The Dean Bros. pump is shown in Figs. 295, 296 and 297.
[Illustration: FIG. 295.]
The auxiliary valve, A, Fig. 297, has in its face two diagonal exhaust
cavities, B B_{1}. The ports, C C_{1}, and the exhaust port, D, are
placed in a triangular position with one another, the diagonal cavities
diverging so that the cavity B, when the valve is in place, connects
the ports, C_{1} and D. Cavity B_{1} connects the ports C and D, when
the valve, A, is at the end of the stroke. The three small cuts show
relation of auxiliary valve to ports.
[Illustration: FIG. 296.]
The piston starts from left to right when the valve, A, moves in an
opposite direction, opens the port, C, admitting steam to the auxiliary
cylinder at the moment the main piston has reached the end of its
stroke. The auxiliary piston, E, is forced to the left, opening the
main port and admitting steam to the main cylinder, reversing the
movement of the main piston the return stroke of the main piston
reverses the movement of the auxiliary valve, whereby the port, C, is
closed, at the moment the main piston reaches the end of its outer
stroke. The port, C_{1}, is opened by the valve, A, and reverses the
valve piston, E, opens the main port and reverses the motion of the
main piston.
[Illustration: FIG. 297.]
This port arrangement admits of a short valve with a long travel. The
stroke of the pump can be regulated by moving the stud up or down in
the segmental slot, Fig. 296, which varies the travel of the auxiliary
valve and reverses the stroke of the main piston as desired. By raising
the stud the pump will make shorter strokes, and by lowering it will
make longer strokes.
The motion of the auxiliary steam slide valve is like that derived
from an eccentric. The ports leading to the valve piston, E, Fig. 297,
remain closed, except at the moment the main piston is reversed; hence,
there can be no waste of steam from leaks when the valve piston becomes
worn.
Having a long stroke and a rapid motion, the auxiliary valve insures a
certain reversal of the piston at the proper time.
_To set the valve_, turn the steam chest upside down. Put valve stem
through the stuffing-box and secure in place the clamp for small
slide valve. The diameter of valve stem is smaller where the clamp is
attached.
Now screw up the stuffing-box nut (having previously removed the
packing), then move the valve and stem so that the small port at right
of valve will be open 1/16 inch and make a scratch upon the stem close
to stuffing-box nut. The valve should then be moved in the opposite
direction to open the other small port 1/16 inch and make a second
scratch upon the valve stem next to stuffing-box nut. Prepare joint
and replace steam chest on cylinder. To square the valve, slacken the
screw in crosshead and move the latter to the end of stroke with edge
of crosshead flush with the end of guide, then set the valve stem so
that the first scratch is flush with the face of nut, same as when the
scratch was made. Tighten screw in set screw under valve rod dog and
move the crosshead to the opposite end of stroke, and note the position
of second scratch. If it does not come to the position in which it was
made, _split the difference by slackening the set screw under valve rod
dog and move the valve rod to equalize the travel of valve_.
In replacing steam chest on cylinder, cover the opening with a thin
board, or piece of sheet iron, before turning it over to prevent the
valve from dropping out of place.
This slide valve has a fixed travel and _the process of setting is
precisely the same as for that upon a steam engine with a plain slide
valve_.