HYDRAULIC TOOLS
Before discussing these in detail we shall do well to trace the history of the Bramah press, which may be said to be their parent, since the principle employed in most hydraulic devices for the workshop, as also the idea of using water as a means of transmitting power under pressure, are justly attributed to Joseph Bramah.
If you take a dive into the sea and fall flat on the surface instead of entering at the graceful angle you intended, you will feel for some time afterwards as if an enemy had slapped you violently on the chest and stomach. You have learnt by sad experience that water, which seems to offer so little resistance to a body drawn slowly through it, is remarkably hard if struck violently. In fact, if enclosed, it becomes more incompressible than steel, without in any way losing its fluidity. We possess in water, therefore, a very useful agent for transmitting energy from one point to another. Shove one end of a column of water, and it gives a push to anything at its other end; but then it must be enclosed in a tube to guide its operation.
By a natural law all fluids press evenly on every unit of a surface that confines them. You may put sand into a bucket with a bottom of cardboard and beat hard upon the surface of the sand without knocking out the bottom. The friction between the sand particles and the bucket's sides entirely absorbs the blow. But if water were substituted for sand and struck with an object that just fitted the bucket so as to prevent the escape of liquid, the bottom, and sides, too, would be ripped open. The writer of this book once fired a candle out of a gun at a hermetically sealed tin of water to see what the effect would be. (Another candle had already been fired through an iron plate 1 / 4 of an inch thick.) The impact slightly compressed the water in the tin, which gave back all the energy in a recoil which split the sheet metal open and flung portions of it many feet into the air. But the candle never got through the side.
This affords a very good idea of the almost absolute incompressibility of a liquid.
We may now return to history. Joseph Bramah was born in 1748 at Barnsley, in Yorkshire. As the son of a farm labourer his lot in life would probably have been to follow the plough had not an accident to his right ankle compelled him to earn his living in some other way. He therefore turned carpenter and developed such an aptitude for mechanics that we find him, when forty years old, manufacturing the locks with which his name is associated, and six years later experimenting with the hydraulic press. This may be described simply as a large cylinder in which works a solid piston of a diameter almost equal to that of the bore, connected to a force pump. Every stroke of the pump drives a little water into the cylinder, and as the water pressure is the same throughout, the total stress on the piston end is equal to that on the pump plunger multiplied by the number of times that the one exceeds the other in area. Suppose, then, that the plunger is one inch in diameter and the piston one foot, and that a man drives down the plunger with a force of 1,000 lbs., then the total pressure on the piston end will be 144 × 1,000 lbs.; but for every inch that the plunger has travelled the piston moves only 1 / 144 of an inch, thus illustrating the law that what is gained in time is lost in power, and vice versâ.
The great difficulty encountered by Bramah was the prevention of leakage between the piston and the cylinder walls. If he packed it so tightly that no water could pass, then the piston jammed; if the packing was eased, then the leak recommenced. Bramah tried all manner of expedients without success. At last his foreman, Henry Maudslay—already mentioned in connection with the lathe slide-rest—conceived an idea which showed real genius by reason of its very simplicity. Why not, he said, let the water itself give sufficient tightness to the packing, which must be a collar of stout leather with an inverted U-shaped section? This suggestion saved the situation. A recess was turned in the neck of the cylinder at the point formerly occupied by the stuffing-box, and into this the collar was set, the edges pointing downwards. When water entered under pressure it forced the edges in different directions, one against the piston, the other against the wall of the recess, with a degree of tightness proportioned to the pressure. As soon as the pressure was removed the collar collapsed, and allowed the piston to pass back into the cylinder without friction. A similar device, to turn to smaller things for a moment, is employed in a cycle tyre inflater, a cup-shaped leather being attached to the rear end of the piston to seal it during the pressure stroke, though acting as an inlet valve for the suction stroke.
What we owe to Joseph Bramah and Henry Maudslay for their joint invention—the honour must be divided, like that of designing the steam hammer between Nasmyth and Wilson—it would indeed be hard to estimate. Wherever steady but enormous effort is required for lifting huge girders, houses, ships; for forcing wheels off their axles; for elevators; for advancing the boring shield of a tunnel; for compressing hay, wool, cotton, wood, even metal; for riveting, bending, drilling steel plates—there you will find some modification of the hydraulic press useful, if not indispensable.
However, as we are now prepared for a consideration of details, we may return to our workshop, and see what water is doing there. Outside stands a cylindrical object many feet broad and high, which can move up and down in vertical guides. If you peep underneath, you notice the shining steel shaft which supports the entire weight of this tank or coffer filled with heavy articles—stones, scrap iron, etc. The shaft is the piston-plunger of a very long cylinder connected by pipes to pumping engines and hydraulic machines. It and the mass it bears up serves as a reservoir of energy. If the pumping engines were coupled up directly to the hydraulic tools, whenever a workman desired to use a press, drill, or stamp, as the case might be, he would have to send a signal to the engine-man to start the pumps, and another signal to tell him when to stop. This would lead to great waste of time, and a danger of injuring the tackle from over driving. But with an accumulator there is always a supply of water under pressure at command, for as soon as the ram is nearly down, the engines are automatically started to pump it up again. In short, the accumulator is to hydraulic machinery what their bag is to bagpipes, or the air reservoir to an organ.
In large towns high-pressure water is distributed through special mains by companies who make a business of supplying factories, engineering works, and other places where there is need for it, though not sufficient need to justify the occupiers in laying down special pumping plant. London can boast five central distributing stations, where engines of 6,500 h.p. are engaged in keeping nine large accumulators full to feed 120 miles of pipes varying in diameter from seven inches downwards. The pressure is 700 lbs. to the square inch. Liverpool has twenty-three miles of pipes under 850 lbs. pressure; Manchester seventeen miles under 1,100 lbs. To these may be added Glasgow, Hull, Birmingham, Geneva, Paris, Berlin, Antwerp, and many other large cities in both Europe and the United States.
For very special purposes, such as making metal forgings, pressures up to twelve tons to the square inch may be required. To produce this "intensifiers" are used, i.e. presses worked from the ordinary hydraulic mains which pump water into a cylinder of larger diameter connected with the forging press.
The largest English forging press is to be found in the Openshaw Works of Sir W. G. Armstrong, Whitworth, and Company. Its duty is to consolidate armour-plate ingots by squeezing, preparatory to their passing through the rolling mills. It has one huge ram 78 inches in diameter, into the cylinder of which water is pumped by engines of 4,000 h.p., under a pressure of 6,720 lbs. to the square inch, which gives a total ram force of 12,000 tons. It has a total height of 33 feet, is 22 feet wide, and 175 feet long, and weighs 1,280 tons. On each side of the anvil is a trench fitted with platforms and machinery for moving the ingot across the ingot block. Two 100-ton electric cranes with hydraulic lifting cylinders serve the press.
A HUGE HYDRAULIC PRESS
The 12,000-ton pressure Whitworth Hydraulic Press, used for consolidating steel ingots for armour-plating. Water is forced into the ram cylinder at a pressure of three tons to the square inch. Notice the man to the left of the press.
The Bethlehem Works "squeezer" has two rams, each of much smaller diameter than the Armstrong-Whitworth, but operated by a 10 1 / 2 tons pressure to the square inch. It handles ingots of over 120 tons weight for armour-plating. In 1895 Mr. William Corey, of Pittsburg, took out a patent for toughening nickel steel plates by subjecting them, while heated to a temperature of 2,000° F., to great compression, which elongates them only slightly, though reducing their thickness considerably. The heating of a large plate takes from ten to twenty hours; it is then ready to be placed between the jaws of the big press, which are about a foot wide. The plate is moved forward between the jaws after each stroke until the entire surface has been treated. At one stroke a 17-inch plate is reduced to 16 inches, and subsequent squeezings give it a final thickness of 14 inches. Its length has meanwhile increased from 16 to 18 1 / 2 feet, or in that proportion, while its breadth has remained practically unaltered. A simple sum shows that metal which originally occupied 32 2 / 3 cubic inches has now been compressed into 31 cubic inches. This alteration being effected without any injury to the surface, a plate very tough inside and very hard outside is made. The plate is next reheated to 1,350° F., and allowed to cool very gradually to a low temperature to "anneal" it. Then once again the furnaces are started to bring it back to 1,350°, when cold water is squirted all over the surface to give it a proper temper. If it bends and warps at all during this process, a slight reheating and a second treatment in the press restores its shape.
The hydraulic press is also used for bending or stamping plates in all manners of forms. You may see 8-inch steel slabs being quietly squeezed in a pair of huge dies till they have attained a semicircular shape, to fit them for the protection of a man-of-war's big-gun turret; or thinner stuff having its ends turned over to make a flange; or still slenderer metal stamped into the shape of a complete steel boat, as easily as the tinsmith stamps tartlet moulds. In another workshop a pair of massive jaws worked by water power are breaking up iron pigs into pieces suitable for the melting furnace.
The manufacture of munitions of war also calls for the aid of this powerful ally. Take the field-gun and its ammunition. "The gun itself is a steel barrel, hydraulically forged, and afterwards wire-wound; the carriage is built up of steel plates, flanged and shaped in hydraulic presses; the wheels have their naves composed of hydraulically flanged and corrugated steel discs, and even the tyres are forced on cold by hydraulic tyre-setters, the rams of which are powerful enough to reduce the diameter of the welded tyre until the latter tightly nips the wheel. The shells for the gun are punched and drawn by powerful hydraulic presses, and the copper driving-bands are fixed on the projectiles in special hydraulic presses. Quick-firing cartridge-cases are capped, drawn, and headed by an hydraulic press, whose huge mass always impresses the uninitiated as absurdly out of proportion to the small size of the finished case, and finally the cordite firing charge is dependent on hydraulic presses for its density and shape."[9]
The press for placing the "driving-band" on a shell is particularly interesting. After the shell has been shaped and its exterior turned smooth and true, a groove is cut round it near the rear end. Into this groove a band of copper is forced to prevent the leakage of gas from the firing charge past the shell, and also to bite the rifling which imparts a rotatory motion to the shell. The press for performing the operation has six cylinders and rams arranged spoke-wise inside a massive steel ring; the rams carrying concave heads which, when the full stroke is made, meet at the centre so as to form a complete circle. "Pressure is admitted," says Mr. Petch, "to the cylinders by copper pipes connected up to a circular distributing pipe. The press takes water from the 700-pounds main for the first 3 / 8 -inch of the stroke, and for the last 1 / 8 -inch water pressure at 3 tons per square inch is used. The total pressure on all the rams to band a 6-inch shell is only 600 tons, but for a 12-inch shell no less than 2,800 tons is necessary."