The Davidson pump is shown complete in Fig. 248; the valve motion
consists principally of _a valve, valve pistons, valve pin and cam_.
The main valve is operated by a positive mechanical connection between
it and the main piston rod, also by the action of steam on the valve
pistons. The engraving, Fig. 249, shows the details of valve gear and
steam cylinder. The steam end consists of the cylinder, M, valve, A,
and valve pistons, B and B. These pistons are connected with sufficient
space between them for the valve, A, covering the steam ports, F and
F_{1}, Fig. 250.
[Illustration: FIG. 248.]
The valve is operated by the steel cam, C, acting on a steel pin, D,
which passes through the valve into the exhaust port, N, in which
the cam is located. In addition to this positive motion steam is
alternately admitted to and exhausted from the ends of the valve piston
through the ports, E and E_{1}, which moves the pistons, B and B_{1}.
[Illustration: FIG. 249.]
Assumed that this pump is at rest with the valve, A, covering the
main steam ports, F and F_{1}, in which position the cam C, holds
the main valve by means of the valve pin, D, so that ports, E and
E_{1}, admit steam to one end of the valve piston at the same time
connects the other end with the exhaust port. The steam, acting on
the valve pistons, moves both, opening the main ports, F and F_{1},
admitting steam to one end of the steam cylinder and opening the other
end to the exhaust. If the valve occupies any other position than the
one described, the main ports, F and F_{1}, will be opened for the
admission and exhaust of steam; consequently it is evident that this
pump will start from all points of the stroke.
[Illustration: FIG. 250.]
On the admission of steam to the cylinder the main port, F, the main
piston, cam and valve will move in the direction indicated by the
arrows. The first movement of the cam oscillates the valve, preparatory
to bringing it into a proper position for the opening of the auxiliary
steam ports, E, to live steam, and E, to exhaust also to close the
valve mechanically just before the main piston reaches the end of
its stroke. This causes a slight cut-off and compression, and fully
opens the auxiliary ports, E, to steam, and E_{1}, to exhaust. By the
admission of steam to one end, the other being open to the exhaust,
the valve pistons move the valve to allow the admission and exhaust of
steam from the cylinder for the return stroke.
This main valve is as much under the control of the piston rod as is
the valve of an ordinary steam engine worked by an eccentric which
insures a positive action, the pump being capable of starting from all
positions and maintaining a uniform and full stroke.
_To set the valve piston_, push the main pistons to the end of the
stroke until the inner edge of the port and the piston coincide, then
loosen the side lever, turn the cam, C, until the valve piston uncovers
the auxiliary steam port, E, leading to the same end of the steam chest
occupied by the main piston.
[Illustration: FIG. 251.]
After setting, secure the cam and then connect the side lever to
the connecting rod. The side lever and cam occupy correct relative
positions, therefore, the lever should be secured to the cam shaft
while in this position. The stroke may be regulated by raising or
lowering the end of the connecting rod in the slotted end of the slide
lever. Raising the connecting rod shortens the stroke and lowering it
lengthens the stroke. When making the foregoing adjustments it is well
to have the connecting rod at or near the bottom of the slot as shown
in the engravings.