裝配圖復合形法減速器優(yōu)化設計,裝配,復合,減速器,優(yōu)化,設計 ELEMENTS OF CAM DESIGN How to plan and produce simple but efficient cams for petrol engines and other mechanisms Cams are among the most versatile mechanisms available．A cam is a simple two-member device．The input member is the cam itself，while the output member is called the follower．Through the use of cams，a simple input motion can be modified into almost any conceivable output motion that is desired．Some of the common applications of cams are ——Camshaft and distributor shaft of automotive engine ——Production machine tools ——Automatic record players ——Printing machines ——Automatic washing machines ——Automatic dishwashers The contour of high-speed cams (cam speed in excess of 1000 rpm) must be determined mathematically．However，the vast majority of cams operate at low speeds(less than 500 rpm) or medium-speed cams can be determined graphically using a large-scale layout．In general，the greater the cam speed and output load，the greater must be the precision with which the cam contour is machined． Cams in some form or other are essential to the operation of many kinds of mechanical devices. Their best-known application is in the valve-operating gear of internal combustion engines, but they play an equally important part in industrial machinery, from printing presses to reaping machines. In general, a cam can be defined as a projection on the face of a disc or the surface of a cylinder for the purpose of producing intermittent reciprocating motion of a contacting member or follower. Most cams operate by rotary motion, but this is not an essential condition and in special cases the motion may be semi-rotary， oscillatory or swinging. Even straight-line motion of the operating member is possible, though the term cam may not be considered properly applicable in such circumstances. Most text books on mechanics give some information on the design of cams and show examples of cam forms plotted to produce various orders of motion. Where neither the operating speed nor the mechanical duty is very high, there is a good deal of latitude in the nermissible design of the cam and it is only necessary to avoid excessively steep contours or abrupt changes which would result in noise, impact shock, and side pressure on the follower. But, with increase of either speed or load, much more exacting demands are made on the cam, calling for the most careful design and, at very high speed, the effect of inertia on the moving parts is most pronounced, so that the further factors of acceleration and rate of lift have to be taken into account and these are rarely dealt with in any detail in the standard text books. The design of the cam follower is also of great importance and bears a definite relation to the shape of the cam itself. This is because the cam cannot make contact with the follower at a single fixed point. Surface contact is necessary to distribute load and avoid excess wear, thus the cam transmits its motion through various points of location on the follower, depending on the shape of the two complementary members. The cams for operating i.c. engine valves present specially difficult problems in design. In the case of racing engines, both the load and speed may be regarded as extreme, because in many engines the rate at which the valves can be effectively controlled is the limiting factor in engine performance. In some respects, cam design of miniature engines is simplified by reason of their lighter working parts (and consequent less inertia) but on the other hand, working friction is usually greater and rotational speeds are generally considerably higher than in full-size practice. In the many designs for small four-stroke engines which I have published, I have sought to simplify valve operation and to provide designs for cams which can be simply and accurately produced with the facilities of the amateur workshop. Numerous engine designs which have been submitted to me by readers have contained errors in the valve gear and particularly in the cams and in view of prevalent misconceptions in the fundamental principles of these items, I am giving some advice on the matter which I trust will help individual designers to obtain the best results from their engines. There have been many engines built with cams of thoroughly bad design but which, in spite of this, have produced results more or less satisfactory to their constructors. It may be said that within certain limits of speed one can get away with murder but in no case can an engine perform efficiently with badly designed cams, or indeed errors in any of its working details. This article is concerned mainly with the design of cams for operating the valves of i.c. engines and, in order to avoid any confusion of terms, Fig. 1 shows the various parts of a cam of this type and explains their functions. The circular, concentric portion of the cam, which has no operative effect, is known as the base circle: the humy of the cam (shown shaded) is known as the lobe, and the flanks on either side rise from the base circle to the nose, which is usually rounded.Lift may be defined as the difference between the radius of the base circle and that of the nose. the anele enclosed between the points where the flanks join the base circle is termed the angular ‘period, representing the proportion of the full cycle during which the cam operates the valve gear. In Fig. 2, typical examples of cams used in i.c. engines are illustrated. The tangent cam, A, has dead straight flanks-which as the name implies form tangents to the base circle. This type of cam is easy to design and produce, the simplest method of machining being by a circular milling process forming a concentric surface on the base circle and running straight out tangentially where the flanks start and finish. It can also be produced by filing and I have in the past described how to make it with the aid of a roller filing rest in the lathe, in conjunction with indexing gear to locate the flank angles. Tangent cams can only work efficiently in conjunction with a convex curved follower, as this is the only way in which the flank can be brought progressively and smoothly into action. Some time ago an engine was described having tangent cams in conjunction with flat followers. This was not intended for extremely high speed and very likely produced all the power required of it, but it is quite clear that the flat face of the tangent cam. On engaging the flat tappet-over the full length of the flank all at once, must produce an abrupt slapping action which is noisy, inefficient and destructive in the long run. Rollers are often used as followers with tangent cams and are satisfactory in respect of their shape, but the idea of introducing rolling motion at this point is not as good as it seems at first sight, because it merely transfers the sliding friction to a much smaller area--that of the pivot pin. It is possible in some cases, however, to use a ball or roller race for the follower and this, at any rate, has the merit of distributing and equalizing the wearing surface. Tangent cams have been used with a certain degree of success for high-performance-engines and were at one time popular on racing motorcycle engines, though usually with some slight modification of shape-often “ designed ” by the tuner with the aid of .a Carborundum slip! Their more common application, however, has been on gas and oil engines running at relatively slow speeds, where they work well in contact with rollers attached to the ends of the valve rockers. Cams with convex flanks are extensively used in motor cars and other mass-produced engines. One important advantage in this respect is that they are suited to manufacture in quantity by a copying process from accurately formed master cams. The fact that hat-based tappets can be used also favours quantity production and they can be designed to work fairly silently. The contour of the flank can be plotted so that violent changes in the acceleration of the cam are avoided and, more important still, the tappet will follow the cam on the return motion without any tendency to bounce or float at quite high speeds. In such cases, it may be necessary to introduce compound curves which are extremely difficult to copy on a small scale, but cams made with flanks formmg true circular arcs will give reasonably efficient results, and are very easily produced in any scale: Concave-flanked cams. Comparatively few examples of concave-flanked cams (Fig. 2c) are to be seen nowadays, though they have been used extensively in the past with the idea of obtaining the most rapid opening and closing of the valves. Theoretically, they can be designed to produce consant-acceleration, but in practice they render valve control very difficult at high speed and their fierce angle of attack produces heavy side pressure on the tappet. The concave flank must always have a substantially greater radius than the follower, or a slapping action like that of a tangent cam on a flat follower is produced. The shape of the nose in most types of cams is dictated mainly by the need to decelerate the follower as smoothly as possible. It is one thing to design it in such a way that ideal conditions are obtained, and quite another to ensure in practice that the follower retains close contact with the cam. If the radius of the nose is too small, the follower will bounce and come down heavily on the return flank of the cam and,. if too great, valve opening efficiency will be reduced. Of the three types of cams, A, B and C, which all have identically equal lift and angular period, the lobe of B encloses the smallest area, and on first sight it might appear that it is the least efficient in producing adequate valve opening, or mean lift area, but owing to the use of a flat based tappet, its lift characteristics are not very different from those of a tangent cam with round-based tappet, and not necessarily inferior to those of a concave-flank cam. Unsymmetrical cams It is not common to make the two flanks of a cam of different contours to produce some particular result which the designer may consider desirable. In some cases, the object is to produce rapid opening and gradual closing, but sometimes the opposite effect is preferred. When all things are considered, however, most attempts to monkey about with cam forms lead to complications which may actually defeat their own object, at least at really high speeds. In many engines, particularly those of motorcycles, the cams operate the valves through levers or rockers which move in an arc instead of in a straight line, as in the orthodox motor car tappet. This may be mechanically efficient, but it modifies the lift characteristic of the cam, as the point at which the latter transmits motion to the follower varies in relation to the radius of the lever arm, (Fig. 3). With the cam rotating in a clockwise direction, the effective length of the lever will be greater in the position. A during valve opening than in position B during closing, as indicated by dimensions X and Y. This amounts to the same as using an unsymmetrical cam, and in the example shown, would result in slow opening and rapid closing of the valve, or vice versa if either the direction of rotation of the cam, or the relative “ hand ” of the lever, is reversed. The shorter the lever, the greater the discrepancy in the rate of movement, Neither the unsymmetrical cam form nor the pivoted lever is condemned as bad design, but I have sought to avoid them in most of the engines I have designed because they are a complicating factor in what is already a very involved problem, and by keeping to fairly simple cams and straight-line tappets, one can be assured that there are not too many snags. The employment of cams with flanks of true circular arc has enabled me to devise means of producing them on the lathe without elaborate attachments and, what is more important still, to produce an entire set of cams for a multi-cylinder engine in correct angular relation to each other by equally simple means. There is no doubt whatever that these methods have enabled many engine constructors (some without previous experience) to tackle successfully a problem which would otherwise have been formidable, to say the least. Many designers have attempted to improve valve efficiency by designing cams which hold the valve at maximum opening for as long a period as possible. This is done by providing dwell or, in other words, making the top of the lobe concentric with the cam axis over a certain angular distance in the centre of its lift. To do this, however, it is necessary to make the flanks excessively steep, thus producing heavy side thrust on the tappet, and making control at high speed more difficult, (Fig. 4A). A little consideration, however, will show that the same result can be achieved, with much less mechanical difficulty, by lifting the valve somewhat higher at an easier rate, as shown at B. This avoids the need for sudden acceleration and deceleration of the tappet and promotes flow efficiency of the valve. The shaded portions of the two cams show the differences in the area of the lobe, showing that nothing is really gained by the dwell. Factors in efficiency High valve lift is a desirable feature, but only if it can be obtained without making extra dificulties in controlling the valve. The maximum port area of a valve is obtained when the lift is equal to one-fourth of the seat diameter, but owing to the baffling effect on the valve head, a higher lift is better for flow efficiency-if it is practicable. Large diameter valves will obviously release and admit gas efficiently but they are more difficult to control and keep cool at high speed than smaller valves. Another point is that the exhaust valve is required to open against a high cylinder pressure, and the larger it is the more the load imposed on the cam, quite apart from the spring load. 凸輪設計的基本內(nèi)容 如何為汽油發(fā)動機和其他機械設計和生產(chǎn)簡單而有效的凸輪 凸輪是被應用的最廣泛的機械結(jié)構(gòu)之一。凸輪是一種僅僅有兩個組件構(gòu)成的設備。主動件本身就是凸輪，而輸出件被稱為從動件。通過使用凸輪，一個簡單的輸入動作可以被修改成幾乎可以想像得到的任何輸出運動。常見的一些關于凸輪應用的例子有： ——凸輪軸和汽車發(fā)動機工程的裝配 ——專用機床 ——自動電唱機 ——印刷機 ——自動的洗衣機 ——自動的洗碗機 高速凸輪(凸輪超過1000 rpm的速度)的輪廓必須從數(shù)學意義上來定義。無論如何，大多數(shù)凸輪以低速(少于500 rpm)運行而中速的凸輪可以通過一個大比例的圖形表示出來。一般說來，凸輪的速度和輸出負載越大，凸輪的輪廓在被床上被加工時就一定要更加精密。 在多種機械裝置的操作中凸輪在某種形式下是必不可少。他們最有名的應用是在內(nèi)燃機閥門操作裝置中，但在工業(yè)機器中，從印刷機到收割機械，凸輪機構(gòu)也是一個相當重要的一部分。 一般來說，一個凸輪可以被定義為一個圓盤面或一個為產(chǎn)生接觸間歇往復運動的零件或從動件。大多數(shù)凸輪的運動是旋轉(zhuǎn)運動，但這不是一個必要條件，在特殊情況下，它的運動是半旋轉(zhuǎn)，振動或擺動。即使原動件可能是直線運動，但在某種情況下凸輪也可能會適當?shù)乇豢紤]，。 在機構(gòu)學大多數(shù)文本書籍中給了關于凸輪設計和凸輪類型的實例設計的一些信息，產(chǎn)生各種規(guī)定的運動。在某種情況運行速度和機械的性能不是非常高，有一個規(guī)律是凸輪機構(gòu)設計很好的協(xié)議，只需要避免過于陡峭的輪廓或從動件產(chǎn)生噪聲，影響沖擊，并側(cè)壓力的突然改變。 然而，凸輪速度或負荷增加或具有更嚴格的要求，尋求更精細的設計，并以極高的速度，在慣性運動部件上的作用最明顯，因此，對舉升的加速度和速度因素都必須考慮，這些很少在任何詳細的標準教科書中得到處理。 凸輪從動件的設計也是非常重要的，并且關系到凸輪自身的形狀。這是因為凸輪與從動件不能在一個固定點上接觸。表面接觸需要分配負荷，避免過度磨損，凸輪傳送運動通過從動件各點位置，這都取決于兩個互補零部件形狀。目前凸輪在發(fā)動機氣門設計的問題上特別困難。在賽車引擎中，無論是負載還是速度都可能會被視為極端，因為在很多發(fā)動機中引擎的閥門上這些限制因素得不到有效控制。在某些方面，微型發(fā)動機凸輪的設計的簡化是由于他們的打火工作部件（以及隨之而來的慣量）的原因，但在另一方面，工作通常具有更大的摩擦和旋轉(zhuǎn)速度，一般都大大高于全尺寸的做法更高。 對于小型四沖程發(fā)動機的許多設計已經(jīng)出版，我力求簡化操作閥門，并為凸輪可以簡單而準確地運用在業(yè)余設計制作車間的設施。許多發(fā)動機的設計已提交，讀者對設計中包含有錯誤的閥門裝置，特別是凸輪和這些項目上的基本原則有普遍的誤解，我給一些對此事的意見，而我的信任將有助于個別設計人員獲得其發(fā)動機的最佳效果。 目前有許多關于發(fā)動機的凸輪設計，盡管如此，這些設計結(jié)果或多或少令它們不滿意?？梢哉f，它的速度在一定限度內(nèi)可以滿足要求，但在某些情況下，凸輪設計不良可以與發(fā)動機配合，但在工作細節(jié)上會出現(xiàn)錯誤??。 本文關注的主要是集成電路的閥門發(fā)動機凸輪設計，并且為了避免一些混淆的術(shù)語，圖1顯示了這種類型的凸輪的各個部分，并解釋它們的功能。凸輪輪廓的同心部分，它沒有操作效果，被稱為基圓；凸輪（顯示陰影）被稱為葉輪，無論是從基圓上升到頂圓，這通常是圓形的側(cè)翼。 推程可以被定義為基圓半徑和頂圓之間的差值。點之間的封閉在兩側(cè)加入基圓的角被稱為周期，代表整個周期運作期間，凸輪齒輪比例閥。 在圖2中，對用于i.c.發(fā)動機凸輪典型例子進行了說明。切線凸輪，一個已經(jīng)固定在直線兩側(cè)，其中顧名思義形式向基圓的切線。這種凸輪式很容易設計和生產(chǎn)，是由一個圓形銑加工過程形成了一個最簡單的方法，它是在表面與基圓同心連續(xù)運行了切向那里的兩翼開始和結(jié)束。它也可以產(chǎn)生和我在過去的描述如何與一個在軋輥車床休息備案，它借助與索引齒輪側(cè)面結(jié)合的角度來定位。 切線凸輪的有效工作只能和一個凸弧形上的從動件一起，因為這是唯一的方式，側(cè)面可以發(fā)揮作用，逐步平穩(wěn)。前一段時間有一種引擎與切線凸輪一起被描述。這是不適合極高的速度的，極有可能產(chǎn)生力的集中現(xiàn)象，但相當清楚的是，它是切線凸輪的盤型平面。平面上推桿以上的側(cè)翼突然變長，運動中突然出現(xiàn)運動噪音，效率低，從長遠來看具有破壞性。 滾子經(jīng)常被用來作為與切線凸輪的從動件，并在其形狀方面令人滿意，但在這一點上引入滾動運動的想法并不好??，因為它似乎是一相情愿，因為它只是轉(zhuǎn)移的滑動摩擦要小得多，即樞軸銷。然而，在某些情況下，有可能要使用一個球或滾子的比賽，對于這一點，無論如何，具有分布和均衡的磨損表面的優(yōu)點。 切線凸輪已經(jīng)使用于高性能型發(fā)動機的成功，在一定程度上，同一時間流行的賽車摩托車發(fā)動機，與一些形狀往往是“設計”所具有的援助略有修改，但通常調(diào)諧器，碳化硅流失了！然而，他們更常見的應用一直在天然氣和石油的發(fā)動機速度相對緩慢，他們在哪里工作接觸良好，附著在滾筒的兩端閥門搖動運行引擎。凸齒面凸輪廣泛應用于汽車及其他大眾生產(chǎn)的發(fā)動機。在這方面的一個重要優(yōu)勢是，它們適合于制造一個復制過程中形成的準確數(shù)量從主凸輪。在這頂帽子的挺桿可以使用的事實也有利于批量生產(chǎn)，他們可以設計相當默默工作。該側(cè)面輪廓可以繪制，使在凸輪加速度變化是避免突變，更重要的是，推桿將不能出現(xiàn)任何反彈或漂浮，在相當高的速度出現(xiàn)回復運動。在這種情況下，可能有必要引入一個小規(guī)模的復合曲線，但凸輪與兩翼圓弧會作出合理有效的結(jié)合，并且在任何規(guī)模很容易的產(chǎn)生凹陷。兩側(cè)凸輪的例子現(xiàn)在是比較少看到的，雖然他們已經(jīng)在過去廣泛地使用，獲取最快速的開啟和關閉的閥門的方法。理論上，他們可以被用來產(chǎn)生永久的加速，但在實踐中非常高的速度和激烈的攻角時，閥門控制產(chǎn)生的側(cè)壓力使沉重挺桿困難。凹側(cè)面必須始終比從動件半徑更大，或者一個平面從動件切線凸輪產(chǎn)生左右運動。 ??大多數(shù)類型的凸輪，形狀主要是出于減速所需要的，盡可能的作平緩運動。這種設計方式可以認為理想的條件，在實踐中又是另一回事確保從動件保持密切與凸輪接觸。如果凸起半徑過小，從動件將會在凸輪側(cè)翼反彈和回落。如果過大，閥門開關效率會降低。 對凸輪的三種類型，A，B和C都具有相同的舉升和角度區(qū)間，B葉包圍面積最小，乍一看，它是可能出現(xiàn)的，它在產(chǎn)生足夠的閥門打開效率最低或平均升力面積，但是由于有挺桿的使用，其舉升特性并不十分明顯，不同于基圓的切線凸輪的挺桿，并不見得遜色于凹凸輪的側(cè)面。 非對稱凸輪 這是一種不常見的凸輪，利用不同的輪廓在凸輪兩側(cè)產(chǎn)生某種特定結(jié)果，這種考慮，設計者是可取的。在某些情況下，該對象產(chǎn)生迅速開放和逐步縮小現(xiàn)象，但有時相反的效果卻為最佳。然而，當所有的因素都考慮在內(nèi)，大多數(shù)試圖與運動有關凸輪形式會引起并發(fā)癥，實際上可能失敗，至少在非常高的速度。 在很多引擎中，特別是摩托車，凸輪通過杠桿或在一個搖滾的操作，用一條直線代替圓弧運動，如正統(tǒng)汽車挺桿。這可能是改善機械效率，但它修改了凸輪的升降特征，當在某點上，后者傳遞運動給從動件和杠桿臂的半徑有關，（圖3）不同的凸輪升程的特性。隨著凸輪順時針方向旋轉(zhuǎn)，杠桿的有效長度將會更大。 A位置開啟和B位置關閉閥門，用X和Y的尺寸來表示。與使用非對稱凸輪是相對等的，如這個例子所示，會產(chǎn)生緩慢開啟和迅速關閉的閥門的情況，或者相反，如果不論是凸輪的旋轉(zhuǎn)方向，或相同于“手”的杠桿的方向?qū)a(chǎn)生相反的效果。桿越短，運動速率的差異就越大，無論是凸輪機構(gòu)的形式還是非對稱凸輪樞軸桿不好的設計，但我一直在努力避免在我所設計的引擎產(chǎn)生這種情況，因為他們是一個非常復雜的問題，并保持相對簡單的凸輪和直線挺桿因素，人們可以放心，不會有太多的障礙。 對于具有圓弧面的凸輪的運用，使沒有詳細說明的情況下，車床生產(chǎn)它們的手段和方法，更重要的是，用正確的關系來生產(chǎn)多汽缸引擎的關系具有同樣簡單的方式。這些方法已使許多發(fā)動機構(gòu)造（不含以前的一些經(jīng)驗）成功地解決一個疑問，至少可以這樣說。 很多設計師都試圖改善凸輪的設計，以最大時間來提高閥門的工作效率。換句話說，使得耳垂超過一定在其升力中心的角距離凸輪軸同心的頂部。但是要做到這一點，有必要作出過度陡峭的側(cè)面，從而產(chǎn)生大量的挺桿側(cè)向推力，在高轉(zhuǎn)速時，使控制更加困難。 然而幾乎沒有考慮，同樣的結(jié)果表明，這種方法是可以實現(xiàn)的，出現(xiàn)很少的機械問題，解除閥也更容易效率也更高，如B所示。這避免了推桿突然加速和減速，并促進流動閥門的工作效率。兩個凸輪的陰影部分顯示了葉面積的差異，顯示出真正的高效性。只有在不進行額外的控制時，氣門的高效升程是非常理想的，閥門的最大開口區(qū)，等于閥座直徑的四分之一，但是由于對閥頭莫名其妙的影響，如果它是可行的，較高的升程流動效率將會更好。 大口徑閥門能夠很明顯釋放和有效得承載，在高速運動下比小閥門更難控制但他們更難以控制。另一點是，排氣閥對高壓缸做出反抗，凸輪上的外加荷載越大反抗就越大，完全不同于彈簧負載。 7