THE HYDRAULIC PRESS.

16. The Applications of the Hydraulic Press.

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Hydraulic Power is now used to a great extent on steamers, for hoisting, steering, and working the guns; an Accumulator however cannot well be carried afloat, on account of its great weight. On land Hydraulic Power is extensively used for cranes and lifts; also on a large scale to replace steam hammers for forging steel by steady squeezing into shape, when a thrust up to 4000 tons is required, and on canals for locks.

On a small scale the Hydraulic Press is useful when applied to jacks, for lifting (fig. 11) or pulling (fig. 12), as manufactured by Tangyes of Birmingham; one great advantage of the machine being that the motion in either direction can be so easily controlled. The Bramah collar in these presses is seen to be replaced by a cupped piece of leather, pressed into shape from a circular sheet.

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THE HYDRAULIC PRESS.

An application to a certain form of weighing machine (Duckham's) may be mentioned here, consisting of a combination of a Bourdon Gauge and of a small Hydraulic Press, suspended from the chain of a crane (fig. 13).

The pressure of the water in the Press is read off on the Bourdon Gauge, graduated so as to show the weight of the body suspended from the ram of the Press.

In fig. 8 the Hydraulic Press is shown as employed for making elongated rifle bullets or lead pipe; a cylindrical wire or tube of lead, in a semi-molten state, is squeezed out through the hole in the fixed plunger D, which fills up the cavity in the ram of the Hydraulic Press, as the ram rises; the length of wire or pipe formed will be to the length of the stroke of the ram as the cross section of the lead cavity to the cross section of the wire or pipe.

The hydrostatic pressure in the molten lead is intensified over the pressure in the water of the Press in the ratio of the cross section of the ram to the cross section of the lead cavity.

THE AMAGAT GAUGE.

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"Pressure Intensifying Apparatus" for other purposes, such as rivetting and pressing cotton, applied in a similar manner to the Hydraulic Press, is described by R. H. Tweddell in the Proc. Inst. Mech. Engineers, 1872, 1878.

For a general description of the applications of Hydraulic Power and of the Hydraulic Press, the reader is referred to the treatises on Hydraulic Machinery by Prof. Robinson and Mr. F. Colyer.

17. The Amagat Gauge.

In this gauge (fig. 14) devised for measuring great pressures, the principle is the reverse of the Hydraulic Press, or Pressure Intensifying Apparatus; in the Amagat Gauge (manomètre à pistons libres) “unequal pressures act on unequal areas, producing equal thrusts"; so that a pressure p(lb/in2) acting over an area a = d2 in2 is measured by a balancing pressure q acting over an area A=D2; and then pa=qA, or p=qA/a=qD2/d2.

(Nature, 21 Feb., 1890; Challenger Scientific Reports on the Compressibility of Water, by Prof. P. G. Tait.)

To allow for the friction in this gauge, or generally in a Hydraulic Press, suppose the collar is h inches high, and that μ is the coefficient of friction between the leather and the metal; then for a pressure of q lb/in2 the total normal thrust between the leather and the metal is Dhq, and the frictional resistance to motion is

μπDhq pounds.

This gauge might be usefully employed either to test or even to replace the Crusher gauges used in artillery for measuring powder pressures; a mechanical fit of the pistons, if made long and provided with cannelures, is found to offer a sufficient frictional resistance to the leakage of the fluid, so that cupped leather or packing may be dispensed with.

(1) What must be the diameter of a safety valve, the

weight at the end of the lever being 60 lb, and its distance from the fulcrum 30 in, the weight of the lever 7 lb and its c.G. at 16 in from the fulcrum, the weight of the valve 3 lb and its c.G. at 3 in from the fulcrum, for the valve to blow off at 70 lb/in2?

Find also the leverage of the weight to allow the steam to blow off at 50 lb/in2.

(2) In a hydraulic press a thrust of 20 lb is applied at the end of a lever at 6 ft from the fulcrum, actuating the plunger of the force-pump which moves in a line 1 ft from the fulcrum; the plunger is in diameter 1 in and the ram is 10 in; find the thrust in tons exerted by the ram.

(3) The plunger of a force pump is 10 in (83) diameter, the length of the stroke is 42 in (30), and the pressure of the water acted upon is 50 lb/in2. Find the number of ft-lb and ft-tons of work performed in each stroke.

(4) The ram of a hydraulic accumulator is 10 ins in diameter, determine the load in tons requisite for a pressure of 700 lb/in2.

Find the fall in the ram in 1 minute, if water is not being supplied, and the water is working an engine of 9 H.P.

(5) Give sketches and describe the construction of a Estimate the volume of ram necessary if a weight of 5 tons is to be lifted

hydraulic crane.

20 ft, the water pressure being 700 lb/in2 and efficiency of machine.

SURFACES OF EQUAL PRESSURE.

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(6) The hydraulic lifts used in the construction of the Forth Bridge had a diameter of 14 inches and a range of 12 inches; the water was supplied at a pressure of 35 cwt/in2, and the lift took 5 hours. Find the lifting force of the ram and its rate of working in terms of a horse-power.

(7) Prove that in consequence of the friction of the collar, the efficiency of the hydraulic press is reduced to

1-4un,

where n denotes the ratio of the height of the collar to its diameter.

18. THEOREM. "In a fluid at rest under gravity, the pressure at any two points in the same horizontal plane is the same; in other words, the surfaces of equal pressure are horizontal planes."

Suppose A and B are any two points in the same horizontal plane; draw two horizontal planes a short distance apart, one above and the other below AB; and consider the equilibrium of the stratum of fluid between these horizontal planes (fig. 15).

Draw the two vertical planes through A and B perpendicular to AB, and two vertical planes parallel to AB on each side of AB a short distance apart; and consider the equilibrium of the prism of the fluid stratum cut out by these vertical planes.

The fluid pressures being normal to the faces of the prism, and the weight acting vertically downwards, the conditions of equilibrium require the thrusts on the faces perpendicular to AB to be equal; and the faces also being equal, the pressures at A and B are equal.

A similar proof holds when the prism is replaced by any thin cylinder on AB as axis, with ends at A and B perpendicular to the axis AB.