[Illustration: FIG. 268.]
[Illustration: 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 motion by
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.
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.