TM 1-411, Airplane Hydraulic Systems and Miscellaneous Equipment: 2 - Hydraulic System Units

SECTION II: HYDRAULIC SYSTEM UNITS

3. General.-a. A unit of hydraulic equipment is a part of the hydraulic system having a distinct function to perform.

b. The names of the several units are generally suggestive of the functions performed by them. Some have names long associated with airplane equipment; a considerable number, however, have names of relatively recent origin.

c. The units discussed in this section are representative of the hydraulic units now in general use. Each of the units treated in the discussion is taken as representative of the simplest device that will

FIGURE 3.-Reservoir.

completely perform the related function. The figures are designed to illustrate the discussion and are not intended to be detail drawings of the units.

4. Reservoir.-a. The reservoir (fig. 3) houses the supply of fluid for the hydraulic system. Fluid is drawn from the reservoir by the hydraulic pumps, forced by them throughout the system, and eventually returned to the reservoir. The reservoir is the unit through which fluid is introduced into the hydraulic system. Not only does the reservoir supply the operating needs of the system, it also replenishes fluid lost through leakage and seepage. Furthermore, the reservoir serves as an overflow basin to receive the excess of fluid forced out of the system by temperature expansion and by piston rod displacement. The reservoir also affords an opportunity for the fluid to purge itself of air bubbles induced into the system by certain operating units. Foreign particles and solids picked up in the system are deposited in the reservoir.

b. Reservoirs are always vented to the atmosphere. The vent line is led overboard and the excess fluid is thereby disposed of. A filter

FIGURE 4-Gear type power pump.

screen may be provided in the filler neck and a sediment trap is located at the lowest point in the reservoir. A standpipe is installed in the reservoir to insure that the hand pump is fed, even though the fluid supply has been depleted to the point of starving the power pump. A fluid quantity gage is sometimes installed as an accessory and may be used as a guide in filling the reservoir. In most cases,

FIGURE 5.-Vane type power pump.

however, the reservoir is simply filled to overflowing. The reservoir is usually located in the system at a higher level than the pump so as to keep the pump primed at all times.

5. Power pump.-a. A power pump is the energizing unit of the hydraulic system. It is the unit with which circulation of the fluid is induced and working pressures developed. The power pump draws fluid from the reservoir and forces it throughout the hydraulic system.

b. A power pump may be engine driven, in which case it is mounted on the accessory section of the engine crankcase, or it may be driven by an electric motor and mounted in any convenient location, usually near the reservoir. The pump may be of the gear type, or it may be of the vane or gerotor types -(see figs. 4, 5, and 6). In any case, only a small volume of fluid is passed per revolution of the pump. Due to the close clearances common in hydraulic pumps, very high pressures may be obtained from them. The capacities of the hydraulic pumps in general use vary from 11/2 to 3 gallons per minute.

FIGURE 6.-Gerotor type power pump.

Maximum working pressures of 1,600 pounds per square inch are obtainable. The smaller capacity pumps absorb approximately 11/2 horsepower at full load and the larger ones approximately 3 horsepower to operate them. Most power pumps have a shear pin incorporated in the drive mechanism. This is to free the pump and prevent injury to the system in case of failure of the system relief valve. Power pumps are not intended to operate at peak loads for periods in excess of 4 to 5 minutes. A power pump is a very specialized piece of equipment and should be repaired only by specially trained personnel.

6. Hand pump.-a. A hand pump is intended to serve as a substitute for the power pump during emergencies in flight and also as a source of power for checking the hydraulic system when the airplane is at rest on the ground. It is fed from the reservoir and discharges into the pressure manifold.

b. The hand pump is a manually operated reciprocating piston type pump. It may be of the single-action type (fig. 7) or it may be of the double-action type (fig. 8). In either case check valves are required on the inport and on the outport sides of the pump.

FIGURE 7.-Single-action band pump.

The check valve on the inport side prevents fluid under pressure in the pump cylinder from flowing back to the reservoir. The check valve on the outport side prevents fluid from being sucked into the pump from the pressure manifold. The band pump is always located so that the handle is readily accessible to the pilot and also to members of the crew. The handle of the pump is of such length and the piston is of such area that working pressures may be developed without undue exertion on the part of the operator. A check valve is usually placed in the pressure manifold upstream from the point where the hand pump discharges into the manifold. This is to concentrate the output of the hand pump into that section of the hydraulic system where it will insure operation of the mechanism in case of failure of power pump.

FIGURE 8.-Double-action hand pump.

7. Pressure manifold.-a. The pressure manifold conducts fluid from the hydraulic pumps to the selector valves. The pressure tank, pressure regulator, system relief valve, etc., are part of or auxiliary to the pressure manifold. It is the main line of the pressure side of the hydraulic system.

b. The pressure, manifold as illustrated in figure 9 may consist of a prefabricated branching tube or it may consist of an assembly of tubes and fittings.

FIGURE 9-Pressure manifold.

8. Surge chamber.-a. A surge chamber is a device for modulating pressure surges in the hydraulic system. When the pressure is on the increase, air or a spring is compressed; as the pressure recedes, the compressed air or spring expands. Thus, sudden pressure

FIGURE 10.-Bladder type surge chamber.

increases that might otherwise impose destructive stresses on the hydraulic system are damped out.

b. Surge chambers may contain an inflatable rubber bladder, or in some cases, a coiled spring and piston. Surge chambers are mounted into the pressure side of the system, often as an adjunct to a relief valve or a power-control valve. The two types are illustrated in figures 10 and 11.

FIGURE 11-Spring type surge chamber.

9. Pressure tank.-a. A pressure tank, or "accumulator" as shown in figure 12, is essentially a very large surge chamber. In addition, however, to damping out pressure surges, a pressure tank is intended to serve as an energy storage device. Under circumstances when the system does not require the total output of the power pump, the excess fluid is forced into the pressure tank. The, air in the pressure tank is compressed by the incoming fluid and absorbs and stores

FIGURE 12-Pressure tank.

the energy required to compress it. When the output of the power pump falls below the requirements of the hydraulic system, the compressed air in the pressure tank forces fluid back into the pressure manifold. This fluid is returned to the pressure manifold under pressure and in sufficient quantities to operate the mechanism for a limited time. The pressure tank ordinarily supplements the power pump at periods of peak load but may also serve as the motivating force during emergencies in flight and during landing when the power pump is inactive or operating at a decreased r. p. m.

b. Pressure tanks are built to withstand high pressures and to contain considerable quantities of fluid. The tank is divided into two compartments, an air compartment and a fluid compartment. The two compartments are separated by a piston or by a synthetic rubber diaphragm.

FIGURE 13-Pressure regulator.

The air compartment is equipped with an air valve for charging the tank with compressed air. By serving the tank with an initial charge of air it assures that the total fluid output of the tank will be at sufficiently high pressures. The pressure tank is usually located near the pressure manifold of which it is an adjunct.

10. Pressure regulator.-a. A pressure regulator is, as the name implies, a device for regulating pressures. When the pressure in the pressure manifold attains the upper limit of the pressure range for which the regulator is adjusted, a valve opens and the output of the power pump is bypassed back to the reservoir. When the pressure in the pressure manifold drops to the lower limit of the predetermined range, the valve closes and the output of the pump is directed into the pressure manifold. The range, of pressures for which the regulator is set to pass fluid into the pressure manifold is relatively broad, in some cases 25 percent of the maximum working pressure. A pressure regulator operates very much as a relief valve except that its control of pressure is over a much wider range and the valve is held open by system pressure instead of pump pressure. This permits the power pump to operate without load after working pressure has been attained and throughout those periods when the output of the power pump is not required to operate mechanism.

b. The principle of operation of a pressure regulator is shown in figure 13. The regulator consists of several chambers. In one end of the unit is a relief valve. In the opposite end of the unit is a piston. Between the two is the bypass chamber. Fluid enters the valve end of the unit from the power pump and, since the spring loaded valve is closed, the fluid continues on to the opposite end of the unit and thence into the pressure manifold. Fluid flows from one end of the unit to the other through a shunt line which includes a check valve. As pressure builds up in the pressure manifold and in the pressure regulator, the pressure of the fluid on the piston impels it to move inward until the piston rod attached to the inner side of the piston rests against the spring loaded valve. When the fluid pressure attains the upper limit of the pressure range for which the regulator is adjusted to operate, the force developed by the pressure of the fluid on the piston overcomes the combined force of the fluid pressure plus the spring tension exerted against the valve to hold it closed, and the valve is pushed off its seat. The output of the pump is now diverted through the open valve, thence through the bypass outlet and back to the reservoir. The check valve in the shunt line prevents the fluid under pressure in the pressure manifold from escaping through the open valve and bypass. The valve remains open and the output of the pump is circulated freely to the reservoir as long as the pressure in the pressure manifold is sufficient to hold the valve open. In case the pressure in the pressure manifold drops below the lower limit of the pressure range for which the regulator is adjusted to operate, the pressure on the valve plus the tension of the spring overcomes the weakened force on the piston and the valve closes. The output of the pump is now directed into the pressure manifold again. The pressure regulator is always located at the extreme upstream end of the pressure manifold.

11. Relief valve.-a. The function of a relief valve is to release pressure in some section of the hydraulic, system when such pressure has attained a predetermined value. Several relief valves may be required in a hydraulic system, each serving some particular section

FIGURE 14.-Relief valve.

of the system. In such cases, it is not uncommon to have the different valves set to open at different pressures. The pressure for which a relief valve is set to open will be well above the minimum pressure required to operate the mechanisms, but below a pressure estimated to be a safe operating pressure. When a relief valve is intended only to relieve pressures caused by increase of temperature, the valve is apt to be termed a temperature expansion valve. Such valves are set to open at relatively high pressures. In any case, a relief valve passes fluid from the pressure side of the system to the return side.

b. A relief valve (fig. 14) depends upon a coil spring to hold the valve closed. The pressure in the system must overcome the tension of the spring to push the valve open. When the excess pressure has been released, the tension of the spring again closes the valve. The tension of the spring is controlled by an external adjustment provided for the purpose.

12. Check valve.-a. The function of a check valve is to confine fluid under pressure within some section of the hydraulic system. It prevents the fluid from reversing its normal direction of flow

FIGURE 15.-Check valve.

and thereby prevents pressure from escaping into an adjacent section of the system. Check valves are used in both the pressure side of the system and in the return side.

b. The valve mechanism of a check valve is held to its seat by either a coiled spring as shown in figure 15, or by its own weight. In the latter case it is called a flap check. When the pressure on the downstream side of the valve exceeds that on the upstream side, the resultant unbalanced force seals the valve closed. When the relative pressures are reversed the valve is forced open against the tension of the spring and fluid passes through. The tension of the spring is relatively weak and is intended to be barely sufficient to support the valve in its proper position. Ordinarily, no means of adjustment of the tension of this spring is provided.

13. Orifice.-An. orifice in a hydraulic line is simply a restriction as illustrated in figure 16. The, object of an orifice is to restrain the rate of flow of the fluid in the line. The mechanism actuated by the fluid is thus caused to move more slowly. An orifice is used in connection with mechanisms the movement of which would be too fast with an unrestrained flow.

14. Orifice check.-a. Since an orifice restrains the flow of fluid similarly in both directions, it cannot be used in those sections of

FIGURE 16-Orifice.

the system where it is required that the flow in one direction only be restrained. In such cases, a modified check valve called an orifice check is used.

b. In the case of an orifice check as shown in figure 17, the valve head is so formed as to prevent a perfect seal in its seat. In one direction the flow is unrestrained, while in the opposite direction the flow is restrained but not completely shut off. A common employment of an orifice check is in the wing flap system where it is desirable that the uptravel of the flaps be delayed against the tendency of the air pressure to force them up. Another employment of the orifice check is in the landing gear system to delay the extension of the gear against the tendency of the weight of the gear to pull it down too fast. An orifice check may be installed in either the upline or the downline to the actuating cylinder. The line in which it is installed and the direction in which it is installed in the line will determine which stroke of the mechanism is slowed up.

15. Bypass check.-A bypass check as illustrated in figure 18 is simply a check valve provided with a. means whereby the valve may be manually opened to permit fluid to flow in either direction. The unit is usually employed in connection with a pressure tank so that the output of the hand pump may be diverted into the pressure tank. The normal setting of the valve is such as to concentrate the output of the hand pump into a restricted section of the pressure manifold. By moving a control lever which unseats the valve, a passage is provided whereby the output of the hand pump can reach the pressure tank. The bypass check is installed in the pressure

FIGURE 17.-Orifice check valve.

manifold somewhere between the pressure tank and the point where the hand pump discharges into the manifold. The control lever is located accessible to the pilot or one of the crew and is normally set in "system" position.

16. Selector valve.-a. The selector valve is the control unit of the hydraulic system. The mechanism moved, the direction it is moved, and the distance it is moved are determined and controlled by manipulation of the selector valve or valves.

FIGURE 18.-Bypass check valve.

b. There are three types of selector valves commonly used in hydraulic systems. They are the rotor type, the piston type, and the poppet type and are illustrated in figures 19, 20, and 21, respectively.

c. The rotor type selector valve is a four-port valve consisting of an inner rotor, an outer case, and a control handle. The four ports in the case are spaced 90° apart. The rotor carries two fluid channels so arranged as to connect adjacent ports. When one port is connected to the pressure manifold, the opposite port is connected to the return manifold. The other two ports are connected to the actuating cylinder, one to one end of the cylinder and the other to the opposite end. It is thus seen that a rotation of 90° of the rotor changes the flow of fluid from one end of the actuating cylinder to the other end, thus reversing the direction of movement of the piston. The fluid purged from the actuating cylinder flows through the selector valve and thence into the return manifold. In this type selector valve, when one port is open, they are all open, and the valve is closed to the passage of fluid midway between port openings. This midport setting of the selector valve may be, used when it is desirable to stop the mechanism before it has reached the end of a stroke. This use of the valve is applicable to wing flap control. The setting is termed the, "neutral" setting of the valve.

FIGURE 19.-Rotor type selector valve.

FIGURE 20.-Piston type selector valve.

FIGURE 21.-Poppet type selector valve.

d. The piston type selector valve consists of a grooved piston within a cylinder. The piston is hollow part of its length. The hole in the piston provides a fluid passage to the outport of the valve.

The inport of the valve is midway of the cylinder and the outport is in one end. The two ports leading to the actuating cylinder are on the side of the cylinder opposite the inport. By moving the piston back and forth in the cylinder, the desired port alinement is obtained.

When none of the ports aline, the valve setting is neutral.

e. The poppet type selector valve consists of a, series of spring loaded cone seat valves. The valves are actuated by cams. The cams are so disposed on the camshaft that rotation of the shaft opens the proper combination of valves to effect the desired control of flow through the valve assembly. A control handle fastened to the camshaft provides a means of rotating the shaft. Each selector valve has two inlet valves and two outlet (unloading) valves. When one of the inlet valves is opened, a companion outlet valve opens. When the alternate inlet valve is opened, the other outlet valve opens. Thus, fluid flow is controlled and directed to the actuating cylinder. When all four valves are closed, the setting is neutral; and the control handle is in the neutral position.

FIGURE 22.-Power control valve.

17. Power control valve.-a. A power control valve is essentially a hand shut-off valve with an automatic turn-on feature. It permits the circulating of fluid from the power pump to the reservoir without imposing on the pump the burden of continuously developing the high pressures required to keep a relief valve open. In this respect the power control valve performs the same function in a system without a pressure tank that a. pressure regulator does in a system that includes a pressure tank.

b. When pressure is not required in the hydraulic system, the output of the power pump is directed through the power control valve, as illustrated in figure 22 and thence to the reservoir. When pressure is required in the system, the knob on the power control is pressed inward as shown in figure 22. This causes a piston to enter the outport of the valve, and thus close the fluid passage through the unit. The output of the power pump is now directed into the system where pressure builds up to actuate the mechanism. When the mechanism has been moved to the limit of its stroke, excess pressure develops in the system and in the power control valve. This excess pressure lifts the spring loaded pin that locks the piston in the closed position. The pressure that lifts the pin also slides the piston out of the outport. This opens the passage in the power control valve and. again permits the fluid to circulate freely to the reservoir. The pressure at which the valve releases is controlled by the tension of the coil spring surmounting the lock pin. An external adjustment is provided for the purpose of regulating the tension of the spring. When the valve releases, the control knob is returned to its original position. Power control valves are usually located in close proximity to the selector valve controls.

c. In case more than one power control unit is installed in an airplane, they are connected in series so that the manipulation of any one of them is equivalent to manipulating all of them in unison, that is, if the knob of any one of the power-control units is pushed inward, the circulation of fluid to the reservoir is shut off and pressure starts to build up in the hydraulic system.

18. Master cylinder.-a. The master cylinder (fig. 23) is the energizing unit of a hydraulic brake system; a system separate and apart from the general hydraulic system. There is one master cylinder for each wheel brake--one distinct hydraulic system for each brake assembly.

b. A master cylinder is a manually operated, single-action reciprocating piston pump. It is operated by a toe pedal mounted one on each of the two rudder pedals. Usually each master cylinder is fed from a reservoir integral in the unit. In some cases, however, the two cylinders are fed from a common reservoir or supply tank. In any case, the reservoir is vented and the feed to the working chamber of the unit is by gravity.

c. When pressure is applied to the toe pedal, the piston is advanced within the cylinder. When pressure is released on the toe pedal, a coil spring forces the piston back to its original position. On the down stroke of the toe pedal, fluid is forced out of the master cylinder through the fluid line and into the brake actuating cylinder. On the return stroke of the toe pedal, the return springs in the brake system, acting against their respective pistons force the fluid back to the master cylinder.

FIGURE 23.-Master cylinder.

It is evident that the fluid in the hydraulic brake system does not circulate; it simply flows back and forth.

d. To preclude the possibility of the brakes being applied by pressure developed by temperature expansion, a compensating valve is placed between the working chamber and the reservoir.

FIGURE 24.-Brake control valve.

This valve is open at all times when the brakes are "off." The valve is an adaptation of a flap valve and simply falls open. During application of the brakes, fluid pressure within the working chamber holds the valve closed. The resiliency of the coil spring absorbs the excess pressure caused by temperature expansion when the brakes are in the "on" position. The spring also assures that the brakes remain "on" in case of fluid contraction caused by decrease of temperature.

e. To enable the brakes to be locked in the applied or "on" position, a mechanism called a parking brake is incorporated in the brake system. This is a manually operated latch by means of which the toe pedals are held in the depressed or "on" position. When the latch is released, the pedals return to the neutral or "off " position.

19. Brake control valve.-a. A brake control valve is used in place of a master cylinder to actuate the wheel brakes of the larger airplanes where high brake pressures are required. It is a device for metering fluid out of the pressure manifold of the hydraulic system at a pressure required to operate the brakes.

FIGURE 25-Actuating cylinder.

b. When pressure is applied to the brake pedal, a piston and valve pin are pushed upward, as shown in R, figure 24. The valve pin unseats, a spring loaded valve in the head of the unit. Fluid enters through the open valve and then flows through a port into the brake line. Fluid will flow into the brake line until the piston pressure and brake pressure are equal. The piston and pin will now be forced down, allowing the ball to seat and relieving the brake from further increase in pressure. A  bar spring in the brake control linkage will accommodate the downward movement of the piston for a given position of the brake pedal. When pressure is released on the brake pedal, the piston moves down, unseating the valve pin as in L, figure 24. As soon as the piston and valve pin are separated, two holes in the piston are uncovered, thus providing an outlet for the fluid to flow into the return manifold of the hydraulic system and relieve the brake pressure.

20. Actuating cylinder.-a. An actuating cylinder is the unit in the hydraulic system where fluid pressure is transposed into mechanical action.

b. An actuating cylinder (fig. 25) consists of a cylinder containing a piston, a piston rod, and piston-rod seals. The piston rod extends through one or both ends of the cylinder. Two ports are located near the ends of the cylinder. Each port is alternately an inport and an outport, depending upon the setting of the control valve.

c. Upon fluid entering one of the ports, the piston is driven toward the opposite end of the cylinder. The piston rod transmits the, motion of the piston, and the mechanism to which the piston rod is fastened is caused to move. The fluid ahead of the piston is pushed out of the cylinder and is returned to the reservoir by way of the selector valve and return manifold. By changing the setting of the selector valve, the direction of flow of the fluid from the selector valve to the actuating cylinder is reversed and the direction of travel of the piston is thereby reversed. Upon completion of the reverse movement, the mechanism has been moved through one complete cycle, or two complete strokes.

d. Although the above. describes a cycle of operation of actuating cylinders in general, the brake actuating cylinder (fig. 26) must, be mentioned as an exception. In this case the cylinder has but one port. As fluid under pressure enters this port, the piston is pushed toward the opposite end of the cylinder. When the stroke has been completed and pressure released on the fluid, the return springs in the brake assembly push the piston back to its original position and force the fluid out of the cylinder and into the fluid line from whence it came.

FIGURE 26.-Brake cylinder.

21. Return manifold.-The return manifold (fig. 27) is, in general, that side of the hydraulic system which returns the fluid from the actuating cylinders, the relief valves, and the power control valves to the reservoir. It parallels, in reverse, the service of the pressure manifold.

FIGURE 27.-Return manifold.

22. Line disconnect.-A line disconnect is a check valve installed in the end of a tube. When the tube is connected to another tube as shown in figure 28(1), the check is held off its seat. The check valve is thus kept open as long as the two tubes are connected. When the two tubes are disconnected, as in figure 28(2), the check immediately falls into its seat and seals the tube in which it is installed. Line disconnects are generally used at the fire wall where connections are made from the engine assembly hydraulic system to the main hydraulic system.

FIGURE 28-Line disconnect valve.

The use of these units obviates the necessity of draining or partially draining the hydraulic system when an engine change is made or the hydraulic power pump is removed.

23. Pressure gage snubber.-a. A pressure gage snubber is a device for damping the oscillations of the pressure gage indicator. The pressure impulses originating in the power pump and transmitted through the fluid to the pressure gage tend to oscillate the indicator. Although these oscillations are of fairly small amplitude, they are nevertheless jerky and disconcerting to the observer of the gage.

b. A snubber (fig. 29) is essentially a passage with a plunger in it. Any increase in pressure forces fluid past the plunger; and due to the close fit of the plunger and passage, the flow of fluid to the gage is restricted, thus introducing a time lag which prevents constant and violent oscillations of the gage pointer. The snubber is mounted in the pressure line to the gage and it is usually placed close to the gage.

FIGURE) 29-Pressure gage snubber.

24. Bleed.-a. A bleed may not properly be termed a unit of hydraulic equipment but it is an important feature of certain hydraulic units.

b. The object of a bleed is to transfer pressure past the seal of a hydraulic unit. It constitutes a perpetual internal leak in the unit. The bleed permits a slow and gradual equalizing of pressures on either side of the seal. It is essentially a device for normalizing pressures.

a A bleed consists of an extremely small hole drilled in the unit in such a manner as to shunt fluid around the seal as shown in figure 18. In case a check or bypass check valve is installed in the pressure manifold, it is not uncommon to provide it with a bleed so that excessive pressures created by temperature expansion in the lower section of the manifold can reach the pressure tank or surge chamber, or other pressure absorbing device in the upper section of the manifold.