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中國(guó)地質(zhì)大學(xué)長(zhǎng)城學(xué)院本科論文外文資料翻譯
中國(guó)地質(zhì)大學(xué)長(zhǎng)城學(xué)院
本科畢業(yè)論文外文資料翻譯
系 別： 工程技術(shù)系
專 業(yè)：機(jī)械設(shè)計(jì)制造及其自動(dòng)化
姓 名： 趙東亮
學(xué) 號(hào)： 05208729
2012年 1 月 30 日
外文資料翻譯譯文
汽車設(shè)計(jì)之車架設(shè)計(jì)
車架是汽車最基本的臺(tái)架，所有的懸架和轉(zhuǎn)向連接部件都安裝在車架上面。如果汽車車架柔性過(guò)大，會(huì)使汽車既無(wú)法轉(zhuǎn)向，也無(wú)法進(jìn)行正常操縱。而如果汽車車架結(jié)構(gòu)剛性過(guò)大，又會(huì)引起不必要的震動(dòng)傳遞給駕駛員和乘客的座艙室。汽車車架和懸架的結(jié)構(gòu)設(shè)計(jì)不僅決定了汽車噪聲大小和震動(dòng)的幅度強(qiáng)度，而且也將影響到汽車的質(zhì)量和車輛的正常操縱。
汽車制造廠商們?cè)谒麄兩a(chǎn)的汽車上都使用了幾種不同的車架結(jié)構(gòu)。其中，整個(gè)七十年代最常使用的是殼體和大梁的分體結(jié)構(gòu)。目前它仍然在大型貨車、小噸位貨車和卡車上應(yīng)用著。在汽車殼體和大梁的分體結(jié)構(gòu)里，發(fā)動(dòng)機(jī)、傳動(dòng)裝置、傳動(dòng)齒輪和車殼都是通過(guò)絕緣裝置固定在車身大梁上。車架內(nèi)部的絕緣裝置是人造橡膠緩沖墊，能夠阻止道路不平和發(fā)動(dòng)機(jī)工作引起的噪音和震動(dòng)傳到駕駛員和乘客的座艙里。
第二種是汽車車架的單體結(jié)構(gòu)。這種設(shè)計(jì)到目前為止在現(xiàn)代汽車上是最常用。單體車架按所需的強(qiáng)度來(lái)分，設(shè)計(jì)有輕型結(jié)構(gòu)。在這種汽車結(jié)構(gòu)中大梁作為車架的一部分被直接焊接到殼體上。底盤(pán)的重量增加了大梁的強(qiáng)度。傳動(dòng)齒輪和傳動(dòng)裝置經(jīng)由大而軟的人造橡膠絕緣墊安裝在單體車架上。絕緣墊減弱了噪聲的傳動(dòng)和震動(dòng)。若絕緣墊太軟，將會(huì)引起傳動(dòng)齒輪和傳動(dòng)裝置位移。這種位移稱為柔量，它會(huì)影響到汽車的操縱性能和控制性能。若絕緣墊太硬，則不能起到應(yīng)有的隔絕噪音和減小震動(dòng)的作用。汽車制造廠商們精心地設(shè)計(jì)絕緣墊，把它們裝置在汽車適當(dāng)?shù)牡胤剑越档驮肼?，緩沖震動(dòng)的傳送，使汽車便于駕駛，駕駛員和乘客乘坐舒適。絕緣墊的性能隨使用年限發(fā)生變化，當(dāng)汽車變舊時(shí)原先的性能也隨之改變。
第三種結(jié)構(gòu)是把前兩種結(jié)構(gòu)的主要特點(diǎn)結(jié)合在一起。它在汽車前艙使用了短車梁，在汽車后艙使用了單體車架。單體部分剛性很大，而短的車梁增強(qiáng)了絕緣作用。 汽車制造廠家們?cè)谄嚿线x擇那種生產(chǎn)成本低而同時(shí)又符合對(duì)噪音震動(dòng)，駕駛操縱性能要求很高的車架結(jié)構(gòu)。老式的大型的車輛、貨車、和卡車通常使用殼體和大梁的分體結(jié)構(gòu)。較新的，較小型的車輛通常使用單體結(jié)構(gòu)的車架。
下邊敘述一些不同的傳動(dòng)裝置系統(tǒng)的基本知識(shí)，在進(jìn)行懸架修理時(shí)可供參考。使用前輪驅(qū)動(dòng)的傳動(dòng)系統(tǒng)經(jīng)常將聯(lián)動(dòng)裝置和后輪傳動(dòng)裝置結(jié)合成一個(gè)裝置。這個(gè)對(duì)中置和后置發(fā)動(dòng)機(jī)的汽車也是很適用的。這個(gè)裝置稱為轉(zhuǎn)換軸。它為兩端各帶有一個(gè)萬(wàn)向節(jié)的短半軸，把轉(zhuǎn)換軸和車輪連接起來(lái)。這些軸當(dāng)懸梁移動(dòng)和轉(zhuǎn)向時(shí)把動(dòng)力從后傳動(dòng)裝置傳送到車輪上。后傳動(dòng)裝置里的差速器分流輸入的動(dòng)力，每個(gè)驅(qū)動(dòng)輪上各分一半。這就使驅(qū)動(dòng)輪在轉(zhuǎn)彎時(shí)會(huì)以不同的速度轉(zhuǎn)動(dòng)。
在前置發(fā)動(dòng)機(jī)后輪驅(qū)動(dòng)的汽車?yán)铮?lián)動(dòng)裝置位于駕駛坐艙的前底板下。傳動(dòng)軸被用來(lái)把發(fā)動(dòng)機(jī)動(dòng)力傳送到后橋上。傳動(dòng)軸每端各有一個(gè)萬(wàn)向節(jié)。當(dāng)懸架移動(dòng)時(shí)，萬(wàn)向節(jié)通過(guò)變化著的傳動(dòng)系統(tǒng)的角度傳送動(dòng)力。驅(qū)動(dòng)輪上帶有獨(dú)立懸架的汽車中有一個(gè)牢固地附加在車身大梁或發(fā)動(dòng)機(jī)上的后傳動(dòng)裝置。在加速時(shí)該裝置在懸架部位上會(huì)產(chǎn)生動(dòng)力，并不產(chǎn)生扭矩。如果剎車裝置安裝在車艙內(nèi)，卡鉗裝到大梁上而不是懸架上，那么剎車裝置也不會(huì)在懸架上產(chǎn)生扭矩。僅用于控制加速和減速扭矩的懸架與必須同時(shí)控制懸架力和扭矩的懸架在汽車設(shè)計(jì)上是完全不相同的。懸架包括彈簧，避震器和控制連桿裝置。它必須能夠足以支撐車身自重和負(fù)載。懸架也應(yīng)能夠承受發(fā)動(dòng)機(jī)和制動(dòng)對(duì)它的反作用力。懸架系統(tǒng)最重要的作用是使輪胎與路面接觸的時(shí)間盡可能的長(zhǎng)。在支撐車體和負(fù)載時(shí)，甚至在高低不平的道路上行駛時(shí)更加應(yīng)如此。這四個(gè)輪胎的胎面是車與路面相接觸的唯一的部位。發(fā)動(dòng)機(jī)全部輸出的動(dòng)力，轉(zhuǎn)向力和制動(dòng)力都通過(guò)與路面相接觸的輪胎的胎面起作用。每當(dāng)輪胎不與路面接觸或汽車開(kāi)始打滑時(shí)，汽車的控制力（動(dòng)力、轉(zhuǎn)向力、制動(dòng)力）就會(huì)減弱甚至喪失。
車體是靠彈簧支撐著，彈簧可分為螺旋型、鋼板型、扭棒型和充氣型。螺旋型彈簧是現(xiàn)代汽車中應(yīng)用最為廣泛的類型。螺旋型、扭棒型和充氣型彈簧都需要用連桿和連桿臂以使車輪就位。鋼板彈簧提供了對(duì)車體的橫向和縱向控制，以防止汽車車輪在行駛時(shí)不必要的位移，它們通常用在載重貨車和卡車上。懸架系統(tǒng)是隨著客運(yùn)汽車的發(fā)展而變化和改進(jìn)著。豪華轎車，特種車輛，小型汽車和輕型卡車的設(shè)計(jì)目的是截然不同的?，F(xiàn)代輪胎的改進(jìn)不斷地改善了車輛的操作性能，它的改進(jìn)是與避震器，轉(zhuǎn)向系統(tǒng)和懸架控制裝置一起同步改進(jìn)的?，F(xiàn)代汽車在各種操縱條件下都需要輪胎與路面接觸，以便安全、正確地控制并行駛汽車。要想要最大限度的安全駕車，要牢記這四個(gè)輪胎必須在任何時(shí)間都與路面相接觸。同時(shí)需要考慮汽車操縱的靈活性，輪胎的抗耐磨性，汽車駕駛的舒適性和行車的安全性，以達(dá)到汽車的有效控制。
懸架系統(tǒng)分為前懸架和后懸架。前懸架的設(shè)計(jì)已得到了飛速發(fā)展。從較為粗糙的硬軸結(jié)構(gòu)發(fā)展到了現(xiàn)代的輕型、高強(qiáng)度、支撐型獨(dú)立懸架結(jié)構(gòu)，并由于增加了連桿裝置而使汽車的性能得到了改善。懸架結(jié)構(gòu)的改進(jìn)是隨著路況的改善和駕駛員的需要而進(jìn)行改進(jìn)的。 大多數(shù)前置發(fā)動(dòng)機(jī)，后輪驅(qū)動(dòng)的汽車都采用一個(gè)簡(jiǎn)單的從屬性后懸架。但后輪驅(qū)動(dòng)的獨(dú)立懸架結(jié)構(gòu)復(fù)雜得多，而且成本極高，因而只用于少數(shù)客車上。 對(duì)于前置發(fā)動(dòng)機(jī)前輪驅(qū)動(dòng)的車輛，通過(guò)把傳動(dòng)裝置移至前部，后懸架僅用來(lái)調(diào)節(jié)駕駛控制力和剎車時(shí)的反作用。這就 導(dǎo)致了簡(jiǎn)化的非獨(dú)立的懸架機(jī)構(gòu)，半獨(dú)立的懸架機(jī)構(gòu)和獨(dú)立的后懸架機(jī)構(gòu)的應(yīng)用，后者大量應(yīng)用于新型車輛的結(jié)構(gòu)設(shè)計(jì)上。
轉(zhuǎn)向統(tǒng)汽車駕駛員通過(guò)對(duì)轉(zhuǎn)向齒輪的控制汽車前輪的方向?，F(xiàn)代的轉(zhuǎn)向齒輪有兩個(gè)主要的部分組成，轉(zhuǎn)向桿和齒輪組。轉(zhuǎn)向桿有一個(gè)被支撐的軸，它把駕駛員的方向盤(pán)與齒輪組連在了一起。齒輪組可將汽車駕駛員的轉(zhuǎn)向力增大，以帶動(dòng)轉(zhuǎn)向連桿裝置。后輪驅(qū)動(dòng)汽車的前輪在一個(gè)心軸上轉(zhuǎn)動(dòng)。心軸是轉(zhuǎn)向節(jié)的一部分。該轉(zhuǎn)向節(jié)與帶有球接頭的前懸橫梁相互連接。球接頭在前懸架上下移動(dòng)時(shí)可以進(jìn)行轉(zhuǎn)向。前輪驅(qū)動(dòng)的汽車的輪轂在轉(zhuǎn)向節(jié)里的軸承內(nèi)的空心軸短軸桿上傳動(dòng)。 汽車方向盤(pán)控制轉(zhuǎn)向齒輪裝置。它依次通過(guò)轉(zhuǎn)向連桿裝置使轉(zhuǎn)向節(jié)開(kāi)始移動(dòng)?，F(xiàn)在使用兩種轉(zhuǎn)向齒輪的結(jié)構(gòu)，即齒輪齒條式結(jié)構(gòu)以及循環(huán)球式結(jié)構(gòu)。現(xiàn)代汽車設(shè)計(jì)了對(duì)速度敏感的轉(zhuǎn)向結(jié)構(gòu)。因此當(dāng)汽車慢速行駛時(shí)需要較大的力才能使汽車轉(zhuǎn)向。于是在很多汽車上裝備了助力轉(zhuǎn)向裝置。由于助力轉(zhuǎn)向裝置起了主要作用，所以轉(zhuǎn)向比降低了，這樣就能夠輕微轉(zhuǎn)動(dòng)方向盤(pán)使得汽車轉(zhuǎn)向。助力轉(zhuǎn)向齒輪類似于標(biāo)準(zhǔn)的轉(zhuǎn)向齒輪。它有承壓面，液體壓力加在其上，以增加汽車駕駛員的轉(zhuǎn)向力。齒輪齒條式轉(zhuǎn)向結(jié)構(gòu)和循環(huán)球齒輪結(jié)構(gòu)都有了動(dòng)力輔助裝置。 轉(zhuǎn)向齒輪的動(dòng)力是由發(fā)動(dòng)機(jī)從動(dòng)泵提供的。該泵使動(dòng)力轉(zhuǎn)向液體流過(guò)一個(gè)由閥體控制的系統(tǒng)。該控制閥能感知汽車駕駛員的轉(zhuǎn)向力。把液體壓力加到轉(zhuǎn)向系統(tǒng)的承壓面上。該液體壓力承接了一些使汽車轉(zhuǎn)向的力?，F(xiàn)在汽車的轉(zhuǎn)向桿有很多個(gè)部件組成。它被用來(lái)分散、抵消汽車碰撞力以保護(hù)駕駛員的切身安全。在有些汽車上轉(zhuǎn)向桿還可以傾斜和伸縮來(lái)調(diào)節(jié)方向盤(pán)的位置使駕駛員感覺(jué)更加舒適。為了減少駕駛員汽車被盜的機(jī)會(huì)，還安裝有一個(gè)轉(zhuǎn)向齒輪的保險(xiǎn)鎖。很多汽車還有一個(gè)變速器保險(xiǎn)鎖。因?yàn)樘幵隈{駛員很容易觸及的范圍內(nèi)，所以轉(zhuǎn)向桿上還可以帶有變速器換擋控制滑桿，轉(zhuǎn)向信號(hào)開(kāi)關(guān)，前大燈和變光開(kāi)關(guān)，刮水器開(kāi)關(guān)，緊急閃爍器開(kāi)關(guān)和速度控制器。
制動(dòng)系使用中的制動(dòng)器應(yīng)能起到制動(dòng)住車輛的作用。制動(dòng)器能使汽車滑行時(shí)能防止行駛速度過(guò)快，在斜坡上制動(dòng)時(shí)能將汽車停在適當(dāng)?shù)奈恢蒙?。汽車剎車的設(shè)計(jì)應(yīng)使駕駛員能調(diào)節(jié)制動(dòng)力以控制汽車。汽車的控制不僅受懸架和轉(zhuǎn)向系統(tǒng)影響，而且也受汽車剎車影響。制動(dòng)系統(tǒng)的故障可導(dǎo)致汽車剎車時(shí)車輪滑脫。要修理懸梁系統(tǒng)，也可能需要將制動(dòng)系統(tǒng)的部件拆卸開(kāi)。為此本文將討論制動(dòng)系統(tǒng)。 制動(dòng)系統(tǒng)應(yīng)給予 汽車駕駛員提供均勻平穩(wěn)的制動(dòng)力。剎車板上所需的力不應(yīng)太大，而使車輪不至于被瞬間剎死。為滿足這些汽車剎車的要求，對(duì)于汽車制動(dòng)已有了最低限度的剎車標(biāo)準(zhǔn)。駕駛員通過(guò)機(jī)械裝置、真空和液壓裝置控制制動(dòng)力。制動(dòng)力是隨著附加在汽車剎車板上的踏板力的增加而增加的。這個(gè)力通過(guò)制動(dòng)系統(tǒng)的傳遞以把固定的汽車剎車片推壓到轉(zhuǎn)動(dòng)的制動(dòng)器表面上。當(dāng)它把動(dòng)能（運(yùn)動(dòng)的能量）轉(zhuǎn)化為熱能（熱）時(shí)，就使汽車減速。制動(dòng)量的最大值就產(chǎn)生于車輪被瞬間閘死而引起的輪胎在路面上滑動(dòng)之前。所以制動(dòng)量的最大值取決于輪胎和路面之間的附著力。當(dāng)輪胎在道路上滑動(dòng)時(shí)，制動(dòng)效果減弱，汽車的方向控制可能就不起作用了。前剎車總成的固定構(gòu)件安裝在前懸架的轉(zhuǎn)向節(jié)上，在后部，它們被安裝在后橋殼或后心軸總成上，鑄鐵剎車鼓或車盤(pán)隨車輪一起轉(zhuǎn)動(dòng)。
汽車的制動(dòng)盤(pán)剎車時(shí)：汽車制動(dòng)盤(pán)剎車有隨車輪一起轉(zhuǎn)動(dòng)的圓盤(pán)。它通常被稱為汽車剎車轉(zhuǎn)子。在固定的卡鉗里的液壓控制的活塞被用來(lái)把汽車的剎車片加在轉(zhuǎn)子的汽車剎車表面上。汽車剎車片和轉(zhuǎn)子之間的摩擦力的大小會(huì)減慢或阻止車輪的轉(zhuǎn)動(dòng)。固定的卡鉗殼體使墊圈被壓在轉(zhuǎn)動(dòng)的汽車剎車盤(pán)上，使之不能轉(zhuǎn)動(dòng)。汽車制動(dòng)盤(pán)剎車墊圈的運(yùn)動(dòng)與剎車轉(zhuǎn)子的表面垂直，這樣會(huì)使它們卡在轉(zhuǎn)子上減慢汽車的車輪運(yùn)動(dòng)?？ㄣQ壓的力與駕駛員加在汽車剎車板上的力成正比。
汽車制動(dòng)鼓剎車：汽車制動(dòng)鼓剎車使用帶有摩擦片的固定的內(nèi)脹式剎車塊。他們被安裝在轉(zhuǎn)動(dòng)的汽車剎車鼓內(nèi)側(cè)。汽車剎車鼓緊箍在輪胎總成和轂總成或輪軸法蘭之間。當(dāng)汽車剎車塊的直徑膨脹至使汽車剎車片與汽車剎車表面相接觸時(shí)，汽車剎車塊就減慢了汽車剎車鼓的轉(zhuǎn)動(dòng)。它是由液壓操縱的汽車剎車分泵來(lái)完成的。來(lái)自剎車總泵的流體壓力被施加到汽車剎車分泵上，使剎車分泵膨脹起來(lái)。汽車剎車分泵的膨脹使剎車塊通過(guò)機(jī)械連桿進(jìn)行移動(dòng)，把汽車剎車片壓到轉(zhuǎn)動(dòng)的剎車鼓上。當(dāng)汽車剎車鼓的轉(zhuǎn)動(dòng)速度減慢時(shí)，就起到了制動(dòng)作用。
外文原文
Automobile Design----Frame Designs
The vehicle frame is the basic platform to which all suspension and steering linkage parts attach. A vehicle will neither steer nor handle well if the frame is too flexible. A rigid frame structure may pass unnecessary vibrations into the passenger compartment. The frame and suspension design will affect the ride quality, handling, and durability, as well as the levels of both noise and vibration. Manufacturers use several different types of construction on their vehicles.
Of these, separate body and frame construction was the most common through the 1970's. It is still used in large vans, pickups and trucks. In this type of construction, the engine, drive line, running gear, and body mount to the frame through insulators. Insulators are synthetic rubber pads that keep road and engine noise and vibration from going into the passenger compartment.
A second type of construction is the unitized body. This, design is by far the most popular in modern vehicles. The unitized design has a lightweight structure with the required strength. In this type of construction, the frame is welded into the body as part of the body structure. Body panels add strength to the frame pieces. The running gear and drive line are mounted to the unitized body through large, soft synthetic rubber insulators. The insulators minimize the transfer of noise and vibration. If the insulators are too soft, they will allow too much running gear and drive line movement. This movement, called compliance, affects vehicle handling and control. If the insulators are too hard, they will not insulate noise and vibration as they should. The manufacturer carefully designs the insulators and puts them where they will be in a vehicle with low noise and vibration transmission that still has proper handling and feel. Insulator properties change with age, changing original characteristics as the vehicle becomes older.
A third type of construction combines the features of the first and second types. It uses a stub frame from the bulkhead forward and a unitized body from the bulkhead back. The unitized part is very rigid, while the stub frame provides a place for good insulation. Manufacturers select the type of construction .that is most economical to build,' while providing the noise, vibration, and ride and handling characteristics they want in the vehicle. Large older vehicles, vans, and trucks generally use separate body and frame construction. The newer, smaller' vehicles generally use unitized construction.
A basic understanding of different drive line assemblies is presented here to give you a working knowledge so that you can do suspension repair. Drive lines with front-wheel drive often combine the transmission and the final drive into one assembly. This is also true of mid-and rear-engine vehicles. The assembly is called a transaxle, Short half-shafts with universal joints at each end connect between the transaxle and the wheels. These shafts carry power from the final drive to the wheels even when the suspension moves and steers. A differential in the final drive splits incoming power, sending half to each drive wheel. This allows the drive wheels to turn at different speeds while rounding corners. The transmission Other parts form the link from one part to while cornering.
In front-engine, rear-wheel drive vehicles, the transmission is located under the front floor of the passenger compartment. A drive shaft is used to carry engine power to the rear axle. The drive shaft has a universal joint at each end. It carries power through the changing drive line angles as the suspension moves. A vehicle with independent suspension at the drive wheels has the final drive attached rigidly to the vehicle frame or the engine. This drive arrangement produces forces, without any torques, on the suspension parts during acceleration. If the brakes are mounted inboard so the caliper mounts to a frame piece and not to a suspension, the brake will also not produce a torque on the suspension. A suspension designed to handle only acceleration and braking torques can be designed differently than one that must handle both suspension forces and torques. Suspension Systems The suspension includes springs, shock absorbers, and control linkages. It must be strong enough to support the vehicle body and load. The suspension must also resist engine and brake reactions. The most important job of the suspension is to keep the tires in contact with the road as much of the time as possible. This is done while supporting the vehicle and its load, even while traveling over rough roads. The four tire footprints are the only place the vehicle touches the road. All of the engine power, steering, and braking forces operate through the tire-to-road footprints. Control of the vehicle ( power, steering and braking) is reduced or lost any time a tire does not stay on the road or when skidding begins.
The vehicle body is supported by springs. The springs can be of the coil, leaf, torsion bar, or pneumatic type. Coil springs are the most popular design used in the modern automobile. Coil, torsion bar,and pneumatic springs all require links and arms to hold the wheel in position. Leaf springs provide lateral and longitudinal control to prevent unwanted wheel motions.They are commonly found on vans and trucks. Suspension systems have been changed and refined as the passenger automobile has developed. Design objectives differ between luxury sedans, performance vehicles, small compact vehicles, and light trucks. Tire improvements, along with improvements in shock absorbers, steering systems, and suspension control devices, have continually upgraded vehicle handling characteristics. Tire-to-road contact is needed for safe, positive vehicle control under all operating conditions. Keep in mind that all four tires must stay in contact with the road at all times for maximum vehicle control. Compromises are made in handling response, tire wear, driver comfort, and ride harshness to achieve positive vehicle control. Suspension systems are divided into front suspension and rear suspension. Front suspension designs have developed from relatively rugged solid-axle designs to the modern lightweight, high-strength , strut-type independent designs. These have been upgraded with added linkage. Suspension design improvements have followed improvements in roadways and driver expectations. Most front-engine, rear-wheel-drive vehicles use a simple dependent rear suspension . Rear-wheel-drive independent suspension is much more complex and expensive. As a result, it is only used on a few passenger vehicles. To front-engine, front-wheel-drive vehicles by moving the drive train to the front, only ride control and braking reactions are controlled by the rear suspension. This has led to the use of simplified dependent suspension , semi-independent suspension and independent rear suspension. The latter is used in a larger number of new vehicle designs.
Steering Systems The driver controls the direction of the front wheels of the vehicle through the steering gear. Modern steering gears have two major units* a steering column and a gear unit. Tin-steering column has a supported shaft that connects the driver's steering wheel to the gem unit. The gear unit multiplies the driver's steering effort to move the steering linkage. The front wheels of rear-wheel-drive vehicles rotate on a spindle. The spindle is part of the steering knuckle . The knuckle is connected to the front suspension members with ball joints. The ball joints allow for steering as the suspension moves up and down. The wheel hubs on front-wheel-drive vehicles rotate on hollow axle stub shafts inside bearings within the steering knuckles. The steering wheel controls the steering gear assembly. This, in turn, moves the knuckle through the steering linkage. Two steering gear designs are in use today, the rack and pinion and recirculating ball. vehicles are designed with responsive steering. As a result, more effort is needed to steer the vehicle when it is moving slowly. Power steering supplies this effort on many vehicles. With power steering doing most of the work, steering ratios are decreased so that the vehicle can be steered with small steering wheel movements. The power steering gear is similar to the standard steering gear. It includes surfaces upon which fluid pressure is applied to aid the driver's steering effort. Both rack and pinion and recirculating ball gears may have power assist. Power for the steering gear is provided by an engine-driven pump. The pump forces power steering fluid through a system controlled by a valve. This control valve can sense the driver's steering effort. It puts fluid pressure against a pressure surface in the steering system. This fluid pressure takes over some of the effort needed to steer the vehicle. The steering column in the modern vehicle has many parts. It is designed to collapse or fold in a vehicle collision to protect the driver. In some installations, it may be tilted and telescoped to adjust the position of the steering wheel for the comfort of the driver. To reduce the chance of theft, it also has a steering gear lock. On many vehicles, it has a transmission lock. Because it is within easy reach of the driver, the steering column may carry the transmission shift control lever, turn signal switch, headlight and dimmer switches, wiper switch, emergency flasher switch, and speed control.
Brake Systems Service brakes must be able to stop the vehicle, prevent excess speed when coasting, and hold the vehicle in position while it is stopped on grades. They are designed so the driver can adjust the braking effort to maintain vehicle control. Vehicle control is influenced by brakes as well as the suspension and steering systems. Faults in the brake system can lead to wheel pull during braking. To repair suspension systems, parts of the brake system may require disassembly. For these reasons, the brake system will be discussed briefly in this text. The brake system must provide smooth stopping power that can be controlled by the driver. The force required on the brake pedal must not be so high that the wheels cannot be locked. To meet these braking requirements, minimum braking standards have been set for vehicle brakes. The driver controls the braking force through mechanical, vacuum, and hydraulic mechanisms. The amount of braking increases as more force is placed on the brake pedal. This force is transferred through the brake system to push stationary brake linings against the rotating brake surface. This slows the vehicle as it turns kinetic energy (energy of motion) into thermal energy (heat). Maximum braking occurs just before the wheels lock to cause the tires to slide on the road surface. Maximum braking, therefore, depends on the adhesion between the tire and the road surface. When the tire slides on the road, braking effect is reduced and directional control of the vehicle may be lost. The stationary parts of the front brake assemblies are mounted on the steering knuckle of the fron t suspension. In the rear, they are mounted on the axle housing or the rear spindle assembly. The cast-iron brake drum or disc rotates with the wheel .
Disc Brake：Disc brakes have discs that rotate with the wheel. The brake disc is usually called a brake rotor. A hydraulically operated piston in a stationary caliper is used to force the lining of the brake pad against the braking surface of the rotor. The friction between the lining and rotor is used to slow or stop wheel rotation. The stationary caliper housing keeps the pads from rotating when they are being forced against the rotating brake disc. Disc brake pads move perpendicular to the face of the brake rotor. In this way, they clamp on the rotor to slow the vehicle motion. The clamping force is proportional to the force the driver puts on the brake pedal.
Drum Brakes: Drum brakes use stationary, internal expanding brake shoes with linings. They are mounted inside a rotating brake drum. The brake drum is fastened between the wheel-tire assembly and the hub assembly or the axle flange. The brake shoes slow drum rotation when the diameter of the shoes is expanded to bring the lining in contact with the brake surface. This is done by a hydraulically operated wheel cylinder. Fluid pressure from the master cylinder is forced into the wheel cylin-ders, expanding them. The expansion of the wheel cylinder moves the brake shoe through mechanical linkage to press the-linings against the rotating brake drum. This provides braking action as it slows the rotation of the drum.
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