The auxiliary valve of this pump is a plain flat slide operated by a
valve stem, the latter being moved back and forth by means of a rocker
shaft, as shown in the engraving, the upper end of which alternately
comes in contact with the collars on the stem.
The outer end of the valve stem passes through a sleeve attached to
a pin in the upper end of the rocker arm, as shown. A knuckle joint
near the stuffing-box permits the rod to vibrate without causing any
derangement in the alignment of valve stem through the stuffing-boxes.
On the valve stem at either end of the auxiliary valve is a spring,
which tends to keep the valve in a central position, so that when the
rocker arm engages one of the collars, the valve is drawn against the
spring toward that end of the stroke. The result is that the stem and
valve follow the rocker arm on the return stroke to its mid-position,
and are started on the latter half of the stroke by the stem, but
without shock or lost motion. This arrangement is particularly valuable
in the case of condensers, and in pumps where the first part of the
stroke is made quickly, and the piston is then suddenly stopped by
coming in contact with a solid body of water, the latter part of the
stroke being made much more slowly. The springs on either side of the
auxiliary valve take up lost motion and keep the parts in absolute
contact, thus preventing shocks and unnecessary wear.
[Illustration: FIG. 281.]
[Illustration: FIG. 282.]
The auxiliary valve controls the admission and exhaust of steam for the
steam chest and valve piston in the manner common to all slide valve
engines. The valve piston is connected to the main valve, which allows
the valve to find its own bearings on the seat and not only takes up
the wear automatically, but produces even wear.
_To set the auxiliary valve_, see that the valve is in its central
position when the rocker arm is plumb, and that the collars on the
valve stem are located at equal distances from each end of the sleeve.
When the piston moves to one end of the stroke, the auxiliary valve
will open the small port at the opposite end, provided the collars on
the valve stem have been properly placed. Setting the collars closer
together shortens the stroke of the piston, and moving them farther
apart lengthens the stroke. The piston should always make a full stroke
without danger of striking the cylinder heads.