第6章-Machine-Element.ppt

1、TEXT READING MATERIAL,Unit 6,TEXT Machine Elements However simple, any machine is a combination of individual components generally referred to 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

2、 shaftsthe building blocks of 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

3、 of elements is combined to form a subassembly, 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 form a toothed wheel. The rotation of this gear on a hub or shaft drives other gears

4、 that may rotate faster or slower, depending upon the number of teeth on the basic wheels.,Fig. 6-1 Nut and bolt Fig. 6-2 Spring,Fig. 6-3 Shaft and bearing,Fig.6-4 Clutch,Other fundamental machine elements have evolved from wheel and lever. A wheel must have a shaft on which it may rotate. The wheel

5、 is fastened to the shafts with couplings. The shaft must rest in bearings, may be turned by a pulley with a 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

6、 upon knowledge of the force on the structure and the strength of the materials employed.,Many machine elements are thoroughly standardized. Testing and practical experience have established the most suitable dimensions for common structural and mechanical parts. Through standardization, uniformity

7、of practice and resulting economics are obtained. Not all machine parts in use are standardized, however. In the automotive industry only fasteners, bearings, bushings, chains, and belts are standardized. Crankshafts and connecting rods are not standardized.,LEARN TO SPEAK Mr. Liu:Good afternoon, Mr

8、. Xie. Mr. Xie:Good afternoon, Mr. Liu. Mr. Liu:May I ask you a few questions? Mr. Xie:Yes, of course. What are the problems? Mr. Liu:How many kinds of gears are there in common use? Mr. Xie:There are four kinds of gears. Mr. Liu:What are they? Mr. Xie:They are spur gears, helical gears, worm gears,

9、 and bevel gears. Mr. Liu:Are gears important in nearly all of machinery? Mr. Xie:Yes, they are.,Mr. Liu:What can the spur gears be used for? Mr. Xie:Spur gears are used to transmit power and rotary motion between parallel shafts. Mr. Liu:How do you distinguish the driving element, and the driven el

10、ement? Mr. Xie:The smaller of two gears in mesh is called driving element, and the other is called driven element. Mr. Liu:Where are the spur gears used for? Mr. Xie:They are used for low speed applications and those situations where noise control is not a problem.,Mr. Liu:And where are the helical

11、gears used for? Mr. Xie:They are used for high speed, large power transmission, or where noise abatement is important. Mr. Liu:Thank you for telling me about these. Mr. Xie:Not at all.,NEW WORDS AND EXPRESSIONS combination kmbnen n. 组合，组合物 component kmpnnt n. 元件，组件 dismantle dsmntl vt. 分解(机器)，拆开 nut

12、 nt n. 螺母，螺帽 v. 上(拧，装)螺母 spring spr n. 弹簧 v. 弹回 cam km n. 凸轮，偏心轮 machinery min()r n. 机器，机械 perform pfm v. 执行，完成,associate set v. 联合，结合 key ki: n. 键 subassembly ,sbsembl n. 组合件，部件 coupling kpl n. 联轴节，联轴器 clutch klt n. 离合器 fundamentally fndmentl adv. (从)根本上 hub hb n. 轮毂 evolve vlv v. 进化，演变 assemble se

13、mb()l v. 安装，装配 n. 组件,rivet rvt n. 铆钉 v. 铆接，铆 weld weld v.&n. 焊接，熔焊 adj. 焊接的 thoroughly rl adv. 完全地，彻底地 standardize stnddaz vt. 标准化 establish stbl vt. 确定，建立 dimension dmen()n n. 尺寸 v. 标尺寸,uniformity ,junfmt n. 均匀性，一致性 obtain bten v. 获得 automotive tmtv adj. 自动的，汽车的 distinguish dstgw v. 区别 abatement be

14、tmnt n. 消除,refer to 指的是，称为 combine with 与相结合 in other instances 在其它情况下 spur gear 直齿轮 driving element 主动元件 driven element 被动元件,NOTES 1Thus, if a machine is completely dismantled, a collection of simple parts remains such as nuts, bolts, springs, gears, cams and shaftsthe building blocks of all machin

15、ery. 因此，如果把机床完全拆开，就可以得到像螺母、螺栓、弹簧、齿轮、凸轮及轴等简单零件这是所有机器的标准元件。 if引导条件状语从句，表示如果。,2The most common example of a machine element is a gear, which, fundamentally, is a combination of the wheel and the lever to form a toothed wheel. The rotation of this gear on a hub or shaft drives other gears that may rotat

16、e faster or slower, depending upon the number of teeth on the basic wheels. 机械零件中最常见的是齿轮，它实际上是由轮子和杆组成的带有齿的轮子。装在轴套轮毂或轴上的旋转齿轮驱动其它齿轮做加速或减速转动，这取决于基准齿轮的齿数。, “which”引导非限制性定语从句，修饰“gear”，不定式“to form a toothed wheel”作状语，表示结果。 分词“depending upon”作状语。 3Not all machine parts in use are standardized, however. 然而，

17、并不是所有已经使用的零件都是标准的。 “Not all”是不完全否定，其含义是“并非所有”。句中“hower”的位置可以比较灵活，可以放在句首，也可以放在句子中间，还可以放在句子后面。,EXERCISES 1Discuss the following questions in English: (1) What is machine element? (2) What do the bolts do? (3) Where are the springs used? (4) Which elements are associated in pairs?,3Place a “T” after se

18、ntences that are true and an “F” after those that are false. (1) Gears are used as building blocks for the construction of most devices, apparatus, and machinery. ( ) (2) A pair of gears with different numbers of teeth will rotate at different speeds. ( ) (3) The most common machine element is the g

19、ear from which other fundamental machine elements have developed. ( ) (4) A coupling is a machine element which joins two shafts. ( ) (5) All machine parts have been developed in to standardized designs. ( ),4Contextual reference. (1) In sentence two,“the building blocks”refers to ( ). (a) machine e

20、lements. (b) devices, apparatus and machinery. (2) In sentence 9, the“it” refers to ( ). (a) the brake. (b) the shaft. (c) the clutch.,(3) In sentence 11, the“it” refers to ( ). (a) the pulley. (b) the shaft. (c) the belt. (4) In sentence 13,“these machine elements” refers to( ). (a) bolts and rivet

21、s. (b) wheels, shafts, bearings, clutches, couplings, etc.,5Rephrasing (1) The most common machine element is the gear, which combines features of the wheel and the lever. (2) The hardness of a gear determines its ability to resist wear. (3) In the design, calculations must make the forces fit the m

22、aterials in the simplest construction. (4) Manufacturing engineers have centered their efforts on the development of standardized elements. (5) These parts are produced in large quantities with a high degree of perfection at reduced cost.,READING MATERIAL Gears The transmission of rotary motion from

23、 one shaft to another occurs in nearly every machine one can imagine. Gears constitute one of the best of the various means available for transmitting this motion.,A gear is virtual a wheel with very accurately shaped teeth. These teeth mesh with the teeth of another gear, thus providing a positive-

24、motion drive. The speed ratio between the shafts carrying a pair of gears depends upon the number of teeth in the gears. For example, if a 20-tooth gear drives a gear of 50 teeth, the smaller gear will have to turn 2 1/2 times to cause the larger one to make 1 turn.,Various types of gearing have bee

25、n developed for different purposes. If the shafts are parallel, any of three types may be used: spur, helical, or herringbone. Spur gears are the simplest and least expensive type. They are generally used on drives requiring moderate speeds. Bevel gears serve to transmit power between two intersecti

26、ng shafts. With a worm-gear drive, it is possible to obtain large speed reduction at a right-angle drive. Rack-and-pinion drives are used where it is desirable to transform the rotary motion of one part into linear motion for the other part, or vice versa(as shown in Fig.6-8).,Fig. 6-5 Spur gear and pinion Fig. 6-6 Bevel gears,Fig. 6-7 W