外文翻譯--故障的分析、尺寸的決定以及凸輪的分析和應(yīng)用

1、<p><b>　　附錄</b></p><p><b>　　附錄1</b></p><p>　　Failure Analysis，Dimensional Determination And Analysis，Applications Of Cams</p><p>　　INTRODUCTION</p>

2、;<p>　　It is absolutely essential that a design engineer know how and why parts fail so that reliable machines that require minimum maintenance can be designed．Sometimes a failure can be serious，such as when a tir

3、e blows out on an automobile traveling at high speed．On the other hand，a failure may be no more than a nuisance．An example is the loosening of the radiator hose in an automobile cooling system．The consequence of this lat

4、ter failure is usually the loss of some radiator coolant，a condition that </p><p>　　The type of load a part absorbs is just as significant as the magnitude．Generally speaking，dynamic loads with direction rev

5、ersals cause greater difficulty than static loads，and therefore，fatigue strength must be considered．Another concern is whether the material is ductile or brittle．For example，brittle materials are considered to be unaccep

6、table where fatigue is involved．</p><p>　　Many people mistakingly interpret the word failure to mean the actual breakage of a part．However，a design engineer must consider a broader understanding of what appr

7、eciable deformation occurs．A ductile material，however will deform a large amount prior to rupture．Excessive deformation，without fracture，may cause a machine to fail because the deformed part interferes with a moving seco

8、nd part．Therefore，a part fails(even if it has not physically broken)whenever it no longer fulfills its required fun</p><p>　　In general，the design engineer must consider all possible modes of failure，which i

9、nclude the following．</p><p><b>　　——Stress</b></p><p>　　——Deformation</p><p><b>　　——Wear</b></p><p>　　——Corrosion</p><p>　　——Vibrat

10、ion</p><p>　　——Environmental damage</p><p>　　——Loosening of fastening devices</p><p>　　The part sizes and shapes selected also must take into account many dimensional factors that p

11、roduce external load effects，such as geometric discontinuities，residual stresses due to forming of desired contours，and the application of interference fit joints．</p><p>　　Cams are among the most versatile

12、mechanisms available．A cam is a simple two-member device．The input member is the cam itself，while the output member is called the follower．Through the use of cams，a simple input motion can be modified into almost any con

13、ceivable output motion that is desired．Some of the common applications of cams are</p><p>　　——Camshaft and distributor shaft of automotive engine </p><p>　　——Production machine tools</p>

14、<p>　　——Automatic record players</p><p>　　——Printing machines</p><p>　　——Automatic washing machines</p><p>　　——Automatic dishwashers</p><p>　　The contour of high-s

15、peed cams (cam speed in excess of 1000 rpm) must be determined mathematically．However，the vast majority of cams operate at low speeds(less than 500 rpm) or medium-speed cams can be determined graphically using a large-sc

16、ale layout．In general，the greater the cam speed and output load，the greater must be the precision with which the cam contour is machined．</p><p>　　DESIGN PROPERTIES OF MATERIALS</p><p>　　The fol

17、lowing design properties of materials are defined as they relate to the tensile test．</p><p>　　Figure 2.7</p><p>　　Static Strength． The strength of a part is the maximum stress that the part can

18、 sustain without losing its ability to perform its required function．Thus the static strength may be considered to be approximately equal to the proportional limit，since no plastic deformation takes place and no damage t

19、heoretically is done to the material．</p><p>　　Stiffness． Stiffness is the deformation-resisting property of a material．The slope of the modulus line and，hence，the modulus of elasticity are measures of the s

20、tiffness of a material．</p><p>　　Resilience． Resilience is the property of a material that permits it to absorb energy without permanent deformation．The amount of energy absorbed is represented by the area u

21、nderneath the stress-strain diagram within the elastic region．</p><p>　　Toughness． Resilience and toughness are similar properties．However，toughness is the ability to absorb energy without rupture．Thus tough

22、ness is represented by the total area underneath the stress-strain diagram， as depicted in Figure 2．8b．Obviously，the toughness and resilience of brittle materials are very low and are approximately equal．</p><

23、p>　　Brittleness． A brittle material is one that ruptures before any appreciable plastic deformation takes place．Brittle materials are generally considered undesirable for machine components because they are unable to

24、yield locally at locations of high stress because of geometric stress raisers such as shoulders，holes，notches，or keyways．</p><p>　　Ductility． A ductility material exhibits a large amount of plastic deformati

25、on prior to rupture．Ductility is measured by the percent of area and percent elongation of a part loaded to rupture．A 5%elongation at rupture is considered to be the dividing line between ductile and brittle materials．&l

26、t;/p><p>　　Malleability． Malleability 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 sheets．</p><p

27、>　　Figure 2.8</p><p>　　Hardness． The hardness of a material is its ability to resist indentation or scratching．Generally speaking，the harder a material，the more brittle it is and，hence，the less resilient．

28、Also，the ultimate strength of a material is roughly proportional to its hardness．</p><p>　　Machinability． Machinability is a measure of the relative ease with which a material can be machined．In general，the

29、harder the material，the more difficult it is to machine． </p><p>　　COMPRESSION AND SHEAR STATIC STRENGTH</p><p>　　In addition to the tensile tests，there are other types of static load testing th

30、at provide valuable information．</p><p>　　Compression Testing． Most ductile materials have approximately the same properties in compression as in tension．The ultimate strength，however，can not be evaluated fo

31、r compression．As a ductile specimen flows plastically in compression，the material bulges out，but there is no physical rupture as is the case in tension．Therefore，a ductile material fails in compression as a result of def

32、ormation，not stress．</p><p>　　Shear Testing． Shafts，bolts，rivets，and welds are located in such a way that shear stresses are produced．A plot of the tensile test．The ultimate shearing strength is defined as t

33、he stress at which failure occurs．The ultimate strength in shear，however，does not equal the ultimate strength in tension．For example，in the case of steel，the ultimate shear strength is approximately 75% of the ultimate s

34、trength in tension．This difference must be taken into account when shear stresses are encountered in ma</p><p>　　DYNAMIC LOADS</p><p>　　An applied force that does not vary in any manner is calle

35、d a static or steady load．It is also common practice to consider applied forces that seldom vary to be static loads．The force that is gradually applied during a tensile test is therefore a static load．</p><p&g

36、t;　　On the other hand，forces that vary frequently in magnitude and direction are called dynamic loads．Dynamic loads can be subdivided to the following three categories．</p><p>　　Varying Load． With varying lo

37、ads，the magnitude changes，but the direction does not．For example，the load may produce high and low tensile stresses but no compressive stresses．</p><p>　　Reversing Load． In this case，both the magnitude and d

38、irection change．These load reversals produce alternately varying tensile and compressive stresses that are commonly referred to as stress reversals．</p><p>　　Shock Load． This type of load is due to impact．On

39、e example is an elevator dropping on a nest of springs at the bottom of a chute．The resulting maximum spring force can be many times greater than the weight of the elevator，The same type of shock load occurs in automobil

40、e springs when a tire hits a bump or hole in the road．</p><p>　　FATIGUE FAILURE-THE ENDURANCE LIMIT DIAGRAM</p><p>　　The test specimen in Figure 2.10a．，after a given number of stress reversals w

41、ill experience a crack at the outer surface where the stress is greatest．The initial crack starts where the stress exceeds the strength of the grain on which it acts．This is usually where there is a small surface defect，

42、such as a material flaw or a tiny scratch．As the number of cycles increases，the initial crack begins to propagate into a continuous series of cracks all around the periphery of the shaft．The conception o</p><p

43、>　　This brings out an interesting fact．When actual machine parts fail as a result of static loads，they normally deform appreciably because of the ductility of the material．</p><p>　　Figure 2.13</p>

44、<p>　　Thus many static failures can be avoided by making frequent visual observations and replacing all deformed parts．However，fatigue failures give to warning．Fatigue fail mated that over 90% of broken automobile

45、parts have failed through fatigue．</p><p>　　The fatigue strength of a material is its ability to resist the propagation of cracks under stress reversals．Endurance limit is a parameter used to measure the fat

46、igue strength of a material．By definition，the endurance limit is the stress value below which an infinite number of cycles will not cause failure．</p><p>　　Let us return our attention to the fatigue testing

47、machine in Figure 2.9．The test is run as follows：A small weight is inserted and the motor is turned on．At failure of the test specimen，the counter registers the number of cycles N，and the corresponding maximum bending st

48、ress is calculated from Equation 2.5．The broken specimen is then replaced by an identical one，and an additional weight is inserted to increase the load．A new value of stress is calculated，and the procedure is repeated un

49、til failu</p><p>　　The relationship depicted in Figure 2.14 is typical for steel，because the curve becomes horizontal as N approaches a very large number．Thus the endurance limit equals the stress level wher

50、e the curve approaches a horizontal tangent．Owing to the large number of cycles involved，N is usually plotted on a logarithmic scale，as shown in Figure 2.14b．When this is done，the endurance limit value can be readily det

51、ected by the horizontal straight line．For steel，the endurance limit equals approximately 50%</p><p>　　The most common type of fatigue is that due to bending．The next most frequent is torsion failure，whereas

52、fatigue due to axial loads occurs very seldom．Spring materials are usually tested by applying variable shear stresses that alternate from zero to a maximum value，simulating the actual stress patterns．</p><p>

53、;　　In the case of some nonferrous metals，the fatigue curve does not level off as the number of cycles becomes very large．This continuing toward zero stress means that a large number of stress reversals will cause failure

54、 regardless of how small the value of stress is．Such a material is said to have no endurance limit．For most nonferrous metals having an endurance limit，the value is about 25% of the ultimate strength．</p><p>

55、;　　EFFECTS OF TEMPERATURE ON YIELD STRENGTH AND MODULUS OF ELASTICITY</p><p>　　Generally speaking，when stating that a material possesses specified values of properties such as modulus of elasticity and yield

56、 strength，it is implied that these values exist at room temperature．At low or elevated temperatures，the properties of materials may be drastically different．For example，many metals are more brittle at low temperatures．In

57、 addition，the modulus of elasticity and yield strength deteriorate as the temperature increases．Figure 2.23 shows that the yield strength for mild steel</p><p>　　Figure 2.24 shows the reduction in the modulu

58、s of elasticity E for mild steel as the temperature increases．As can be seen from the graph，a 30% reduction in modulus of elasticity occurs in going from room temperature to 1000oF．In this figure，we also can see that a p

59、art loaded below the proportional limit at room temperature can be permanently deformed under the same load at elevated temperatures．</p><p>　　Figure 2.24</p><p>　　CREEP: A PLASTIC PHENOMENON<

60、;/p><p>　　Temperature effects bring us to a phenomenon called creep，which is the increasing plastic deformation of a part under constant load as a function of time．Creep also occurs at room temperature，but the

61、process is so slow that it rarely becomes significant during the expected life of the temperature is raised to 300oC or more，the increasing plastic deformation can become significant within a relatively short period of t

62、ime．The creep strength of a material is its ability to resist creep，and creep st</p><p>　　Since creep is a plastic deformation phenomenon，the dimensions of a part experiencing creep are permanently altered．T

63、hus，if a part operates with tight clearances，the design engineer must accurately predict the amount of creep that will occur during the life of the machine．Otherwise，problems such binding or interference can occur． </

64、p><p>　　Creep also can be a problem in the case where bolts are used to clamp tow parts together at elevated temperatures．The bolts，under tension，will creep as a function of time．Since the deformation is plasti

65、c，loss of clamping force will result in an undesirable loosening of the bolted joint．The extent of this particular phenomenon，called relaxation，can be determined by running appropriate creep strength tests．</p>&l

66、t;p>　　Figure 2.25 shows typical creep curves for three samples of a mild steel part under a constant tensile load．Notice that for the high-temperature case the creep tends to accelerate until the part fails．The time l

67、ine in the graph (the x-axis) may represent a period of 10 years，the anticipated life of the product．</p><p>　　Figure 2.25</p><p><b>　　SUMMARY</b></p><p>　　The machine d

68、esigner must understand the purpose of the static tensile strength test．This test determines a number of mechanical properties of metals that are used in design equations．Such terms as modulus of elasticity，proportional

69、limit，yield strength，ultimate strength，resilience，and ductility define properties that can be determined from the tensile test．</p><p>　　Dynamic loads are those which vary in magnitude and direction and may

70、require an investigation of the machine part’s resistance to failure．Stress reversals may require that the allowable design stress be based on the endurance limit of the material rather than on the yield strength or ulti

71、mate strength．</p><p>　　Stress concentration occurs at locations where a machine part changes size，such as a hole in a flat plate or a sudden change in width of a flat plate or a groove or fillet on a circul

72、ar shaft．Note that for the case of a hole in a flat or bar，the value of the maximum stress becomes much larger in relation to the average stress as the size of the hole decreases．Methods of reducing the effect of stress

73、concentration usually involve making the shape change more gradual．</p><p>　　Machine parts are designed to operate at some allowable stress below the yield strength or ultimate strength．This approach is used

74、 to take care of such unknown factors as material property variations and residual stresses produced during manufacture and the fact that the equations used may be approximate rather that exact．The factor of safety is ap

75、plied to the yield strength or the ultimate strength to determine the allowable stress．</p><p>　　Temperature can affect the mechanical properties of metals．Increases in temperature may cause a metal to expan

76、d and creep and may reduce its yield strength and its modulus of elasticity．If most metals are not allowed to expand or contract with a change in temperature，then stresses are set up that may be added to the stresses fro

77、m the load．This phenomenon is useful in assembling parts by means of interference fits．A hub or ring has an inside diameter slightly smaller than the mating shaft or post</p><p>　　TYPES OF CAM CONFIGURATIONS

78、</p><p>　　Plate Cams．This type of cam is the most popular type because it is easy to design and manufacture．Figure 6．1 shows a plate cam．Notice that the follower moves perpendicular to the axis of rotation o

79、f the camshaft．All cams operate on the principle that no two objects can occupy the same space at the same time．Thus，as the cam rotates ( in this case，counterclockwise )，the follower must either move upward or bind insid

80、e the guide．We will focus our attention on the prevention of binding and attainment</p><p>　　Figure 6.2 illustrates a plate cam with a pointed follower．Complex motions can be produced with this type of follo

81、wer because the point can follow precisely any sudden changes in cam contour．However，this design is limited to applications in which the loads are very light；otherwise the contact point of both members will wear prematur

82、ely，with subsequent failure．</p><p>　　Two additional variations of the plate cam are the pivoted follower and the offset sliding follower，which are illustrated in Figure 6.3．A pivoted follower is used when r

83、otary output motion is desired．Referring to the offset follower，note that the amount of offset used depends on such parameters as pressure angle and cam profile flatness，which will be covered later．A follower that has no

84、 offset is called an in-line follower．</p><p>　　Figure 6.3</p><p>　　Translation Cams．Figure 6.4 depicts a translation cam．The follower slides up and down as the cam translates motion in the hori

85、zontal direction．Note that a pivoted follower can be used as well as a sliding-type follower．This type of action is used in certain production machines in which the pattern of the product is used as the cam．A variation o

86、n this design would be a three-dimensional cam that rotates as well as translates．For example，a hand-constructed rifle stock is placed in a special lathe．</p><p>　　Figure 6.4</p><p>　　Positive-M

87、otion Cams．In the foregoing cam designs，the contact between the cam and the follower is ensured by the action of the spring forces during the return stroke．However，in high-speed cams，the spring force required to maintain

88、 contact may become excessive when added to the dynamic forces generated as a result of accelerations．This situation can result in unacceptably large stress at the contact surface，which in turn can result in premature we

89、ar．Positive-motion cams require no spring because </p><p>　　Cylindrical Cam．The cylindrical cam shown in Figure 6.5 produces reciprocating follower motion，whereas the one shown in Figure 6.6 illustrates the

90、application of a pivoted follower．The cam groove can be designed such that several camshaft revolutions are required to produce one complete follower cycle．</p><p>　　Grooved-plate Cam．In Figure 6.8 we see a

91、matched-plate cam with a pivoted follower，although the design also can be used with a translation follower．Cams E and F rotate together about the camshaft B．Cam E is always in contact with roller C，while cam F maintains

92、contact with roller D．Rollers C and D are mounted on a bell-crank lever，which is the follower oscillating about point A．Cam E is designed to provide the desired motion of roller C，while cam F provides the desired motion

93、of roller D．</p><p>　　Scotch Yoke Cam．This type of cam，which is depicted in Figure 6.9，consists of a circular cam mounted eccentrically on its camshaft．The stroke of the follower equals two times the eccentr

94、icity e of the cam．This cam produces simple harmonic motion with no dwell times．Refer to Section 6.8 for further discussion．</p><p>　　CAM TERMINOLOGY</p><p>　　Before we become involved with the

95、design of cams，it is desirable to know the various terms used to identify important cam design parameters．The following terms refer to Figure 6.11．The descriptions will be more understandable if you visualize the cam as

96、stationary and the follower as moving around the cam．</p><p>　　Trace Point．The end point of a knife-edge follower or the center of the roller of a roller-type follower．</p><p>　　Cam Contour．The

97、actual shape of the cam．</p><p>　　Base Circle．The smallest circle that can be drawn tangent to the cam contour．Its center is also the center of the camshaft．The smallest radial size of the cam stars at the b

98、ase circle．</p><p>　　Pitch Curve．The path of the trace point，assuming the cam is stationary and the follower rotates about the cam．</p><p>　　Prime Circle．The smallest circle that can be drawn ta