parts, the hydraulic press is used for squeezing the juices from
vegetable substances, such as beetroots, &c., for pressing oils from
seeds, and, in fact, all purposes where a powerful, steady, and easily
regulated pressure is needed. Cannons and steam boilers are tested by
hydraulic pressure, by forcing water into them by means of a force-pump,
just as it is forced into the cylinder of the hydraulic press described
above. This mode of testing the strength has several great advantages;
for not only can the pressure be regulated and its amount accurately
known; but in case the cannon or steam boiler should give way, there is
no danger, for it does not explode—the metal is simply ruptured, and the
moment this takes place, the water flows out and the strain at once
ceases.
The strength of bars, chains, cables, and anchors is also tested by
hydraulic power, and the engraving at the head of this article, Fig.
163, represents the hydraulic testing machine at the works of Messrs.
Brown and Lenox, the eminent chain and anchor manufacturers, of
Millwall. Immediately in front of the spectator are the force-pumps, and
the steam engine by which they are driven. It will be observed that four
plungers are attached to an oscillating beam in such a manner that the
water is continuously forced into the hydraulic cylinder. The outer pair
of plungers are of much larger diameter than the inner pair, in order
that the supply of water may be cut off from the former when the
pressure is approaching the desired limit, and the smaller pair alone
then go on pumping in the water, the pressure being thus more gradually
increased. Behind the engine and forcing pump is the massive iron
cylinder, where the pressure is made to act on a piston, which is forced
towards that end of the cylinder seen in the drawing. The piston is
attached to a very thick piston-rod, moving through a water-tight collar
at the other end of the cylinder. The effect of the hydraulic pressure
is, therefore, to draw the piston-rod into the cylinder, and not, as in
the apparatus represented in Fig. 166, to force a plunger out. The head
of the piston-rod is provided with a strong shackle, to which the chains
to be tested can be attached. In a line with the axis of the cylinder is
a trough, some 90 ft. long, to hold the chain, and at the farther end of
the trough is another very strong shackle, to which the other end of the
chain is made fast. A peculiarity of Messrs. Brown and Lenox’s machine
is the mode in which the tension is measured. In many cases it is deemed
sufficient to ascertain by some kind of gauge the pressure of the water
in the hydraulic cylinder, and from that to deduce the pull upon the
chain; but the Messrs. Brown have found that every form of gauge is
liable to give fallacious indications, from variations of temperature
and other circumstances, and they prefer to measure the strain directly.
This is accomplished by attaching the shackle at the farther extremity
of the trough to the short arm of a lever, turning upon hard steel
bearings, the long arm of this lever acting upon the short arm of
another, and so on until the weight of 1 lb. at the end of the last
lever will balance a pull on the chain of 2,240 lbs., or 1 ton. The
tension is thus directly measured by a system of levers, exactly
resembling those used in a common weighing machine, and this is done so
accurately that even when a chain is being subjected to a strain of many
tons, an additional pull, such as one can give to the shackle-link with
one hand, at once shows itself in the weighing-room. The person who has
charge of this part of the machine places on the end of the lever a
weight of as many pounds as the number of tons strain to which the chain
to be tested has to be submitted. The engineer sets the pump in action,
the water is rapidly forced into the cylinder, the piston is thrust
inwards, and the strain upon the chain begins; the engineer then cuts
off the water supply from the larger force-pumps, and the smaller pair
go on until the strain becomes sufficient to raise the weight, and then
the person in the weighing-room, by pulling a wire, opens a valve in
connection with the hydraulic cylinder, which allows the water to
escape, and the strain is at once taken off. This testing machine, which
is capable of testing cables up to 200 tons or more, was originally
designed by Sir T. Brown, the late head of the firm, and not only was
the first constructed in the country, but remains unsurpassed in the
precision of its indications.
The testing of cables, which we have just described, is a matter of the
highest importance, for the failure of cables and anchors places ships
and men’s life in great danger, since vessels have frequently to ride
out a storm at anchor, and should the cables give way, a ship would then
be almost entirely at the mercy of the winds and waves. Hence the
Government have, with regard to cables and anchors, very properly made
certain stringent regulations, which apply not only to the navy but to
merchant shipping. The chain-cable is itself a comparatively modern
application of iron, for sixty years ago our line-of-battle ships
carried only huge hempen cables of some 8 in. or 9 in. diameter.
Chain-cables have now almost entirely superseded ropes, though some
ships carry a hempen cable, for use under peculiar circumstances. The
largest chain-cables have links in which the iron has a diameter of
nearly 3 in., and these cables are considered good and sound when they
can bear a strain of 136 tons. Such are the cables used in the British
navy for the largest ships. Of course, there are many smaller-sized
cables also in use, and the strains to which these are subjected when
they are tested in the Government dockyards vary according to the
thickness of the iron; but it is found that nearly one out of every four
cables supplied to the Admiralty proves defective in some part, which
has to be replaced by a sounder piece. The chain-cables made by Messrs.
Brown and Lenox for the _Great Eastern_ are, as might have been
expected, of the very stoutest construction; the best workmanship and
the finest quality of iron having been employed in their manufacture.
These cables were tested up to 148 tons, a greater strain than had ever
before been applied as a test to any chain, and it was found that a pull
represented by at least 172 tons was required to break them. It is
difficult to believe that a teacup-full of cold water shoved down a
narrow pipe is able to rend asunder the massive links which more than
suffice to hold the huge ship securely to her anchors, but such is
nevertheless the sober fact. The regulations of the Board of Trade
require that every cable or anchor sold for use in merchant ships is to
be previously tested by an authorized and licensed tester, who, if he
finds it bears the proper strain, stamps upon it a certain mark.
The means which is afforded by hydraulic power of applying enormous
pressures has been taken advantage of in a great many of the arts, of
which, indeed, there are few that have not, directly or indirectly,
benefited by this mode of modifying force. An illustration, taken at
random, may be found in the machinery employed at Woolwich for making
elongated rifle-bullets. The bullets are formed by forcing into dies,
which give the required shape, little cylinders of solid lead, cut off
by the machine itself from a continuous cylindrical rod of the metal.
The rod, or rather filament, of lead is wound like a rope on large
reels, from which it is fed to the machine. It is in the production of
this solid leaden rope or filament that hydraulic pressure is used.
About 4 cwt. of melted lead is poured into a very massive iron cylinder,
the inside of which has a diameter of 7½ in., while the external
diameter is no less than 2 ft. 6 in., so that the sides of the cylinder
are actually 11¼ in. thick. When the lead has cooled so far as that it
has passed into a half solid state, a ram or plunger, accurately fitting
the bore of the cylinder, is forced down by hydraulic pressure upon the
semi-fluid metal. This plunger is provided with a round hole throughout
its entire length, and as it is urged against the half solidified metal
with enormous pressure, the lead yields, and is forced out through the
hole in the plunger, making its appearance at the top as a continuous
cylindrical filament, quite solid, but still hot. This is wound upon the
large iron reels as fast as it emerges from the opening in the plunger,
and these reels are then taken to the bullet-shaping machine, which
snips off length after length of the leaden cord, and fashions it into
bullets for the Martini-Henry rifle. The leaden pipes which are so much
used for conveying water and gas in houses are made in a similar manner,
metal being forced out of an annular opening, which is formed by putting
an iron rod, having its diameter of the required bore of the pipe, in
the middle of the circular opening. The lead in escaping between the rod
and the sides of the opening takes the form of a pipe, and is wound upon
large iron reels, as in the former case.
[Illustration:
FIG. 167.—_Section of Hydraulic Lift Graving Dock._
]
[Illustration:
FIG. 168.—_Section of Column._
]
Another interesting application of hydraulic power is to the raising of
ships vertically out of the water, in order to examine the bottoms of
their hulls, and effect any necessary repairs. The hydraulic lift
graving dock, in which this is done, is the invention of Mr. E. Clark,
who, under the direction of Mr. Robert Stephenson, designed the
machinery and superintended the raising of the tubes of the Britannia
Bridge, where a weight of 1,800 tons was lifted by only three presses.
The suitability of the hydraulic press for such work as slowly raising a
vessel was doubtless suggested to him in connection with this
circumstance, and the durability, economy, and small loss of power which
occurs in the action of the press, pointed it out as particularly
adapted for this purpose. The ordinary dry dock is simply an excavation,
lined with timber or masonry, from which the tide is excluded by a gate,
which, after the vessel has entered the dock at high water, is closed;
and when the tide has ebbed, and left the vessel dry, the sluice through
which the water has escaped is also closed. In a tideless harbour the
water has to be pumped out of the dock, and this last method is also
adopted even in tidal waters, so that the docks may be independent of
the state of the tides. The lift of Clark’s graving dock is a direct
application of the power of the hydraulic press, and we select for
description the graving dock constructed at the Victoria Docks for the
Thames Graving Dock Company, whose works occupy 26 acres. Fig. 167 is a
transverse section of this hydraulic lift graving dock, in which there
are two rows of cast iron columns, 5 ft. in diameter at the base, where
they are sunk 12 ft. in the ground, and 4 ft. in diameter above the
ground. The clear distance between the two rows is 60 ft., and the
columns are placed 20 ft. apart from centre to centre, sixteen columns
in each row, thus giving a length of 310 ft. to the platform, but
vessels of 350 ft. in length may practically be lifted. The bases of the
columns, one of which is represented in section in Fig. 168, are filled
with concrete, on which the feet of the hydraulic cylinders rest. The
outer columns support no weight, but act merely as guides for the
crossheads attached to the plungers. The height of the columns is 68½
ft., and a wrought iron framed platform connects the columns at the top.
In order that any inequalities in the height of the rams may be
detected, a scale is painted on each column, to mark the positions of
the crossheads. The hydraulic cylinders, which are within these columns,
have solid rams of 10 in. diameter, with a stroke of 25 ft., and on the
tops of these are fastened the crossheads, 7½ ft. long, made of wrought
iron, and supporting at the ends bars of iron, to the other ends of
which the girders of the platform are suspended. The girders are,
therefore, sixteen in number, and together form a gridiron platform,
which can be raised or lowered with the vessel upon it. The thirty-two
hydraulic cylinders were tested at a pressure of more than 3 tons per
square inch. The water is admitted immediately beneath the collars at
the top (this being the most accessible position) by pipes of only ½ in.
diameter, leading from the force-pumps, of which there are twelve, of 1⅞
in. diameter, directly worked by a fifty horse-power steam engine. The
presses are worked in three groups—one of sixteen, and two of eight
presses,—so arranged that their centres of action form a sort of tripod
support, and the presses of each group are so connected that perfect
uniformity of pressure is maintained. The raising of a vessel is
accomplished in about twenty-five minutes, by placing below the vessel a
pontoon, filled in the first instance with water, and then raising the
pontoon with the vessel on it, while the water is allowed to escape from
the pontoon through certain valves; then when the girders are again
lowered, the pontoon, with the vessel on it, remains afloat. Thus in
thirty minutes a ship drawing, say, 18 ft. of water is lifted on a
shallow pontoon, drawing, perhaps, only 5 ft., and the whole is floated
to a shallow dock, where, surrounded with workshops, the vessel, now
high and dry, is ready to receive the necessary repairs. The number of
vessels which can thus be docked is limited only by the number of
pontoons, and thus the same lift serves to raise and lower any number of
ships, which are floated on and off its platform by the pontoons. With a
pressure in the hydraulic cylinders of about 2 tons upon each square
inch, the combined action of these thirty-two presses would raise a ship
weighing 5,000 tons.
Hydraulic power has been used not only for graving docks, as shown in
the above figures, but also for dragging ships out of the water up an
inclined plane. The machinery for this purpose was invented by Mr.
Miller for hauling ships up the inclined plane of “Martin’s slip,” at
the upper end of which the press cylinder is placed, at the same slope
as the inclined plane, and the ship is attached, by means of chains, to
a crosshead fixed on the plunger. Hydraulic power has also been used for
launching ships, and the launch of the _Great Eastern_ is a memorable
instance; for the great ship stuck fast, and it was only by the
application of an immense pressure, exerted by hydraulic apparatus, that
she could be induced to take to the water. Water pressure is also
applied to hoists for raising and lowering heavy bodies, and in such
cases the pressure which is obtained by simply taking the water supply
from an elevated source, or from the water-main of a town, is sometimes
made use of, instead of that obtained by a forcing pump. The lift at the
Albert Hall, South Kensington, by which persons may pass to and from the
gallery without making use of the stairs, is worked by hydraulic
pressure in the manner just mentioned. In such lifts or hoists there is
a vertical cylinder, in which works a leather-packed piston, having a
piston-rod passing upwards through a stuffing-box in the top of the
cylinder. The upper end of the piston-rod has a pulley of 30 in. or 36
in. diameter, attached to it, and round this pulley is passed a chain,
one end of which is fixed, and the other fastened to the movable cage or
frame. So that the cage moves with twice the speed of the piston, and
the length of the stroke of the latter is one-half of the range of the
cage.
Sir William Armstrong has applied hydraulic power to cranes and other
machines in combination with chains and pulleys. His hydraulic crane is
represented by the diagram, Fig. 169, intended to show only the general
disposition of the principal parts of this machine, which is so
admirably arranged that one man can raise, lower, or swing round the
heaviest load with a readiness and apparent ease marvellous to behold.
Here it is proper to mention once for all, that the pressure for the
hydraulic machines is obtained not only by natural heads of water, or by
forcing-pumps worked by hand, but very frequently by forcing-pumps
worked by steam power. It is usual to have a set of three pumps with
their plungers connected respectively with three cranks on one shaft,
making angles of 120° with each other. A special feature of Sir W.
Armstrong’s hydraulic crane is the arrangement by which the engines are
made to be always storing up power by forcing water into the vessel,
_a_, called the “accumulator.” The accumulator—which in the diagram is
not shown in its true position—may be placed in any convenient place
near the crane, and consists of a large cast iron cylinder, _b_, fitted
with a plunger, _c_, moving water-tight through the neck of the
cylinder. To the head of the plunger is attached by iron cross-bars, _d
d_, a strong iron case filled with heavy materials, so as to load the
plunger, _c_, with a weight that will produce a pressure of about 600
lbs. upon each square inch of the inner surface of the cylinder. The
water is pumped into the cylinder by the pumping engines through the
pipe, _f_, and then the piston rises, carrying with it the loaded case,
guided by the timber framework, _g_, until it reaches the top of its
range, when it moves a lever that cuts off the supply of steam from the
pumping engine. When the crane is working the water passes out of the
cylinder, _a_, by the pipe, _h_, and exerts its pressures on the
plungers of the smaller cylinders; and the plunger of the accumulator,
in beginning its descent again, moves the lever in connection with the
throttle-valve of the engine, and thus again starts the pumps, which
therefore at once begin to supply more water to the accumulator. The
latter is, however, large enough to keep all the several smaller
cylinders of the machine at work even when they are all in operation at
once. Fig. 169 shows a sketch elevation and a ground plan of the crane
as constructed to carry loads of 1 ton, but the size of the cylinders is
somewhat exaggerated, and all details, such as pipes, guides, valves,
rods, &c., are omitted. The hydraulic apparatus is entirely below the
flooring—only the levers by which the valves are opened and closed
appearing above the surface. The crane-post, _i_, is made of wrought
iron: it is hollow and stationary; the jib, _k_, is connected with the
ties, _l_, by side-pieces, _n_, which are joined by a cross-piece at
_m_, turning on a swivel and bearing the pulley, _u_. The jib and the
side-pieces are attached at _o_ to a piece turning round the crane-post,
and provided with a friction roller, _p_, which receives the thrust of
the jib against the crane-post; the same piece is carried below the
flooring and is surrounded with a groove, which the links of the chain,
_q_, fit. This chain serves to swing the crane round, and for this
purpose the hydraulic cylinders, _r_, _r´_, come into operation. The
plungers of these have each a pulley, over which passes the chain _q_,
having its ends fastened to the cylinders, so that when, by the pressure
of the water, one plunger is forced out, the other is pushed in, and the
chain passing round the groove at _s_ swings the jib round. The
cylinders are supplied with water by pipes—omitted in the sketch, as are
also those by which the water leaves the cylinders. These pipes are
connected with valves—also omitted on account of the scale of the
diagram being too small to show their details—so that the movement of a
lever, _t_, in one or the other direction at the same time connects one
cylinder with the supply and the other with the exit-pipe. When the
crane is swinging round, the sudden closing of the valves would produce
an injurious shock, and to prevent this relief-valves are provided on
both the supply and exit-pipes communicating with each cylinder. When,
therefore, the valves are closed, the impetus of the jib and its load
acting on the chain, and through that on the plungers, continues to move
the latter, the motion is permitted to take place by the relief-valves
opening, and allowing water to enter or leave the cylinders against the
pressure of the water. There is also a self-acting arrangement by which,
when these plungers have moved to the extent of their range in either
direction, the valves are closed. The chain of the crane rests on guide
pulleys, and passing over the pulley _u_, goes down the centre of the
crane-post to the pulley _v_, and thence passes backwards and forwards
over a series of three pulleys at _w_ and two at _x_, and is fastened at
its end to the cylinder, _y_. As there are thus six lines of chain, when
the plunger of the lifting cylinder comes 1 ft. out, 6 ft. of chain pass
over the guide pulley, _u_. The plunger, when near the end of its stroke
in either direction, is made to move a bar—not shown—which closes the
valve. When the crane is loaded, the load is lowered by simply opening
the exhaust-valve, when the lift-plunger will be forced back into its
cylinder by the pull on the chain. But as the chain may require to be
lowered when there is no load upon it, although a bob is provided at _z_
to draw the chain down, it would be disadvantageous to increase the
weight of this to the extent required for forcing back the lifting
plunger. A _return_ cylinder is therefore made use of, the plunger of
which has but a small diameter, and is connected with the head of the
lift-plunger, so that it forces the latter back when the lift-cylinder
is put in communication with the exhaust-pipe. The water is admitted to
the lifting cylinder from the accumulator by a valve worked by a lever,
which, when moved the other way, closes the communication and opens the
exhaust-pipe, and then the pressure in the return cylinder, which is
constant, drives in the plunger of the lifting cylinder. The principle
of the accumulator may plainly be used with great advantage even when
manual labour is employed, for a less number of men will be required for
working the pumps to produce the effect than if their efforts had to be
applied to the machine only at the time it is in actual operation, for
in the intervals they would, in the last case, be standing idle.
Apparatus on the same plan has been used with advantage for opening and
shutting dock gates, moving swing bridges, turn-tables, and for other
purposes where a considerable power has to be occasionally applied.
[Illustration:
FIG. 169.—_Sir W. Armstrong’s Hydraulic Crane._
]
[Illustration:
FIG. 170.—_Raising Tubes of the Britannia Bridge._
]
[Illustration:
FIG. 171.—_Press for Raising the Tubes._
]
[Illustration:
FIG. 172.—_Head of Link-Bars._
]