機械專業(yè)外文翻譯--故障的分析、尺寸的決定以及凸輪的分析和應用

1、<p>　　Failure Analysis，Dimensional Determination And Analysis，Applications Of Cams——故障的分析、尺寸的決定以及凸輪的分析和應用</p><p>　　It is absolutely essential that a design engineer know how and why parts fail so that r

2、eliable machines that require minimum maintenance can be designed．Sometimes a failure can be serious，such as when a tire blows out on an automobile traveling at high speed．On the other hand，a failure may be no more than

3、a nuisance．An example is the loosening of the radiator hose in an automobile cooling system．The consequence of this latter failure is usually the loss of some radiator coolant，a condition that </p><p>　　The

4、type of load a part absorbs is just as significant as the magnitude．Generally speaking，dynamic loads with direction reversals cause greater difficulty than static loads，and therefore，fatigue strength must be considered．A

5、nother concern is whether the material is ductile or brittle．For example，brittle materials are considered to be unacceptable where fatigue is involved．</p><p>　　Many people mistakingly interpret the word fai

6、lure to mean the actual breakage of a part．However，a design engineer must consider a broader understanding of what appreciable deformation occurs．A ductile material，however will deform a large amount prior to rupture．Exc

7、essive deformation，without fracture，may cause a machine to fail because the deformed part interferes with a moving second part．Therefore，a part fails(even if it has not physically broken)whenever it no longer fulfills it

8、s required fun</p><p>　　In general，the design engineer must consider all possible modes of failure，which include the following．</p><p><b>　　——Stress</b></p><p>　　——Defor

9、mation</p><p><b>　　——Wear</b></p><p>　　——Corrosion</p><p>　　——Vibration</p><p>　　——Environmental damage</p><p>　　——Loosening of fastening devic

10、es</p><p>　　The part sizes and shapes selected also must take into account many dimensional factors that produce external load effects，such as geometric discontinuities，residual stresses due to forming of de

11、sired contours，and the application of interference fit joints．</p><p>　　Cams are among the most versatile mechanisms available．A cam is a simple two-member device．The input member is the cam itself，while the

12、 output member is called the follower．Through the use of cams，a simple input motion can be modified into almost any conceivable output motion that is desired．Some of the common applications of cams are</p><p&g

13、t;　　——Camshaft and distributor shaft of automotive engine</p><p>　　——Production machine tools</p><p>　　——Automatic record players</p><p>　　——Automatic washing machines</p>&l

14、t;p>　　——Automatic dishwashers</p><p>　　The contour of high-speed cams (cam speed in excess of 1000 rpm) must be determined mathematically．However，the vast majority of cams operate at low speeds(less than

15、500 rpm) or medium-speed cams can be determined graphically using a large-scale 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&

16、gt;<p>　　DESIGN PROPERTIES OF MATERIALS</p><p>　　The following design properties of materials are defined as they relate to the tensile test．</p><p>　　Figure 2.7</p><p>　　Sta

17、tic Strength． The strength of a part is the maximum stress that the part can sustain without losing its ability to perform its required function．Thus the static strength may be considered to be approximately equal t

18、o the proportional limit，since no plastic deformation takes place and no damage theoretically is done to the material．</p><p>　　Stiffness． Stiffness is the deformation-resisting property of a material．T

19、he slope of the modulus line and，hence，the modulus of elasticity are measures of the stiffness of a material．</p><p>　　Resilience． Resilience is the property of a material that permits it to absorb ener

20、gy without permanent deformation．The amount of energy absorbed is represented by the area underneath the stress-strain diagram within the elastic region．</p><p>　　Toughness． Resilience and toughness are

21、 similar properties．However，toughness is the ability to absorb energy without rupture．Thus toughness is represented by the total area underneath the stress-strain diagram， as depicted in Figure 2．8b．Obviously，the toughne

22、ss and resilience of brittle materials are very low and are approximately equal．</p><p>　　Brittleness． A brittle material is one that ruptures before any appreciable plastic deformation takes place．Brit

23、tle materials are generally considered undesirable for machine components because they are unable to yield locally at locations of high stress because of geometric stress raisers such as shoulders，holes，notches，or keyway

24、s．</p><p>　　Ductility． A ductility material exhibits a large amount of plastic deformation prior to rupture．Ductility is measured by the percent of area and percent elongation of a part loaded to ruptur

25、e．A 5%elongation at rupture is considered to be the dividing line between ductile and brittle materials．</p><p>　　Malleability． Malleability is essentially a measure of the compressive ductility of a ma

26、terial and，as such，is an important characteristic of metals that are to be rolled into sheets．</p><p>　　Figure 2.8</p><p>　　Hardness． The hardness of a material is its ability to resist ind

27、entation or scratching．Generally speaking，the harder a material，the more brittle it is and，hence，the less resilient．Also，the ultimate strength of a material is roughly proportional to its hardness．</p><p>　　

28、Machinability． Machinability is a measure of the relative ease with which a material can be machined．In general，the harder the material，the more difficult it is to machine．</p><p>　　COMPRESSION AND SHEA

29、R STATIC STRENGTH</p><p>　　In addition to the tensile tests，there are other types of static load testing that provide valuable information．</p><p>　　Compression Testing． Most ductile materi

30、als have approximately the same properties in compression as in tension．The ultimate strength，however，can not be evaluated for compression．As a ductile specimen flows plastically in compression，the material bulges out，bu

31、t there is no physical rupture as is the case in tension．Therefore，a ductile material fails in compression as a result of deformation，not stress．</p><p>　　Shear Testing． Shafts，bolts，rivets，and welds ar

32、e located in such a way that shear stresses are produced．A plot of the tensile test．The ultimate shearing strength is defined as the stress at which failure occurs．The ultimate strength in shear，however，does not equal th

33、e ultimate strength in tension．For example，in the case of steel，the ultimate shear strength is approximately 75% of the ultimate strength in tension．This difference must be taken into account when shear stresses are enco

34、untered in ma</p><p>　　DYNAMIC LOADS</p><p>　　An applied force that does not vary in any manner is called a static or steady load．It is also common practice to consider applied forces that seldo

35、m vary to be static loads．The force that is gradually applied during a tensile test is therefore a static load．</p><p>　　On the other hand，forces that vary frequently in magnitude and direction are called dy

36、namic loads．Dynamic loads can be subdivided to the following three categories．</p><p>　　Varying Load． With varying loads，the magnitude changes，but the direction does not．For example，the load may produce

37、 high and low tensile stresses but no compressive stresses．</p><p>　　Reversing Load． In this case，both the magnitude and direction change．These load reversals produce alternately varying tensile and com

38、pressive stresses that are commonly referred to as stress reversals．</p><p>　　Shock Load． This type of load is due to impact．One example is an elevator dropping on a nest of springs at the bottom of a c

39、hute．The 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 road．</p><p>

40、　　FATIGUE FAILURE-THE ENDURANCE LIMIT DIAGRAM</p><p>　　The test specimen in Figure 2.10a．，after a given number of stress reversals will experience a crack at the outer surface where the stress is greatest．Th

41、e 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，such as a material flaw or a tiny scratch．As the number of cycles increases

42、，the initial crack begins to propagate into a continuous series of cracks all around the periphery of the shaft．The conception o</p><p>　　This brings out an interesting fact．When actual machine parts fail as

43、 a result of static loads，they normally deform appreciably because of the ductility of the material．</p><p>　　Figure 2.13</p><p>　　Thus many static failures can be avoided by making frequent vis

44、ual observations and replacing all deformed parts．However，fatigue failures give to warning．Fatigue fail mated that over 90% of broken automobile parts have failed through fatigue．</p><p>　　The fatigue streng

45、th of a material is its ability to resist the propagation of cracks under stress reversals．Endurance limit is a parameter used to measure the fatigue strength of a material．By definition，the endurance limit is the stress

46、 value below which an infinite number of cycles will not cause failure．</p><p>　　Let us return our attention to the fatigue testing machine in Figure 2.9．The test is run as follows：A small weight is inserted

47、 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 stress is calculated from Equation 2.5．The broken specimen is then replaced b

48、y 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 until failu</p><p>　　The relationship depicted in Figure 2.14 is

49、 typical for steel，because the curve becomes horizontal as N approaches a very large number．Thus the endurance limit equals the stress level where the curve approaches a horizontal tangent．Owing to the large number of cy

50、cles 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 detected by the horizontal straight line．For steel，the endurance limit equals

51、approximately 50%</p><p>　　The most common type of fatigue is that due to bending．The next most frequent is torsion failure，whereas fatigue due to axial loads occurs very seldom．Spring materials are usually

52、tested by applying variable shear stresses that alternate from zero to a maximum value，simulating the actual stress patterns．</p><p>　　In the case of some nonferrous metals，the fatigue curve does not level o

53、ff as the number of cycles becomes very large．This continuing toward zero stress means that a large number of stress reversals will cause failure regardless of how small the value of stress is．Such a material is said to

54、have no endurance limit．For most nonferrous metals having an endurance limit，the value is about 25% of the ultimate strength．</p><p>　　EFFECTS OF TEMPERATURE ON YIELD STRENGTH AND MODULUS OF ELASTICITY</p

55、><p>　　Generally 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 temperature．At low

56、or elevated temperatures，the properties of materials may be drastically different．For example，many metals are more brittle at low temperatures．In addition，the modulus of elasticity and yield strength deteriorate as the t

57、emperature increases．Figure 2.23 shows that the yield strength for mild steel</p><p>　　Figure 2.24 shows the reduction in the modulus of elasticity E for mild steel as the temperature increases．As can be see

58、n 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 part loaded below the proportional limit at room temperature can be permanen

59、tly deformed under the same load at elevated temperatures．</p><p>　　Figure 2.24</p><p>　　CREEP: A PLASTIC PHENOMENON</p><p>　　Temperature effects bring us to a phenomenon called cre

60、ep，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 process is so slow that it rarely becomes significant during the expected l

61、ife of the temperature is raised to 300oC or more，the increasing plastic deformation can become significant within a relatively short period of time．The creep strength of a material is its ability to resist creep，and cre

62、ep st</p><p>　　Since creep is a plastic deformation phenomenon，the dimensions of a part experiencing creep are permanently altered．Thus，if a part operates with tight clearances，the design engineer must accur

63、ately predict the amount of creep that will occur during the life of the machine．Otherwise，problems such binding or interference can occur．</p><p>　　Creep also can be a problem in the case where bolts are us

64、ed to clamp tow parts together at elevated temperatures．The bolts，under tension，will creep as a function of time．Since the deformation is plastic，loss of clamping force will result in an undesirable loosening of the bolt

65、ed joint．The extent of this particular phenomenon，called relaxation，can be determined by running appropriate creep strength tests．</p><p>　　Figure 2.25 shows typical creep curves for three samples of a mild

66、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 line in the graph (the x-axis) may represent a period of 10 years，the anticip

67、ated life of the product．</p><p>　　Figure 2.25</p><p><b>　　SUMMARY</b></p><p>　　The machine designer must understand the purpose of the static tensile strength test．This

68、 test determines a number of mechanical properties of metals that are used in design equations．Such terms as modulus of elasticity，proportional limit，yield strength，ultimate strength，resilience，and ductility define prope

69、rties that can be determined from the tensile test．</p><p>　　Dynamic loads are those which vary in magnitude and direction and may require an investigation of the machine part’s resistance to failure．Stress

70、reversals may require that the allowable design stress be based on the endurance limit of the material rather than on the yield strength or ultimate strength．</p><p>　　Stress concentration occurs at location

71、s 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 circular shaft．Note that for the case of a hole in a flat or bar，the value of the

72、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 concentration usually involve making the shape change more gradual．</p>

73、;<p>　　Machine parts are designed to operate at some allowable stress below the yield strength or ultimate strength．This approach is used to take care of such unknown factors as material property variations and re

74、sidual stresses produced during manufacture and the fact that the equations used may be approximate rather that exact．The factor of safety is applied to the yield strength or the ultimate strength to determine the allowa

75、ble stress．</p><p>　　Temperature can affect the mechanical properties of metals．Increases in temperature may cause a metal to expand and creep and may reduce its yield strength and its modulus of elasticity．

76、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 from the load．This phenomenon is useful in assembling parts by means of interfe

77、rence fits．A hub or ring has an inside diameter slightly smaller than the mating shaft or post</p><p>　　TYPES OF CAM CONFIGURATIONS</p><p>　　Plate Cams．This type of cam is the most popular type

78、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 of the camshaft．All cams operate on the principle that no two objects can occ

79、upy the same space at the same time．Thus，as the cam rotates ( in this case，counterclockwise )，the follower must either move upward or bind inside the guide．We will focus our attention on the prevention of binding and att

80、ainment</p><p>　　Figure 6.2 illustrates a plate cam with a pointed follower．Complex motions can be produced with this type of follower because the point can follow precisely any sudden changes in cam contour

81、．However，this design is limited to applications in which the loads are very light；otherwise the contact point of both members will wear prematurely，with subsequent failure．</p><p>　　Two additional variations

82、 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 rotary output motion is desired．Referring to the offset follower，note that th

83、e 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 offset is called an in-line follower．</p><p>　　Figure 6.3</

84、p><p>　　Translation Cams．Figure 6.4 depicts a translation cam．The follower slides up and down as the cam translates motion in the horizontal direction．Note that a pivoted follower can be used as well as a slidi

85、ng-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 on this design would be a three-dimensional cam that rotates as well as trans

86、lates．For example，a hand-constructed rifle stock is placed in a special lathe．</p><p>　　Figure 6.4</p><p>　　Positive-Motion Cams．In the foregoing cam designs，the contact between the cam and the

87、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 contact may become excessive when added to the dynamic forces generated as

88、a result of accelerations．This situation can result in unacceptably large stress at the contact surface，which in turn can result in premature wear．Positive-motion cams require no spring because </p><p>　　Cyl

89、indrical Cam．The cylindrical cam shown in Figure 6.5 produces reciprocating follower motion，whereas the one shown in Figure 6.6 illustrates the application of a pivoted follower．The cam groove can be designed such that s

90、everal camshaft revolutions are required to produce one complete follower cycle．</p><p>　　Grooved-plate Cam．In Figure 6.8 we see a matched-plate cam with a pivoted follower，although the design also can be us

91、ed 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 contact with roller D．Rollers C and D are mounted on a bell-crank lever，whic

92、h 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 of roller D．</p><p>　　Scotch Yoke Cam．This type of cam，which is

93、 depicted in Figure 6.9，consists of a circular cam mounted eccentrically on its camshaft．The stroke of the follower equals two times the eccentricity e of the cam．This cam produces simple harmonic motion with no dwell ti

94、mes．Refer to Section 6.8 for further discussion．</p><p>　　CAM TERMINOLOGY</p><p>　　Before we become involved with the design of cams，it is desirable to know the various terms used to identify im

95、portant cam design parameters．The following terms refer to Figure 6.11．The descriptions will be more understandable if you visualize the cam as stationary and the follower as moving around the cam．</p><p>　　

96、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 actual shape of the cam．</p><p>　　Base Circle．The smallest circ

97、le 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 base circle．</p><p>　　Pitch Curve．The path of the trace point，as

98、suming the cam is stationary and the follower rotates about the cam．</p><p>　　Prime Circle．The smallest circle that can be drawn tangent to the pitch curve．Its center is also the center of the camshaft．</