機(jī)械專(zhuān)業(yè)畢業(yè)設(shè)計(jì)-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>　　Failure Analysis，Dimensional Determination And Analysis，Applications Of Cams</p><p>　　INTRODUCTION</p><p>　　It is absolutely essential that a design engineer know how and why parts fa

2、il so that reliable 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 n

3、o more than 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><

4、;p>　　The 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

5、considered．Another 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

6、the word failure 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

7、 rupture．Excessive 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

8、 fulfills its 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&

9、gt;　　——Deformation</p><p><b>　　——Wear</b></p><p>　　——Corrosion</p><p>　　——Vibration</p><p>　　——Environmental damage</p><p>　　——Loosening of fas

10、tening devices</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 f

11、orming of desired 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 itse

12、lf，while the 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&

13、gt;<p>　　——Camshaft and distributor shaft of automotive engine</p><p>　　——Production machine tools</p><p>　　——Automatic record players</p><p>　　——Printing machines</p>

14、<p>　　——Automatic washing machines</p><p>　　——Automatic dishwashers</p><p>　　The contour of high-speed cams (cam speed in excess of 1000 rpm) must be determined mathematically．However，the

15、vast majority of cams operate at low speeds(less than 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 pr

16、ecision with which the cam contour is machined．</p><p>　　DESIGN PROPERTIES OF MATERIALS</p><p>　　The following design properties of materials are defined as they relate to the tensile test．</

17、p><p>　　Figure 2.7</p><p>　　Static 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 stren

18、gth 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 material．</p><p>　　Stiffness．Stiffness is the de

19、formation-resisting property of a material．The 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

20、material that permits it to absorb energy without permanent deformation．The amount of energy absorbed is represented by the area underneath the stress-strain diagram within the elastic region．</p><p>　　Tough

21、ness．Resilience and toughness are 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

22、 Figure 2．8b．Obviously，the toughness 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 plasti

23、c deformation takes place．Brittle 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 sho

24、ulders，holes，notches，or keyways．</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

25、of a part loaded to rupture．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 compress

26、ive ductility of a material and，as such，is an important characteristic of metals that are to be rolled into sheets．</p><p>　　Hardness．The hardness of a material is its ability to resist indentation or scra

27、tching．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>　　Machinability．M

28、achinability 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>　　Figure 2.8</p><p>　　COMPRESS

29、ION AND SHEAR 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

30、 materials 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

31、 out，but 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

32、are 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

33、the 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 en

34、countered 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 sel

35、dom 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

36、dynamic 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 hi

37、gh 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 compressive

38、 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 chute．The resu

39、lting 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>　　FATIGUE FAI

40、LURE-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．The initial cra

41、ck 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，the initial

42、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 a result of

43、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 visual observati

44、ons 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 strength of a mater

45、ial 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 value below

46、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 and the moto

47、r 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 by an identica

48、l 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 typical for

49、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 cycles involved

50、，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 approximately

51、 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 tested by app

52、lying 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 off as the num

53、ber 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 have no endur

54、ance 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><p&g

55、t;　　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 or elevated t

56、emperatures，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 temperature in

57、creases．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 seen from the gr

58、aph，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 permanently deformed

59、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 creep，which is t

60、he 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 life of the te

61、mperature 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 creep st</p&g

62、t;<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 accurately predict

63、 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 used to clamp t

64、ow 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 bolted joint．The

65、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 steel part un

66、der 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 anticipated life of

67、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 test determi

68、nes 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 properties that ca

69、n 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 reversals may

70、 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 locations where a mac

71、hine 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 maximum stres

72、s 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><p>　

73、　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 residual stress

74、es 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 allowable stress．&l

75、t;/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．If most metal

76、s 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 interference fits．A

77、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 because it is

78、 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 occupy the same

79、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 attainment</p

80、><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．However，this

81、 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 of the plate

82、 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 the amount of o

83、ffset 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 </p>

84、<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 sliding-type

85、 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 translates．F

86、or 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 followe

87、r 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 a resul

88、t 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>　　Cylindrica

89、l 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 several

90、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 used with

91、 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，which is th

92、e 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 depict

93、ed 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 times．Ref

94、er 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 important

95、 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>　　Trace P

96、oint．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 circle that

97、 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，assuming

98、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．</p>