外文翻譯---機(jī)械零件的強(qiáng)度

1、<p><b>　　附錄：</b></p><p><b>　　1 外文資料譯文</b></p><p><b>　　機(jī)械零件的強(qiáng)度</b></p><p>　　在設(shè)計任何機(jī)器或者結(jié)構(gòu)時，所考慮的主要問題之一是其強(qiáng)度應(yīng)該比它所承受的應(yīng)力要大得多，以確保安全與可靠性。要保證機(jī)械零件在使用過程中

2、不失效，就必須知道它們在某些時候為什么會失效的原因，然后，我們才能將應(yīng)力與強(qiáng)度聯(lián)系起來，以確保其安全。</p><p>　　設(shè)計任何機(jī)械零件的理想情況為，工程師可以利用大量的他所選用的這種材料的強(qiáng)度試驗數(shù)據(jù)。這些試驗應(yīng)該采用與所設(shè)計的零件有著相同的熱處理，表面形貌度和尺寸大小的試件進(jìn)行，而且試驗應(yīng)該在與零件使用過程中承受的載荷完全相同的情況下進(jìn)行。這表明，如果零件將要承受彎曲載荷，那么就應(yīng)該進(jìn)行彎曲載荷的試驗。如

3、果零件將要承受彎曲載荷，那么久應(yīng)該進(jìn)行彎曲載荷的試驗。如果零件將要承受彎曲和扭轉(zhuǎn)的復(fù)合載荷，那么就應(yīng)該進(jìn)行彎曲和扭轉(zhuǎn)復(fù)合載荷的試驗。這些種類的試驗可以提供非常有用和精確的數(shù)據(jù)。它們可以告訴工程師應(yīng)該使用的安全系數(shù)和對于給定使用壽命時的可靠性。在設(shè)計過程中，只要能夠獲得這種數(shù)據(jù)，工程師就可以盡可能好地進(jìn)行工程設(shè)計工作。</p><p>　　如果零件的失效可能會危害人地生命安全，或者零件有足夠大的產(chǎn)量有足夠大的產(chǎn)量，

4、則在設(shè)計前收集這樣廣泛的數(shù)據(jù)所花費(fèi)的費(fèi)用是值得的。例如汽車和冰箱的零件的產(chǎn)量非常大，可以在生產(chǎn)之前對它們進(jìn)行大量的試驗，使其具有較高的可靠性。如果把進(jìn)行這些試驗的費(fèi)用分?jǐn)偟剿a(chǎn)的零件攤到的費(fèi)用是非常低的。</p><p>　　你可以對下列四種類型的設(shè)計做出評價：</p><p>　?。?）零件的失效可能會危害人地生命安全，或者零件的產(chǎn)量非常大，因此在設(shè)計時安排一個完善的試驗程序會被認(rèn)為是

5、合理的。</p><p>　?。?）零件的產(chǎn)量足夠大，可以進(jìn)行適當(dāng)?shù)南盗性囼灐?lt;/p><p>　?。?）零件的產(chǎn)量非常小，以至于進(jìn)行試驗根本不合算；或者要求很快地完成很快地完成設(shè)計，以至于沒有足夠的時間進(jìn)行試驗。</p><p>　?。?）零件已經(jīng)完成了設(shè)計，制造和試驗，但其結(jié)果不能令人滿意。這時候需要采用分析的方法來弄清楚不能令人滿意的原因和應(yīng)該如何進(jìn)行改進(jìn)。&

6、lt;/p><p>　　我們將主要對后三種類型進(jìn)行討論。這就是說，設(shè)計人員通常只能利用那些公開發(fā)表的屈服強(qiáng)度，極限強(qiáng)度和延伸率等數(shù)據(jù)資料。人們期望著工程師在利用這些不是很多的數(shù)據(jù)資料的基礎(chǔ)上，對靜載荷與動載荷，兩維應(yīng)力狀態(tài)與三維應(yīng)力狀態(tài)，高溫與低溫以及大零件與小零件進(jìn)行設(shè)計！而設(shè)計中所能利用的數(shù)據(jù)通常是從簡單的拉伸試驗中得到的。其載荷與動載荷是逐漸加上去的，有充分的時間產(chǎn)生應(yīng)變。到目前為止，還必須利用這些數(shù)據(jù)來設(shè)計每

7、分鐘承受幾千次復(fù)雜的動載荷的作用零件，因此機(jī)械零件有時會失效是不足為奇的。</p><p>　　概括地說，設(shè)計人員所遇到的基本問題是，不論對于哪一種應(yīng)力狀態(tài)或者載荷情況，能利用通過簡單拉伸試驗所獲得數(shù)據(jù)并將與其零件的強(qiáng)度聯(lián)系起來。</p><p>　　可能會有兩種具有完全相同的強(qiáng)度和硬度值得金屬，其中的一種由于其本身的延伸性而具有很好的承受超載荷的能力。延展性是用材料斷裂時的延伸率來量度的

8、。通常將5﹪的延伸率定義為延展性與脆性的分界線。斷裂時延伸率小于5﹪的材料稱為脆性材料，大于5﹪的稱為延性材料。</p><p>　　材料的伸長量通常是在50mm的計量的。因為這并不是對實際應(yīng)變量的測量，所以有時也采用另一種測量延展性的方法。這個方法是在試件斷裂后，測量其斷裂處的橫截面的面積。因此，延展性可以表示為橫截面的收縮率。</p><p>　　延性材料能夠承受較大的超載荷這個特性是

9、設(shè)計中的一個附加的安全因素。延性材料的重要性在于它是材料冷變形性能的衡量尺度。諸如彎曲和拉延這類金屬加工都需要采用延性材料。</p><p>　　在選用抗磨損，抗侵蝕或者抗塑性變形的材料時，硬度通常是最主要的性能。有幾種可供選用的硬度試驗方法，采用哪一種方法取決于最希望測量的材料特性。最常用的四種硬度數(shù)值是布氏硬度﹑洛氏硬度﹑維氏硬度和努氏硬度。</p><p>　　大多數(shù)硬度試驗系統(tǒng)是將

10、一個標(biāo)準(zhǔn)的載荷加在與被試驗材料相接觸的小球或者凌錐上。因此，硬度可以表示為所產(chǎn)生的壓痕尺寸的函數(shù)。這表明由于硬度是非破壞性試驗，而且不需要專門的試件，因而硬度是一個容易測量的性能。通常可以直接在實際的機(jī)械零件上進(jìn)行硬度試驗。</p><p><b>　　軸與聯(lián)軸器</b></p><p>　　實際上，幾乎所有的機(jī)器中都裝有軸。軸最常見的形狀是圓形，其截面可以是實心的，

11、也可以是空心的（空心軸可以減輕重量）。有時也采用矩形軸，例如，螺絲起子的頭、套筒扳手和控制旋扭的桿。</p><p>　　為了在傳遞扭矩時不發(fā)生過載，軸應(yīng)該具有適當(dāng)?shù)目古?qiáng)度。軸還應(yīng)該具有足夠的抗扭剛度，以使同一個軸上的兩個傳動零件之間的相對角度不會過大。一般來說，在軸的長度等于其直徑的20倍時，扭轉(zhuǎn)角不應(yīng)該超過1度。</p><p>　　軸安裝在軸承中，通過齒輪、皮帶輪、凸輪和離合器等零

12、件傳遞動力。通過這些零件傳來的力可能會使軸產(chǎn)生彎曲變形。因此，軸應(yīng)該有足夠的剛度以防止支承軸承受力過大?？偠灾趦蓚€軸承支承之間，軸在每英尺長度上的彎曲變形不應(yīng)該超過0.01英寸。</p><p>　　此外，軸還必須能夠承受彎矩和扭矩的組合作用。因此，要考慮扭矩與彎矩的當(dāng)量載荷。因為扭矩和彎矩會產(chǎn)生交變應(yīng)力，在許用應(yīng)力中也應(yīng)該有一個考慮疲勞現(xiàn)象的安全系數(shù)。</p><p>　　直徑小于

13、3英寸的軸可以采用含碳量大約為0.4%的冷軋鋼，直徑在3~5英寸之間的軸可以采用冷軋鋼或鍛造毛坯。當(dāng)直徑大于5英寸時，則要采用鍛造毛坯，然后機(jī)械加工到所要求的尺寸。輕載時，廣泛采用塑料軸。由于塑料是電的不良導(dǎo)體，在電器中用它作軸比較安全。</p><p>　　齒輪和皮帶輪等零件通過鍵連接在軸上。在對鍵及軸上與之相應(yīng)的鍵槽進(jìn)行設(shè)計時，必須進(jìn)行認(rèn)真的計算。例如，軸上的鍵槽會引起應(yīng)力集中，由于鍵槽的存在使軸的橫截面積減

14、小，會進(jìn)一步減弱軸的強(qiáng)度。</p><p>　　如果軸以臨界速度轉(zhuǎn)動，將會發(fā)生強(qiáng)烈的振動，可能會毀壞整臺機(jī)器。知道這些臨界速度的大小是很重要的，因為這樣可以避開它。一般憑經(jīng)驗來說，工作速度與臨界速度之間至少硬挨相差20%.</p><p>　　軸的設(shè)計工作的另一個重要方面是軸與軸之間的鏈接方法。這是由剛性或者彈性聯(lián)軸器來實現(xiàn)的。</p><p>　　聯(lián)軸器是用來把兩

15、個相鄰軸端鏈接起來的裝置。在機(jī)械結(jié)構(gòu)中，聯(lián)軸器被用來實現(xiàn)相鄰的兩根轉(zhuǎn)軸之間的半永久性聯(lián)接。在機(jī)器的正常使用期間內(nèi)，這種聯(lián)接一般不必拆開，在這種意義上，可以說聯(lián)軸器的聯(lián)接是永久性的。但是在緊急情況下，或者需要更換已磨損的零件時，可以先把聯(lián)軸器拆開，然后再聯(lián)接上。</p><p>　　聯(lián)軸器有幾種類型，它們的特征隨其用途而定。如果制造工廠中或者船舶的螺旋槳需要一根特別長的軸，可以采用分段的方式將其制造出來，然后采用剛

16、性聯(lián)軸器將各段聯(lián)接起來。一種常用的聯(lián)軸器是由兩個配對的法蘭（盤）組成，這兩個法蘭盤借助靠鍵傳動的兩根軸通常是靠法蘭面上的槽口來進(jìn)行找正的。</p><p>　　在把屬于不同的設(shè)備（例如一個電動機(jī)和一個變速箱）的軸聯(lián)接起來的時候，要把這些軸精確地對準(zhǔn)是比較困難的，此時可以采用彈性聯(lián)軸器。這種聯(lián)軸器連接軸的方式可以把由于被聯(lián)接軸之間的軸線不重合所造成的有害影響減少到最低程度。彈性聯(lián)軸器也允許被聯(lián)接的軸在它們各自的載荷

17、系統(tǒng)作用下產(chǎn)生偏斜或在軸線方向自由移動（浮動）而不至于相互干擾。彈性聯(lián)軸器也可以用來減輕從一根軸傳到另一根軸上的沖擊載荷和振動的強(qiáng)度。</p><p><b>　　2 外文資料原文</b></p><p>　　The Strength of Mechanical Elements</p><p>　　One of the primary con

18、siderations in designing any machine or structure is that the strength must be sufficiently greater than the stress to assure both safety and reliability. To assure that mechanical parts do not fail in service, it is nec

19、essary to learn why they sometimes do fail. Then we shall be able to relate the stresses with the strengths to achieve safety.</p><p>　　Ideally, in designing any machine element, the engineer should have at

20、his disposal the results of a great many strength tests of the particular material chosen. These tests should have been made on specimens having the same heat treatment, surface texture, and size as the element he propos

21、es to design; and the tests should be made under exactly the same loading conditions as the part will experience in service. This means that, if the part is to experience a bending load, it should be tested wi</p>

22、<p>　　The cost of gathering such extensive data prior to design is justified if failure of the part may endanger human life, or if the part is manufactured insufficiently large quantities .Automobiles and refrigera

23、tors, for example, have very good reliabilities because the parts are made in such large quantities that they can be thoroughly tested in advance of manufacture. The cost of making these tests is very low when it is divi

24、ded by the total number of parts manufactured.</p><p>　　You can now appreciate the following four design categories:</p><p>　　(1)Failure of the part would endanger human life, or the part is ma

25、de in extremely large quantities; consequently, an elaborate testing program is justified during design.</p><p>　　(2)The part is made in large enough quantities so that a moderate series of tests is feasibl

26、e.</p><p>　　(3)The part is made in such small quantities that testing is not justified at all; or the design must be completed so rapidly that these is not enough time for testing.</p><p>　　(4)

27、The part has already been designed, manufactured, and tested and found to be unsatisfactory. Analysis is required to understand why the part is unsatisfactory and what to do to improve it.</p><p>　　It is wi

28、th the last three categories that we shall be mostly concerned. This means that the designer will usually have only published values of yield strength, ultimate strength, and percentage elongation. With this meager infor

29、mation the engineer is expected to design against static and dynamic loads, biaxial stress states, high and low temperatures, and large and small parts! The data usually available for design have been obtained form the s

30、imple tension test, where the load was applied gradu</p><p>　　To sum up, the fundamental problem of the designer is to use the simple tension-test data and relate them to the strength of the part, regardless

31、 of the stress state or the loading situation. It is possible for two metals to have exactly the same strength and hardness, yet one of these metals may have a superior ability to absorb overloads, because of the prope

32、rty called ductility. Ductility is measured by the percentage elongation which occurs in the material at fracture. The usual dividing l</p><p>　　Shafts and Couplings</p><p>　　Virtually all machi

33、nes shafts. The most common shape for shafts is circular and the cross section can be either solid or hollow(hollow shafts can result in weight savings). Rectangular shafts are sometimes used, as in screwdriver blades, s

34、ocket wrenches and control knob stems.</p><p>　　A shaft must have adequate torsional strength to transmit torque and not be overstressed. It must also be torsionally stiff enough so that one mounted componen

35、t does not deviate excessively from its original angular position relative to a second component mounted on the same shaft. Generally speaking, the angle of twist should not exceed one degree in a shaft length equal to 2

36、0 diameters.</p><p>　　Shafts are mounted in bearings and transmit power through such devices as gears, pulleys, cams and clutches. These devices introduce forces which attempt to bend the shaft; hence, the s

37、haft must be rigid enough to prevent overloading of the supporting bearings. In general, the bending deflection of a shaft should not exceed 0.01 in. per ft of length between bearing supports.</p><p>　　In ad

38、dition, the shaft must be able to sustain a combination of bending and torsional loads. Thus an equivalent load must be considered which takes into account both torsion and bending. Also, the allowable stress must contai

39、n a factor of safety which includes fatigue, since torsional and bending stress reversals occur.</p><p>　　For diameters less than 3 in, the usual shaft material is cold-rolled steel containing about 0.4 perc

40、ent carbon.. Shafts are either cold-rolled or forged in sizes from 3 in. to 5 in. For sizes above 5 in. shaft are forged and machined to size. Plastic shafts are widely used for light load applications. One advantage of

41、using plastic is safety in electrical applications, since plastic is a poor conductor of electricity.</p><p>　　Components such as gear and pulleys are mounted on shafts by means of key. The design of the key

42、 and the corresponding keyway in the shaft must be properly evaluated. For example, stress concentrations occur in shafts due to keyways, and the material removed to form the keyway further weakens the shaft.</p>

43、<p>　　If shafts are run at critical speeds, severe vibrations can occur which can seriously damage a machine. It is important to know the magnitude of these difference between the operating speed and the critical sp

44、eed should be at least 20 percent.</p><p>　　Another important aspect of shaft design is the method of directly conneoting one shaft to another. This is accomplished by devices such as rigid and flexible coup

45、lings.</p><p>　　A coupling is a device for connecting the ends of adjacent shafts. In machine construction, couplings are used to effect a semipermanent connection between adjacent rotating shafts. The conne

46、ction is permanent in the sense that it is not meant to be broken during the useful life of the machine, but it can be broken and restored in an emergency or when worn parts are replaced.</p><p>　　There are

47、several types of shaft couplings, their characteristics depend on the purpose for which they are used. If an exceptionally long shaft is required in a manufacturing plant or a propeller shaft on a ship, it is made in sec

48、tions that are coupled together with rigid couplings. A common type of rigid coupling consists of two mating radial flanges (disks) that are attached by key-driven hubs to the ends of adjacent shaft sections and bolted t

49、ogether through the flanges to form a rigid connec</p><p>　　Inconnecting shafts belonging to separate devices (such as an electric motor and a gearbox), precise aligning of the shafts is difficult and a flex

50、ible coupling is used. This coupling connects the shafts in such a way as to minimize the harmful effects of shaft misalignment. Flexible couplings also permit the shafts to deflect under their separate systems of loads