机械专业毕业设计外文翻译5.doc

实际上，几乎所有的机器中都装有轴。轴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）。有时也采用矩形轴，例如，螺丝起子的头、套筒扳手和控制旋扭的杆。为了在传递转矩时不发生过载，轴应该具有适当的抗扭强度。轴还应该具有足够的抗扭刚度，以使同一个轴上的两个传动零件之间的相对转角不回过大。一般来说，在轴的长度等于其直径的20倍时，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里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，在两个轴承支承之间，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，在许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碳量大约为 0.4%的冷轧钢，直径在 35 英寸之间的轴可以采用冷轧钢或锻造毛坯。当直径大于5 英寸大，则要采用锻造毛坯，然后机械加工到所要求的尺寸。轻载时，广泛采用塑料轴。由于塑料是电的不良导体，在电器中用它做轴比较安全。 齿轮和皮带轮等零件通过键联接在轴上。在对键及轴上与之相应的键槽进行设计时，必须进行认真的计算。例如，轴上的键槽会引起应力集中，由于键槽的存在使轴的横截面积间隙哦啊，会进一步减弱轴的强度。如果轴以临界速度转动，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%。 轴的设计工作的另一个重要方面是轴与轴之间的直接联接方法。这是有刚性或者弹性联轴器来实现的。联轴器是用来把两个相邻轴端联接起来的装置。在机械结构中，联轴器被用来实现相邻的两根转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的。但是在紧急情况下，或者需要更换以磨损的零件时，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浆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的槽口来对准的。在把属于不同的设备（例如一个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。这种联轴器联接轴的方式可以把由于被联接的轴之间的轴线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）而不至于相互干扰。弹性联轴器也可以用来减轻从一根轴传到另一根轴上的冲击载荷和振动的强度。而几乎所有的轴都装有滚动轴承。对于球轴承和滚子轴承，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）噪音。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大约900hv、整体淬火的钢来制造的。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大约 600hv。滚动轴承由于在工作中会产生高的应力，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。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，即所承担的载荷小于这个值时，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轴承制造厂家，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要的过盈良好，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振腐蚀磨损从而导致严重的故障。内圈通常是通过紧靠在轴肩上进行轴向定位的。轴肩处的圆弧半径重要是为了避免应力集中。在轴承内圈上加工出一个圆狐或者倒角，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的或者进行缓慢转动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，当它作用在轴承上时，滚动体与滚道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%。这相当于直径位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，许多轴承才能成功地实现它们的功用。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系统的一部分，只有根据适当轴承理论才能设计出令人满意的密封装置。虽然它们很重要，与轴承其他方面的研究工作相比，到目前为止在密封装置的研究方面所做出的工作还是比较少的。virtually 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 screwdriver blades, socket wrenches and control knob stems . a shaft must have adequate torsion strength to transmit torque and not be overstressed. it also be torsion ally stiff enough so that one mounted component does not deviate excessively from its original angular position relative to a second component mounted on the same shaft. generally speaking, the angle of twist should not 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; hence, the shaft must be rigid enough to prevent overloading of the supporting bearings. in general, the bending deflection of a shaft should not 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 tensional loads. thus an equivalent load must be considered 2 which take into account both torsion and bending. also, the allowable stress must contain a factor of safety which includes fatigue, since tensional and bending stress reversals occur. for diameters less than 3 in., the usual shaft material is cold-rolled steel containing about 0.4 percent carbon. shafts are either cold-rolled or forged in sizes from 3 in. to 5 in., shafts are forged and machined to size. plastic shafts are widely used for light load applications. one advantage of using plastic is safety 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 the shaft. 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 difference between the operating speed and the critical speed should be at least 20 percent. 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. 3 a coupling is a device for connecting the ads of adjacent shafts. in machine construction, couplings are used to effect a semi permanent 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 (disks) that are attached by key-driven hubs to the ads 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 an electric motor and a gearbox), precise aligning of the shafts is difficult and a flexible 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 with one another. flexible couplings can also serve to reduce the intensity of shock loads and vibrations transmitted from one shaft to another.virtually all shafts contain rolling contact bearings. 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 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 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 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 machine designer becomes one of selection rather than design. rolling-contact bearings are generally made with steel which is through-hardened to about 900hv, 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 5 based on accepted values of life and it is general practice in the bearing industry to define the load capacity of the bearing as that value below which 90 percent of a batch will exceed a lift of anew 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 provides 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 6 determined b