steam flow; the passage area being equal to that of the old slide valve ports. Q. How is increased steam efficiency in high-speed engines now obtained? A. Among other ways, by so proportioning the openings for admission, and shaping the passages, as to guide the steam in stream lines to the working cylinders. This has the advantage of making a smaller valve effective, lessening the weight, and on locomotives, where this Area at x = Area of y 4x- JA Fig. 137. Cylinder Port Arrangement. method is particularly applicable, cutting down the valvegear stresses, which on many engines are so great as to call for power reversing gear. This new method also reduces back pressure by giving a freer exhaust, and makes what the man in the cab calls a "smarter" engine. Fig. 137 shows the "stream-line" system for new engines. For each bushing port Y the areas for the steam passage increase from 1x to 2x on one side and even 4x on the other. CHAPTER XLVI RELIEF VALVES Q. What is a relief valve? A. An outward opening safety-valve on the cylinder head or the end of the steam chest, which relieves any excessive pressure due to over-compression of steam, or the presence of water in the chest or cylinders to which it is applied. Q. What are the disadvantages of relief valves? A. If they leak badly, they obscure the engineman's view. Q. What prevents air and cinders being sucked into the steam chest through the exhaust pipes, when steam is off, and the piston working? A. A relief valve in the end of the steam-chest, opening inward into the chest, and permitting air to enter the chest through it, instead of coming by way of the exhaust pipes and drawing cinders therewith. Q. When the engine is drifting, what is the danger unless specially prevented? A. That hot gases or cinders are drawn into the cylinder, the lubricating oil carbonized and the pistons caused to knock. Q. How is this prevented? A. By an automatic vacuum breaker, Fig. 138. When the pressure in chests or cylinders drops below ten pounds, the boiler pressure against the plunger 6 moves it and with it the piston 4 far enough to uncover the passage B and let live steam into the cylinders until ten pounds pressure is built up, when the plunger returns. CHAPTER XLVII THE PISTON Q. What is the piston? A. A reciprocating member formed of a piston-head and a piston-rod, playing together, lengthwise of the cylinder, freely but practically steam-tight. Q. How is the piston-head made? A. There are dozens of designs. One of the most common is a spider consisting of a ring, hub, and radial arms, and a follower plate or follower fastened to the spider by the follower-bolts. This built-up head works slightly loose in the cylinder, but has a pair of rings set out by bolts from the inside of the spider so that they press with any desired degree of force against the cylinder-walls; the rings being cut across to permit being opened out by the packing-bolts. The joint or cut in one ring is on the opposite side of the piston-head from that in the other, so as not to make a continuous cut through which steam might pass. Fig. 139 shows a piston-head in one solid piece with two cast-iron spring rings 8, 8, let into grooves in its periphery. Fig. 140 shows a head made of a spider 1 or head proper, and a follower 2, fastened thereto by follower-bolts 3 and follower-bolt nuts 4; the rings 10 in this case being of brass and composition held out by piston-springs 11, the force of which may be varied by spring-studs and nuts 12. In Fig. 141 there is also a spider or head 1, a follower 2 and bolts and nuts 4, but there is a cast-iron T-ring 9, and cast-iron spring-rings. These three show the principal kinds of packing used. In Fig. 139, the piston is fastened on with a nut 7; in Figs. 140 and 141, by a key. Q. Of what material are these rings? A. Of cast-iron, or of brass or gun-metal, or of either of these two with babbitt-metal run in to lessen friction. 248 Figs. 139, 140 and 141. Piston and Packing Rings. 1. Piston-head. 2. Piston-follower. 3. Piston-follower Bolts. 4. Pistol-follower Bolt-nuts. Piston-Rod. spring Rings (Cast-iron). 9. 5. Piston T-ring (Cast-iron). Brass and Composition Pistonrings. 10. 11. Piston-spring. 12. Piston-spring Studs and Nuts. 6. Piston-rod Key. 7. Piston-rod Nut. |