CHAPTER XIV. ENGINE CONSTRUCTION*-DETAILS. The Cylinder. THE Cylinder Barrel should be stiffened by external flanges or webs, at about 12 times the thickness of metal apart; these webs should be 1.5 ׃thick, and stand at least 0.75 Xƒ beyond the surface of the cylinder. Some engineers, however, prefer to do without these stiffening webs, and make the cylinder somewhat thicker instead. The following rules are for the cylinder and its connections: D is the diameter of the cylinder in inches. p, the load on the safety-valves in lbs. per square inch. P1, the absolute pressure of steam in the boiler. ƒ, a constant multiplier = thickness of barrel +.25 inch. Thickness of metal of cylinder barrel or liner, not to be less than XD ÷ 3000 when of cast iron. Thickness of cylinder-barrel PX D - 5000 +0.6 inch. Thickness of liner when of steel pX D÷6000 +0.5. Thickness of metal of valve-box sides=0.65 Xƒ. * Seaton. Thickness of metal of cylinder covers = 1.0 Xƒ, if single thickness. Thickness of metal of cylinder covers =0.6 × ƒ, if double thickness. Thickness of cylinder flange = 1.4 Xƒ. Thickness of cylinder cover flange = 1.3 Xf. Thickness of cylinder valve-box flange = 1.0 Xƒ. Thickness of cylinder face over ports = 1.0 X f, when there is a false face. Thickness of cylinder false face over ports = 0.8 ׃, when cast iron. Thickness of cylinder false face over ports =0.6 Xƒ, when steel or bronze. Main Steam Pipe.-The main steam pipe, which supplies a cylinder with steam, should be of such a size that the mean velocity of flow through it does not exceed 8,000 feet per minute. When this is not exceeded, the loss of pressure between the boiler and the vaive-chest is very slight indeed. If, however, the valve-chest is large, and the cut-off in the cylinder is before half stroke, the area of transverse section of this pipe may be smaller than given by the above rule, inasmuch as the piston speed is below the mean velocity at the early part of the stroke, and the space in the steam-chest acts as a reservoir for steam, so as to keep up a steady supply during admission. Taking 8, 100 feet as the mean velocity, S the mean speed of piston in feet per minute, and D the diameter of the cylinder, then, Diameter of main steam pipe = D2 X S D VS. 8100 90 Example.-To find the diameter of the main steam pipe to a cylinder 45 inches diameter and 60 inches stroke, the revolutions at full speed to be 60 per minute. Here S 2 X 5 X 60=600, and D=45 inches. Cylinder Liner.-In order that a suitable material may be supplied to resist the rubbing action of the piston without wearing away, and one that shall be capable of taking and retaining a polished surface, so as to minimize the friction of the piston, an inner bush or false barrel is fitted, usually called the cylinder liner. This liner should be made of a hard, close-grained metal having considerable strength, but not so hard as to resist the action of a cutting tool or file; it should also be such that the expansion caused by heat is very nearly the same as the cast iron of which the cylinder itself is made. It is usual to make these liners of cast iron, strengthened, closed and hardened by mixing with it certain kinds of pig iron, pressed or other equally good steel, hammered out to the proper size for boring; and some engineers use cast steel. Although the compressed steel gives good results, it can be equalled by the specially-made cast iron, so far as good wearing is concerned, but, of course, it far exceeds cast iron in strength; this latter quality is necessary to a higher degree for the horizontal engine than for the vertical engine. In the merchant service, with the vertical engine, the cast iron liner does exceedingly well, and is not likely to be superseded by steel, even FIG. 25.-SECTION THROUGH CYLINDER. if this material can be manufactured much cheaper than at present. Liners are usually made with an inside flange at the bottom end (Fig. 25), which fits into a recess in the cylinder end, and is secured there by screw-bolts. The upper end is turned for a few inches, so as to fit tightly into the cylinder shell at that part. The joint at the cylinder bottom is made with red lead paint, while leakage between the liner and the cylinder shell is prevented at the other end by stuffing a few rounds of gasket, rope, or Tuck's packing into a recess formed for that purpose, and preventing it from coming out by securing a flat wrought-iron ring to the liner so as to cover the packing. Sometimes in lieu of a stuffing-box, the outer edge of the liner and the edge of the turned part of the cylinder shell are chamfered so as to form a groove; into this groove a turn of Tuck's packing or abestos rope is pressed with a ring as before. Some engineers, preferring to rely on metallic contact, turn a slight recess instead of chamfering the edge of the liner, and caulk into it a strip of soft copper. The liners are sometimes secured without a flange at the bottom, by screwing studs through the cylinder shell and liner, and making the ends steam-tight as before. The space between the liner and shell should not be less than 1 inch, and may be filled with steam so as to heat the steam during expansion. If the cylinder has to be jacketed, this is really a better plan of doing it than by casting the cylinder and inner cylinder together, as was very generally done formerly. Pistons.-Pistons of marine engines above 12 inches diameter for a high-pressure, and 20 inches |