Fig. 275.
When the piston reaches the end of its stroke it lifts one of the tappet levers and with it the corresponding valve is raised from its seat, opening the port leading from the end of the steam chest to the main exhaust port. The pressure is thus relieved on one end of the valve piston and the steam pressing on the opposite end forces the valve piston to the opposite end of its stroke, thus reversing the distribution of steam to the cylinder and starting the piston on its return stroke. The valve piston thus moves back and forth, the ends of the steam chest being filled with steam at initial pressure. Steam escaping from one end of the steam chest causes a difference of pressure on the two ends of the valve piston, from which we realize the power to move the main valve. The tappet valves, having a very slight lift, operate without shock or noise. The main valve is connected with the valve piston so that all lost motion is taken up automatically.
A rocker shaft, extending through the steam chest carries a toe moving in a slot in the top of the valve piston, so that the valve can be moved by hand.
To set the valves of this pump. Simply keep the valves in order. The motion of the piston as it nears the end of the stroke opens and closes the valves.
THE KNOWLES.
In the Knowles pump a valve piston, G, [Fig. 277], in the steam chest moves the main valve. This valve piston is driven alternately backward and forward by the pressure of steam, carrying with it the main valve, which admits steam to the main steam piston that operates the pump. The main valve is a plain slide whose section is of B form, working on a flat seat.
Fig. 276.