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畢業(yè)論文開題報告
姓 名
張海燕
學(xué) 號
0411020125
專 業(yè)
機械設(shè)計制造及自動化
班 級
0204班
題 目
汽車液壓制動驅(qū)動機構(gòu)的設(shè)計
1．本選題的目或意義，該選題目前研究現(xiàn)狀、水平和發(fā)展趨勢
汽車 制動系是用以強制行駛中的汽車減速或停車、使下坡行駛的汽車的車速保持穩(wěn)定以及使已停駛的汽車在原地駐留不動的機構(gòu)。隨著高速公路的迅速發(fā)展和車速的提高以及車流密度的日益增大，為了保證行車安全，汽車制動系的工作可靠性顯得日益重要。也只有制動性能良好、制動系工作可靠的汽車，才能充分發(fā)揮其動力性能。
最I(lǐng) 最原始的制動控制只是駕駛員操縱一組簡單的機械裝置向制動器施加作用力，這時的 車輛的質(zhì)量比較小，速度比較低，機械制動雖已滿足車輛制動的需要，但隨著汽車自質(zhì)量 的增加，助力裝置對機械制動器來說已顯得十分必要。 隨著科學(xué)技術(shù)的發(fā)展及汽車工業(yè) 的發(fā)展，尤其是軍用車輛及軍用技術(shù)的發(fā)展，車輛制動有了新的突破，液壓制動是繼機械 制動后的又一重大革新。當(dāng)考慮基本的制動功能量，液壓操縱仍然是最可靠、最經(jīng)濟(jì)的方 法。即使增加了防抱 制動功能后，傳統(tǒng)的“油液制動系統(tǒng)”仍然占有優(yōu)勢地位?！鹘y(tǒng)的 制動控制系統(tǒng)只做一樣事情，即均勻分配油液壓力。當(dāng)制動踏板踏下時，主缸就將等量的 油液送到通往每個制動器的管路，并通過一個比例閥使前后平衡。
目前，液壓驅(qū)動機構(gòu)已發(fā)展成為成熟的產(chǎn)品，并在各種車輛上得到了廣泛的應(yīng)用。但是 ，制動控制系統(tǒng)仍然存在著嚴(yán)重的問題和不足。車輛制動控制系統(tǒng)的發(fā)展主要是控制技術(shù)的發(fā)展。一方面是擴大控制范圍、增加控制功能；另一方面是采用優(yōu)化控制理論，實施伺服控制和高精度控制。
2．研究目標(biāo)、研究內(nèi)容和擬解決的關(guān)鍵問題
本課題主要研究汽車制動系統(tǒng)的工作原理及整體結(jié)構(gòu)。要求根據(jù)生產(chǎn)實際結(jié)合調(diào)查研究，選出最優(yōu)方案并對其主要工作部件進(jìn)行詳細(xì)理論計算，繪制整機的裝配圖及相關(guān)的零件圖，設(shè)計說明書詳細(xì)周密,具有科學(xué)性。
對汽車的行車制動系統(tǒng)采取液壓控制回路，對于制動系統(tǒng)采取雙回路控制系統(tǒng)，可以使一回路損壞后，剩余制動達(dá)到總制動力的50%。
汽車制動驅(qū)動結(jié)構(gòu)和各缸參數(shù)和整個系統(tǒng)。
通過制動力的計算來對液壓控制系統(tǒng)進(jìn)行涉及計算。
3．?dāng)M采取的研究方法、技術(shù)路線、實驗方案及可行性分析
技術(shù)路線：通過查閱資料和試驗，應(yīng)該了解制動系統(tǒng)得結(jié)構(gòu)和工作原理，來確定各輪缸的輸入力，進(jìn)而確定主缸輸入力。擬定制動控制系統(tǒng)的工作路線及回路布置，由回路的供油情況計算設(shè)計出各缸的尺寸大小，根據(jù)液壓設(shè)計手冊選定各缸的材料和輔助件。由所學(xué)的知識選擇裝配方案并用AUTOCAD系統(tǒng)繪制出裝配圖和各個零件圖。
為解決以上問題，我選定了以下方案：
1.選用雙回路控制系統(tǒng)，以提高系統(tǒng)得工作安全性，
2.選用X型回路，當(dāng)一回路失效后確保另一回路能正常工作。
3.對各缸采取彈簧回位，減輕了制動機構(gòu)的重量，
可行性分析：
本系統(tǒng)解決了以下問題：
1. 制動協(xié)調(diào)時間符合規(guī)定。
2. 制動力大小符合規(guī)定。
3. 制動性能的優(yōu)化，制動安全性提高。
4．與本選題相關(guān)的主要參考文獻(xiàn)（列出作者、論文名稱、期刊號、出版年月，參考文獻(xiàn)應(yīng)在5篇及以上）
[1] 王望予、張建文、史文庫等 汽車設(shè)計[M]-4版.-北京：機械工程出版社，2004.8
[2] 余志生、趙六奇、夏群生等 汽車?yán)碚揫M]-3版.-北京：機械工業(yè)出版社，2000.10
[3] 張大成、戴波南等 [M] 上海桑塔納2000系列轎車維修手冊 –北京:北京理工大學(xué)出版社 2001.12
[4] 趙英勛、劉明等 汽車檢測與診斷技術(shù)[M] -北京：機械工業(yè)出版社.2003.1
[5] 劉長榮、肖念新等 工程力學(xué) [M] –北京：中國農(nóng)業(yè)科技出版社 2002
[6] 鄭玉才、劉長榮等 機械設(shè)計基礎(chǔ) [M] –北京：中國農(nóng)業(yè)科技出版社 2002
[7] 成大先、李長順、姜勇等 機械設(shè)計手冊 –北京：化學(xué)工學(xué)出版社，2004.1
[8] 佟玲等 液壓傳動 [M] –北京：中國農(nóng)業(yè)科技出版社 95.2
[9] 雷天覺、楊爾莊等 [M] 新編液壓工程手冊 -北京：北京理工大學(xué)出版社 98.2
[10] 卜炎等 機械傳動裝置設(shè)計手冊 [M] -北京：機械工程出版社98.12
[11] 陳家瑞等 汽車構(gòu)造 [M] -北京：人民交通出版社2002.2
5．指導(dǎo)教師意見：（對本課題的深度、廣度及工作量的意見）
?
?指導(dǎo)教師簽名：
年 月 日
5
河北科技師范學(xué)院
畢 業(yè) 論 文（設(shè) 計）
題 目：
完 成 人：
指 導(dǎo) 教師：
系（院）別：
專業(yè)、班級：
完 成 時間：
河北科技師范學(xué)院教務(wù)處制
一、資料目錄
1．畢業(yè)論文（設(shè)計）計劃書 ……………………………1~ 頁
2．畢業(yè)論文（設(shè)計） ……………………………………1~ 頁
3．文獻(xiàn)綜述 ………………………………………………1~ 頁
4．外文翻譯資料 …………………………………………1~ 頁
5．畢業(yè)論文（設(shè)計）工作總結(jié) …………………………1~ 頁
6．答辯記錄 ………………………………………………1~ 頁
其他反映研究成果的資料（如公開發(fā)表的論文復(fù)印件
及效益證明等）……………………………………… 1~ 頁
二、指導(dǎo)教師評語
成績：
項 目
成 績
工作表現(xiàn)
結(jié)論:
經(jīng)審查,該畢業(yè)論文(設(shè)計)合格（不合格）,可以（不可以）提交系(部)答辯委員會,參加答辯。
指導(dǎo)教師簽名：
年 月 日
三、論文質(zhì)量
成績:
項 目
得分
權(quán)重
成績
項 目
得分
權(quán)重
成績
選 題
0.15
參考文獻(xiàn)
0.05
試驗設(shè)計
0.15
能力運用
0.10
實施過程
0.10
創(chuàng) 新
0.10
分析與結(jié)論
0.15
研究價值
0.05
撰寫規(guī)范
0.15
總 計
評 審 人
專業(yè)答辯小組組長簽名: 年 月 日
四、答辯質(zhì)量
成績:
項 目
得 分
權(quán) 重
成 績
儀 表
0.15
語 言
0.25
表現(xiàn)效果
0.60
總 計
專業(yè)答辯小組組長簽名： 秘書簽名：
專業(yè)答辯小組成員簽名：
年 月 日
五、系（部）答辯委員會意見
評語:
綜合成績:
系（部）答辯委員會主任簽字： 系（部）蓋章
年 月 日
河北科技師范學(xué)院
畢業(yè)論文（設(shè)計）計劃書
題 目：
學(xué) 生 姓名：
指 導(dǎo) 教師：
系（院）別：
專業(yè)、班級
填 表 時間：
河北科技師范學(xué)院教務(wù)處制
一、立題依據(jù)（目的意義，國內(nèi)外研究現(xiàn)狀、水平與發(fā)展趨勢）
二、研究內(nèi)容、方法、技術(shù)路線及進(jìn)程安排
三、準(zhǔn)備情況（含基礎(chǔ)條件）及（易耗品）經(jīng)費預(yù)算
四、指導(dǎo)教師審核（或推薦）意見
指導(dǎo)教師簽名：
年 月 日
五、系（部）意見
系（部）主任簽名：
年 月 日
河北科技師范學(xué)院
學(xué)生畢業(yè)論文（設(shè)計）工作總結(jié)
學(xué) 生 姓名：
指 導(dǎo) 教師：
系（院）別：
專業(yè)、班級：
完 成 時間：
河北科技師范學(xué)院教務(wù)處制
畢業(yè)論文(設(shè)計)工作總結(jié)
一、計劃的實施情況及工作的詳細(xì)進(jìn)程
二、研究結(jié)果及意義
三、主要收獲及體會
四、存在問題及努力方向
指導(dǎo)教師評價意見：
指導(dǎo)教師簽名：
年 月 日
(可另附白紙)
河北科技師范學(xué)院畢業(yè)論文（設(shè)計）答辯記錄
答辯主持人
答辯組
成 員
答辯記錄人
地 點
時 間
年 月 日
答辯過程及內(nèi)容
組長簽名:
成員簽名:
（可另附白紙）
汽車液壓制動系統(tǒng)的發(fā)展
張海燕
（河北科技師范學(xué)院 機械電子系 機械設(shè)計制造及其自動化）
摘要:闡述了汽車制動控制統(tǒng)統(tǒng)的歷史、現(xiàn)狀和發(fā)展趨勢,著重分析了人力液壓制動系統(tǒng)的工作原理,討論了ABS防抱死系統(tǒng)的魯棒性和BBW系統(tǒng)的基本原理,介紹了液壓制動系統(tǒng)中液壓油的使用要求。
關(guān)鍵詞:制動系統(tǒng);制動缸;制動控制
從汽車自問世以來,制動系統(tǒng)在車輛行車和駐車的安全方面扮演著至關(guān)重要的角色.近年來,隨著車輛技術(shù)的進(jìn)步和汽車行駛速度的提高,這種重要性表現(xiàn)得越來越明顯.眾多的汽車工程師在改進(jìn)汽車制動性能的研究中傾注了大量的心血.目前關(guān)于汽車制動的研究主要集中在制動控制方面[1],包括制動控制的理論和方法,以及新技術(shù)推廣應(yīng)用.。
1．制動系統(tǒng)的歷史
對汽車的制動作用.由于那時的車輛的重量比較小,速度比較低,機械制動也能滿足車輛制動的需要,但隨著汽車自身重量的增加,助力裝置對機械制動器來說已顯得十分必要.于是開始出現(xiàn)了真空助力裝置.1932年生產(chǎn)的質(zhì)量為2860kg的凱迪拉克轎車四輪采用直徑419.1mm的鼓式制動器,并有制動踏板控制的真空助力裝置.林肯公司也在同年推出同種類型的轎車,該車采用了通過四根軟索控制真空助力器的鼓式制動器[2].
隨著科學(xué)技術(shù)的發(fā)展及汽車工業(yè)的發(fā)展,尤其是軍用車輛及軍用技術(shù)的發(fā)展,車輛制動有了新的突破,液壓制動是繼機械制動后的又一重大突破.DuesenbergEight車率先使用了轎車液壓制動器.克萊斯勒的四輪液壓制動器于1924年問世.通用和福特分別于1934年和1939年采用了液壓制動技術(shù).到20世紀(jì)50年代,液壓助力制動器開始在制動系統(tǒng)中應(yīng)用[3].
20世紀(jì)30年代后期,隨著電子技術(shù)的發(fā)展,世界汽車技術(shù)領(lǐng)域最顯著的成就是ABS防抱死制動系統(tǒng)(antilockbrakingsystem)[1]的使用和推廣.ABS集微電子技術(shù)、精密加工技術(shù)、液壓控制技術(shù)為一體,是機電一體化的高技術(shù)產(chǎn)品.它的使用大大提高了汽車的主動安全性和操縱性.防抱死控制裝置一般包括三部分:傳感器、控制器(ABSECU)與壓力調(diào)節(jié)器.傳感器接受運動參數(shù),如車輪角速度、角加速度、車速等傳送給控制裝置,控制裝置進(jìn)行計算并與規(guī)定的數(shù)值進(jìn)行比較后,給壓力調(diào)節(jié)器發(fā)出指令.
1936年,博世公司申請一項電液控制的ABS裝置專利,促進(jìn)了防抱死制動系統(tǒng)在汽車上的應(yīng)用.1969年福特使用了真空助力的ABS制動器;1971年,克萊斯勒車采用了四輪電子控制的ABS裝置.這些早期的ABS裝置性能有限,可靠性不夠理想,且成本高.
1979年,默·本茨推出了一種性能可靠、帶有獨立液壓助力器的全數(shù)字電子系統(tǒng)控制的ABS制動裝置.1985年美國開發(fā)出帶有數(shù)字顯示微處理器、復(fù)合主缸、液壓制動助力器、電磁閥及執(zhí)行器“一體化”的ABS防抱死裝置.隨著大規(guī)模集成電路和超大規(guī)模集成電路技術(shù)的出現(xiàn),以及電子信息處理技術(shù)的高速發(fā)展,ABS已成為性能可靠、成本日趨下降的具有廣泛應(yīng)用前景的成熟產(chǎn)品.一些國家和地區(qū)(如歐洲、日本、美國等)已制定法規(guī),使ABS成為汽車的標(biāo)準(zhǔn)設(shè)備[4].
2.制動控制系統(tǒng)的發(fā)展現(xiàn)狀
傳統(tǒng)的制動控制系統(tǒng)的主要特點是能均勻分配油液壓力.當(dāng)制動踏板踏下時,制動主缸就將等量的油液送到每個制動器的制動輪缸中,并通過一個比例閥使前后平衡.而ABS或其他制動干預(yù)系統(tǒng)則根據(jù)每個制動器的需要對油液壓力進(jìn)行調(diào)節(jié).目前ABS系統(tǒng)已發(fā)展成為成熟的產(chǎn)品,并在各種車輛上得到了廣泛的應(yīng)用,但是這些產(chǎn)品基本都是基于車輪加、減速門限及參考滑移率方法設(shè)計的.方法雖然簡單實用,但是其調(diào)試比較困難,不同的車輛需要不同的匹配技術(shù),在許多不同的道路上加以驗證.
制動控制系統(tǒng)的重要問題在于控制的穩(wěn)定性,即系統(tǒng)魯棒性(robustness),應(yīng)保證在各種條件下制動系統(tǒng)的控制作用不失效.防抱死系統(tǒng)要求高可靠性,否則會導(dǎo)致人身傷亡及車輛損壞.因此,發(fā)展魯棒性的ABS控制系統(tǒng)成為關(guān)鍵所在.現(xiàn)在,多種魯棒控制系統(tǒng)都應(yīng)用到ABS的控制邏輯中來[4].除了傳統(tǒng)的邏輯門限方法外,增益調(diào)度PID控制、變結(jié)構(gòu)控制和模糊控制是常用的魯棒控制系統(tǒng),是目前所采用的以滑移率為目標(biāo)的連續(xù)控制系統(tǒng).模糊控制法是基于經(jīng)驗規(guī)則的控制,與系統(tǒng)的模型無關(guān),具有很好的魯棒性和控制規(guī)則的靈活性,但調(diào)整控制參數(shù)比較困難,尚無理論計算公式,基本上是靠試湊的方法.然而對大多數(shù)基于目標(biāo)值的控制而言,控制規(guī)律有一定的規(guī)律.
車輪的驅(qū)動打滑與制動抱死是很類似的問題.在汽車起動或加速時,因驅(qū)動力過大而使驅(qū)動輪高速旋轉(zhuǎn)、超過摩擦極限而引起打滑.此時,車輪同樣不具有足夠的側(cè)向力來保持車輛的穩(wěn)定,車輪切向力也減少,影響加速性能.由此看出,防止車輪打滑與抱死都是要控制汽車的滑移率,所以在ABS的基礎(chǔ)上發(fā)展了驅(qū)動防滑系統(tǒng)(ASR)[6].ASR是ABS的邏輯和功能擴展.ABS在增加了ASR功能后,主要的變化是在電子控制單元中增加了驅(qū)動防滑邏輯系統(tǒng),來監(jiān)測驅(qū)動輪的轉(zhuǎn)速.ASR大多借用ABS的硬件,兩者共存一體,發(fā)展成為ABS/ASR系統(tǒng).
今天,ABS/ASR已經(jīng)成為歐美和日本等發(fā)達(dá)國家汽車的標(biāo)準(zhǔn)設(shè)備,并且已在歐洲新載貨車中普遍使用,歐共體法規(guī)EEC/71/320已強制性規(guī)定在總質(zhì)量大于3.5t的某些載貨車上使用,重型車輛首先裝用的.隨著技術(shù)的成熟和發(fā)展,現(xiàn)在的高級轎車也開始采用ABS/ASR系統(tǒng).然而ABS/ASR只是解決了緊急制動時附著系數(shù)的利用,可獲得較短的制動距離及制動方向穩(wěn)定性,但是不能解決制動系統(tǒng)中的所有缺陷.
3.制動控制系統(tǒng)的發(fā)展趨勢
經(jīng)過一百多年的發(fā)展,汽車制動系統(tǒng)的結(jié)構(gòu)已經(jīng)基本固定下來.隨著電子技術(shù),特別是大規(guī)模、大規(guī)模集成電路的發(fā)展,汽車制動系統(tǒng)的形式也將發(fā)生變化.凱西-海斯(K-H)公司在一輛實驗車上安裝了一種電-液(EH)制動系統(tǒng),該系統(tǒng)徹底改變了制動系統(tǒng)的工作原理.通過采用4個比例閥和電力電子控制裝置,該系統(tǒng)能處理基本制動、ABS、牽引力控制、巡航控制制動干預(yù)等情況,而不需增加任何一種附加裝置.該系統(tǒng)潛在的優(yōu)點是比標(biāo)準(zhǔn)制動器能更加有效地分配基本制動力,從而使制動距離縮短5%.一種完全無油液、完全通過電路控制的BBW(Brake-By-Wire)[7]系統(tǒng)的開發(fā)將使傳統(tǒng)的液壓制動控制系統(tǒng)成為歷史.
BBW系統(tǒng)是未來制動控制系統(tǒng)的發(fā)展方向.全電制動系統(tǒng)不同于傳統(tǒng)的制動系統(tǒng),因為其傳遞的是電流,而不是液壓油或壓縮空氣,可以省去許多管路和傳感器,縮短制動反應(yīng)時間.
4.汽車制動液的使用
制動液是液壓制動系統(tǒng)的重要組成部分,它的質(zhì)量好壞對制動系統(tǒng)的工作可靠性有很大的影響.因此制動液應(yīng)具有高溫下不易液化、低溫下流動性良好、吸水性差而溶水性良好、能潤滑液壓系統(tǒng)的運動件而不會使其腐蝕,不會使橡膠件發(fā)生膨脹、變硬和損壞等特點.
在選用制動液時應(yīng)根據(jù)制動系統(tǒng)是否安裝ABS裝置來選擇.因為沒有安裝ABS的車輛在緊急制動時往往會使車輪抱死,安裝ABS后,可使汽車在緊急制動時車輪不會被抱死而防止出現(xiàn)側(cè)滑,因此ABS裝置工作時,制動系統(tǒng)產(chǎn)生的摩擦熱比未裝ABS裝置的車高,制動液的惡化變質(zhì)也會相對早地出現(xiàn),適合于ABS的只能是高沸點的制動液,如果使用低沸點的制動液,會因容易產(chǎn)生氣阻而使汽車處于非常危險的狀態(tài).
如果在制動液變質(zhì)的情況下繼續(xù)使用,將會使制動主缸、制動輪缸、油壓控制器等產(chǎn)生損傷、吸濕率增加,使制動力下降[8].因此,要對裝有ABS裝置車輛的制動液進(jìn)行從嚴(yán)選用,嚴(yán)格遵守汽車制造廠商推薦的更換周期.另外還要根據(jù)使用條件,在必要時提前進(jìn)行更換.
6.結(jié)束語
介紹汽車制動控制系統(tǒng)的進(jìn)展歷史,詳細(xì)闡述制動控制系統(tǒng)由簡單的機械制動裝置發(fā)展到ABS防抱死制動控制系統(tǒng),可以嘗試模糊控制方法以改進(jìn)汽車控制系統(tǒng)的性能.現(xiàn)代汽車制動控制技術(shù)正朝著電子制動控制方向發(fā)展.全電制動控制因其巨大的優(yōu)越性,將取代傳統(tǒng)的以液壓為主的傳統(tǒng)制動控制系統(tǒng).同時,隨著其他汽車電子技術(shù)特別是超大規(guī)模集成電路的發(fā)展,電子元件的成本及尺寸不斷下降.另外汽車電子制動控制系統(tǒng)將與其他汽車電子系統(tǒng)融合在一起成為綜合的汽車電子控制系統(tǒng),未來的汽車中就不存在孤立的制動控制系統(tǒng),各種控制單元集中在一個ECU中,并將逐漸代替常規(guī)的控制系統(tǒng),實現(xiàn)車輛控制的智能化.
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[2]KazemiR.DevelopmentofanewABSforpassengercarsusingdynamicsurfacecontrolmethod[J].ProceedingsoftheAmericanControlConference,2001,(2):677683.
[3] 陳家瑞.汽車構(gòu)造[M].北京:機械工業(yè)出版社,2000.
[4] 沙奇林.ABS控制邏輯對車輛動力學(xué)模型參數(shù)及滑移率的規(guī)避分析[J].汽車工程,1996,16(6):329333.
[5] 陳軍.防抱死制動系統(tǒng)不同控制方法的模擬研究[J].汽車技術(shù),1989,(11):2326.
[6] 馬岳軍.基于ABS的ABSASR集成液壓系統(tǒng)設(shè)計[J].液壓與氣動,2004,(6):2729.
[7] 戴冠軍、周軍、孫偉等[M]汽車底盤維修大全 –杭州：浙江科學(xué)技術(shù)出版社 2000.4
[8] 張大成、戴波南等 [M] 上海桑塔納2000系列轎車維修手冊 –北京:北京理工大學(xué)出版社 2001.12
[9]ParkShinsuk.Numericalevaluationofbrakingfeeltodesignoptimalbrake-by-wiresystem[J].InternationalJournalofVehicleDesign,2005,37(1):123.
[10] 陳家瑞等 汽車構(gòu)造 [M] -北京：人民交通出版社20
Automobile hydraulic brake system development
Zhang Haiyan
(Dept.of Machinery and Electron,Hebei Normal University of Science & Technology)
Abstract: Elaborated the automobile brake control entirely history, the present situation and the trend of development, have emphatically analyzed the manpower hydraulic brake system principle of work, discussed ABS to guard against holds the dead system robustness and the BBW system basic principle, introduced in the hydraulic brake system the hydraulic fluid operation requirements.
Key words: Braking system; Applies the brake the cylinder;
5
The Brake System
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The braking system is the most important system in your car. If your brakes fail, the result can be disastrous. Brakes are actually energy conversion devices, which convert the kinetic energy (momentum) of your vehicle into thermal energy (heat). When you step on the brakes, you command a stopping force ten times as powerful as the force that puts the car in motion. The braking system can exert thousands of pounds of pressure on each of the four brakes. In modern systems, the master cylinder is power-assisted by the engine. All newer cars have dual systems, with two wheels' brakes operated by each subsystem. That way, if one subsystem fails, the other can provide reasonably adequate braking power. Safety systems like this make modern brakes more complex, but also much safer than earlier braking systems. The brake system is composed of the following basic components: The "master cylinder" which is located under the hood, and is directly connected to the brake pedal, converts your foot's mechanical pressure into hydraulic pressure. Steel "brake lines" and flexible "brake hoses" connect the master cylinder to the "slave cylinders" located at each wheel. Brake fluid, specially designed to work in extreme conditions, fills the system. "Shoes" and "pads" are pushed by the slave cylinders to contact the "drums" and "rotors" thus causing drag, which (hopefully) slows the car.
In recent years, brakes have changed greatly in design. Disc brakes, used for years for front wheel applications, are fast replacing drum brakes on the rear wheels of modern cars. This is generally due to their simpler design, lighter weight and better braking performance. The greatest advantage of disc brakes is that they provide significantly better resistance to "brake fade" compared to drum type braking systems. Brake fade is a temporary condition caused by high temperatures generated by repeated hard braking. It occurs when the pads or shoes "glaze" due to the great pressure and heat of hard use. Once they cool, the condition subsides. Disc brakes allow greater air ventilation (cooling) compared to drum brakes. Drum brakes are not internally ventilated because if they were, water could accumulate in them. Disc brakes can rapidly fling off any water that they are exposed to, and so they can be well ventilated.
"Boosters" are present in "power brake" systems, and use the engine's energy to add pressure to the master cylinder. "Anti-lock" (ABS) systems, originally developed for aircraft braking systems, use computer controlled valves to limit the pressure delivered to each slave cylinder. If a wheel locks up, steering input cannot affect the car's direction. With ABS, no matter how hard the pedal is pressed, each wheel is prevented from locking up. This prevents skidding (and allows the driver to steer while panic-braking).
As impressive as these advances are, the basic process of converting a vehicle's momentum into (wasted) heat energy has not changed since the days of the horse and buggy. To stop a horse drawn carriage, the driver would pull on a lever which would rub on the wheel. But today, with the advent of regenerating brakes on electric vehicles, new ways of recapturing this lost energy are being developed. In these types of electric cars, when you step on the brakes, the motor switches into "generator mode", and stores the car's momentum as chemical energy in the battery, to be used again when the light turns green!
Disc Brakes
Disc brakes use a clamping action to produce friction between the "rotor" and the "pads" mounted in the "caliper" attached to the suspension members. Inside the calipers, pistons press against the pads due to pressure generated in the master cylinder. The pads then rub against the rotor, slowing the vehicle. Disc brakes work using much the same basic principle as the brakes on a bicycle; as the caliper pinches the wheel with pads on both sides, it slows the bicycle. Disc brakes offer higher performance braking, simpler design, lighter weight, and better resistance to water interference than drum brakes.
Disc brakes, like many automotive innovations, were originally developed for auto racing, but are now standard equipment on virtually every car made. On most cars, the front brakes are of the disc type, and the rear brakes are of the "drum" type. Drum brakes use two semi-circular shoes to press outward against the inner surfaces of a steel drum. Older cars often had drum brakes on all four wheels, and many new cars now have 4-wheel disc brakes.
Because disc brakes can fling off water more easily than drum brakes, they work much better in wet conditions. This is not to say that water does not affect them, it definitely does. If you splash through a puddle and then try to apply the brakes, your brakes may not work at all for a few seconds! Disc brakes also allow better airflow cooling, which also increases their effectiveness. Some high performance disc brakes have drilled or slotted holes through the face of the rotor, which helps to prevent the pads from "glazing" (becoming hardened due to heat). Disc brakes were introduced as standard equipment on most cars in the early seventies.
Brake Drums
The brake drum is a heavy flat-topped cylinder, which is sandwiched between the wheel rim and the wheel hub. The inside surface of the drum is acted upon by the linings of the brake shoes. When the brakes are applied, the brake shoes are forced into contact with the inside surface of the brake drums to slow the rotation of the wheels.
The drums are usually covered with fins on their outer surfaces to increase cooling. They are not cooled internally, because water could enter through the air vent cooling holes and braking would then be greatly impaired.
Drum brakes are found on the rear wheels of most older cars, but they are increasingly being fazed out in favor of rear disc brakes. Drum brakes were standard equipment on all four wheels of most cars until the early 70's.
Brake Calipers
The caliper works like a C-clamp to pinch the pads onto the rotor. It straddles the rotor and contains the hydraulic "slave cylinder" or "wheel cylinder" piston(s). One caliper is mounted to the suspension members on each wheel. The caliper is usually mounted onto the spindle, allowing it to deliver the torsional force of the wheel to the chassis via the control arms. Brake hoses connect the caliper to the brake lines leading to the master cylinder. A "bleeder valve" is located on each caliper to allow air bubbles to be purged from the system.
"Floating caliper" disc brakes, the most common variety, allow the caliper to move from side to side slightly when the brakes are applied. This is because only one pad moves (in relation to the caliper). Some calipers contain two or four seperate pistons. These calipers are fixed in place; i.e., there is no lateral movement like the floating caliper, the pistons take up the slack on each side of the rotor. These are called "dual cylinder" or "dual piston" calipers, and are standard equipment on many performance cars.
Wheel (Slave) Cylinder
Wheel cylinders, also called the "slave" cylinders, are cylinders in which movable piston(s) convert hydraulic brake fluid pressure into mechanical force. Hydraulic pressure against the piston(s) within the wheel cylinder forces the brake shoes or pads against the machined surfaces of the drum or rotor. There is one cylinder (or more in some systems) for each wheel. Drum brake wheel cylinders are usually made up of a cylindrical casting, an internal compression spring, two pistons, two rubber cups or seals, and two rubber boots to prevent entry of dirt and water. This type of wheel cylinder is fitted with push rods that extend from the outer side of each piston through a rubber boot, where they bear against the brake shoes. In disc brakes, the wheel cylinder is built into the caliper. All wheel cylinders have bleeder screws (or bleeder valves) to allow the system to be purged of air bubbles.
As the brake pedal is depressed, it moves pistons within the master cylinder, pressurizing the brake fluid in the brake lines and slave cylinders at each wheel. The fluid pressure causes the wheel cylinders' pistons to move, which forces the shoes or pads against the brake drums or rotors. Drum brakes use return springs to pull the pistons back away from the drum when the pressure is released. On disc brakes, the calipers' piston seals are designed to retract the piston slightly, thus allowing the pads to clear the rotor and thereby reduce rolling friction.
Parking (Emergency) Brakes
The parking brake (sometimes called the emergency brake) is a cable-activated system used to hold the brakes continuously in the applied position. The parking brake activates the brakes on the rear wheels. Instead of hydraulic pressure, a cable (mechanical) linkage is used to engage the brake shoes or discs. When the parking-brake pedal is pressed (or, in many cars, a hand lever is pulled), a steel cable draws the brake shoes or pads firmly against the drums or rotors. The release lever or button slackens the cables and disengages the brake shoes. The parking brake is self adjusting on most systems. An automatic adjuster compensates for lining (brake shoe) wear. On many cars, the parking brake is used to re-adjust the brake shoes as they wear in, or when the shoes are replaced. In these systems, the adjustment is made by repeatedly applying the parking brake while backing up.
The parking brake can be useful while driving up hills: If you're driving a manual transmission car, and you pull up to a stop on an incline, you might notice that you don't have enough feet to operate the clutch, brake, and gas at the same time. In other words, you will likely roll backwards slightly while getting started again. If a someone pulls up right behind you, this can be a problem. Your parking brake is useful in this situation: Apply the parking brake after you stop. When you want to go, release the clutch while pressing the gas, and release the parking brake. This keeps you from having to quickly switch your left foot from the brake to the clutch, or your right foot from the brake to the gas pedal. A little practice, and you'll be able to do it smoothly. Also, remember if you pull up behind someone who is stopped on a hill, give them extra room to roll back a little. Especially if it's a truck.
Some cars have no parking brake release! They automatically release the parking brake when the car is placed in drive or reverse.
Remember, it's a good idea to test the parking brake periodically and keep it in good condition. It may save your life if the main braking system fails!
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