LAIDLAW-DUNN-GORDON.
The single cylinder pumps of this make are equipped with the gear illustrated inFig. 252, in sizes varying from 4 inches in diameter by 5 inches stroke to 28 inches in diameter by 24 inches stroke.
The arrangement of valves and ports is shown in the engravings,Figs. 253and254.
The admission of live steam to the cylinder and of exhaust steam to the atmosphere is controlled by a valve piston, A, shown inFig. 252.
Fig. 252.
Fig. 252.
Assume that the piston is in position shown,Fig. 253, and that both the main and auxiliary valves cover their respective steam ports. By means of a starting bar, operating through a stuffing-box in the valve chest, the piston valve, A, is moved toward the head of the steam chest, D, thus opening the ports, E and L, and admitting live steam through L, from the cavities, S, of the valve piston to the housing end of the main steam cylinder, through the port, F,Fig. 255, forcing the main piston, P, toward the opposite end of the stroke, or toward the left in the figure. The port, E,Fig. 253, being open, the exhauststeam escapes from front of the main piston through the port, F,Fig. 255, into the main exhaust port, G, through the port, E. The piston, P, travels to its extreme left position and the auxiliary slide valve has been drawn to such a position in the direction indicated by the arrow in the smaller drawing inFig. 252, as to bring valve piston, A, toward the opposite end; the exhaust steam from the steam chest escapes from before it, through the exhaust port, K, the opening of which into the chest is at such a distance from the head as will permit sufficient exhaust steam to remain to afford a cushion to the valve piston.
Fig. 253.
Fig. 253.
Fig. 254.
Fig. 254.
Fig. 255.
Fig. 255.
Fig. 256.
Fig. 256.
With the auxiliary slide valve in position to bring the hole, H, over the port, J,Fig. 256, it is plain that the exhaust through the port, K, will pass into the main exhaust through the port, L. With the main piston at its extreme travel toward the right, the ports, E and L, which correspond to F and F, respectively, inFig. 255, are opened in such a manner as to exhaust steam to the atmosphere from the housing end of the steam cylinder through the port, F, and live steamfrom the chest to the head end of the main cylinder, through the port, F, thus driving the main piston, P, toward the housing end of the cylinder, or toward the right. The piston and reciprocating parts traveling in this direction move the auxiliary slide valve to its maximum point of travel in the opposite direction, thus opening the opposite auxiliary steam and exhaust ports and again driving the valve piston toward the head, D, of the steam chest, whence a new stroke begins.
Lost motion in the valve gear is taken up by adjustable links, on all sizes above 7 inches diameter by 10 inches stroke and on some smaller sizes.
Cushioning of the steam pistons in the larger sizes and upwards is accomplished by means of suitable valves called cushion valves. In the smaller sizes sufficient cushioning is done by exhaust steam passing from the clearance space next the head through a small hole drilled into the main steam port.
To set the valveof this pump it is only necessary to place the piston in its central position and adjust the lever so that the valve will occupy its central position. By this proceeding the travel of the valve is equalized.
Fig. 257.
Fig. 257.
The Foster single cylinder pump valve motion is a compound valve piston and slide valve in one piece and performs the officeof both a main and auxiliary valve. It seats over the main steam cylinder on the web or casting connecting together the valve pistons, and is provided at the bottom with vents or openings,AandB,Fig. 257, for opening and closing the main steam ports to the main steam cylinder. The sides of this valve are cut to form the recesses,CandD, which are for the purpose of opening and closing the small steam ports, which admit steam to the valve pistons; it is also provided with L shape slots for the purpose of alternately exhausting steam from the valve pistons through the steam ports.
Fig. 258.
Fig. 258.
Fig. 259.
Fig. 259.
Fig. 260.
Fig. 260.
In operation the main piston rod commences its forward stroke, motion is communicated to the vertical arm, which moves forward on the rod until it engages with one of the cams,E,Fig. 259, adjusted on the valve stem. The arm coming in contact with the inclined faces of the cam imparts a rolling or oscillating movement to the stem and valve, opensone of the recesses,CorD, cut in the side of the valve and admits steam through a port to one of the valve pistons. At the same time this oscillating movement of the valve opens the slot,F,Fig. 260, opposite the first, and exhausts steam through the alternate port from the other piston. Steam thus admitted to one of the pistons (while exhausts from the other piston), carries the valve over its seat, and by means of the vents,AandB, steam is admitted through one of the main ports and exhausts through the other. When the main piston has completed one stroke, it moves backward on its return; the arm strikes against the other cam on the valve stem and communicates a reverse motion to the valve which closes and opens the alternate ports to admit and exhaust steam to and from the pistons. It also cushions the piston at the end of each stroke which prevents the piston from striking the heads.
To set the valves.This can best be done by starting the pump slowly and adjusting the cams,E, so as to open the valve at the proper time to compel the piston to make a full stroke. A little experimenting combined with good judgment is all that is necessary, unless the piston valve be badly worn, in which casea new valve must be substitutedand fitted to the valve chest.