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一 題目及專題 1 題目 車床主軸傳動(dòng)系統(tǒng)設(shè)計(jì) 2 專題 單主軸變速機(jī)構(gòu)設(shè)計(jì) 二 課題來源及選題依據(jù) 課題來源為無錫某機(jī)械有限公司 通過畢業(yè)設(shè)計(jì)是為了培養(yǎng)學(xué)生開發(fā) 和創(chuàng)新機(jī)械產(chǎn)品的能力 要求學(xué)生能夠針對(duì)實(shí)際使用過程中存在的機(jī)床選擇 問題 綜合所學(xué)的機(jī)械理論設(shè)計(jì)與方法 專用機(jī)床設(shè)計(jì)方法 在機(jī)床基礎(chǔ)型 系的基礎(chǔ)上 設(shè)計(jì)一個(gè)簡單的主傳動(dòng)變速系統(tǒng) 從而達(dá)到解決實(shí)際工程問題 的能力 在設(shè)計(jì)傳動(dòng)件時(shí) 在滿足產(chǎn)品工作要求的情況下 應(yīng)盡可能多的采用標(biāo) 準(zhǔn)件 提高其互換性要求 以減少產(chǎn)品的設(shè)計(jì)生產(chǎn)成本 三 本設(shè)計(jì) 論文或其他 應(yīng)達(dá)到的要求 1 該部件工作時(shí) 能運(yùn)轉(zhuǎn)正常 2 擬定工作機(jī)構(gòu)和驅(qū)動(dòng)系統(tǒng)的運(yùn)動(dòng)方案 并進(jìn)行多方案對(duì)比分析 3 根據(jù)車床的加工要求 設(shè)計(jì)出機(jī)床主傳動(dòng)系統(tǒng)的原理及繪制主傳 動(dòng)系 統(tǒng)的裝配圖 4 對(duì)所設(shè)計(jì)的主傳動(dòng)系統(tǒng)進(jìn)行必要的驗(yàn)算和推導(dǎo) 5 繪制車床的主傳動(dòng)軸的零件圖 6 繪制車床主傳動(dòng)系轉(zhuǎn)速圖和變速傳動(dòng)系圖 7 編制設(shè)計(jì)說明書 1 份 四 接受任務(wù)學(xué)生 I 摘要 車床主要是為了進(jìn)行車外圓 車端面和鏜孔等項(xiàng)工作而設(shè)計(jì)的機(jī)床 車削很少在其 他種類的機(jī)床上進(jìn)行 而且任何一種其他機(jī)床都不能像車床那樣方便地進(jìn)行車削加工 由于車床還可以用來鉆孔和鉸孔 車床的多功能性可以使工件在一次安裝中完成幾種加 工 因此 在生產(chǎn)中使用的各種車床比任何其他種類的機(jī)床都多 車床的基本部件有 床身 主軸箱組件 尾座組件 溜板組件 絲杠和光杠 主軸箱安裝在內(nèi)側(cè)導(dǎo)軌的固定位置上 一般在床身的左端 它提供動(dòng)力 并可使工 件在各種速度下回轉(zhuǎn) 它基本上由一個(gè)安裝在精密軸承中的空心主軸和一系列變速齒輪 類似于卡車變速箱 所組成 通過變速齒輪 主軸可以在許多種轉(zhuǎn)速下旋轉(zhuǎn) 大多數(shù)車 床有 8 12 種轉(zhuǎn)速 一般按等比級(jí)數(shù)排列 而且在現(xiàn)代機(jī)床上只需扳動(dòng) 2 4 個(gè)手柄 就能 得到全部轉(zhuǎn)速 一種正在不斷增長的趨勢是通過電氣的或者機(jī)械的裝置進(jìn)行無級(jí)變速 由于機(jī)床的精度在很大程度上取決于主軸 因此 主軸的結(jié)構(gòu)尺寸較大 通常安裝 在預(yù)緊后的重型圓錐滾子軸承或球軸承中 主軸中有一個(gè)貫穿全長的通孔 長棒料可以 通過該孔送料 主軸孔的大小是車床的一個(gè)重要尺寸 因此當(dāng)工件必須通過主軸孔供料 時(shí) 它確定了能夠加工的棒料毛坯的最大尺寸 關(guān)鍵字 車床 主軸箱組件 主軸 無級(jí)變速 II Abstract Lathes are machine tools designed primarily to do turning facing and boring Very little turning is done on other types of machine tools and none can do it with equal facility Because lathes also can do drilling and reaming their versatility permits several operations to be done with a single setup of the work piece Consequently more lathes of various types are used in manufacturing than any other machine tool The essential components of a lathe are the bed headstock assembly tailstock assembly and the leads crew and feed rod The headstock is mounted in a foxed position on the inner ways usually at the left end of the bed It provides a powered means of rotating the word at various speeds Essentially it consists of a hollow spindle mounted in accurate bearings and a set of transmission gears similar to a truck transmission through which the spindle can be rotated at a number of speeds Most lathes provide from 8 to 18 speeds usually in a geometric ratio and on modern lathes all the speeds can be obtained merely by moving from two to four levers An increasing trend is to provide a continuously variable speed range through electrical or mechanical drives Because the accuracy of a lathe is greatly dependent on the spindle it is of heavy construction and mounted in heavy bearings usually preloaded tapered roller or ball types The spindle has a hole extending through its length through which long bar stock can be fed The size of maximum size of bar stock that can be machined when the material must be fed through spindle Key words Lathes headstock assembly variable speed 目錄 摘要 III ABSTRACT IV 1 緒論 1 1 1 畢業(yè)設(shè)計(jì)的目的 1 1 2 機(jī)床主傳動(dòng)系統(tǒng)設(shè)計(jì)要求 1 2 車床參數(shù)的擬定 2 2 1 車床主參數(shù)和基本參數(shù) 2 2 1 1 極限切削速度 Vmax Vmin 的確定 2 2 1 2 主軸的極限轉(zhuǎn)速的確定 2 2 1 3 主軸轉(zhuǎn)速級(jí)數(shù)和公比的確定 3 2 1 4 主電機(jī)的選擇 3 3 主傳動(dòng)系統(tǒng)設(shè)計(jì) 6 3 1 傳動(dòng)結(jié)構(gòu)式 結(jié)構(gòu)網(wǎng) 轉(zhuǎn)速圖的確定 6 3 1 1 傳動(dòng)形式的確定 6 3 1 2 傳動(dòng)組及各傳動(dòng)組中傳動(dòng)副的數(shù)目 6 3 1 3 傳動(dòng)系統(tǒng)擴(kuò)大順序的安排 7 3 1 4 繪制轉(zhuǎn)速圖 7 3 1 5 轉(zhuǎn)速圖的擬定 7 3 1 6 分配降速比 8 3 2 齒輪齒數(shù)的確定及傳動(dòng)系統(tǒng)圖的繪制 10 3 2 1 帶輪確定 10 3 2 2 齒輪齒數(shù)的確定的要求 14 4 強(qiáng)度計(jì)算和結(jié)構(gòu)草圖設(shè)計(jì) 17 4 1 確定計(jì)算轉(zhuǎn)速 17 4 1 1 各軸 齒輪的計(jì)算轉(zhuǎn)速 17 4 2 軸的估算和驗(yàn)算 19 4 2 1 主軸的設(shè)計(jì)與計(jì)算 19 4 2 2 傳動(dòng)軸直徑的估算 22 4 3 齒輪模數(shù)的估算和計(jì)算 24 4 3 1 齒輪模數(shù)的估算 24 4 3 2 齒輪模數(shù)的驗(yàn)算 26 I 4 4 軸承的選擇與校核 31 4 4 1 一般傳動(dòng)軸上的軸承選擇 31 4 4 2 主軸軸承的類型 31 4 4 3 軸承間隙調(diào)整和預(yù)緊 33 4 4 4 軸承的較核 34 4 4 5 軸承的密封和潤滑 36 4 5 片式摩擦離合器的選擇與驗(yàn)算 36 4 5 1 按扭矩選擇 36 4 5 2 片式離合器的計(jì)算 36 4 5 3 計(jì)算摩擦面的對(duì)數(shù) Z 37 5 主軸箱的箱體設(shè)計(jì) 39 6 結(jié)論與展望 40 6 1 結(jié)論 40 6 2 展望 40 致 謝 41 參考文獻(xiàn) 42 0 1 緒論 1 1 畢業(yè)設(shè)計(jì)的目的 通過機(jī)床主運(yùn)動(dòng)機(jī)械變速傳動(dòng)系統(tǒng)的結(jié)構(gòu)設(shè)計(jì) 在擬定傳動(dòng)和變速的結(jié)構(gòu)方案過程 中 得到設(shè)計(jì)構(gòu)思 分析方案 結(jié)構(gòu)工藝性 機(jī)械制圖 零件計(jì)算 編寫技術(shù)要求文件 和查閱級(jí)數(shù)資料等方面的綜合訓(xùn)練 樹立正確的設(shè)計(jì)思想 掌握機(jī)床設(shè)計(jì)的過程和方法 使原有的知識(shí)有了進(jìn)一步的加深 1 課程設(shè)計(jì)屬于機(jī)械系統(tǒng)設(shè)計(jì)課程的延續(xù) 通過設(shè)計(jì)實(shí)踐 進(jìn)一步學(xué)習(xí)掌握機(jī)械系 統(tǒng)的一般方法 2 培養(yǎng)綜合運(yùn)用機(jī)械制圖 機(jī)械設(shè)計(jì)基礎(chǔ) 精度設(shè)計(jì) 金屬工藝學(xué) 材料熱處理及 結(jié)構(gòu)工藝等相關(guān)知識(shí) 進(jìn)行工程設(shè)計(jì)的能力 3 培養(yǎng)使用手冊 圖冊 有關(guān)資料及設(shè)計(jì)標(biāo)準(zhǔn)規(guī)范的能力 4 提高技術(shù)總結(jié)及編制技術(shù)文件的能力 5 為進(jìn)入工廠打下基礎(chǔ) 1 2 機(jī)床主傳動(dòng)系統(tǒng)設(shè)計(jì)要求 1 主軸具有一定的轉(zhuǎn)速和足夠的轉(zhuǎn)速范圍 轉(zhuǎn)速級(jí)別 能夠?qū)崿F(xiàn)運(yùn)動(dòng)的開停 變速 換向和制動(dòng)等 以滿足機(jī)床的運(yùn)動(dòng)要求 2 主電動(dòng)機(jī)具有足夠的功率 全部機(jī)構(gòu)和元件具有足夠的強(qiáng)度和剛度 以滿足 機(jī)床的傳動(dòng)要求 3 主運(yùn)動(dòng)的有關(guān)機(jī)構(gòu) 特別是主軸組件有足夠的精度 抗振性 溫升小和噪音 小 傳動(dòng)效率高 以滿足機(jī)床的工作性能要求 4 操作靈活可靠 調(diào)整維修方便 潤滑密封良好 以滿足機(jī)床的使用要求 5 結(jié)構(gòu)緊湊簡單 工藝性好 成本低 以滿足經(jīng)濟(jì)要求 三 車床主要參數(shù) 規(guī)格尺寸 最大工件回轉(zhuǎn)直徑 D mm 400 刀架上最大回轉(zhuǎn)直徑 D1 200 主軸通孔直徑 d 50 主軸頭號(hào) JB2521 79 6 最大工件長度 L 750 2000 外文翻譯 1 1 Summarize Outline uses a more compact design along with the engine and has in a big way the engine produces the waste heat density also obviously increases along with it Some essential regions if around a row of tyre valve radiates the question to have first to consider the cooling system even if appears the small breakdown also possibly to create the disaster in such region consequence The engine cooling system radiation ability generally should satisfy when the engine full load radiation demand because this time engine produces the quantity of heat is biggest However when partial loads the current capacity which the cooling system can have the power loss which the water pumping station provides the refrigerant current capacity surpasses needs We hoped starts the starting time to be as far as possible short Because engine time discharges pollutant more the oil consumption is also big The cooling system structure has a more tremendous influence to the engine cold starting time 2 Characteristics of modern engine cooling system Modern engines series characteristic tradition cooling system function reliably protects the engine but also should have the function which the improvement fuel economy and reduces discharges Therefore the modern cooling system must synthesize under the consideration the factor Engine interior friction loss Cooling system water pump power Burning boundary condition like combustion chamber temperature complete density complete temperature The advanced cooling system uses systematized the modular design method the overall plan considered each influence factor causes the cooling system both to guarantee the engine normal work and enhances the engine efficiency and the reduction discharges 2 1 The temperatures set point Temperatures hypotheses firing in bursts motive operating temperature limit value are decided by a row of tire valve the peripheral region maximum temperature The most ideal situation is according to the metal temperature but is not the refrigerant temperature control cooling system like this can protect the engine well Because the cooling system hypothesis cooling temperature is by the full load time most is big is the foundation therefore engine and cooling system in partial loads time is at not too the perfect condition when urban district travel and low speed travel can have the high oil consumption and discharge Supposes the fixed point through the change refrigerant temperature to be possible to improve the engine and the cooling system in partial loads time performance According to a row of tyre valve the peripheral region temperature limit value may elevate either reduce the refrigerant or the metal temperature supposes the fixed point Elevates or reduced temperature all respectively has the characteristic this is decided the goal which achieved to the hope 2 2 Enhances the temperature Enhances the temperature to suppose the fixed point enhancement operating temperature to suppose the fixed point is one kind of comparison the method which welcome Enhances the temperature to have many merits it directly affects the engine loss and the cooling system effect as well as the engine discharging formation Will enhance the operating temperature to enhance the engine Mac reduce the engine to rub wears reduces the engine fuel oil consumption The research indicated that the engine operating temperature to rubs the loss to have the very tremendous influence Discharges the temperature the refrigerant to enhance to 150 causes the cylinder temperature to elevate to 195 the oil consumption drops 4 6 Maintains the refrigerant temperature in 90 115 scope causes the engine machine oil the maximum temperature is 140 then oil consumption in partial loads time drops 10 Enhances the operating temperature also obvious influence cooling system the potency Enhances the refrigerant or the metal temperature can improve the engine and disperse the steam heat transfer transmission the effect reduces the refrigerant the speed of flow reduces the water pump the rated power thus reduces the engine the power dissipation In addition may select the different method further reduces the refrigerant the speed of flow 2 3 Reduce the temperatures set point Reduced temperatures suppose the fixed point to reduce the cooling system the operating temperature to be possible to enhance the engine charge efficiency reduces the inlet temperature This to the combustion process 外文翻譯 2 the fuel oil efficiency and discharges advantageously The reduced temperature supposes the fixed point to be allowed to save the engine movement cost enhances the part service life The research indicated if the cylinder head temperature reduces to 50 the ignition angle of advance may 3 A but not have the engine knock ahead of time the charge efficiency enhances 2 the engine operational factor improvement is helpful to the optimized compression ratio and the parameter choice obtains the better fuel oil efficiency and discharges the performance 2 4 Precise cooling system Precise cooling systems precise cooling system mainly manifests in the cooling jacket structural design and in the refrigerant speed of flow design In precise cooling system hot essential area if around a row of tyre valve the refrigerant has an greater speed of flow the heat transfer efficiency is high the refrigerant gradient of temperature changes slightly Such effect comes from to reduce these place refrigerant channels the lateral section enhances the speed of flow reduces the current capacity The precise cooling system design key lies in the determination cooling jacket the size the choice match cooling water pump guaranteed the system the radiation ability can satisfy when the low speed big load essential region operating temperature demand The engine refrigerant speed of flow range of variation is quite big from time 1 m s to maximum work rate time 5 m s Therefore should considered the cooling jacket and the cooling system whole that mutually supplemented display biggest potential The research indicated that uses the precise cooling system in the engine entire work rotational speed scope the refrigerant current capacity may drop 40 Covers the cooling jacket to the air cylinder the precise design may make the ordinary speed of flow to enhance from 1 4m s to 4 m s greatly enhances the cylinder cover or cap thermal conductivity cylinder cover or cap metal temperature drop to 60 2 5 Divergences types cooling system Divergences types cooling system divergence type cooling system for other one kind of cooling system In this kind of cooling system the hine oil temperature will cylinder cover or cap friendly cylinder body cools by respective return route the cylinder cover or cap friendly cylinder body has the different temperature The divergence like cooling system has the unique superiority may cause engine each part to suppose the fixed point work at the most superior temperature The cooling system overall efficiency achieves in a big way Each cooling return route will suppose under the fixed point or the speed of flow in the different cooling temperature works will create the ideal engine temperature distribution The ideal engine hot active status is the cylinder head temperature lower but the air cylinder body temperature relative is higher The cylinder head temperature is lower may enhance the charge efficiency increases The temperature is low also greatly may promote completely to burn reduces CO HC and the NOx formation also enhances the output The higher air cylinder body temperature can reduce the friction to lose directly improves the fuel oil efficiency indirectly reduces in the cylinder the peak value pressure and the temperature The divergence type cooling system may cause the cylinder cover and the cylinder body temperature differs 100 The cylinder temperature may reach as high as 150 but the cylinder cover temperature may reduce 50 reduces the cylinder body to rub loses reduces the oil consumption The higher cylinder body temperature causes the oil consumption to reduce 4 6 when partial loads HC reduces 20 35 When the damper all opens the cylinder cover and the cylinder body temperature supposes the definite value to be possible to move to 50 and 90 improves the fuel oil consumption the power output from the whole and discharges 2 6 Controllable cooling system Controllable engine cooling system tradition engine cooling system belongs to the passive form the structure simple or the cost is low The controllable cooling system may make up at present cooling system the insufficiency Now the cooling system design standard solves time the full load radiation problem thus partially shoulders time the oversized radiation ability will cause the engine power waste This to the light vehicle said especially obvious these vehicles majority time all under the partial loads go in the urban district only uses the partial engine power causes a cooling system higher loss In order to solve the engine to get 外文翻譯 3 down the hot question in the peculiar circumstance the present cooling system volume was bigger causes the evaporation efficiency to reduce has increased the cooling system power demand lengthened the engine during warm machine hour The controllable engine cooling system generally includes the sensor the execution and the electrically controlled module The controllable cooling system can act according to the engine working condition adjustment cooling quantity reduces the engine power loss In the controllable cooling system the execution for the cooling water pump and the thermostat generally and the control valve is composed by the electrically operated water pump may act according to requests to adjust the cooling quantity Temperature sensor for a system part but rapidly bequeaths the engine hot condition the controller Controllable installment if the electrically operated water pump may suppose the temperature the fixed point from 90 to enhance to 110 saves 2 5 fuel oil CO reduces 20 HC reduces 10 When steady state the metal temperature ratio tradition cooling system is high 10 the controllable cooling system has the quicker response ability may cool the temperature to maintain is supposing the fixed point 2 the scope From 110 drops to 100 only needs 2 s The engine during warm machine hour reduces 200s the cooling system operating region draws close to the work limit region can reduce the engine cooling temperature and the metal temperature undulation scope reduces circulates the fatigue of metal which the hot load creates lengthens the component life 3 Conclusion In front of 3 conclusions introduced several kind of advanced cooling systems have the improvement cooling system performance the potential can enhance the fuel oil efficiency and discharge the performance The cooling system can control the nature is improves the cooling system the key can the controlling expression to the engine structure protection essential parameter like the metal temperature the refrigerant temperature and the machine oil temperature and so on can control guarantees the engine to work in the safety margin scope The cooling system can make the rapid reaction to the different operating mode the most earth saves the fuel reduces discharges but does not affect the engine overall performance Looked from the design and the operational performance angle that divergence type cooling and precise cooling unifies has the very good prospects for development both can provide the ideal engine protection and can enhance the fuel oil efficiency and discharge the nature This kind of structure is advantageous to forming the engine ideal temperature distribution Directly to a cylinder cover or cap row of tyre valve around the supplies refrigerant reduced the cylinder head temperature change causes the cylinder cover temperature distribution to be evener also can maintains the machine oil and the cylinder body temperature at the design operating region has a lower friction to damage the pollution withdrawal cooling system function and maintenance maintenance method as follows 1st the cooling system function is part of quantity of heats which absorbs the engine part carries off guaranteed the diesel engine various components maintain in the normal temperature range 2nd the cooling water should be does not contain dissolves the Xie salt the soft water like clean river water rain water and so on Do not use hard water and so on the well water water seepage or sea water guards against produces causes the engine to radiate not good question occurrence and so on air cylinder heat 3rd with the funnel when joins the cooling water the water tank must prevent the water splashes to on the engine and the radiator prevented on the radiator fin and the organism accumulates the dust smears affects the cooling effect 4th if when the engine lacks the water causes the hyperpyrexia cannot immediately add water should cause the engine idling speed to revolve 10 15 minutes after the uniform temperature slightly reduces slowly does not join the cooling water in the engine situation 5th the winter the water tank planted agent adds the hot water After the start should surpass 40 degree hour the slow revolution to the water temperature to be able to work After the work had ended must put the completely cooling water 外文翻譯 4 6th must regularly eliminate in the water tank must frequently scour the sludge to the forced air cooling engine radiator fin dirty is filthy The radiator fin cannot damage after if damages must promptly replace in order to avoid influence radiation effect 4 Lathes Lathes are machine tools designed primarily to do turning facing and boring Very little turning is done on other types of machine tools and none can do it with equal facility Because lathes also can do drilling and reaming their versatility permits several operations to be done with a single setup of the work piece Consequently more lathes of various types are used in manufacturing than any other machine tool The essential components of a lathe are the bed headstock assembly tailstock assembly and the leads crew and feed rod The bed is the backbone of a lathe It usually is made of well normalized or aged gray or nodular cast iron and provides s heavy rigid frame on which all the other basic components are mounted Two sets of parallel longitudinal ways inner and outer are contained on the bed usually on the upper side Some makers use an inverted V shape for all four ways whereas others utilize one inverted V and one flat way in one or both sets They are precision machined to assure accuracy of alignment On most modern lathes the way are surface hardened to resist wear and abrasion but precaution should be taken in operating a lathe to assure that the ways are not damaged Any inaccuracy in them usually means that the accuracy of the entire lathe is destroyed The headstock is mounted in a foxed position on the inner ways usually at the left end of the bed It provides a powered means of rotating the word at various speeds Essentially it consists of a hollow spindle mounted in accurate bearings and a set of transmission gears similar to a truck transmission through which the spindle can be rotated at a number of speeds Most lathes provide from 8 to 18 speeds usually in a geometric ratio and on modern lathes all the speeds can be obtained merely by moving from two to four levers An increasing trend is to provide a continuously variable speed range through electrical or mechanical drives Because the accuracy of a lathe is greatly dependent on the spindle it is of heavy construction and mounted in heavy bearings usually preloaded tapered roller or ball types The spindle has a hole extending through its length through which long bar stock can be fed The size of maximum size of bar stock that can be machined when the material must be fed through spindle The tailsticd assembly consists essentially of three parts A lower casting fits on the inner ways of the bed and can slide longitudinally thereon with a means for clamping the entire assembly in any desired location An upper casting fits on the lower one and can be moved transversely upon it on some type of keyed ways to permit aligning the assembly is the tailstock quill This is a hollow steel cylinder usually about 51 to 76mm 2to 3 inches in diameter that can be moved several inches longitudinally in and out of the upper casting by means of a hand wheel and screw The size of a lathe is designated by two dimensions The first is known as the swing This is the maximum diameter of work that can be rotated on a lathe It is approximately twice the distance between the line connecting the lathe centers and the nearest point on the ways The second size dimension is the maximum distance between centers The swing thus indicates the maximum work piece diameter that can be turned in the lathe while the distance between centers indicates the maximum length of work piece that can be mounted between centers Engine lathes are the type most frequently used in manufacturing They are heavy duty machine tools with all the components described previously and have power drive for all tool movements except on the compound rest They commonly range in size from 305 to 610 mm 12 to 24 inches swing and from 610 to 1219 mm 24 to 48 inches center distances but swings up to 1270 mm 50 inches and center distances up to 3658mm 12 feet are not uncommon Most have chip pans and a built in coolant circulating system Smaller engine lathes with swings usually not over 330 mm 13 inches also are available in bench type designed for the bed to be mounted on a bench on a bench or cabinet Although engine lathes are versatile and very useful because of the time required for changing and setting tools and for making measurements on the work piece thy are not suitable for quantity production Often the actual chip production tine is less than 30 of the total cycle time In addition a skilled machinist is 外文翻譯 5 required for all the operations and such persons are costly and often in short supply However much of the operator s time is consumed by simple repetitious adjustments and in watching chips being made Consequently to reduce or eliminate the amount of skilled labor that is required turret lathes screw machines and other types of semiautomatic and automatic lathes have been highly developed and are widely used in manufacturing 5 Limits and Tolerances Machine parts are manufactured so they are interchangeable In other words each part of a machine or mechanism is made to a certain size and shape so will fit into any other machine or mechanism of the same type To make the part interchangeable each individual part must be made to a size that will fit the mating part in the correct way It is not only impossible but also impractical to make many parts to an exact size This is because machines are not perfect and the tools become worn A slight variation from the exact size is always allowed The amount of this variation depends on the kind of part being manufactured For examples part might be made 6 in long with a variation allowed of 0 003 three thousandths in above and below this size Therefore the part could be 5 997 to 6 003 in and still be the correct size These are known as the limits The difference between upper and lower limits is called the tolerance 外文翻譯 6 外文翻譯 7 1 概述 隨著發(fā)動(dòng)機(jī)采用更加緊湊的設(shè)計(jì)和具有更大的比功率 發(fā)動(dòng)機(jī)產(chǎn)生的廢熱密度也隨之明顯增大 一 些關(guān)鍵區(qū)域 如排氣門周圍散熱問題需優(yōu)先考慮 冷卻系統(tǒng)即便出現(xiàn)小的故障也可能在這樣的區(qū)域造 成災(zāi)難性的后果 發(fā)動(dòng)機(jī)冷卻系統(tǒng)的散熱能力一般應(yīng)滿足發(fā)動(dòng)機(jī)滿負(fù)荷時(shí)的散熱需求 因?yàn)榇藭r(shí)發(fā)動(dòng) 機(jī)產(chǎn)生的熱量最大 然而 在部分負(fù)荷時(shí) 冷卻系統(tǒng)會(huì)發(fā)生功率損失 水泵所提供的冷卻液流量超過 所需的流量 我們希望發(fā)動(dòng)機(jī)冷啟動(dòng)時(shí)間盡可能短 因?yàn)榘l(fā)動(dòng)機(jī)怠速時(shí)排放的污染物較多 油耗也大 冷卻系統(tǒng)的結(jié)構(gòu)對(duì)發(fā)動(dòng)機(jī)的冷啟動(dòng)時(shí)間有較大的影響 2 現(xiàn)代發(fā)動(dòng)機(jī)冷卻系統(tǒng)的特點(diǎn) 傳統(tǒng)冷卻系統(tǒng)的作用是可靠地保護(hù)發(fā)動(dòng)機(jī) 而還應(yīng)具有改善燃料經(jīng)濟(jì)性和降低排放的作用 為此 現(xiàn)代冷卻系統(tǒng)要綜合考慮下面的因素 發(fā)動(dòng)機(jī)內(nèi)部的摩擦損失 冷卻系統(tǒng)水泵的功率 燃燒邊界條件 如燃燒室溫度 充量密度 充量溫度 先進(jìn)的冷卻系統(tǒng)采用系統(tǒng)化 模塊化設(shè)計(jì)方法 統(tǒng)籌考慮每項(xiàng)影響因素 使冷卻系統(tǒng)既保證發(fā)動(dòng) 機(jī)正常工作 又提高發(fā)動(dòng)機(jī)效率和減少排放 2 1 溫度設(shè)定點(diǎn) 發(fā)動(dòng)機(jī)工作溫度的極限值取決于排氣門周圍區(qū)域最高溫度 最理想的情況是按金屬溫度而不是冷 卻液溫度控制冷卻系統(tǒng) 這樣才能更好地保護(hù)發(fā)動(dòng)機(jī) 由于冷卻系統(tǒng)設(shè)定的冷卻溫度是以滿負(fù)荷時(shí)最 大散熱率為基礎(chǔ) 因此 發(fā)動(dòng)機(jī)和冷卻系統(tǒng)在部分負(fù)荷時(shí)處于不太理想狀態(tài) 如市區(qū)行駛和低速行駛 時(shí) 會(huì)產(chǎn)生高油耗和排