MPA Logo, San Francisco Maritime National Park Association, USS Pampanito, Historic Ships at Hyde Street Pier, Education Programs Maritime Park Association Home Page Maritime Park Association Home Page Events Maritime Park Association Home Page Maritime Park Association Home Page Maritime Park Association Home Page Volunteer Membership Donate Maritime Park Association Home Page USS Pampanito Submarine Historic Ships at Hyde Street Pier Education Programs About Maritime Park Association Home Page Directions to Maritime Jobs at Maritime Facility Rental at Maritime Trustees of the Association Calendar Press Room Store Maritime Map
1
PRINCIPLES OF HYDRAULICS
 
A. INTRODUCTION
 
1A1. Increasing use of hydraulic power in modern submarines. In the development of the submarine from pre-war classes, many changes and improvements have occurred. One of the outstanding differences is the large variety of submarine devices which are now operated by hydraulic power. In early classes, there was no hydraulic system, and power requirements were met by means of air or electricity. Along with constantly improving submarine design has gone a constant extension and diversification of the use of hydraulic power.

1A2. Other sources of power available on submarines. Why this noticeable trend toward hydraulics? Obviously hydraulic actuation is not the only means of transmitting power throughout the submarine, and the tasks now being done by the hydraulic system were originally performed by hand, electricity, or compressed air.

a. Hand power. Some equipment on a submarine is still operated exclusively by hand, but this practice is rapidly disappearing. This is because the power requirements exceed that which manual effort can provide over long periods of time, and because power operation is faster and can be remotely controlled, thus greatly reducing the communication necessary between crew members.

b. Electric power. Since the electrical plant occupies such a prominent place in the submarine power system and must be used for propulsion in any event, it would be reasonable to expect that electricity would also be used to operate all of the auxiliary equipment as well.

Electricity is ideally adapted for submarine equipment that has few or no moving parts, such as lamps, radios, cooking facilities, and similar devices. But electricity is not so ideal when it is necessary to move heavy apparatus such as rudders, and bow and stern planes, because heavy, bulky

  electrical units are required. Also it is not ideal when instantaneous stopping of a driving mechanism is demanded, since electric motors have a tendency to "overtravel," or "drift," making fine control difficult to achieve. A further disadvantage in the operation of electrical units is the noise made by relays and magnetic brakes in starting and stopping, and by shafting and other mechanical power transmission units.

c. Pneumatic power. Since compressed air must also be used aboard a submarine for certain functions, this system, which consists of the compressors, high and low pressure air bottles and air lines, provides another source of auxiliary power. However, pneumatic or compressed-air power also has definite shortcomings. Pressure drop caused by leakage, and the mere fact that air is a compressible substance, may result in "sponginess" or lag in operation. The high pressure necessary for compressed-air storage increases the hazard from ruptured lines, with consequent danger to personnel and equipment. Another disadvantage of air systems is that the air compressors require greater maintenance and are relatively inefficient.

d. Comparative advantages of hydraulic power. Hydraulic systems possess numerous advantages over other systems of power operation. They are light in weight; they are simple and extremely reliable, requiring a minimum of attention and maintenance. Hydraulic controls are sensitive, and afford precise controllability. Because of the low inertia of moving parts, they start and stop in complete obedience to the desires of the operator, and their operation is positive. Hydraulic systems are self-lubricated; consequently there is little wear or corrosion. Their operation is not apt to be interrupted by salt spray or water. Finally, hydraulic units are relatively quiet in operation, an important consideration when detection by the enemy must be prevented.

 
1

Therefore, in spite of the presence of the two power sources just described, hydraulic power makes its appearance on the submarine because of the fact that its operational advantages, when weighed against the disadvantages enumerated for electricity and air in the preceding paragraphs, fully justify the   addition of this third source of power to those available in the modern submarine.

e. Comparative summary. If we draw up a table of the characteristics of the three power systems, a comparison will reveal the superiority of hydraulics for the operation of auxiliary mechanisms.

FACTORAIRELECTRICITYHYDRAULICS
ReliabilityPoorGoodGood
WeightLightHeavyLight
InstallationSimpleSimpleSimple
Control MechanismValvesSwitches and solenoidsValves
MaintenanceConstant attention necessaryDifficult, requiring skilled personnelSimple
VulnerabilityHigh pressure bottle dangerous; broken lines cause failure and danger to personnel and equipmentGoodSafe; broken lines cause failure
ResponseSlow for both starting and stoppingRapid starting, slow stoppingInstant starting and stopping
ControllabilityPoorFairGood
Quietness of OperationPoorPoorGood
 
B. THEORY OF HYDRAULICS
 
1B1. Familiarity of hydraulic principles. For many centuries, man has utilized hydraulic principles to satisfy common, everyday needs. Opening a faucet to fill a sink with water a practical application of hydraulics. Water moves through a dam in accordance with well-known principles of fluid motion. There are hydraulic principles that explain the action of fluids in motion and others for fluids at rest.

We are chiefly concerned, however; with that branch of hydromechanics which is called simply Hydraulics and is defined in engineering textbooks as the engineering application of fluid mechanics. It includes the study of the behavior of enclosed liquids under pressure, and the harnessing of the forces existing in fluids to do some practical task such as steering a submarine or opening the outer door of a torpedo tube.

Examples of hydraulically operated equipment are familiar to all. Barber or

  dentist chairs are raised and lowered hydraulically; so is an automobile when placed on a hydraulic rack for a grease job. Stepping on the brake pedal in an automobile creates the hydraulic power which stops the rotation of the four wheels and brings the car to a halt.

For an understanding of how a hydraulic system works, we must know the basic principles, or laws, of hydraulics, that is, of confined liquids under pressure. This will be made easier, however, if we first examine the somewhat simpler laws governing the behavior of liquids when unconfined, that is, in open containers.

1B2. Liquids in open containers. a. Density and specific gravity. The first characteristic of an unconfined liquid which interests us is its density. The density of a fluid is the weight of a unit volume of it. The unit of volume normally used in this text is the cubic foot; the unit of weight normally used is the pound. The standard of density, to which the

 
2

densities of all other liquids are referred, is that of pure water at zero degrees centigrade (32 degrees Fahrenheit), and at sea-level atmospheric pressure.

Let us fill a container with a cubic foot of pure water (see Figure 1-1). We weigh

Figure 1-1. Liquids of different densities.
Figure 1-1. Liquids of different densities.

the contents and find it to be 62.4 pounds. This is the density of water. Under the same conditions, a similar volume of oil, such as is used in a submarine's hydraulic system weighs approximately 50 pounds; therefore its density is less than that of water. Under the same conditions, a cubic foot of mercury weighs 845.9 pounds; its density obviously exceeds that of water.

When we speak of the weight of substance, we actually mean the force, or gravitational pull, exerted on the substance at the earth's surface. Every material responds to the earth's gravitational attraction. To express the relative density, or specific gravity, of various liquids and solids, the

  gravitational pull upon them is compared to the gravitational pull upon an equal volume of water. Water, therefore, is said to have a specific gravity of 1 and the specific gravity of any other substance is its density relative to that of water. Oil has a specific gravity of (50 x 1)/62.4, or approximately 0.8; that is, its density is 0.8 of that of water. This explains why oil floats on water. Mercury, on the other hand, has a specific gravity of (845.9 x 1)/62.4 or about 13.5; that is, its density is 13.5 times as great as that of water; consequently, it sinks rapidly.

These calculations of the weights of water, oil, and mercury were made at zero degrees centigrade (32 degrees Fahrenheit) and at sea level. At other temperatures and altitudes, different results would be obtained. In some engineering calculations, cubic centimeters and grams are used instead of cubic feet and pounds. This does not affect specific gravity, as the relationship between the weight of a unit volume of any other material and of water would be the same no matter what measuring unit were used.

b. Force and pressure. A liquid has no shape of its own. It acquires the shape of its container up to the level to which it fills the container. However, we know that liquids have weight. This weight exerts a force upon

Figure 1-2. Weight of an isolated column of water.
Figure 1-2. Weight of an isolated column of water.

 
3

all sides of the container, and this force can be measured.

Let us measure this force in a given container of water (see Figure 1-2).

Figure 1-3. Weight=Total Force.
Figure 1-3. Weight=Total Force.

Theoretically, we isolate a vertical column of water whose base is 1 square inch, extending from the bottom of the container to the surface of the liquid. If it were possible to weigh this

  pressure, when not otherwise qualified, means pressure in pounds per square inch.

If the bottom of the container has an area of 10 square inches and the pressure on each square inch is 2 pounds, then the force exerted by the water on the bottom of the container is 20 pounds (see Figure 1-3). This is called the total force and is obtained by the formula:

Total Force = Pressure X Area

The pressure exerted by a liquid on the bottom of a container is independent of the shape of the container, and depends only on the height and density of the liquid. In all the dissimilar vessels shown in Figure 1-4, the pressures are identical as long as the liquid levels are equal in height.

What happens if the levels are not equal? Then we do have a difference in pressure. Suppose we have two containers in which the fluid in A is twice as high as in B (see Figure 1-5). Let us again assume that we have

Figure 1-4. Equal levels produce equal pressures.
Figure 1-4. Equal levels produce equal pressures.
column and we found the weight to be 2 pounds, we would be able to say that the one inch-square column of water exerts a pressure of 2 pounds per square inch.

Therefore, for unconfined liquids, that is, liquids in open containers, the pressure in pounds per square inch exerted by the liquid on the bottom of the container is equal to the weight of the liquid on each square inch of the bottom of the container. It must be emphasized that the weight of the liquid is here thought of as a force exerted on the bottom of the container. Expressed as a formula, we have:

Pressure = Force per unit area

In this text, as in general engineering practice, it is understood that the word

  weighed a one-inch square column from each container. The column from jar A weighs 2 pounds and the column from jar B weighs only 1 pound; therefore the pressure in A is

Figure 1-5. Unequal levels produce unequal pressures.
Figure 1-5. Unequal levels produce unequal pressures.

 
4

2 pounds per square inch while the pressure in B is only half of that, or 1 pound per square inch.

Figure 1-6. Pressure on submerged body increases with increasing depth.
Figure 1-6. Pressure on submerged body increases with increasing depth.

In other words, the greater the depth, the greater the pressure will be at that depth. A practical example of the working of this law is seen when a submarine submerges. The deeper the submarine goes, the greater the pressure exerted on its hull by the surrounding water (see Figure 1-6).

The difference in liquid pressures at various levels can also be illustrated in the following way: If we have a tank with openings of equal size at different heights, as shown in Figure 1-7, we find that the liquid will flow out of the lowest opening, where the pressure is greatest, with much greater velocity than from the top opening, where the pressure is lowest.

Figure 1-7. Pressure increases with depth.
Figure 1-7. Pressure increases with depth.

The importance of this principle of hydraulics can be better understood by considering its following application.

Figure 1-8 shows two containers. In one container, we have a pressure of 1 pound per square inch exerted on an area of 10 square inches; the total force is 10 pounds. In the

  other container we have a pressure of 2 pounds per square inch applied to an area of only 5 square inches; and the total force is again 10 pounds. We see, therefore, that a high pressure directed against a small area can be just as effective as a low pressure directed against a large area. It follows from this important law that we are able to reduce the size of hydraulic units by merely increasing the pressures in order to obtain the same required working force-one of the many great advantages offered by hydraulic power for applications where the saving of space is a consideration.

Figure 1-8. Equal total forces from unequal pressures.
Figure 1-8. Equal total forces from unequal pressures.

1B3. Liquids in enclosed systems. Some of the general properties of liquids in open containers have been described. It remains to discuss how a liquid will behave when confined, for, example, in an enclosed hydraulic system.

a. Liquids are practically incompressible. The following two basic principles will help to explain the behavior of liquids when enclosed:

1. Liquids are practically incompressible in the pressure ranges being considered. Stated simply, this means that a liquid cannot be squeezed into a smaller space than it already occupies.

2. Therefore, an increase in pressure on any part of a confined liquid is transmitted undiminished in all directions throughout the liquid (Pascal's principle). For example, if pressure is applied at one end of a long pipe, the liquid, being practically incompressible, will transmit the pressure equally to every portion of the pipe.

 
5

Figure 1-9 shows a simple experiment which illustrates both these principles. A thin bottle is filled to the top with a liquid and tightly corked. A lever is pressed against the

Figure 1-9. Applied pressure Is exerted equally in all directions.
Figure 1-9. Applied pressure Is exerted equally in all directions.

cork to apply a downward force. If sufficient pressure is exerted, the bottle will suddenly shatter into a number of pieces, showing that:

a) Liquids are practically incompressible.
b) The applied pressure is transmitted equally in all directions at once.

Figure 1-10 illustrates the application of these principles to a closed hydraulic system. Two cylinder each having a base whose area is 1 square inch, are connected by a tube. The cylinders are filled with liquid to the level shown, and a piston with a base of the same area (1 square inch) is placed on top of each column of liquid. Then a downward force of 1 pound is applied to one of the pistons. Since this piston has an area of 1 square inch, the pressure upon it is 1 pound per square inch; and since the other piston is of equal area, the same pressure, 1 pound per square inch, will be imposed upward upon it.

b. Increase of force with area. We are now ready to consider a remarkable fact which follows from the principles just discussed, and which is illustrated in a simplified manner in Figure 1-11. Here a cylinder whose base has an area of 1 square inch is connected to another cylinder whose base has an area of 10 square inches. Again a force of 1 pound is applied to

  the piston in the smaller cylinder; and again the pressure exerted is 1 pound per square inch. Now, since this pressure is transmitted equally in all directions throughout the confined liquid, an upward pressure of 1 pound per square inch will be exerted on the piston in the larger cylinder; and since this larger piston has a total area of 10 square inches, the total force exerted on the larger piston is 10 pounds. Actually, what is happening is that an upward force of 1 pound is being exerted against each square inch of bottom surface of the larger piston; and since the area of this surface is 10 square inches, the total force is equal to the downward pressure on the small piston (1 pound per square inch) multiplied by the area of the larger piston (10 square inches); or, 1 (pounds per square inch) X 10 (square inches) = 10 pounds (total force exerted on larger piston). In other words, the ratio between the force applied to the smaller piston and the force applied to the

Figure 1-10. Transmission of equal pressures to equal areas.
Figure 1-10. Transmission of equal pressures to equal areas.

 
6

larger piston is the same as the ratio between the area of the smaller piston and the area of the larger piston. Expressed as a proportion, then, we have:

Force on larger piston/Force on smaller piston =
Area of larger piston/Area of smaller piston

This means that the mechanical advantage obtainable by such an arrangement is equal to the ratio between the areas of the two pistons.

Figure 1-11. Equal pressure transmitted to larger area.
Figure 1-11. Equal pressure transmitted to larger area.

It is this principle, discovered by Pascal, which makes possible the tremendous forces

  attainable in certain hydraulic devices, such as the hydraulic press, and hydraulic hoists.

Figure 1-12. Multiple units from a single source
of power.
Figure 1-12. Multiple units from a single source of power.

Now let us once more consider the arrangement shown in Figure 1-10. Since the cylinders (and pistons) are of equal area, pushing the liquid down a distance of 1 inch in one cylinder will force it upward a distance of 1 inch in the other cylinder. In other words, the displacements of liquid are equal. But, in Figure 1-11, since the area of the larger cylinder is 10 times as great as that of the smaller cylinder, pushing the smaller piston downward

Figure 1-13. Automobile hydraulic-brake system.<br>
1) Brake pedal; 2) piston; 3) master cylinder; 4) hydraulic line; 5) brake cylinder; 6) brake piston; 7) brake band; 8) wheel; 9) return spring.
Figure 1-13. Automobile hydraulic-brake system.
1) Brake pedal; 2) piston; 3) master cylinder; 4) hydraulic line; 5) brake cylinder; 6) brake piston; 7) brake band; 8) wheel; 9) return spring.
 
7

a distance of 1 inch will move the larger piston upward only 1/10 of an inch. The ratio between the displacement of liquid in the smaller cylinder and the displacement of liquid in the larger cylinder is once again equal to the ratio between their areas.

Therefore, we may say that what the larger piston gains in force, it loses in distance traveled, so that the amount of work (force X distance) done by the larger piston is exactly the same as the amount done by the smaller piston.

c. Multiple units. It is not necessary to confine our system to a single line from the source of hydraulic power. Hydraulic power may be transmitted in many directions to do multiple jobs.

Let us connect one cylinder to four others as in Figure 1-12. Here we apply a force against the piston in the large cylinder. The pressure from the large cylinder is transmitted equally to each of the pistons in the other four cylinders.

This is actually the method of operation of an automobile hydraulic-brake system (see Figure 1-13). The foot pressure on the brake pedal (1) depresses a piston (2) in the master cylinder (3). Fluid is forced through the lines (4) into each of the brake cylinders (5). At the brake cylinder, two opposed pistons (6) attached to the brake shoes are forced outward, pressing the brake bands (7) against the inside of the wheels (8) to stop their rotation by friction. Removal of the foot pressure allows springs (9) at each wheel to restore the pistons to their original positions and returns the fluid to the master cylinder where it is stored in preparation for the next braking operation.

1B4. Pumps. a. Need for pumps. In all our illustrations, we have seen that in an enclosed system a working force was created by the displacement of fluid. A weight, acting on a piston in one cylinder, forced fluid through a line, thus moving a piston elsewhere in the system. In the hydraulic brake system, foot pressure on the pedal displaced the fluid in the master cylinder and forced it into the brake cylinders to stop wheel rotation. These

  elementary methods are practical enough where small forces or small volumes of fluid are required. However, more often a far greater passage of energy, more or less continuous in its delivery of fluid, is needed in a system.

In other words, in practice we usually need some device which will deliver, over a

Figure 1-14. Principle of a suction pump.
Figure 1-14. Principle of a suction pump.

period of time, a definite volume of fluid at the required pressure, and which will continue to deliver it as long as we desire it to do so. Such a device is called a pump.

b. Basic principles of pumps. A hydraulic pump is a mechanical device which

 
8

forcibly moves, or displaces, fluids. Various pumping principles are employed in the different types of hydraulic pumps, but one fundamental principle applies to all: a volume of fluid entering the intake opening, or port, is moved by mechanical action and forced out the discharge port.

The basic principle underlying the action of a hydraulic pump is illustrated by the simplified device shown in Figure 1-14. The larger chamber, or reservoir, is connected by a pipe to the smaller chamber, or cylinder. A piston, free to slide up or down within this cylinder, is connected by a piston rod to a pump handle (not shown). The reservoir is filled with liquid to the height shown.

The illustration shows the device in three different conditions. At A, the piston is assumed to be resting squarely on top of the column of liquid, that is, there is no intervening space between piston and liquid. At B, the piston has just been pulled upward by the pump handle, creating a lower pressure in the lower half of the cylinder, that is, in the space now left between the bottom face of the piston and the top of the column of liquid. At C, the pressure of the atmosphere, acting on the surface of the liquid in the reservoir, has forced the liquid up into the cylinder, filling the empty space with a compensating amount of liquid out of the reservoir; the level in the reservoir consequently falls, as shown.

It should be clearly understood that the illustration (Figure 1-14) greatly exaggerates the size of the empty space, or partial, vacuum, left by the, piston as it rises in the cylinder. Actually, if a working model of the illustrated device were to be constructed of glass, no space of any kind could be observed because as the piston rises in the cylinder, the liquid rushes in practically instantaneously follow the rise of the piston.

c. The reciprocating pump. The simplest practical application of this principle is seen in the hand-operated reciprocating pump, a simplified version of which is illustrated in Figure 1-15. Here the inlet and outlet ports in the cylinder, or pump body, are both in the same side of the piston. The piston makes a close sliding fit within the cylinder, reducing

  leakage to a minimum, since excessive leakage destroys the efficiency of a pump. Both the inlet and outlet ports are equipped with check valves which permit the liquid to flow in one direction only, as shown by the arrows.

Figure 1-15. Hand-operated reciprocating pump.
Figure 1-15. Hand-operated reciprocating pump.

Assume that the intake side of the pump is connected to a supply of liquid. When we move the piston to the right, lower pressure is created in the chamber formed by the piston. Higher pressure on the fluid outside the chamber forces fluid in through the inlet port and fills the chamber. Moving the handle forward in the opposite direction forces the fluid out. A check valve at the inlet port prevents flow there and, since the fluid must find an outlet somewhere, it is forced out through the discharge port. The check valve at the discharge port prevents the entrance of fluid into the pump on the subsequent suction stroke. The back-and-forth movement of the piston in the pump is referred to as reciprocating motion, and this type of pump is generally known as a reciprocating-type piston pump. It may have a single piston or be multi-pistoned. It may be hand-actuated or power-driven. The reciprocating piston principle is conceded to be the most effective for developing high fluid pressures.

 
9

d. The theory of suction. In a discussion of reciprocating pumps, the word suction may be frequently used. Some writers use it as though it referred to an independent force created in the pump itself. It must be emphasized that suction is merely an expression of the difference between two unequal pressures. In this case, the atmospheric pressure, amounting to 14.7 pounds per square inch at sea level, acts as a downward force on the liquid in the reservoir.

Raising the piston, that is, pulling it away from the surface of the liquid, creates a partial vacuum, or an area of lower pressure, between the liquid and the bottom surface of the piston.

Therefore, as the piston moves upward in the cylinder, atmospheric pressure forces the liquid in the connecting pipe to follow the piston. This fact is the basis of a simple pumping operation involving "suction." It also explains why there is a limit to the height to which a suction pump can move a liquid under atmospheric pressure, since the liquid cannot be "pulled" to a greater height by the pump than atmospheric pressure will push it.

For water at sea level this limiting height is theoretically 33 feet, but this figure is never attainable in practice. The imperfections of actual pumps reduce the limiting height to 25 feet or less, depending on the efficiency of the individual pump.

For liquids other than water, the limiting height varies inversely as the density (weight per cubic foot) of the liquid; in other words, the lighter the liquid, the higher atmospheric pressure will push it when the liquid is pumped.

e. The gear pump. Another widely used type of pump is the rotary gear pump whose

  operating principle is illustrated, in simplified form, in Figure 1-16. Here the mechanical action which moves the fluid is furnished by the teeth of the rotary gears. The oil is trapped by the gear teeth and carried by them around the outside channels of the pump body. This sucks in oil at the inlet port (the left-hand port in the figure), and discharges it at the outlet port (the right-hand port in the figure). The oil cannot get back through the outer channels to the inlet side of the pump because the gear teeth fit too closely against the pump body. On the other hand, the oil cannot pass back between the gear teeth themselves at the point where they mesh with each other because they mesh so closely that, in effect, they form a continuous seal at this point. Therefore a continuous flow of oil is set up in the direction shown by the arrows. This flow continues as long as the gears continue to rotate. Pumps using the gear principle are popular because of their quiet performance and because their simplicity of design results in relative freedom from service troubles.

1B5. Hydraulic fluids. Almost any free-flowing liquid is suitable as a hydraulic fluid, as long as it will not chemically injure the hydraulic equipment. For example, an acid, although free-flowing, would obviously be unsuitable because it would corrode the metallic parts of the system.

Water, except for its universal availability, suffers from a number of serious defects as a possible hydraulic fluid. One such defect is that it freezes at a relatively high temperature, and, in freezing, expands with tremendous force, destroying pipes and other equipment. Also, it rusts steel parts; and it is rather heavy, creating considerable amount of inertia in a system of any size.

Figure 1-16. Rotary gear pump.
Figure 1-16. Rotary gear pump.
 
10

The hydraulic fluid used in submarine hydraulic systems is a light, fast-flowing lubricating oil, which does not freeze or even lose its fluidity to any marked degree even at low temperatures, and which possesses the additional advantage of lubricating the internal moving parts of the hydraulic units through which it circulates.

Since this oil, a petroleum derivative, causes rapid deterioration of natural rubber, synthetic rubber is specified for use in these systems as packing and oil seals.

1B6. A simple hydraulic system. On the basis of the explanation of basic hydraulic principles just given, it is possible to construct a simple, workable hydraulic system which will operate some mechanical device. For example, such a system might open and close a door, and hold it in either position for any desired interval.

a. Basic units of a hydraulic system. Such a system is illustrated in Figure 1-17. It necessarily includes the following basic equipment, which, in one form or another, will be found in every hydraulic system:

1. A reservoir, or supply tank, containing oil which is supplied to the system as needed and into which the oil from the return line flows.

  2. A pump, which supplies the necessary working pressure.

3. A hydraulic cylinder, or actuating cylinder, which uses the hydraulic energy developed in the pump to move the door.

4. A cut-out valve, by means of which the pressure in the actuating cylinder may be maintained or released as desired.

5. A check valve, placed in the return line to permit fluid to move in only one direction.

6. "Hydraulic lines," such as piping or hose, to connect the units to each other.

The supply tank must have a capacity large enough to keep the entire system filled with oil and furnish additional oil to make good the inevitable losses from leakage. The tank is vented to the atmosphere; thus atmospheric pressure (14.7 pounds per square inch) forces the oil into the inlet, or suction, side of the pump, in accordance with the principle explained in connection with Figure 1-14. The tank is generally placed at a higher level than the other units in the system, so that gravity assists in feeding oil into other units.

The pump is the hand-operated, reciprocating piston type illustrated in Figure 1-15.

Figure 1-17. A simple hydraulic system.
Figure 1-17. A simple hydraulic system.
 
11

The surface of the pump piston in contact with the hydraulic fluid has an area of 1 square inch.

The hydraulic cylinder (see Figure 1-18), which is the simplest type of hydraulic motor, contains a spring-loaded piston, with a piston. rod that extends through one end of the

Figure 1-18. Single acting hydraulic cylinder.
Figure 1-18. Single acting hydraulic cylinder.

cylinder. This piston rod, when connected to the door, supplies the mechanical motion which opens and closes the door. The surface of the piston in contact with the hydraulic fluid has an area of 2 square inches.

The cut-out valve is hand-operated. When closed, it shuts off the line between the actuating cylinder and the supply tank, preventing the oil under pressure in the cylinder from escaping into the return line; when opened, it releases this pressure, allowing the loading spring inside the cylinder to expand, and the oil in the cylinder to escape back into the supply tank.

The check valve (see Figure 1-19) is of the ball spring type. It is shown in two positions. At A, fluid entering the right-hand port under pressure sufficient to overcome the tension of the spring has unseated the ball, allowing oil to pass out through the other port in the direction shown by the arrow. At B, lower pressure on the line entering, the right-hand port has caused the oil pressure and tension spring to reseat the ball check, blocking off the right-hand port, and preventing movement of oil in that direction. The ball, machined to a smooth finish, fits closely into the seat, making a tight seal.

b. Operation of the system. Let us assume that the force necessary to move the door is 200 pounds. Let us further assume that the mechanical advantage of the handle and the muscular effort applied to it result in a force of 100 pounds exerted against the pump piston. Therefore, oil from the piston

  is forced into the actuating cylinder at a pressure of 100 pounds per square inch. This, then, is the working pressure of the system, the pressure at which fluid is delivered to the actuating cylinder.

Since the piston in the actuating cylinder presents an area of 2 square inches to the fluid -twice as great as the area presented by the pump piston- the total force acting against the piston of the actuating cylinder is 200 pounds, enough to overcome the resistance of the loading spring and close the door. To operate the system, the cut-out valve is closed and the pump handle is moved to the right, drawing in a quantity of oil from the reservoir ("suction stroke"). Then the handle is moved in the opposite direction ("pressure stroke"). The check valve to the reservoir line closes and the check valve to the pressure line leading to the actuating cylinder opens, delivering oil to the actuating cylinder at a pressure of 100 pounds per square inch. The check valve in the actuating cylinder opens,

Figure 1-19. Ball check valve.
Figure 1-19. Ball check valve.

allowing the oil to enter. The closed cut-out valve prevents the oil from entering the return line, and the oil, acting against the actuating cylinder piston with a total force of 200 pounds, pushes it to the left, overcoming the resistance of the loading spring and closing the door.

 
12

The door will remain shut as long as the cut-out valve is in the closed position. As soon as the valve is turned to OPEN, the piston in the actuating cylinder is returned to its original position by the spring. The door opens. Fluid that was locked in the cylinder will be forced out through the return line back to the reservoir. It cannot return through the pump because of the check valve. Back-flow of the fluid from the tank into the return line is also prevented by a check valve.

1B7. A power-driven hydraulic system. The door-operating system illustrated in Figure 1-17 is far simpler than is usually found in actual service. It has the obvious disadvantage that instantaneous opening of the door is not possible because pressure is built up slowly by hand pumping.

a. Units of a power-driven hydraulic system. Figure 1-20 illustrates a system in which a motor-driven pump is substituted for the hand pump, a double acting actuating cylinder for the spring-loaded single acting cylinder in Figure 1-17, and including a control valve, an unloading valve, and an automatic relief valve, in addition to the supply tank, or reservoir, and the return line check valve, which are the same as in the first system.

  Automatic pumping will give immediate pressure for use at the actuating cylinder whenever it is needed.

In the simplified system, the door was actuated by a single acting cylinder. Oil was kept in or released from the cylinder by a simple "on-and-off" valve. For more efficient and positive actuation, this will be replaced by a double acting cylinder (see Figure 1-21). In such a cylinder, the piston can move in either direction to open or close the door. The piston is locked in the desired position by the hydraulic fluid, which enters either side of the piston as required and remains there until forced out. Since the flow of the fluid must be directed to either of two sides, a valve, which selects the direction of flow, is installed in the line. This is called a control valve. Control valves vary with the specific application, but generally they are equipped with four ports. Two are connected to the actuating cylinder at either side of the piston. A third port is the pressure port and receives fluid from the pump. The fourth port returns surplus fluid either back to the reservoir or elsewhere in the system. Figure 1-22 shows a piston-type, or spool-type, control valve, so called because of the internal piston, or spool, which, as it slides into various positions

Figure 1-20. Power-driven hydraulic system.
Figure 1-20. Power-driven hydraulic system.
 
13

inside the valve body, directs the flow of fluid by opening and closing the desired combination of ports. The grooves permit flow between two of the ports, while the lands at

Figure 1-21. Double acting hydraulic system.
Figure 1-21. Double acting hydraulic system.

both ends of the spool block off the remaining ports.

NOTE: There are types of spool valves other than the type shown in Figure 1-22.

In order to have pressure at all times for the immediate operation of the door, the power-driven pump turns continuously. However, a pressure of 100 pounds per square inch

  in the cylinder is all that is necessary to move the door, and any pressure greatly in excess of this may damage some of the equipment. To guard against this danger, a relief valve is placed in the pressure line beyond the pump.

The usual construction of a relief valve consists of a valve body containing a valve which is held against a seat by a spring whose tension can be adjusted for any desired operating pressure (see Figure 1-23, A). When the fluid pressure is greater than the spring tension, the spring is compressed and unseats the valve (see Figure 1-23, B), thus bypassing the fluid back into the reservoir.

b. Friction, turbulence, and thermal expansion. Oil, or in fact any liquid driven at high speed through an enclosed system, soon rises in temperature. This is caused by two factors:

1. Friction of the oil against the interior of the pipe lines, valves, and other parts. 2. Turbulence of the oil itself; for example, the swirls and eddies caused in the oil by its coming into contact at relatively high velocity with internal bends, its sudden emergence into wider or narrower places in the system, and so forth.

Friction is caused by the collision of individual oil molecules with the solid walls of pipes and other parts.

Turbulence causes another kind of friction, which is the result of the collisions of oil molecules with each other. Both kinds of

Figure 1-22. Spool-type control valve.
Figure 1-22. Spool-type control valve.
 
14

friction cause a loss of power through heat. The rise in temperature of the oil is caused by this friction-heat. The heat also causes a thermal expansion of the oil. Therefore, both heating and expansion inevitably occur whenever hydraulic fluid is pumped continuously through the system, even though it is not in use.

c. Power losses. When we compute the power necessary to operate our system, allowance must be made for power losses which

Figure 1-23. Principle of the relief valve.
Figure 1-23. Principle of the relief valve.

cannot be prevented. A pressure of 100 pounds per square inch acting upon a piston whose base has an area of 2 square inches should theoretically provide a working force of 200 pounds. However, this is possible only theoretically.

In practice, as fluid passes through the hydraulic lines, it meets resistance from the inner pipe walls. Some of the energy imparted to the oil by the pump is lost in friction. At low rates of flow, the fluid will flow

  in fairly straight lines. At high rates of flow, the flow becomes turbulent and friction losses increase. Friction and turbulence losses usually range between 10 percent and 20 percent of the developed power. Instead of getting a 200-pound force to open the door in our system, we may obtain a force of only 160 pounds because of these losses. Therefore, in conformance with good hydraulic design, we must either increase the pump pressure, enlarge the piston area in the actuating cylinder, or increase the size of the pipes and passages to compensate for the loss of energy.

d. Need for a bypass valve in a power-driven system. Since friction always increases with rate of flow, it follows that the greater the rate at which oil circulates in the system -all other things being equal- the more the oil will be heated. Also, the greater the length of the circuit traveled by the fluid during this free, or no-load, circulation, the greater the friction surface and consequent heating. To reduce both the pressure and the length of circuit to a minimum, a bypass valve is provided. This valve returns the oil from the pressure side of the pump directly to the reservoir, or supply tank, without its first having traveled through the rest of the system. Thus, the bypass valve in effect "short-circuits" the oil pressure from the pump, leaving the oil in the remainder of the system inactive, and reducing the pressure at which the oil circulates to atmospheric pressure.

A bypass valve may be operated by hand or automatically in the same manner as a relief valve, or by remote control. When automatic, it is known as an automatic bypass or unloading valve. In actual practice, an automatic bypass arrangement requires more complex equipment than is shown in Figure 1-23. It is shown here merely in a schematic view, greatly simplified for explanatory purposes.

e. Operation of system using power-driven pump. Since the power-driven pump has been turned on and has come up to its operating speed, hydraulic power at the working pressure becomes constantly and instantaneously available. The automatic bypass,

 
15

or unloading valve, and the relief valve will relieve any pressure greatly in excess of this.

To close the door, the control valve handle is turned so that fluid under pressure is directed to the side of the actuating cylinder which is marked d; the movement of the piston closes the door. It also pushes out the fluid which has been trapped on the side of the cylinder marked o. The expelled fluid reenters the system through the return line of the control valve and flows back to the reservoir. To lock the door shut, the control valve handle is turned to its neutral position; the door will then remain shut until the control valve is moved to the OPEN position.

To open the door, the control valve is turned so that fluid enters the actuating cylinder at o. This moves the piston back to the d side of the cylinder and forces out the fluid, which was delivered there when the door was originally closed. The fluid is then returned to the system.

Observe that the two lines connecting the actuating cylinder to the control valve have a dual function. Depending upon which way the hydraulic fluid is directed, one side becomes the pressure line and the other the return line. A change in direction reverses their functions.

During those intervals between opening and closing the door, the fluid circulates between the pump and the reservoir; the automatic bypass valve short-circuits the pressure from the pump, as explained above.

1B8. Practical hydraulics on the submarine. In an extremely simplified form, we have, just described a basic hydraulic system. In actual appearance the hydraulic equipment installed aboard a submarine may not closely resemble such basic units. Nevertheless, the same principles govern both systems.

In a submarine, a single system actuates a multitude of devices and appears to be far more complete. Stripped to its essentials, each unit is moved by a hydraulic motor which receives its power in the form of fluid pressure from a central pumping plant. The liquid moves through pipes and its flow is directed by valves. Essentially, therefore, the submarine hydraulic system does not deviate

  in principle from the simple system we designed and discussed.

Actually a submarine employs not one, but four separate hydraulic systems:

1. The steering system, which operates the rudder.

2. The stern plane system, which tilts the stern diving planes to dive or rise.

3. The bow plane tilting system, which tilts the bow diving planes to rise or dive.

4. The main hydraulic system, which operates the following equipment.

a.Flood and vent valves.
b.Main air induction valve.
c.Bow plane rigging.
d.Windlass-and-capstan in bow.
e.Main engine outboard exhaust valves (in some installations hydro-pneumatic).
f.Torpedo tube outer doors.
g.Emergency power for steering system if failure occurs.
h.Emergency power for bow plane tilting system.
i.Emergency power for stern plane tilting system.
j.Periscope hoists.
k.Vertical antenna hoist.
l.Sound heads.

These functions may vary somewhat among different submarine classes. They represent an accurate picture of the usefulness of hydraulics as applied to the submarine. Moreover, the functions of hydraulics are constantly increasing because hydraulics has proved to be superior as a source of power. Let us summarize its advantages:

a.Lighter weight of units.
b.Controllability in small movements.
c.Low inertia of moving parts.
d.Simplicity.
e.Positive operation.
f.Self-lubrication.
g.Little wear or corrosion.
h.Relatively silent operation.
i.System not apt to be disrupted by salt spray or water.
j.Less maintenance.

We are now ready in succeeding chapters to examine each of the systems in detail to see how each system works and how to keep it working in the vent of trouble.

 
16

Sub Elec. Home
Sub Hydr. Home Page
Next chapter
Next chapter


Copyright © 2013, Maritime Park Association
All Rights Reserved
Legal Notices and Privacy Policy
Version 1.10, 22 Oct 04