THE NATIONAL.
Fig. 268.
Fig. 268.
Fig. 269.
Fig. 269.
The main valve in this pump is moved by steam acting upon a valve piston. The steam admitted to and released from the auxiliary cylinder is controlled by a small slide valve operated by the valve gear. This small slide valve is in every essential feature identical with the slide valve of a steam engine, admitting and releasing the steam in precisely the same manner. In this pump the usual collars and tappets on the valve stem are dispensed with, the stem receiving motionby means of a roller carried by a slide block to which the valve stem is attached.
The roller is given a lateral motion by set screws in the forked ends of the rocker arm, as shown in the engraving,Fig. 269. By adjusting these set screws the travel of the small slide valve is made to suit the speed of the pump, thus preventing the main piston from striking the cylinder heads.
The roller is given a lateral motion by set screws in the forked ends of the rocker arm, as shown in the engraving,Fig. 269. By adjusting these set screws the travel of the small slide valve is made to suit the speed of the pump, thus preventing the main piston from striking the cylinder heads.
A sectional elevation of the cylinder and main valve is also shown; the connection of the valve piston to the main valve, also the construction of the small slide valve. The valve gear is provided with roller bearings to reduce the friction.
To set the valve.The main piston must be moved to the end of the stroke, so that the face of the piston and the edge of the steam port are flush, as a preliminary step to set the small slide valve, then the set screw in the rocker arm nearest the roller on the valve stem should be adjusted so that when the roller is in contact with the set screw the small slide valve will have opened the steam port corresponding to the position of the main piston.
Now push the main piston to a corresponding position at the opposite end of the stroke and adjust the other set screw nearest the roller on the valve stem in the same manner as the first one.
Mark the striking points of piston on the piston rod close to the stuffing-boxes. Start the pump slowly, and if the parts work smoothly, gradually increase the speed, keeping close watch of the striking points to see that the piston has ample clearance.
If the stroke is too short, the set screws should be backed out until the stroke is found to be right. Screwing the set screws in shortens the stroke and increases the clearance at the ends of the stroke. A full stroke should always be required of every pump.