3.1 OVERVIEW
As 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 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 details 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 machine 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 construction and operation of machines.To communicate effectively 1
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 process.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 issues.Finally,this chapter is a good place for you to begin the journey of understanding the inner workings of mechanical hardware.It's 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 .
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。
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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或更好。
3.2 ROLLING ELEMENT BEARINGS
Bearing 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
conditions.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 directions.
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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 elements 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 within 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 4
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 bearing’s inner race rotate together , while the outer race and the transmission’s housing are stationary . When a shaft is supported in this way , the bearing’s 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 power , 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 of 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 . 5
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)。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 Figure 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
block’s bolt holes , this type 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。
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Ball Bearings
The most common type 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 bearing’s connections to the (rotating) shaft and the (stationary) housing . The separator (which can also be called the cage or retainer) keeps the balls evenly spaced around the bearing’s perimeter and prevents them from contacting one another .
Otherwise , if the bearing was used at high speed or subjected to large forces , friction 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,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 7
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 . 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 bearing 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 Bearings
While 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) , cylindrical rollers can also be used in a rolling element bearing in order to better distribute forces over the bearing’s races . Figure 3.6 8
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 essence 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 separator is again used to space roller around the bearing’s 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 bearings
Instead 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 ['k?n?k(?)l]adj. 圆锥的；圆锥形的 9
rollers all have the same taper angle and their centerlines intersect 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 tapered 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 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平稳。
Although they may not be as familiar to you as ball
bearings,tapered roller bearings are quite common in mechanical 10
engineering.they are intended for applications where both thrust and radial forces are present—the wheel bearings in an automobile are a prime example .Wheel bearings must support both the weight of the vehicle (which is a radial force between the bearing and the wheel’s 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 roller 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 bearings
While 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 11
bearings carry loads that are directed primarily along a shaft’s 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 radially 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。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 couplings
When the output shaft from one machine is connected to the input shaft of another ，you might initially expect that the shafts would always be rigidly 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.
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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
configuration 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 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。
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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, 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 train's 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 increase 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 a foundation in place for analyzing force, 14
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，（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 gear's tooth is mathematically defined and
precisely machined according to codes and standards that have been established by industry. 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 and
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 15
of machine design situations, however, 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 single ''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 task 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 gears
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Spur gears are the simplest type of practical engineering-gear. As shown in Fig3.14, spur gears are cut from cylindrical blanks and their teeth have 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 ―pinion‖, and the other (driven) one is simply called the ―gear‖. The pinion and gear are said to be mesh as the point where the teeth approach, contact one another, and then separate.
直齿轮是工程所用齿轮之中最简单的齿轮。如图3.14所示，直齿轮是由圆柱齿胚切削加工得到的，并且直齿轮的齿面与安装齿轮的轴相平行。对于外齿轮来说，齿轮的齿成型在圆柱的外侧；相反对于内齿轮来说，齿轮的齿位于内侧。当两个具有相同齿型的齿轮啮合时，这对齿轮被称之为形成了一个齿轮副。按照惯例，小（主动）齿轮被称之为“pinion”而大（被动）齿轮被称之为“gear”。当两个齿轮的两个齿相互接近、接触然后分离时，主动齿轮和被动齿轮被称之为啮合。
Conceptually, the pinion and gear are regarded as two cylinders that are pressed against one another and that smoothly roll together, in contrast to a collection of many discrete teeth that are
continuously contacting, engaging, and disengaging. The cylinders 17
roll on the outside of one another for two external gears, or one can roll within the other ( one external and one internal gear ). The effective radius of a spur gear, which is also the radius of its
conceptual rolling cylinder, is called the pitch radius. Continuous contact between the pinion and gear imagined to tack place on the two pitch circles. The pitch radius is not the distance from the gear’s center to either the top or bottom lands of a tooth. Instead, the pitch radius is simply the radius that an equivalent cylinder would have if it rotated at the same speed as the pinion or gear.
在理论和概念上，与实际的接触，啮合和分离的一对齿轮啮合相对应，可以将主动齿轮和被动齿轮 看成两个相互压在一起并且光滑滚动的圆柱。对于两个外齿轮而言，两个圆柱体都在外侧滚动，对于一个外齿轮和另一个内齿轮而言，一个圆柱在另一个圆柱内滚动。直齿轮的有效半径，也是理想化滚动圆柱体的半径，被称之为节圆半径。主动齿轮和被动齿轮之间的连续接触假想发生在节圆上。节圆半径并不是由齿轮中心到齿顶或齿底的距离。反之简言之，节圆半径是如果主动齿轮和被动齿轮以相同的切线速度转动时的等效圆柱体的半径。
The thickness of a tooth, and the spacing between adjacent teeth , are each measured along the gear’s pitch circle. The tooth-to–tooth spacing will be slightly larger than the tooth’s thickness itself in order to prevent the teeth from binding against one another as the pinion and gear rotate. On the other hand, if the space between the 18
teeth is too large, the free play could cause undesirable rattle, vibration, and speed fluctuations.
齿轮的厚度和两个齿之间的间隔都是沿着节圆测量的。为了防止当主动齿轮和被动齿轮转动时两个齿互相卡住，两个齿之间的间隔稍大于这个齿轮的厚度。从另一方面讲，如果两个齿之间的间隔过大，自由的咣动将引起不符合需要的响声、振动和速度波动。
In the USCS,the proximity of teeth to one another is mearsured by the diametral pitch
p=N/2r.
where N is the number of teeth on the gear.The diametral pitch therefore has the units of teeth per inch of diameter.In order for
pinion and gear to be compatible,they must have the same value of p or the gearset will bind as the shafts begin to rotate.The product catalogs supplied by gear manufactures generally group compatible gears according to their diametral pitch.By convention,values of p<20 teeth/in.are regarded as being"coarse"gears,and gears with p>=20 teeth/in.are said to be "fine"pitched.Because r and N are
proportional in Equation(3.1)for gears of the same diametral pitch.if a gear's radius is doubled,the number of teeth will likewise double.In the SI,the spacing between teeth is instead measuredd by a quanity called the module m,whith is the reciprocal of the daimetral pitch:m=2r/N.and it has the units of millimeters.
19
在美国USCS单位制中，轮齿间的相似度由径节来度量 p=N/2r，N是齿轮的齿数。径节因此有每英寸直径上齿数的单位，为了让大小齿轮相兼容，他们必须有相同的P值，不然当轴开始转动时齿轮系就会被卡住。由齿轮制造商提供的产品目录通常根据径节对相兼容的齿轮进行分类。按照惯例，P值小于20齿/英寸的被称为粗齿，大于20的被称为细齿，因为对于P相同的齿，半径和齿数在方程中是成比例的，如果半径扩大一倍，齿数也增大一倍。在国际单位制中，齿间距却是由一个称作模数m的量来衡量的，m是P的倒数，m=2r/N；它的单位是毫米。
In principle,a pinion and gear having teeth with any
complementary shape are capable of transmitting rotation between their shafts.In the past,water-and wind-powererd mills,for
instance,ofen used"gears"with teeth that were nothing more than wooden pegs attached to a rotating disk.As you might
imagine,however,such cog teeth did not provide smooth and efficient operation.With arbitrarily shaped gear teeth,if
the(driving)pinion were to turn at a constant speed,the(driven)output gear would be observed to rotate with sight variations in its speed as the teeth engage and disengage.Although perhaps satisfactory for an eighteenth-century grain mill,such chances in output speed are unacceptable for modern high-speed machinery,and they would cause noise,vibration,wear,and inefficiency.
20
原则上来说，一对有任何互补形状的齿的齿轮组是能够在轴之间传递转动的。在过去，水能和风能的风车就使用“齿轮”，它的齿只不过是些木桩。可以想象，如此粗糙的齿不可能提供平稳和有效的运动传递。如果采用任意形状的齿形，当(驱动)齿轮,在恒定的速度下转动,那么可以观察到，在转动，啮合，分离时，输出轮的速度有轻微的波动。尽管这样的齿轮能够满足18世纪的谷粒风车，但是对于现代的高速机械是不能接受的，它们会引起噪音，振动，磨损和低效。
Figure3.18 shows a sector of a spur gear set during three stages of its rotation.As the pinion and gear mesh,the two teeth roll on the surface of one another and their point of contact moves from one side of the pitch circles to the other.Because of the special
mathematical shape of the teeth on a spur gear,however,motion is smoothly transferred between the pinion and gear.The
cross-sectional shape of the tooth is called the involute profile ,and it compensates for the fact that the tooth-to-tooth contact point moves during meshing.The term involute is synonymous with intricate,and this shape ensures that the(output)gear will turn at a constant speed if the(input)pinion does.
图3.18展示了一个直齿齿轮系在三个转动阶段的啮合过程。当大小齿轮啮合时，一对轮齿互相在表面滚动，接触点从一个齿节圆的一侧移动到同一个齿节圆的另一侧。因为齿特殊的数学形状，运动平稳地在小轮和大轮之间 21
转移。齿轮的横截面形状叫做渐开线齿形，渐开线齿形的特点是接触点在啮合时没有相对移动。这个术语“渐开线”与错综复杂同义，而且这个形状能确保在小齿轮匀速转动时大齿轮也以恒定的速度转动。
You can sketch the involute gear tooth profile by drawing a
circle(called the base circle)of any radius that you choose,and then laying s short length of string's along the circle.Hold one end of the string fixed,and gradually unwrap the string's other end,tangent to the ciecle.Mark down the curve that the string's free end follows,and you will have drawn the involute's shape.This geometric
construction is illustrated in Figure3.19,where the base circle is slightly smaller than the pitch circle.The mathematical
characteristics of the involute profile establish what has become known as the fundamental property of gearsets,and this principle enables engineers to view them as cylineders rolling on one
another:For spur gears with involute-shaped teeth,if one gear turus at a constant speed,so will the other.
你能够画出渐开线轮齿轮廓，通过在基圆上画一个任何半径的圆，放一根不长的线，在基圆上固定住线的一端，然后始终与基圆相切渐渐移动线的另一端。画出线活动端点的曲线形状，就得到了渐开线的形状。这种几何结构在图3.19中展示出了，基圆稍微比节圆小些。渐开线轮廓的这个数学特点形成了齿轮系的基本性质，而且这一原理使得工程师把它们当做互相在节圆表面滚动的圆柱体。对于有渐开线形状的齿的直齿齿轮，如果一个以恒 22
定速度转动，那么另一个也是这样。
Rank and Pinion
Gears are sometimes used to convert the rotational motion of a shaft into the straight-line motion of a slider and vice versa.The rack and pinion form the limiting case of a gearset in which the gear has an infinite radius.This configuration is shown in Figure 3.20,where the pinion meshes with a straight toothed rack.With the center point of the pinion being fixed,the rack will move horizontally as the pinion rotates,leftward in the figure as the pinion turns clockwise and vice versa.The rack itself(Figure 3.21)can be supported by rollers,or it might slide on a hard surface lubricated with oil.Racks and pinions are often used in the mechanism for steering an automobile’s front wheels,and they also positioned read/write heads above magnetic disks in vintage computer storage drives,A collection of spur gear and rack components is shown in Figure 3.22.
齿条与齿轮
齿轮有时用来把轴的旋转运动转换为一个滑块的直线运动,或者相反。齿条与齿轮是齿轮组的一个极限情况，即齿轮的半径是无穷大的。这种 形状如图3.20所示，小齿轮与一个直的有齿的齿条。当小齿轮的中心点固定了，齿轮转动时，齿条就会水平运动，当齿轮顺时针动，齿条就向左运动，反之亦然。齿条能被滚轴支撑，或者在被油润滑过的硬表面上滑动。齿条与齿轮经常用于操纵汽车前轮的变速箱机构，它们也用于定位磁盘的读写头。 23
图3.22展示了一些正齿齿轮和齿条的组件。
Bevel Gears
Whereas the teeth of spur gears are arranged on a cylinder,a bevel gear is produced by instead forming teeth on a truncated cone.Figure
3.23is a photograph and cross-sectional drawing of a bevel
gearset;you can see how it is able to redirect the shafts by 90.Bevel gears (Figure 3.24)are appropriate for applications in which two shafts must be connected at a right angle and where extensions to the shaft centerlines would intersect.
伞齿轮
与源于圆柱体的正齿齿轮相反，伞齿是由截头圆锥制造出的，图3.23是伞齿的照片和横截面图。你可以看出它是怎样把轴的转动方向改变90度的，伞齿适用于两个转动传动轴必须是直角，而且中心线的延长线相交的情况。 Helical Gears
Because the teeth on the spur gears of Figure 3.14are straight with faces parallel to their shafts,as two teeth approach one another,they make contact along the full width of each tooth.Similarly,in the
meshing sequence of Figure3.18,the teeth separate and lose contact along the tooth’s entire width at once.Those relatively sudden engagements and disengagements cause spur gears to produce more drivetrain noise and vibration than other types of gears. 24
斜齿轮
因为正齿齿轮的齿面是直的而且平行于轴的，当齿互相靠近时，齿的整个宽度都会接触到，相似地，在啮合的过程中，齿一分开，沿着齿的整个宽度立刻就没有接触，那些相对突然的啮合和分离就会引起齿轮产生比其他类型的齿轮更多的动力传动噪音和振动。
Helical gears are an alternative to spur gears;they offer the
advantage of meshing more smoothly.Helical gears are similar to their spur counterparts in the sense that teeth are still formed on a cylinder,but in this case the teeth are not parallel to the gear’s
shaft.Instead,and as their name implies,the teeth on a helical gear are inclined at an angle so that each tooth wraps on the gear’s cylinder in the shape of a shallow helix.A single helical gear is shown in Figure
3.25.With the same objective of having teeth mesh gradually,bevel gears (as in Figure 3.24) can likewise be formed with spiral instead of straight teeth.
斜齿轮可替换直齿轮，他们具有啮合更平稳的优势，斜齿轮与直齿有相似的地方，如都是在圆柱体上形成的，但是在斜齿轮的情形中齿面与轴不是平行的。相反，如同它的名字所暗示的，斜齿轮的齿是有倾角的，因此齿是以小的螺旋角包裹圆柱体的，要使伞齿同样达到逐渐啮合的目的，伞齿也能做成螺旋状而不是直齿。
Helical gears are evidently more complex to analyze and
manufacture than spur gears.On the other hand,helical gear sets do 25
have the advantage that they produce less noise and vibration in machinery.Automobile automatic transmissions,for instance,are typically constructed using both external and internal helical gears for precisely that reason.In a helical gear set,tooth-to-tooth contact starts at one edge of a tooth and proceeds gradually across its width,thus smoothing out the engagement and disengagement of teeth.Another attribute of helical gears is that they are capable of carrying greater torque and power when compared to similarly sized spur gears because the tooth-to-tooth contact forces are spread over more surface area.
很明显， 斜齿轮在分析和制造方面，比直齿齿轮更复杂，另外，斜齿轮确实有产生更少噪音和振动的优点，例如，汽车的自动变速器，是典型的应用内外斜齿轮的例子。在斜齿轮中，接触在一个齿的边缘开始并且继续渐渐地达到整个宽度，以至使啮合与分离都平滑。另一个特质是，与相似尺寸的直齿齿轮比，因为外力被分散到了更大的面积上，它能够承受更多的扭矩和功率。
We have seen how helix-shaped teeth can be formed on gears that are mounted on parallel shafts,but gears that are attached to
perpendicular shafts can also incorporate helical teeth.The crossed helical gears of Figure 3.26 connect two perpendicular shafts,but unlike the bevel gear set application,the shafts here are offset from 26
one another and extensions to their centerlines do not intersect.A sampler of helical gears is shown in Figure 3.27。
我们已经知道了斜齿轮是怎样在平行的轴上形成的，但是安装在垂直轴上的齿轮也能使用斜齿，图3.26中交叉斜齿圆柱齿轮连接两条相互垂直的轴,但与伞齿轮的应用不同, 该交叉斜齿轮的轴相互偏移，而且中心线的延长线不相交。斜齿轮的一个实例如图3.27所示。
Worm Gearsets
If the helix angle on a pair of crossed helical gears is large
enough,the resulting pair is called a worm and worm gear.Figure 3.28 illustrates this type of gear set where the worm itself has only one tooth that wraps several times around a cylindrical body,similar to a screw. for each revolution of the worm(Figure 3.29),the worm gear(Figure 3.30) advances by just one tooth in its rotation.
如果一对交叉斜齿圆柱齿轮的螺旋角足够大,由此产生的一对啮合的齿轮被称为蜗轮蜗杆。图3.28展示了这种类型的齿轮,蜗杆本身只有一个齿，在一个圆柱体上绕几圈,类似于一个丝杠。对于蜗杆每旋转一圈(图3.29),蜗轮(图3.30)在它的前进方向只旋转一个齿。
Worm gear sets (Figure 3.31) can operate with large
speed-reduction ratios.For instance,if the worm gear has 50 teeth,the speed reduction fo a gear set as in Figure 3.28 would be 50-fold.The ability to package a gear train with large speed reduction into a small physical space is an attractive feature of worm gear sets.However,the 27
tooth profiles within worm gear sets are not involutes.Significant sliding occurs between teeth during meshing,and that fiction results in power loss,heating, and inefficiency compared to other types of gears.
蜗轮蜗杆(图3.31)可以产生很大的减速比。例如,如果蜗轮有50个齿,如图
3.28的蜗轮蜗杆减速比将是50。能够将大减速比齿轮系放到一个小的物理空间是蜗轮蜗杆传动的颇有吸引力的特点。然而,蜗轮齿面不是渐开线形状。在啮合过程中齿面之间发生明显的滑动，相比其他类型的齿轮，滑动产生的摩擦将导致功率损耗,效率低下和发热。
It is also possible to design worm gearsets such that they can be driven in only one direction ,namely,from the worm to the worm gear.for such a self-locking gearset,it would not be possible to reverse the power flow and cause the worm gear to back drive the worm.This one-way drive capability is sometimes exploited in such applications as hoists起重机 or jacks 千斤顶where it is necessary to mechanically prevent the system from being back driven.Not all worm gear sets are self-locking,this characteristic depends on such factors as the helix angle,the amount of friction between the worm and worm gear,and the presence of vibration.
设计蜗轮蜗杆传动时,只能沿着一个方向传动,顾名思义，只能从蜗杆到蜗轮传动。像这样具有自锁性质的传动，它不可能使功率反向传输，既从蜗轮到蜗杆的传动。这种单向传动能力有时用在起重机或千斤顶上，机械地防 28
止系统反向驱动。并不是所有的蜗轮蜗杆传动都是自锁的,这个特性取决于螺旋角,蜗轮蜗杆之间摩擦力的大小,是否存在振动等因素。
3.5 BELT AND CHAIN DRIVES
Like gear sets,belt and chain drives are also available for
transferring rotation,torque,and power between shafts.They are used in such applications as compressors,appliances,machine
tools,automotive engines(Figure 3.32),and sheet metal rolling
mills.Some attributes of belt and chain drives include the abilities to isolate different elements in a drivetrain form shock,to have relatively long working distances between shaft centers,and to tolerate misalignment between adjacent shafts.These favorable
characteristics stem largely from the belt’s flexibility.
像齿轮传动一样，皮带和链条传动也能在轴与轴之间传递旋转运动，扭矩和功率，它们常常用在压缩机、家用电器、机床、汽车发动机（如图3.32）和钢板滚轧设备上。皮带和链传动的属性包括：能在一个动力传动系统中隔离不同的元素之间形成的冲击,允许轴中心线之间有较长的距离,包容相邻轴之间的不对中。这些有利的特点主要源自皮带的柔软性。
The common type of power transmission belt shown in Figure 3.33 is called a v-belt,named appropriately after the wedge-shaped
appearance of its cross section.The grooved pulleys on which the v-belt rides are called sheaves.IN order to have efficient transfer of power between shafts,the belt must be tensioned and have good 29
frictional contact with its sheaves.In fact,the cross section is
designed to wedge the belt down into the groove of the sheaves and increase the friction force between the tow.The capability of v-belts to transfer load between shafts is determined by the wedge angle,β，and the point at which the belt will begin to slip on a sheave,an event that is often accompanied by high-pitched squeal noise.The exterior of a v-belt is made form a synthetic rubber material to generate
friction between the belt and sheaves.These belts are also internally reinforced with fiber or wire cords,which carry the bulk of the tension.
普通类型的电力传输带如图3.33被称为三角皮带,是根据横截面的楔形外形而命名的。带v形槽的滑轮被称为皮带轮。为了在轴之间有效地传递动力,必须将橡胶带张紧，并与皮带轮之间具有良好的摩擦接触。事实上,皮带横截面设计成楔型可以增加牵引皮带与皮带轮之间的摩擦力。三角带在轴之间传递负载的能力取决于楔角β和开始打滑的临界点,皮带打滑往往伴随着高声尖叫的噪声。v形带的外面是由合成橡胶材料制成，以便在皮带和皮带轮之间产生摩擦力。皮带的内部加有纤维或钢丝绳芯,以便承受较大的拉力。 While v-belts are well suited for transmitting power,some amount of slippage invariably occurs between the belt and sheaves because the only contact between the two is friction.If you initially place small paint marks adjacent to one another on the belt and sheave,those marks will shift and separate from one another after some
30
operation.Slippage between gears,on the other hand,cannot occur because of the direct mechanical engagement between their teeth.Gear trains are said to be a synchronous method of
rotation,meaning that the input and output shafts rotate exactly together.
虽然三角带适合动力传输,但由于在皮带和皮带轮之间是靠摩擦力接触的，一定量的打滑总是不可避免的。如果你最初在相邻的皮带和皮带轮之间用记号笔画上印，经过一段时间的运行，这些标记彼此分开。而在齿轮之间,,由于齿之间是机械啮合，不能发生打滑。齿轮可以实现同步旋转,这意味着输入和输出轴能精确地一起旋转。
Belt slippage in not a problem if one is interested only in power transmission,as in a gasoline engine that drives a compressor or generator.On the other hand,for such precision applications as robotic manipulators and calve timing in automotive engines,shaft rotations must remain perfectly synchronized.To address that need,timing belts are formed with molded teeth that mesh into matching grooves on their sheaves.Timing belts,shown in Figure
3.35,combine some of the best features of belts__mechanical
isolation and long working distances between shaft centers__with a gear set’s ability to provide synchronous motion.
如果只是动力传输，皮带打滑不是问题,就像在带动压缩机或发电机的汽油发动机中那样。而对于这样的精密应用，如机器人控制器和汽车发动机 31
同步带，轴的旋转运动必须保持完全同步。为了满足这种需要,在皮带上模塑成齿,与皮带轮上的凹槽匹配。如图3.35的正时皮带,将皮带的最好功能--机械隔离和长距离传输，与同步运动的能力相结合。
Chain drives(Figure 3.36)can also be used when synchronous motion is required and,further,when high torque or power must be applied.Because of their metallic link construction,chains can generally carry greater forces than belts,and they are able to withstand high-temperature environments as well.
当需要同步运动和高转矩、功率时，也可以采用链传动(图3.36)。而且。因为它们是金属链接、链条一般可以传递更大的载荷,它们也能够承受高温的环境。
3.6 MACHINE TOOLS
In addition to being familiar with the hardware components
described in the previous sections,to be an effective engineer you should also have some hands-on experience with the machine tools that one day will be used to fabricate the products that you design. In this section, we discuss drill presses,band aws,lathes,and mills,which are some of the tools available to perform standard
machining operations in prototyping and production shops. Each of these tools is based on the principle of removing unwanted material 32
from a work piece through the cutting action of sharpened bits or blades.
3.6 机床
要想成为一名优秀的工程师，除了熟悉前面章节所描述的各部分硬件以外，你应该有机床方面的实际经验，以便有一天你要用这些机床来制造你所设计的产品。在这一段里，我们讨论钻床、带锯、车床、铣床，这些都是能够在样品车间和生产车间进行标准加工操作的工具。这些工具中的每一个都是基于通过尖锐的钻头和刀片的切削作用从工件上移去多余材料的原理进行工作的。
The fabrication techniques that an engineer seletcts for a certain product will depend,in part ,on the time and expence of setting up the tooling . Some device(for instance ,air-conditioner compressors ,microprocessors ,hydraulic valves ,and tires) are mass-produced ,implying a process that is based on
widespread mechanical automation (Figure3.37). Mass production might be what you first imagine to take place in a large factory , and the manufacture of autumobile eigines is a prime example.The assembly line in that case comprises custom tools , fixtures,and
specialized processes that are capable in the end of producing only certain types of engines for certain vehicles. The assembly line will be set up so that only a small number of operations need to be 33
performed in any one work are before the engine moves on to the next stage.Because finished engines will be produced at a relatively high rate,it is cost-effective for a company to allocate a large amount of factory floor space and many expensive machine tools to the
production line,each of which might only drill a few holes or make a few welds .Aside from hardware that is produced through mass manufacturing ,other products are either unique(for instance ,the Hubble space telescope),or made in relatively small quantities (such as commercial jetliners).The best production method for a given product will ultimately depend on the quantity to be produced ,the allowable cost, and the level of part-to-part variability that is acceptable .
工程师为某一特定产品所选择的加工工艺,部分地取决于安装工具所花费的时间和经费。某些装置（例如:空调压缩机、微处理器、液压阀件和轮胎）是大批量生产的，它取决于机械自动化生产。你首先可能想象到的批量生产发生在大的工厂,而汽车发动机的生产是首选实例。在这种情况下，装配生产线由特制的工具、固定的装置和最终能够生产指定车型的指定型号发动机的专业化生产过程组成。设计和建造生产线时，使得在发动机转到下一个工序之前，在某一工作面上尽可能完成少量的操作。 因为生产的效率很高，所以公司会花费大成本建造大的厂房使用昂贵的机床来组成生产线，每条生产线可能只是做个小洞或者焊接几条缝。除了大批量生产的产品,其他产品要么独一无二的(例如,哈勃太空望远镜),或在相对产量少的(如商用客 34
机)。决定采用哪种类型的生产线取决于生产产品的数量、允许的费用、可以接受的产品互换性。
The drill press shown in Figure 3.38 bores holes into a work piece.A drill bit is held in the rotating chuck , and as a machinist turns the pilot wheel, the bit is lowered into the surface of the work piece .The bit removes material in small chip and creates a hole such as those shown in the cross sections of Figure3.39.As should be the case whenever metal is machined, the point where the bit cuts into the work piece is lubricated;the oil reduces friction,and it also helps to remove heat from the cutting region.For safety
reasons,vises and clamps securely hold the work piece as the hole is bored in order to prevent material from shifting unexpectedly.
钻床---如图3.38所示的在工件表面钻孔的钻床。钻床在工件上钻孔。钻头被安装在转动的卡盘之中并且当机器使主轴转动时，钻头向下运动到工件的表面。钻头移走了小片的材料形成了孔。尽管在任何时候金属都能被加工，但是钻头切削进入工件的位置是被润滑的。油减少了摩擦而且油有助于从切削区域带走热量。为了安全起见，以防工件发生不希望的偏移，当钻孔操作时，用老虎钳和卡具牢固地固定着工件。
A band saw makes rough cuts through metal or plastic stock. The configuration of a typical industrial band saw is shown in
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Figure3.40.The blade is a long, continuous loop having sharp teeth on one edge, and it rides on the drive and idler wheels.A
variable-speed motor enables the operator to adjust the blade's speed depending on the type and thickness of material that will be cut. A tilting table is used for support. The machinist feeds the work piece into the blade, and guides it by hand, to make straight or
slightly curved cuts. When the blade becomes dull and needs to be replaced , or if it should break, the band saw's internal blade grinder and welder are used to clean up the blade's ends, connect them, and form a loop.
带锯主要用于粗加工金属和塑料材料。典型的工业带锯的形状如图
3.40。刀具是长的连续环形边缘带有锋利锯齿的，它靠驱动力和中间齿轮工作。一个可变速电机可以让操作者根据要切割的材料的形状和厚度调整带锯（刀具）的切割速度。机械操作者将工件靠近刀具，用手引导，进行直线或曲线的切削。当刀片变得迟钝,需要更换,或如果它损坏，可以用带锯内部的刀锋研磨机和焊机清理带锯的末端，把它们连接起来形成一个循环
A machinist’s lathe holds a work piece and rotates it as the sharpened edge of a tool cuts and removes material. Some
applications of a lathe include the production of shafts, the turning and resurfacing of disk brake rotors, and thread cutting. A lathe with typical features is shown in Figure 3.41. The headstock and chuck 36
hold one end of a piece of solid round bar stock, for instance, and drive mechanism spins rapidly about its centerline. The tailstock can be used to provide support to the otherwise free end of a long work piece. As a cutting tool is fed against the bar stock and moved along is length, the diameter of work piece is reduced to a desired
dimension. Shoulders that will locate bearings on a shaft, grooves for holding retaining clips, and sharp changes in diameter of a stepped shaft can each be made in this manner.
车床---车床卡住一个工件旋转，并用锋利的刀具来切削，同时刀具做直线进给运动。车床的应用包括轴的生产，发动机旋转和盘式刹车片的修复和断面边缘切削。具有典型特征的车床如图3.41所示。床头箱主轴和卡盘卡主工件的一端，驱动结构沿工件的中心线迅速的旋转工件的一端。尾座用来支撑长的加工工件的自由端。当切削的工具沿着轴线长度方向切削进给时，工件的直径被加工成确定的尺寸。用来放置轴承的台阶，在这种情况下锋利的变化也可以实现。
A mill is useful for machining the rough surfaces of work piece flat and smooth and for shaping them with slots, grooves, and
holes .Shown in Figure 3.42, the mills a versatile machine tool .The work piece is held by a vise on an adjustable table so that the part can be accurately moved in three directions (in the plane of the table and perpendicular to it) to locate the work piece beneath the cutting 37
bit. In a typical application, a piece of metal plate would be cut to approximate shape with a bandsaw, and the mill would be used to machine the surfaces and edges smooth, square, and to the final dimensions.
铣床---被称作铣床的机器经常被用来精确加工一些粗糙的工件上的平的槽、孔、洞。如图3.42所示，铣床是一种万能的机床。工件被钳子夹住在一个可调节的工作台上以便能精确的在在三个方向移动来定位在钻头下的工件。典型的应用是：一块金属被带锯锯切呈大致的形状，然后用铣床加工表面和光滑边角、方块和最终的尺寸。
A machine component known as a lead screw is often used in the mill’s internal mechanisms for positioning the work table beneath spindle. One such application is shown in Figure 3.44 depicts several lead screw converts rotation of a shaft, produced either by a hand wheel or an electric motor, into the straight-line motion of the work table. Figure 3.44 depicts lead screws. As the lead screw turns and engages the nut (which id not allowed to rotate) , the nut moves along the screw with each rotation by an amount equal to the distance between threads.
丝扛—一个被称为丝杠的部件经常被用在车床的内部主轴下的部位工作台用手轮、电动机产生螺杆的运动变成工作台的直线运动。图3.44所示是几种丝杠。当丝杠旋转时螺杆不移动，螺母就和工作台沿水平在移动相应的距离。
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By using electric motors to drive lead screws and position the work piece or cutting bit, mills and other machine tools can be controlled by computers. In that manner, a shop operation can be automated to achieve high precision or to complete a repetitive task on a large number of parts. Figure 3.45(on the previous page) shows an example of such a computer numerically controlled (CNC)mill, A CNC mill performs the same types of operations as a conventional mill, but instead of being manually operated, it is programmed either through entry on a keypad or by downloading machining instructions that were created by computer-aided engineering software. CNC machine tools offer the potential to seamlessly produce physical hardware directly from a computer-generated drawing. With the ability to quickly reprogram machine tools, even a small
general-purpose shop can produce a variety of high-quality machine components. In Chapter 8, we explore a case study in which a suite of computer-aided engineering software packages was used to design, analyze, and ultimately manufacture a mechanical
component.
CNC—铣床---通过电脑用电机驱动丝扛和工件的位置和切削钻头、铣床和其他工具的位置。通过这种方式，自动化的制造可以进行高精度、大批量的操作制造部件。图3.45所示的是一个这样的电脑控制的数字化的铣床。CNC取代手工操作来完成和常规铣床一样的操作，取而代之的是用下载机械 39
指令或者是机械工程师编写的程序。CNC可以提供流畅的生产零部件，甚至一个很小的工厂可以生产多样的高质量的机械零件。在第8章中，我们探索了这样一个例子，在这个例子中计算机辅助硬件包被用来设计、分析、最终制造机械零件。
SUMMERARY
In this chapter, we have described a sampler of the machine components and tools that arise in mechanical engineering.
Although not exhaustive, these summaries of shop tools on gears, and power transmission components on the one hand –and of shop tools on the other hand---should help you understand and better describe to others the anatomy and function of common machines. Bearings, gears, shafts, and the like each have special properties and terminology, and mechanical engineers need to select the
component that is best suited to a particular application.
In Chapter 4-7, we will apply physical laws and the tools of engineering science (namely, the numbers and equations side of engineering) to analyze engineering problems that are based in part on some of the components and tools described I n this chapter later, in Chapter 8, we return to the subject of mechanical design and 40
investigate the major steps involved in defining a design problem, brainstorming potential solution developing a detailed design, and protecting the new technology through patens.
总结--在这章中我们已经描述了在机械制造领域中遇到一些典型的机器零件和机床。虽然不详尽,这些轴承、齿轮、动力传动部件以及车间里的机床,能够很好的帮助你理解和向他人描述常见的机器的内部结构和功能。轴承、齿轮、轴等每一种零部件各有其独特的性能和术语,机械工程师要非常熟悉它们，以便为特定的应用场合选择最适合的零部件。
Self-study and review
1. Explain the differences between ball, straight roller, tapered roller, and thrust bearings. Give examples of real situations in which you would select one to be used instead of another.
2. What is the purpose of a bearing’s separator?
3. Make a cross-sectional drawing of a tapered roller bearing.
4. Why are flexible couplings sometimes used when connecting shafts in a drive train?
5. Make an accurate sketch for the shape of a spur gear’s teeth?
6. What is the fundamental property?
7. Define the terms diametral pitch and module?
8. What is rack and pinion?
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9. what are the characteristics of helical gears, and how do they differ
from spur gears?
10. Make a sketch to show the difference in shaft orientations when bevel gears and crossed helical gears are used?
11. Under what circumstances would you select a v-belt to be used instead of a timing belt in a machine and vice versa?
12. Briefly describe the operation of a drill press, band saw,lathe,and mill.
13. What is a lead screw?
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