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1、Unit 2 Some Rules for Mechanical DesignContentslNew Words & Expressions lText & TranslationlComplex Sentence AnalysislKey to ExercisesNew Words & Expressionslapparatus preits n. 装置,设备,器具ldurability djurbiliti n. 耐久性,耐用性loptimum utilization 最适度利用linertia in:i n. 惯性llubrication lu:brikein n. 润滑，润滑作用lh
2、ydrodynamic haidrudainmik adj. 流体动力学的;水力的lsales appeal 销售吸引力lintimately intimitli adv. 密切地New Words & Expressionslbuoyancy binsi n. 浮力lcentrifugal force 离心力lincline inklain n. & v. 倾斜lprinciple of lever and inclined plane 杠杆原理和斜面原理lthermal expansion 热膨胀 ldetrimental detrimentl adj. 有害的lfluctuating s
3、tress 交变应力lstress concentration 应力集中lfillet filit n. 倒角lcrank krk n. 曲柄，曲轴New Words & Expressionslcam km n. 凸轮lgear shaft 齿轮轴lresonant vibration 共振ldeterminable dit:minbl adj. 可决定的, 可确定的lpertinent p:tinnt adj. 有关的, 相关的lweldability weld bilti n. 焊接性lliability laibiliti n. 责任, 义务lcompactness kmpktnis
4、n. 紧密, 紧密度, 体积小, 小型linterference intfirns n. 冲突,干涉ltapped hole 螺纹孔Text & TranslationlThe 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 functio
5、n previously done by men，such as 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 freeh
6、and sketching can record ones thoughts and is helpful in communicating your ideas to others. It can also visualize ones ideas，thus stimulating his creativity.Text & TranslationlWhen the general shape and a few dimensions of the several components become apparent，the relevant analysis can be commence
7、d. The analysis will have as its objective 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 comp
8、onents. Materials and their treatment 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 elasti
9、city and strength of materials for stress and deflection；and of fluid mechanics for lubrication and hydrodynamic drives.Text & TranslationlFinally，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 produc
10、ed in quantity designers should undergo 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 in
11、dicate alterations. Sales appeal，customer satisfaction，and manufacture cost are all related to design，and ability in design is intimately involved in the success of an engineering innovation.Text & TranslationlThe following tips may be helpful in stimulating creative ideas of the designers.l（1）Apply
12、 ingenuity to utilize desired physical 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，frict
13、ion，viscosity，fluid pressure，and thermal 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
14、 at the clutch face but not in the clutch 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 & Translationl（2）Provide for favorable stress distribute and stiffness with minimum weight. On
15、components subjected to fluctuating stress，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
16、 rolling and shallow hardening. Hollow 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 encl
17、osures and frames containing bearing supports. The stiffness of shafts and other components must be suitable to avoid resonant vibrations.Text & Translationl（3）Use basic equations to calculate and optimize dimensions. The fundamental equations of mechanics and the other sciences are the accepted bas
18、es for calculations. These fundamental 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 no
19、t analytically determinable，e.g.，on thin 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 giv
20、es results “on the safe side”. In situations where data are incomplete，equations of the sciences may be used as proportioning guides to extend a satisfactory design to new capacities.Text & Translationl（4）Choose materials with the consideration of the combination of different properties. Materials s
21、hould be chosen for a combination of pertinent 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
22、t e m p e r a t u r e s， a n d r e q u i r e d c o a t i n g .Text & Translationl（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 performa
23、nce and reliability requirements and is adaptable without additional 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
24、if there is a failure in any one of the machines parts. In other cases，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 & Translationl（6）Provide for accurate location and non-inte
25、rference of parts in assembly. A good design provides for the correct 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.
26、 Interferences，as between screws in tapped holes，and between linkages 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 & Translationl设计是从实际或者假想的
27、需要开始的。对于现有的设备可能需要在耐用性、效率、质量、速度或成本等方面做进一些改进工作；也可能需要新的设备完成以前由人来做的工作, 例如估算、装配或维修。当目标完全或部分被确定以后,下一个设计步骤是对能够完成所需要功能的机构及其布局进行总体设计。对于此项工作, 徒手画的草图是很有价值的, 它不仅可以记录下我们的想法,而且还有助于与别人进行讨论,特别是和自己的大脑进行交流, 从而促进创新想法的产生。Text & Translationl当一些零件的大致形状和几个尺寸被确定后,就可以开始认真地分析工作。分析工作的目的是要在重量最轻、成本最低的情况下,获得令人满意的,即优良的工作性能, 并且还要安
28、全耐用。 对于每个关键承载截面,应该寻求最佳的比例和尺寸,同时要对这几个零件的受力进行平衡，要对材料和处理方式进行选择。只有根据力学原理进行分析才能达到这些重要目的。这些分析包括根据静力学原理分析反作用力和摩擦力, 根据动力学原理分析关于惯性、加速度和能量；根据弹性力学和材料力学分析应力和变形；根据流体力学分析润滑和流体传动。Text & Translationl最后,完成基本功能要求和可靠性所进行的设计, 且要制作一台样机。如果试验结果令人满意,而且该装置将要进行批量生产,就应该对最初提出的设计方案做一些修改, 使其能以较低的成本进行批量生产。在以后的制造和使用期内, 如果产生了新的想法或者
29、根据试验和经验所做的进一步分析结果表明,可以有更好的替代方案,则很可能对原设计方案进行修改。销售吸引力、客户的满意程度和制造成本均与设计有关,而设计能力则与工程创新的实现是密切相关的。Text & Translationl为激发创造性思维,建议设计人员遵循下列准则。l（）创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。可以利用自然法则或物质的性能（例如柔性、强度、重力、惯性、浮力、离心力、杠杆原理和斜面原理、摩擦、粘性、流体压力和热膨胀）和许多电学、光学和化学现象来满足一台机器的设计要求。一种性能在某种场合下可能是有用的, 而在另外一种场合下则可能是有害的。阀门的弹簧应该有柔性,而阀门的
30、凸轮轴就不需要柔性。离合器结合面上需要有摩擦,而离合器轴承却不需要摩擦。设计时, 需要创造性地利用和控制所要的物理性能,将不需要的物理性能减至最小。Text & Translationl（）在质量最轻的情况下，提供合理的应力分布和刚度。对于承受交变应力的零件,应该特别注意减轻应力集中和提高圆角,螺纹和配合处的强度。改变零件的形状,可以降低它所承受的应力,对零件施加预应力,如表面滚压和浅表面硬化,均可使其得到强化。空心轴和空心管道,箱形截面能获得有力的应力分布,同时具有强度高而重量最轻的特点。曲轴,凸轮轴以及含有轴承支座的外壳和构架都应有足够的刚度以保证直线对中精度和接触表面之间的压力均匀分布。
31、轴和其他零件须有适当的刚度,避免产生共振。Text & Translationl（3）利用基本公式进行尺寸计算和尺寸优化。力学和其他学科的基本公式是公认的计算依据。有时需要将这些公式进行移项而化成特殊形式,以简化尺寸的计算或者对尺寸进行优化。例如,用梁的表面应力公式来计算齿轮的轮齿尺寸。在不能采用解析法计算的情况下,可以在基本公式内引入系数。例如,对于薄壁钢管,考虑到腐蚀性,可将根据压力求得的厚度增加一些。当必须用一个基本公式来确定形状、材料和使用条件,而这些被确定的量仅仅与在公式推导中的假设比较接近时,要采取措施使结果“偏于安全”。当数据不完全时,可以应用理论公式作为尺寸的指南,在扩展后的范
32、围内获得令人满意的设计结果。Text & Translationl（）根据性能组合选择材料。选择材料时需要考虑有关的性能组合,不仅考虑强度,硬度和重量,而且有时还要考虑抗冲击性,抗腐蚀性和耐高温或低温的能力。成本和制造性能应该是考虑的因素, 这些因素包括可焊接性,机械加工性能,对热处理温度变化的敏感性和所需要的涂层等。Text & Translationl（）在现有零件和整体零件之间进行认真的选择。若一个以前研制的零件能够满足性能要求和可靠性要求,并适用于所设计的那台机器而无须附加的研制费用,那么设计人员及其公司通常会从零件制造厂的现货中选取该零件。但是，只有充分了解其性能，才能进行认真的选择
33、工作，因为任何一个机器零件的失效都会影响公司的信誉，并使公司承受相应的责任。在其他情况下，若机器设计人员自己来设计零件，则零件的强度,可靠性和成本等方面的要求就可以更好地得到满足。可将某个零件与其他零件设计成一个整体零件，例如，将几个齿轮设计为一个锻件或者将齿轮与轴设计为一体，这种方法的主要优点是紧凑。Text & Translationl（）保证零件在装配中准确定位和不发生干涉。一个良好的设计能够保证零件定位准确,装配和修理方便容易。轴肩和导向表面在装配过程中不需要测量就能够提供准确定位。零件的形状应该被设计得保证这个零件不会被装反或装错位置。必须能够预见和防止诸如不同的螺纹孔中的螺钉之间的
34、干涉和不同的连杆机构之间的干涉。必须避免部件之间的找正对中误差和定位误差，或者必须采取措施，减小任何由此引起的不利的位移和应力。Complex Sentence Analysisl1With 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. 当目标完全或部分被确定以后,下一个设计步骤是对能够完成所需要功能的机构及其布局进行总体设计
35、。l句中 “With the objective wholly or partly defined” 是独立主格成分，充当条件状语l句中 “that will perform the needed functions”是限制性定语从句，修饰先行词 “mechanisms and their arrangements”。 译为：能够完成所需要功能的机构及其布局Complex Sentence Analysisl2These important objectives can be attained only by analysis based upon the principles of mech
36、anics, 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; and of fluid mechanics for lubrication and hydrodynamic drives. 只有根据力学原理进行分析才能达到这些重要目
37、的。这些分析包括根据静力学原理分析反作用力和充分利用摩擦力, 根据动力学原理分析惯性、加速度和能量；根据弹性力学和材料力学分析应力和变形；根据流体力学分析润滑和流体传动等。l句中 “based upon the principles of mechanics” 为过去分词短语作后置定语，修饰analysis, 可以译为：根据力学原理进行的分析。l 在“such as those of statics for reaction forces and for the optimum utilization of friction; of dynamics for inertia, accelera
38、tion, and energy; of elasticity and strength of materials for stress and deflection; and of fluid mechanics for lubrication and hydrodynamic drives”句中, those后面跟了四个并列的介词短语成分, 分别是 “of statics for reaction forces and for the optimum utilization of friction”; “of dynamics for inertia, acceleration, and
39、energy”; “of elasticity and strength of materials for stress and deflection”; 以及 “of fluid mechanics for lubrication and hydrodynamic drives”。Complex Sentence Analysisl3The performance requirements of a machine are met by utilizing laws of nature or properties of matter (e. g., flexibility, strength
40、, 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. 可以利用自然法则或物质的性能（例如柔性、强度、重力、惯性、浮力、离心力、杠杆原理和斜面原理、摩擦、黏性、流体压力和热膨胀性）和许多电学、光学和化学现象来满足一台机器
41、的设计要求。l句中 “by utilizing laws of nature or properties of matter”是由介词by引起的动名词短语，表示手段和方法。Complex Sentence Analysisl4When 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 resu
42、lts “on the safe side”. 当必须用一个基本公式来确定形状、材料和使用条件,而这些被确定的量仅仅与在公式推导中的假设比较接近时, 要采取措施使结果“偏于安全”。l句中 “which only approximate the assumptions for its derivation”是由which引导的限制性定语从句，修饰先行词shapes, materials, or conditions。l which gives results “on the safe side” 也是限制性定语从句，修饰先行词manner，短语on the safe side是安全可靠的意思。K
43、ey to Exercisesl. Translate the following phrases into Chinese or English.l1.最适度利用l2.sales appeal l3.地心引力l4.principle of lever and inclined plane l5.热膨胀l6.fluctuating stress l7.应力集中l8.gear shaft l9.共振l10.tapped hole Key to Exercisesl. Mark the following statement with True (T) or False (F) according
44、 to the text.l1. Fl2. Fl3. Tl4. Fl5. FKey to Exercisesl. Fill in the blanks with the suitable words or phrases given below.l1. fundamentall2. facilitatel3. surface stressl4. determinablel5. in a mannerl6. on the safe sideKey to Exercisesl.Translate the Chinese parts given in the brackets into English.l1. superior performancel2. based upon the principles of mechanicsl3. undergo certain modificationsl4. by utilizing laws of nature or properties of matterl5. resistance to impact, corrosion, and low or high temperatureslThe end!
