or so-called "oscillating" or "rock valve," controlling two end-ports and an exhaust-port, is a development of the flat slide, but the objection to this forum is that it wears to an arc of shorter radius, while its seat wears to one of a larger radius, so that its tightness cannot be maintained.
modification of the flat slide is the piston-valve, a cyl inder moving lengthwise of time engine, and so ported as to act, as regards distribution, exactly like a flat slide, its advantage being that it is not pressed to its seat by the steam in the chest, as in the case of the flat slide, although many consider that this advantage is offset by the difficulty of keeping it tight.
The three-port slide, commonly called the " D-valve,'' and its equiva lents the rock- and the piston-valve, have the merit of extreme simpli city, but their steam distribution, which will be explained farther on, is defective. An improvement gives the exhaust a separate valve or valves, with time of opening or closing not dependent upon load or point of cut off; while the admission and cut-off are effected by another and a separate valve or valves.
The flat slide, the piston-valve, and the oscillating valve never escape from the control of the driving-mechanism; the poppet is usually closed by its own weight or by a spring; and of the "plug" type, those used for admission only are generally opened by positive means and closed by a spring or a weight, while those employed for exhaust alone or for both admission and exhaust are never released from the driving-inechanism. Figure i (iv. 83) shows a double-seat poppet-valve.
The slide-valve may be wholly or partially "balanced "—that is, relieved from the pressure of the steafit in the chest—by a plate or ring playing, against the lid of the chest, and thus lessening the area upon which unbalanced pressure may be exerted. Most stationary and all loco motive engines employ the slide. The oscillating or " rock " type is used on a few stationary, the "plug" on high-grade automatic cut-off station ary, and the "poppet " principally on the better class of marine engines, although some recent stationary engines, like many of the old beam engines, employ poppets. The piston type is used very largely on direct acting steam-pumps, and is slowly coining into favor for small marine engines. There may be in double-acting engines but two cylinder-ports, one for each end, each acting alternately for admission and exhaust; or there may be two for admission and two for exhaust. Where there are
four ports, those for the exhaust may be controlled by the same kind of valves as those for admission, or by a different kind.
il/ever's /iv-fable Cut-of Slidc-vahr.--Figure 2 shows a variable cut-off valve-motion (Meyer's); the upper valve is moved by an eccen tric-rod, but it is placed directly upon the back of a three-port slide-valve, also opergted by an eccentric-rod. The time at which live steam is cut off by the tipper valve (earlier or later according as the load on the engine is light or heavy) is varied by moving the two parts of the upper valve far ther apart or closer together by a right- and a left-hand screw operated by hand as nmy be desired by the engineer. A throttling governor might be used with this movement to make the engine automatic tinder varying loads and pressures; but the Meyer valve is usually employed where there are no sudden changes either in load or in pressure.
Farcot' s —Figure 3 (pi. 32) shows the successive stages of distribution by the Farcot slide-valve motion, in which a riding cut-off valve is actuated through a cam (f ) by the governor. At I the live steam has been cut off by the main valve from the left-hand end-port d', while the right-hand end-port d is also closed by the same valve as an exhaust port. The position of the cut-off valve is therefore immaterial. In // the right-hand end-port d of the seat and the end-port of the main valve are open to live steam, and the left-hand end-port of the seat is partly open for exhaust. In ///, although the passages b and d are in co:nmuni cation, the right-hand end-port is closed to live steam by the action of the end c of the cut-off plate worked by the cam f. The left-hand end-port is exhausting. In IV the valve has commenced to move to the right, but while the left-hand end-port a° is still in exhaust communication with the exhaust passage o through the valve-arch a, the right-hand passages b and a' are still closed by c to live steam. In If the end c' of the cut-off plate has uncovered the left-hand end-passage b', so that live steam can enter it, but the main valve has not yet opened the left-hand end-port ail to live steam, although it has closed the right-hand end-port a'. This is an automatic cut-off system.