PUMPS AS HYDRAULIC APPARATUS.
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-pumpsuch as is used in lectures, but which has essentially the same arrangement as the pumps in common use. It consists, 1st, of aglass cylinder, B, at the bottom of which is a valve, S, opening upwards; 2nd, of asuction-tube, A, which dips into the reservoir from which water is to be raised; 3rd, of apiston, 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.
Fig. 153.
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.
Fig. 154.
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 theair-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 Jao, 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 riveteris shown in Fig. below; it is capable of exerting on the rivet a pressure of 40 tons or more; the machine has aworking pressureof 1,500 pounds per square inch. Working pressures of 5,000 to 10,000 pounds per square inch are used inhydraulic forging presses, but in the riveter much less pressure is required.
large fixed hydraulic riveter
Note.—The proportionsof 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.
Note.—The proportionsof 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.