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【正文】 eavy 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 4 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 cam forms lead to plications 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 plicating factor in what is already a very involved problem, and by keeping to fairly simple cams and straightline 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 multicylinder 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
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