to find position of crank-pin, notice that the line a d on valve cuts the centre line, FC, at b, which extended down to small circle, cuts it at e. The line a e D is now evidently the centre line of eccentric. Taking the distance, B C, in a pair of compasses, and measuring back from D, we find the point A, which is the position of the crankpin when steam is cut off. The figures on the large circle give the number of inches traveled when this occurs. The valve now continues to move from left to right till edge of exhaust cavity g, comes to edge of port h, when the exhaust commences. The centre line on valve d a now cuts line FC at a; this, extended down, cuts eccentric circle at m. A line, a mj G, now represents the centre line of eccentric; laying off the distance, B C, from G, we find the point H of crank-pin, when exhaust commences. The crank now travels on the lower half of its circle. The valve continues to travel from left to right, opening into the exhaust until the edge of exhaust cavity again comes to h (edge of steam port, the valve now traveling from right to left), the exhaust is closed and compression commences. This occurs, as before stated, when g, on valve, comes to h, on ports; the line da, on valve, then cuts the line F C at a, and the line a i K represents centre line of eccentric. Lay off from K the distance B C, and we find I, the position of crank when com pression commences. Similar points on the opposition stroke can be found by commencing with crank pin at Finstead of C, noting that centre line of eccentric is down instead of up. If the eccentric's motion is transferred to valve through a rocker shaft, the lap and lead must be laid off toward the crank-pin instead of from it. These points, we assume, are understood by the reader, as it would require too much space to explain them all. The irregularity of the valve's motion (more expansion occuring on one end than the other) can be ascertained by this diagram. Unless the eccentric rod is very short, no notice may be taken of its irregularity due to radius (shown in line b c, being struck from length of eccentric rod), as it is so little that, generally, a straight line, as bc, will suffice for practical purposes. We trust that this may be of use to some who have worried their brains with impracticable formulas and diagrams produced by men more scientific than practical, and which resemble puzzles. MOTION CURVES. The laying off of motion curves presents to the eye all of the movements of the valve at a glance. Fig. 26 shows the diagram after it is completed, ready to file away for future reference. Elevations of the ports are laid out to a scale and in length equal to the stroke of the piston, with the inches marked as shown. We start with the valve V, as shown at the commencement of the stroke, and, by means of Fig. 26, get the position of the valve for several positions of the piston, or for each inch, as shown at a, b, c, d. When all of the positions are obtained and jotted down on the port elevations, a curve that cuts them all is drawn, as are similar curves for the other edges of the valve. The opposite curves can also be laid down, and the movements of the valve for the opposite stroke obtained. In the diagram shown, we see that the port S' was wide open at h, or 41⁄2 inches of piston travel; that steam was cut off at c, 18 inches; exhaust opened at f, 191⁄2 inches, and compression commenced at g, 191⁄2 inches also, the valve being "line and line." Other points can be obtained by inspection, and the diagram forms valuable data for the builder and user. AND EXHAUST AFTER EXPANSION. TABLE BY WHICH TO FIND THE RELATIVE STATE OF PISTON the stroke. the end of the end of the end of the end of Steam cut off Steam cut off Steam cut off Steam cut off at from at 4 from at % from at % from the stroke. the stroke. the stroke. Cover on the exhausting side of the valve in parts of the length of its stroke. Distance of the piston from the end of its relative position will be the following: side of the valve being 1-32d of its stroke, the the end of its stroke, the cover on the exhaust steam cut off when the piston is one-third from with a stroke of six feet, or 72 inches, and the Example from the Table.—Suppose an engine |