REGULATION OF STEAM.
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a ring of metal, the bearing surface on the back of the valve-casingbeing merely the thickness of the metal forming the ring. It is
held to the face on the valve-casing with two flat springs placedinside of the valve, thus pressing the valve and the piston ringto their respective faces. A pin is inserted in the valve and ringto prevent the latter turning round; the valve is fitted with a hoopto which the valve rod is attached. This arrangement is aboutas effective as any. Some engineers have split the piston ring;others consider, however, that this is not required, as it is a morepreferable plan to make the piston steam-tight with light Steel ringsrecessed into the piston as already described, as the ring of itselfwith a good fit would nearly be steam-tight, while the Steel ringsmake it perfectly so. This valve is admirably suited for the loco-motive engine; the rubbing surface on the packing ring is verysmall, and there can be no doubt that this is a benefit. Care mustbe taken, however, to have ample provision made for running offany water that may collect when the engine is standing still, so thatthe narrow rubbing surface may be kept quite dry. In the largemarine compound engines, piston valves are now being used.
There are other plans for tightening up the slide-valve rings.The casing, for instance, is pro-vided with a cover on the back,its inside face being truly planedand scraped. To the valve isfitted a ring with snugs cast 011it, each snug being providedwith a ratchet screw bolt andspring.. This ring carries twopacking rings, which are pressedup against the valve ehest door with the set screws. There areholes tapped in the valve-casing door for the reception of screwedplugs. These holes correspond with the snugs and screws for
Fig. 74.—Ratchet-bolt for Valve Rings.