机械零件设计外文翻译.docx

附录1 机械零件设计设计任何机械零件的理想情况为，工程师可以用大量的他所选用的这种材料的强度试验数据。这些实验应该采用与所设计的零件有相同的热处理，表面光洁度和尺寸大小的试件进行，而且试验应该在与零件使用过程中承受的载荷完全相同的情况下进行。这表明，如果零件将要承受弯曲载荷，那么就应该进行弯曲载荷实验。如果零件将要承受弯曲和扭转的复合载荷，那么就应该进行弯曲和扭转的复合实验。这些种类的试验可以提供非常游泳和精确的数据。它们可以告诉工程师应该使用的安全系数和对于给顶的使用寿命的可靠性。在设计工作中，只要能够获得这种数据，工程师就可以尽可能好的进行设计工作。如果零件的失效可能会危害人的生命安全，或者零件有足够大的产量，则在设计前收集这样的数据所花费的费用是值得的。例如，汽车和冰箱的零件产量非常大，可以在生产之前对它们进行大量的实验，使其具有较高的可靠性能。如果把进行这些试验的费用摊到所生产的零件上的话，则每一个零件的费用是非常低的。你可以对下列四类的设计作出评价：1.零件的失效可能危害人的生命安全，或者零件的产量非常大，为此在设计时安排一个完善的试验程序是被认为合理的。2.零件的产量足够大，可以进行适当的系列试验。3.零件的产量非常小，以至于进行试验根本不合算；或者要求很快完成设计，以至于么有足够的时间进行试验。4.零件已经完成设计、制造和试验，但结果不能令人满意。这时候需要采用分析法来弄清楚不能令人满意的原因和应该如何进行改进。我们主要对后三种类型进行讨论。这就是说，设计人员通常只能利用那些公开的屈服强度、极限强度和延伸率等数据资料。人们期望着工程师在利用这些不是很多的数据资料的基础上，对静载荷与动载荷、两维应力状态与三维应力状太、高温有低温以及大零件与小零件进行设计。而设计中的，有充分的时间产生应变。到目前为止，还必须利用这些数据来设计每分钟承受几千次复杂的动载荷的作用的零件，因此机械零件有时会失效是不足为奇的。 概括来说，设计人员所遇到的基本问题是，不论对于哪一种应力状态或者载荷情况，都有利用通过简单拉伸实验所获得的数据并将其与零件的强度联系起来。可能会有两种完全相同的强度和硬度值的金属，其中一种由于其本身的延展性而具有很好的承受载荷的能力。延展性是用材料断裂时的延伸率来量度的。通常将5%的延伸率定义为延展性和脆性的分界线。断裂时候延展率小于5%的材料成为脆性材料，大于5%的成为延性材料。材料的伸长量通常是在50mm的计算长度上测量的。因为这并不是对实际应变量的测量，所以有时候也采用一种测量延展性的方法。这个方法是在试件断裂后，测量其断裂处的横截面的面积。因此，延展性可以表示为横截面的收缩率。延展材料能够承受较大的载荷这特性是设计中的一个附加的安全因素。延展材料的重要性在于他是材料冷变形性能的衡量尺度。诸如弯曲和拉延这类金属加工过程中需要采用延性材料。在选用抗磨损、抗侵蚀或者抗塑性变形的材料时，硬度通常是最主要的性能。有几种可供选用的硬度试验方法，采用哪一种方法取决于最希望测量的材料的性能特性。最常用的四种硬度数值是布氏硬度、洛氏硬度、维氏硬度和努氏硬度。大多数硬度实验系统是将一个标准的载荷加在与被试验材料相接触的小球或者棱锥上。因此，硬度可以表示为所生产的压痕尺寸的函数。这表明由于硬度是非破坏性试验，而且不需要专门的试件，因而硬度是一个容易测量的性能。通常可以直接在实际的机械零件上进行硬度试验。对于球轴承和磙子轴承，一个机械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.噪音。对于中等载荷和转速，根据额定负荷选定一个标准轴承，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，将会变得重要起来。在转速高的场合需要有专门的冷却装置，而这可能会增大摩擦阻力。磨损主要是由于污染进入引起的，必须采用密封装置防止周围环境的不良影响。因为大批量生产这种方式决定了球轴承和磙子轴承不但质量高，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。滚动轴承通常是用硬度为900HV、整体淬火钢制成。但在许多机构上不使用专门的套圈，而是将相互作用的表面淬硬到600HV。滚动轴承由于在工作时会产生高的应力，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。抽成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，即承担的载荷小于这个值时，一批轴承中的会有90%的轴承具有超过100万转的寿命。尽管球轴承和磙子轴承的基本设计责任不在轴承的制造厂家，机器设计人员必须对轴承所要完成的任务做出正确的评价，不仅要考虑轴承的选择，而且还要考虑轴承的正确安装条件。轴承套圈与轴或轴承坐的配合非常重要，因为他们之间的配合不仅仅应该保证所需要的过盈量，而且也应该保证轴承的内部间隙。不正确的过盈会产生微震腐蚀从而导致严重的故障。内圈通常是通过靠近在轴肩上进行轴向定位的。轴肩处的圆弧半径主要是为了避免应力集中。在轴承内圈上加工出一个圆弧或者倒角，用来提供容纳轴肩处圆弧半径的空间。在使用寿命不是设计中的决定因素的场合，通常根据轴承受载荷时产生的变形量来确定其最大载荷。因此“静态承载能力”这个概念可以理解为对处于静止状态或者进行缓慢转动的轴承所能施加的载荷。这个载荷对轴承在随后进行旋转运动时的质量没有不利影响。按照实践经验确定，静态承载能力是这样一个载荷，当他作用在轴承上时，滚动体与套圈在任何一个接触点处的总变形量不超过滚动体直径的0.01%。这相当于为25mm的球产生0.0025mm的永久变形。只有将轴承与周围环境适当地隔离开，许多轴承才能够成功的实现他们的作用。在某些情况下，必须保护环境，使其不受润滑剂和轴承表面磨损生成污染物的污染。轴承设计的一个重要组成部分是使密封装置起到应有作用。此外，对摩擦学研究人员来说，为了任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设计出令人满意的密封系统。虽然它们很重要，与轴承其他方面的研究工作相比，在密封装置的研究方面所做的工作还是比较少的。附录2 Machine element designing Ideally in designing any machine element, the engineer should have at his diposal the results of a great many strengh tests of the particular material chosen. These tests should have been made on specimens having the same heat treatment, surface finish, and size as the element he propose to design; and the tests should be made under exactly the same loading conditions as the part will experience in service. This means that, if the part is to experience a bending load, it should be tested with a bending load. If it is to be subjected to a combined bending and torsion, it should be tested under combined bending and torsion. Such tests will provide very useful and precise information. They tell the engineer what factor of safely to use and what the reliability is for a given service life. Whenever such date are available for design purposes, theengineer can be assured that he is doing the best possible job of engineering. The cost of gathering such extensive date prior to design is justified if failure of the part may endanger human life, or if the part is manufactured in sufficiently large quantities. Automobiles and refrigerators, for example, have very good reliabilities because the parts are made in such large quantities that they can be thoroughly tested in advance of manufacture. The cost of making these tests is very low when it is divided by the total number of parts manufactured. You can now appreciate the following four design categories: (1) Failure of the part would endanger human life, or the part is made in extremely large quantities; consequently, an elaborae testing program is justified during design. (2) The part is made in large enough quantities so that a moderate series of tests is feasible. (3) The part is made in such small quantities that testing is not justified at all; or the design must be completed so rapidly that there is not enough time of testing. (4) The part has already been designed, manuactured, and tested and found to be unsatisfactiry. Analysis is required to understand why the part is unsatisfactory and what to do to improve it. It is with the last three categories that we shall be mostly concerned. This means that the designer will usually have only published values of yield strenth, ultimate strenth, and percentage elongation. With this meger information the engineer is expected to design against static and dynamic loads, biaxial and triaxial stress states, high and low temperratures, and large and small pars! The date usually available for design have been obtained from the simple tension test, where the load was applied gradually and the strain given time to develop. Yet these same date must be used in designing parts with complicated dynamic loads applied thousands of times per minute. No wonder machine parts sometimes fail. To sum up, the fundamental problem of the designer is to use the simple tension-test date and relate thenm to the strength of the part, regardless of the stress state of the loading situation. It is possible for two metals to have exactly the same strength and hardness, yet one of these metals may have a superior ability to absorb overloads, because of the property called ductility. Ductility is measured by the percentage elongation which occurs in the material at fracture. The usual dividing line between ductility and brittleness is 5 percent elongation. A material having less than 5 percent elongation at fracture is said to be brittle, while one having more is said to be ductile. The elongation of a material is usually measured over 50mm gauge length. Since this is not a measure of the actual strain, another method of determining ductility is sometimes used. After the specimen has been fractured, measurements are made of the area of the cross section at the fracture. Ductility can then be expressed as the percentage reduction in cross-sectional area. The characteristic of a ductile material which permits it to absorb large overloads is an addition safety factor in design. Ductility is also important because it is a measure of that property of a material which metal-processing operations which require ductile materials. When a material is to be selected to resist wear, erosion, or plastic deformation, hardness is generally the most important priperty. Several methods of hardness testing are available, depending upon which particular property is most desired. The four hardness numbers in greatest use are the Brinell, Rockwell, Vickers, and Knoop. Most hardness-testing systerm employ a standard load which is applied to a ball or pytamid in contact with the material to be tested. The hardness is then expressed as a function of the size of the resulting indentation. This means that hardness is an easy property to measure, because the test is nondestructive and test specimens are not required. Usually the test can be conducted directly on an actual machine element.The concern of a machine designer with ball and roller 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 seletion 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 loads 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. Wear 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 ball and roller bearings depends on quantity production, the task of the designer becomes one of selection rather than design. Rolling-contact bearings are generally made with steel which is though-hardened to above 900 HV, although in many mechanisms special races are not provided and the interacting surfaces are hardened to above 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 industy 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 milion 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 conditions for correct instalation. 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 a