【正文】 hanical properties of materials are:(1) Strength (in tension[5tenFEn], pression, shear and bending)(2) Stiffness[5stifnis](3) Ductility[dQk5tiliti](4) Impact strength(5) Hardness(6) Toughness[5tQfnis]1. Strength or mechanical strength of a material may be defined as the ability of the material to sustain loads without undue[5Qn5dju:] distortion[dis5tC:FEn] or failure. Material should have adequate[5Adikwit] strength when subjected[5sQbdVikt] to tension, pression, shear, bending or torsion[5tC:FEn] as per the intended use.For example the crankshaft[5krANkFB:ft] of an automobile[5C:tEmEubi:l, 7C:tE5mEubil, 7C:tEmE5bi:l] should have proper torsion strength.2. Stiffness. Stiffness is the ability of a material or shape to resist elastic[I5lAstIk] deflection[di5flekFEn]. For identical[ai5dentikEl] shapes, the stiffness is proportional[prE5pC:FEnl] to the modulus[5mCdjulEs] of elasticity[IlAs5tIsItI]. A material which deforms less under a given load is more stiff than on which deforms more.8. Gears and ReducersGears[^iE] are vital[5vaitl] factors in machinery[mE5Fi:nEri]. One of the first mechanism[5mekEnizEm] invented using gears was the clocks. In fact, a clock is little more than a train of gears. Considerable[kEn5sidErEbl] study and research have been made on gears in recent years because of their wide use under exacting[i^5zAktiN] conditions. They have to transmit[trAnz5mit] heavier loads and run at higher speeds than ever before. The engineers and the machinists[mE5Fi:nist] all consider gearing the prime element in nearly all classes of machinery.1. Spur[spE:] gearsSpur gears are used to transmit power and rotary[5rEutEri] motion between parallel[5pArElel] shafts. The teeth are cut parallel to the axis of the shaft on which the gears are mounted. The smaller of two gears in mesh[meF] is called the pinion[5pinjEn], and the larger is customarily[`kQstEmErElI。 and worm [wE:m], 80. For lightly loaded, instrument[5instrumEnt], and positioning [pE5ziFEniN] gears, these rations can be exceeded[ik5si:d]. Rations as high as 400 or higher can be obtained with gears that resemble tapered [5teipE] worms meshing with hypoid gear. For heavily loaded gear, the given rations may be so high that a reasonable gear size precludes [pri5klu:d] a satisfactory pinion.Since the ratio in a single pair of gears is the quotient [5kwEuFEnt] of the tooth number, and since there usually are limitations on both the minimum and maximum numbers of teeth on the available [E5veilEbl] gears, it follows that the number of ratios obtainable in a single pair is limited. To enlarge the coverage [5kQvEridV] it is necessary to use multiple[5mQltipl] pairs, or trains. The overall[5EuvErC:l] speed ratio in a train is the product of the ratios in each pair. In certain cases an exact ratio cannot be obtained with gears, but by using two or more parts, the desired ratio can be approximated[E5prCksimeit] to any degree of precision.As a convenience for machine builders and users, packaged[5pAkidV] speed reducers, following an industryaccepted pattern [5pAtEn], are manufactured[7mAnju5fAktFE] in a wide variety of types, configurations [kEn7fi^ju5reiFEn], speed ratios, and capacities。兩塊薄板或厚板以邊沿稍稍搭疊的方式放在一起，對金屬板施加強(qiáng)壓，氧化皮和陷入薄板中間任何雜質(zhì)都會被擠出，而完成了焊縫連接.6. Heat Treatment and Hot Working of MetalsWe can alter[5C:ltE] the characteristics[7kAriktE5ristik] of steel in various ways. In the first place, steel which contains very little carbon will be milder than steel which contains a higher percentage of carbon, up to the limit of about %. Secondly, we can heat the steel above a certain critical[5kritikEl] temperature, and then allow it to cool at different rates. At this critical temperature, changes begin to take place in the molecular[mEu5lekjulE] structure of the metal. In the process known as annealing[A5ni:liN], we heat the steel above the critical temperature and permit it to cool very slowly. This causes the metal to bee softer than before, and much easier to machine. Annealing has a second advantage。在這一系列重要的焊接過程中，電弧可為熔化金屬供熱，而焊劑起著保護(hù)和清理焊縫的作用，而且常常也是起著熔煉控制作用。此外，加熱鑄鐵或鋼到高溫會引起氧化井在焊接表面形成氧化皮。軟釬焊中最重要的工序之一就是清理焊縫表面。鍛造將在大約與輥軋同樣的溫度下進(jìn)行，這種工藝正像軋制那樣可改善金屬的物理性能。然而落錘鍛造可以鍛制形狀復(fù)雜而不規(guī)則的鍛件，因而較為經(jīng)濟(jì)。模具可由各白獨(dú)立的單元裝配而成，即把所有個別模腔都放到一起，組成整體。4. ForgingPress forging[5fC:dViN] employs a slow squeezing[skwi:z] action in deforming[di:5fC:m] the plastic metal, as contrasted with the rapidimpact[5impAkt] blows of a hammer. The squeezing action is carried pletely to the center of the part being pressed, thoroughly working the entire section[5sekFEn]. These presses are the vertical[5vE:tikEl] type and may be either mechanically[mi5kAnikEli] or hydraulically[hai5drC:lik] operated. The mechanical presses, which are faster operating and most monly used, range in capacity[kE5pAsiti] from 500 to 1000 tons.與錘鍛的快速沖擊不同，壓力機(jī)鍛造是用緩慢的擠壓作用使塑性金屬變形。這些合金可以進(jìn)一步進(jìn)行分類為低溫類合金和高溫類合金。金屬模是由兩個中空的部件組成，它們應(yīng)被聯(lián)結(jié)在一起以便將金屬液澆入模箱中。實(shí)際上所有金屬在成分上也是變化的，而合金也可以鑄造。它容許大量生產(chǎn)單位成本低廉的模制式器件，例如，各種容器。例如，跟金屬相比較，塑料既有優(yōu)點(diǎn)也有缺點(diǎn)。對于某種用途而言，這些性能使得塑料比傳統(tǒng)材料更為可取。It is their plasticity[plAs5tisiti] at certain temperatures[5temprItFE(r)] which gives plastics their main advantages over many other materials. It permits the largescale production of molded[mEuld] articles, such as containers, at an economic unit cost, where other materials require laborious[lE5bC:riEs] and often costly processes involving cutting, shaping, machining, assembly[E5sembli] and decoration.在某一溫度下塑料是處于塑性狀態(tài)的，這就使塑料具備超過許多其他材料的主要優(yōu)點(diǎn)。鑄造的產(chǎn)品是鑄件，鑄件可能從零點(diǎn)幾公斤到幾百噸范圍變化。The molds may be of two kinds: sand molds and metal molds. A metal mold consists of two hollow parts which should be joined for pouring the metal into it. The inside of this mold is covered with carbon or graphite[5^rAfait] so that the metal could not stick[stik] to the wall of the form. When the metal has solidified[sE5lidifai], these hollow parts are disjoined and the casting is taken out. There are also special molds in which large blocks of steel can be cast. These molds are usually m