机械外文翻译---机械设计

1、附录一machinery design is either to formulate an engineering plan for the satisfaction of a specified need or to solve an engineering problem. it involves a range of disciplines in materials, mechanics, heat, flow, control, electronics and production.machinery design may be simple or enormously complex

2、, easy or difficult, mathematical or nonmathematical, it may involve a trivial problem or one of great importance. good design is the orderly and interesting arrangement of an idea to provide certain results or effects. a well-designed product is functional, efficient, and dependable. such a product

3、 is less expensive than a similar poorly designed product that does not function properly and must constantly be repaired.people who perform the various functions of machinery design are typically called industrial designers. he or she must first carefully define the problem, using an engineering ap

4、proach, to ensure that any proposed solution will solve the right problem. 2it is important that the designer begins by identifying exactly how he or she will recognize a satisfactory alternative, and how to distinguish between two satisfactory alternatives in order to identify the better. so indust

5、rial designers must have creative imagination, knowledge of engineering, production techniques, tools, machines, and materials to design a new product for manufacture, or to improve an existing product.in the modern industrialized world, the wealth and living standards of a nation are closely linked

6、 with their capabilities to design and manufacture engineering products. it can be claimed that the advancement of machinery design and manufacturing can remarkably promote theoverall level of a countrys industrization. our country is playing a more and more vital role in the global manufacturing in

7、dustry. to accelerate such an industrializing process, highly skilled design engineers having extensive knowledge and expertise are neededthe major part of a machine is the mechanical system. 3and the mechanical system is decomposed into mechanisms, which can be further decomposed into mechanical co

8、mponents. in this sense, the mechanical components are the fundamental elements of machinery. on the whole, mechanical components can be classified as universal and special components. bolts, gear, and chains are the typical examples of the universal components, which can be used extensively in diff

9、erent machines across various industrial sectors. turbine blades, crankshaft and aircraft propeller are the examples of the special components, which are designed for some specific purposes.sometimes, design begins when a designer recognizes a need and decides to do something about it. the need is o

10、ften not evident at all; recognition is usually triggered by a particular adverse circumstance or a set of random circumstances, which arise almost simultaneously. identification of need usually consists of an undefined and vague problem statement.definition of problem is necessary to fully define a

11、nd understand the problem, after which it is possible to restate the goal in a more reasonable and realistic way than the original problem statement. definition of the problem must include all the specifications for the thing that is to be designed. obvious items in the specifications are the speeds

12、, feeds, temperature limitations, maximum range, expected variation in the variables, and dimensional and weight limitations.the synthesis is one in which as many alternative possible design approaches are sought, usually without regard for their value or quality. this is also sometimes called the i

13、deation and invention step in which the largest possible number of creative solutions is generated. the synthesis activity includes the specification of material, addition of geometric features, and inclusion of greater dimensional detail to the aggregate design. analysis is a method of determining

14、or describing the nature of something by separating it into its parts. in the process the elements, or nature of the design, are analyzed to determine the fit between the proposed design and the original design goals.evaluation is the final proof of a successful design and usually involves the testi

15、ng of a prototype in the laboratory. here we wish to discover if the design really satisfies the needs. the above description may give an erroneous impression that this process can be accomplished in a linear fashion as listed. on the contrary, iteration is required within the entire process, moving

16、 from any step back to any previous step, in all possible combinations, and doing this repeatedly.communicating the design to others is the finial, vital presentation step in the design process. basically, there are only three means of communication. these are the written, the oral, and the graphica

17、l forms. a successful engineer will be technically competent and versatile in all three forms of communication. the competent engineer should not be afraid of the possibility of not succeeding in a presentation. in fact, the greatest gains are obtained by those willing to risk defeat.it is absolutel

18、y essential that a design engineer know how and why parts fail so that reliable machines that require minimum maintenance can be designedsometimes a failure can be serious，such as when a tire blows out on an automobile traveling at high speedon the other hand，a failure may be no more than a nuisance

19、an example is the loosening of the radiator hose in an automobile cooling systemthe consequence of this latter failure is usually the loss of some radiator coolant，a condition that is readily detected and correctedthe type of load a part absorbs is just as significant as the magnitudegenerally speak

20、ing，dynamic loads with direction reversals cause greater difficulty than static loads，and therefore，fatigue strength must be consideredanother concern is whether the material is ductile or brittlefor example，brittle materials are considered to be unacceptable where fatigue is involvedmany people mis

21、takingly interpret the word failure to mean the actual breakage of a parthowever，a design engineer must consider a broader understanding of what appreciable deformation occursa ductile material，however will deform a large amount prior to ruptureexcessive deformation，without fracture，may cause a mach

22、ine to fail because the deformed part interferes with a moving second parttherefore，a part fails(even if it has not physically broken)whenever it no longer fulfills its required functionsometimes failure may be due to abnormal friction or vibration between two mating partsfailure also may be due to

23、a phenomenon called creep，which is the plastic flow of a material under load at elevated temperaturesin addition，the actual shape of a part may be responsible for failurefor example，stress concentrations due to sudden changes in contour must be taken into accountevaluation of stress considerations i

24、s especially important when there are dynamic loads with direction reversals and the material is not very ductilein general，the design engineer must consider all possible modes of failure，which include the followingthe part sizes and shapes selected also must take into account many dimensional facto

25、rs that produce external load effects，such as geometric discontinuities，residual stresses due to forming of desired contours，and the application of interference fit jointscams are among the most versatile mechanisms availablea cam is a simple two-member devicethe input member is the cam itself，while

26、 the output member is called the followerthrough the use of cams，a simple input motion can be modified into almost any conceivable output motion that is desiredsome of the common applications of cams arecamshaft and distributor shaft of automotive engineproduction machine toolsautomatic record playe

27、rsprinting machinesautomatic washing machinesautomatic dishwashersthe contour of high-speed cams (cam speed in excess of 1000 rpm) must be determined mathematicallyhowever，the vast majority of cams operate at low speeds(less than 500 rpm) or medium-speed cams can be determined graphically using a la

28、rge-scale layoutin general，the greater the cam speed and output load，the greater must be the precision with which the cam contour is machinedthe following design properties of materials are defined as they relate to the tensile teststatic strengththe strength of a part is the maximum stress that the

29、 part can sustain without losing its ability to perform its required functionthus the static strength may be considered to be approximately equal to the proportional limit，since no plastic deformation takes place and no damage theoretically is done to the materialstiffnessstiffness is the deformatio

30、n-resisting property of a materialthe slope of the modulus line and，hence，the modulus of elasticity are measures of the stiffness of a materialresilienceresilience is the property of a material that permits it to absorb energy without permanent deformationthe amount of energy absorbed is represented

31、 by the area underneath the stress-strain diagram within the elastic regiontoughnessresilience and toughness are similar propertieshowever，toughness is the ability to absorb energy without rupturethus toughness is represented by the total area underneath the stress-strain diagram， as depicted in fig

32、ure 28bobviously，the toughness and resilience of brittle materials are very low and are approximately equalbrittlenessa brittle material is one that ruptures before any appreciable plastic deformation takes placebrittle materials are generally considered undesirable for machine components because th

33、ey are unable to yield locally at locations of high stress because of geometric stress raisers such as shoulders，holes，notches，or keywaysductilitya ductility material exhibits a large amount of plastic deformation prior to ruptureductility is measured by the percent of area and percent elongation of

34、 a part loaded to rupturea 5%elongation at rupture is considered to be the dividing line between ductile and brittle materialsmalleabilitymalleability is essentially a measure of the compressive ductility of a material and，as such，is an important characteristic of metals that are to be rolled into s

35、heetshardnessthe hardness of a material is its ability to resist indentation or scratchinggenerally speaking，the harder a material，the more brittle it is and，hence，the less resilientalso，the ultimate strength of a material is roughly proportional to its hardnessmachinabilitymachinability is a measur

36、e of the relative ease with which a material can be machinedin general，the harder the material，the more difficult it is to machinecompression and shear static strengthin addition to the tensile tests，there are other types of static load testing that provide valuable informationcompression testingmos

37、t ductile materials have approximately the same properties in compression as in tensionthe ultimate strength，however，can not be evaluated for compressionas a ductile specimen flows plastically in compression，the material bulges out，but there is no physical rupture as is the case in tensiontherefore，

38、a ductile material fails in compression as a result of deformation，not stressshear testingshafts，bolts，rivets，and welds are located in such a way that shear stresses are produceda plot of the tensile testthe ultimate shearing strength is defined as the stress at which failure occursthe ultimate stre

39、ngth in shear，however，does not equal the ultimate strength in tensionfor example，in the case of steel，the ultimate shear strength is approximately 75% of the ultimate strength in tensionthis difference must be taken into account when shear stresses are encountered in machine componentsdynamic loadsa

40、n applied force that does not vary in any manner is called a static or steady loadit is also common practice to consider applied forces that seldom vary to be static loadsthe force that is gradually applied during a tensile test is therefore a static loadon the other hand，forces that vary frequently

41、 in magnitude and direction are called dynamic loadsdynamic loads can be subdivided to the following three categoriesvarying loadwith varying loads，the magnitude changes，but the direction does notfor example，the load may produce high and low tensile stresses but no compressive stressesreversing load

42、in this case，both the magnitude and direction changethese load reversals produce alternately varying tensile and compressive stresses that are commonly referred to as stress reversalsshock loadthis type of load is due to impactone example is an elevator dropping on a nest of springs at the bottom of

43、 a chutethe resulting maximum spring force can be many times greater than the weight of the elevator，the same type of shock load occurs in automobile springs when a tire hits a bump or hole in the roadfatigue failure-the endurance limit diagramthe test specimen in figure 2.10a，after a given number o

44、f stress reversals will experience a crack at the outer surface where the stress is greatestthe initial crack starts where the stress exceeds the strength of the grain on which it actsthis is usually where there is a small surface defect，such as a material flaw or a tiny scratchas the number of cycl

45、es increases，the initial crack begins to propagate into a continuous series of cracks all around the periphery of the shaftthe conception of the initial crack is itself a stress concentration that accelerates the crack propagation phenomenononce the entire periphery becomes cracked，the cracks start

46、to move toward the center of the shaftfinally，when the remaining solid inner area becomes small enough，the stress exceeds the ultimate strength and the shaft suddenly breaksinspection of the break reveals a very interesting pattern，as shown in figure 2.13the outer annular area is relatively smooth b

47、ecause mating cracked surfaces had rubbed against each otherhowever，the center portion is rough，indicating a sudden rupture similar to that experienced with the fracture of brittle materialsthis brings out an interesting factwhen actual machine parts fail as a result of static loads，they normally de

48、form appreciably because of the ductility of the materialthus many static failures can be avoided by making frequent visual observations and replacing all deformed partshowever，fatigue failures give to warningfatigue fail mated that over 90% of broken automobile parts have failed through fatiguethe

49、fatigue strength of a material is its ability to resist the propagation of cracks under stress reversalsendurance limit is a parameter used to measure the fatigue strength of a materialby definition，the endurance limit is the stress value below which an infinite number of cycles will not cause failu

50、relet us return our attention to the fatigue testing machine in figure 2.9the test is run as follows：a small weight is inserted and the motor is turned onat failure of the test specimen，the counter registers the number of cycles n，and the corresponding maximum bending stress is calculated from equat

51、ion 2.5the broken specimen is then replaced by an identical one，and an additional weight is inserted to increase the loada new value of stress is calculated，and the procedure is repeated until failure requires only one complete cyclea plot is then made of stress versus number of cycles to failurefig

52、ure 2.14a shows the plot，which is called the endurance limit or s-n curvesince it would take forever to achieve an infinite number of cycles，1 million cycles is used as a referencehence the endurance limit can be found from figure 2.14a by noting that it is the stress level below which the material

53、can sustain 1 million cycles without failurethe relationship depicted in figure 2.14 is typical for steel，because the curve becomes horizontal as n approaches a very large numberthus the endurance limit equals the stress level where the curve approaches a horizontal tangentowing to the large number

54、of cycles involved，n is usually plotted on a logarithmic scale，as shown in figure 2.14bwhen this is done，the endurance limit value can be readily detected by the horizontal straight linefor steel，the endurance limit equals approximately 50% of the ultimate strengthhowever，if the surface finish is no

55、t of polished equality，the value of the endurance limit will be lowerfor example，for steel parts with a machined surface finish of 63 microinches ( in)，the percentage drops to about 40%for rough surfaces (300inor greater)，the percentage may be as low as 25%the most common type of fatigue is that due

56、 to bendingthe next most frequent is torsion failure，whereas fatigue due to axial loads occurs very seldomspring materials are usually tested by applying variable shear stresses that alternate from zero to a maximum value，simulating the actual stress patternsin the case of some nonferrous metals，the

57、 fatigue curve does not level off as the number of cycles becomes very largethis continuing toward zero stress means that a large number of stress reversals will cause failure regardless of how small the value of stress issuch a material is said to have no endurance limitfor most nonferrous metals h

58、aving an endurance limit，the value is about 25% of the ultimate strengthgenerally speaking，when stating that a material possesses specified values of properties such as modulus of elasticity and yield strength，it is implied that these values exist at room temperatureat low or elevated temperatures，t

59、he properties of materials may be drastically differentfor example，many metals are more brittle at low temperaturesin addition，the modulus of elasticity and yield strength deteriorate as the temperature increasesfigure 2.23 shows that the yield strength for mild steel is reduced by about 70% in going from room tem