Granting the necessity for proper means to regulate the inflow and outgo of the charge and consequent products of combustion, as exemplified by the valves, the next most important part is the one which controls the movement of the valve, and is, therefore, essential to the success of the latter. This is what is known as a cam and in the usual case amounts to an extension of or projection from the so-called camshaft. Inasmuch as the valve functions only come into play upon every other stroke of the crankshaft, this camshaft is gear-driven from the crankshaft, so as to rotate at half the speed of the latter. This is very simply effected by having the cam gear twice as large as the crankshaft gear, that is, with twice as many teeth. As the same valve is never used for both the inlet and the exhaust, so the cams are seldom made to do more than the one thing, namely, operate one of the valves. From this has grown the custom of referring to them according to the function of the valve which they operate - inlet cam, exhaust cam, etc.
Fig. 14. Method of Laying Out Cams.
In order to lay out a set of cams, not only must the cycle be fixed, but the clearance as well. Fig. 14 shows the way to go about this; the size of the shaft is simply determined, and if other means fail, the empirical formula may be used:
Camshaft diameter = .625D - 1/8" in which D is the clear diameter of the valve opening in inches.
Having the camshaft diameter fixed, lay it out and about it circumscribe the cam thickness. This may be from one-eighth of an inch, upon very small, light-weight engines, to three-eighths inch on larger and heavier motors. Around this, in turn, describe another circle, distant from the cam surface a distance equal to the clearance. A fourth circle representing the height is shown only partly complete.
From the cycle previously determined upon, the total angle of inlet valve opening, for instance, is found by simple addition and subtraction; thus, if the inlet is to open 10 degrees past the upper center and close 20 degrees past the lower center, this makes the valve remain open a total of 190 degrees upon the crankshaft. As the camshaft turns but half as fast and, therefore, but half as far in the same length of time, for the cam this angle will be halved, or 95 degrees. Proceed to lay out half of this, or 47½ degrees, on each side of the vertical center line. A line forming this angle with the center line will intersect the line representing the clearance in a point. Through this point draw a line which will be tangent to the circle representing the surface of the cam, and prolong it upward to meet the upper circle. Drawing in a round corner completes the cam layout. By sketching in the cam roller the progression is shown. Figs. 15 and 16 illustrate the complete valve action very well, the former, that of the Locomobile Company of America, Bridgeport, Connecticut, showing the form in which the cam works against a roller in the bottom of the push rod, [this working upward in the push rod guide with a dirt excluding arrangement at the top. The top of the push rod bears against the bottom of the valve stem with an adjustable hardened screw forming the contact. The valve is held down on its seat in the cylinder by means of a strong spring, which the upward movement of the push rod opposes. The valve is guided in and has its bearing in the valve guide, made long to give large bearing surface. As the Locomobile motor is of the T-head type the exhaust and inlet valves are on opposite sides of the cylinders and are operated by separate camshafts. The valve mechanism is completely enclosed. The second figure shows the valve action used on Haynes cars, made by the Haynes Automobile Company, Kokomo, Indiana. The difference is in the elimination of the roller at the bottom of the push rod, which forms the point of contact with the cam. In this form, a flat hardened surface makes the push rod more simple and reduces the number of parts. It has been said against this form that the cam scrapes across the push rod face and thus wears it, but in actual use it has been found that the push rod rotates and in this way the wear is distributed over the whole flat face, which in this construction can be made much larger than can the face of the roller. The push rods are of the "mushroom" type and are made of nickel steel. The push rod adjustments are completely enclosed but may be readily reached without disturbing any other unit. They may be removed and replaced without removing the valve springs or valves.
Fig. 15. Complete Valve Motion with Roller Push Rod.
Courtesy of Locomobile Company of America, Bridgeport, Connecticut.
Fig. 16. Complete Valve Motion without Roller in Push Rod.
Courtesy of Haynes Automobile Company, Kokomo, Indiana.
Neither of these systems is in decided favor, designers being about equally divided between them.
The construction and operation of the cam mechanism is the same whether used in connection with an exhaust or an inlet valve, as the same line of reasoning and the same method of procedure in both cases would lead to the same results. It will be noticed in Figs. 14 and 17 that the straight sided cam has been chosen to illustrate the elements of design.
Fig. 17. Straight-Sided or Ordinary Cam.
Fig. 18. Lay-Out for Uniform Acceleration Cam.
It has many times been tried and still more often urged that the straight surface of the side of the cam is not conducive to the best results, because of the fact that when the first straight portion of the cam surface strikes the cam roller it does so with so much force that it tends to wear the latter in that direction. As for the receding face, it has been urged that the ordinary closing of the valve is too slow, and that the straight surface, as shown in Fig. 17, can be altered so as to allow of speeding up the downward movement of the valve. This idea works out into a curve, Fig. 18, the back of the surface being hollowed out so that as soon as the cam roller passes the center it drops vertically, due to the tension of the spring. This method has been tried, but without success.