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Automotive engine camshaft
Brief introduction
The camshaft is a part of the piston engine. Its role is to control the opening and closing operation of the valve. Although the camshaft rotational speed in a four-stroke engine is a half of the crankshaft (the same as the camshaft rotational speed in a two-stroke engine with the crankshaft), but usually it is still very high speed, but also need to withstand the large torque, so the design right demanding camshaft in terms of strength and support material is generally a special cast iron, occasionally using forgings. Valve motion law related to engine power and operation characteristics, the design in the design process of the engine camshaft occupies a very important position.
Structure
The main body of the camshaft is the same as the one with the cylinder length of the cylindrical rod. The above sets have several cam for driving the valve. One end of the camshaft camshaft bearing support and the other end is connected to the drive wheels.
Cam side was egg-shaped. The design aims to ensure the the cylinder sufficient air intake and exhaust, specifically, within the shortest possible time to complete the valve opening and closing movements. In addition, taking into account the durability of the engine and the smoothness of operation, the valve can not be generated due to the deceleration process of opening and closing movements too much too large the impact of serious wear and tear of the valve, otherwise it will cause an increase in noise or other serious consequences. Therefore, the cam and the power of the engine torque output as well as the operation of the ride there is a direct relationship.
Generally inline engine, a cam corresponding to a valve V-type engine or horizontal opposed type engine, every two valves share a cam. The rotary engine the valveless with gas engine because of its special structure, does not need to cam
Position
In the long period of time, the bottom-mounted camshaft in an internal combustion engine is most common. Typically such engines, the valve is located in the top of the engine camshaft machine, i.e., so-called the OHV (Over Head Valve, OHV) engines. Usually camshaft located on the side of the crankcase, through the gas distribution agencies (such as tappet, push rod, rocker, etc.) valve control. Bottom-mounted camshaft general also called side-mounted camshaft. Far distance valve, and each cylinder is usually only two valves in such an engine camshaft, so the speed is usually slower, ride comfort is poor, the output power is also relatively low. However, the engine output torque and low-speed performance of this structure is relatively good, relatively simple structure and easy maintenance.
Now most of the production car's engine is equipped with overhead camshaft. The overhead camshafts structure closer to the camshaft valve, to reduce the kinetic energy of the waste caused by the bottom-mounted camshaft due to the larger distance between the camshaft and the valve shuttle. Overhead camshaft of the engine valve opening and closing action is relatively rapid, and hence higher speed, and the smooth running is also better. The engine of the the overhead camshafts structure appeared earlier the SOHC (Single Over Head Cam, overhead single camshaft) engine. This engine is only installed at the top of a camshaft, and therefore generally only two to three valves of each cylinder (the intake air a to two exhaust), the high-speed performance has been limited. Technology updates DOHC (Double Over Head Cam, double overhead camshaft) engine, this engine with a two camshafts per cylinder can be installed four to five valves (intake two to three, Pai gas two), high-speed performance significantly improved, but at the same time the low-speed performance will be affected to some degree, the structure will be complicated and difficult to repair.
Classification
According to the the camshaft number of how many, can be divided into single overhead camshaft (SOHC) and double overhead camshaft (DOHC), two kinds. The single overhead camshaft camshaft is only one camshaft, double overhead camshaft is two, this is too straightforward explanation.
The single overhead camshaft with a camshaft in the cylinder head, direct drive into the exhaust valve, it has a simple structure, suitable for high-speed engine. Generally used in the past side camshaft, the camshaft in the cylinder side, is driven directly by a timing gear. The valve lifter to the rotation of the camshaft is converted into reciprocating motion of the valve must be used to transfer power. Thus, more parts of the reciprocating motion, the inertial mass, is not conducive to high-speed movement of the engine. Moreover, the slender tappet has a certain degree of flexibility, prone to vibration, accelerated component wear, even the valve control is lost.
DOHC cylinder head equipped with two camshafts, one is used to drive the intake valve, the other for driving the exhaust valve. Double overhead camshaft camshaft and valve spring design less demanding, especially for the hemispherical combustion chamber of the valve V-shaped configuration, but also facilitate and used in conjunction with four-valve gas distribution agencies.
Fault
Camshaft common faults including abnormal wear and tear, abnormal wear of the symptoms often first appear before the occurrence of abnormal sound as well as fracture, abnormal sound and fracture.
(1) Camshaft almost at the end of the engine lubrication system, lubrication situation is not optimistic. If the oil pump is too long and so insufficient oil pressure or the lubricants Road blockage caused by lubricating oil can not reach the camshaft bearing cap fastening bolts tightening torque caused by excessive oil can not enter the the camshaft gap will causing abnormal wear of the camshaft.
(2) the abnormal wear of the camshaft causes the gap increases between the camshaft bearing, the camshaft movement occurs when the axial displacement, resulting in abnormal noise. Abnormal wear will lead to increased gap between the drive cam with hydraulic tappets, cam combined with hydraulic tappets will collide, resulting in abnormal noise.
(3) camshaft sometimes fracture and other serious fault, common causes of hydraulic tappet cracked or severely worn, serious poor lubrication the camshaft poor quality and camshaft timing gear rupture.
(4) In some cases, the failure of the camshaft is man-made causes, in particular the maintenance of the engine camshaft not correct disassembly. Such as demolition of the camshaft bearing caps with a hammer strength knocking or prying with a screwdriver, or install the bearing cap installed the wrong position does not match the result in the bearing cap and bearing, or bearing cover the fastening bolt tightening torque is too large. Install bearing cap should pay attention to the direction of the arrow and the position number marked on the surface of the bearing cap, and in strict accordance with the provisions of torque using the torque wrench tighten the bearing cap fastening bolts.
Refit
In order to enhance the power of the engine, some converted stores a modified camshaft engine face lift high angle camshaft (Hi-camshaft CAM) is a common form of modified method. This modification operation is not complicated, but because of the lack of understanding of some modification cam on the camshaft angle and works so that the modified effect is not obvious even lead to the deterioration of the performance of the engine.
High angle camshaft relative to ordinary camshaft cam angle of about 240°, high angle camshaft cam angle can often reach over 280°. The large angle of the camshaft can extend the valve open time, increase the valve lift, the intake valve and the exhaust valve open as early and late off, so that more air into the cylinder, in order to improve the engine, the power of the high speed output. Should choose for civilian vehicles, modified cam camshaft angle 278, will be a significant increase in working an angle greater than 278°camshaft valve overlap angle, so that the power of the engine high speed improve a lot, but engine cylinder seal is not good at low speed and cause the idling serious jitter or even turn off, so that the vehicle can not adapt to everyday use, and can only be used for competition purposes.
Production technology
The camshaft is one of the key parts of the engine, the hardness of the camshaft peach apical and white layer depth is to determine the key technical indicators camshaft life and engine efficiency. , Should be considered to ensure that the cam has a sufficiently high hardness and a fairly deep white layer premise journal does not appear high carbide, so that it has a better cutting performance.
Currently, the main method of domestic and foreign production camshaft: steel forging blank by cutting the cam peach tip martensitic layer formed some of the high-frequency quenching process. The end of the 1970s, Germany and France have developed a new camshaft argon arc remelting process; hardened cast iron camshaft otherwise dominated by the United States; chilled cast iron camshaft mainly to Japan and France; well cam parts of the Cr-Mn-Mo alloy coatings casting surface alloying production.
汽車發(fā)動(dòng)機(jī)凸輪軸
簡(jiǎn)介
凸輪軸是活塞發(fā)動(dòng)機(jī)里的一個(gè)部件。它的作用是控制氣門的開(kāi)啟和閉合動(dòng)作。雖然在四沖程發(fā)動(dòng)機(jī)里凸輪軸的轉(zhuǎn)速是曲軸的一半（在二沖程發(fā)動(dòng)機(jī)中凸輪軸的轉(zhuǎn)速與曲軸相同），不過(guò)通常它的轉(zhuǎn)速依然很高，而且需要承受很大的扭矩，因此設(shè)計(jì)中對(duì)凸輪軸在強(qiáng)度和支撐方面的要求很高，其材質(zhì)一般是特種鑄鐵，偶爾也有采用鍛件的。由于氣門運(yùn)動(dòng)規(guī)律關(guān)系到一臺(tái)發(fā)動(dòng)機(jī)的動(dòng)力和運(yùn)轉(zhuǎn)特性，因此凸輪軸設(shè)計(jì)在發(fā)動(dòng)機(jī)的設(shè)計(jì)過(guò)程中占據(jù)著十分重要的地位。
構(gòu)造
凸輪軸的主體是一根與汽缸組長(zhǎng)度相同的圓柱形棒體。上面套有若干個(gè)凸輪，用于驅(qū)動(dòng)氣門。凸輪軸的一端是軸承支撐點(diǎn)，另一端與驅(qū)動(dòng)輪相連接?！ ?
凸輪的側(cè)面呈雞蛋形。其設(shè)計(jì)的目的在于保證汽缸充分的進(jìn)氣和排氣，具體來(lái)說(shuō)就是在盡可能短的時(shí)間內(nèi)完成氣門的開(kāi)、閉動(dòng)作。另外考慮到發(fā)動(dòng)機(jī)的耐久性和運(yùn)轉(zhuǎn)的平順性，氣門也不能因開(kāi)閉動(dòng)作中的加減速過(guò)程產(chǎn)生過(guò)多過(guò)大的沖擊，否則就會(huì)造成氣門的嚴(yán)重磨損、噪聲增加或是其它嚴(yán)重后果。因此，凸輪和發(fā)動(dòng)機(jī)的功率、扭矩輸出以及運(yùn)轉(zhuǎn)的平順性有很直接的關(guān)系?！ ?
一般來(lái)說(shuō)直列式發(fā)動(dòng)機(jī)中，一個(gè)凸輪都對(duì)應(yīng)一個(gè)氣門，V型發(fā)動(dòng)機(jī)或水平對(duì)置式發(fā)動(dòng)機(jī)則是每?jī)蓚€(gè)氣門共享一個(gè)凸輪。而轉(zhuǎn)子發(fā)動(dòng)機(jī)和無(wú)閥配氣發(fā)動(dòng)機(jī)由于其特殊的結(jié)構(gòu)，并不需要凸輪。
位置
在以前很長(zhǎng)的一段時(shí)間里，底置式凸輪軸在內(nèi)燃機(jī)中最為常見(jiàn)。通常這樣的發(fā)動(dòng)機(jī)中，氣門位于發(fā)動(dòng) 機(jī)的頂部，即所謂的OHV（Over Head Valve，頂置氣門）式發(fā)動(dòng)機(jī)。此時(shí)通常凸輪軸位于曲軸箱的側(cè)面，通過(guò)配氣機(jī)構(gòu)（如挺桿、推桿、搖臂等）對(duì)氣門進(jìn)行控制。因此底置式凸輪軸一般也叫側(cè)置式凸輪軸。由于在這樣的發(fā)動(dòng)機(jī)中凸輪軸距離氣門較遠(yuǎn)，而且每個(gè)氣缸通常只有兩個(gè)氣門，因此轉(zhuǎn)速通常較慢，平順性不佳，輸出功率也比較低。不過(guò)這種結(jié)構(gòu)的引擎輸出扭矩和低速性能比較出色，結(jié)構(gòu)也比較簡(jiǎn)單，易于維修。 現(xiàn)在大多數(shù)量產(chǎn)車的發(fā)動(dòng)機(jī)配備的是頂置式凸輪軸。頂置式凸輪軸結(jié)構(gòu)使凸輪軸更加接近氣門，減少了底置式凸輪軸由于凸輪軸和氣門之間較大的距離而造成的往返動(dòng)能的浪費(fèi)。頂置式凸輪軸的發(fā)動(dòng)機(jī)由于氣門開(kāi)閉動(dòng)作比較迅速，因而轉(zhuǎn)速更高，運(yùn)行的平穩(wěn)度也比較好。較早出現(xiàn)的頂置式凸輪軸結(jié)構(gòu)的發(fā)動(dòng)機(jī)是SOHC（Single Over Head Cam，頂置單凸輪軸）式發(fā)動(dòng)機(jī)。這種發(fā)動(dòng)機(jī)在頂部只安裝了一根凸輪軸，因此一般每個(gè)汽缸只有兩到三個(gè)氣門（進(jìn)氣一到兩個(gè)，排氣一個(gè)），高速性能受到了限制。而技術(shù)更新一些的則是DOHC式（Double Over Head Cam，頂置雙凸輪軸）發(fā)動(dòng)機(jī)，這種發(fā)動(dòng)機(jī)由于配備了兩根凸輪軸，每個(gè)汽缸可以安裝四到五個(gè)氣門（進(jìn)氣二到三個(gè)，排氣二個(gè)），高速性能得到了顯著的提升，不過(guò)與此同時(shí)低速性能會(huì)受到一定的影響，結(jié)構(gòu)也會(huì)變得復(fù)雜，不易維修。
分類
按凸輪軸數(shù)目的多少，可分為單頂置凸輪軸(SOHC)和雙頂置凸輪軸(DOHC)兩種。單頂置凸輪軸就是 只有一根凸輪軸，雙頂置凸輪軸就是有兩根，這是太直白的解釋?！ ?
單頂置凸輪軸在氣缸蓋上用一根凸輪軸，直接驅(qū)動(dòng)進(jìn)、排氣門，它具有結(jié)構(gòu)簡(jiǎn)單，適用于高速發(fā)動(dòng)機(jī)。以往一般采用的側(cè)置凸輪軸，即凸輪軸在氣缸側(cè)面，由正時(shí)齒輪直接驅(qū)動(dòng)。為了把凸輪軸的轉(zhuǎn)動(dòng)變換為氣門的往復(fù)運(yùn)動(dòng)，必須使用氣門挺桿來(lái)傳遞動(dòng)力。這樣，往復(fù)運(yùn)動(dòng)的零件較多，慣性質(zhì)量大，不利于發(fā)動(dòng)機(jī)高速運(yùn)動(dòng)。而且，細(xì)長(zhǎng)的挺桿具有一定的彈性，容易引起振動(dòng)，加速零件磨損，甚至使氣門失去控制。
頂置雙凸輪軸是在缸蓋上裝有兩根凸輪軸，一根用于驅(qū)動(dòng)進(jìn)氣門，另一根用于驅(qū)動(dòng)排氣門。采用雙頂置凸輪軸對(duì)凸輪軸和氣門彈簧的設(shè)計(jì)要求不高，特別適用于氣門V形配置的半球形燃燒室，也便于和四氣門配氣機(jī)構(gòu)配合使用。
傳動(dòng)
底置式凸輪軸通常采用星形齒輪組（即所謂的“控制輪”），輥?zhàn)渔溁螨X條與曲軸相連。為了控制噪聲，直徑較大的凸輪軸端傳動(dòng)輪通常由塑料或者輕金屬制造，而相對(duì)直徑較小的曲軸端傳動(dòng)輪則大多采用鋼材。
鏈條連接也比較多見(jiàn)。這種方式在底置式和頂置式凸輪軸上都可以看到。為了減小噪聲（一般是鏈條在運(yùn)動(dòng)中產(chǎn)生的“振擺噪聲”），通常還會(huì)附帶一個(gè)液壓壓緊裝置和塑料材質(zhì)的導(dǎo)軌。
頂置式凸輪軸結(jié)構(gòu)中比較多見(jiàn)的是用一個(gè)塑料齒條鏈連接。這個(gè)齒條鏈位于發(fā)動(dòng)機(jī)機(jī)油腔外，附帶有鋼質(zhì)的嵌入部件，通過(guò)一個(gè)可調(diào)的輥?zhàn)訋椭鷱埦o。
還有一種結(jié)構(gòu)由于動(dòng)力在傳輸過(guò)程中損耗過(guò)大且過(guò)于復(fù)雜，現(xiàn)在已經(jīng)比較少見(jiàn)。這種結(jié)構(gòu)通過(guò)一個(gè)偏心連桿、星形齒輪組或帶中間軸的錐形齒輪組來(lái)連接頂置式凸輪軸與曲軸。 凸輪軸與曲軸之間的常見(jiàn)傳動(dòng)方式包括齒輪傳動(dòng)、鏈條傳動(dòng)以及齒形膠帶傳動(dòng)。下置凸輪軸和中置凸輪軸與曲軸之間的傳動(dòng)大多采用圓柱形正時(shí)齒輪傳動(dòng)，一般從曲軸到凸輪軸只需要1對(duì)齒輪傳動(dòng)，如果傳動(dòng)齒輪直徑過(guò)大，可以再增加1個(gè)中間惰輪。為了嚙合平穩(wěn)并降低工作噪聲，正時(shí)齒輪大多采用斜齒輪。
鏈條傳動(dòng)常見(jiàn)于頂置凸輪軸與曲軸之間，但其工作可靠性和耐久性不如齒輪傳動(dòng)。近年來(lái)在高轉(zhuǎn)速發(fā)動(dòng)機(jī)上廣泛使用齒形膠帶代替?zhèn)鲃?dòng)鏈條，但在一些大功率發(fā)動(dòng)機(jī)上仍然使用鏈條傳動(dòng)。齒形膠帶具有工作噪聲小、工作可靠以及成本低等特點(diǎn)。對(duì)于雙頂置凸輪軸，一般是排氣凸輪軸通過(guò)正時(shí)齒形膠帶或鏈條由曲軸驅(qū)動(dòng)，進(jìn)氣凸輪軸通過(guò)金屬鏈條由排氣凸輪軸驅(qū)動(dòng)，或進(jìn)氣凸輪軸和排氣凸輪軸均由曲軸通過(guò)齒形膠帶或鏈條驅(qū)動(dòng)。
安裝凸輪軸時(shí)，一定要注意凸輪軸帶輪或鏈輪上的正時(shí)標(biāo)記。有些發(fā)動(dòng)機(jī)沒(méi)有明顯的正時(shí)標(biāo)記，維修人員可以在拆卸凸輪軸之前標(biāo)記出曲軸和凸輪軸的準(zhǔn)確位置，有些發(fā)動(dòng)機(jī)則是需要專用工具才能進(jìn)行正時(shí)的調(diào)校。
故障
凸輪軸的常見(jiàn)故障包括異常磨損、異響以及斷裂，異響和斷裂發(fā)生之前往往先出現(xiàn)異常磨損的癥狀。
凸輪軸幾乎位于發(fā)動(dòng)機(jī)潤(rùn)滑系統(tǒng)的末端，因此潤(rùn)滑狀況不容樂(lè)觀。如果機(jī)油泵因?yàn)槭褂脮r(shí)間過(guò)長(zhǎng)等原因出現(xiàn)供油壓力不足，或潤(rùn)滑油道堵塞造成潤(rùn)滑油無(wú)法到達(dá)凸輪軸，或軸承蓋緊固螺栓擰緊力矩過(guò)大造成潤(rùn)滑油無(wú)法進(jìn)入凸輪軸間隙，均會(huì)造成凸輪軸的異常磨損?！ ? 凸輪軸的異常磨損會(huì)導(dǎo)致凸輪軸與軸承座之間的間隙增大，凸輪軸運(yùn)動(dòng)時(shí)會(huì)發(fā)生軸向位移，從而產(chǎn)生異響。異常磨損還會(huì)導(dǎo)致驅(qū)動(dòng)凸輪與液壓挺桿之間的間隙增大，凸輪與液壓挺桿結(jié)合時(shí)會(huì)發(fā)生撞擊，從而產(chǎn)生異響?！ ?
凸輪軸有時(shí)會(huì)出現(xiàn)斷裂等嚴(yán)重故障，常見(jiàn)原因有液壓挺桿碎裂或嚴(yán)重磨損、嚴(yán)重的潤(rùn)滑不良、凸輪軸質(zhì)量差以及凸輪軸正時(shí)齒輪破裂等?！ ?
有些情況下，凸輪軸的故障是人為原因引起的，特別是維修發(fā)動(dòng)機(jī)時(shí)對(duì)凸輪軸沒(méi)有進(jìn)行正確的拆裝。例如拆卸凸輪軸軸承蓋時(shí)用錘子強(qiáng)力敲擊或用改錐撬壓，或安裝軸承蓋時(shí)將位置裝錯(cuò)導(dǎo)致軸承蓋與軸承座不匹配，或軸承蓋緊固螺栓擰緊力矩過(guò)大等。安裝軸承蓋時(shí)應(yīng)注意軸承蓋表面上的方向箭頭和位置號(hào)等標(biāo)記，并嚴(yán)格按照規(guī)定力矩使用扭力扳手?jǐn)Q緊軸承蓋緊固螺栓。
改裝
為了提升發(fā)動(dòng)機(jī)的動(dòng)力，有些改裝店對(duì)發(fā)動(dòng)機(jī)的凸輪軸進(jìn)行了改裝，其中換裝高角度凸輪軸（Hi－CAM）是常見(jiàn)的一種改裝方法。這種改裝操作并不復(fù)雜，但由于一些改裝人員對(duì)凸輪軸上凸輪的工作角度和工作原理了解不足，使得改裝后的效果并不明顯甚至導(dǎo)致發(fā)動(dòng)機(jī)的性能惡化。
高角度凸輪軸是相對(duì)于普通凸輪軸的240°左右的凸輪工作角度而言的，高角度凸輪軸的凸輪工作角度通常可以達(dá)到280°以上。大角度的凸輪軸可以延長(zhǎng)氣門的開(kāi)啟時(shí)間，增大氣門的升程，使進(jìn)氣門和排氣門實(shí)現(xiàn)早開(kāi)和晚關(guān)，使更多空氣進(jìn)入氣缸，以提高發(fā)動(dòng)機(jī)中、高轉(zhuǎn)速的動(dòng)力輸出。對(duì)于民用車來(lái)說(shuō)，改裝時(shí)應(yīng)該選擇凸輪工作角度在278°以下的凸輪軸，因?yàn)楣ぷ鹘嵌却笥?78°的凸輪軸會(huì)大幅度增加氣門重疊角，使發(fā)動(dòng)機(jī)高轉(zhuǎn)速時(shí)的動(dòng)力提升很多，但發(fā)動(dòng)機(jī)在低轉(zhuǎn)速時(shí)會(huì)因?yàn)闅飧酌芊庑圆缓枚鴮?dǎo)致怠速嚴(yán)重抖動(dòng)甚至熄火，這樣的車輛無(wú)法適應(yīng)日常使用，而只能用于競(jìng)賽用途。
生產(chǎn)技術(shù)
凸輪軸是發(fā)動(dòng)機(jī)的關(guān)鍵零件之一，凸輪軸桃尖部位的硬度和白口層深度是決定凸輪軸使用壽命和發(fā)動(dòng)機(jī)效率的關(guān)鍵技術(shù)指標(biāo)。在保證凸輪有足夠高的硬度和相當(dāng)深的白口層的前提下，還應(yīng)考慮軸頸不出現(xiàn)較高的碳化物，使其具有較好的切削加工性能?！ ∧壳?，國(guó)內(nèi)外生產(chǎn)凸輪軸的主要方法有：采用鋼質(zhì)鍛造毛坯經(jīng)切削加工后，凸輪桃尖部分經(jīng)高頻淬火形成馬氏體層的工藝。20世紀(jì) 70年代末，德國(guó)和法國(guó)相繼開(kāi)發(fā)了凸輪軸氬弧重熔新工藝；另有以美國(guó)為主的可淬硬鑄鐵凸輪軸；以日本和法國(guó)為主的冷硬鑄鐵凸輪軸；以及凸輪部位用 Cr-Mn-Mo 合金涂料進(jìn)行鑄件表面合金化的生產(chǎn)工藝等。