THE GUILD AND GARRISON.
Fig. 273.
Fig. 273.
Fig. 274.
Fig. 274.
The steam chest of this pump differs somewhat from other pumps in which valve pistons are employed to operate the main valve. See accompanying engraving,Fig. 272. This pump has a steam chest with a cover for inspection and repairs. This chest is bored at each end to form suitable cylinders to receive the valve piston, E. By the side of the valve piston, E, in the steam chest is an auxiliary slide valve, G,Figs. 273and274, which admits and releases the steam from the ends of the valve piston. The valve piston, E, has two slots at the center, the lower one receiving the lug on the back of the main valve and the upper one the toe on the rocker shaft, D. The rocker shaft has two toes, the larger one, F, engaging with the valve piston, and the smaller one with the auxiliary slide valve, G, as shown inFig. 274. The auxiliary, as well as the main, valves are plain slides designed to take up the wear automatically. The pendulum lever, J,Fig. 273, is connected with the piston rod as shown and rotates the shaft, D, and by means of the toes the valves move together.
The auxiliary slide valve, G, admits and exhausts the steam to and from the ends of the valve piston. Steam being admitted, the piston moves toward one end. The two valves also move in the same direction by means of the rocker shaft and the toes. This movement continues until the piston has nearly completed its stroke, when the auxiliary valve opens one of the small ports leading to the end of the valve piston, thus admitting steam at one end and exhausting it from the other. The valve piston moving from one end of the steam chest to the other, shifts the position of the main valve and reverses the motion of the main piston. The valve piston is moved the greater part of its travel by the toe on the rocker shaft, thus reversing the steam distribution, by steam pressure, which brings the opposite end of the slot in the driver to a second engagement by the toe on the rocker shaft which begins the return stroke.
The steam acts to throw the main valve only near the ends of the valve travel, and being already in motion the valve requires but very slight help to complete its stroke quickly without shock.
The motion of the auxiliary valve is synonymous with the main piston so there can be no practical dead center. The valves are very durable under such easy motion.
With this arrangement of valves no setting is necessarybecause when assembling the parts the centers of the pendulum lever and the toes on the rocker shaft are adjusted parallel with one another.