PUTTING RIVERS TO WORK
FLOWING water exerts a strange fascination upon mankind, even to the present day. Tourists travel hundreds of miles to view the glorious spectacle of a riotous tumbling cataract. Is it strange, then, that in the olden times, when the world was peopled with gods and genii and strange spirits, the ancients looked upon ceaselessly flowing rivers as the symbol of life? It was most natural for them to covet the endless power of a river and eventually, despite their superstitions, to try to utilize some of its energy.It may be that sailboats antedate the first water wheel, but it seems much more probable that flowing water was the first inanimate power harnessed by man. Windmills were certainly a later development. They possessed the advantage that they could be located anywhere while the water mill had of necessity to be built along the bank of a stream. However, the power of the wind is so unreliable and fluctuates so widely that it was little used, except in flat countries, where there was little if any available water power.
Water power predominated until the steam engine was introduced, when it had to give way to an even more reliable power and one which could be located at any place to which fuel could be transported. Now, however, we are going back to our first power, seeking it out in the most inaccessible mountainous regions, because we have discovered the means of taking the power it yields and transmitting it hundreds of miles, over hills and plains to the point where we can put it to useful service. Hydroelectric power has very aptly been termed “white coal.”
The first prime motor was the current wheel, that is, a wheel fitted with paddles, which was journaled over a stream with the paddles projecting into the water. This was a very inefficient machine; it converted very little of the energy of a stream into useful mechanical power. The idea of damming the stream and letting the waters flow over the dam through a raceway upon a water wheel was a much later development.
WATER WHEELSThree types of water wheel which were in universal use before the advent of the steam engine were the undershot wheel, the overshot wheel, and the breast wheel (Figure 30). In the undershot wheel the water stored back of a dam is let out near the bottom of the dam and strikes the under side of the wheel, so that the top of the wheel turns toward the dam. In the overshot wheel, the water flows over the wheel striking the paddles or buckets on top and on the forward side, so that the wheel turns forward. In the breast wheel, the water strikes the paddles half way up the wheel on the rear side and drives the wheel in the same direction as that of the undershot wheel.
When we speak of water power we are apt to think of the water as actually furnishing the energy. As a matter of fact, it is not water but gravity that drives the wheel, the water being merely the medium that gravity acts upon. By having the water drop from a great height, its velocity is greatly increased and the power it imparts to the wheel is much higher. In mountainous regions it is easy to obtain a high head of water and thus generate a great deal of power from a relatively small stream. However, the ancient type of wheel with its paddles or buckets has now practically passed out of existence, being superseded by the Pelton wheel for high heads and the turbine for low heads of water.
INVENTION OF THE PELTON WHEEL
A 4,000-FOOT HEAD OF WATERA notable illustration of such a plant is the great installation at Big Creek, Cal. Big Creek, despite its name, used to be a small stream flowing down the mountains into a canyon. One would hardly suppose that it was capable of yielding much power, but it had its source high up in the Sierras and was fed mainly by melting snows. In the springtime, it swelled to a good-sized torrent. By building three dams near the top of the mountain, a lake was formed in which the water of the melting snow was impounded, so that a steady stream of water could be supplied the year round for power purposes. But even so, the stream hardly amounts to very much if we consider only the quantity of water that passes through it. The particular advantage of this installation is the fact that in a distance of six miles from the dam the creek falls 4,000 feet.
An inhabitant of the Eastern States who is unused to mountain heights may gain some conception of the meaning of this elevation by gazing up to the pinnacle of the Woolworth Tower, which rises 795 feet above street level, then mentally multiplying its altitude by five. Evidently even a small stream of water dropping from such an elevation would develop an enormous amount of power. In fact, it was considered inexpedient to use the entire fall at a single drop and so it was divided into two stages. The water is carried through a tunnel three-quarters of a mile long and then through a flow pipe along the face of the mountain to a point where it may drop 2,000 feet to the first power plant. After passing through this plant the water is discharged into the creek and is then diverted into a second tunnel four miles long and a series of steel conduits to a point from which it may drop 2,000 feet more to the second power plant. In each power house there are two electric generators, each fitted with a pair of Pelton wheels. These wheels are a little less than eight feet in diameter and each one develops 23,000 horsepower.
The water is directed into the buckets of the Pelton wheel in a stream six inches in diameter, and it issues from the nozzle with a velocity of 300 feet per second or about 210 miles per hour. A jet of water is almost like a solid bar of wood. In fact, it is impossible to chop through it with an ax. The water would swing the ax out of one’s hand before it got part way through the jet. Traveling at such a high speed the friction is so great that it would tear the skin off one’s hands, if it did not actually tear the hand off the arm, and yet it strikes the buckets of the wheels with no shock at all, for the first part of the bucket it touches is nearly parallel to the jet, and as the water sweeps around the curved face of the bucket it loses practically all of its pressure and velocity and falls into the tail race. The electric power generated by the two plants is stepped up to 150,000 volts and sent out over transmission lines to points of service. The street cars of Los Angeles are connected by a 240-mile electric harness to the hydraulic horses of Big Creek.
Powerful as this stream is, a still higher head is used in Switzerland, at Lake Fully, where there is a drop of over a mile in a distance of 2.8 miles. The water is carried by a short tunnel through the mountain, and then makes a drop of over 5,000 feet to the power plant, where it strikes the Pelton wheels at a velocity of 400 miles per hour, or about seven times the speed of a fast express train.
HARNESSING THE MISSISSIPPIIn contrast to such high heads, we have the low-head power plants which are employed where a large volume of water is available. The most notable installation of this type, and the largest in the world, is that at Keokuk, Iowa, where a dam has been thrown across the Mississippi River. For many years it was thought impossible to make any use of the vast volume of water that flows through this great river. But above Keokuk there used to be a rapid extending back about twelve miles. By building a dam across the river just below the rapid it was possible to obtain a working head of about thirty-two feet, and with the enormous volume of water available this provided sufficient energy to make the development worth while. In marked contrast to the installation at Big Creek, it is volume rather than velocity that is employed, and hence turbines rather than Pelton wheels are used. More water goes through a single turbine than is used in the whole of the city of New York with all its elaborate aqueduct system. Enormous turbines are used, fifteen feet in diameter, and when the installation is complete there will be thirty units, each yielding 10,000 horsepower, or a total of 300,000 horsepower. A turbine, it may be explained, differs from the ordinary water wheel in the fact that the water runs through the wheel instead of around it (Figure 32). The water may enter at the center and then flow out at the periphery, or it may enter at the periphery and then be discharged from the center of the wheel, or it may run axially through the wheel. In a Pelton wheel there is a single jet which strikes but one pair of buckets at a time, but in a turbine there are many jets distributed all around the circumference of the wheel. The water is divided into a series of jets by being forced through a stationary set of curved vanes. The blades of the rotor or revolving part of the turbine are oppositely curved. If the rotor were immovable the jets would have to change their direction in passing through the rotor, but as the rotor is free to turn, the jets react against these blades and set the wheel to revolving. The turbine may be designed to run either on a horizontal axis or on a vertical one.
The highest velocity of a wheel is naturally at the periphery and the advantage of an inflowing turbine such as this is that the water is traveling at its highest velocity when it strikes the periphery of the rotor. As it loses its velocity it flows in toward the slower-moving portions of the rotor. Finally it reaches the center, after giving up practically all its energy, and falls into the tail pool through a draft tube at the center of the rotor.
The scroll chambers at Keokuk are thirty-nine feet in diameter and the draft tubes are eighteen feet in diameter. Water enters the scroll chambers with a velocity of fourteen feet per second and comes out of the draft tubes into the tail pool with its velocity cut down to but four feet per second. Compare this with the velocity of the water jets at Big Creek!
The current generated at Keokuk goes to St. Louis and surrounding towns and serves a population of 1,120,000.
Now that we have learned how to transmit electrical power without serious loss over enormous distances, it is only a question of time before all the water power in the world is harnessed and put to the service of man. The power costs nothing after once the plant has been built; the only expense is that of maintaining the machinery and keeping it in repair. It is estimated that there is some 200,000,000 horsepower available in this country, but this includes all flowing water, much of which it would be impracticable, if not almost impossible, to utilize. However, there is about 60,000,000 horsepower commercially available, according to the figures of the U. S. Geological Survey, of which we have developed so far only 6,000,000 horsepower.
The ancients used flowing streams not so much for power purposes as to lift water to a higher level so that it would flow into their irrigating ditches. Nowadays, electricity, steam, or air is used for elevating water, but we have a very ingenious machine which makes the stream lift a part of itself. This machine is very different in principle from the old Egyptian noria. It depends upon the kinetic energy of water in motion. You cannot push a nail into a piece of wood with a hammer but you can easily drive it in by striking it with the hammer. As the hammer is swung it acquires what we term kinetic energy or energy of motion.
SETTING KITCHEN FAUCETS TO WORKIt is not generally realized that water in motion also acquires kinetic energy. Whenever a faucet is turned off very quickly, there is a hammering sound which is due to the fact that the moving water in the water pipe is brought to an abrupt stop. This puts a severe strain on the piping. A great deal of trouble was experienced from this source in the early days of plumbing. At a hospital in Bristol, England, there was a lead pipe leading from a cistern in one of the upper stories to the kitchen. Every time the faucets were turned off abruptly the momentum of the water caused the lead pipe to expand, and every now and then the pipe was burst. In order to relieve the situation, a plumber connected a pipe to the faucet and carried it up the side of the building to the level of the cistern. His idea was that whenever the water was turned off suddenly it would have a vent leading up to the level of the water reservoir. Much to his surprise, the water issued from the pipe in a jet of considerable height. To prevent the escape of the water, he extended the pipe considerably, and still a jet of water would issue from it. Eventually the relief pipe was carried up twice the height of the cistern and even then the water would squirt out occasionally when the faucets in the kitchen were turned off very suddenly. Then the idea was conceived of placing a reservoir on one of the upper floors of the hospital and letting the jet of water fill this reservoir. Every time the faucet was operated in the kitchen a certain amount of water flowed into the new cistern, and in this way it was kept supplied with enough water to furnish that which was required for the upper floors of the hospital.
There is a very ingenious water-driven motor which is employed merely to record the amount of water flowing through it. This is the Thomson water meter which is illustrated in Figure 34. It consists of a circular chamber with inwardly dished or conical top and bottom walls. In the chamber is a flat disk with a ball and socket bearing. At one side there is a vertical diaphragm in the chamber which passes through a slot in the disk. This prevents the disk from revolving, but it is free to oscillate. It has a motion similar to the gyrations of a top when it is beginning to lose speed and die down, except that the disk does not revolve. When the disk is in contact with the bottom wall of the chamber on one side it contacts with the top wall on the other so that the chamber is virtually divided into two compartments by the disk, but by gyrating the disk these compartments are made to revolve. Water enters at one side and discharges at the other side of the vertical diaphragm. Now, if the disk is in the position shown in Figure 34, the water, on entering, bears upon the upper face of the inclined disk and wedges its way between the disk and the upper wall of the chamber, making the disk oscillate on its ball center. As the edge of the disk rises across the face of inlet port the water entering the chamber bears against the under side of the disk, continuing the gyratory motion. The water cut off on the upper side of the disk is carried around to the outlet and discharges, while a fresh supply flows in on the other side of the vertical partition and at the next half turn the water in the lower compartment discharges at the outlet side of the partition, while the compartment is filling on the other side of the partition. A measured amount of water flows through the chamber at each gyratory oscillation of the disk. A train of gearing is driven by the gyrating disk which operates a set of dial pointers and a measure of the amount of water passing through the meter is indicated.
DIGGING WITH WATER JETSWe have referred to the enormous velocity of the jets used to drive Pelton wheels. Where high heads of water are obtainable water jets are used very effectively for excavating purposes, particularly in mining plants for washing down gold-bearing gravel banks. If water is not found near such banks expensive canals, flumes, and pipe lines are constructed and even tunnels are bored to bring the water to the point where it can be utilized. Some of the giant nozzles spout streams from 2 to 8 inches in diameter with a pressure of from 50 to 200 pounds per square inch. The powerful streams tear into the gravel banks, washing them away into sluices in which riffle boxes are placed to catch the precious metal. The back pressure of these nozzles is very heavy and the larger ones have to be provided with strong anchorages. Water in motion resists any change of direction and long levers have to be provided to permit the miners to guide the nozzles.
The hydraulic jet is also used for general excavating wherever water power is available. Sometimes it is employed under water when clearing a channel to level down piles of stones that are too large to be picked up by a suction dredge. Hollow iron piles are driven into a sandy bottom by means of hydraulic jets. No hammer is needed. Water is pumped into the pile and on issuing from the bottom of the pile it carries sand with it, making a hole into which the pile sinks. Wooden piles are driven in the same way by loosely attaching a water pipe to them so that the pipe may be withdrawn when the pile has been driven far enough. The pile is grooved at the lower end so that the pipe outlet may be centered at the bottom of the pile.
COMPRESSING AIR WITH WATER
One advantage of this type of air compressor is that it cools the air while compressing it. This was hardly an advantage in the Catalan forges, but when the air is used to drive machinery it is important that it be precooled. When air is compressed by mechanical means a great deal of heat is generated and the machines must be water jacketed to extract this heat, but in the hydraulic compressor the air bubbles are compressed as they pass down with the water to the reservoir and the water absorbs the heat, delivering cool compressed air at the bottom of the intake pipe.
LIQUID LEVERS AND GEARS
MANY an inventor has strayed off into the delusive pursuit of perpetual motion because he did not know that the pressure in a body of water at any given point is equal in all directions, upward, downward, or laterally.A cubic foot of water weighs 62½ pounds. Take a hollow column with an internal cross-sectional area of one square foot and if it be filled with water to a depth of ten feet there will be a weight of 625 pounds of water in the column and hence a pressure of 625 on the bottom of the tube or 4.34 pounds on every square inch of the bottom. But the water presses on the sides of the tube as well and the amount of this pressure depends upon the depth or “head” of water and not upon the quantity of water. At the bottom of the tube the pressure on the side walls is 625 pounds per square foot or 4.34 pounds per square inch; at a depth of one foot the pressure on the side walls is 62.5 pounds per square foot or .434 pound per square inch; at a depth of two feet it will be .864 pound per square inch, etc. The pressure on each square inch depends not upon the mass of the water but upon its depth. If the column of water had a cross-sectional area of a mile or a thousand miles, the pressure at a depth of one foot would always be .434 pound per square inch. (Of course there are slight variations from this figure due to salt or other substances dissolved in water or to changes in density produced by variations of temperature, but we need not consider such minute differences here.) That is why a dam which is strong enough to hold back the waters of a pond will be just as able to hold back the waters of the whole ocean if it be placed in a sheltered bay where ocean waves cannot tear it to pieces. The ocean, despite its enormous mass, can exert no more pressure per foot of depth than the water in a cistern.
WHY A SHIP FLOATSIt is because the pressure of water at a given depth is exerted upward, as well as laterally and downward, that a ship floats. It is the upward pressure of the water that holds up the boat. When an object is placed in a reservoir of water it sinks into zones of increasing pressure until it finally reaches a depth at which the pressure on the bottom of the object balances the weight of the body. If the body is entirely submerged before reaching such a point, it will continue to sink to the bottom of the reservoir because water will flow over the top of the object and keep adding downward pressure to offset the increasing upward pressure. The amount of water in the reservoir makes no difference. A battleship will float just as high in a flooded dry dock as it will in the open ocean. If the dry dock were so narrow as to leave a clearance of but a few inches of water around the ship, the latter would still float even though the ship weighed considerably more than the water in the dock.
There is a big difference, then, between the weight of water and the pressure it exerts. In Figure 38 we have an L-shaped receptacle with the lower arm of the L terminating in a chamber A. The top wall B of this chamber measures ten square inches. The tube C has a cross sectional area of one square inch. If tube C is filled to a height of twelve inches above wall B we shall have an upward pressure of 0.434 pound on every square inch of wall B, or a total of 4.34 pounds. If by means of a plunger D we add a hundred pounds of pressure to the column of water in tube C, we shall be adding a thousand pounds to the pressure on the wall B. The side walls and bottom of the chamber A will also be subjected to a pressure of 1,000 pounds per inch plus the pressure due to the depth or head of water.
HYDRAULIC LEVERAGEHowever, we must remember that in mechanics, as in all walks of life, we cannot get “something for nothing.” If we multiply the pressure or force, we must pay for it in some way, otherwise we should be getting more work out of the press than we put in it, which is what the perpetual motion crank is ever trying to do. As the cross-sectional area of the plunger D is only 1/30th of that of the ram, the plunger must descend thirty inches to raise the ram one inch. We need not consider the difference in the head of water because it would not amount to more than a few ounces at most, nor need we consider frictional losses. The case is parallel to that of the lever. In fact, we may consider the hydraulic press as a fluid lever with the water in tube C as the effort arm and that in cylinder A as the weight arm. The two arms are here so proportioned that the power arm must move thirty times as far as the weight arm. The work put into the press is exactly balanced by that we get out of it. An effort of 100 pounds exerted through a distance of thirty inches is exactly balanced by the moving of 3,000 pounds through a distance of one inch.
It is a decided disadvantage to have to move the plunger so far and in actual commercial practice hand-operated hydraulic presses are not worked in that way. A pump is used to force water into the cylinder so that a great many short strokes may be taken in place of one long one, and the pump handle provides an added leverage, enabling a man with little effort to exert an enormous lift. The water enters the ram cylinder through a valve, and the pressure is maintained on the ram until relieved by the opening of an outlet port.
MAUDSLEY’S LEATHER COLLARAll this seems very simple and one would suppose that the inventor of the hydraulic press must have been exceptionally free from the troubles and trials that beset most inventors. However, there is a vast difference between a laboratory apparatus and a commercial machine. When, towards the close of the eighteenth century, Joseph Bramah, the eminent British tool builder, invented the hydraulic press, he experienced all sorts of difficulty in holding the water in the ram cylinder. Of course, the ram has to slide freely into and out of the cylinder, but how could he prevent the water from leaking out past the ram? He resorted to all the plumbing expedients of the day. He used a stuffing box and gland, but when this was packed tight enough to hold the water in, it gripped the ram so tightly that the latter would not move down into the cylinder on the return stroke. Bramah had in his employ a very clever young mechanic named Henry Maudsley, who later became famous as an inventor and designer of machine tools. We read of him in Chapter III. Maudsley attacked the baffling problem of the hydraulic press and provided a solution that survives to this day. In place of the stuffing box which is a means of jamming a mass of cotton waste about the collar, he provided a cupped leather collar. When the pressure was applied it expanded the collar and made it bear tightly against the ram, but on relieving the hydraulic pressure the pressure of the cup leather was also reduced automatically.
There are many machines analogous to the hydraulic press in principle. They do not use water in every case for the fluid lever. Where the fluid is used over and over again oil is frequently employed. The compactness of this form of lever makes it most useful wherever an operation calls for the overcoming of a very heavy load or resistance through a relatively short distance. For instance, there are machines for bending pipe, for curving railroad rails, for punching holes in metal, for pulling wheels off their shafts, for jacking up heavy weights, for baling cotton, paper, and other materials, all of which operate on the same principle as the hydraulic press. Water pressure is supplied sometimes by a hand pump, sometimes by a power-driven pump, and sometimes it is taken from a reservoir in which compressed air imparts the requisite pressure to the water.
BLASTING WITH WATERA novel use of water pressure has been developed in England. In certain mines it is dangerous to use dynamite for blasting purposes owing to the presence of explosive gases, and successful experiments have been made with hydraulic cartridges. This consists of a cylinder of steel fitted with a series of little plungers arranged in a row in the cylindrical wall of the cartridge. As in powder blasting, a series of holes are drilled in the face of the rock and the cartridges with the plungers retracted are fitted into the holes. Then the cartridges are connected to a high-pressure water supply. The water forces the plungers out, exerting enough pressure to burst the rock. Not only is this system perfectly safe, but it is economical, because the gallery does not have to be cleared of workmen before every blast. There is more certainty in the use of water cartridges, and the danger, common where dynamite is used, of having the rock drill or pick strike and explode a stick of dynamite which failed to go off with the rest of the charge in a previous blast is avoided.
Hydraulic pressure is also used in jacks for lifting heavy weights. The principle of the hydraulic jack is the same as that of the hydraulic press.
RAISING A BRIDGE SPAN WITH WATERAn interesting illustration of the use of these liquid levers was afforded in the construction of the Quebec Bridge. This huge bridge, it will be recalled, consists of two cantilevers which stretch out from opposite shores of the St. Lawrence River and support between them a center span 640 feet long and weighing 5,400 tons. The span was built on barges, towed down to position between the cantilever arms and then lifted up 150 feet to the floor level of the bridge. Eight 1,000-ton hydraulic lifting jacks were used, two at each corner of the span. They were ideally suited to this kind of work. The bridge span had to be lifted with utmost care and the motion had to be simultaneous on all four corners. If one corner were raised faster than another the span would be twisted and subjected to serious strains. In the first attempt the fastening gave way at one corner and the span crumpled up and plunged down into the river. But a year later at the second attempt the span was hoisted successfully to position. The jacks had a lift of two feet, and, counting the time required to secure the huge plate chains by which the span was suspended, move the jacks down and give them a fresh hold, it took fourteen minutes to complete each two-foot lift. The work was done only during the daylight hours and on the third day the span was finally brought into position and made fast to the cantilever arms by means of twelve-inch pins driven home at each corner.
Hydraulic power is used very largely in the operation of cranes. As the plunger or ram has a very limited range of motion some means of multiplying distance of travel is required. One simple scheme is to use a set of pulley wheels or sheaves attached to the cylinder and another set to the ram and pass the hoisting chain around them. If we refer back to Figure 16, on page 34, we shall see that seven feet of rope must be pulled in at E in order to raise the lower pulley block one foot. It is very evident that the process can be reversed. Power might be applied to spread the two-pulley blocks apart when a movement of one foot would produce a travel of seven feet at E. That is what is done on the hydraulic crane. The ram is lifted, spreading the pulley blocks apart and thus multiplying the motion of the lifting cable to any extent, depending upon the number of sheaves, so that a travel of but a few feet will result in lifting a load forty or fifty feet.
HYDRAULIC ELEVATORSThe same principle is used in hydraulic elevators. The hydraulic cylinder lies horizontally on the basement floor and by means of pulley gearing a short motion of the plunger is sufficient to raise the car several stories.
In a more modern type of elevator the plunger acts directly on the car. The plunger is long enough to reach to the top story of the building which means that it must sink far into the ground in order to let the car down to the first story or basement. A steel pipe is sunk into the ground and serves as the cylinder and in this the long plunger operates. In one of New York’s tall buildings the cars are lifted to a height of 282 feet. The plungers are 6½ inches in diameter and they travel at a speed of over 400 feet per minute. The car which, when loaded, weighs 1,617 pounds, is supported on the top of the plunger. However, counterweights are provided which balance the weight of the car, so that practically the only weight lifted by the plunger is that of the passengers. The advantage of this type of elevator is that it reduces the danger of accident. The motion of the car is steady and easily controlled. The car cannot move down above a given speed and the only possible danger is that the plunger might break. However, a shaft of steel 6½ inches in diameter, even if hollow, is hardly likely to give way even under the most extraordinary loads to which it might be subjected in elevator service.
LIFT LOCKS FOR CANALSThere is another type of elevator that completely dwarfs anything used in office buildings. On some canals where it is necessary to make a sudden change of level of considerable extent, instead of using a flight of locks, a hydraulic elevator is employed to lift or lower not only the vessel but the water it is floated in. There are two huge tanks which ply between the upper and lower levels of the canal. These are so connected that as one rises the other descends. The tanks are big enough to take in the largest vessels that are likely to use the canal. They are fitted with water-tight gates at each end, and after a vessel has entered one of them, say at the lower level, the gate behind it is closed. Then hydraulic mechanism is operated to lift the tank, and when the top is reached the other gate is opened and the vessel sails out into the upper level. It seems like a tremendous undertaking to lift a heavy ship, but the two tanks balance each other and the only work that has to be done is to overcome the friction and inertia. No matter how heavy a vessel may be, it will not disturb the balance between the two tanks. Nor is it necessary to have a ship in each tank, for the same depth of water is maintained in the two tanks, and when a ship enters the weight of the tank is not increased, for the ship displaces its own weight of water.
HYDRAULIC VARIABLE SPEED GEARWater is frequently used to take the place of toothed gearing for the transmission of power, particularly where it is desirable to vary speed. One of the disadvantages of toothed gearing is that a change of speed can only be made by shifting gears. The speed cannot be varied gradually but is changed by abrupt steps. The guns of a battleship must be kept trained on the target, while the ship rolls in the waves under them, so as to be ready for a broadside at any instant. A telescope is secured to the barrel of the gun and a man known as a “pointer” tries to keep the cross-hairs of a telescope on the target by constantly elevating or depressing the gun. The speed of the elevating machinery must be constantly changing; now running at high speed, the next instant barely moving, and the next moment reversing.
To produce these variations a hydraulic variable speed gear is used. The construction is somewhat complicated, but the general principle of operation is simple. In a common pump water is lifted by operating a plunger in and out of a cylinder. It will be readily understood that the operation may be reversed, and if power is applied to the water to force it through the pump, it will make the plungers move in and out of the cylinder. In the hydraulic variable speed gear the driving shaft is made to operate a set of little pumps which are connected to a second series of pumps that act upon the driven shaft. Instead of water oil is used, and the oil pumped by the driving pumps is forced into the driven pumps. The latter pumps are so connected to the driven shaft as to cause it to rotate. But by a simple mechanical expedient the stroke of the driving pumps can be varied at will from maximum to zero, and the motion of the pumps can also be reversed. The variable amount of oil pumped into the driven pumps varies the speed of the driven shaft, for if it takes two strokes of a driving pump to fill the cylinder of a driven pump the driven shaft will travel at only half the speed of the driving shaft.
In turbine-driven vessels some sort of gearing is required between the propeller and the turbine. In order to operate at its best efficiency a turbine must run at very high speed, but this speed is entirely too high for the propeller. There is a limit to the rate at which a propeller may be driven. If this limit is exceeded, the propeller merely bores a hole in the water, forming a vacuum which produces a drag on the ship. It is customary to interpose some form of gearing between the propeller and the turbine shaft, but because of the high speed and the vast amount of power to be transmitted it is a difficult matter to design gearing that will be reliable. Furthermore, it is desirable to vary the speed of the vessel and even to reverse it without slowing down the turbine engine. In some cases a specially designed system of toothed gearing has been employed, in another the power of the turbine is converted into electricity and then reconverted into mechanical power by means of a motor on the propeller shaft. This provides a very efficient transmission, because the electricity can be very conveniently controlled to accelerate, retard, or reverse the speed of the propeller motor. The same thing can also be done by using a hydraulic transmission gear. This, although quite different from the variable-speed gear, yet operates on the same general principle. The turbine drives a centrifugal pump and the water thus pumped is fed to a horizontal water turbine on the propeller shaft. By an ingenious arrangement of turbine wheels the speed of the propeller shaft may be varied at will.
Hydraulic machinery is notable for its reliability. Sometimes water or oil is used as a convenient means of transmitting pressure from one part of a machine to another. If we take a tube filled with water and fit a plunger in each end, then, when one plunger is depressed or pushed in, the other will be expressed or pushed out. The pipe may be twisted around in any direction or be tied up in a double bowknot, and yet pressure applied to one cylinder will immediately be felt by the other. This method of transmission may do away with a vast number of gears or levers.
Water is comparatively incompressible and hence not a very adaptable means of transmitting power, but a certain amount of flexibility is secured in some systems by the use of what is termed an “accumulator.” This consists of a large cylinder fitted with a plunger. The plunger is heavily weighted and it maintains a constant pressure upon the column of water in the cylinder. If, for instance, a hydraulic crane is being used and suddenly a larger quantity of water is required than would normally be delivered by a pump, it is automatically supplied from the cylinder of the accumulator, and at a constant pressure. The accumulator is arranged to control the throttle valve of the steam pumping engine so that when the plunger of the accumulator reaches a predetermined height the stream is cut off and the engine stops pumping.
TRANSMITTING POWER WITH SOUND WAVESDuring the war a new use of water for power transmission was discovered and it is now being developed for the operation of mining machinery. The inventor of the new transmission is George Constantinesco, a Rumanian engineer. His first application of the invention was to a mechanism for synchronizing the firing of a machine gun with the rotation of an airplane propeller, so that it was possible to fire through the propeller without danger of striking its blades. There were several existing methods of gearing the propeller to the machine gun, but Constantinesco’s system proved so much more reliable that it was adopted and widely used by the British in their battle planes.
Instead of gears and levers, the Rumanian engineer used a column of water in a heavy steel tube to conduct impulses from the propeller to the machine gun. The pulsations produced by the propeller were too rapid to cause an actual displacement of the whole column of water in the tube, but a wave of pressure traveled through the water column at an enormously high velocity.
We are wont to think of liquids as incompressible, but they are actually slightly compressible and highly elastic. This is demonstrated by the submarine telephone or the submarine bell. Water, we know, is an excellent medium for the transmission of sound, but sound, we know, is produced by a succession of pressure waves. If water were absolutely incompressible, it could not convey sound from one point to another. When sound travels through a speaking tube the whole column of air does not move back and forth simultaneously, but it is divided into a series of waves. Each particle of air has a local oscillatory motion which it communicates to its neighbor, producing alternate compression and rarefication, and it is this wave action that travels through the column. The same is true of water, except that the rate of travel of the pressure wave in water is much higher than in air, namely 4,800 feet per second. It is this wave transmission that Constantinesco employed and which he calls “sonic” wave transmission.
To-day sonic wave transmission is used to operate rock drills. A wave generator is used, which acts somewhat on the principle of a pump. A pair of plungers are reciprocated at a rate of forty strokes per second by means of an electric or gasoline motor, and they produce a train of pressure waves in a column of water. The water is contained in a specially designed flexible steel piping which runs to the rock drill. The pressure waves operate a plunger in the drill and the plunger carries the drill steel. The latter pounds the rock under the wave impulses at the rate of forty strokes per second.
Sonic wave transmission is analogous to alternating current transmission of electricity. There are direct equivalents in the wave transmission, of volts, amperes, frequency, angle of phase, induction, inductance, capacity, resistance, condensers, transformers, and single-phase or polyphase systems.
by A. Russell Bond