It will be seen that by drawing a line forming a tangent to the circle GHJ at F and another at E, and producing these, they will meet at point K. Consequently, as the side shaft rotates in the direction indicated, the lever L will begin to open the valve V when the cam is in the position shown in fig. 29, reach a maximum opening at K, and finally close when the cam has moved so that point E is now where F was. With a cam of this shape, however, a considerable portion of the stroke would have passed before the valve was raised any appreciable distance off its seat; it would only be fully open for an instant, viz., when K was passing over R, and would begin to close again directly.
Moreover, if the engine were running at even a slow speed, the motion imparted to lever L would be indefinite; and this, especially if the governor is fitted to the air valve lever, as in fig. 25, is very undesirable. Therefore, to obtain a definite opening we must set out the cam, as shown in fig. 31. In this diagram the roller is shown standing clear of the back of cam by about 1⁄16 in. A line MN is then drawn, forming a tangent to both roller R and circle GHJ at points F and O respectively. This gives us the opening portion of cam. Then from the centre S with radius SF describe the arc FE (shown dotted in fig. 31), and set off the angle required (ABD, fig. 30), as previously explained. Through point E draw a line forming a tangent to circle GHJ, and produce it towards P. This line gives us the closing portion of cam. The distance W is of course variable, according to the amount of lift we give the valve. By comparing these two diagrams it will be seen that in both cases the valve will be opened the same length of time, but in first case the motion will be indefinite and uncertain. In practice the corners are rounded off somewhat, in order to obtain a steady motion; and when the air cam is also the governing cam, it is advisable to round off the opening face, as indicated in fig. 32. Upon the shape of this face both the sensitiveness and the life of the governor gear depends. If it is nicely rounded off, giving a gradual rise, very little tension (or compression, as the case may be) of the controlling spring will be necessary to give the required speed to engine; whereas, if the rise is sudden, the spring will have to be screwed up tighter, and, if uneven and lumpy (i.e., not a fair curve), the result will, of course, be erratic governing.
Fig. 32.
A certain amount of clearance should always be provided between the roller and the back of cam (compare figs. 29 and 31), that is, the roller should not bear against the cam, except during that portion of the stroke in which it is actually operating the valve, viz., from F to E (fig. 31). A small stop interposed between the lever and some convenient part of the engine, such as the side-shaft bracket bearing, answers this purpose.
Fig. 33.
Fig. 34.
The size and shape of the exhaust cam is found in the same manner as above described; the angle through which it operates is greater than that of the air cam, and is shown in fig. 33. A fair margin should be allowed for filing or machining these castings up; the shape and sizes arrived at by the above described method being finished measurements. Fig. 34 gives the outline of an exhaust cam worked out from the setting diagram, fig. 33.