Note 20 — Appendix

The exhaust valve operating cam was even more ingenious. To obtain operation on a 4-stroke cycle and still avoid the addition of a half-speed camshaft, a cam traveling at crankshaft speed was made to operate the exhaust valve every other revolution (see Figure 17). It consisted of a very slim quarter-moon outline fastened to a disc on the crankshaft by a single bearing bolt through its middle which served as the pivot about which it moved. Just enough clearance was provided between the inside of the quarter-moon and the crankshaft to allow the passage of the cam-follower roller. The quarter-moon, statically balanced and free to move about its pivot, basically had two positions. In one the leading edge was touching the shaft (Figure 17b), so that when the cam came to the cam follower, the follower was forced to go over the top of the cam, thus opening the exhaust valve. When the cam pivot point had passed the roller, the pressure of the exhaust valve spring forced the following edge of the cam into contact with the shaft and this movement, which separated the leading edge of the cam from the shaft, provided sufficient space between it and the shaft for the roller to enter (Figure 17c). Thus, when the leading edge of the cam next reached the roller, the roller, being held against the crankshaft by the valve spring pressure (Figure 17d), entered the space between the cam and the shaft and there was no actuation of the valve. In exiting from the space, it raised the trailing edge of the cam, forcing the leading edge against the shaft (Figure 17a) so that at the next meeting a normal valve opening would take place. The cam was maintained by friction alone in the position in which it was set by the roller, but since the amount of this could be adjusted to any value, it could be easily maintained sufficient to offset the small centrifugal force tending to put the cam in a neutral position.[20]