In Figs. 153 and 154 are shown representations of certain apparatus,
long used in schools, to explain the rather obscure operation, of even
the simplest of pumps; these models are made of glass so that all the
movements of the valves, etc., may be clearly noted. Credit is due to
Monsieur Ganot, author of Elements of Physics, for the following.
_Fig. 153 represents a model of a suction-pump_ such as is used in
lectures, but which has essentially the same arrangement as the
pumps in common use. It consists, 1st, of a _glass cylinder_, B,
at the bottom of which is a valve, S, opening upwards; 2nd, of a
_suction-tube_, A, which dips into the reservoir from which water is
to be raised; 3rd, of a _piston_, which is moved up and down by a rod
worked by a handle, P. The piston has a hole in its center; this upper
aperture is closed by a valve, O opening upwards.
[Illustration: FIG. 153.]
When the piston rises from the bottom of the cylinder B, a vacuum is
produced below, and the valve O is kept closed by the atmospheric
pressure, while the air in the pipe A, in consequence of its
elasticity, raises the valve S, and part of it passes into the
cylinder. The air being thus rarefied, water rises in the pipe until
the pressure of the liquid column, together with the pressure of the
rarefied air which remains in the tube, counterbalances the pressure of
the atmosphere on the water in the reservoir.
When the piston descends, the valve S closes by its own weight, and
prevents the return of the air from the cylinder into the tube A. The
air compressed by the piston opens the valve O, and escapes into the
atmosphere by the pipe C. With a second stroke, the same series of
phenomena is produced, until after a few strokes the water reaches the
cylinder. The effect is now somewhat modified; during the descent of
the piston the valve S closes, and the water raises the valve O, and
passes above the piston by which it is lifted into the upper reservoir
D. There is now no more air in the pump, and the water forced by the
atmospheric pressure rises with the piston, provided that when it is at
the summit of its course it is not more than 34 feet above the level of
the water into which the tube A dips.
[Illustration: FIG. 154.]
_In practice the height of the tube A does not exceed 26 to 28 feet_;
for although the atmospheric pressure can support a higher column,
the vacuum produced in the barrel is not perfect, owing to the fact
that the piston does not fit exactly on the bottom of the barrel. But
when the water has passed the piston, it is the ascending force of
the latter which raises it, and the height to which it can be brought
depends on the power which works the piston.
The action of this pump, a model of which is represented in Fig. 154,
_depends both on exhaustion and on pressure_. At the base of the
barrel, where it is connected with the tube A, there is a valve, S,
which opens upwards. Another valve, O, opening in the same direction,
closes the aperture of a conduit, which discharges from a hole, _o_,
near the valve S, into a vessel, M, which is called the _air-chamber_.
From this chamber there is another tube, D, up which the water is
forced.
At each ascent of the piston B, which is solid, the water rises through
the tube A into the barrel. When the piston sinks the valve S closes,
the water is forced through the valve O into the reservoir M, and
thence into the tube D. The height to which it can be elevated in this
tube depends solely on the motive power which works the pump.
If the tube D were a prolongation of the tube J_ao_, the flow would
be intermittent; it would take place when the piston descended, and
would cease as soon as it ascended. But between these motions there is
an interval, which, by means of the air in the reservoir M, ensures
a continuous flow. The water forced into the reservoir M separates
into two parts, one of which, rising in D, presses on the water in
the reservoir by its weight; while the other, by virtue of this
pressure, rises in the reservoir above the lower orifice of the tube
D, compressing the air above. Consequently, when the piston ascends,
it no longer forces the water into M, the air of the reservoir, by the
pressure it has received, reacts on the liquid, and raises it in the
tube D, until the piston again descends, so that the jet is continuous.
_Hydraulic Machine Tools._ Probably in no department of engineering
has the use of hydraulic power met with more success than in its
application to certain machine tools. This success is owing to the
peculiar suitability of pressure—water as the motive agent for the
performance of a certain class of operations requiring the exertion of
a great force with comparative slow motion, as in punching, riveting,
forging and the like.
The wide spread and successful use of hydraulic machines—of which a few
only have been described and illustrated upon the pages of this book—is
due to the necessity for such tools and the inventive ability of our
tool designers.
_A large fixed hydraulic riveter_ is shown in Fig. below; it is capable
of exerting on the rivet a pressure of 40 tons or more; the machine has
a _working pressure_ of 1,500 pounds per square inch. Working pressures
of 5,000 to 10,000 pounds per square inch are used in _hydraulic
forging presses_, but in the riveter much less pressure is required.
[Illustration: Large fixed hydraulic riveter.]
NOTE.—_The proportions_ of this machine are immense. The platform
weighs 22,500 lbs. and is operated by a single lever shown in the
side view. The “_gap_” is 8 feet across. The machine has a large
steel “_stake_” carrying the stationary die; this is held in tension
strain by the two steel bolts shown, one upon each side of the
machine. The other part of the jaw is cast iron.
CLASSIFICATION
OF PUMPS
[Illustration: FIG. 155.—THE BELLOWS PUMP.]