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齒輪和軸的介紹
摘 要:在傳統(tǒng)機械和現(xiàn)代機械中齒輪和軸的重要地位是不可動搖的。齒輪和軸主要安裝在主軸箱來傳遞力的方向。通過加工制造它們可以分為許多的型號，分別用于許多的場合。所以我們對齒輪和軸的了解和認識必須是多層次多方位的。
關鍵詞:齒輪；軸
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。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。齒的形狀是一濺開線螺旋面。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，在血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，當齒進入更多的嚙合時，它就變成線。在直齒圓柱齒輪中，接觸是平行于回轉軸線的。在斜齒輪中，該先是跨過齒面的對角線。它是齒輪逐漸進行嚙合并平穩(wěn)的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。當軸向推力變的大了或由于別的原因而產(chǎn)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效。他們產(chǎn)生相反的軸向推力作用，這樣就消除了軸向推力。當兩個或更多個單向齒斜齒輪被在同一軸上時，齒輪的齒向應作選擇，以便產(chǎn)生最小的軸向推力。
交錯軸斜齒輪或螺旋齒輪，他們是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捏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合。在交錯軸斜齒設計中，當該齒的斜角相等時所產(chǎn)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(shù)，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www.mapeng.net 馬棚網(wǎng)
存在齒間有較大滑移速度的缺點，正像交錯軸斜齒輪那樣。
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。著兩者之間的重要區(qū)別是，在雙包圍蝸輪組的輪齒間有面接觸，而在單包圍的蝸輪組的輪齒間有線接觸。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，因此習慣常規(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度的軸線交角。
當齒輪要用來傳遞相交軸之間的運動時，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(chǎn)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，實際上是由于齒輪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(chǎn)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，螺旋錐齒輪比直齒輪能產(chǎn)生平穩(wěn)的多的嚙合作用，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，因為它們的節(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(chǎn)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。
軸是一種轉動或靜止的桿件。通常有圓形橫截面。在軸上安裝像齒輪，皮帶輪，飛輪，曲柄，鏈輪和其他動力傳遞零件。軸能夠承受彎曲，拉伸，壓縮或扭轉載荷，這些力相結合時，人們期望找到靜強度和疲勞強度作為設計的重要依據(jù)。因為單根軸可以承受靜壓力，變應力和交變應力，所有的應力作用都是同時發(fā)生的。
“軸”這個詞包含著多種含義，例如心軸和主軸。心軸也是軸，既可以旋轉也可以靜止的軸，但不承受扭轉載荷。短的轉動軸常常被稱為主軸。
當軸的彎曲或扭轉變形必需被限制于很小的范圍內時，其尺寸應根據(jù)變形來確定，然后進行應力分析。因此，如若軸要做得有足夠的剛度以致?lián)锨惶?，那么合應力符合安全要求那是完全可能的。但決不意味著設計者要保證；它們是安全的，軸幾乎總是要進行計算的，知道它們是處在可以接受的允許的極限以內。因之，設計者無論何時，動力傳遞零件，如齒輪或皮帶輪都應該設置在靠近支持軸承附近。這就減低了彎矩，因而減小變形和彎曲應力。
雖然來自M.H.G方法在設計軸中難于應用，但它可能用來準確預示實際失效。這樣，它是一個檢驗已經(jīng)設計好了的軸的或者發(fā)現(xiàn)具體軸在運轉中發(fā)生損壞原因的好方法。進而有著大量的關于設計的問題，其中由于別的考慮例如剛度考慮，尺寸已得到較好的限制。
設計者去查找關于圓角尺寸、熱處理、表面光潔度和是否要進行噴丸處理等資料，那真正的唯一的需要是實現(xiàn)所要求的壽命和可靠性。
由于他們的功能相似，將離合器和制動器一起處理。簡化摩擦離合器或制動器的動力學表達式中，各自以角速度w1和w2運動的兩個轉動慣量I1和I2，在制動器情況下其中之一可能是零，由于接上離合器或制動器而最終要導致同樣的速度。因為兩個構件開始以不同速度運轉而使打滑發(fā)生了，并且在作用過程中能量散失，結果導致溫升。在分析這些裝置的性能時，我們應注意到作用力，傳遞的扭矩，散失的能量和溫升。所傳遞的扭矩關系到作用力，摩擦系數(shù)和離www.mapeng.net 馬棚網(wǎng)
合器或制動器的幾何狀況。這是一個靜力學問題。這個問題將必須對每個幾何機構形狀分別進行研究。然而溫升與能量損失有關，研究溫升可能與制動器或離合器的類型無關。因為幾何形狀的重要性是散熱表面。各種各樣的離合器和制動器可作如下分類：
1．輪緣式內膨脹制凍塊；
2．輪緣式外接觸制動塊；
3．條帶式；
4．盤型或軸向式；
5．圓錐型；
6．混合式。
分析摩擦離合器和制動器的各種形式都應用一般的同樣的程序，下面的步驟是必需的：
1．假定或確定摩擦表面上壓力分布；
2．找出最大壓力和任一點處壓力之間的關系；
3．應用靜平衡條件去找尋（a）作用力；（b）扭矩；(c)支反力。
混合式離合器包括幾個類型，例如強制接觸離合器、超載釋放保護離合器、超越離合器、磁液離合器等等。
強制接觸離合器由一個變位桿和兩個夾爪組成。各種強制接觸離合器之間最大的區(qū)別與夾爪的設計有關。為了在結合過程中給變換作用予較長時間周期，夾爪可以是棘輪式的，螺旋型或齒型的。有時使用許多齒或夾爪。他們可能在圓周面上加工齒，以便他們以圓柱周向配合來結合或者在配合元件的端面上加工齒來結合。
雖然強制離合器不像摩擦接觸離合器用的那么廣泛，但它們確實有很重要的運用。離合器需要同步操作。
有些裝置例如線性驅動裝置或電機操作螺桿驅動器必須運行到一定的限度然后停頓下來。為著這些用途就需要超載釋放保護離合器。這些離合器通常用彈簧加載，以使得在達到預定的力矩時釋放。當?shù)竭_超載點時聽到的“喀嚓”聲就被認定為是所希望的信號聲。
超越離合器或連軸器允許機器的被動構件“空轉”或“超越”，因為主動驅動件停頓了或者因為另一個動力源使被動構件增加了速度。這種離合器通常使用裝在外套筒和內軸件之間的滾子或滾珠。該內軸件，在它的周邊加工了數(shù)個平面。驅動作用是靠在套筒和平面之間契入的滾子來獲得。因此該離合器與具有一定數(shù)量齒的棘輪棘爪機構等效。
磁液離合器或制動器相對來說是一個新的發(fā)展，它們具有兩平行的磁極板。這些磁極板之間有磁粉混合物潤滑。電磁線圈被裝入磁路中的某處。借助激勵該線圈，磁液混合物的剪切強度可被精確的控制。這樣從充分滑移到完全鎖住的任何狀態(tài)都可以獲得。
加工基礎
作為產(chǎn)生形狀的一種加工方法，機械加
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工是所有制造過程中最普遍使用的而且是最重要的方法。機械加工過程是一個產(chǎn)生形狀的過程，在這過程中，驅動裝置使工件上的一些材料以切屑的形式被去除。盡管在某些場合，工件無承受情況下，使用移動式裝備來實現(xiàn)加工，但大多數(shù)的機械加工是通過既支承工件又支承刀具的裝備來完成。
機械加工在知道過程中具備兩方面。小批生產(chǎn)低費用。對于鑄造、鍛造和壓力加工，每一個要生產(chǎn)的具體工件形狀，即使是一個零件，幾乎都要花費高額的加工費用。靠焊接來產(chǎn)生的結構形狀，在很大程度上取決于有效的原材料的形式。一般來說，通過利用貴重設備而又無需特種加工條件下，幾乎可以以任何種類原材料開始，借助機械加工把原材料加工成任意所需要的結構形狀，只要外部尺寸足夠大，那都是可能的。因此對于生產(chǎn)一個零件，甚至當零件結構及要生產(chǎn)的批量大小上按原來都適于用鑄造、鍛造或者壓力加工來生產(chǎn)的，但通常寧可選擇機械加工。
嚴密的精度和良好的表面光潔度，機械加工的第二方面用途是建立在高精度和可能的表面光潔度基礎上。許多零件，如果用別的其他方法來生產(chǎn)屬于大批量生產(chǎn)的話，那么在機械加工中則是屬于低公差且又能滿足要求的小批量生產(chǎn)了。另方面，許多零件靠較粗的生產(chǎn)加工工藝提高其一般表面形狀，而僅僅是在需要高精度的且選擇過的表面才進行機械加工。例如內螺紋，除了機械加工之外，幾乎沒有別的加工方法能進行加工。又如已鍛工件上的小孔加工，也是被鍛后緊接著進行機械加工才完成的。
基本的機械加工參數(shù)
切削中工件與刀具的基本關系是以以下四個要素來充分描述的：刀具的幾何形狀，切削速度，進給速度，和吃刀深度。
切削刀具必須用一種合適的材料來制造，它必須是強固、韌性好、堅硬而且耐磨的。刀具的幾何形狀——以刀尖平面和刀具角為特征——對于每一種切削工藝都必須是正確的。
切削速度是切削刃通過工件表面的速率，它是以每分鐘英寸來表示。為了有效地加工，切削速度高低必須適應特定的工件——刀具配合。一般來說，工件材料越硬，速度越低。
進給速度是刀具切進工件的速度。若工件或刀具作旋轉運動，進給量是以每轉轉過的英寸數(shù)目來度量的。當?shù)毒呋蚬ぜ魍鶑瓦\動時，進給量是以每一行程走過的英寸數(shù)度量的。一般來說，在其他條件相同時，進給量與切削速度成反比。
吃刀深度——以英寸計——是刀具進入工件的距離。它等于旋削中的切屑寬度或者等于線性切削中的切屑的厚度。粗加工比起精加工來，吃刀深度較深。
切削參數(shù)的改變對切削溫度的影響
金屬切削操作中，熱是在主變形區(qū)和副變形區(qū)發(fā)生的。這結果導致復雜的溫度分布遍及刀具、工件和切屑。圖中顯示了一組典型等溫曲線，從中可以看出：像所能預料的那樣，當工件材料在主變形區(qū)被切削時，沿著整個切屑的寬度上有著很大的溫度梯度，而當在副變形區(qū)，切屑被切落時，切屑附近的前刀面上就有更高的溫度。這導致了前刀面和切屑離切削刃很近的地方切削溫度較高。
實質上由于在金屬切削中所做的全部功能都被轉化為熱，那就可以預料：被切離金屬的單位體積功率消耗曾家的這些因素就將使切削溫度升高。這樣刀具前角的增加而所有其他參數(shù)不變時，將使切離金屬的單位體積所耗功率減小，因而切削溫度也將降低。當考慮到未變形切屑厚度增加和切削速度，這情形就更是復雜。未變形切屑厚度的增加趨勢必導致通過工件的熱的總數(shù)上產(chǎn)生比例效應，刀具和切屑仍保持著固定的比例，而切削溫度變化傾向于降低。然而切削速度的增加，傳導到工件上的熱的數(shù)量減少而這又增加主變形區(qū)中的切屑溫升。進而副變形區(qū)勢必更小，這將在該區(qū)內產(chǎn)生升溫效應。其他切削參數(shù)的變化，實質上對于被切離的單位體積消耗上并沒有什么影響，因此實際上對切削溫度沒有什么作
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用。因為事實已經(jīng)表明：切削溫度即使有小小的變化對刀具磨損率都將有實質意義的影響作用。這表明如何人從切削參數(shù)來確定切削溫度那是很合適的。
為著測定高速鋼刀具溫度的最直接和最精確的方法是W&T法，這方法也就是可提供高速鋼刀具溫度分布的詳細信息的方法。該項技術是建立在高速鋼刀具截面金相顯微測試基礎上，目的是要建立顯微結構變化與熱變化規(guī)律圖線關系式。當要加工廣泛的工件材料時，Trent已經(jīng)論述過測定高速鋼刀具的切削溫度及溫度分布的方法。這項技術由于利用電子顯微掃描技術已經(jīng)進一步發(fā)展，目的是要研究將已回過火和各種馬氏體結構的高速鋼再回火引起的微觀顯微結構變化情況。這項技術亦用于研究高速鋼單點車刀和麻花鉆的溫度分布。
刀具磨損
從已經(jīng)被處理過的無數(shù)脆裂和刃口裂紋的刀具中可知，刀具磨損基本上有三種形式：后刀面磨損，前刀面磨損和V型凹口磨損。后刀面磨損既發(fā)生在主刀刃上也發(fā)生副刀刃上。關于主刀刃，因其擔負切除大部金屬切屑任務，這就導致增加切削力和提高切削溫度，如果聽任而不加以檢查處理，那可能導致刀具和工件發(fā)生振動且使有效切削的條件可能不再存在。關于副刀刃，那是決定著工件的尺寸和表面光潔度的，后刀面磨損可能造成尺寸不合格的產(chǎn)品而且表面光潔度也差。在大多數(shù)實際切削條件下，由于主前刀面先于副前刀面磨損，磨損到達足夠大時，刀具將實效，結果是制成不合格零件。
由于刀具表面上的應力分布不均勻，切屑和前刀面之間滑動接觸區(qū)應力，在滑動接觸區(qū)的起始處最大，而在接觸區(qū)的尾部為零，這樣磨蝕性磨損在這個區(qū)域發(fā)生了。這是因為在切削卡住區(qū)附近比刀刃附近發(fā)生更嚴重的磨損，而刀刃附近因切屑與前刀面失去接觸而磨損較輕。這結果離切削刃一定距離處的前刀面上形成麻點凹坑，這些通常被認為是前刀面的磨損。通常情況下，這磨損橫斷面是圓弧形的。在許多情況中和對于實際的切削狀況而言，前刀面磨損比起后刀面磨損要輕，因此后刀面磨損更普遍地作為刀具失效的尺度標志。然而因許多作者已經(jīng)表示過的那樣在增加切削速度情況下，前刀面上的溫度比后刀面上的溫度升得更快，而且又因任何形式的磨損率實質上是受到溫度變化的重大影響。因此前刀面的磨損通常在高速切削時發(fā)生的。
刀具的主后刀面磨損帶的尾部是跟未加工過的工件表面相接觸，因此后刀面磨損比沿著磨損帶末端處更為明顯，那是最普通的。這是因為局部效應，這像未加工表面上的已硬化層，這效應是由前面的切削引起的工件硬化造成的。不只是切削，還有像氧化皮，刀刃產(chǎn)生的局部高溫也都會引起這種效應。這種局部磨損通常稱作為凹坑性磨損，而且偶爾是非常嚴重的。盡管凹坑的出現(xiàn)對刀具的切削性質無實質意義的影響，但凹坑常常逐漸變深，如果切削在繼續(xù)進行的話，那么刀具就存在斷裂的危機。
如果任何進行性形式 的磨損任由繼續(xù)發(fā)展，最終磨損速率明顯地增加而刀具將會有摧毀性失效破壞，即刀具將不能再用作切削，造成工件報廢，那算是好的，嚴重的可造成機床破壞。對于各種硬質合金刀具和對于各種類型的磨損，在發(fā)生嚴重失效前，就認為已達到刀具的使用壽命周期的終點。然而對于各種高速鋼刀具，其磨損是屬于非均勻性磨損，已經(jīng)發(fā)現(xiàn)：當其磨損允許連續(xù)甚至到嚴重失效開始，最有意義的是該刀具可以獲得重磨使用，當然，在實際上，切削時間遠比使用到失效的時間短。以下幾種現(xiàn)象之一均是刀具嚴重失效開始的特征：最普遍的是切削力突然增加，在工件上出現(xiàn)燒損環(huán)紋和噪音嚴重增加等。
自動夾具設計
用做裝配設備的傳統(tǒng)同步夾具把零件移動到夾具中心上，以確保零件從傳送機上或從設備盤上取出后置于已定位置上。然而在某些應用場合、強制零件移動到中心線上時，可能引起零件或設備破壞。當零件易損
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而且小小振動可能導致報廢時，或當其位置是由機床主軸或模具來具體時，再或者當公差要求很精密時，那寧可讓夾具去適應零件位置，而不是相反。為著這些工作任務，美國俄亥俄州Elyria的Zaytran公司已經(jīng)開發(fā)了一般性功能數(shù)據(jù)的非同步西類柔順性夾具。因為夾具作用力和同步化裝置是各自獨立的，該同步裝置可以用精密的滑移裝置來替換而不影響夾具作用力。夾具規(guī)格范圍是從0.2英寸行程，5英鎊夾緊力到6英寸行程、400英寸夾緊力。
現(xiàn)代生產(chǎn)的特征是批量變得越來越小而產(chǎn)品的各種規(guī)格變化最大。因此，生產(chǎn)的最后階段，裝配因生產(chǎn)計劃、批量和產(chǎn)品設計的變更而顯得特別脆弱。這種情形正迫使許多公司更多地致力于廣泛的合理化改革和前面提到過情況那樣裝配自動化。盡管柔性夾具的發(fā)展很快落后與柔性運輸處理裝置的發(fā)展，如落后于工業(yè)機器人的發(fā)展，但仍然試圖指望增加夾具的柔順性。事實上夾具的重要的裝置——生產(chǎn)裝置的專向投資就加強了使夾具更加柔性化在經(jīng)濟上的支持。
根據(jù)它們柔順性，夾具可以分為：專用夾具、組合夾具、標準夾具、高柔性夾具。柔性夾具是以它們對不同工件的高適應性和以少更換低費用為特征的。
結構形式可變換的柔性夾具裝有可變更結構排列的零件（例如針形頰板，多片式零件和片狀頰板），標準工件的非專用夾持或夾緊元件（例如：啟動標準夾持夾具和帶有可移動元件的夾具配套件），或者裝有陶瓷或硬化了的中介物質（如：流動粒子床夾具和熱夾具緊夾具）。為了生產(chǎn)，零件要在夾具中被緊固，需要產(chǎn)生夾緊作用，其有幾個與夾具柔順性無關的步驟：
根據(jù)被加工的即基礎的部分和工作特點，確定工件在夾具中的所需的位置，接著必須選擇若干穩(wěn)定平面的組合，這些穩(wěn)定平面就構成工件被固定在夾具中確定位置上的夾持狀輪廓結構，均衡所有各力和力矩，而且保證接近工件工作特點。最后，必須計算、調整、組裝可拆裝的或標準夾具元件的所需位置，以便使工件牢牢地被夾緊在夾具中。依據(jù)這樣的程序，夾具的輪廓結構和裝合的規(guī)劃和記錄過程可以進行自動化控制。
結構造型任務就是要產(chǎn)生若干穩(wěn)定平面的組合，這樣在這些平面上的各夾緊力將使工件和夾具穩(wěn)定。按慣例，這個任務可用人—機對話即幾乎完全自動化的方式來完成。一人—機對話即以自動化方式確定夾具結構造型的優(yōu)點是可以有組織有規(guī)劃進行夾具設計，減少所需的設計人員，縮短研究周期和能更好地配置工作條件。簡言之，可成功地達到顯著提高夾具生產(chǎn)效率和效益。
在充分準備了構造方案和一批材料情況下，在完成首次組裝可以成功實現(xiàn)節(jié)約時間達60%。
因此夾具機構造型過程的目的是產(chǎn)生合適的編程文件。
GEAR AND SHAFT INTRODUCTION
GEAR AND SHAFT INTRODUCTION
Abstract: The important position of the wheel gear and shaft can't falter in traditional machine and modern machines. The wheel gear and shafts mainly install the direction that delivers the dint at the principal axis box. The passing to process to make them can is divided into many model numbers, useding for many situations respectively. So we must be the multilayers to the understanding of the wheel gear and shaft in many ways .
Key words: Wheel gear; Shaft
In the force analysis of spur gears, the forces are assumed to act in a single plane. 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 also 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 helicoid. If a piece of paper cut in the shape of a parallelogram 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 helicoid.
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 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 reactions 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 fears 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 on difference between a crossed heli
cal 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 be 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 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.
Worm 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 90-deg. Shaft angle.
When gears are to be used to transmit motion between intersecting shaft, some of bevel gear is required. Although bevel gear are usually made for a shaft angle of 90 deg. They may be produced for almost any shaft angle. The teeth may be cast, 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 teeth. Another difficulty, which occurs in predicting the stress in bevel-gear teeth, is the fact 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 squr gears, however, they become noisy at higher values of the pitch-line velocity. In these cases it is often go
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od design practice to go to the 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.
A shaft is a rotating or stationary member, usually of circular cross section, having mounted upon it such elementsas gears, pulleys, flywheels, cranks, sprockets, and other power-transmission elements. Shaft may be subjected to bending, tension, compression, or torsional loads, acting singly or in combination with one another. When they are combined, one may expect to find both static and fatigue strength to be important design considerations, since a single shaft may be subjected to static stresses, completely reversed, and repeated stresses, all acting at the same time.
The word “shaft” covers numerous variations, such as axles and spindles. Anaxle is a shaft, wither stationary or rotating, nor subjected to torsion load. A shirt rotating shaft is often called a spindle.
When either the lateral or the torsional deflection of a shaft must be held to close limits, the shaft must be sized on the basis of deflection before analyzing the stresses. The reason for this is that, if the shaft is made stiff enough so that the deflection is not too large, it is probable that the resulting stresses will be safe. But by no means should the designer assume that they are safe; it is almost always necessary to calculate them so that he knows they are within acceptable limits. Whenever possible, the power-transmission elements, such as gears or pullets, should be located close to the supporting bearings, This reduces the bending moment, and hence the deflection and bending stress.
Although the von Mises-Hencky-Goodman method is difficult to use in design of shaft, it probably comes closest to predicting actual failure. Thus it is a good way of checking a shaft that has already been designed or of discovering why a particular shaft has failed in service. Furthermore, there are a considerable number of shaft-design problems in which the dimension are pretty well limited by other considerations, such as rigidity, and it is only necessary for the designer to discover something about the fillet sizes, heat-treatment, and surface finish and whether or not shot peening is necessary in order to achieve the required life and reliability.
Because of the similarity of their functions, clutches and brakes are treated together. In a simplified dynamic representation of a friction clutch, or brake, two in
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ertias I1 and I2 traveling at the respective angular velocities W1 and W2, one of which may be zero in the case of brake, are to be brought to the same speed by engaging the clutch or brake. Slippage occurs because the two elements are running at different speeds and energy is dissipated during actuation, resulting in a temperature rise. In analyzing the performance of these devices we shall be interested in the actuating force, the torque transmitted, the energy loss and the temperature rise. The torque transmitted is related to the actuating force, the coefficient of friction, and the geometry of the clutch or brake. This is problem in static, which will have to be studied separately for eath geometric configuration. However, temperature rise is related to energy loss and can be studied without regard to the type of brake or clutch because the geometry of interest is the heat-dissipating surfaces. The various types of clutches and brakes may be classified as fllows:
1. Rim type with internally expanding shoes
2. Rim type with externally contracting shoes
3. Band type
4. Disk or axial type
5. Cone type
6. Miscellaneous type
The analysis of all type of friction clutches and brakes use the same general procedure. The following step are necessary:
1. Assume or determine the distribution of pressure on the frictional surfaces.
2. Find a relation between the maximum pressure and the pressure at any point
3. Apply the condition of statical equilibrium to find (a) the actuating force, (b) the torque, and (c) the support reactions.
Miscellaneous clutches include several types, such as the positive-contact clutches, overload-release clutches, overrunning clutches, magnetic fluid clutches, and others.
A positive-contact clutch consists of a shift lever and two jaws. The greatest differences between the various types of positive clutches are concerned with the design of the jaws. To provide a longer period of time for shift action during engagement, the jaws may be ratchet-shaped, or gear-tooth-shaped. Sometimes a great many teeth or jaws are used, and they may be cut either circumferentially, so that they engage by cylindrical mating, or on the faces of the mating elements.
Although positive clutches are not used to the extent of the frictional-contact type, they do have important applications where synchronous operation is required.
Devices such as linear drives or motor-operated screw drivers must run to definite limit and then come to a stop. An overload-release type of clutch is required for these applications. These clutches are usually spring-loaded so as to release at a predetermined toque. The clicking sound which is heard when the overload point is reached is considered to be a desirable signal.
An overrunning clutch or coupling permits the driven member of a machine to “freewheel” or “overrun” because the driver is stopped or because another source of power increase the speed of the driven. This
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type of clutch usually uses rollers or balls mounted between an outer sleeve and an inner member having flats machined around the periphery. Driving action is obtained by wedging the rollers