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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. 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
