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1、Lesson 1 Machine ElementsHowever simple, any machine is a combination of individual components generally known as machine elements or parts. Thus, if a machine is completely dismantled, a collection of simple parts remains, such as nuts, bolts, springs, gears, cams and shafts-the building blocks of
2、all machinery. A machine element is, therefore, a single unit designed to perform a specific function and capable of combining with other elements. Sometimes certain elements are associated in pairs, such as nuts and bolts or keys and shafts. In other instances, a group of elements is combined to fo
3、rm a assembly, such as bearings, couplings, and clutches. The most common example of a machine element is a gear, which, fundamentally, is a combination of the wheel and the lever to from a toothed wheel. The rotation of this gear on a hub of shaft drives other gears which may rotate faster or slowe
4、r, depending upon the number of teeth on the basic wheel. The material from which the gear is made establishes its strength, and the hardness of its surface determines its resistance to wear. Knowledge of the forces on the gear makes possible the determination of its size. Change in its shape allow
5、modification in its use. These applications, as in most machine elements, have developed into many standard forms, such as spur, bevel, helical, and worm gears. Each of these forms has required the development of a special technology for its production and use. Other fundamental machine elements hav
6、e evolved from wheels and levers. A wheel must have a shaft on which it may rotate. The wheel is fastened to the shaft with a key, and the shaft is joined to other shafts with couplings. The shaft must rest in bearings, such as journal bearings, ball bearings, or roller bearings. The shaft may be st
7、arted by a clutch or stopped with a brake. It may be turned by pulley with a flat belt, a V belt, or a chain connecting it to a pulley on a second shaft. The supporting structure may be assembled with bolts or rivets or by welding. Proper application of these machine elements depends upon a knowledg
8、e of the forces on the structure and the strength of the materials employed. In the design, calculations must accommodate the forces to the materials in the simplest construction. Other machine elements have been evolved whose applications are more specific in construction. Machine parts which are c
9、ommonly used have been developed into standardized designs. Manufacturing specialists have concentrated upon the development of standard elements and have mass-produced these parts with a high degree of perfection at reduced cost. The course “machine elements”, as a branch of engineering, outlines t
10、he methods , rules and standards for designing elements on the basis of the conditions of their operation in a machine, with a view to giving them the most advantageous forms and sizes , choosing the required materials , the degree of accuracy and surface finish and providing for adequate conditions
11、 of manufacture. In doing this, one of the main tasks of the designer should be to ensure that material is economized wherever possible. 然而简单,任何机器是一个结合成独立的部件通常被认为是机械元件或零件。因此,如果一个机器人是完全拆除、一本简单的部分仍然存在,例如螺母、螺栓、弹簧、齿轮、凸轮和shafts-the积木各类机械设备。一种机械零件,因此,一个单一的单位被设计来完成特定的功能,并且有能力结合其他元素。有时某些元素有关,如在双螺母和螺栓或钥匙和轴心。
12、在其他情况下,一群元素结合在一起,形成一个装配,如轴承、联轴器、离合器。最普通的一个例子就是齿轮机械元件,从根本上说,它是一个结合轮子和杠杆的车轮从一个齿。齿轮旋转中心轴的驱动其他齿轮旋转得快些或慢些,这取决于的齿数基本轮。齿轮的材料是由它的力量,并建立了其表面的硬度和耐磨性决定了它。知识的齿轮上的力就有可能确定它的大小。改变它的形状允许使用中对它进行修正。这些应用程序,大多数机器元件,现已发展成许多标准形式,例如柱齿、传动齿轮、螺旋齿轮、蜗轮蜗杆。每一种形式的齿轮都需要一种特殊的技术的发展为其生产和使用。其他的基本机器元件的祖先来自轮子和杠杆。一个轮子必须要有轴,这样它才能旋转。轮子被固定于
13、轴上,这一轴又同其他的轴相偶联。在轴承的轴必须休息,如径向轴承、深沟球轴承、或滚柱轴承。轴可能先由离合器启动,停下了刹车。它可能成为世界上最大的滑轮和一个平皮带传动,V带、链连接到一个滑轮第二轴上。支架结构可聚集在螺钉或铆钉或用焊接。正确运用这些机器元件取决于知识的结构上的力和实力的材料聘用。在设计中,必须考虑调节作用与材料的力在最简单的建设。其他的机器元件的应用相对更专业。机器零件一般用于已发展成为标准的设计。制造业专家现在已集中于开发标准元件并已批量生产的这些零部件高的完善程度降低成本。本课程”、“机器零件的工程的一个分支,概述其方法、规则和标准的基础上,设计元素对其运作的条件在机器中,为
14、了给他们最有利的各种形状和尺寸,选择需要的材料,能达到的精度和表面光洁度和提供足够的条件的制造。在这样做时,最主要的任务之一的设计者必须确保资料是尽可能的节约。New words and expressionsdismantle v.拆除(卸, 下, 开, 散, 掉)bolt n.螺栓, 螺钉 , 插销, 索紧件 v. 栓接, 拧紧associated a.联合的, 关联的, 协同的assembly n. 组合, 装配, 组件coupling n. 联轴器nut n. 螺母key n. 键clutch n. 离合器hub n. (轮)毂, 衬套bevel n. 斜角, 倾斜 a. 斜的, 倾斜
15、的 bevel gear 伞齿轮mass-produced 大量生产的helical a. 螺旋的, 螺旋状的 helical gear 斜齿轮journal n. 轴颈, 耳轴, 枢轴 journal bearing 轴颈轴承rivet n. 铆钉v. 铆接, 铆accommodate v.调节, 适应Lesson 2 GearsGears are direct contact bodies, operating in pairs, that transmit motion and force from one rotating shaft to another, or from a sha
16、ft to a slide (rack), by means of successively engaging projections called teeth. A gear having tooth elements that are straight and parallel to its axis is known as a spur gear. Parallel shafts, however, can also be connected by gears of another type, and a spur gear can be mated with a gear of a d
17、ifferent type. Since the pitch circles roll on one another, the spacing of the teeth on these circles on a mating pair must be equal. This spacing, which is known as the circular pitch and is a measure of tooth size, is the distance between corresponding points on adjacent teeth, measured on the pit
18、ch circle. To prevent jamming as a result of thermal expansion, to aid lubrication, and to compensate for unavoidable inaccuracies in manufacture, all power-transmitting gears must have backlash. This means that on the pitch circles of a mating pair, the space width on the pinion must be slightly gr
19、eater than the tooth thickness on the gear, and vice versa. On instrument gears, backlash can be eliminated by using a gear split down its middle, one half being rotatable relative to the other. A spring forces the split gear teeth to occupy the full width of pinion space. If an involute spur pinion
20、 were made of rubber and twisted uniformly so that the ends rotated about the axis relative to one another, the elements of the teeth, initially straight and parallel to the axis, would become helices. The pinion then in effect would become a helical gear. Helical gears have certain advantages; for
21、example when connecting parallel shafts they have a higher load-carrying capacity than spur gears with the same tooth numbers and cut with the same cutter. Because of the overlapping action of the teeth, they are smoother in action and can operate at higher pitch-line velocities than spur gears. The
22、 pitch-line velocity is the velocity of the pitch circle. Since the teeth are inclined to the axis of rotation, helical gears create an axial thrust .If used singly, this thrust must be absorbed in the shaft bearing. The thrust problem can be overcome by cutting two sets of opposed helical teeth on
23、the same blank. Depending on the method of manufacture, the gear may be of the continuous-tooth herringbone variety or a double-helical gear with a space between the two halves to permit the cutting tool to run out. Helical gears can also be used to connect nonparallel, non-intersecting shafts at an
24、y angle to one another. Ninety degree is the commonest angle at which such gears are used. When the shafts are parallel, the contact between the teeth on mating gears is “line contact” regardless of whether the teeth are straight or helical. When the shafts are inclined, the contact becomes “point c
25、ontact”. For this reason, crossed-axis helical gears do not have as much load-carrying capacity as parallel-shaft helicals. They are relatively insensitive to misalignment, however, and are frequently employed in instruments and positioning mechanisms where friction is the only force opposing their
26、motion. As stated above, the rolling-pitch-circle concept, which applies to gears on parallel shafts, does not apply to gears on nonparallel, non-intersecting shafts. This means that a large speed ratio on one pair of gears, 100 for example, is more easily obtained when the axes are crossed than whe
27、n they are parallel. With parallel shafts, the pinion pitch diameter would have to be 1/100 of the gear pitch diameter, an impractical proportion. With crossed axes, the pinion could have only one helical tooth, or thread, and be as large as necessary for adequate strength. The pinion would look lik
28、e a screw, and the gear would have 100 teeth. In order to achieve line contact and improve the load-carrying capacity of the crossed-axis helical gears, the gear can be made to curve partially around the pinion, in somewhat the same way that a nut envelops a screw. The result would be a cylindrical
29、worm and gear. Worms are also made in the shape of an hourglass, instead of cylindrical, so that they partially envelop the gear. This results in a further increase in load-carrying capacity.Worm gears provide the simplest means of obtaining large ratios in a single pair. They are usually less effic
30、ient than parallel-shaft gears, however, because of an additional sliding movement along the teeth. Because of their similarity, the efficiency of a worm and gear depends on the same factors as the efficiency of screw. Single-thread worms of large diameter have small lead angles and low efficiencies
31、. Multiple-thread worms have larger lead angles and higher efficiencies. For lead angles of about 15 degrees and a coefficient of friction less than 0.15, the efficiency ranges from about 55 percent to 95 percent, and the gear can drive the worm. Such units make compact speed increasers; they have b
32、een used for driving superchargers on aircraft engines. In self-locking worms, the gear cannot drive the worm, and the efficiency is less than 50 percent. For transmitting rotary motion and torque around corners, bevel gears are commonly used. The connected shafts, whose axes would intersect if exte
33、nded, are usually but not necessarily at right angles to one another. The pitch surfaces of bevel gears are rolling, truncated cones, and the teeth, which must be tapered in both thickness and height, are either straight or curved. Although curved tooth bevel gears are called spiral bevel gears, the
34、 curve of the teeth is usually a circular arc. The curvature of the teeth results in overlapping tooth action and a smoother transmission of power than with straight teeth. For high speeds and torques, spiral bevel gears are superior to straight gears in much the same way that helical gears are supe
35、rior to spur gears for connecting parallel shafts. When adapted for shafts that do not intersect, spiral bevel gears are called hypoid gears. The pitch surfaces of these gears are not rolling cones, and the ratio of their mean diameters is not equal to the speed ratio .Consequently, the pinion may h
36、ave few teeth and be made as large as necessary to carry the load. This permits higher speed ratios than with intersecting axes, just as crossed-axis helicals and worm gears can provide higher ratios than parallel helicals. The absence of the proportional rolling-pitch surface requirement is a benef
37、it. Hypoid gears are used on automobiles to connect the drive shaft to the rear axles. The axis of the pinion on the drive shaft is below the gear axis, which permits lowering of the engine and the center of gravity of the vehicle. Since the shafts do not intersect, several gear shafts may be driven
38、 from pinions mounted on a single pinion shaft, as in tandem axles for trucks. In some cases, the desired reduction in angular velocity is too great to achieve using only two gears, when this occurs, several gears must be connected together to give what is known as a gear train. In many gear trains,
39、 it is necessary to be able to shift gears in and out of mesh so as to obtain different combinations of speeds. A good example of this is the automobile transmission.A change-speed gearbox usually comprises the driving shaft end, the layshaft, and the driven shaft, which are installed in the gearcas
40、e. A gear is rigidly mounted on the driving shaft end which protrudes into the gearcase. This gear is driven directly by the engine, through the clutch, and therefore rotates at the speed of the engine. It drives a second, somewhat larger gear which is mounted on the layshaft, so that this shaft rot
41、ates at a lower speed. The driven shaft is mounted in line with the driving shaft and carries the longitudinally movable driven gears corresponding to the various speeds. 齿轮是直接接触物体、操作成双,运动和力的传递到另一个,从一个旋转的轴,或者从一轴滑动(架),通过对预测先后从事叫作齿。有一个齿轮的牙齿的要素,它笔直平行轴是众所周知的齿轮。然而,平行轴齿轮也可以连接的另一种类型,和一个齿轮可以交配齿轮不同类型。因为球场圈卷放
42、在另一个人,间隔的牙齿在这些圆圈在交配必须相等。这个间距,众所周知的圆形场地和牙齿大小是衡量,是对应点之间的距离,测量牙上相邻节圆。防止干扰由于热膨胀,来帮助润滑之关系,并以补偿不可避免的错误,所有power-transmitting生产齿轮必须有反作用。这意味着在球场上的生活圈子交配,空间宽度等因素对齿轮必须稍微比在齿轮齿厚,反之亦然。在仪器的齿轮、反向间隙可消除利用齿轮分裂当中,有一半人会减少其被可相对于其它。一个春天的分裂势力占满轮齿齿轮空间宽度。如果一个渐开线直齿圆柱齿轮是由橡胶和扭曲的统一,使结束关于轴旋转,相对于彼此的元素,最初的牙齿笔直平行轴,将成为一圈。齿轮然后在效果将会变成一
43、个螺旋齿轮。有一定的优越性斜齿圆柱齿轮;例如当连接平行但他们有更高的承载力比直齿圆柱齿轮具有相同的牙齿编号、划痕和相同的刀具。由于重叠的动作,他们是光滑牙齿的行动,并可以操作的速度更高的pitch-line比直齿圆柱齿轮。pitch-line流速的节圆的速度。 自从牙齿斜轴的旋转,创造一个轴向推力圆柱斜齿轮,如果使用个位数,这必须吸收推力轴承。问题是可以克服的推力两套反对通过切割斜齿轮在相同的空白。根据的方法可以制造、齿轮的continuous-tooth人字形品种以及double-helical齿轮间隔空间的两半,允许刀具消耗殆尽。斜齿圆柱齿轮也可以被用来连接不平行,non-intersec
44、ting轴任意角度彼此。90度是最常见的角度,这样的齿轮正在使用。当轴平行,之间的联系进行交配齿轮齿线接触”是“无论是否自己的牙齿直或螺旋。当轴斜,接触变得“点联络”。因为这个原因,crossed-axis斜齿圆柱齿轮不拥有尽可能多的承载能力,为parallel-shaft helicals。 他们是相当敏感的,不过,是错位,经常用于仪器和定位机制摩擦是唯一在他们的运动方向相反的力。为了实现线接触,提高承载力的crossed-axis圆柱斜齿轮,齿轮的曲线,就可以将部分地绕齿轮,以多多少少相同的方式包围着一个螺丝螺母。其结果将是一个圆柱蜗杆和齿轮。蠕虫是也出现了一个沙漏的形状,而不是圆柱形,以
45、致他们部分信封齿轮。这样就导致了进一步增长的承载力。涡轮齿轮提供最简单的意味着获得大的比例,在一个单一的一对。他们通常是更有效率的比parallel-shaft齿轮,然而,由于额外的一个滑动运动沿牙齿。因为他们的相似度、效率和齿轮的虫子一样的因素取决于效率的螺丝。四头蠕虫的大直径有小角度和低效率领先。蠕虫有较大的领先Multiple-thread角度和更高的效率。铅的角度大约15度和摩擦系数小于0.15%,效率的范围很广,从大约55% 95%,齿轮可以驱动虫。increasers此类装置使紧凑的速度,因为他们已经被用来驾驶增压器在飞机引擎。在自锁蠕虫、齿轮不能把虫,效率是小于50%。旋转运动和
46、扭矩传输给每一个角落,伞齿轮,被普遍使用。轴的连接,其轴相交时,如延伸,将通常是,但不一定成直角彼此。球场上的表面,伞齿轮,连绵起伏,锥状,牙齿,必须在两个厚度和高度趋缓,要么是直或弯曲的。虽然曲线齿锥齿轮,被称为螺旋锥齿轮的曲线通常是一个圆弧的牙齿。结果的曲率在牙齿牙作用和光滑重叠的传输功率比直的牙齿。对于高速度和扭力、螺旋伞齿轮优于straig当改编为轴不相交,螺旋伞齿轮,被称为准双曲面齿轮。该齿轮的球场表面滚锥不是率与平均直径不等于.Consequently齿轮速比,可能很少有牙齿,使大如有必要带负荷。这允许更高的速度比和相交轴比值,正如crossed-axis helicals及涡轮齿
47、轮能提供更高的比率比平行helicals。比例rolling-pitch的缺位是一项惠及表面要求。准双曲面齿轮连接用于汽车驱动轴向后方轴。小齿轮轴上的驱动轴齿轮轴以下,它允许降低发动机和重心的车辆。自从不相交,轴驱动齿轮轴可能是几个小齿轮安装在从一个单一的齿轮轴,轴为卡车中串联。在某些情况下,预期的减少角速度也不嫌多只利用两个齿轮,实现当这种情况发生时,几个齿轮必须连接在一起,给了所谓的齿轮火车。在许多齿轮火车是有必要的,它可以改变齿轮的啮合从而得到不同的组合,速度。一个很好的例子,这是汽车变速箱。通常包括一个change-speed变速箱,最后layshaft驱动轴,和驱动轴,这是安装在某轧
48、钢厂齿轮箱。一个齿轮固定安装在驱动轴上结束,到某轧钢厂齿轮箱,凸现。该装置由发动机直接驱动,通过离合器,因此旋转的速度引擎。它驱动一秒钟,有点大齿轮是安装在layshaft转动轴,因此这些以较低的速度。驱动轴安装在符合在驱动轴和携带的纵向移动驱动齿轮对应于不同的速度。New words and expressionsrack n. 齿条, 机架, 导轨involute n. 渐开线pinion n. 小齿轮, 副齿轮pitch n. 节距helical a.螺旋的, 螺旋状的 helical gear 斜齿轮helices n. (pl.) 螺旋线herringbone n. &a.人字形(的)backlash n. 齿侧间隙twist n&.v. 扭曲, 扭转misalignment n.不对准, 不同轴度cylindrical a.圆柱形的, 圆柱体的hourglass n.(古代计时器)沙漏supercharger n. 增压器, 超装器curvature n. 弯曲, 曲率truncate vt.切掉的头, 截短 a.截头的tandem a.&ad.级联的, 串联的overlap v.重叠,
