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黄河科技学院毕业设计(文献翻译） 第 7 页轴、联轴器、材料的选择及传动方式June, 1992 by Keith Briere轴和联轴器实际上, 几乎所有的机器中都装有轴。 轴的最常见的形状是圆形,其截面可以是实心的,也可以是空心的(空心轴可以减轻重量).有时也采用矩形2轴,例如,螺丝起子的头部, 套筒扳手和控制旋钮的杆。 为了在传递扭矩时不发生过载，轴应该具有适当的抗扭矩度。轴还应该具有足够的抗扭刚度，以使在同一个轴上的两个传动零件之间的相对转角不会过大。一般说来，在长度等于轴的直径的202倍时，轴的扭转角不应该超过1度。轴安装在轴承中，通过齿轮，皮带轮，凸轮和离合器等零件传递动力。通过这些零件传来的力可能会使轴产生弯曲变形。因此，轴应该有足够的刚度以防止支撑轴承受力过大。总而言之，在两个轴承支撑之间，轴在每英尺长度上的弯曲变形不应该超过0.01英寸。 此外，轴还必须能够承受弯矩和扭矩的组合作用。因此。要考虑扭矩与弯矩的当量载荷。因为扭矩和弯矩会产生交变应力，在需用应力中也应该有一个考虑疲劳现象的安全系数。 直径小于3英寸的轴可以采用含炭量大约为0.4的冷轧钢，直径在35英寸之间的轴卡一采用冷轧钢或锻造毛坯。当直径大于5英寸时，则要采用锻造毛坯，然后机械加工到所要求得尺寸。轻载时，广泛采用塑料轴。由于塑料是电的不良导体，在电器中采用塑料比较安全。 齿轮和皮带轮等零件通过键联接在轴上。在键及轴上与之相对应的键槽的设计中。必须进行认真的计算。例如，轴上的键槽会引起应力集中，由于键槽的存在会使轴的横截面积减小，会进一步减弱轴的强度。 如果轴以临界速度传动，将会发生强烈的振动，可能会毁坏整台机器。知道这些临界速度的大小是很重要的，因为这样可以避开它。一般凭经验来说，工作速度与临界速度之间至少应该相差20 许多轴需要三个或更多的轴承来支撑，这就意味着它是一个超静定问题。材料力学教科书介绍了求解这类问题的方法。但是，设计工作应该与特定场合的经济性相符合。例如，需要一根由三个或更多个轴承来支撑的主传动轴，可以对力矩做出保守的假定，按照静定轴对其进行设计，其成本可能会更低一些。由于轴的尺寸增加的成本可能会比进行复杂，精细的设计分析工作所多花费的成本要低一些。轴的设计工作中的另一个重要方面是一根轴与另一根轴之间的直接联接方法。这是由刚性或者弹性联轴来实现的。 联轴器是用来把相邻的两个轴端联接起来的装置。在机械机构中，联轴器被用来实现相邻的两根轴之间的半永久性联接。在机器的正常使用期间内，这种联接一般不必拆开，在这种意义上，可以说联轴器的联接是永久性联接。但是在紧急情况下 或者在需要更换已磨损的零件时，可以先把联轴器拆开，然后再联接上。 联轴器有几种类型，它们的特性随其用途而定。如果制造工厂中或者船舶的螺旋桨需要一根特别长的轴，可以采用分段的方式将其制造出来，然后采用刚性联轴器将各段联接起来。一种常用的联轴器是由两个配对的法兰盘组成。这两个法兰盘借助靠键传动的轴套联接到相邻两节轴的两端。然后用螺栓穿过法兰盘联接起来形成刚性接头。相互联接的两根轴通常是靠法兰面上的槽口来对准的。 在把属于不同的设备（例如一个电动机和一个变速箱）的轴联接起来的时候，要把这些轴精确地对准是比较困难的，此时可以采用弹性联轴器联接轴的方式可以把由于被联接的轴之间的轴线的不重合所造成的有害影响减少到最低程度。弹性联轴器也允许被联接的轴在它们各自的载荷系统作用下产生偏斜或在轴线方向自由移动（浮动）而不致于产生相互干扰。弹性联轴器也可以用来减轻从一根轴到另一根轴的冲击载荷和振动的强度。滚动轴承对于求轴承和滚子轴承，一个机器设计人员应该考虑下面五个方面：（a）寿命与载荷的关系；（b）刚度，也就是在载荷作用下的变形；（c）摩擦；（d）磨损；（e）噪声。对于中等载荷和转速，根据额定负荷选择一个标准轴承，通常都可以保证其具有令人满意的工作性能。当载荷较大时，轴承零件的变形，尽管它通常小于轴和其他与轴承一起工作的零部件的变形，将会变的重要起来。在转速高的场合需要有专门的冷却装置，而着可能会增大摩擦阻力。磨损主要是由于污染物的进入引起的，必须选用密封装置以防止周围环境的不良影响。因为大批量生产这种方式就决定了球轴承和滚子轴承不但质量高而且价格低，因而机器设计人员的任务是选择而不是设计轴承。滚动接触轴承通常是采用硬度约为900HV、整体淬火的钢来制造。但在许多机构上不使用专门的套圈，而将相互作用的表面淬硬到600HV。滚动轴承由于在工作中会产生高的压力，其主要失效形式是金属疲劳，这一点并不奇怪，目前正在进行大量的工作以求改进这种轴承的可靠性。轴承设计可以基于能够被人们所接受的寿命值来进行。在轴承行业中，通常将轴承的承载能力定义为这样的值，即所承担载荷小于这个值时，一批轴承中将会有90%的轴承具有超过一百万转的寿命。 尽管球轴承和滚子轴承的基本设计责任在生产厂家，机器设计人员必须对轴承所要完成的任务进行正确的评价，不仅要考虑轴承的选择，而且还要考虑轴承的正确的安装条件。轴承套圈与轴或轴承座的配合非常重要，因为它们之间的配合不仅应该保证所需的过盈量，而且也应该保证轴承的内部间隙。不正确的过盈量会产生微动腐蚀从而导致严重的故障。内圈通常是通过紧靠在轴肩上进行周详定位的。轴肩处的圆弧半径主要是未了避免应力集中。在轴承内圈上加工一个圆弧或倒角，用来提供轴肩处圆弧半径的空间。 在使用寿命不是设计中决定因素的场合，统称根据轴承受灾何时产生的变形量来确定其最大载荷。因此“静态载荷能力”这个概念可以理解为对处于静止状态或进行缓慢转动的轴承所能够施加的载荷。这个载荷在轴承随后进行的旋转运动时的质量没有不利影响。按照实践经验确定，静载承载能力是这样一个载荷，当他作用在轴承时，滚动体与滚道在一个接触点处的总变形量不超过滚动体直径的0.01%。这相当于直径为25 mm 的球产生0.0025mm的永久变形。 只有将轴承与周围的环境适当的隔离开，许多轴承才能成功地实现他们的功能。在某些情况下，必须保护环境，使其不受润滑剂和轴承表面摩擦生成物的污染。轴承设计的一个重要组成部分实施密封装置起到应有的作用。此外，对摩擦学研究人员来说，未料任何目的而应用于运动零部件上的密封装置都是他们感兴趣的。因为密封装置是轴承的一部分，只有根据适当的轴承理论才能使基础令人满意的密封系统。虽然他们很重要，与轴承其它方面的研究工作相比，在密封装置研究方面所作的工作还是比较少的。齿轮齿轮是从一个轴相另一个轴传递旋转运动在几乎所能想象的每一种机器都存在。齿轮便是能够用来传递这种运动的最好方法之一。齿轮实际上是带有精确成型齿的轮子。这些齿与另一个齿轮的齿啮合，因而就提供拉强制运动的驱动。装有一对齿轮的轴间的传速比取决于齿轮的齿数。例如，一个齿的齿轮，驱动一个齿的齿轮，较小的齿轮转.圈，可使较大的齿轮转一圈。为了不同的用途，人们已经研制不同类型的齿轮。如果两个轴平行可采用直齿圆柱齿轮、斜齿轮或人字齿轮三类齿轮中任意一种。直齿圆柱齿轮是最简单和最便宜的，它一般用在需要中速驱动的装置。锥齿轮用在两个交叉轴之间的传递动力。在希望一个零件的旋转运动转换为其他零件的线性运动时采用齿条和小齿轮驱动，反之亦然。直齿圆柱齿轮用于平行轴之间传递旋转运动，他们通常是圆柱形的，且齿是直的并且平行与旋转轴。齿轮常用术语定义：（） 分度圆是一个假想圆柱的正截面（节圆柱），带齿的齿轮要考虑替换。分度圆的直径为节径。（） 齿顶圆是经过所有齿端的圆，齿顶高是齿顶圆与分度圆之间的径向距离。（） 齿根高和齿根圆限制拉齿间的间隙，分度圆和齿根圆之间的距离成为齿根高。（） 一个齿轮的齿根高和啮合齿轮的齿顶高之间的公差是间隙配合。（） 一个齿的顶面、底面分别称为顶端面、底端面。（） 齿面是分度圆柱和齿顶圆柱之间的齿的一部分沿齿向宽度称为齿面宽。（） 齿的齿根面是节圆柱和齿根圆柱之间的齿的一部分。（） 齿厚是沿着节度圆弧测量的齿的厚度。（） 齿槽宽是沿着节圆测量的两连续齿间的齿距。（） 齿隙是一个齿轮的齿槽宽与配合齿轮齿厚之间的间隙。（） 周节距是沿着节圆测量的齿厚与齿槽宽之和。如果表示节圆直径，是一个齿轮的齿数，周节距c由（）来计算。 ./（）（）分度周节d是单位分度圆直径齿轮的齿数，由式（）计算。 d/（）分度周节d的倒数是齿轮模数m。由上两式可知节圆和直径节圆之间的关系由（）获得。 d. （）（）小齿轮是啮合齿轮副小的齿轮。（）节点是一对啮合节圆相切的点，公切线是在节点处切于节圆的一条切线。（）啮合线是接触点处两啮合轮廓的法线。（）接触啮合路径是连接接触节点的路径。（）压力角是公切线与啮合线之间的角度。材料的选择近些年来，工程材料的选择已经显得非常重要。此外，选择过程应该是一个对材料的连续不断得重新评价过程。新材料不断出现，而一些原有的材料的可以被利用的数量可能会减少。环境污染，材料的回收利用，工人的健康及安全等方面经常会对材料的选择附加新的限制条件。为了减轻重量或者节约能源，可能要求使用不同的材料。来自国内和国际的竞争，对产品维修保养方便性要求的提高和顾客的反馈等方面的压力，都会促使人们对材料进行重新评价。由于材料选择不当造成的产品责任诉讼，已经产生深刻的影响。此外，材料与材料加工之间的相互依赖关系已经被人们认识的更清楚。新的加工方法的出现，通常会促使人们对被加工材料进行重新评价。因此，为了能在合理的成本和确保质量的前提下，获得满意的结果，设计工程师和制造工程师都必须认真仔细的选择，确定和使用材料。制造任何产品的第一部工作都是设计。设计通常可以分为几个明确的阶段：（a）概念设计；(b)功能设计；（c）生产设计。在概念设计阶段，设计者着重考虑产品应该具有的功能。 通常要设想和考虑几个方案做进一步的改进。在此阶段，关于材料选择唯一需要考虑的问题是：是否有性能符合要求的材料可供选择；如果没有的话，是否有较大的把握在和时间都允许的限度内研制出一种新材料。在功能设计或工程设计阶段，要做出一个切实可行的设计。在这个阶段需要绘制相当完整的图纸，选择并且确定各种零件的材料。通常要制造出样机或者实物模型，并对其进行试验，评价产品的功能，可靠性，外观和维修保养性等。虽然这种试验可能表明，在产品进入生产阶段之前，应该更换某些材料，但是，绝对不能将这一点作为不认真选择材料的借口。应该结合产品的功能，认真仔细的考虑产品的外观，成本和可靠性。一个很有成就的公司在制造所有样机时，所选用的材料应该和其在生产中使用的材料相同，并尽可能使用同样的技术。这样做，对公司是很有好处的。功能完备的样机如果不能根据预期的销售量经济地制造出来，或者样机与正式生产的装置在质量和可靠性方面有很大不同，则这种样机就没有多大的价值。设计工程师最好能在这一阶段全部完成材料的分析，选择和确定工作，而不是将其留到生产设计阶段去做。因为，在生产设计阶段材料的更换是由其他人进行的，这些人对产品的所有功能的了解可能不如设计工程师。在生产设计阶段中，与材料有关的主要问题是应该把材料完全确定下来，使他们与现有的设备相适应，能够利用现有设备经济地进行加工，而且材料的数量能够比较容易地保证供应。在制造过程中，不可避免地会出现对使用中的材料做一些更改的情况。经验表明，可以采用某些便宜材料作为替代品。然而，大多数情况下，在进行生产以后改换材料比在开始生产前改换材料所花费的代价要高。在生产设计阶段做好材料选择工作，可以避免大多数的这种材料更换情况。在生产制造开始后出现了可供使用的新材料是更换材料的最常见的原因。当然，这些新材料可能降低成本，改进产品性能。但是，必须对新材料进行认真的评价，以确保其所有性能都被人们所了解。应当时刻牢记，新材料的性能和可靠性很少能像现有的材料那样为人们所了解。大部分的产品失效和产品责任事故案件是由于在选用新材料作为替代材料之前，没有真正了解它们的长期使用性能而引起的。产品的责任诉讼迫使设计人员和公司在选材料时，采用最好的程序。在材料选择过程中，五个最常见的问题：（a）不了解或者未能利用关于材料应用方面的最新和最好的信息资料；（b）未能预见和考虑产品可能的合理用途（若有可能，设计人员还应进一步预测和考虑由于产品使用方法不当造成的后果。在近年来的许多产品责任诉讼案件中，由于错误的使用产品而受到伤害的原告控告生产厂家， 并且赢得判决）；（c）所使用的材料的数据不全或者有些数据不确定，尤其当其长期性能数据是如此的时候；（d）质量控制方法不适当和未经验证；（e）由一些完全不称职得人员选择材料。通过对上述五个问题的分析，可以得出这些问题是没有充分理由存在的结论。对这些问题的分析和研究可以给避免这些问题的出现指明方向。尽管采用最好的材料选择办法爷不能避免发生产品责任诉讼，设计人员和工作界按照适当的程序进行材料选择，可以大大减少诉讼的数量。从上面的讨论可以看出，选择材料的人们应该对材料的性质，特点和加工方法有一个全面而基本的了解。7SHAFTS、COUPLINGS、MATERIAL SELECTION AND TRANSMISSION METHODJune, 1992 by Keith BriereShafts and couplingsVirtually all machines contain shafts. The most common shape for shafts is circular and the cross section can be either solid or hollow (hollow shafts can result in weight savings). Rectangular shafts are sometimes used ,as in screw driver blades, socket wreches and control knob stems.A shaft must have adequate torsional strength to transmit torque and not be over stressed . It must also be torsionally stiff enough so that one mounted component does not deviat excessively from its original angular position relative to a second component mounted on the same shaft . Generally speaking , the angle of twist should ont exceed one degree in a shaft length equal to 20 diameters.Shafts are mounted in bearings and transmit power through such devices as gears , pulleys , cams and clutches .These devices introduce forces which attempt to bend the shaft ; bence , the bending deflection of a shaft should ont exceed 0.01 in per ft of length between bearing supports .In addition , the shaft must be able to sustain a combination of bending and torsional loads . Thus an equivalent load must be considered which takes into account both torsion and bending . Also , the allowable stress must contain a factor of safety which includes fatigue , since torsional and bending stress reversaks occur .For diameters less than 3 in. , the usual shaft material is cold-rolled steel containing about 0.4 percent xarbon . Shafts are either cold-rolled or forged in size . Plastic shafts are widely used for light load applications . One advantage of using plastic is safely in electrical applications , since plastic is a poor conductor of electricity .Components such as gears and pulleys are mounted on shafts by means of key . The design of the key and the corresponding keyway in the shaft must be properly evaluated . For example ,stress concentrations occur in shafts due to keyways , and the material removed to form the keyway further weakens theshaft .If shafts are run at critical speeds , severe vibrations can occur which can seriously damage a machine . It is important to know the magnitude of these critical speeds so that they can be avoided . As a general rule of thumb , the diference between the oprating speed and the critical speed should be at least 20 percent . Many shafts are supported by three or more bearings , which means that the problem is statically indeterminate . Texts on strength of materials give methods of solving such problems . The design effort should be in keeping with the economics of a given situation . For example , if one line shaft supported by three or more bearings is needed , it probably would be cheaper to make conservative assumptions as to moments and design it as though it were determinate . The extra cost of an oversize shaft may be less than the extra cost of an elaborate design analysis .Another important aspect of shaft design is the method of directly connecting one shaft to another . This is accomplished by devices such as rigid and flexible couplings.A coupling is a device for connecting the ends of adjacent shafts . In machine construction , couplings are used to effect a semipermanent connection between adjacent rotating shafts . The connection is permanent in the sense that it is not meant to be broken during the useful life of the machine , but it can be broken and restored in an emergency or when worn parts are replaced .There are several types of shaft couplings , their characteristics depend on the purpose for which they are used . If an exceptionally long shaft is required in a manufacturing plant or a propeller shaft on a ship , it is made in sections that are coupled together with rigid couplings . A common type of rigid coupling consists of two mating radial flanges (disk) that are attached by key driven hubs to the ends of adjacent shaft sections and bolted together through the flanges to form a rigid connection .Alignment of the connected shafts is usually effected by means of a rabbet joint on the face of the flanges .In connecting shafts belonging to separate devices (such as electric motor and a gearbox ) , precise aligning of the shafts is difficult and a flrxible coupling is used . This coupling connects the shafts in such a way as to minimize the harmful effects of shaft misalignment . Flexible couplings also permit the shafts to deflect under their separate systems of loads and to move freely (float) in the axial direction without interfering with one another . Flexible coupling can also serve to reduce the intensity of shock loads and vibrations transmitted from one shaft to another .Rolling Contact Bearing The concern of a machine designer with ball and rolling bearings is fivefold as follows:(a) life in relation to load; (b) stiffness, i.e. deflections under load;(c) friction; (d) wear; (e) noise. For moderate loads and speeds the correct selection of a standard bearing on the basis of load rating will usually secure satisfactory performance. the deflection of the bearing elements will become important where load are high, although this is usually of less magnitude than that of the shafts or other components associated with the bearing. Where speeds are high special cooling arrangements become necessary which may increase frictional drag. Wearing is primarily associated with the introduction of contaminants, and sealing arrangements must be chosen with regard to the hostility of the environment.Because the high quality and low price of the ball and roller bearings depends on quantity production, the task of the machine designer becomes one of selection rather than design. Rolling-contact bearings are generally made with steel which is through-hardened to about 900 HV, although in many mechanisms special races are not provided and the interacting surfaces are hardened to about 600 HV. It is not surprising that , owing to the high stresses involved, a predominant form of failure should be metal fatigue, and a good deal of work is currently in progress intended to improve the reliability of this type of bearing. Design can be based on accepted values of life and it is general practice in the bearing industry to define to define the load capacity of the bearing as that value below which 90 per cent of a batch will exceed a life of one million revolutions.Notwithstanding the fact that responsibility for the basic design of ball and roller bearings rests with the bearing manufacturer, the machine designer must form a correct appreciation of the duty to be performed by the bearing and be concerned not only with bearing selection but with the conditions for correct installation.The fit of the bearing races onto the shaft or onto the housings is of critical importance because of their combined effect on the internal clearance of the bearing as well as preserving the desired degree of interference fit. Inadequate interference can induce serious trouble from fretting corrosion. The inner race is frequently located axially by abutting against a shoulder. A radius at this point is essential for the avoidance of stress concentration and ball races are provided with a radius or chamfer to allow space for this. Where life is not the determining factor in design, it is usual to determine maximum loading by the amount to which a bearing will deflect under load. Thus the concept of” static load-carrying capacity” is understood to mean the load that can be applied to a bearing which is either stationary or subject to slight swiveling motions, without impairing its running qualities for subsequent rotational motion. This has been determined by practical experience as the load which when applied to a bearing result in a total deformation of the rolling element and raceway at any point of contact not exceeding 0.01 per cent of the rolling-element diameter. This would correspond to a permanent deformation of 0.0025 mm for a ball 25 mm in diameter. The successful functioning of many bearings depends upon providing them with adequate protection against their environment, and in some circumstances the environment must be protected from lubricants or products of deterioration of the bearing surfaces. Achievement of the correct functioning of seals is an essential part of bearing design. Moreover, seals which are applied to moving parts for any purpose are of interest to tribologists because they are components of bearing systems and can only be designed satisfactorily on the basis of the appropriate bearing theory. Notwithstanding their importance, the amount of research effort that has been devoted to the understanding of the behavior of seals has been small when compared with that devoted to other aspects of bearing technology.GearsThe transmission of rotary motion from one shaft to another occurs in nearly every machine one can imagine. Gears constitute one of the best of the various means available for transmitting this motion. A gear is virtually a wheel with very accurately shaped teeth. These teeth mesh with teeth of another gear, thus providing as positive-motion drive. The speed ratio between shafts carrying a pair of gears depends upon the numbers of teeth in the gear. For example, a 20tooth gear drives a gear of 50 teeth, the smaller gear will have to turn 2.5 times to cause the larger one to make 1 turn. Various types of gearing have been developed for different purposes. If the shafts are parallel, any of these types may be used, spur, bevel, or herring-bone. Spur gears are the simplest and least expensive type. They are generally used on drives requiring moderate speeds. Bevel gears serve to transmit power between tow intersecting shafts. Rack-and-pinion drives are used where it is desirable to transform the rotary motion of one part into linear motion for the other part or vice versa.Spur gears are used to transmit rotary motion between parallel shafts; they are usually cylindrical, and the teeth are straight and parallel to the axis of rotation.The following are definitions for some common terms used in study of gears.(1) The pitch circle is a right section of an imaginary cylinder (pitch cylinder), that the toothed gear may be considered for replacement. The diameter of the circle is called pitch diameter.(2) The addendum circle is a circle which passes through all the tooth ends, and the addendum is the radial distance between the pitch circle and addendum circle.(3) The dedendum or root circle bounds the spaces between the teeth, and the distance between the pitch circle and the dedendum circle is termed as dedendumm.(4) The tolerate between the dedendum of one gear and addendum of the mating gear is the clearance.(5) The top and bottom surfaces of a tooth are known as top land and bottom land respectively.(6) The face of the tooth is the part of the tooth between the pitch cylinder and addendum cylinder, and its width along the tooth element is known as face width.(7) The flank of the tooth is the part of the tooth lying between pitch cylinder and addendum cylinder。(8) The tooth thickness is the thickness of the tooth measured along the arc of the pitch circle.(9) The tooth space is the circular distance between two successive teeth measured along the pitch circle.(10) Back ash is the difference between the tooth space of one gear and tooth thickness of the mating gear.(11) The circular pitch, Pc is the sum of tooth thickness and tooth space, measured along the pitch circle. If D is the pitch diameter, and T is the number of the teeth of a gear, the circular pitch Pc is given by Eqs (3-1) Pc=3.14D/T (3-1)(12) The diametral pitch, Pd is the number of the teeth of a gear per unit pitch diameter.Hence by Eqs Pd=T/D (3-2)The inverse of diametric pitch Pd is called module m of the gear.From Eqs (3-1) and (3-2), the relation between circular and diametric pitches can be obtained as (3-3) Pc Pd =3.1415926 (3-3)(13) The pinion is the smaller gear of a mating gear pair.(14) The pitch point is the point of tangency of the pitch circles ora pair of mating gear wheels, and the common tangent is a tangent to the pitch circles at the pitch point.(15) The line of action is a line normal to both the mating profiles at the contact point.(16) The path of contact is the path traced by the contact point.(17) The pressure angle between the common tangent and line of action.Material SelectionDuring recent years the selection of engineering materials has assumed great importance . Moreover , the process should be one of continual reevaluation . New materials often become available and there may be a decreasing availability of others . Concerns regarding environmental pollution , recycling and worker health and safety often impose new constraints .The desire for weight reduction or energy savings may dictate the use of different materials . Pressures from domestic and foreign competition , increased serviceability requirements , and customer feedback may all promote materials reevaluation . The extent of product liability actions , often the result of improper material use , has had a marked impact .In addition , the interdependence between materials and their processing has become better recognized . The development of new processes often forces reevaluation of the materials being processed . Therefore , it is imperative that design and manufacturing engineers exercise considerable care in selecting , specifying ,and utilizing materials if they are to achieve satisfactory results at reasonable cost and still assure quality .The first step in the manufacture of any product is design , which usually takes place in several distinct stages : (a) conceptual ;(b) functional ;(c) production . During the conceptual-design stage , the designer is concerned primarily with the functions the product is to fulfill .Usually several concepts are visualized and considered ,and a decision is made either that the idea is not practical or that the idea is sound and one or more of the conceptual designs should be developed further . Here ,the only concern for materials is that materials exist that can provide the desired properties . If no such materials are available ,consideration is given as to whether there is a reasonable prospect that new one could be developed within cost and time limitations . At the functional or engineering-design stage , a practical ,workable design is developed .Fairly complete drawings are made , and materials are selected and specified for the various components . Often a prototype or working model is made that can be tested to permit evaluation of the product as to function ,reliability , appearance , serviceability , and so on . Although it is expected that such testing might show that some changes may have to be made in materials before the product is advanced to the product is advanced to the production-design stage ,this should not be taken as an excuse for not doing a thorough job of material selection . Appearance , cost ,and reliability factors should be considered in detail , together with the functional factors . There is much merit to the practice of one very successful company which requires that all prototypes be built with the same materials that will be used in production and ,insofar as possible , with the same manufacturing techniques . It is of little value to have a perfectly functioning prototype that cannot be manufactured economically in the expected sales volume , or one that is substantially different from what the production units will be in regard to quality and reliability . Also , it is much better for design engineers to do a complete job of material analysis , selection , and specification at the development stage of design rather than to leave it to the production-design stage , where changes may be made by others , possibly less knowledgeable about all of the functional aspects of the product . At the production-design stage , the primary concern relative to materials should be that they are specified fully , that they are compatible with , and can be processed economically by , existing equipment , and that they are readily available in the needed quantities .As manufacturing progresses , it is inevitable that situations will arise that may require modifications of the materials being used . Experience may reveal that substitution of c
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