調(diào)速杠桿(135調(diào)速器)的機(jī)械工藝規(guī)程和夾具設(shè)計(jì)【鏜孔夾具】

資源目錄里展示的全都有，所見即所得。下載后全都有,請(qǐng)放心下載。原稿可自行編輯修改=【QQ：401339828 或11970985 有疑問可加】 畢業(yè)設(shè)計(jì)(論文)外文資料翻譯 系　 部： 專 業(yè)： 姓 名： 學(xué) 號(hào)： (用外文寫) 外文出處： Design of machine elements 附 件：1.外文資料翻譯譯文；2.外文原文。 指導(dǎo)教師評(píng)語： 譯文基本能翻譯表達(dá)出原文的內(nèi)容，條理較為分明，語句基本通順，總體譯文質(zhì)量尚可，但少數(shù)專業(yè)術(shù)語翻譯不夠準(zhǔn)確，一些語句比較生硬。 簽名： 年 月 日 附件1：外文資料翻譯譯文 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，毫無疑問，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)材料通過試驗(yàn)測(cè)定的物理性能。國際上的設(shè)計(jì)方法試圖通過從一些相對(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： ①最初的設(shè)計(jì)理念 ②受力分析 ③材料的選擇 ④外形 ⑤制造 ⑥安全性 ⑦環(huán)境影響 ⑧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： ①漸變力 ②瞬時(shí)力 ③沖擊力 ④不斷變化的力 ⑤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(cè)并改正過來。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： ①應(yīng)力 ②應(yīng)變 ③外形 ④腐蝕 ⑤震動(dòng) ⑥外部環(huán)境破壞 ⑦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，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(chǔ)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比例極限取決于測(cè)量設(shè)備的靈敏度。 當(dāng)負(fù)載超過彈性極限時(shí)，塑性變形開始，逐漸的試件被硬化。變形比負(fù)載增加得更快時(shí)的點(diǎn)被稱成為屈服點(diǎn)Q。金屬開始抵抗負(fù)載轉(zhuǎn)變成快速變形，這時(shí)的屈服力成為屈服極限Ay。 試件的延伸率 繼續(xù)由Q到T再到，在這種塑性流動(dòng)時(shí)，應(yīng)力—應(yīng)變關(guān)系在曲線上處于QRST區(qū)域。在點(diǎn)，試件破壞且這種負(fù)載稱為破壞負(fù)載。最大負(fù)載S除以試件初始的截面積，被定義為這種金屬的最終拉伸極限或試樣的拉伸強(qiáng)度Au。 按邏輯說，在應(yīng)力不增加的情況下，一旦超出彈性極限，金屬開始屈服，并最終破壞。但是當(dāng)超出彈性極限后，在紀(jì)錄曲線上應(yīng)增大。 這種變化主要有兩個(gè)原因： ①材料的應(yīng)力硬化 ②由于塑性變形而引起的試件橫截面積的變小 由于加工硬化，金屬塑性變化越大，硬化越嚴(yán)重。金屬拉伸越長，他的直徑（橫截面積）越小。直到到達(dá)點(diǎn)為止。點(diǎn)之后，減少的速率開始變化，超過了應(yīng)力增加的速率，應(yīng)變很大以至于在局部的某些點(diǎn)的面積減少，被稱為頸縮。橫截面積減少得非?？?，以至于抗負(fù)載的能力下降，即ST階段。破壞發(fā)生在T點(diǎn)。延伸率A和截面積變化率u被描述成材料的延展性和塑性： a=(L0-L)/L0*100% u=(A0-A)/A0*100% 在這里，L0和L分別是試件的最初和最終長度，A0和A分別是試件的最初截面積和最終截面積。 質(zhì)量保證與控制 產(chǎn)品質(zhì)量是生產(chǎn)中最重要的。如果放任質(zhì)量惡化下去，生產(chǎn)者會(huì)很快發(fā)現(xiàn)銷售量銳減，可能從而會(huì)導(dǎo)致產(chǎn)業(yè)的失敗。用戶期望他們買的產(chǎn)品質(zhì)量性能好，而且如果制造商想建立并維持其信譽(yù)，必須在產(chǎn)品制造前制造過程中及制造過程后進(jìn)行質(zhì)量控制和質(zhì)量保證。一般來說，質(zhì)量保證包括所有的活動(dòng)，其包括質(zhì)量建立和質(zhì)量控制。質(zhì)量保證可以被分為三個(gè)主要領(lǐng)域，他們?nèi)缦滤觯? ①制造之前的原材料的檢查 ②在制造加工過程中的質(zhì)量控制 ③制造之后的質(zhì)量保證 生產(chǎn)制造后的質(zhì)量控制包括保證書和面對(duì)產(chǎn)品用戶的服務(wù)。 生產(chǎn)制造之前的原材料檢驗(yàn) 質(zhì)量保證常常在實(shí)際生產(chǎn)制造之前就開始了。這些都是生產(chǎn)者在供應(yīng)原材料、散件或配件的車間里進(jìn)行檢驗(yàn)。生產(chǎn)制造公司的原材料檢驗(yàn)員到供應(yīng)廠并且檢查原材料及于制造的另配件。原材料檢驗(yàn)為生產(chǎn)者提供了一次機(jī)會(huì)，那就是在原料及散件被運(yùn)到生產(chǎn)車間之前先進(jìn)行挑選淘汰。原料檢察員的責(zé)任是去檢查原料和零件是否達(dá)到設(shè)計(jì)規(guī)格并且淘汰那些未達(dá)到特殊指標(biāo)的原料。原料檢驗(yàn)有很多于檢查產(chǎn)品相同的檢驗(yàn)。其如下所述： ①目測(cè) ②冶金測(cè)試 ③尺寸測(cè)試 ④損傷檢驗(yàn) ⑤性能檢驗(yàn) 目測(cè) 目測(cè)檢驗(yàn)一種產(chǎn)品或原料的某些特征，如顏色、紋理、表面光潔度或部件的總體外觀，從而判斷其是否具有明顯的缺損。 冶金測(cè)試 冶金測(cè)試常常是原料間嚴(yán)厲的一個(gè)很重要的部分，尤其是像棒料、建筑材料毛坯一些原材料。金屬測(cè)試包含所有主要的檢驗(yàn)類型，其中有目測(cè)，化學(xué)檢驗(yàn)，光譜檢驗(yàn)和機(jī)械性能檢驗(yàn)，其包括硬度、伸縮性能、剪切性能、壓縮性能和合成成分的光譜分析。冶金測(cè)試既可用于成品件也可用于預(yù)制件。 尺寸檢驗(yàn) 質(zhì)量控制的一些領(lǐng)域是重要的生產(chǎn)件的要求尺寸。尺寸在檢驗(yàn)過程中，像其在生產(chǎn)過程中一樣重要。如果這些零件是為總成供應(yīng)的，那尺寸尤其嚴(yán)格。一些尺寸在生產(chǎn)車間用標(biāo)準(zhǔn)測(cè)量工具進(jìn)行檢驗(yàn)，像特種接頭、造型和需求的功能標(biāo)準(zhǔn)度量。符合尺寸規(guī)格對(duì)所制造多部件的互換性和對(duì)多部件成功組裝成復(fù)雜的裝置，如汽車、輪船、飛機(jī)和其他多部件產(chǎn)品都地極其重要的。 損傷檢驗(yàn) 在一些情況下，對(duì)原材料或零部件采取損傷測(cè)試的原始測(cè)驗(yàn)是很必要的。特別是涉及到大批的原材料時(shí)。例如，在被運(yùn)到生產(chǎn)車間作最終機(jī)器之前，對(duì)鑄件進(jìn)行X-射線、電磁離子、染色滲透劑技術(shù)的探傷是很必要的，又對(duì)機(jī)器總成的電子或持久運(yùn)作測(cè)試而確定的規(guī)格，是無損測(cè)試的又一例證。有時(shí)，對(duì)材料及零件的測(cè)試是很必要的，但由于無損測(cè)試的花費(fèi)和成本及時(shí)間不是任何時(shí)候都允許的。 例如，有壓力測(cè)試決定在設(shè)計(jì)中其是否安全。損傷測(cè)試經(jīng)常用于設(shè)計(jì)樣機(jī)的測(cè)試，而不是原材料或零件的常規(guī)檢驗(yàn)。一旦設(shè)計(jì)達(dá)到了所希望的材料強(qiáng)度，通常對(duì)零件做進(jìn)一步的損傷測(cè)試是不必要的，除非他們確實(shí)存在疑點(diǎn)。 性能測(cè)試 性能測(cè)試在零部件被其他產(chǎn)品被安裝之前，檢查部件的功能，尤其是那些機(jī)械構(gòu)造復(fù)雜的部件。例如電子設(shè)備零件，飛機(jī)和汽車發(fā)動(dòng)機(jī)，泵、閥及其他需要在裝運(yùn)和最后安裝前進(jìn)行性能測(cè)驗(yàn)的機(jī)械系統(tǒng)。 附件2：外文原文（復(fù)印件） 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: ① Initial design conception; ② Strength analysis; ③ Materials selection; ④ Appearance; ⑤ Manufacturing; ⑥ Safety; ⑦ Environment effects; ⑨ Reliability and life; Strength is a measure of the ability to resist, without fails, forces which cause stresses and strains. The forces may be; ① Gradually applied; ② Suddenly applied; ③ Applied under impact; ④ Applied with continuous direction reversals; ⑤ 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: ① Stress; ② Deformation; ③ Wear; ④ Corrosion; ⑤ Vibration; ⑥ Environmental damage; ⑦ 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 layman’s 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 hookes’s 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: ①The strain hardening of the material; ②The 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: ①Source and receiving inspection before manufacturing; ②In-process quality control during manufacturing; ③Quality 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 manufacturer’s 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 manufacturer’s 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: ①Visual inspection; ②Metallurgical testing; ③Dimensional inspection; ④Destructive and nondestructive inspection; ⑤Performance 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.