外文翻譯--機(jī)器和機(jī)器零件的設(shè)計(jì)

Machine design Machine design is the art of planning or devising new or improved machines to accomplish specific purposes. In general, a machine will consist of a combination of several different mechanical elements properly designed and arranged to work together, as a whole. During the initial planning of a machine, fundamental decisions must be made concerning loading, type of kinematic elements to be used, and correct utilization of the properties of engineering materials. Economic considerations are usually of prime importance when the design of new machinery is undertaken. In general, the lowest over-all costs are designed. Consideration should be given not only to the cost of design, manufacture the necessary safety features and be of pleasing external appearance. The objective is to produce a machine which is not only sufficiently rugged to function properly for a reasonable life, but is at the same time cheap enough to be economically feasible. The engineer in charge of the design of a machine should not only have adequate technical training, but must be a man of sound judgment and wide experience, qualities which are usually acquired only after considerable time has been spent in actual professional work. Design of machine elements The principles of design are, of course, universal. The same theory or equations may be applied to a very small part, as in an instrument, or, to a larger but similar part used in a piece of heavy equipment. In no ease, however, should mathematical calculations be looked upon as absolute and final. They are all subject to the accuracy of the various assumptions, which must necessarily be made in engineering work. Sometimes only a portion of the total number of parts in a machine are designed on the basis of analytic calculations. The form and size of the remaining parts are designed on the basis of analytic calculations. On the other hand, if the machine is very expensive, or if weight is a factor, as in airplanes, design computations may then be made for almost all the parts. The purpose of the design calculations is, of course, to attempt to predict the stress or deformation in the part in order that it may sagely carry the loads, which will be imposed on it, and that it may last for the expected life of the machine. All calculations are, of course, dependent on the physical properties of the construction materials as determined by laboratory tests. A rational method of design attempts to take the results of relatively simple and fundamental tests such as tension, compression, torsion, and fatigue and apply them to all the complicated and involved situations encountered in present-day machinery. In addition, it has been amply proved that such details as surface condition, fillets, notches, manufacturing tolerances, and heat treatment have a market effect on the strength and useful life of a machine part. The design and drafting departments must specify completely all such particulars, must specify completely all such particulars, and thus exercise the necessary close control over the finished product. As mentioned above, machine design is a vast field of engineering technology. As such, it begins with the conception of an idea and follows through the various phases of design analysis, manufacturing, marketing and consumerism. The following is a list of the major areas of consideration in the general field of machine design: Initial design conception； Strength analysis； Materials selection； Appearance； Manufacturing； Safety； Environment effects； Reliability and life； Strength is a measure of the ability to resist, without fails, forces which cause stresses and strains. The forces may be； Gradually applied； Suddenly applied； Applied under impact； Applied with continuous direction reversals； Applied at low or elevated temperatures. If a critical part of a machine fails, the whole machine must be shut down until a repair is made. Thus, when designing a new machine, it is extremely important that critical parts be made strong enough to prevent failure. The designer should determine as precisely as possible the nature, magnitude, direction and point of application of all forces. Machine design is mot, however, an exact science and it is, therefore, rarely possible to determine exactly all the applied forces. In addition, different samples of a specified material will exhibit somewhat different abilities to resist loads, temperatures and other environment conditions. In spite of this, design calculations based on appropriate assumptions are invaluable in the proper design of machine. Moreover, it is absolutely essential that a design engineer knows how and why parts fail so that reliable machines which require minimum maintenance can be designed. Sometimes, a failure can be serious, such as when a tire blows out on an automobile traveling at high speeds. On the other hand, a failure may be no more than a nuisance. An example is the loosening of the radiator hose in the automobile cooling system. The consequence of this latter failure is usually the loss of some radiator coolant, a condition which is readily detected and corrected. The type of load a part absorbs is just as significant as the magnitude. Generally speaking, dynamic loads with direction reversals cause greater difficulties than static loads and, therefore, fatigue strength must be considered. Another concern is whether the material is ductile or brittle. For example, brittle materials are considered to be unacceptable where fatigue is involved. In general, the design engineer must consider all possible modes of failure, which include the following: Stress； Deformation； Wear； Corrosion； Vibration； Environmental damage； Loosening of fastening devices. The part sizes and shapes selected must also take into account many dimensional factors which produce external load effects such as geometric discontinuities, residual stresses due to forming of desired contours, and the application of interference fit joint. Selected from” design of machine elements”, 6th edition, m. f. sports, prentice-hall, inc., 1985 and “machine design”, Anthony Esposito, charles e., Merrill publishing company, 1975. Mechanical properties of materials The material properties can be classified into three major headings: (1) physical, (2) chemical, (3) mechanical Physical properties Density or specific gravity, moisture content, etc., can be classified under this category. Chemical properties Many chemical properties come under this category. These include acidity or alkalinity, react6ivity and corrosion. The most important of these is corrosion which can be explained in laymans terms as the resistance of the material to decay while in continuous use in a particular atmosphere. Mechanical properties Mechanical properties include in the strength properties like tensile, compression, shear, torsion, impact, fatigue and creep. The tensile strength of a material is obtained by dividing the maximum load, which the specimen bears by the area of cross-section of the specimen. This is a curve plotted between the stress along the This is a curve plotted between the stress along the Y-axis(ordinate) and the strain along the X-axis (abscissa) in a tensile test. A material tends to change or changes its dimensions when it is loaded, depending upon the magnitude of the load. When the load is removed it can be seen that the deformation disappears. For many materials this occurs op to a certain value of the stress called the elastic limit Ap. This is depicted by the straight line relationship and a small deviation thereafter, in the stress-strain curve (fig.3.1) . Within the elastic range, the limiting value of the stress up to which the stress and strain are proportional, is called the limit of proportionality Ap. In this region, the metal obeys hookess law, which states that the stress is proportional to strain in the elastic range of loading, (the material completely regains its original dimensions after the load is removed). In the actual plotting of the curve, the proportionality limit is obtained at a slightly lower value of the load than the elastic limit. This may be attributed to the time-lagin the regaining of the original dimensions of the material. This effect is very frequently noticed in some non-ferrous metals. Which iron and nickel exhibit clear ranges of elasticity, copper, zinc, tin, are found to be imperfectly elastic even at relatively low values low values of stresses. Actually the elastic limit is distinguishable from the proportionality limit more clearly depending upon the sensitivity of the measuring instrument. When the load is increased beyond the elastic limit, plastic deformation starts. Simultaneously the specimen gets work-hardened. A point is reached when the deformation starts to occur more rapidly than the increasing load. This point is called they yield point Q. the metal which was resisting the load till then, starts to deform somewhat rapidly, i. e., yield. The yield stress is called yield limit Ay. The elongation of the specimen continues from Q to S and then to T. The stress-strain relation in this plastic flow period is indicated by the portion QRST of the curve. At the specimen breaks, and this load is called the breaking load. The value of the maximum load S divided by the original cross-sectional area of the specimen is referred to as the ultimate tensile strength of the metal or simply the tensile strength Au. Logically speaking, once the elastic limit is exceeded, the metal should start to yield, and finally break, without any increase in the value of stress. But the curve records an increased stress even after the elastic limit is exceeded. Two reasons can be given for this behavior: The strain hardening of the material； The diminishing cross-sectional area of the specimen, suffered on account of the plastic deformation. The more plastic deformation the metal undergoes, the harder it becomes, due to work-hardening. The more the metal gets elongated the more its diameter (and hence, cross-sectional area) is decreased. This continues until the point S is reached. After S, the rate at which the reduction in area takes place, exceeds the rate at which the stress increases. Strain becomes so high that the reduction in area begins to produce a localized effect at some point. This is called necking. Reduction in cross-sectional area takes place very rapidly； so rapidly that the load value actually drops. This is indicated by ST. failure occurs at this point T. Then percentage elongation A and reduction in reduction in area W indicate the ductility or plasticity of the material: A=(L-L0)/L0*100% W=(A0-A)/A0*100% Where L0 and L are the original and the final length of the specimen； A0 and A are the original and the final cross-section area. 1 機(jī)器和機(jī)器零件的設(shè)計(jì) 機(jī)器設(shè)計(jì) 機(jī)器設(shè)計(jì)為了 特定的目的而發(fā)明或改進(jìn)機(jī)器的一種藝術(shù)。一般來講，機(jī)器時有多種不同的合理設(shè)計(jì)并有序裝配在一起的部件構(gòu)成的，在最初的機(jī)器設(shè)計(jì)階段，必須基本明確負(fù)載、元件的運(yùn)動情況、工程材料的合理使用性能。負(fù)責(zé)新機(jī)器的設(shè)計(jì)最初的最重要的是經(jīng)濟(jì)性考慮。一般來說，選擇總成本最低的設(shè)計(jì)方案，不僅要考慮設(shè)計(jì)、制造、銷售、安裝的成本。還要考慮服務(wù)的費(fèi)用，機(jī)械要保證必要的安全性能和美觀的外形。制造機(jī)器的目標(biāo)不僅要追求保證只用功能的合理壽命，還要保證足夠便宜以同時保證其經(jīng)濟(jì)的可行性。負(fù)責(zé)設(shè)計(jì)機(jī)器的工程師，不僅要經(jīng)過專業(yè)的培訓(xùn)，而且必須是一個準(zhǔn) 確判斷而又有豐富經(jīng)驗(yàn)的人，具有一種有足夠時間從事專門的實(shí)際工作的素質(zhì)。 機(jī)器零件的設(shè)計(jì) 相同的理論或方程可應(yīng)用在一個一起的非常小的零件上，也可用在一個復(fù)雜的設(shè)備的大型相似件上，既然如此，毫無疑問，數(shù)學(xué)計(jì)算是絕對的和最終的。他們都符合不同的設(shè)想，這必須由工程量決定。有時，一臺機(jī)器的零件全部計(jì)算僅僅是設(shè)計(jì)的一部分。零件的結(jié)構(gòu)和尺寸通常根據(jù)實(shí)際考慮。另一方面，如果機(jī)器和昂貴，或者質(zhì)量很重要，例如飛機(jī)，那麼每一個零件都要設(shè)計(jì)計(jì)算。 當(dāng)然，設(shè)計(jì)計(jì)算的目的是試圖預(yù)測零件的應(yīng)力和變形，以保證其安全的帶動負(fù)載，這是必要的， 并且其也許影響到機(jī)器的最終壽命。當(dāng)然，所有的計(jì)算依賴于這些結(jié)構(gòu)材料通過試驗(yàn)測定的物理性能。國際上的設(shè) 2 計(jì)方法試圖通過從一些相對簡單的而基本的實(shí)驗(yàn)中得到一些結(jié)果，這些試驗(yàn)，例如結(jié)構(gòu)復(fù)雜的及現(xiàn)代機(jī)械設(shè)計(jì)到的電壓、轉(zhuǎn)矩和疲勞強(qiáng)度。 另外，可以充分證明，一些細(xì)節(jié)，如表面粗糙度、圓角、開槽、制造公差和熱處理都對機(jī)械零件的強(qiáng)度及使用壽命有影響。設(shè)計(jì)和構(gòu)建布局要完全詳細(xì)地說明每一個細(xì)節(jié)，并且對最終產(chǎn)品進(jìn)行必要的測試。 綜上所述，機(jī)械設(shè)計(jì)是一個非常寬的工程技術(shù)領(lǐng)域。例如，從設(shè)計(jì)理念到設(shè)計(jì)分析的每一個階段，制造，市場，銷售。以 下是機(jī)械設(shè)計(jì)的一般領(lǐng)域應(yīng)考慮的主要方面的清單： 最初的設(shè)計(jì)理念 受力分析 材料的選擇 外形 制造 安全性 環(huán)境影響 可靠性及壽命 在沒有破壞的情況下，強(qiáng)度是抵抗引起應(yīng)力和應(yīng)變的一種量度。這些力可能是： 漸變力 瞬時力 沖擊力 不斷變化的力 溫差 如果一個機(jī)器的關(guān)鍵件損壞，整個機(jī)器必須關(guān)閉，直到修理好為止。設(shè)計(jì)一臺新機(jī)器時，關(guān)鍵件具有足夠的抵抗破壞的能力是非常重要的。設(shè)計(jì)者應(yīng)盡可能準(zhǔn)確地確定所有的性質(zhì)、大 小、方向及作用點(diǎn)。機(jī)器設(shè)計(jì)不是這樣，但精確的科學(xué)是這樣，因此很難準(zhǔn)確地確定所有力。另外，一種特殊材料的不同樣本會顯現(xiàn)出不同的性能，像抗負(fù)載、溫度和其他外部條件。盡管如此，在機(jī)械設(shè)計(jì)中給予合理綜合的設(shè)計(jì)計(jì)算是非常有用的。 3 此外，顯而易見的是一個知道零件是如何和為什麼破壞的設(shè)計(jì)師可以設(shè)計(jì)出需要很少維修的可靠機(jī)器。有時，一次失敗是嚴(yán)重的，例如高速行駛的汽車的輪胎爆裂。另一方面，失敗未必是麻煩。例如，汽車的冷卻系統(tǒng)的散熱器皮帶管松開。這種破壞的后果通常是損失一些散熱片，可以探測并改正過來。零件負(fù)載類型是一個重要的標(biāo) 志。一般而言，變化的動負(fù)載比靜負(fù)載會引起更大的差異。因此，疲勞強(qiáng)度必須符合。另一個關(guān)心的方面是這種材