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.
[Illustration: FIG. 276.]
[Illustration: FIG. 277.]
The valve piston is slightly rotated back and forth by the rocker bar,
H; this rotative movement places the small steam ports, D E F, Fig.
276, which are located in the under side of the valve piston in proper
position with reference to the corresponding ports, A B, cut in the
steam chest. Steam enters through the port at one end and fills the
space between the valve piston and the head, drives the valve piston
to the end of its stroke and carries the main slide valve with it.
When the valve piston has traveled a certain distance, a corresponding
port in the opposite end is uncovered and steam enters, stopping its
progress by giving it the necessary cushion. In other words, when the
reciprocating rotative motion is given to the valve piston through the
outside mechanism, it opens the port to steam admission on one end, and
at the same time opens the port on the other end to the exhaust. There
is no point in the stroke at which either the valve piston or the main
piston is not open to direct steam pressure, hence the immunity from
any dead position or dead center.
The operation of the pump is as follows: The piston rod with its tappet
arm, J, Fig. 277, moves backward and forward with the piston. At the
lower part of this tappet arm is attached a stud or bolt, K, on which
is a friction roller, I. This friction roller, lowered or raised,
adjusts the pump for a longer or shorter stroke. This roller coming in
contact with the rocker bar at the end of each stroke, and this motion
is transmitted to the valve stem, causing the valve to roll slightly.
This action opens the ports, admits steam and moves the valve piston,
which carries with it the main slide valve which admits steam to the
main piston. The upper end of the tappet arm does not come in contact
with the tappets, L M, on the valve rod, unless the steam pressure
from any cause should fail to move the valve piston, in which case the
tappet arm moves it mechanically.
_To set the valve_, loosen the set screws in the tappets on the valve
stem. Then place the piston at mid-stroke, and have the rocker bar, H,
in a horizontal position, as shown in the engraving. The valve piston
should then occupy the position shown at C, Fig. 276. The valve piston
may be rotated slightly in order to obtain this position by adjusting
the length of connection between the rocker bar, H, and the valve stem.
Then turn the valve piston, G, one way or the other to its extreme
position, put on the chest cover, and start the pump slowly.
If the pump should make a longer stroke on one end than on the other,
simply lengthen or shorten the rocker connection so that the rocker
bar, H, will touch the rocker roller, I, equally distant from the
center pin, J.
In case the pump hesitates in making its return stroke, it is because
the rocker roller, I, is too low and does not come in contact with the
rocker bar, H, soon enough. To raise it, take out the rocker roller
stud, K, give the set screw in this stud a sufficient downward turn,
and the stud with its roller may at once be raised to its proper height.
[Illustration: FIG. 278.]
When the valve rod has a tendency to tremble, slightly tighten the
valve rod stuffing-box nut. When the valve motion is properly adjusted,
the vertical arm should not quite touch the collar, L, and the clamp,
M. Rocker roller, I, coming in contact with the rocker bar, H, reverses
the stroke.
The water piston is packed by means of segments that will be seen by
taking off the follower. The packing may be quickly set out to insure
a good vacuum and at the same time not to be so tight as to bind. All
wearing parts are made adjustable.