valve covers it partly when at full stroke, as seen in Fig. 22. Make the bars 34", so that they will cast well. When the travel of valve is determined, the length, e ƒ, of the face may be fixed. The valve should travel over the face at f, so as not to "shoulder.” We have now found the full opening of port, and have decided upon the point of cut-off-34. From these we can find the lap and travel of valve. Draw the vertical y y perpendicular to a a cutting it in e. From e make e b equal to port opening. From 180° subtract one-half the arc traveled through by crank pin previous to cut-off. Divide the remainder by 2, and make the angle e b c the quotient. The arc traveled through by the crank pin previous to 34 stroke is 120°, one-half of which is 60°. Make the angle e b c= Bisect b cat n, Extend b c until it cuts y y in c. and extend the perpendicular n d until it cuts a Zin d. With d as a center, with a radius equal to d b, describe the circle hc From c draw c d-also cg-parallel to a be is the port opening at full throw; e d, the lap; bh, the travel of valve; circle h c, the path of eccentric; c, position of eccentric centers, when crank pin is at h. The angle g d c, is the angular advance of eccentric. The cg, the linear advance of the eccen tric, gi, is to the travel of valve as that part of the piston stroke, after the closure and opening of exhaust, is to the whole piston stroke. The angle c d b is the one-half arc traveled through by tne crank pin previous to cut-off. Angle d cy measures the whole arc traveled through by crank pin previous to cut-off. In the above there is supposed to be no lap on the exhaust side and no lead on the steam side. Neither is the angularity of the connecting rod taken into account. But the latter has nothing to do with the travel of valves or the proportions of valves and seats, but should be considered in the setting of valves. By this simple process we have the following: Cylinder 16" x 30" Exhaust port 2' x II" Throw of valve 3" Lap outside 34" Full port opening 34" Angular advance of eccentric 30° Length e ƒ of face (Fig. 22) 2/Linear advance 34" Bars 34" Exhaust closure 2.2" CHAPTER VIII. VALVE MOTION DIAGRAM. THE practical man has no time for formulas when he can travel (to him) a much straighter road to what he wants. The plan is simple, and any one can understand why he does this or that; he knows he is right, and he has no conclusions to jump at. H A B 301 VE G The outside circle is the crank-pin circle, tne smaller one the eccentric's centre circle. The crank-pin circle is divided off into the inches of travel of piston, by a tram or pair of compasses set to represent the rod's length. SS, in Fig. 24, are the steam ports, E the exhaust port, and X X the bridges. Fig. 25 is a sectional eleva tion of the valve divided by the line a d. The valve drawing is cut out at the line fe, and placed on the line representing the valve-seat. The laps and lead are laid off (from the pin C, the valve rod running directly from eccentric to valve) a b from centre of circles, a. The line b c is erected perpendicular to centre line F C, and through where it cuts eccentric circle at ca line is drawn, as a B. This line is the centre line of eccentric, and the angle Ca B-the angle formed by eccentric and crank c. This angle is constant in whatever position the crank-pin may be. Place the valve, Fig. 25, on the seat so that line da coincides with line c b, and you have position of valve when crank is on centre. As the crank moves, in direction of arrow, from C, the valve moves from right to left, and back again, till right hand edge of valve comes to The steam is now cut off, and, edge of port, 2. |