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中北大學2006屆本科畢業(yè)設計說明書
外文文獻原文
Helical，Worm and Bevel Gears
In the force analysis of spur gars, the forces are assumed to act in a single plain. In this lesson we shall study gears in which the forces have three dimensions. The reason for this, in the case of helical gears, is that the teeth are not parallel to the axis of rotation. And in the case of bevel gears, the rotational axes are not parallel to each other. There are other reasons, as we shall learn.
Helical gears are used to transmit motion between parallel shafts. The helix angle is the same on each gear, but one gear must have a right—hand helix and the other a left—hand helix. The shape of the tooth is an involute helicoids. If a piece of paper cut in the shape of a parallclogram is wrapped around a cylinder, the angular edge of the paper becomes a helix. If we unwind this paper, each point on the angular edge generates an involute curve. The surface obtained when every point on the edge generates an involute is called an involute helicoids.
The initial contact of spur—gear teeth is a line extending all the way across the face of the tooth. The initial contact of helical gear teeth is a point,which changes into a line as the teeth come into more engagement. In spur gears the line of contact is parallel to the axis of the rotation; in helical gears, the line is diagonal across the face of the tooth.It is this gradual engagement of the teeth and the smooth transfer of load from one tooth to another ,which give helical gears the ability to transmit heavy loads at high speeds. Helical gears subject the shaft bearings to both radial and thrust loads. When the thrust loads become high or are objectionable for other reasons, it may be desirable to use double helical gears. A double helical gear（herringbone）is equivalent to two helical gears of opposite hand, mounted side by side on the same shaft. They develop opposite thrust reaction and thus cancel out the thrust load. When two or more single helical gears are mounted on the same shaft, the hand of the gears should be selected so as to produce the minimum thrust load.
Crossed—helical, or spiral, gears are those in which the shaft centerlines are neither parallel nor intersecting. The teeth of crossed-helical gears have point contact with each other, which changes to line contact as the gears wear in. For this reason they will carry out very small loads and are mainly for instrumental applications, and are definitely not recommended for use in the transmission of power. There is no difference between a crossed helical gear and a helical gear until they are mounted in mesh with each other. They are manufactured in the same way. A pair of meshed crossed helical gears usually have the same hand; that is, a right-hand driver goes with a right hand driven. In the design of crossed-helical gears, the minimum sliding velocity is obtained when the helix angle are equal. However, when the helix angle are not equal, the gear with the larger helix angle should he used as the driver if both gears have the same hand.
Worm gears are similar to crossed helical gears. The pinion or worm has a small number of teeth, usually one to four, and since they completely wrap around the pitch cylinder they are called threads. Its mating gear is called a worm gear, which is not a true helical gear. A worm and worm gear are used to provide a high angular-velocity reduction between nonintersecting shafts which are usually at right angle. The worm gear is not a helical gear because its face is made concave to fit the curvature, nature of the worm in order to provide line contact instead of point contact. However, a disadvantage of worm gearing is the high sliding velocities across the teeth, the same as with crossed helical gears.
Worn gearing are either single or double enveloping. A single enveloping gearing is one in which the gear wraps around or partially encloses the worm, A gearing in which each element partially encloses the other is, of course, a double enveloping worm gearing. The important difference between the two is that area contact exists between the teeth of double enveloping gears while only line contact between those of single-enveloping gears. The worm and worm gear of a set have the same hand of helix as for crossed helical gears, but the helix angles are usually quite different. The helix angle on the worm is generally quite large, and that on the gear very small. Because of this, it is usual to specify the lead angle on the worm, which is the complement of the worm helix angle, and the helix angle on the gear; the two angles are equal for a 9O deg. shaft angle.
When gears are to be used to transmit motion between intersecting shafts, some form of bevel gear is required. Although bevel gears are usually made for a shaft angle of 9O deg., they may be produced for almost any shaft angle. The teeth may be east, milled, or generated. Only the generated teeth may be classed as accurate. In a typical bevel gear mounting, one of the gear is often mounted outboard of the bearing. This means that shaft deflection can be more pronounced and have a greater effect on the contact of the teeth. Another difficulty, which occurs in predicting the stress in bevel gear teeth, is the fact that the teeth are tapered.
Straight bevel gears are easy to design and simple to manufacture and give very good results in service if they are mounted accurately and positively. As in the case of spur gears, however, they become noisy at higher values of the pitch-line velocity. In these eases it is often good design practice to go to ~he spiral bevel gear, which is the bevel counterpart of the helical gear, as in the case of helical gears, spiral bevel gears give a much smoother tooth action than straight bevel gears, and hence are useful where high speed are encountered.
It is frequently desirable, as in the case of automotive differential applications, to have gearing similar to bevel gears but with the shaft offset. Such gears are called hypoid gears because their pitch surfaces are hyperboloids of revolution. The tooth action between such gears is a combination of rolling and sliding along a straight line and has much in common with that of worm gears
SAND CASTING
Most metal casting are made by pouring molten metal into a prepared cavity and allowing it to solidify. The process dates from antiquity. The largest bronze statue in existence to-day is the great Sun Buddha in Nara, Japan. Cast in the eighth century, it weighs 551 tons(500 metric tons) and is more than 71 ft (21m) high. Artisans of the Shang Dynasty in China ( 1766 - 1222B. C. ) created art works of bronze with delicate filigree as sophisticated as anything that is designed and produced today.
There are many casting processes available today, mid selecting the best one to produced particular part depends on several basic factors, such as cost, size. production rate. finish, tolerance, section thickness, physical-mechanical properties, intricacy of design mach inability, and weld ability.
Sand casting. the oldest and still the most widely used casting process. will be presented in more detail than the other processes since many of the concepts carry over into those processes as well.
Green Sand
Green sand generally consists of silica sand and additives coated by rubbing the sand grains together with clay uniformly wetted with water. More stable and refractory sands have been developed, such as fused silica, zircon, and mullets, which replace lower-cost silica and have only 2% linear expansion at ferrous metal temperatures. Also, relatively un-stable water and clay bonds are being replaced with synthetic resins, which are much mores table at elevated temperatures.
Green sand molding is used to produce a wide variety of castings in sizes of less than around to as large as several tons. This versatile process is applicable to both ferrous and nonferrous materials.
Green sand can be used to produce intricate molds since it provides for rapid collapsibility: that is, the mold is much less resistant to the contraction of the casting as it solidifies than are other molding processes. This results in less stress and strain in the casting.
The sand is rammed or compacted around the pattern high a variety of methods, including hand or pneumatic-tool ramming, jolting (abrupt mechanical shaking), squeezing (com-pressing the top and bottom mold surfaces), and driving the sand into the mold at high velocities (sad slinging). Sand slings are usually resented for use in making very large casting where great volumes of sand are handled.
For smaller casting, a two-part metal box or flask referred to as a cope and drag issued. First the pattern is positioned on a mold board. and the drag or lower half of the flask is positioned over it. Parting powder is sprinkled on the paten and the box is filled with sand. A jolt squeeze machine quick]y compacts the sand. The flask is then turned over and again parting powder is dusted on it. The cope is then positioned on the top half of the flask and is filled with sand, and the two-part mold with the patter board sandwiched in between is squeezed.
Patterns
Patterns for sand casting have traditionally been made of wood or metal. However, it has been found that wood patterns change as much as 3% due to heat and moisture. This factor alone would put many casting out of acceptable tolerance for more exacting specifications. Now, patterns are often made from epoxies and from cold-setting rubber with stabilizing inserts. Patterns of simple design, with one or more flat surface, can be molded in one piece, provided that they can be withdrawn without disturbing the compacted sand. Other patterns may be split into two or more parts to facilitate their removal from the sand when using two-part flasks. The pattern must be tapered to permit easy removal from the sand. The taper is referred to as draft. When a part does not have some natural draft, it must be added. A more recent innovation in patterns for sand casting has been to make them out of foamed polystyrene that is vaporized by the molten metal. This type of casting, known as the full-mold process, does not require pattern draft.
Spruces, Runners, and Gates.
Access to the mold cavity for entry of the molten metal is provided by sprees, runners, and gates, as shown in Fig. 7 I. A pouring basin can be carved in the sand at the top of the spree, or a pour box, which provides a large opening, may be laid over the spree to facilitate pouring. After the metal is poured, it cools most rapidly in the sand mold. Thus the outer surface forms a shell that permits the still molten metal near the center to flow toward it. As a result, the last portion of the casting to freeze will be deficient in metal and, in the absence oaf supplemental metal-feed source, will result in some form of shrinkage.2 This shrinkage may take the form of gross shrinkage (large cavities) or the more subtle micro shrinkage ( finely dispersed porosity). These porous spots can be avoided by the use of risers, as shown in Fig.7-1, which provide molten metal to make up for shrinkage losses.
Cores
Cores are placed in molds wherever it is necessary to preserve the space it occupies in the mold as a void in the resulting castings. As sown in Fig.7-1, the core will be put in place after the pastern is removed. To ensure its proper location, the pattern has extensions known as core prints that leave cavities in the mold into which the core is seated. Sometimes the core may be molded integrally with the green sand and is then referred to as a green-sand core. Generally, the core is made of sand bonded with core oil, some organic bonding materials, and water. These materials are thoroughly blended and placed in a mold or core box. After forming, they are removed and baked at 350°to 450°F ( 177°to 232°C). Cores that consist of two or more parts are pasted together after baking.
CO2 Cores
CO2 cores are made by ramming up moist sand in a core box. Sodium silicate is used as a binder, which is quickly hardened by blowing CO2 gas over it. The C02 system has the advantage of making the cores immediately available.
Pouring the Metal
Several types of containers are used to move the molten metal from the furnace to the pouring area. Large castings of the floor-and-pit type are poured with a ladle that has a plug in the button, or, as it is called, a bottom-pouring ladle. It is also employed in mechanized operations where the molds are moved along a line and each is poured as it is momentarily stopped beneath the large bottom-pour ladle.
ladles used for pouring ferrous metals are lined with a high alumina-content refractory. After long use and oxidation, it can be broken out and replaced. Ladles used in handling ferrous metals most be preheated with gas flames to approximately 2600° to 2700°F ( 1427° to 1482°C) before filling. Once the ladle is filled, it is used constantly until it has been emptied.
For nonferrous metals, simple clay-graphite crucibles are used. While they are quite susceptible to breakage, they are very resistant to the metal and will hold up a long time under normal condition. They usually do not require preheating, although care must he taken to avoid moisture pickup. For this reason they are sometimes baked out to assure dryness.
The pouring process must he carefully controlled, since the temperature of the melt greatly affects the degree of liquid contraction before solidification, the rate of solidification, which in turn affects the around of columnar growth present at the mold wall, the extent and nature of the dendrite growth, the degree of alloy burnout, and the feeding characteristics of the rise ring system.
Finishing Operations
After the castings have solidified and cooled somewhat. they are placed on a shakeout table or grating on which the sand mold is broken up, leaving the casting free to be picked out. The casting is then taken to the finishing room where the gates and risers are removed. Small gates and risers may he broken off with a hammer if the material is bride. Larger ones requiem sawing, cutting with a roach, or shearing. Unwanted metal protrusions such as fins, bosses, and small portions of gates and risers need to be smoothed off to blend with the surface. Most of this work is done with a heavy-duty grinder and the process is known as snagging or snag grinding. On large castings it is easier to move the grinder than the work, so swing-type grinders are used. Smaller castings are brought to stand or bench-type grinders. Hans and pneumatic chisels are also used to trim castings. A more recent method of removing excess metal from famous castings is with a carbon air torch. This consists of a carbon rod and high-amperage current with a stream of compressed air blowing at the base of it. This oxidizes and removes the metal as soon as it is molten, In many foundries this method has replaced nearly all chipping and grinding operation.
譯文
斜齒輪、蝸桿蝸輪和錐齒輪
在直齒圓柱齒輪的受力分析中，是假定各力作用在單一平面的。在這一課題中，我們將研究作用力具有三維坐標的齒輪。因此，在斜齒輪的情況下，其齒向是不平行于回轉軸線的。而在錐齒輪的情況中各回轉軸線互相不平行。像我們將要討論的那樣，尚有其他道理需要學習、掌握。
斜齒輪用于傳遞平行軸之間的運動。傾斜角度每個齒輪都一樣，但一個必須右旋斜齒，而另一個必須是左旋斜齒。齒的形狀是一漸開線螺旋面。如果一張被剪成平行四邊形(矩形)的紙張包圍在齒輪圓柱體上，紙上印出齒的角刃邊就變成斜線。如果我展開這張紙，在斜角刃邊上的每一個點就發(fā)生一漸開線曲線。
直齒圓柱齒輪輪齒的初始接觸處是跨過整個齒面而伸展開來的線。斜齒輪輪齒的初始接觸是一點，當齒進入更多的嚙臺時，它就變成線。在直齒圓柱齒輪中，接觸線是平行于回轉軸線的。在斜齒輪中，該線是跨過齒面的對角線。它是輪齒逐漸進行嚙臺并平穩(wěn)地從一個齒到另一個齒傳遞運動，那樣就使斜齒輪具有高速重載下平穩(wěn)傳遞運動的能力。斜齒輪使軸的軸承承受徑向和軸向力。當軸向推力變得大了或由于別的原因而產生某些影響時，那就可以使用人字齒輪。雙斜齒輪(人字齒輪)是與反向的并排地裝在同一軸上的兩個斜齒輪等敬。他們產生相反的軸向推力作用，這樣就消除了軸向推力。當兩個或更多的單向齒斜齒輪被裝在同一軸上時，齒輪的齒向應作選擇，以便產生最小的軸向推力。
交錯軸斜齒輪或螺旋齒輪，他們的軸中心線既不相交也不平行。交錯軸斜齒輪的齒彼此之間發(fā)生點接觸，它隨著齒輪的磨合而變成線接觸。因此他們只能傳遞小的載荷和主要用于儀器設備中，而且肯定不能推薦在動力傳動中使用。交錯軸斜齒輪與斜齒輪之間在被安裝后互相嚙合之前是沒有任何區(qū)別的。它們是以同樣的方法進行制造。一對相嚙合的交錯軸斜齒輪通常具有同樣的齒向，即左旋主動齒輪跟右旋從動齒輪相嚙舍。在交錯軸斜齒設計中，當該齒的斜角相等時所產生滑移速度最小。然而當該齒的斜角不相等時，如果兩個齒輪具有相同齒向的話，大斜角齒輪應該用作主動齒輪。
蝸輪與交錯軸斜齒輪相似。小齒輪即蝸桿具有較小的齒數，通常是一到四齒．由于它們完全纏繞在節(jié)圓柱上，因此它們又被稱為螺紋齒。與其相配的齒輪叫做蝸輪，蝸輪不是真正的斜齒輪。蝸桿和蝸輪通常是用于向垂直相交軸之間的傳動提供大的角速度減速比。蝸輪不是斜齒輪，因為其齒頂面做成中凹形狀以適配蝸桿曲率，目的是要形成線接觸而不是點接觸。然而蝸桿蝸輪傳動機構中存在齒問有較大滑移速度的缺點，正像變錯軸斜齒輪那樣。 蝸桿蝸輪機構有單包圍和雙包圍機構。單包圍機構就是蝸輪包裹著蝸桿或部分地包圍著蝸桿的一種機構。當然，如果每個構件各自局部地包圍著對方的蝸輪機構就是雙包圍蝸輪蝸桿機構。這兩者之間的重要區(qū)別是，在雙包圍蝸輪組的輪齒間有面接觸，而在單包圍蝸輪組的輪齒間只有線接觸。一個裝置中的蝸桿和蝸輪正像交錯軸斜齒輪那樣具有相同的齒向，但是其斜齒齒角的角度是極不相同的。蝸桿上的齒斜角度通常很大，而蝸輪上的則極小。因此慣常規(guī)定蝸桿的導角，那就是蝸桿齒斜角的余角；也規(guī)定了蝸輪上的齒斜角，該兩角之和就等于90。的軸線交角。 當齒輪要用來傳遞相交軸之網的運動時，就需要某種形式的錐齒輪。雖然錐齒輪通常制造成能構成90度軸交角，但它們也可產生任何角度的軸交角。輪齒可以鑄出、銑制或滾切加工。僅就滾齒而言就可達一級精度。在典型的錐齒輪安裝中，其中一個錐齒輪常常裝于支承的外側。這意味著軸的撓曲情況更加明顯而使在輪齒接觸上具有更大的影響。
另外一個難題，發(fā)生在難于預示錐齒輪輪齒上的應力．實際上是由于輪齒被加工成錐狀造成的。
直齒錐齒輪易于設計且制造簡單，如果他們安裝的精密而確定，在運轉中會產生良好效果。然而在直齒圓柱齒輪情況下，在節(jié)線速度較高時，他們將發(fā)出噪音。在這些情況下，通常設計使用螺旋錐齒輪，實踐證明是切實可行的，那是和配對斜齒輪很相似的配對錐齒輪。當在斜齒輪情況下，螺旋錐齒輪比直齒輪能產生平
穩(wěn)得多的嚙合作用，因此碰到高速運轉的場合那是很有用的。當在汽車的各種不同用途中，有一個帶偏心軸的類似錐齒輪的機構，那是常常所希望的。這樣的齒輪機構叫做準雙曲面齒輪機構，因為他們的節(jié)面是雙曲回轉面。這種齒輪之間的輪齒作用是沿著一根直線上產生滾動與滑動相結合的運動并和蝸輪蝸桿的輪齒作用有著更多的共同之處。
砂型鑄造
大多數金屬鑄件。是通過將熔化的金屬注入預先做好的型腔凝固而成的，這
種方法可溯及古代, 現存最大的青銅鑄件是日本奈良市的太陽大佛．它鑄于八世紀，重551(美國)叫(500噸)．高度超過71英尺(21米) 小國商朝(公元前1766—1222年)的工匠們制造的精美的青銅制品．其復雜程度可與當代設計制造的工藝品媲美‘
目前，有許多鑄造方法，對特定鑄件所選擇的最好的鑄造方法，取決于幾個基本因素。比如成本、尺寸、生產率、光潔度(我國標準名詞術語現稱作表面粗糙度——譯者)、公差、截面厚度、物理化學降性、設計難度、可加工件和可焊件等
砂則鑄造是最古老且仍廣泛應用的鑄造方法。本文將詳細地介紹這種方法，因為它的許多概念也適用于其他方法
型砂
型砂通常含有石英砂和添加劑、通過砂粒與用水均勻濺濕的粘土的攪拌、使砂粒及添加劑表面包復,層粘結薄膜 更穩(wěn)定耐熔的砂子，如熔融石英砂、鈷土砂、富鋁石砂已開始使用、用來替代低成本石英砂。它們在澆注溫度下僅有2%的線件擴張,問時用在高溫下更穩(wěn)定的合成樹脂來取代相對不穩(wěn)定的水和粘土粘結劑。
型砂鑄型可用來制造重量從小于1磅到幾噸的許多鑄件．可適用于黑色金屬和有色金屬材料、 型砂可用來制造復雜鑄型．因為它具有很好的退讓性，即鑄型對鑄件凝固時的收縮抗力比其他鑄型要小，這樣鑄件中的應力、應變就小. 可用許多力法將模型周圍的砂子搗實和壓緊、包括手工壓緊、氣錘壓緊、振動緊實(劇烈地機械振動)、擠壓壓緊(壓緊模型上,下表面)和將型砂高速加入型腔(拋砂)。拋砂機通常用于制造很大的鑄件,此時要用很多型砂。
對較小鑄件、使用兩箱(即上箱和下箱)來造型，首先。將模型放在型板上，再將下箱放于板上，在模型上撤分型砂并將砂箱填滿型砂, 振動造型機快速壓緊型砂、然后將砂箱翻轉并再在上面撤分型砂，再將上箱放于上面并填滿型砂、將兩箱鑄件壓緊.
模型
傳統(tǒng)方法采用木頭和金屬來制造砂型鑄件的模型 然而，已發(fā)現木模因熱量和溫度引起的變化達3%之多，這個因素會使許多有較高精度規(guī)定的鑄件超出了要求的許用公差，現在、模型通常采用環(huán)氧樹脂和帶有穩(wěn)定劑的冷塑化橡膠制造 設計簡單的含一個或多個平面的模型，如果取模時不破壞壓緊的型砂．可整體造型—對其他模型．當用兩箱造型時．模型可分成兩塊或多塊以便從砂中取出。模型必須做出錐度以使取模容易、這個錐度稱為拔模斜度. 當零件沒有拔模斜度時、必須另外加上最近對砂型鑄造的模型作的革新是用發(fā)泡聚本乙烯來制造模型、當熔化金屬澆入時模型將蒸發(fā) 這種鑄造方法稱為整模造型．模型不需要拔模斜度。
直澆道，橫澆道和內澆口
熔化的金屬可通過直澆道、橫澆道和內澆口進入型腔，如圖7—1所示。可在直澆道或澆注箱上部的型秒中開出一個澆口杯．以提供一個大開口幫助撓注、金屬澆注完后，在砂型中快速冷卻．因此．在外表層形成一個殼體，使中心附近的熔融金屬向表層流動 結果,鑄件最后凝固的部分會缺少金屬，在缺少補充金屬的情況下．會產生某種形式的縮孔 這鐘縮孔可能是集中縮孔(大孔洞)．或者是更多的細微的微扎(分散的縮松) 冒口可以克服這些縮孔，如圖7—]所示、冒口提供了熔融金屆來補充收縮損失
型芯
型芯放在鑄型中需要之處、保持空間．以便在最后的鑄件中形成孔洞 如圖7—1所示，型芯將在取走模型后放入.為保證它的正確定位,模型具有稱為型芯頭的外伸端。在鑄型中形成空腔以安放型心 有時型芯可用型砂整體造型，制成型砂測芯.通常,做型芯用的芯砂是用型芯油、一些有機粘結材料和水將砂于粘在一起,這些材料徹底混合，放在鑄型或型芯箱中, 成型后．拿山來在350°F—450°F(177°C—232°C)溫度下焙燒，含有兩件或多件的型芯在焙燒后粘在一起
CO2型芯
CO2型芯的制成是在型芯箱中預填濕砂．鈉硅酸酯作為粘結劑．向其吹CO2氣體時，可使它迅速硬化，CO2系統(tǒng)具有快速制成型芯的優(yōu)點.
澆注金屬
可用幾種形式的容器將熔融金屬從爐中移至澆注區(qū)，落地式大型鑄件用底部帶有柱塞的鐵水包(或稱為底部澆注鐵水包)澆注、這種鐵水包也用于機械化操作中，鑄型在生產線上移動，到達底部澆注鐵水包下面瞬即停止．進行澆注，
用于澆注黑色金屬的鐵水包用高氧化鋁的耐熔物做內襯．在長時間使用并氧化后，內襯可打碎更換，用來澆注黑色金屬的鐵水包在澆注前必須用氣焰預熱到2600°F—2700°F(1427°C—1482°C)。一旦鋼水包充滿。要連續(xù)使用自至倒空.對有色金屬，用簡單的粘土石墨坩堝爐來熔化、雖然它們很易破裂，但它們能耐高溫金屬，在正常條件下、可保持很長時間, 它們通常不需預熱．但必須小心．避免潮濕的影響，因此．有時將它們烘烤以保證干燥.
必須小心控制澆注過程，因為熔化溫度大大影響凝固前液態(tài)金屬收縮的程度和凝固速率，并將影響鑄型中針狀物成長的數量、程度和樹枝狀成長物的性質，合金熔蝕的程度和冒口系統(tǒng)的補縮持性.
修整
在鑄件凝固和冷卻到一定程度后．將它們置于落砂臺上或篩子上．破碎砂型，露出鑄件，以便取出。然后將鑄件拿到修整間，將澆口和冒口除去、對脆性材料，小的澆口和冒口可用錘子敲掉，大的冒口和澆口需要鋸去、用噴焰機切去或者剪去。不需要的金屬凸出物如毛翅、凸臺和小的澆口和冒口需要去除使其表面平齊。這類下作多用強力砂輪磨頭完成、稱為打磨或清鏟磨.對大型鑄件，移動磨頭比移動工件容易，故使用擺動磨頭 小的鑄件被拿到立式或臺式磨頭上打磨,也可用手動和氣動鑿刀修磨鑄件、從黑色金屬鑄件上去除金屬的一種新方法是使用碳一氣火焰槍，火焰槍用一根碳棒通入大電流。并用壓縮空氣從底部吹向碳棒，這會使金屬在熔化時氧化并去除留在許多鑄造廠。這種方法幾乎已全部取代清理和打磨方法。
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