机械工程导论翻译第三章

1、3.1 OVERVIEWAs introduced in Chapter 1,the primary objective of mechanical engineers is to make machines that work, that are useful, and that improve society in some way. Machines in general,and power transmission equipment in particular, are often constructed of standardized components or building

2、blocks.Just as an electrical engineer might select off-the-shelf resistors,capacitors,and transistors as the elements of a circuit。mechanical engineers have good intuition for specifying the different available types of bearing ,shafts ,gears,belts and other components in their designs.Once the deta

3、ils of a machine have been worked out and the components for it have been selected ,the product then needs to be built.Mechanical engineers also devise manufacturing and assembly processes,and at the conclusion of this chapter we will introduce several common machine tools.A solid appreciation of ma

4、chine components and tools is valuable at three levels.First,and from our primary standpoint in this textbook,a working knowledge of this hardware is important for you to develop a technical vocabulary!Mechanical engineering has its own precise language,and specialized terms are used to describe the

5、 construction and operation of machines.To communicate effectively with other engineers,you will need to learn,adopt,and share that language.Second,having such a background is necessary to select the poper component for an application and to specify the appropriate machine tool for a production proc

6、ess.Of course,it is not possible to list and describe every machine and component that embodies mechanical engineering principles,and that is not our intention in this chapter.However,by examining just a few machine components and tools,you will develop a growing appreciation for practical machinery

7、 issues.Finally,this chapter is a good place for you to begin the journey of understanding the inner workings of mechanical hardware.Its intellectually healthy for you to be curious about machines,wonder how they were made,dissect them,and think about they could have been made differently or better

8、.3.1概述正如第1章介绍，机械工程师的首要目标是制造机器，制造能够工作的、有用的并以某种方式，改善社会的机器。普通机器，以及像电力传输设备这样的特殊机器设备，往往是由各种标准化的部件或称为“模块”组成的。就像一个电气工程师可能会选择现成的电阻，电容和晶体管做为电路元件，机械工程师在他的设计中可以得心应手的选择合适类型的轴承、轴、齿轮、皮带和其它机械零件。一旦一种机器的设计方案制订出来，它的组件被选定，然后需要制造产品。机械工程师也设计制造和装配工艺过程，在本章结尾，我们将介绍几种常见的机床。 牢固全面地了解各种机器零件和机床，在三方面是有价值的。首先，也是本教科书的最初目的，了解硬件工作方面

9、的知识，对扩展你的技术词汇是非常重要的。机械工程有它自己独特的语言，需要使用一些专业术语来描述机械的结构和操作。为了有效地与其他工程师沟通交流，你需要学习，采纳，并分享这种语言。其次，为某种实际应用场合选择合适的部件，为某种生产工艺选择合适的机床，都需要你具有各种机械零件和各种机床的知识。当然，我们不可能列出并描述机械工程学科所包括的所有机器和零件，这也不是本章的目的。然而，仅仅通过学习几种机械零件和机床的知识，将增进你对实际机械问题的了解和掌握。最后，通过本章的学习，能让你很好地了解机械零件的内部结构和工作原理。激发起你对机器的好奇心，想想他们是如何制成的，剖析他们，研究他们如何能够作出不同

10、或更好。3.2 ROLLING ELEMENT BEARINGSBearing are used to hold shafts that must rotate relative to supports that are fixed for instance,the housing of a motor,gearbox,or transmission.A wide variety of bearing exist,and each particular type is appropriate for a different set of installation and operating c

11、onditions.Although a ball bearing might be the best choice for one application,it might not have enough load-carrying capability for another.The common feature of all bearing is that they enable shafts to rotate with little resistance while being well supported against forces acting in other directi

12、ons. Bearing are classified into two broad groups,called rolling contact and journal.In this chapter,we discuss only rolling contact bearings;these typically comprise an inner race;an outer race;rolling elements in the form of balls,cylinders,or cones,and a separator that prevents the rolling elemen

13、ts in the form rubbing against one another,Rolling contact bearing are ubiquitous in machine design,and they are found in applications as diverse as bicycle wheels,robotic joint,and automobile transmissions.Journal bearing,on the other hand,have no rolling elements.Instead,the shaft simply rotates w

14、ithin a polished sleeve that is lubricated by oil or another fluid.Just as the puck on an air hockey table slides smoothly over a thin film of oil.Also ubiquitous,journal bearing support crankshafts in internal combustion engines and shafts inside pumps and compressors.3.2滚动轴承 轴承是用来支撑轴的，而轴又必须相对于固定的支

15、撑物旋转，例如，一个电机、变速箱的壳体（轴承座）就是这样固定的支撑物。目前，有各种类型的轴承存在，每一种特定的类型都适合不同的安装和工作条件。尽管球轴承可能是某种应用场合的最佳选择，但它对另一种场合却没有足够的承载能力。所有球轴承的共同特点是，它们使轴在转动时阻力最小，同时能承受其它方向的载荷。 轴承可分为两大类，称为滚动轴承和滑动轴承。在这一章中，我们只讨论滚动接触的轴承。滚动轴承通常包括内圈、外圈、滚动体和分离架，其中滚动体常见的有：球，圆柱体，圆锥体，而分离架的作用是防止滚动体相互接触而产生摩擦。滚动接触轴承在机械设计中无处不在，在自行车的车轮，机器人关节，汽车零件等场合随处可以看到它们

16、的身影。另一方面对于滑动轴承来讲，它没有滚动体。取而代之的是，轴只简单地在抛光的套筒内旋转，由油或空气形成的薄膜来润滑。Just as the puck on an air hockey table slides smoothly over a thin film of oil.就像在冰球桌上的冰球光滑地在油膜上滑动一样。滑动轴承的应用也是无处不在，如内燃机曲轴、泵和压缩机的轴都是由滑动轴承支撑的。A sample installation of a rolling contact bearing is illustrated in Figure 3.2 . The shaft and the

17、 bearings inner race rotate together , while the outer race and the transmissions housing are stationary . When a shaft is supported in this way , the bearings outer race will fit tightly into a matching circular recess that is formed in the housing . As the shaft in Figure 3.2 turns and transmits p

18、ower , the bearing could be subjected to forces directed either along the shaft (called a thrust force) or perpendicular to it (a radial force) . An engineer will make a decision on the type of bearing to be used in a machine depending on whether thrust forces , radial forces , or some combination o

19、f the two act on the bearing . Thus , engineers apply the properties of force systems (as described in Chapter 4) when they analyze and select bearings and other machine components .滚动轴承安装示意图如图3.2所示。轴和轴承的内圈一起旋转,而外圈和传输箱是静止的。当轴以这种方式安装时,轴承的外圈与传动箱紧密配合。当图3.2的轴旋转并传输功率时,轴承可能承受沿着轴线方向的力(称为轴向力)或垂直于轴线的力(称为径向力)

20、。工程师将根据轴承承受力的情况来选择轴承的类型，如轴向力，径向力，或轴向力和径向力同时存在。因此, 当工程师分析和选择轴承和其他机器部件时。应当首先了解结构的受力情况 (详见第4章) 。Rolling contact bearings can also support shafts that are mounted on the outside of a frame or structure . In those cases , it may not be possible to press the bearing into a recess such as that shown in Fig

21、ure 3.2 . One solution for such situations is to use a pillow block mount , as depicted in Figure 3.3 . The bearing itself is contained within the block , which in turn can simply be bolted to a suitable surface . Because of the clearances that are present in the pillow blocks bolt holes , this type

22、 of mounting arrangement generally does not provide close tolerance for positioning and aligning the shaft .滚动轴承也能支撑安装在箱体和结构外边的轴。在这些情况下，轴承的外圈不能像图3.2那样与固定箱体紧密配合。对这种情形的解决方案是使用轴承座,如图3.3所示。轴承本身包含在轴承座里, 轴承座可以简单地用螺栓固定到一个适当的支撑面上。由于轴承座的螺栓孔存在间隙,这种类型的安装布置一般对有定位和校准要求的轴不能提供足够的精确度。Ball BearingsThe most common ty

23、pe of rolling element bearing is the ball bearing , which incorporates hardened , precision-ground steel spheres . Figure 3.4 depicts the major elements of a standard ball bearing : the inner race , outer race , balls , and separator . The inner and outer races are the bearings connections to the (r

24、otating) shaft and the (stationary) housing . The separator (which can also be called the cage or retainer) keeps the balls evenly spaced around the bearings perimeter and prevents them from contacting one another . Otherwise , if the bearing was used at high speed or subjected to large forces , fri

25、ction could cause it to overheat and become damaged . In some cases , the gap between the inner and outer races will be sealed by a rubber or plastic ring to keep grease in the bearing and dirt and debris out of it . Thus , bearings can either be sealed or unsealed .球轴承最常见的类型的滚动轴承是球轴承,这种轴承包含了若干个硬化的,

26、精密研磨的钢球体。图3.4描述了一个标准的球轴承的主要组成部件:内圈、外圈,钢球和分离器。轴承的内圈和外圈分别连接(旋转的)轴和(固定的)箱体。分离器使钢球在轴承的圆周方向等间距分布,以避免它们彼此接触。否则,如果轴承承受高速或大的载荷时,摩擦可能导致过热,而使轴承受损。在某些情况下, 轴承的内圈和外圈之间的间隙用橡胶或塑料环密封，保持润滑脂在轴承内并防止污垢和碎片进入轴承内部。轴承可以密封也可以不密封。In principle , the balls press against the inner and outer races of the bearing at single points

27、 . The force that each ball transfers between the inner and outer races is concentrated on those surfaces in an intense and relatively sharp manner , as indicated in Figure 3.5(a) . If the force between the rolling elements and races were instead spread over a larger area , we might expect the beari

28、ng to last longer . Rolling contact bearings that incorporate cylindrical rollers or tapered cones in place of spherical balls are one solution for more evenly distributing forces , and we discuss those types of bearings next .理论上,钢球与球轴承的内圈和外圈之间是点接触。每个球在球轴承的内圈和外圈之间传递的力集中在一点上，如图3.5(a)所示。如果滚动体与轴承的内圈和外

29、圈之间传递的力能分布在更大面积上,我们就可以期望轴承的寿命更长。滚动轴承,将圆柱滚轴或锥形锥代替钢球就是一个这样的解决方案，下面我们将讨论这些类型的轴承。Straight Roller BearingsWhile ball bearings may be inexpensive , they have a relatively modest capacity for carrying forces because of the point contacts present between the balls and races . As shown in Figure 3.5(b) , cyl

30、indrical rollers can also be used in a rolling element bearing in order to better distribute forces over the bearings races . Figure 3.6 illustrates the structure of a straight roller bearing . If you place a few pens or pencils between your hands , and then rub your hands together , you have the es

31、sence of a straight roller bearing . 圆柱滚子轴承虽然球轴承可能是便宜的, 但由于钢球与球轴承的内圈和外圈之间是点接触，球轴承的承载能力很有限，。如图3.5(b),圆柱辊子也可以用做滚动轴承的滚动体，能更好地在滚动体和轴承的内圈和外圈之间分散载荷。图3.6展示了圆柱滚子轴承的结构。如果你把几只笔或铅笔放在双手之间,然后揉搓双手,你就对圆柱滚子轴承的本质有一个直观的了解。In Figure 3.6 , the centerlines of the rollers and the races are parallel to one another , and a

32、 separator is again used to space roller around the bearings circumference . Straight roller bearings are used in machinery when large radial forces must be carried , but they are not well suited for supporting thrust loads .在图3.6中, 各个滚动体和轴承的内圈和外圈的中心线彼此平行, 分离器使圆柱滚子在轴承的圆周方向等间距分布,以避免它们彼此接触。圆柱滚子轴承用于承受较

33、大径向力的机械结构中,但它们并不适合承受轴向负荷。Tapered roller bearingsInstead of having spheres or cylinders roll on the races,a tapered roller bearing uses rollers that are shaped like truncated cones,as seen in figure 3.7. the conical knk()ladj. 圆锥的；圆锥形的rollers all have the same taper angle and their centerlines inters

34、ect at a single point on the shaft,located some distance away from the bearing itself . separators in the bearings of figures 3.7and 3.8 prevent friction from roller-to-roller contact and better distribute forces around the bearing. As was the case for straight roller bearings,the rollers of a taper

35、ed roller bearing contact the outer race(which is often called the cup)and the inner race (the cone)along lines,rather than points ,in order to lower wear .The lifetime of straight and tapered roller bearing is generally further improved by giving the rollers a slight crown or barrel shape,a feature

36、 again intended to smooth out the transfer of forces between the races.圆锥滚子轴承在轴承的内外圈之间，取代钢球或圆柱滚子，圆锥滚子轴承使用截短的圆锥作为滚动体(见图3.7)。这个锥形辊都有相同的锥角，并且每个锥形辊的中心线相交于轴线上, 与轴承本身有一定的距离。在图3.7和3.8中的分离架可以防止锥形辊之间相互接触而产生摩擦。就像圆柱滚子轴承一样,圆锥滚子轴承与内外圈之间是线接触,而不是点接触,以降低磨损。为了进一步提高圆柱和圆锥滚子轴承的寿命，通常将圆柱和圆锥滚子做成轻微曲线或桶的形状,该特性使得力在内外圈之间的传递更加

37、平稳。Although they may not be as familiar to you as ball bearings,tapered roller bearings are quite common in mechanical engineering.they are intended for applications where both thrust and radial forces are presentthe wheel bearings in an automobile are a prime example .Wheel bearings must support bo

38、th the weight of the vehicle (which is a radial force between the bearing and the wheels shaft)and the cornering force generated during turns(which is a thrust force).圆锥滚子轴承对你来讲，虽然不像球轴承那么熟悉，但它在机械工程领域是相当常见的。它们多用于既有轴向力又有径向力的场合，汽车轮毂轴承是一个典型的例子。轮轴承必须同时支持汽车的重量(这是一个径向力)和在转弯过程中产生的转弯力 (这是一个推力或轴向力)。 Tapered r

39、oller bearing are regarded as having an intermediate cost ,a high capacity for radial forces, and a moderate capacity for thrust forces .in other high-load and heavy duty application ,matched tapered roller bearings are combined into two-row or four-row sets,such as those shown in figure 3.9. 圆锥滚子轴承

40、的特点是成本适中, 能承受较高径向力,以及适当的轴向力。在其它重负荷和重型机械的应用中,可以将圆锥滚子轴承成对匹配，组合成双排或4排的集合体,如图3.9所示。 Thrust roller bearingsWhile straight roller bearings are well suited for supporting loads that are directed mostly radially,and tapered roller bearings can support a combination of radial and thrust forces ,thrust roller

41、 bearings carry loads that are directed primarily along a shafts centerline.one type of thrust bearing is shown in figure 3.10.the rolling elements in this type of bearing are cylinders having a slight barrel shape.in contrast to the straight roller bearing of figure 3.6,these rollers are placed rad

42、ially relative to the shaft .thrust bearings are appropriate for such applications as a turntable that Must support the dead weight of cargo but which also needs to turn freely. The thrust bearing shown in figure 3.11 uses tapered rollers.推力滚子轴承相对于圆柱轴承适合承受径向荷载,圆锥滚子轴承可以同时承受径向和轴向力，推力滚子轴承可以承受沿着轴中心线轴向力。

43、典型的推力轴承如图3.10。在这种类型的轴承中的滚动体是轻微桶形的锥形辊。相比于图3.6的圆柱滚子轴承,这些辊相对于轴的径向放置。推力轴承适合这样的应用场合，如一个转盘,必须支持货物的重量,但也需要自由旋转。如图3.11的推力轴承，使用锥形滚动体。3.3 Flexible shaft couplingsWhen the output shaft from one machine is connected to the input shaft of another ，you might initially expect that the shafts would always be rigidl

44、y connected。 From a practica1 standpoint, however, it is difficult to align two shafts precisely, and any (even slight) angular or offset misalignment can cause the bearings to become over loaded and damaged.3.3 柔性联轴器当一台机器的输出轴与另一根输入轴联接时，最初你可能希望这两根轴最好是刚性的联接在一起。然而，实际情况是，使两根轴准确地处于同一轴线上是困难的，而且任何角度的安装误差或

45、轴线的安装偏移（尽管它们很小）都会使得轴承处于过载和损坏的危险状态。 Consider the arrangement in which the shaft of an electric motor will be connected to another shaft that serves as the input to a gearbox. The electric motor in this case has its own shaft that is mounted on bearings internal to the motor. Our conceptual configurat

46、ion is shown in Figure 3. l2(a), where the shafts of the motor and the transmission are ideally aligned. In practice, however, small but significant imperfections due to manufacturing and assembly tolerances will be unavoidable. If the two shafts were rigidly connected, the misalignment wou1d cause

47、the shafts to bend. In turn, the bearings would be subjected to relatively large forces, and they would not be expected to have a long sevice life.考虑电动机的轴和齿轮箱的输入轴相联接的情况。在这种情况下，电动机上有安装在电动机内轴承上的轴。理想中的安装情况如图3.12a所示，在图中电动机的轴和传动轴是理想的处于同一轴线上的。然而，实际上制造误差和安装误差将是不可避免的。如图3.12b所示，如果将两轴刚性的连接在一起，两轴的不同轴会使轴发生弯曲，接下

48、来就是轴承将承受相当大的力，而且可以预料到轴承的寿命不会太长。 To overcome this problem, a flexible coupling can be used to connect the two shafts. Several types are shown in Figure 3. l3. These couplings effectively transmit rotation, and at the same time they are relatively compliant with respect to sideways bending. In short,

49、flexible couplings accommodate shafts that need to be connected but that might be slightly misaligned, and in so doing they reduce forces acting on the drive trains bearings.为了解决这个问题，可以采用柔性联轴器联接两轴。几种形式的柔性联接如图3.13所示。这些联轴节有效的传递着转动并且同时与轴两端的弯曲相适应。简言之，柔性联轴器不仅联接了所需要的两根轴而且允许它们的轴线少量的不同轴，使用柔性联轴器还可以减少作用在动力系统轴

50、承上的力。3.4 GEAR TYPES AND TERMINOLOGY Gears, and the belt and chain drives that are described in Section 3.5, transmit rotation, torque, and power between shafts. Geartrains can be used to increase rotation speed but decrease torque, to keep speed and torque constant, or to reduce rotation speed but i

51、ncrease torque. Mechanisms incorporating gears are common in the design of machinery, and they have applications as diverse as electric can openers and helicopter transmissions. The mathematical analysis of geartrains and other machinery is postponed until Chapter 7, at which point we will have set

52、a foundation in place for analyzing force, torque, and power. In this section, our objective is simply to explore various types of gears with an emphasis on their characteristics and the terminology used to describe them.3.4 齿轮的类型和术语在3.5节所描述的齿轮、皮带和链条，传递着两根轴之间的转动、转矩和动力。采用齿轮变速箱有三种用途：（1）能够增加转速、但是减小转矩，（

53、2）保持转速和转矩为常数，（3）减小转速、但是增加转矩。在机械设计中，包括各种齿轮的机构是常见的，而且他们具有不同的用途，如电开罐器和直升飞机的传动。对于动力系统和其它机械的分析将延至到第七章。在第七章中，我们将分析力、转矩和功率。在这一节中，我们的目的就是简单地介绍各种各样的齿轮，并侧重了解它们的特性和技术术语。The shape of a gears tooth is mathematically defined and precisely machined according to codes and standards that have been established by ind

54、ustry. The American Gear Manufacturers Association, for instance, has developed guidelines for standardizing the design and production of gears. Mechanical engineers are able to purchase pairs of loose gears directly from gear manufactures and suppliers, or they can obtain prefabricated gearboxes an

55、d transmissions suitable for the task at hand. There are exceptions for circumstances where standard gears might not offer sufficient performance (such as 1ow noise and vibration), and in those cases, contract machine shops can custom-produce gears. In the majority of machine design situations, howe

56、ver, gears and gearboxes are se1ected as ”off the-shelf components.齿轮的形状是由数学函数曲线定义的，并按照工业领域所建立的规范和标准精确地加工出来的。例如，美国齿轮制造协会已经制定出了齿轮设计和生产标准化的指导性文件。机械工程师能够直接从制造商和供货商那里购买到普通精度的齿轮，或者能够得到事先预制的齿轮箱和传动箱。也会存在标准齿轮不能很好履行任务（如低噪音和振动小）的例外情况。在这种情况下，契约机械商店能够定制和生产特殊要求的齿轮。然而，在大多数的机械设计情况中，将齿轮和齿轮箱做为脱离商品货架（即不销售）的零部件。 No si

57、ngle best type of gear exists, and each variant is we1l suited to a different application. In the fo1lowing subsections, the gears known as spur, rack, bevel, helical, and worm are described. The type that an engineer ultimately chooses for a product will reflect a balance between expense and the ta

58、sk that the gear is expected to perform. 不存在哪种类型的齿轮是最好的说法，每一种类型的齿轮都能很好地适应着不同的用途。在以下各节中将描述直齿轮、齿条、伞齿轮、斜齿轮和蜗轮蜗杆。工程师最终所选择的产品类型将反映出费用和希望齿轮所执行的任务之间的平衡。Spur gearsSpur gears are the simplest type of practical engineering-gear. As shown in Fig3.14, spur gears are cut from cylindrical blanks and their teeth h

59、ave faces that are oriented parallel to the shaft on which the gear is mounted. For the external gears, the teeth formed on the outside of the cylinder; conversely, for an internal gear, the teeth are located on the inside. When two gears having complementary teeth engage and motion is transmitted from one shaft to another, the two gears are said to form a gearset. By convention, the smaller (driving) gear is called the “pini