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THE HYDRAULIC PRESS.

The difficulty of keeping the pistons tight against the leakage of the liquid prevented the practical application of Pascal's invention, until Bramah (in 1796) replaced the pistons by plungers (fig. 8) and made a water-tight joint by his invention of the cupped collar CC, pressed into U shape in cross section from an annular sheet of leather, which effectually prevents the escape of the water.

The applied thrust P can be applied, directly or by a lever, to the plunger of a force pump, provided with a stuffing box, the invention of Sir Samuel Morland, 1675; and then repeated strokes of the pump will cause the thrust W exerted by the head of the ram to act through any required distance.

In some portable forms, required for instance for punching or rail bending, the pressure is produced and kept up by a plunger P which advances on a screw thread.

For testing gauges Messrs. Schaffer and Budenberg employ an instrument consisting of a small ram working in a horizontal barrel full of water, the traverse of the ram being effected by its revolution in a screw. The gauge to be tested or graduated and the standard gauge are attached to the barrel and each registers the pressure of the water. The machine can even be used for testing vacuum gauges by turning the ram the reverse way, so as to diminish the pressure of the water below the atmospheric pressure.

The Hydrostatic Bellows was devised by Pascal as a mere lecture experiment to illustrate his Principle of the Transmissibility of Pressure; the large cylinder in fig. 7 is replaced by leather fastened to W, as in bellows, while the small cylinder is prolonged upwards by a pipe to a certain vertical height; and the thrust P is produced by

THE HYDRAULIC PRESS.

19

the head of water poured in at the top of the pipe by a man standing on a ladder. In this way a small quantity of water poured in the pipe is shown lifting a considerable weight W supported by the bellows, and leakage is

C

Fig. 8.

avoided. For a diagram consult Ganot's Physique; the instrument is of no practical use, except for Nasmyth's attempt to replace the Hydraulic Press by his patent Steel Mattress (Engineer, 23 May, 1890, p. 426).

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ENERGY DUE TO PRESSURE.

13. The Principle of Virtual Velocities.

Pascal's Principle of the Transmissibility of Pressure was applied by him to verify the Principle of Virtual Velocities in the case of an incompressible liquid, thus showing that a liquid can be made to take the place of a complicated system of levers, in transmitting and multiplying a thrust.

For taking a closed vessel, filled with incompressible liquid, and fitted with cylindrical openings closed by pistons, of areas A, B, C,... ft2; then if the pistons move inwards through distances a, b, c, ... feet respectively, the condition that the volume of liquid is unchanged requires Aa+Bb+Cc+...=0,

that

some of the quantities a, b, c, ... being positive and some negative.

But if P, Q, R,... denote the thrust in lb on the pistons, then P/A=Q/B=R/C=...

=

= the uniform pressure in lb/ft2 of the liquid,

and therefore

Pa+Qb+Re+...=0,

a verification of the Principle of Virtual Velocities.

14. The Energy of Liquid due to Pressure.

We have supposed the fluid employed to be incompressible liquid: for if a compressible gas had been used to transmit power, part of the energy would be used up in compressing the gas, if used to transmit power; so that a gas would behave like a machine composed of elastic levers.

But with an incompressible liquid the energy is entirely due to the pressure; and if the pressure is p lb/ft2, the energy of the liquid is p ft-lb per cubic foot (or p ft-lb/ft3).

THE HYDROSTATIC PARADOX.

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As a practical illustration of Pascal's Principle, applied to a closed vessel and a number of pistons, the Hydraulic Power Company of London supply water in mains for the purpose of lifts and domestic motors, at a pressure of 750 lb/in2, or 108000 lb/ft2, equivalent to an artificial head of 1728 ft, if a cubic foot of water is taken as weighing 1000 oz or 62.5 lb.

This gives an energy of 1728 ft-lb per lb of water, or 17,280 ft-lb per gallon of 10 lb; so that if water at this pressure is used at the rate of 2 gallons per minute, it furnishes energy at the rate of 34,560 ft-lb per minute, say one horse-power of 33,000 ft-lb per minute, allowing for friction in the pipe, estimated at a velocity of 5 f/s. With a consumption of 4 million gallons, or 640,000 cubic feet per 24 hours, this gives

640,000 × 108,000=6.912 x 1010

ft-lb per 24 hours, equivalent to nearly 1500 H.P.

15. The Hydrostatic Paradox.

The fact that a thrust of P lb exerted on a piston of area A ft2, fitting into a vessel filled with incompressible liquid, produces a pressure p=P/A lb/ft2 throughout the liquid and an energy of

p ft-lb/ft3, or pv ft-lb in v ft3

was considered paradoxical by early writers on Hydrostatics; and numerous experiments, similar in principle to the Hydraulic Press, were devised to exhibit this so-called HYDROSTATIC PARADOX (Hon. R. Boyle, Hydrostatical paradoxes made out of new experiments, for the most part physical and easy, 1666); and at the present time the Keeley Motor in America is a paradoxical instrument, devised with the intention of utilizing the hydrostatic energy of pressure.

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THE ACCUMULATOR.

But this hydrostatic energy is unavailable for continued use, unless replenished as fast as it is used, as by the force pump with the Hydraulic Press; or unless the energy is stored up by the ACCUMULATOR (figs. 9, 10), which consists of a vertical piston or ram B, loaded with weights W so as to produce the requisite pressure p.

up

in the Accumu

Then ρυ ft-lb of energy are stored lator when the ram is raised so as to displace v ft3 less of water; that is, if the ram is raised v/A ft, where A is the cross section of the ram in ft2.

In fig. 10, Mr. Tweddell's form of Accumulator, the area A must be taken as the horizontal cross section of the shoulder at DD.

The Accumulator thus acts as the flywheel of Hydraulic Power; so that an engine working continually and storing up unused energy in the Accumulator can replace a larger engine working only occasionally.