Having described the functions of the valves in their respective cylinders, we can next consider how their operation is accomplished, at the proper time, by mechanical connection with the crankshaft.
This is accomplished by cams which are attached to, or formed integral with, a shaft termed the "camshaft;" which in Turn is driven through a gear, which meshes with an idler gear, or with the crankshaft gear. When an idler or intermediate gear is used it is so placed that it will mesh with both the crankshaft and camshaft gears. It is only used when the latter gears cannot be meshed directly, due to the difference in centers between cylinders and valves and the two shafts.
The camshaft is so designed that the rise of the cam is brought into operation at the proper time, causing the valve to be raised from its seat and to permit a charge to enter or escape from the cylinder. This cam only causes the valve to raise from its seat; in other words, causes the valve to open, while the valve spring performs the function of closing it and holding it down on its seat until the cam again opens the valve. In the discussion of the two and four-cycle motors it was pointed out that one valve opens during each revolution of the crankshaft, the valves alternating, first one and then the other. From this it can readily be understood that the camshaft must revolve at one-half of the crankshaft speed, thus necessitating a two to one reduction in the timing gears.
In this case we also have to convert a rotary motion into a reciprocating motion. However, as no permanent connection is made with the camshaft, this becomes a simple matter by providing what is termed the pushrod, one end of which communicates with the valve stem and other end rests on the cam. It is provided with a suitable bearing so that its relation with the camshaft can always be maintained.
These pushrods may either have the bearing or guide in the crank case or in the base of the cylinders. When the valve stems are enclosed it is more advantageous to place them in the cylinders, as it somewhat simplifies the machining operations of the case and also the assembling of the motor. These pushrods are always provided with adjusting screws, so that as little lost motion as is practical may be present between the valve stem and the cam. This feature can also be maintained through these screws which are locked in position by lock washers and nuts.
All T-head motors require two camshafts, while the balance of the types illustrated require but one shaft, as all valves can be operated from one side of the motor. Two camshafts could be used in the other types, excepting the L-head, but this of course would add complications in the motor. The train of gears for operating the valve and the accessories is dependent upon the number of shafts and the general distribution of the accessories. They are generally provided with helical teeth and located in a housing at the forward end of the crank ease. They must be completely enclosed so that they may be effectively lubricated and protected from dust. This housing is generally built into the case and provided with a cover plate.
The crank case, as mentioned in the previous installment, serves as the main structural part of the motor, carrying the cylinders, crankshaft, camshaft and accessories, and is in turn supported in the vehicle frame. It also forms a housing for these important parts, protecting them against dust and mud, and also performs important functions in connection with the lubrication of the motor. The crank case generally forms either a cylindrical or box-shaped housing of sufficient size to enable the crank pins with their respective connecting rods to rotate freely within it and sufficiently long to accommodate all of the cylinders of the motor, which are bolted over the opening in the top of the case.
The general design of this case depends, of course, upon the number of cylinders, the valve location and the size of the crankshaft and its bearings.
There are various methods of mounting the motor in the vehicle frame, each having its advantages and disadvantages. The case is generally provided with four arms, which may either be cast integral or bolted to it, when main frame mounting is resorted to, while for sub-frame mounting they are always cast integral.
Lately there has been quite a tendency toward the use of a three-point support for the motor so as to eliminate stresses in the case, due to frame weaving from road irregularities. This type of motor support may either be incorporated in the motor, by using a separate arm pivoting around the crankshaft center at either the front or rear end, when main frame mounting is resorted to and by incorporating this support at either the front or rear end of the sub-frame when the latter mounting is used.
Crank cases may either be formed in one piece with a separate oil reservoir or in two pieces divided on the horizontal center of the crankshaft. They may either be cast iron or aluminum and in several instances manganese bronze has been used. Aluminum is the most popular, provided with separate steel supporting arms which are bolted to the case, owing to the heavy vibrations duo to the solid tires.