The Gasoline Motor

by Harold Whiting Slauson

As has been mentioned before, the cam serves only to lift the valve, the return of the latter to its seat being obtained by the force from a spring that is coiled around the stem. Thus the spring holds the end of the push rod at all times against the periphery of the cam. This push rod, in some instances, is a small bar of special steel that slides in guides of long-wearing bearing alloy. The upper end of the push rod is in contact with the lower end of the valve stem, while its other extremity is oftentimes designed in the form of a small steel roller that thus serves to create a rolling contact with the periphery of the cam.

In other designs, the lower extremity of the push rod may be in the form of a specially-hardened steel pin with a rounded end, while still a third type consists of a flat disc slightly "offset" on the end of the push rod so that various points of its surface will come in contact with the periphery of the cam and the wear will be evenly distributed. Whatever the particular design, however, the cam is well lubricated and both it and the push rod are intended to last as long as any part of the motor.

Many motors are designed with one valve at the side and the other, usually the intake, in the head. There are also many motors manufactured that have both the intake and the exhaust valves located in the head, in which case the valve pockets, or projections, are eliminated. Such valves may be operated by the same type of cams and cam shaft as those used to open the valves at the side. As the opening of a valve located in the head is downward, however, the motion produced by the cam on the push rod must be reversed in direction. This reversal of motion is obtained by means of a lever mounted at its center and placed in contact with the upper extremity of the push rod at its outer end. The other end of this lever operates in contact with the end of the valve stem, and thus an upward push on the rod is converted into a downward thrust on the stem. This lever that reverses the direction of the push rod motion is known as a rocker arm and is mounted in a yoke cast with the cylinder head.

Inasmuch as a spring is used to keep the valve tightly closed when the cam is not lifting the latter, it is the contact of the valve head with its seat that must form the stop to the motion of the spring. It will be seen that the force of the spring is communicated through the valve stem to the push rod, and thence to the periphery of the cam when the latter is in a position to lift the valve. The push rod should not be forced tightly against the periphery of the cam when the valve is closed, however, for this would prevent perfect contact between the valve and its seat. Consequently there should be a certain amount of "play" between the end of the push rod and valve stem so that it will be certain that the head is forced against the seat with the full power of the spring and without the cam serving as a stop.

On the other hand, this play should not be too great, for the cam and push rod will then move an appreciable distance before the valve is raised. This will cause the opening of the valve to occur late and will reduce the distance that the stem is raised, thus restricting the size of the opening. Furthermore, an undue amount of play between the ends of the push rod and stem will result in a pound or "hammer blow" between the two that is liable to wear the surfaces rapidly.

The "happy medium" that will give the best results may be obtained by properly setting the small valve "tappets" that are secured to the end of the stems or push rods. By turning the nut of the tappet in one direction, the length of the push rod will be reduced, while the reverse operation will increase the length of the rod or stem. This is primarily intended for taking up any wear that may occur at the ends of the push rod or valve stem. In the case of engines having the valves in the head, the long push rod of each valve should be so loose as to move perceptibly when shoved up and down by the thumb and finger.

When the rocker arm is pressed down against the valve stem, the space between the other end of the rocker arm and the push rod should be sufficiently wide to admit a piece of tissue paper. The same test may be made in connection with valves located at the side, after first ascertaining that the end of the short push rod is resting firmly against the periphery of the cam. The play will be apparent, of course, only when the valve is tightly closed, and in order to make certain that their cams are in the "inactive" position, the piston should be set at the beginning of the explosion stroke when testing the intake or exhaust valve. This is at the point of ignition and is the time at which both valves should be tightly closed.

The cam shaft to which the cams that operate the valves are attached is generally placed inside the crank case. If the motor is of the "T"-head type, having valves on opposite sides of the cylinders, the cam shaft operating the exhaust valves will be found on one side of the crank case, while that for opening the inlet valves will be located on the other. If the motor is of the "L"-head type, all the cams will be placed on the one shaft. The cams are sometimes forged with their shaft in a solid piece, while in other designs they are keyed in place, but whatever type is used, the cams and their shaft may be considered as integral with each other.

The cam shafts are generally driven by a gear meshing with a smaller one attached to the front end of the crank shaft of the motor, which forms one of the forward train of gears that are enclosed in an aluminum case. If the cam shaft is driven at the same speed as is the crank shaft of the motor, it will be seen that the valves will open once at every revolution of the flywheel. In a four-cycle motor, however, the explosion and other events occur but once in each cylinder for every two revolutions of the flywheel, and consequently the cam shaft must be driven at one-half the speed of the crank shaft.

To obtain the proper speed ratio, each cam shaft is driven by a "two-to-one" gear, which means that the gear on the end of the crank shaft has but one-half as many teeth as have those attached to the cam shafts. There is thus one revolution of each cam shaft gear for every two of the crank shaft gear, and consequently each cam shaft is driven at the required half speed.