George George distributing system (pl. 83, fig. 3) has a plate consisting of two parts placed one above the other, and arranged between a three-pdrt slide-valve and an expansion-valve which is moved by a special eccentric-rod. This double plate divides the steam-chest, as in Figure 4, into two portions communicating by ports which can be closed by the tipper valve. The time of cut-off (consequently the degree of expansion) can be changed by displacing the two central portions of the upper part of the plate by a rack and a cog wheel, the cog-wheel being movable by a rod passing through the steam chest cover. The main valve, which serves solely for admission and exhaust, works steam-tight both upon the main valve-seat and against the lower side of the partition. As shown, this becomes an automatic cut-off system when the eccentric operating the upper valve has its position on the shaft changed angularly, or the amount of its throw varied, by a governor.
Gonfenbach 4 shows the Gonzenbach valve. The ordinary three-ported slide A gets its reciprocating motion on the valve seat B through an eccentric-rod which puts first one and then the other of the end-ports in communication with the steam in the steam-chest, while the third port is placed in communication with the exhaust open ing through which the steam passes into the air or into a condenser. The upper valve E, which is in a separate steam-chest (C) and is also moved by an eccentric-rod, permits " live " steam from the boiler to go through the orifice D, but as soon as D is closed the steam is worked by expansion only. If the upper valve has constant travel, this type will require a throttling governor to make it automatic under varying loads and pres sures, but if the eccentric throw or its position on the shaft is varied by the governor, the whole becomes an automatic cut-off system.
Poppet-valves.—In the Collmann poppet-valve system there are inde pendent exhaust-valves placed below, one for each end, and the so-called " steam-valves " which control the admission are placed on top, one at each end, being opened by eccentrics at regular intervals and closed at variable times by a trip-motion controlled by the governor. Figure II gives the details of the Sulzer poppet-valve motion. The exhaust valves below and the admission-valves above are all poppets, and are opened at fixed times by eccentric-rods. Each exhaust-valve has a fixed time of closure, but the admission by each steam-valve is cut off by a spring at periods determined by the position of the governor, tendency to increase of speed in the engine causing earlier cut-off. In the Nolet valve system the exhausts are below, by means of a slide-valve at each end, with fixed periods of opening and closing; admission is at the side by poppets (one at each end) which have a fixed time of opening, but which are dropped by the influence of a weight at variable tunes deter mined by the position of a centrifugal governor.
Figures 6 to 9 (N. 83) show details of the valve-motion of oscillating engines. Figure io represents the Meyer throttling-valve motion, which controls the passage into the steam-chcst by a conical valve, whose times of opening and closing are controlled by a stein bearing a yoke o],e rated by a revolving double cam, which is raised and lowered, according to the position of the governor, earlier or later in the stroke.
valves may be operated (I) by a beam, (2) by one or more eccentrics or cranks on the main shaft, (3) from the connecting-rod, (4) by the piston-rod or the cross-head, (5) through a shaft driven by gears from the main shaft, (6) by toes on a rock-shaft worked by a beam, or (7) by cams giving a positive motion. The eccen tric-rod may be attached directly to the valye-stem or to a crank-arm fas tened thereto, or to a rocker-arm connected therewith, or it may drive through a so-called "link," which may he connected to the valve-stem or to a rocker-arm. The eccentric is practically a crank-pin so enlarged as to embrace the shaft. In all motions where a rotating shaft drives a recip rocating member, it acts exactly as does the crank, while dispensing with the necessity of cutting or betiding the shaft, and permitting of its eccen tricity being increased or diminished and its position on the shaft being varied by rotation; but it is not available for driving a rotating shaft from a reciprocating part.
Where there is an oscillating link it may have a fixed centre of oscil lation and, a sliding block, by varying the position of which the amount of travel of the valve may be altered; or the same effect may be produced by varying the point of suspension of the link. The link may be either curved or straight, and straight its convexity may be turned either toward or from the eccentric. Usually the amount of variation of the motion of the link or of the block is sufficient to give the valve move ments, varying from a maximum in the direction which will drive the engine forward to a maximum in the direction which will run it backward, there being an intermediate point at which the motion of the eccentric pro duces no motion in the valve. Most eccentric-and-link engines are thus reversible, this being essential in locomotive and marine practice and for hoisting-engines. Reversal may also be accomplished (r) by throwing the eccentric around on the main shaft, so that it shall follow, instead of pre ceding, the crank, or (2) by sliding it across the shaft to reach the same position and to produce the same result.