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 FLOATS

It 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.

Here, then, we have a convenient means of multiplying force or effort and it is a means that is used very largely in certain classes of machinery. Figure 39 is a diagrammatic representation of a hydraulic press. It consists of a cylinder A in which is fitted a ram B. An L-shaped tube C connects with the cylinder and is fitted with a plunger D. The cylinder and tube are filled with water and then when the plunger is depressed the ram B has to rise, If the area of the plunger is one square inch and that of the ram thirty square inches, a 100 pounds pressure on the plunger will exert 3,000 pounds of lift on the ram.

HYDRAULIC LEVERAGE

However, 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 COLLAR

All 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 WATER

A 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 WATER

An 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 ELEVATORS

The 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 CANALS

There 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 GEAR

Water 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 WAVES

During 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.

---