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2025-01-27 07:43本頁面
【正文】 l cases the motion may be semirotary， oscillatory or swinging. Even straightline 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. 2 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 plementary members. The cams for operating . 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 fullsize practice. In the many designs for small fourstroke 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 . 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 may be defined as the difference between the radius of the base circle and that of the nose. the anele enclosed between the points
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