces and motions are transmitted and modified in a machine. The way in which the parts of a machine are interconnected and guided to produce a required output motion from a given input motion is known as the mechanism of the machine. The piston, connecting rod, and crankshaft in a reciprocating engine constitute a mechanism for changing the rectilinear motion of the piston into the rotary motion of the crankshaft.
Although both forces and motions are involved in the operation of machines, the primary function of a machine may be either the amplification of force or the modification of motion. A lever is essentially a force increase, while a gearbox is most often used as a speed reducer. The motions and forces in a machine are inseparable, however, and are always in an inverse ratio. The output force on a lever is greater than the input force, but the output motion is less than the input motion. Similarly, the output speed of a gear reducer is less than the input torque. In the first case a gain in force is accompanied by a loss in motion, while in the second case a loss in motion is accompanied by a gain in torque.
Although the primary function of some machines can be indentified, it would be difficult to classify all machines as either force or motion modifiers; some machines belong in both categories. All machines, however, must perform a motion-modifying function, since if the parts of a mechanical device do not move, it is a structure, not a machine. It is customary for machinery designers, when studying the motions of the parts, to speak of the mechanism of a machine.
While all machines have a mechanism, and consequently perform a motion-modifying function, some machines do not have a planned force-modifying purpose; the forces that exist are caused by friction and inertia of the moving masses and do not appear as a useful output effort. This group would include measuring instruments and clocks.
The “word” referred to in the definition will be interpreted in its scientific sense. In the science of mechanics, word is something that forces do when they move in the direction in which they are acting, and it is equal to the product of the average force and the distance moved. If a man carries a weight along a horizontal path, he does no work according to this definition, since the force and the motion are at right angles to one another; that is, the force is vertical and the motion horizontal. If he carries the weight up a flight of stairs or a ladder, he does work, since he is moving in the same direction in which he is applying a force. Mathematically, if F equals force (in pounds or kilograms), and S equals distance (in feet or meters), work is then equal to the applied force F multiplied by the distance this force moves S; or WORK=F*S.
When a force causes a body to rotate about a fixed axis, or pivot, the work done is obtained by multiplying the torque (T)by the angle of rotation.
These concepts of work are fundamental in defining the mechanical work function of machines in terms of force and motions, and they bring out the inseparability of force and motions in machines. Because of friction, the work output from a machine is always less than the work input, and the efficiency, which is the ratio of the two, is always less than 100 percent.
The ratio of the output to input forces is the mechanical advantage (MA), and it defines the force-modifying function, while the ratio of the input to output motions is the velocity ratio (VR), and it defines the motion-modifying function. When the efficiency is high, these ratios are approximately equal; if the output force is ten times the input force, the input motion must be ten times the output motion, i.e., what is gained in force is lost in motion. Friction affects the mechanical advantage but not the velocity ratio.
To calculate the efficiency from the ratio of output to input work, it would be necessary to know the work done by the output and input forces over a specified distance. Since this would entail the determination of average forces over the interval, it would be inconvenient. The efficiency of a machine is more easily determined from instantaneous values of load and the rate at which the load is moving. For this purpose, power formulas are most useful.
Power is the rate at which work is don. If a man carries a ten-pound weight a vertical height of 12 feet (i.e,up a ladder or stairs) in half a minute, his power expenditure is 10*12 or 120 foot-pounds in half a minute; his rate of doing word is then 240 foot-pounds per minute.
The unit of power or rate of doing word in English speaking countries is the horsepower (hp), which is equal to 33,000 foot-pounds per minute, so that 240 foot-pounds per minute equal 240/33,000=0.00727 hp.
In dealing with simple force-amplifying machines such as the lever and the wheel and axle, it is convenient to call the input force the “effort” and the output force the “l(fā)oad.” The mechanical advantage is then the ratio of the load to the effort, and the velocity ratio is the motion (displacement or velocity) of the effort divided by the corresponding motion of the load.
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