一級(jí)擺線錐齒輪減速器設(shè)計(jì)掘進(jìn)機(jī)之減速器設(shè)計(jì),一級(jí),擺線,齒輪,減速器,設(shè)計(jì),掘進(jìn)機(jī) 故障的分析、尺寸的決定以及凸輪的分析和應(yīng)用 凸輪是被應(yīng)用的最廣泛的機(jī)械結(jié)構(gòu)之一。凸輪是一種僅僅有兩個(gè)組件構(gòu)成的設(shè)備。主動(dòng)件本身就是凸輪，而輸出件被稱為從動(dòng)件。通過使用凸輪，一個(gè)簡(jiǎn)單的輸入動(dòng)作可以被修改成幾乎可以想像得到的任何輸出運(yùn)動(dòng)。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(dòng)機(jī)工程的裝配 ——專用機(jī)床 ——自動(dòng)電唱機(jī) ——印刷機(jī) ——自動(dòng)的洗衣機(jī) ——自動(dòng)的洗碗機(jī) 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(wú)論如何，大多數(shù)凸輪以低速(少于500 rpm)運(yùn)行而中速的凸輪可以通過一個(gè)大比例的圖形表示出來。一般說來，凸輪的速度和輸出負(fù)載越大，凸輪的輪廓在被床上被加工時(shí)就一定要更加精密。 材料的設(shè)計(jì)屬性 當(dāng)他們與抗拉的試驗(yàn)有關(guān)時(shí)，材料的下列設(shè)計(jì)特性被定義如下。 靜強(qiáng)度： 一個(gè)零件的強(qiáng)度是指零件在不會(huì)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(qiáng)度可以被認(rèn)為是大約等于比例極限，從理論上來說，我們可以認(rèn)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(wú)論如何，韌性是一種可以吸收能量并且不會(huì)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(rèn)為不適合用來做機(jī)床的零部件，因?yàn)楫?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(shí)，脆性材料是無(wú)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(huì)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(zhǎng)度的百分比來測(cè)量的。一個(gè)在發(fā)生破裂的零件，其伸長(zhǎng)量如果為5%，則認(rèn)為該伸長(zhǎng)量就是可延展性和脆性材料分界線。 可鍛性： 可鍛性從根本上來說是指材料的一種在承受擠壓或壓縮是可以發(fā)生塑性變形的能力，同時(shí)，它也是一種在金屬被滾壓成鋼板時(shí)所需金屬的重要性能。 硬度： 一種材料的硬度是指它抵抗擠壓或者拉伸它的能力。一般說來，材料越硬，它的脆性也越大，因此，彈性越小。同樣，一種材料的極限強(qiáng)度粗略與它的硬度成正比。 機(jī)械加工性能（或切削性）： 機(jī)械加工性能是指材料的一種容易被加工的性能。通常，材料越硬，越難以加工。 壓應(yīng)力和剪應(yīng)力 除抗拉的試驗(yàn)之外，還有其它一些可以提供有用信息的靜載荷的實(shí)驗(yàn)類型。 壓縮測(cè)試： 大多數(shù)可延展材料大約有相同特性，當(dāng)它們處于受壓狀態(tài)的緊張狀態(tài)時(shí)。極限強(qiáng)度，無(wú)論如何，不能夠被用于評(píng)價(jià)壓力狀態(tài)。當(dāng)一件具有可延展性的樣品受壓發(fā)生塑性變形時(shí)，材料的其它部分會(huì)凸出來，但是在這種緊張的狀態(tài)下，材料通常不會(huì)發(fā)生物理上的破裂。因此，一種可延展的材料通常是由于變形受壓而損壞的，并不是壓力的原因。 剪應(yīng)力測(cè)試： 軸，螺釘，鉚釘和焊接件被用這樣一種方式定位以致于生產(chǎn)了剪應(yīng)力。一張抗拉試驗(yàn)的試驗(yàn)圖紙就可以說明問題。當(dāng)壓力大到可以使材料發(fā)生永久變形或發(fā)生破壞時(shí)，這時(shí)的壓力就被定義為極限剪切強(qiáng)度。極限剪切強(qiáng)度，無(wú)論如何，不等于處于緊張狀態(tài)的極限強(qiáng)度。例如，以鋼的材料為例，最后的剪切強(qiáng)度是處于緊張狀態(tài)大約極限強(qiáng)度的75%。當(dāng)在機(jī)器零部件里遇到剪應(yīng)力時(shí)，這個(gè)差別就一定要考慮到了。 動(dòng)力載荷 不會(huì)在各種不同的形式的力之間不停發(fā)生變化的作用力被叫作靜載荷或者穩(wěn)定載荷。此外，我們通常也把很少發(fā)生變化的作用力叫作靜載荷。在拉伸實(shí)驗(yàn)中，被分次、逐漸的加載的作用力也被叫作靜載荷。 另一方面，在大小和方向上經(jīng)常發(fā)生變化的力則被稱為動(dòng)載荷。動(dòng)載荷可以被再細(xì)分為以下的3種類型。 變載荷： 所謂變載荷，就是說載荷的大小在變，但是方向不變的載荷。比如說，變載荷會(huì)產(chǎn)生忽大忽小的張應(yīng)力，但不會(huì)產(chǎn)生壓應(yīng)力。 周期性載荷： 像這樣的話，如果大小和方向同時(shí)改變，則就是說這種載荷會(huì)反復(fù)周期性的產(chǎn)生變化的拉應(yīng)力和壓應(yīng)力，這種現(xiàn)象往往就伴隨著應(yīng)力在方向和大小上的周期性變化。 沖擊載荷： 這類載荷是由于沖擊作用產(chǎn)生的。一個(gè)例子就是一臺(tái)升降機(jī)墜落到位于通道底部的一套彈簧裝置上，這套裝置產(chǎn)生的力會(huì)比升降機(jī)本身的重量大上好幾倍。當(dāng)汽車的一個(gè)輪胎碰撞到道路上的一個(gè)突起或者路上的一個(gè)洞時(shí)，相同的沖擊荷載的類型也會(huì)在汽車的減震器彈簧上發(fā)生。 疲勞失效－疲勞極限線圖 正如圖2.10a所示，如果材料的某處經(jīng)常會(huì)產(chǎn)生大量的周期性作用力，那么在材料的表面就很可能會(huì)出現(xiàn)裂縫。裂縫最初是在應(yīng)力超過它極限壓力的地方開始出現(xiàn)的，而通常這往往是有微小的表面缺陷的地方，例如有一處材料出現(xiàn)瑕疵或者一道極小的劃痕。當(dāng)循環(huán)的次數(shù)增加時(shí)，最初的裂縫開始在軸的周圍的逐漸產(chǎn)生許多類似的裂縫。所以說，第一道裂縫的意義就是指應(yīng)力集中的地方，它會(huì)加速其它裂縫的產(chǎn)生。一旦整個(gè)的外圍斗出現(xiàn)了裂縫，裂縫就會(huì)開始向軸的中心轉(zhuǎn)移。最后，當(dāng)剩下的固體的內(nèi)部地區(qū)變得足夠小，且當(dāng)壓力超過極限強(qiáng)度時(shí)，軸就會(huì)突然發(fā)生斷裂。對(duì)斷面的檢查可以發(fā)現(xiàn)一種非常有趣的圖案，如圖2.13中所示。外部的一個(gè)環(huán)形部分相對(duì)光滑一些，因?yàn)樵瓉肀砻嫔舷嗷ソ诲e(cuò)的裂縫之間不斷地發(fā)生磨擦導(dǎo)致了這種現(xiàn)象的產(chǎn)生。無(wú)論如何，中心部分是粗糙的，表明中心是突然發(fā)生了斷裂，類似于脆性材料斷裂時(shí)的現(xiàn)象。 這就表明了一個(gè)有趣的事實(shí)。當(dāng)正在使用的機(jī)器零件由于靜載荷的原因出現(xiàn)問題時(shí)，由于材料具有的延展性，他們通常會(huì)發(fā)生一定程度的變形。 盡管許多地由于靜壓力導(dǎo)致的零件故障可以通過頻繁的做實(shí)際的觀察并且替換全部發(fā)生變形的零件來避免。不管怎樣，疲勞失效有助于起到警告的作用。汽車中發(fā)生故障的零件中的90%的原因都是因?yàn)槠诘淖饔谩? 一種材料的疲勞強(qiáng)度是指在壓力的反復(fù)作用下的抵抗產(chǎn)生裂縫的能力。持久極限是用來評(píng)價(jià)一種材料的疲勞強(qiáng)度的一個(gè)重要參數(shù)。進(jìn)一步說明就是，持久極限就是指在無(wú)限循環(huán)的作用力下不引起失效的壓力值。 讓我們回頭來看在圖2.9 所示的疲勞試驗(yàn)機(jī)器的。試驗(yàn)是這樣被進(jìn)行的：一件小的重物被插入，電動(dòng)機(jī)被啟動(dòng)。在試樣的失效過程中，由計(jì)算寄存器記錄下循環(huán)的次數(shù)N，并且彎曲壓力的相應(yīng)最大量由第2.5 方程式計(jì)算。然后用一個(gè)新的樣品替換掉被毀壞的樣品，并且將另一個(gè)重物插入以增加負(fù)荷量。壓力的新的數(shù)值再次被計(jì)算，并且相同的程序再次被重復(fù)進(jìn)行，直到零件的失效只需要一個(gè)完整周期時(shí)為止。然后根據(jù)壓力值和所需的循環(huán)的次數(shù)來繪制一個(gè)圖。正如圖表2.14a所示圖形，該圖被稱為持久極限曲線或者S-N 曲線。由于這需要的前提是要進(jìn)行無(wú)限次的循環(huán)，所以我們可以以100萬(wàn)個(gè)循環(huán)用來作循環(huán)參考單位。因此，持久極限可以從圖表2.14a那里看到，該材料是在承受了100萬(wàn)個(gè)循環(huán)后而沒有發(fā)生失效的。 用圖2.14 描繪的關(guān)系對(duì)于鋼的材料來說更為典型，因?yàn)楫?dāng)N 接近非常大的數(shù)字時(shí)，曲線就會(huì)變得水平。因此持久極限等于曲線接近一條水平的切線時(shí)的壓力水平。由于包含了大量的循環(huán)，在繪圖時(shí)，N通常會(huì)被按照對(duì)數(shù)標(biāo)度來畫，如圖2.14 b中所示。當(dāng)采用這樣的方法做時(shí)，水平的直線就可以更容易發(fā)現(xiàn)材料的持久極限值。對(duì)于鋼的材料來說，持久極限值大約等于極限強(qiáng)度的50%。無(wú)論如何，已經(jīng)加工完成的表面如果不是一樣的光滑，持久極限的值就會(huì)被降低。例如，對(duì)于鋼材料的零件來說，63 微英寸（ μin ）的機(jī)械加工的表面，零件的持久極限占理論的持久極限的百分比降低到了大約40%。而對(duì)于粗糙的表面來說 （300μin，甚至更多），百分比可能降低到25%左右的水平。 最常見的疲勞損壞的類型通常是由于彎曲應(yīng)力所引起的。其次就是扭應(yīng)力導(dǎo)致的失效，而由于軸向負(fù)載引起的疲勞失效卻極少發(fā)生。彈性材料通常使用從零到最大值之間變化的剪應(yīng)力值來做實(shí)驗(yàn)，以此來模擬材料實(shí)際的受力方式。 就一些有色金屬而論，當(dāng)循環(huán)的次數(shù)變得非常大時(shí)，疲勞曲線不會(huì)隨著循環(huán)次數(shù)的增大而變得水平。，而疲勞曲線的繼續(xù)變小，表明不管作用力有多么的小，多次的應(yīng)力反復(fù)作用都會(huì)引起零件的失效。這樣的一種材料據(jù)說沒有持久極限。對(duì)于大多數(shù)有色金屬來說，它們都有一個(gè)持久極限，數(shù)值大小大約是極限強(qiáng)度的25%。 溫度對(duì)屈服強(qiáng)度和彈性模數(shù)的影響 一般說來，當(dāng)在說明一種擁有特殊的屬性的材料時(shí)，如彈性模數(shù)和屈服強(qiáng)度，表示這些性能在室溫環(huán)境下就可以存在。在低的或者較高的溫度下，材料的特性可能會(huì)有很大的不同。例如，很多金屬在低溫時(shí)會(huì)變得更脆。此外，當(dāng)溫度升高時(shí)，材料的彈性模數(shù)和屈服強(qiáng)度都會(huì)變差。圖2.23 顯示了低碳鋼的屈服強(qiáng)度在從室溫升高到1000oC過程中被降低了大約70%。 當(dāng)溫度升高時(shí)，圖2.24顯示了低碳鋼在彈性模數(shù)E方面的削減。正如從圖上可以看見的那樣，彈性模數(shù)在從室溫升高到1000oC過程中大約降低了30%。從這張圖表中，我們也能看到在室溫下承受了一定載荷而不會(huì)發(fā)生變形的零件卻可能在高溫時(shí)承受相同載荷時(shí)發(fā)生永久變形。 Failure Analysis，Dimensional Determination And Analysis，Applications Of Cams 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． Figure 2.7 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． Figure 2.8 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． Figure 2.13 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 ( μin．)，the percentage drops to about 40%．For rough surfaces (300μin．or greater)，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．