故障的分析、尺寸的决定以及凸轮的分析和应用--中英文翻译

外 文 翻 译故障的分析、尺寸的决定以及凸轮的分析和应用Failure Analysis，Dimensional Determination And Analysis，Applications Of CamsAbstract：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；Cams are among the most versatile mechanisms availableA cam is a simple two-member deviceThe input member is the cam itself，while 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 desiredKey words: failure high-speed cams design propertiesINTRODUCTIONIt is absolutely 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 nuisanceAn 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 speaking，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 mistakingly 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 machine 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 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 is 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 followingStressDeformationWearCorrosionVibrationEnvironmental damageLoosening of fastening devicesThe 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 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 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 engine Production machine toolsAutomatic record playersPrinting 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 large-scale layoutIn general，the greater the cam speed and output load，the greater must be the precision with which the cam contour is machinedDESIGN PROPERTIES OF MATERIALSThe following design properties of materials are defined as they relate to the tensile testStatic Strength The strength of a part is the maximum stress that the 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 materialStiffness Stiffness is the deformation-resisting property of a materialThe slope of the modulus line and，hence，the modulus of elasticity are measures of the stiffness of a materialResilience Resilience is the property of a material that permits it to absorb energy without permanent deformationThe amount of energy absorbed is represented by the area underneath the stress-strain diagram within the elastic regionToughness Resilience 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 Figure 28bObviously，the toughness and resilience of brittle materials are very low and are approximately equalBrittleness A brittle material is one that ruptures before any appreciable plastic deformation takes placeBrittle 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 keywaysDuctility A ductility material exhibits a large amount of plastic deformation prior to ruptureDuctility is measured by the percent of area and percent elongation of a part loaded to ruptureA 5%elongation at rupture is considered to be the dividing line between ductile and brittle materialsMalleability 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 sheetsHardness The 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 hardnessMachinability Machinability is a measure of the relative ease with which a material can be machinedIn general，the harder the material，the more difficult it is to machine COMPRESSION AND SHEAR STATIC STRENGTHIn addition to the tensile tests，there are other types of static load testing that provide valuable informationCompression Testing Most 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，a ductile material fails in compression as a result of deformation，not stressShear Testing Shafts，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 strength 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 LOADSAn 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 in magnitude and direction are called dynamic loadsDynamic loads can be subdivided to the following three categoriesVarying Load With 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 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 Load This type of load is due to impactOne example is an elevator dropping on a nest of springs at the bottom of 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 of 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 cycles 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 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 because 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 materials This brings out an interesting factWhen actual machine parts fail as a result of static loads，they normally deform appreciably because of the ductility of the material.Thus 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 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 failureLet 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 Equation 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 failureFigure 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 can sustain 1 million cycles without failureThe relationship depicted in Figure 2.14 is typical for steel，because the curve becomes horizontal as Napproaches a very large numberThus the endurance limit equals the stress level where the curve approaches a horizontal tangentOwing to the large number 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 not of polished equality，the value of the endurance limit will be lowerFor example，for steel parts with a machined surface finish of 63 microinches ，the percentage drops to about 40%For rough surfaces，the percentage may be as low as 25% The most common type of fatigue is that due 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 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 having an endurance limit，the value is about 25% of the ultimate strengthEFFECTS OF TEMPERATURE ON YIELD STRENGTH AND MODULUS OF ELASTICITYGenerally 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，the 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 temperature to 1000oFFigure 2.24 shows the reduction in the modulus of elasticity E for mild steel as the temperature increasesAs can be seen from the graph，a 30% reduction in modulus of elasticity occurs in going from room temperature to 1000oFIn this figure，we also can see that a part loaded below the proportional limit at room temperature can be permanently deformed under the same load at elevated temperaturesCREEP: A PLASTIC PHENOMENONTemperature effects bring us to a phenomenon called creep，which is the increasing plastic deformation of a part under constant load as a function of timeCreep also occurs at room temperature，but the 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 timeThe creep strength of a material is its ability to resist creep，and creep strength data can be obtained by conducting long-time creep tests simulating actual part operating conditionsDuring the test，the plastic strain is monitored for given material at specified temperaturesSince creep is a plastic deformation phenomenon，the dimensions of a part experiencing creep are permanently alteredThus，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 machineOtherwise，problems such binding or interference can occur Creep also can be a problem in the case where bolts are used to clamp tow parts together at elevated temperaturesThe bolts，under tension，will creep as a function of timeSince the deformation is plastic，loss of clamping force will result in an undesirable loosening of the bolted jointThe extent of this particular phenomenon，called relaxation，can be determined by running appropriate creep strength testsFigure 2.25 shows typical creep curves for three samples of a mild steel part under a constant tensile loadNotice that for the high-temperature case the creep tends to accelerate until the part failsThe time line in the graph (the x-axis) may represent a period of 10 years，the anticipated life of the productSUMMARYThe machine designer must understand the purpose of the static tensile strength testThis test determines a number of mechanical properties of metals that are used in design equationsSuch terms as modulus of elasticity，proportional limit，yield strength，ultimate strength，resilience，and ductility define properties that can be determined from the tensile testDynamic loads are those which vary in magnitude and direction and may require an investigation of the machine parts resistance to failureStress 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 strengthStress 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 circular shaftNote 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 decreasesMethods of reducing the effect of stress concentration usually involve making the shape change more gradualMachine parts are designed to operate at some allowable stress below the yield strength or ultimate strengthThis approach is used 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 exactThe factor of safety is applied to the yield strength or the ultimate strength to determine the allowable stressTemperature can affect the mechanical properties of metalsIncreases in temperature may cause a metal to expand and creep and may reduce its yield strength and its modulus of elasticityIf 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 loadThis phenomenon is useful in assembling parts by means of interference fitsA hub or ring has an inside diameter slightly smaller than the mating shaft or postThe hub is then heated so that it expands enough to slip over the shaftWhen it cools，it exerts a pressure on the shaft resulting in a strong frictional force that prevents loosening 故障的分析、尺寸的决定以及凸轮的分析和应用摘要：作为一名设计工程师有必要知道零件如何发生和为什么会发生故障，以便通过进行最低限度的维修以保证机器的可靠性；凸轮是被应用的最广泛的机械结构之一，是一种仅仅有两个组件构成的设备。主动件本身就是凸轮，而输出件被称为从动件。通过使用凸轮，一个简单的输入动作可以被修改成几乎可以想像得到的任何输出运动。关键词：故障 高速凸轮 设计属性前言介绍：作为一名设计工程师有必要知道零件如何发生和为什么会发生故障，以便通过进行最低限度的维修以保证机器的可靠性。有时一次零件的故障或者失效可能是很严重的一件事情，比如，当一辆汽车正在高速行驶的时候，突然汽车的轮胎发生爆炸等。另一方面，一个零件发生故障也可能只是一件微不足道的小事，只是给你造成了一点小麻烦。一个例子是在一个汽车冷却系统里的暖气装置软管的松动。后者发生的这次故障造成的结果通常只不过是一些暖气装置里冷却剂的损失，是一种很容易被发现并且被改正的情况。能够被零件进行吸收的载荷是相当重要的。一般说来，与静载重相比较，有两个相反方向的动载荷将会引起更大的问题，因此，疲劳强度必须被考虑。另一个关键是材料是可延展性的还是脆性的。例如，脆的材料被认为在存在疲劳的地方是不能够被使用的。很多人错误的把一个零件发生故障或者失效理解成这样就意味着一个零件遭到了实际的物理破损。无论如何，一名设计工程师必须从一个更广泛的范围来考虑和理解变形是究竟如何发生的。一种具有延展性的材料，在破裂之前必将发生很大程度的变形。发生了过度的变形，但并没有产生裂缝，也可能会引起一台机器出毛病，因为发生畸变的零件会干扰下一个零件的移动。因此，每当它不能够再履行它要求达到的性能的时候，一个零件就都算是被毁坏了（即使它的表面没有被损毁）。有时故障可能是由于两个两个相互搭配的零件之间的不正常的磨擦或者异常的振动引起的。故障也可能是由一种叫蠕变的现象引起的，这种现象是指金属在高温下时一种材料的塑性流动。此外，一个零件的实际形状可能会引起故障的发生。例如，应力的集中可能就是由于轮廓的突然变化引起的，这一点也需要被考虑到。当有用两个相反方向的动载荷，材料不具有很好的可延展性时，对应力考虑的评估就特别重要。 一般说来，设计工程师必须考虑故障可能发生的全部方式，包括如下一些方面：压力变形磨损腐蚀振动环境破坏固定设备松动在选择零件的大小与形状的时候，也必须考虑到一些可能会产生外部负载影响的空间因素，例如几何学间断性，为了达到要求的外形轮廓及使用相关的连接件，也会产生相应的残余应力。凸轮是被应用的最广泛的机械结构之一，是一种仅仅有两个组件构成的设备。主动件本身就是凸轮，而输出件被称为从动件。通过使用凸轮，一个简单的输入动作可以被修改成几乎可以想象得到的任何输出运动。常见的一些关于凸轮应用的例子有：凸轮轴和汽车发动机工程的装配专用机床自动电唱机印刷机自动的洗衣机自动的洗碗机高速凸轮(凸轮超过1000 rpm的速度)的轮廓必须从数学意义上来定义。无论如何，大多数凸轮以低速(少于500 rpm)运行而中速的凸轮可以通过一个大比例的图形表示出来。一般说来，凸轮的速度和输出负载越大，凸轮的轮廓在被床上被加工时就一定要更加精密。材料的设计属性当他们与抗拉的试验有关时，材料的下列设计特性被定义如下。静强度：一个零件的强度是指零件在不会失去它被要求的能力的前提下能够承受的最大应力。因此静强度可以被认为是大约等于比例极限，从理论上来说，我们可以认为在这种情况下，材料没有发生塑性变形和物理破坏。刚度：刚度是指材料抵抗变形的一种属性。这条斜的模数线以及弹性模数是一种衡量材料的刚度的一种方法。弹性：弹性是指零件能够吸收能量但并没有发生永久变形的一种材料的属性。吸收的能量的多少可以通过下面弹性区域内的应力图表来描述出来。韧性：韧性和弹性是两种相似的特性。无论如何，韧性是一种可以吸收能量并且不会发生破裂的能力。因此可以通