A valve is opened once during two revolutions of the crank shaft; therefore the cam cannot be placed on the crank shaft, for, if it were, the valve would be opened every revolution. The cam is placed on a separate shaft which is driven by the crank shaft at half its speed. This is usually done with gears, a gear on the crank shaft meshing with a gear on the cam shaft having twice as many teeth; the crank shaft gear must make two revolutions in turning the cam shaft gear once.
The valve in [Figure 13] is held on its seat by a spring. The cam bears against the end of the valve stem, and as it revolves its bulge forces the valve stem and valve to move endways and thus to uncover the valve opening.
As the movement of the piston depends on the crank shaft, the valve can be made to open at the right time by a proper setting of the gears that drive the cam shaft.
The length of time that the cam will hold the valve open depends on the shape of the bulge of the cam. It can be seen that the pointed cam of [Figure 13] will not hold the valve open for as long a time as the flat-end cam of [Figure 14].
Fig. 15.—“Twin City” Tractor Engine
In the design shown in [Figure 13] the cam bears directly against the end of the valve stem, the cam shaft in this case lying along the cylinder head. In the construction shown in [Figure 15] the valves are not placed in the cylinder head, but are in an extension or valve pocket projecting from the combustion chamber; this cam shaft is near the crank shaft. It would not be practicable to make the valve stem long enough to reach down to the cam, so a length of rod, called a push rod, or tappet, is placed between them; the cam moves the push rod and the push rod in turn moves the valve. This is a construction frequently used for automobile engines.
Fig. 16.—“Hart-Parr” Valve Mechanism
In tractor engines the cam shaft is usually placed near the crank shaft, as in [Figure 15], and the valves are in the head, so that a valve moves in the opposite direction to the movement of the push rod. This requires still another part to be used, called the rocker arm. It is shown in [Figure 16]. It is a short bar, pivoted at or near the center, with one end at the push rod and the other at the valve stem. When it is moved by the push rod it in turn moves the valve.