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1、关于凸轮的外文资料ELEMENTS OF CAM DESIGNHow to plan and produce simple but efficient cams for petrol engines and other mechanismsCams are among the most versatile mechanisms availableA cam is a simple two-member deviceThe input member is the cam itself，while the output member is called the followerThrough th
2、e use of cams，a simple input motion can be modified into almost any conceivable output motion that is desiredSome of the common applications of cams areCamshaft and distributor shaft of automotive engine Production machine toolsAutomatic record playersPrinting machinesAutomatic washing machinesAutom
3、atic dishwashersThe contour of high-speed cams (cam speed in excess of 1000 rpm) must be determined mathematicallyHowever，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 layoutIn general，the greater the cam spe
4、ed and output load，the greater must be the precision with which the cam contour is machinedCams 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 equal
5、ly 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 opera
6、te 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
7、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
8、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 i
9、nertia 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
10、 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
11、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 limi
12、ting 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-si
13、ze 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 t
14、o 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.
15、 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
16、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
17、. 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 betw
18、een 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.
19、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 an
20、d 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 efficien
21、tly 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 li
22、kely 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 us
23、ed 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
24、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, t
25、hough 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 th
26、e 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
27、 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
28、 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-flank
29、ed 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 constant-acceleration, but in
30、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 prod
31、uced.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
32、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 enc
33、loses 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
34、 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
35、 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
36、 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 t
37、he 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
38、, 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 unsymmetr
39、ical 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
40、 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 r
41、elation 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 improv
42、e 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 center of its lift. To do this, however, i
43、t 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
44、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 dwel
45、l. Factors in efficiency High valve lift is a desirable feature, but only if it can be obtained without making extra difficulties 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
46、valve head, a higher lift is better for flow efficiency-if it is practicable. 中文翻译凸轮设计的基本内容如何为汽油发动机和其他机械设计和生产简单有效的凸轮凸轮是被应用的最广泛的机械结构之一。凸轮是一种仅仅有两个组件构成的设备。主动件本身就是凸轮，而输出件被称为从动件。通过使用凸轮，一个简单的输入动作可以被修改成几乎可以想像得到的任何输出运动。常见的一些关于凸轮应用的例子有：凸轮轴和汽车发动机工程的装配专用机床自动电唱机印刷机自动的洗衣机自动的洗碗机高速凸轮(凸轮超过1000 rpm的速度)的轮廓必须从数学意义上来定义
47、。无论如何，大多数凸轮以低速(少于500 rpm)运行而中速的凸轮可以通过一个大比例的图形表示出来。一般说来，凸轮的速度和输出负载越大，凸轮的轮廓在被床上被加工时就一定要更加精密。在多种机械装置的操作中凸轮在某种形式下是必不可少。他们最有名的应用是在内燃机阀门操作装置中，但在工业机器中，从印刷机到收割机械，凸轮机构也是一个相当重要的一部分。一般来说，一个凸轮可以被定义为一个圆盘面或一个为产生接触间歇往复运动的零件或从动件。大多数凸轮的运动是旋转运动，但这不是一个必要条件，在特殊情况下，它的运动是半旋转，振动或摆动。即使原动件可能是直线运动，但在某种情况下凸轮也可能会适当地被考虑，。在机构学大多
48、数文本书籍中给了关于凸轮设计和凸轮类型的实例设计的一些信息，产生各种规定的运动。在某种情况运行速度和机械的性能不是非常高，有一个规律是凸轮机构设计很好的协议，只需要避免过于陡峭的轮廓或从动件产生噪声，影响冲击，并侧压力的突然改变。然而，凸轮速度或负荷增加或具有更严格的要求，寻求更精细的设计，并以极高的速度，在惯性运动部件上的作用最明显，因此，对举升的加速度和速度因素都必须考虑，这些很少在任何详细的标准教科书中得到处理。凸轮从动件的设计也是非常重要的，并且关系到凸轮自身的形状。这是因为凸轮与从动件不能在一个固定点上接触。表面接触需要分配负荷，避免过度磨损，凸轮传送运动通过从动件各点位置，这都取决
49、于两个互补零部件形状。目前凸轮在发动机气门设计的问题上特别困难。在赛车引擎中，无论是负载还是速度都可能会被视为极端，因为在很多发动机中引擎的阀门上这些限制因素得不到有效控制。在某些方面，微型发动机凸轮的设计的简化是由于他们的打火工作部件（以及随之而来的惯量）的原因，但在另一方面，工作通常具有更大的摩擦和旋转速度，一般都大大高于全尺寸的做法更高。对于小型四冲程发动机的许多设计已经出版，我力求简化操作阀门，并为凸轮可以简单而准确地运用在业余设计制作车间的设施。许多发动机的设计已提交，读者对设计中包含有错误的阀门装置，特别是凸轮和这些项目上的基本原则有普遍的误解，我给一些对此事的意见，而我的信任将有助于个别设计人员获得其发动机的最佳效果。 待添加的隐藏文字内容3目前有许多关于发动机的凸轮设计，尽管如此，这些设计结果或多或少令它们不满意。可以说，它的速度在一定限度内可以满足要求，但在某些情况下，凸轮设计不良可以与发动机配合，但在工作细节上会出现错误。 本文关注的主要是集成电路的阀门发动机凸轮设计，并且为了避免一些混淆的术语，图
