機電專業(yè)英語Unit-2-Some-Rules-for-Mechanical-Design課件

1、Unit 2 Some Rules for Mechanical DesignContentsNew Words & Expressions Text & TranslationComplex Sentence AnalysisKey to ExercisesNew Words & Expressionsapparatus preits n. 裝置,設(shè)備,器具durability djurbiliti n. 耐久性,耐用性optimum utilization 最適度利用inertia in:i n. 慣性lubrication lu:brikein n. 潤滑，潤滑作用hydrodynami

2、c haidrudainmik adj. 流體動力學(xué)的;水力的sales appeal 銷售吸引力intimately intimitli adv. 密切地New Words & Expressionsbuoyancy binsi n. 浮力centrifugal force 離心力incline inklain n. & v. 傾斜principle of lever and inclined plane 杠桿原理和斜面原理thermal expansion 熱膨脹 detrimental detrimentl adj. 有害的fluctuating stress 交變應(yīng)力stress co

3、ncentration 應(yīng)力集中fillet filit n. 倒角crank krk n. 曲柄，曲軸New Words & Expressionscam km n. 凸輪gear shaft 齒輪軸resonant vibration 共振determinable dit:minbl adj. 可決定的, 可確定的pertinent p:tinnt adj. 有關(guān)的, 相關(guān)的weldability weld bilti n. 焊接性liability laibiliti n. 責任, 義務(wù)compactness kmpktnis n. 緊密, 緊密度, 體積小, 小型interferenc

4、e intfirns n. 沖突,干涉tapped hole 螺紋孔Text & TranslationThe purpose of design is to meet a kind of need，which can be real or imagined. Existing apparatus may need improvements in durability，efficiency，weight，speed，or cost. New apparatus may be needed to perform a function previously done by men，such as

5、computation，assembly，or servicing. 1With the objective wholly or partly defined，the next step in design is the conception of mechanisms and their arrangements that will perform the needed functions. For this purpose，freehand sketching is commonly adopted. Since freehand sketching can record ones tho

6、ughts and is helpful in communicating your ideas to others. It can also visualize ones ideas，thus stimulating his creativity.Text & TranslationWhen the general shape and a few dimensions of the several components become apparent，the relevant analysis can be commenced. The analysis will have as its o

7、bjective satisfactory or superior performance，plus safety and durability with minimum weight，and a competitive cost. Optimum proportions and dimensions will be sought for each critically loaded section，together with a balance between the strength of the several components. Materials and their treatm

8、ent will be chosen. 2These important objectives can be attained only by analysis based upon the principles of mechanics，such as those of statics for reaction forces and for the optimum utilization of friction；of dynamics for inertia，acceleration，and energy；of elasticity and strength of materials for

9、 stress and deflection；and of fluid mechanics for lubrication and hydrodynamic drives.Text & TranslationFinally，a design based upon function and reliability will be completed，and a prototype may be built. If its tests are desirable，and if the device is to be produced in quantity designers should und

10、ergo certain modifications that enable it to be manufactured in quantity at a lower cost. During subsequent years of manufacture and service，the design is likely to undergo changes as new ideas are conceived or as further analysis based upon tests and experience indicate alterations. Sales appeal，cu

11、stomer satisfaction，and manufacture cost are all related to design，and ability in design is intimately involved in the success of an engineering innovation.Text & TranslationThe following tips may be helpful in stimulating creative ideas of the designers.（1）Apply ingenuity to utilize desired physica

12、l properties and to limit undesired ones. 3The performance requirements of a machine are met by utilizing laws of nature or properties of matter (e.g.，flexibility，strength，gravity，inertia，buoyancy，centrifugal force，principles of the lever and inclined plane，friction，viscosity，fluid pressure，and ther

13、mal expansion)，also the many electrical，optical，thermal，and chemical phenomena. However，what may be useful in one application may be detrimental in the next. For example，flexibility is desirable in valve springs but not in the valve camshaft；friction is desirable at the clutch face but not in the cl

14、utch bearing. Ingenuity in design should be applied to utilize and control the physical properties that are desirable and to minimize those that are not desirable.Text & Translation（2）Provide for favorable stress distribute and stiffness with minimum weight. On components subjected to fluctuating st

15、ress，particular attention is given to a reduction in stress concentration，and to an increase of strength at fillets，threads，holes，and fits. Stress reduction is made by modification in shape，and strengthening may be done by prestressing treatments such as surface rolling and shallow hardening. Hollow

16、 shafts and tubing，and box sections give a favorable stress distribution，together with stiffness and minimum weight. Sufficient stiffness to maintain alignment and uniform pressure between contacting surfaces should be provided for crank，cam，gear shafts，and enclosures and frames containing bearing s

17、upports. The stiffness of shafts and other components must be suitable to avoid resonant vibrations.Text & Translation（3）Use basic equations to calculate and optimize dimensions. The fundamental equations of mechanics and the other sciences are the accepted bases for calculations. These fundamental

18、equations sometimes may be rearranged in special orders to facilitate the determination or optimization of dimensions，such as the beam and surface stress equations for determining gear tooth size. Factors may be added to a fundamental equation for conditions not analytically determinable，e.g.，on thi

19、n steel tubes，an allowance for corrosion added to the thickness based on pressure. 4When it is necessary to apply a fundamental equation to shapes，materials，or conditions which only approximate the assumptions for its derivation，it is done in a manner which gives results “on the safe side”. In situa

20、tions where data are incomplete，equations of the sciences may be used as proportioning guides to extend a satisfactory design to new capacities.Text & Translation（4）Choose materials with the consideration of the combination of different properties. Materials should be chosen for a combination of per

21、tinent properties，not only for strengths，hardness，and weight，but sometimes for resistance to impact，corrosion，and low or high temperatures. Cost and fabrication properties are factors，such as weldability，machinabitity，sensitivity to variation in heat treating temperatures，and required coating.Text &

22、 Translation（5）Select carefully between stock and integral components. A previously developed component is frequently selected by a designer and his company from the stocks of parts manufacturers，if the component meets the performance and reliability requirements and is adaptable without additional

23、development costs to the particular machine being designed. However，the selection of these components should be carefully made with a full understanding of their properties，since the reputation and liability of the company suffers if there is a failure in any one of the machines parts. In other case

24、s，the strength，reliability，and cost requirements are better met if the designer of the machine also designs the component，with the particular advantage of compactness.Text & Translation（6）Provide for accurate location and non-interference of parts in assembly. A good design provides for the correct

25、locating of parts and for easy assembly and repair. Shoulders and pilot surfaces give accurate location without measurement during assembly. Shapes can be designed so that parts cannot be assembled backwards or in the wrong place. Interferences，as between screws in tapped holes，and between linkages

26、must be foreseen and prevented. Inaccurate alignment and positioning between such assemblies must be avoided，or provision must be made to minimize any resulting detrimental displacements and stresses.Text & Translation設(shè)計是從實際或者假想的需要開始的。對于現(xiàn)有的設(shè)備可能需要在耐用性、效率、質(zhì)量、速度或成本等方面做進一些改進工作；也可能需要新的設(shè)備完成以前由人來做的工作, 例如估算

27、、裝配或維修。當目標完全或部分被確定以后,下一個設(shè)計步驟是對能夠完成所需要功能的機構(gòu)及其布局進行總體設(shè)計。對于此項工作, 徒手畫的草圖是很有價值的, 它不僅可以記錄下我們的想法,而且還有助于與別人進行討論,特別是和自己的大腦進行交流, 從而促進創(chuàng)新想法的產(chǎn)生。Text & Translation當一些零件的大致形狀和幾個尺寸被確定后,就可以開始認真地分析工作。分析工作的目的是要在重量最輕、成本最低的情況下,獲得令人滿意的,即優(yōu)良的工作性能, 并且還要安全耐用。 對于每個關(guān)鍵承載截面,應(yīng)該尋求最佳的比例和尺寸,同時要對這幾個零件的受力進行平衡，要對材料和處理方式進行選擇。只有根據(jù)力學(xué)原理進行分析

28、才能達到這些重要目的。這些分析包括根據(jù)靜力學(xué)原理分析反作用力和摩擦力, 根據(jù)動力學(xué)原理分析關(guān)于慣性、加速度和能量；根據(jù)彈性力學(xué)和材料力學(xué)分析應(yīng)力和變形；根據(jù)流體力學(xué)分析潤滑和流體傳動。Text & Translation最后,完成基本功能要求和可靠性所進行的設(shè)計, 且要制作一臺樣機。如果試驗結(jié)果令人滿意,而且該裝置將要進行批量生產(chǎn),就應(yīng)該對最初提出的設(shè)計方案做一些修改, 使其能以較低的成本進行批量生產(chǎn)。在以后的制造和使用期內(nèi), 如果產(chǎn)生了新的想法或者根據(jù)試驗和經(jīng)驗所做的進一步分析結(jié)果表明,可以有更好的替代方案,則很可能對原設(shè)計方案進行修改。銷售吸引力、客戶的滿意程度和制造成本均與設(shè)計有關(guān),而設(shè)

29、計能力則與工程創(chuàng)新的實現(xiàn)是密切相關(guān)的。Text & Translation為激發(fā)創(chuàng)造性思維,建議設(shè)計人員遵循下列準則。（）創(chuàng)造性利用所需要的物理性能和控制不需要的物理性能功能。可以利用自然法則或物質(zhì)的性能（例如柔性、強度、重力、慣性、浮力、離心力、杠桿原理和斜面原理、摩擦、粘性、流體壓力和熱膨脹）和許多電學(xué)、光學(xué)和化學(xué)現(xiàn)象來滿足一臺機器的設(shè)計要求。一種性能在某種場合下可能是有用的, 而在另外一種場合下則可能是有害的。閥門的彈簧應(yīng)該有柔性,而閥門的凸輪軸就不需要柔性。離合器結(jié)合面上需要有摩擦,而離合器軸承卻不需要摩擦。設(shè)計時, 需要創(chuàng)造性地利用和控制所要的物理性能,將不需要的物理性能減至最小。T

30、ext & Translation（）在質(zhì)量最輕的情況下，提供合理的應(yīng)力分布和剛度。對于承受交變應(yīng)力的零件,應(yīng)該特別注意減輕應(yīng)力集中和提高圓角,螺紋和配合處的強度。改變零件的形狀,可以降低它所承受的應(yīng)力,對零件施加預(yù)應(yīng)力,如表面滾壓和淺表面硬化,均可使其得到強化。空心軸和空心管道,箱形截面能獲得有力的應(yīng)力分布,同時具有強度高而重量最輕的特點。曲軸,凸輪軸以及含有軸承支座的外殼和構(gòu)架都應(yīng)有足夠的剛度以保證直線對中精度和接觸表面之間的壓力均勻分布。軸和其他零件須有適當?shù)膭偠?避免產(chǎn)生共振。Text & Translation（3）利用基本公式進行尺寸計算和尺寸優(yōu)化。力學(xué)和其他學(xué)科的基本公式是公認的

31、計算依據(jù)。有時需要將這些公式進行移項而化成特殊形式,以簡化尺寸的計算或者對尺寸進行優(yōu)化。例如,用梁的表面應(yīng)力公式來計算齒輪的輪齒尺寸。在不能采用解析法計算的情況下,可以在基本公式內(nèi)引入系數(shù)。例如,對于薄壁鋼管,考慮到腐蝕性,可將根據(jù)壓力求得的厚度增加一些。當必須用一個基本公式來確定形狀、材料和使用條件,而這些被確定的量僅僅與在公式推導(dǎo)中的假設(shè)比較接近時,要采取措施使結(jié)果“偏于安全”。當數(shù)據(jù)不完全時,可以應(yīng)用理論公式作為尺寸的指南,在擴展后的范圍內(nèi)獲得令人滿意的設(shè)計結(jié)果。Text & Translation（）根據(jù)性能組合選擇材料。選擇材料時需要考慮有關(guān)的性能組合,不僅考慮強度,硬度和重量,而且

32、有時還要考慮抗沖擊性,抗腐蝕性和耐高溫或低溫的能力。成本和制造性能應(yīng)該是考慮的因素, 這些因素包括可焊接性,機械加工性能,對熱處理溫度變化的敏感性和所需要的涂層等。Text & Translation（）在現(xiàn)有零件和整體零件之間進行認真的選擇。若一個以前研制的零件能夠滿足性能要求和可靠性要求,并適用于所設(shè)計的那臺機器而無須附加的研制費用,那么設(shè)計人員及其公司通常會從零件制造廠的現(xiàn)貨中選取該零件。但是，只有充分了解其性能，才能進行認真的選擇工作，因為任何一個機器零件的失效都會影響公司的信譽，并使公司承受相應(yīng)的責任。在其他情況下，若機器設(shè)計人員自己來設(shè)計零件，則零件的強度,可靠性和成本等方面的要求

33、就可以更好地得到滿足。可將某個零件與其他零件設(shè)計成一個整體零件，例如，將幾個齒輪設(shè)計為一個鍛件或者將齒輪與軸設(shè)計為一體，這種方法的主要優(yōu)點是緊湊。Text & Translation（）保證零件在裝配中準確定位和不發(fā)生干涉。一個良好的設(shè)計能夠保證零件定位準確,裝配和修理方便容易。軸肩和導(dǎo)向表面在裝配過程中不需要測量就能夠提供準確定位。零件的形狀應(yīng)該被設(shè)計得保證這個零件不會被裝反或裝錯位置。必須能夠預(yù)見和防止諸如不同的螺紋孔中的螺釘之間的干涉和不同的連桿機構(gòu)之間的干涉。必須避免部件之間的找正對中誤差和定位誤差，或者必須采取措施，減小任何由此引起的不利的位移和應(yīng)力。Complex Sentence

34、 Analysis1With the objective wholly or partly defined, the next step in design is the conception of mechanisms and their arrangements that will perform the needed functions. 當目標完全或部分被確定以后,下一個設(shè)計步驟是對能夠完成所需要功能的機構(gòu)及其布局進行總體設(shè)計。句中 “With the objective wholly or partly defined” 是獨立主格成分，充當條件狀語句中 “that will per

35、form the needed functions”是限制性定語從句，修飾先行詞 “mechanisms and their arrangements”。 譯為：能夠完成所需要功能的機構(gòu)及其布局Complex Sentence Analysis2These important objectives can be attained only by analysis based upon the principles of mechanics, such as those of statics for reaction forces and for the optimum utilization

36、of friction; of dynamics for inertia, acceleration, and energy; of elasticity and strength of materials for stress and deflection; and of fluid mechanics for lubrication and hydrodynamic drives. 只有根據(jù)力學(xué)原理進行分析才能達到這些重要目的。這些分析包括根據(jù)靜力學(xué)原理分析反作用力和充分利用摩擦力, 根據(jù)動力學(xué)原理分析慣性、加速度和能量；根據(jù)彈性力學(xué)和材料力學(xué)分析應(yīng)力和變形；根據(jù)流體力學(xué)分析潤滑和流體傳動

37、等。句中 “based upon the principles of mechanics” 為過去分詞短語作后置定語，修飾analysis, 可以譯為：根據(jù)力學(xué)原理進行的分析。 在“such as those of statics for reaction forces and for the optimum utilization of friction; of dynamics for inertia, acceleration, and energy; of elasticity and strength of materials for stress and deflection; a

38、nd of fluid mechanics for lubrication and hydrodynamic drives”句中, those后面跟了四個并列的介詞短語成分, 分別是 “of statics for reaction forces and for the optimum utilization of friction”; “of dynamics for inertia, acceleration, and energy”; “of elasticity and strength of materials for stress and deflection”; 以及 “of f

39、luid mechanics for lubrication and hydrodynamic drives”。Complex Sentence Analysis3The performance requirements of a machine are met by utilizing laws of nature or properties of matter (e. g., flexibility, strength, gravity, inertia, buoyancy, centrifugal force, principles of the lever inclined plane, friction, viscosity, fluid pressure, and thermal expansion), also the many electrical, optical, thermal, and chemical phenomena. 可以利用自然法則或物質(zhì)的性能（例如柔性、強度、重力、慣性、浮力、離心力、杠桿原理和斜面原理、摩擦、黏性、流體壓力和熱膨脹性）和許多電學(xué)、光學(xué)和化學(xué)現(xiàn)象來滿足一臺機器的設(shè)計要求。句中 “by utilizing laws of nature or properties of matter”是由介