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譯 文 學(xué) 院 機械工程學(xué)院 專 業(yè) 機械設(shè)計制造及其自動化 學(xué) 號 0545501141 姓 名 周 煒 指導(dǎo)教師 李欽奉 教授 Failure Analysis Dimensional Determination And Analysis Applications Of Cams Jack Bauble Abstract 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 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 conceivable output motion that is desired Key words failure high speed cams design properties INTRODUCTION 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 tire 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 latter failure is usually the loss of some radiator coolant a condition that is readily detected and corrected 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 considered Another concern is whether the material is ductile or brittle For example brittle materials are considered to be unacceptable where fatigue is involved 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 appreciable 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 second part Therefore a part fails even if it has not physically broken whenever it no longer fulfills its required function Sometimes failure may be due to abnormal friction or vibration between two mating parts Failure also may be due to a phenomenon called creep which is the plastic flow of a material under load at elevated temperatures In addition the actual shape of a part may be responsible for failure For example stress concentrations due to sudden changes in contour must be taken into account Evaluation of stress considerations is especially important when there are dynamic loads with direction reversals and the material is not very ductile In general the design engineer must consider all possible modes of failure which include the following Stress Deformation Wear Corrosion Vibration Environmental damage Loosening of fastening devices 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 desired contours and the application of interference fit joints 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 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 Camshaft and distributor shaft of automotive engine Production machine tools Automatic record players Printing machines Automatic washing machines Automatic dishwashers 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 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 DESIGN PROPERTIES OF MATERIALS The following design properties of materials are defined as they relate to the tensile test 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 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 material 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 stiffness of a material 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 underneath the stress strain diagram within the elastic region Toughness 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 Figure 2 8b Obviously the toughness and resilience of brittle materials are very low and are approximately equal 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 yield locally at locations of high stress because of geometric stress raisers such as shoulders holes notches or keyways 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 rupture A 5 elongation at rupture is considered to be the dividing line between ductile and brittle materials 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 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 Also the ultimate strength of a material is roughly proportional to its hardness 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 COMPRESSION AND SHEAR STATIC STRENGTH In addition to the tensile tests there are other types of static load testing that provide valuable information Compression Testing Most ductile 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 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 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 the 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 strength in tension This difference must be taken into account when shear stresses are encountered in machine components DYNAMIC LOADS 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 seldom vary to be static loads The force that is gradually applied during a tensile test is therefore a static load 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 Varying Load With varying loads the magnitude changes but the direction does not For example the load may produce high and low tensile stresses but no compressive stresses Reversing Load In this case both the magnitude and direction change These load reversals produce alternately varying tensile and compressive stresses that are commonly referred to as stress reversals 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 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 FATIGUE FAILURE THE ENDURANCE LIMIT DIAGRAM 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 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 the initial crack begins to propagate into a continuous series of cracks all around the periphery of the shaft The conception of the initial crack is itself a stress concentration that accelerates the crack propagation phenomenon Once the entire periphery becomes cracked the cracks start to move toward the center of the shaft Finally when the remaining solid inner area becomes small enough the stress exceeds the ultimate strength and the shaft suddenly breaks Inspection of the break reveals a very interesting pattern as shown in Figure 2 13 The outer annular area is relatively smooth because mating cracked surfaces had rubbed against each other However the center portion is rough indicating a sudden rupture similar to that experienced with the fracture of brittle materials 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 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 parts have failed through fatigue 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 fatigue strength of a material By definition the endurance limit is the stress value below which an infinite number of cycles will not cause failure 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 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 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 until failure requires only one complete cycle A plot is then made of stress versus number of cycles to failure Figure 2 14a shows the plot which is called the endurance limit or S N curve Since it would take forever to achieve an infinite number of cycles 1 million cycles is used as a reference Hence 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 failure 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 where 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 detected by the horizontal straight line For steel the endurance limit equals approximately 50 of the ultimate strength However if the surface finish is not of polished equality the value of the endurance limit will be lower For 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 bending The next most frequent is torsion failure whereas 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 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 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 EFFECTS OF TEMPERATURE ON YIELD STRENGTH AND MODULUS OF ELASTICITY 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 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 temperature increases Figure 2 23 shows that the yield strength for mild steel is reduced by about 70 in going from room temperature to 1000oF 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 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 permanently deformed under the same load at elevated temperatures CREEP A PLASTIC PHENOMENON 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 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 time The 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 conditions During the test the plastic strain is monitored for given material at specified temperatures 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 the amount of creep that will occur during the life of the machine Otherwise 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 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 extent of this particular phenomenon called relaxation can be determined by running appropriate creep strength tests 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 line in the graph the x axis may represent a period of 10 years the anticipated life of the product SUMMARY The machine designer 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 limit yield strength ultimate strength resilience and ductility define properties that can be determined from the tensile test 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 require that the allowable design stress be based on the endurance limit of the material rather than on the yield strength or ultimate strength 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 circular 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 concentration usually involve making the shape change more gradual 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 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 allowable stress 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 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 interference fits A hub or ring has an inside diameter slightly smaller than the mating shaft or post The hub is then heated so that it expands enough to slip over the shaft When it cools it exerts a pressure on the shaft resulting in a strong frictional force that prevents loosening 故障的分析 尺寸的決定以及凸輪的分析和應(yīng)用 摘要 作為一名設(shè)計工程師有必要知道零件如何發(fā)生和為什么會發(fā)生故障 以便 通過進行最低限度的維修以保證機器的可靠性 凸輪是被應(yīng)用的最廣泛的機械結(jié)構(gòu)之 一 是一種僅僅有兩個組件構(gòu)成的設(shè)備 主動件本身就是凸輪 而輸出件被稱為從動 件 通過使用凸輪 一個簡單的輸入動作可以被修改成幾乎可以想像得到的任何輸出 運動 關(guān)鍵詞 故障 高速凸輪 設(shè)計屬性 前言介紹 作為一名設(shè)計工程師有必要知道零件如何發(fā)生和為什么會發(fā)生故障 以便通過進 行最低限度的維修以保證機器的可靠性 有時一次零件的故障或者失效可能是很嚴重 的一件事情 比如 當(dāng)一輛汽車正在高速行駛的時候 突然汽車的輪胎發(fā)生爆炸等 另一方面 一個零件發(fā)生故障也可能只是一件微不足道的小事 只是給你造成了一點 小麻煩 一個例子是在一個汽車冷卻系統(tǒng)里的暖氣裝置軟管的松動 后者發(fā)生的這次 故障造成的結(jié)果通常只不過是一些暖氣裝置里冷卻劑的損失 是一種很容易被發(fā)現(xiàn)并 且被改正的情況 能夠被零件進行吸收的載荷是相當(dāng)重要的 一般說來 與靜載重相比較 有兩個 相反方向的動載荷將會引起更大的問題 因此 疲勞強度必須被考慮 另一個關(guān)鍵是 材料是可延展性的還是脆性的 例如 脆的材料被認為在存在疲勞的地方是不能夠被 使用的 很多人錯誤的把一個零件發(fā)生故障或者失效理解成這樣就意味著一個零件遭到了 實際的物理破損 無論如何 一名設(shè)計工程師必須從一個更廣泛的范圍來考慮和理解 變形是究竟如何發(fā)生的 一種具有延展性的材料 在破裂之前必將發(fā)生很大程度的變 形 發(fā)生了過度的變形 但并沒有產(chǎn)生裂縫 也可能會引起一臺機器出毛病 因為發(fā) 生畸變的零件會干擾下一個零件的移動 因此 每當(dāng)它不能夠再履行它要求達到的性 能的時候 一個零件就都算是被毀壞了 即使它的表面沒有被損毀 有時故障可能 是由于兩個兩個相互搭配的零件之間的不正常的磨擦或者異常的振動引起的 故障也可能是由一種叫蠕變的現(xiàn)象引起的 這種現(xiàn)象是指金屬在高溫下時一種材料的 塑性流動 此外 一個零件的實際形狀可能會引起故障的發(fā)生 例如 應(yīng)力的集中可 能就是由于輪廓的突然變化引起的 這一點也需要被考慮到 當(dāng)有用兩個相反方向的 動載荷 材料不具有很好的可延展性時 對應(yīng)力考慮的評估就特別重要 一般說來 設(shè)計工程師必須考慮故障可能發(fā)生的全部方式 包括如下一些方面 壓力 變形 磨損 腐蝕 振動 環(huán)境破壞 固定設(shè)備松動 在選擇零件的大小與形狀的時候 也必須考慮到一些可能會產(chǎn)生外部負載影響的 空間因素 例如幾何學(xué)間斷性 為了達到要求的外形輪廓及使用相關(guān)的連接件 也會 產(chǎn)生相應(yīng)的殘余應(yīng)力 凸輪是被應(yīng)用的最廣泛的機械結(jié)構(gòu)之一 是一種僅僅有兩個組件構(gòu)成的設(shè)備 主 動件本身就是凸輪 而輸出件被稱為從動件 通過使用凸輪 一個簡單的輸入動作可 以被修改成幾乎可以想象得到的任何輸出運動 常見的一些關(guān)于凸輪應(yīng)用的例子有 凸輪軸和汽車發(fā)動機工程的裝配 專用機床 自動電唱機 印刷機 自動的洗衣機 自動的洗碗機 高速凸輪 凸輪超過 1000 rpm的速度 的輪廓必須從數(shù)學(xué)意義上來定義 無論如何 大多數(shù)凸輪以低速 少于 500 rpm 運行而中速的凸輪可以通過一個大比例的圖形表示出 來 一般說來 凸輪的速度和輸出負載越大 凸輪的輪廓在被床上被加工時就一定要 更加精密 材料的設(shè)計屬性 當(dāng)他們與抗拉的試驗有關(guān)時 材料的下列設(shè)計特性被定義如下 靜強度 一個零件的強度是指零件在不會失去它被要求的能力的前提下能夠承受 的最大應(yīng)力 因此靜強度可以被認為是大約等于比例極限 從理論上來說 我們可以 認為在這種情況下 材料沒有發(fā)生塑性變形和物理破壞 剛度 剛度是指材料抵抗變形的一種屬性 這條斜的模數(shù)線以及彈性模數(shù)是一種 衡量材料的剛度的一種方法 彈性 彈性是指零件能夠吸收能量但并沒有發(fā)生永久變形的一種材料的屬性 吸 收的能量的多少可以通過下面彈性區(qū)域內(nèi)的應(yīng)力圖表來描述出來 韌性 韌性和彈性是兩種相似的特性 無論如何 韌性是一種可以吸收能量并且 不會發(fā)生破裂的能力 因此可以通過應(yīng)力圖里面的總面積來描述韌性 就像用圖 2 8 b 描繪的那樣 顯而易見 脆性材料的韌性和彈性非常低 并且大約相等 脆性 一種脆性的材料就是指在任何可以被看出來的塑性變形之前就發(fā)生破裂的 材料 脆性的材料一般被認為不適合用來做機床的零部件 因為當(dāng)遇到由軸肩 孔 槽 或者鍵槽等幾何應(yīng)力集中源引起的高的應(yīng)力時 脆性材料是無法來產(chǎn)生局部屈服 的現(xiàn)象以適應(yīng)高的應(yīng)力環(huán)境的 延展性 一種延展性材料會在破裂之前表現(xiàn)出很大程度上的塑性變形現(xiàn)象 延展 性是通過可延展的零件在發(fā)生破裂前后的面積和長度的百分比來測量的 一個在