【文章內(nèi)容簡介】 t 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 highperformanceengines and were at one time popular on racing motorcycle engines, though usually with some slight modification of shapeoften “ designed ” by the tuner with the aid of .a Carborundum slip! Their more mon 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 massproduced 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 hatbased 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 pound 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: Concaveflanked cams. Comparatively few examples of concaveflanked 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 constantacceleration, 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 e 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 roundbased tappet, and not necessarily inferior to those of a concaveflank cam. Unsymmetrical cams It is not mon 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 ca