外文翻譯--機(jī)器零件的設(shè)計(jì)

、 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. 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 max imum 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. Quality assurance and control Product quality is of paramount importance in manufacturing. If quality is allowed deteriorate, then a manufacturer will soon find sales dropping off followed by a possible business failure. Customers expect quality in the products they buy, and if a manufacturer expects to establish and maintain a name in the business, quality control and assurance functions must be established and maintained before, throughout, and after the production process. Generally speaking, quality assurance encompasses all activities aimed at maintaining quality, including quality control. Quality assurance can be divided into three major areas. These include the following: Source and receiving inspection before manufacturing； In-process quality control during manufacturing； Quality assurance after manufacturing. Quality control after manufacture includes warranties and product service extended to the users of the product. Source and receiving inspection before manufacturing Quality assurance often begins ling before any actual manufacturing takes place. This may be done through source inspections conducted at the plants that supply materials, discrete parts, or subassemblies to manufacturer. The manufacturers source inspector travels to the supplier factory and inspects raw material or premanufactured parts and assemblies. Source inspections present an opportunity for the manufacturer to sort out and reject raw materials or parts before they are shipped to the manufacturers production facility. The responsibility of the source inspector is to check materials and parts against design specifications and to reject the item if specifications are not met. Source inspections may include many of the same inspections that will be used during production. Included in these are: Visual inspection； Metallurgical testing； Dimensional inspection； Destructive and nondestructive inspection； Performance inspection. Visual inspections Visual inspections examine a product or material for such specifications as color, texture, surface finish, or overall appearance of an assembly to determine if there are any obvious deletions of major parts or hardware. Metallurgical testing Metallurgical testing is often an important part of source inspection, especially if the primary raw material for manufacturing is stock metal such as bar stock or structural materials. Metals testing can involve all the major types of inspections including visual, chemical, spectrographic, and mechanical, which include hardness, tensile, shear, compression, and spectr5ographic analysis for alloy content. Metallurgical testing can be either destructive or nondestructive. Dimensional inspection Few areas of quality control are as important in manufactured products as dimensional requirements. Dimensions are as important in source inspection as they are in the manufacturing process. This is especially critical if the source supplies parts for an assembly. Dimensions are inspected at the source factory using standard measuring tools plus special fit, form, and function gages that may required. Meeting dimensional specifications is critical to interchangeability of manufactured parts and to the successful assembly of many parts into complex assemblies such as autos, ships, aircraft, and other multipart products. Destructive and nondestructive inspection In some cases it may be necessary for the source inspections to call for destructive or nondestructive tests on raw materials or p0arts and assemblies. This is particularly true when large amounts of stock raw materials are involved. For example it may be necessary to inspect castings for flaws by radiographic, magnetic particle, or dye penetrant techniques before they are shipped to the manufacturer for final machining. Specifications calling for burn-in time for electronics or endurance run tests for mechanical components are further examples of nondestructive tests. It is sometimes necessary to test material and parts to destruction, but because of the costs and time involved destructive testing is avoided whenever possible. Examples include pressure tests to determine if safety factors are adequate in the design. Destructive tests are probably more frequent in the testing of prototype designs than in routine inspection of raw material or parts. Once design specifications are known to be met in regard to the strength of materials, it is often not necessary to test further parts to destruction unless they are genuinely suspect. Performance inspection Performance inspections involve checking the function of assemblies, especially those of complex mechanical systems, prior to installation in other products. Examples include electronic equipment subcomponents, aircraft and auto engines, pumps, valves, and other mechanical systems requiring performance evaluation prior to their shipment and final installation. 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(gè)一起的非常小的零件上，也可用在一個(gè)復(fù)雜的設(shè)備的大型相似件上，既然如此，毫無(wú)疑問(wèn)，數(shù)學(xué)計(jì)算是絕對(duì)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(shí)，一臺(tái)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)，那么每一個(gè)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(cè)零件的應(yīng)力和變形，以保證其安全的帶動(dòng)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(guò)試驗(yàn)測(cè)定的物理性能 。國(guó)際上的設(shè)計(jì)方法試圖通過(guò)從一些相對(duì)簡(jiǎn)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(kāi)槽、制造公差和熱處理都對(duì)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(shuō)明每一個(gè)細(xì)節(jié)，并且對(duì)最終產(chǎn)品進(jìn)行必要的測(cè)試。 綜上所述，機(jī)械設(shè)計(jì)是一個(gè)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(gè)階段，制造，市場(chǎng)，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(méi)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(shí)力 沖擊力 不斷變化的力 溫差 如果一個(gè)機(jī)器的關(guān)鍵件損壞，整個(gè)機(jī)器必須關(guān)閉，直到修理好為止。設(shè)計(jì)一臺(tái)新機(jī)器時(shí)，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(huì)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。 此外，顯而易見(jiàn)的是一個(gè)知道零件是如何和為什么破壞的設(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(shí)，一次失敗是嚴(yán)重的，例如高速行駛的汽車(chē)的輪胎爆裂。另一方面，失敗未必是麻煩。例如，汽車(chē)的冷卻系統(tǒng)的散熱器皮帶管松開(kāi)。這種破壞的后果通常是損失一些散熱片，可以探測(cè)并改正過(guò)來(lái)。零件負(fù)載類型是一個(gè)重要的標(biāo)志。一般而言，變化的動(dòng)負(fù)載比靜負(fù)載會(huì)引起更大的差異。因此，疲勞強(qiáng)度必須符合。另一個(gè)關(guān)心的 方面是這種材料是否直或易碎。例如有疲勞破壞的地方不易使用易碎的材料。一般的，設(shè)計(jì)師要靠考慮所有破壞情況，其包括以下方面： 應(yīng)力 應(yīng)變 外形 腐蝕 震動(dòng) 外部環(huán)境破壞 緊固件的松脫 零件的尺寸和外形的選擇也有很多因素。外部負(fù)荷的影響，如幾何間斷，由于輪廓而產(chǎn)生的殘余應(yīng)力和組合件干涉。 材料的機(jī)械性能 材料的機(jī)械性能可以被分成三個(gè)方面：物理性能，化學(xué)性能，機(jī)械性能。 物理性能 密度或比重、溫度等可以歸為這一類。 化學(xué)性能 這一種類包括很多化學(xué)性能。其 中包括酸堿性、化學(xué)反應(yīng)性、腐蝕性。其中最重要的是腐蝕性，在外行人看來(lái)，腐蝕性被解釋為在某處的零件抵抗腐蝕的能力。 機(jī)械性能 機(jī)械性能包括拉伸性能、壓縮性能、剪切性能、扭轉(zhuǎn)性能、沖擊性能、疲勞性能和蠕變。材料的拉伸強(qiáng)度可以通過(guò)試件的橫截面積出試件承受的最大載荷得到，這是在拉伸試驗(yàn)中，應(yīng)力沿 Y 軸，應(yīng)邊沿 X 軸變化的曲線。一種材料加載時(shí)開(kāi)始發(fā)生變化的初值取決于負(fù)載的大小。當(dāng)負(fù)載去掉時(shí)可以看到變形消失。對(duì)于很多材料而言，在達(dá)到彈性極限的一定應(yīng)力值 A 之前，一直表現(xiàn)為這樣。在應(yīng)力 -應(yīng)變圖中，這是可以用線性關(guān)系來(lái)描述的。 這之后又一個(gè)小的偏移。 在彈性范圍內(nèi)，達(dá)到應(yīng)力的極限之前，應(yīng)力和應(yīng)變是成比例的，這被稱為比例極限 Ap。在這個(gè)區(qū)域，零件符合胡克定律，即應(yīng)力與應(yīng)變是成比例的，在彈性范圍內(nèi)（材料能完全恢復(fù)到最初的尺寸，當(dāng)負(fù)載去掉時(shí)）。曲線中的實(shí)際點(diǎn)，比例極限在彈性極限處。這可以認(rèn)為是材料恢復(fù)初值時(shí)落后于前者。這種影響在不含鐵的材料中經(jīng)常提到。 鐵和鎳有明顯的彈性范圍，而銅、鋅、錫等，即使在相對(duì)低的應(yīng)力下也表現(xiàn)為不完全彈性。實(shí)際上，能否清楚地分辯彈性極限和