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48 AMERICAN SEMI-PLUG PISTON VALVE. _ This valve is called Semi-plug, be- "cause while it is without steam it is a snap-ring valve, that is, the pack- ing rings are expansible, and fit themselves to the valve chamber, but when the throttle is opened, the steam is admitted to the chest to enter the space below the rings, and the action of this pressure is to lock the snap rings in a fixed diameter, making practically a plug of it during the time the pressure is on. This is all- - important is. a piston: valve' for to secure proper service, it is necessary to maintain true cages in order to maintain steam-tight valves. Steam- tight valves cannot be secured with- out proper design and construction of rings to begin with, and the abso- lute regulation of their frictional con- Ss Tar Marine REVIEW with the wide ring, number 3, inter- locked into each snap ring forms the complete packing. The wide rings perform two im- portant functions: First, it carries the snap rings across. ports while drifting, and, second, it keeps the snap rings parallel with each other. Now, having the principle of the valve fixed clearly in mind, we will note its operation. Wedge ring, No. 4, is put in under tension; its ten- dency, therefore, is to crowd the two solid wall rings laterally against the cone-sides of the two snap rings,» No. 1. This prevents lateral wear of all rings. The degree of angle on the cones, it will be observed, is much greater on the double-tapered wedge ring than on the snap rings. These angles are so calculated that, while the pressure is underneath all the ez LLL, oi) eee ee ZEB ~ SEMI-PLUG PISTON = VALVE. tact against the cage to prevent wear of the cages while the valve is work- ing at short cut-off. In addition to this it is absolutely necessary to prevent lateral wear. In order. to accomplish all these necessary features, we have designed this Semi-plug piston valve on the principle of leverage by wedges, the pressure acting upon the wedges, In the valve the wedges take the form of cones, or circular wedges, The outside walls of the snap rings, Oumbeted On the print = "'l', sare straight, and fit against the straight wall of the follower and spool. The inner walls of these snap rings are beveled, forming a cone. Next to the snap rings are wall rings, No. 2, the sides of which are beveled to fit the cones of the snap rings. These are called wall rings, because they form the inner walls of the snap rings, These wall rings are uncut, non-ex- pansible 'steel rings. Between these wall rings, in the center, is placed a double-coned expansible'"ting, 'called a wedge ring, number 4, and which, leverage of the rings, the double- tapered wedge ring, crowding the solid wall rings against the cones of the snap fms. is° just sufficient: to prevent the snap rings from further expansion, but not sufficient to re- duce the snap rings in diameter. By a little consideration of the effect of changing the degrees of angles, it will be observed that the frictional contact of the snap rings against the valve chamber depends entirely upon these angles, and it can, therefore, be regulated to any desired cegree. Following the action of this valve, when steam is adthitted to the steam chest, it passes through the small. holes around the spool, and finds an outlet, first, under the first snap ring. and, second, under the central wedge rng. From 14 to 18 holes in the end of a valve. The velocity of the steam from these holes -against' the first snap ring insures its fitting the valve chamber, and the action against the wedge ring is to place it in position for the pressure to lock up the rings. The packing consists of the com- bination of rings, which are free to move up and down on the spool, that 'the rings may fit the cage Perfectly correct, regardless of any variation in the position of the spool. Ag it j, disastrous to the valve cage to allow the spool to ride on it, as it wears the cage out of true, and, therefore, destroys the perfection of valve ser. vice, we make no provisions for car- rying the spool on our rings, and the spool must, therefore, be carried on the valve rod. This can be accom- plished in any way desired, but is the one feature essential in using this valve. By observing the locking effect of the double-tapered wedge ring, when it is. expanded by presence under- neath, thereby crowding the two solid wall rings laterally and holding them, as it were, with a_ pre-determined force against the cone sides of the snap rings, it will readily be reakized that by putting sharper cones on the sides of the snap rings and making the solid wall rings to correspond, the force of the double-tapered wedge ring, crowding the wall rings laterally on a sharper cone, would decrease the diameter of the snap ring, regard- - less of the pressure under it, under which conditions the leverage would be too great, and would permit a blow over the outside of the snap ring. On the other hand, if the de- gree of angle on the snap ring were lessened and the cone made flatter, with wall rings to correspond, then the wedging power of the central wedge ring would not be sufficient | to hold the snap rings from expand- ing by the pressure underneath them, and this would result in excess fric- tion against the valve chamber, caused by inefficient leverage to lock the rings. The same effect can be reached in either direction by chang- ing the degree of 'angie on the double-tapered wedge ring. With this action clearly in one's mind, it will be seen that at whatever diameter of the cage the snap ring is locked up, in that diameter it: will remain, unless it is locked up Mma large part of a worn cage, 'for in- stance, and the movement of the valve forces it down into a smaller part of the cage, under which condi- tions the snap ring wcu'd be forced shut to the diameter of the cage at the smallest part. Under these con- ditions the snap ring would remain the smallest diameter of the cagé: while the valve traveled back to the position of the original locking, and in that position there would be a blow over the outside diameter of the snap