Assuming that we are still dealing with the Otto cycle engine, the cam or side shaft will revolve at precisely half the speed of the crank shaft. This 2 to 1 motion is obtained by means of toothed wheels, or a screw gear. In the former case, where plain or bevel cog-wheels are employed, the one fixed on the crank shaft must be exactly half the diameter of the one on the side shaft, i.e., it must have one half the number of teeth. On the other hand, if a screw gear is used, the relative diameters of the two wheels may vary, but the pitch of the teeth on the one must be twice that of the other. These wheels sometimes have the teeth or thread formed in the casting, and sometimes they are cut after a plain casting has been made. The latter kind are, needless to say, better than the former, which often require filing up in order to make every tooth alike, and ensure sweet running.
We know already in what positions our crank has to be at the opening and closing of the three valves, and with the aid of the diagram, fig. 28, we can determine the size of the cams. In fig. 29, S is the side shaft to which the cams have to be keyed, R the roller on valve lever, the latter being represented by the centre lines LL, as all we require to find is the motion this lever will transmit to the valve, the spindle of which is shown at V.
Fig. 30 shows diagrammatically the position of crank at the opening and closing of the air valve. From this we see that the angle through which the crank travels during the time the air valve is open is equal to the obtuse angle ABC. Now, as the side shaft S revolves at half the speed of crank, it is obvious that the former will travel through only half that angle in the same space of time, i.e., through an angle equal to ABD. We can now transfer this angle on to S, fig. 29, and draw two lines SE, SF, cutting a circle GHJ, representing the back of the cam, which latter passes in front of the roller R without causing any movement of the lever L.
Fig. 29.
Fig. 30.
Fig. 31.