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Updated: May 5, 2025
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.
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. 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.
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.
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