= i; net area of a single cylinder-head bolt whose nominal diameter is e = AP 4,000 n' where A = area of cylinder head in square inches; P = steam pressure; n = number of bolts. The pitch of the bolts may be from 4.5 to 5.5 in., but should never be more than 5ƒ. 1.5 i; g= .04 D+.125". Take the nearest nominal size pipe tap. twice the outside diameter of drain pipe. h i = .0003 PD + .375'', where P is the steam pressure. If the steam pressure is less than 100 lb., make P = 100. .85 i; j n = m+6e, never less. Here, e is the nominal diameter of the bolt. o = the nominal diameter of steam-chest bolts. The net area of a single steam-chest bolt = A'P 4,000 n' si. This is required only when the length of the port is greater than 12 in. t = 1.25 i. When D is greater than 24 in., use 4 bolts in the standard and make t = 1.1 i. u = 1.5 i; v.25'' (constant). The dimensions of the steam ports, exhaust ports, and other steam passages depend on the velocity of the flow of steam. The ports and passages must be large enough to allow the steam to follow up the advancing piston without loss of pressure. The maximum allowable velocity of the steam in the passages, when they are short, is about 160 ft. per sec. But, with the ordinary ratio between the length of connecting-rod and length of crank, the average velocity is about five-eighths of the maximum. Hence, the allowable average velocities are 100 to 125 ft. per sec. for long and short passages, respectively. Let l b A S v average piston speed in feet per second; average velocity of steam in feet per second. Then, area of port X velocity of steam = area of piston X velocity of piston, or lb v = AS; whence, The constant 100 may be used for v, when designing plain slide-valve engines of the ordinary type, which cut off late in the stroke, and 125 may be used for high-speed engines with early cut-off, and for the Corliss type. The area of the exhaust port or ports may be from 1 to 2 times the area of a steam port. The area of the cross-section of the steam pipe is approximately equal to the area of the steam port; likewise, the area of the exhaust pipe should be equal to that of the exhaust port. The length of the port may be .6 D to .9 D for slide-valve engines, and about .9 D to D for the Corliss type. The height w, Fig. 1, of the valve seat must be such that the area of the most contracted part of the exhaust port is not less than 75% of the area of the steam port. THE STEAM CHEST. Fig. 2 shows a steam chest for the cylinder illustrated in Fig. 1. The principal dimensions are to be determined by the following proportions, which are based on the thickness i of the cylinder walls, and on the travel and dimensions of the valve: a = length of valve + travel of valve + twice the clearance between the valve and the steam chest at ends of valve travel; b = breadth of valve + twice the clearance between one valve and steam chest; c = .75 i; סרס FIG. 2. d = 2.75 o, where o is the nominal diameter of the steamchest bolts, as in Fig. 1; .04 VA' + .125" for all areas above 100 sq. in. A' = area of steam chest, outside measurement, in square inches; 1.3 e; g= .85 i; h height of valve + necessary clearance; t = .85 i; j = 2.5 i. NOTE. When the area of the steam-chest cover is less than 100 sq. in., its thickness e may be made equal to i. If the area of the steam-chest cover exceeds 600 sq. fn., the height of the ribs should be 3:5 i, and their number should be increased. Fig. 3 shows a design for a steam-chest cover when the steam-pipe flange is on one side of the steam chest. Determine the thickness e by the same formula and rules as for the cover in Fig. 2. The other dimensions are found as follows: p should never exceed the distance in inches given by the maximum pitch of the ribs for a cover 18 in. thick, subjected to a steam pressure of 160 lb. per sq. in. SOLUTION.-Substituting in the formula for p, we have p = 40 X e12 V Pg Fig. 4 shows a Corliss engine cylinder that may be designed according to the following proportions: D = diameter of cylinder. |