Some Rules of Mechanical Design
Designing starts with a need ,real or imagined.Existing apparatus may need some improvements in durability,efficiently,weight,speed,or cost.New apparatus may be needed to perform a function previously done by men,such as computation,assembly,or servicing.With 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 ,freehand sketching is of great value ,not only as a record of one’s thoughts and as an aid in discussion with others ,but particularly for communication with one’s own mind ,as a stimulant for creative ideas.
When the general shape and a few dimensions of the several components become apparent ,analysis can begin in earnest.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 components. Materials and their treatment will be chosen.These important objectives can be attained only by analysis based upon the principles of friction ;of dynamics for inertia ,acceleration,and energy;of elasticity 。
Of years 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 altertions .Sales appeal,customer satisfaction,and manufacture cost are all related to design ,and ability ,and ability in design is intimately involved in the success of an engineering venture .
To stimulate creative thought ,the following rules are suggested for the designer .
1.Apply ingenuty to utilize desired physical properties and to control undeired ones .The performance requiement 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 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.Flexibility in the valve springs but not in the valve camshaft ;friction is deired at the cluch face but not in the cluch bearing.Ingenuity in design should be applied to utilize and control the phsical proerties that are desired and to minimize those that are not deired .
2.Provide for favorable stress distribute and stiffiness with minimum weight.Oncomponents subjeted to fluctuating stress,particlar attention is given a reduction in stress concentration ,and to an increase of strength at fillets ,theads,holes ,and fits.Stress reduction are made by modification in shape ,and strengthening may be done .by prestressing treatments such as surface rolling and shallow hardening.Hollow shafts and tubing and box sections five a favorable stress distribution,together with stiffness and minimum weight .Sufficient stiffiness to maintain alignment and uniform pressure between contacting surfaces should be provided to crank ,cam ,and gear shafts and for enclosures.and frames containing bearing supports .The stiffiness of shafts and other components must be suitable to avoid resonant vibrations.
3.Use basic equations to calculate and optimize dimensions .The fundamental equations of mechanics and the other sciences are the accepted bases for calculations They are sometimes rearrannged in special forms to facilitate the determination or ptimization pf dimensions ,such as the beam and surface stress equations for determination for determining gear-tooth size .Factors may be added to a fundamental equation for conditions not analtically deteminable .e.g,on thin steed tubes an allowance for corrosion added to the thickness based on pressure . On thin steel tubes ,an allowance for corrosion added to the thickness based on pressure.When it is necessary to apply a fundamental equation to shapes ,materials,or conditions which only approximate the assumptions for its derivation,it is sone in a mannerwhich gives results on the safe side .In situations where data are incomplete ,equations of the science may be used as propotioning guides to extend a satisfactory design to new capacities .
4.Choose material for a combination of properties.Materials should be chosen for a combination of pertinent properties ,not only for stengths ,hardness,and weight,but sometimes for resistance to impact ,corrosion, and low or high temperatures Cost and fabrication properties are factors ,such as weldability,machinability,sensitivity to variation in heat-treating temperatures ,and required coating.
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 manufactures,if the important meets the performance and reliability requirements and is adaptable without additional development costs to the particular machine being designed.However ,its selection should be carefully made with a full knowledge of its properties ,since the reputation and liability of the company suffer if there is a failure in any one of the machine`s parts .In other cases the strength ,reliability,and cost requirement are better met if the designer of the macine also designs the component , with the particular advantage of compactness if it designs integral with other components ,e.g.,gears to be forged in clusters or integral with a shaft .
6.Provide for accurate location and non-interence 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 Shapers can be designed so that parts cannot be assembled backwards or in the wrong place .Interference ,as between screws in tapped holes ,and between linkage must be avoided ,or provision must be made to minimize any resulting detrimental displacements and stresses.
机械设计准则
设计是从实际或者假想的需要开始的。对于现有的设备可能需要在耐用性，效率，重量，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件，例如，计算或者装配。当目标完全或部分被确定以后，下一个设计步骤是对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，他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。
当一些零件的大致形状和几个尺寸确定后，就可以开始认真的分析工作。分析工作的目的是要在重量最轻，成本最低的情况下，获得令人满意及优良的工作性能，并且还要安全耐用。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。这些分析包括静力学原理进行分析反作用力和充利用分摩擦力，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应变；根据流体力学分析润滑和流体传动。
最后，完成基于功能要求和可靠性所进行的设计，且要制作一台样机。如果实验结果令人满意，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根据实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。
为激发创造性思维，建议设计人员遵循下列准则。
1.创造性的利用所需要的物理性能和控制不需要的物理性能。
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。阀门的弹簧应该有弹性，阀门的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，需要创造性的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。
2.载重量最轻的情况下，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，箱型截面能获得有力的应力分布同时具有强度高而重量轻的特点。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。轴和其他零件须有适当的刚度避免产生共振。
3. 利用基本公式进行尺寸计算和尺寸优化。
力学和其他的基本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，可将根据求的厚度增加一些。当必须应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，要采取措施是结果偏于安全。当数据不完全时，可以应用理论公式作为尺寸的指南，在扩展后的范围内获得令人满意的设计结果。
4.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。成本和制造性能都是应该考虑的因素，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图层等。
5.在现有零件和整体零件之间进行认真的选择。
若一个以前研制的文件能过满足性能要求和可靠性要求，并使用于所设计的那台机器而无须附加的研制费用，那么设计人员和公司通常会从零件制造厂的现货中选取零件。但是，只有了解其性能，才能进行认真的选择工作，因为任何一个机器零件的失效都会影响公司的信誉，并使公司承担相应的责任。在其他情况下，若及其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的得到满足。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。
6.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提供准确定位。零件的形状应该设计的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，或者必须采用措施，减小任何由此引起的不利的位移和应力。
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