Outlet Inlet Outlet Inlet port port port port Drilled passage Simple-type in-line check valve (ball-type) Orifice-type in-line check valve (ball-type) Inlet Outlet I] Flow direction marking on simple- Flow direction marking on orificetype in-line check valve type in-line check valve FICURE 8-22. Typical in-line check valves.
of a mechanism by providing free flow of fluid in one direction, while allowing limited operating speed through restricted flow of fluid in the oppo site direction. The operation of the orifice-type in-line check valve is the same as the simple-type in-line check valve, except for the restricted flow allowed when closed. This is accomplished by a second opening in the valve seat that is never closed, so that some reverse flow can take place through the valve. The second opening is much smaller than the opening in the valve seat. As a rule, this opening is of a specific size thus maintaining close control over the rate at which fluid can flow back through the valve. This type of valve is sometimes called a damping valve.
The direction of fluid flow through in-line check valves is normally indicated by stamped arrow markings on the housings (figure 8-22C and D).
On the simple-type in-line check valve, a single arrow points in the direction which fluid can flow.
The orifice-type in-line check valve is usually marked with two arrows. One arrow is more pronounced than the other, and indicates the direction of unrestricted flow. The other arrow is either of smaller size than the first or of broken-line construction, and points in the direction of restricted reverse fluid flow.
In addition to the ball-type in-line check valves shown in figure 8-22, other types of valves, such as disks, needles, and poppets are used.
The operating principles of integral check valves are the same as the operating principles of in-line check valves.
Line-Disconnect or Quick-Disconnect Valves These valves are installed in hydraulic lines to prevent loss of fluid when units are removed. Such valves are installed in the pressure and suction lines of the system just in front of and immediately behind the power pump. These valves can also be used in other ways than just for unit replacement.
A power pump can be disconnected from the system and a hydraulic test stand connected in its place. These valve units consist of two interconnecting sections coupled together by a nut when installed in the system. Each valve section has a piston and poppet assembly. These are spring loaded to the CLOSED position when the unit is disconnected.
Spring Piston
b. Spring e. Piston
c. Poppet f. Poppet FIGURE 8-23. Line disconnect valve.
The top illustration of figure 8-23 shows the valve in the LINE-DISCONNECTED position. The two springs (a and b) hold both poppets (c and f) in the CLOSED position as shown. This prevents loss of fluid through the disconnected line. The bottom illustration of figure 8-23 shows the valve in the LINE-CONNECTED position. When the valve is being connected, the coupling nut draws the two sections together. The extension (d or e) on one of the pistons forces the opposite piston back against its spring. This action moves the poppet off its seat and permits the fluid to flow through that section of the valve. As the nut is drawn up tighter, one piston hits a stop; now the other piston moves back against its spring and, in turn, allows fluid to flow. Thus, fluid is allowed to continue through the valve and on through the system.
Bear in mind that the above disconnect valve is only one of the many types presently used. Although all line-disconnect valves operate on the same principle, the details will vary. Each manufacturer has his own design features.
A very important factor in the use of the linedisconnect valve is its proper connection. Hydraulic pumps can be seriously damaged if the line disconnects are not properly connected. If you are in doubt about the line disconnect's operation, consult the aircraft maintenance manual.
The extent of maintenance to be performed on a quick disconnect valve is very limited. The internal parts of this type valve are precision built and factory assembled. They are made to very close tolerances, therefore, no attempt should be made to disassemble or replace internal parts in either coupling half. However, the coupling halves:
lock-springs, union nuts, and dust caps may be replaced. When replacing the assembly or any of the parts, follow the instructions in the applicable maintenance manual.
ACTUATING CYLINDERS An actuating cylinder transforms energy in the form of fluid pressure into mechanical force, or action, to perform work. It is used to impart powered linear motion to some movable object or mechanism.
A typical actuating cylinder consists fundamentally of a cylinder housing, one or more pistons and piston rods, and some seals. The cylinder housing contains a polished bore in which the piston operates, and one or more ports through which fluid enters and leaves the bore. The piston and rod form an assembly. The piston moves forward and backward within the cylinder bore and an attached piston rod moves into and out of the cylinder housing through an opening in one end of the cylinder housing. Seals are used to prevent leakage between the piston and the cylinder bore, and between the piston rod and the end of the cylinder. Both the cylinder housing and the piston rod have provisions for mounting and for attachment to an object or mechanism which is to be moved by the actuating cylinder.
Actuating cylinders are of two major types:
(1) Single-action and (2) Double-action. The single-action (single port) actuating cylinder is capable of producing powered movement in one direction only. The double-action (two port) actuating cylinder is capable of producing powered movement in two directions.
Single-Action Actuating Cylinder A single-action actuating cylinder is illustrated in figure 8-24. Fluid under pressure enters the port at the left and pushes against the face of the piston, forcing the piston to the right. As the piston moves, air is forced out of the spring chamber through the vent hole, compressing the spring.
When pressure on the fluid is released to the point that it exerts less force than is present in the compressed spring, the spring pushes the piston toward the left. As the piston moves to the left, fluid is forced out of the fluid port. At the same time, the moving piston pulls air into the spring chamber through the venthole. A three-way control valve is normally used for controlling the operation of a single-action actuating cylinder Piston O-ring packing Fluid port Return spring Vent Mounting holes FICURE 8-24. Single-action actuating cylinder.
Double-Action Actuating Cylinder A double-action (two-port) actuating cylinder is illustrated in figure 8-25. The operation of a double-action actuating cylinder is usually controlled by a four-way selector valve. Figure 8-26 shows an actuating cylinder interconnected with a selector valve. Operation of the selector valve and actuating cylinder is discussed below.
Placing the selector valve in the "on" position (figure 8-26A) admits fluid pressure to the letthand chamber of the actuating cylinder. This results in the piston being forced toward the right.
Fluid port O-ring Fluid port FIcURE 8-25. Double-action actuating cylinder.
As the piston moves toward the right, it pushes return fluid out of the right-hand chamber and through the selector valve to the reservoir.
When the selector valve is placed in its other "on" position, as illustrated in figure 8-26B, fluid pressure enters the right-hand chamber, forcing the piston toward the left. As the piston moves toward the left, it pushes return fluid out of the left-hand chamber and through the selector valve to the reservoir. Besides having the ability to move a load into position, a double-acting cylinder also has the ability to hold a load in position. This capability exists because when the selector valve used to control operation of the actuating cylinder is placed in the "off" position, fluid is trapped in the chambers on both sides of the actuating cylinder piston. Selector valve Actuating cylinder Hoses R P In addition to the two general design types of actuating cylinders discussed (single-action and double-action), other types are available. Figure 8-27 shows three additional types.
SELECTOR VALVES Selector valves are used to control the direction of movement of an actuating unit. A selector valve provides a pathway for the simultaneous flow of hydraulic fluid into and out of a connected actuating unit. A selector valve also provides a means of immediately and conveniently switching the directions in which the fuid Hows through the actuator, reversing the direction of movement.
One port of the typical selector valve is connecter?
with a system pressure line for the input of fluid pressure. A second port of the valve is connected to a system return line for the return of fluid to the reservoir. The ports of an actuating unit through which fluid enters and leaves the actuating unit are connected by lines to other ports of the selector valve.
Selector valves have various numbers of ports.
The number of ports is determined by the particular requirements of the system in which the valve is Port 1 Port 2 Port 3 Three-port actuating cylinder Actuating cylinder having ports in piston rod Double-action actuating cylinder having two exposed piston rod ends FIGURE 8-26. Control of actuating cylinder movement. FIGURE 8-27. Types of actuating cylinders.
used. Selector valves having four ports are the most commonly used. The term four-way is often used instead of four-port in referring to selector valves.
The ports of selector valves (figure 8-28) are individually marked to provide ready identification.
The most commonly used markings are: PRES- SURE (or PRESS, or P), RETURN (or RET, or R), CYLINDER 1 (or CYL 1), and CYLINDER 2 (or CYL 2). The use of the word "cylinder" in the designation of selector valve ports does not indicate, as it may suggest, that only hydraulic cylinders are to be connected to the ports so marked. In fact, any type of hydraulic actuating unit may be connected to the ports. The numbers 1 and 2 are a convenient means of differentiating between two ports of the selector valve.
Cy! 1: — Ret Typical port markings on selector valve having rotary movement Press Cyl 1 Cyl 2 Typical port markings on selector valve having slide movement FIcURE 8-28. Typical port markings on selector valves, Four-Way Closed-Center Selector Valve Because the four-way, closed-center selector valve is one of the most commonly used selector valves in an aircraft hydraulic system, it is discussed in detail in the following paragraphs. Valving devices of various kinds, such as balls, poppets, rotors, or spools, are used in the four-way, closed-center selector valves.
Figure 8-29A illustrates a four-way, closed-center selector valve in the "off" position. All of the valve ports are blocked, and fluid can not flow into or out of the valve.
In figure 8-29B, the selector valve is placed in one of its "on" positions. The PRESS port and CYL 1 port become interconnected within the valve.
As a result, fluid flows from the pump into the selector valve PRESS port, out of the selector valve CYL 1 port, and into port A of the motor. This flow of fluid causes the motor to turn in a clockwise direction. Simultaneously, return fluid is forced out of port B of the motor and enters the selector valve CYL 2 port. Fluid then proceeds through the passage in the valve rotor and leaves the valve through the RET port.
In figure 8-29C, the selector valve is placed in the other "on" position. The PRESS port and CYL 2 port become interconnected. This causes fluid pressure to be delivered to port B of the motor, which results in the motor turning counterclockwise. Return fluid leaves port A of the motor, enters the selector valve CYL 1 port, and leaves through the selector valve RET port.
Spool-Type Selector Valve The valving device of the spool-type selector valve is spool-shaped (figure 8-30). The spool is a one-piece, leak-tight, free-sliding fit in the selector valve housing and can be moved lengthwise in the housing by means of the extended end which projects through the housing. A drilled passage in the spool interconnects the two end chambers of the selector valve. Selector valve spools are sometimes called pilot valves.
When the spool is moved to the selector valve "off" position, the two cylinder ports are directly blocked by the lands (flanges) of the spool (figure 8-30A). This indirectly blocks the PRESS and RET ports and fluid can not flow into or out of the valve.
Moving the spool toward the right moves the spool lands away from the CYL 1 and CYL 2 porte (figure 8-23B). The PRESS port and CYL 2 port then become interconnected. This permits fluid pressure to pass on to the actuating unit. The RET port and CYL 1 port also become interconnected. This provides an open route for the return of fluid from the actuating unit to the system reservoir.
Moving the spool toward the left moves the spool lands away from the CYL 1 and CYL 2 ports (figure 8-30C). The PRESS port and CYL 1 port then become interconnected. This permits fluid pressure to flow to the actuating unit. The RET port and CYL 2 port also become interconnected, provid ing a route for the return of fluid from the actuating unit to the reservoir.
Relief valve Reser voir valve (off). Selector Press. port, Pump Selector valve (on) valve (on). Selector ¡Cyl. 2 Cyl.1 R port port Port A (Turning (idle ) Motor Port B (Turning clockwise)? clockwise) counter- B FIGURE 8-29. Typical rotor-type, closed-center selector valve operation.
AIRCRAFT PNEUMATIC SYSTEMS Some aircraft manufacturers equip their aircraft with a pneumatic system. Such systems operate a great deal like hydraulic systems, except they employ air instead of a liquid for transmitting power.
Pneumatic systems are sometimes used for:
(1) Brakes.
(2). Opening and closing doors.
(3) Driving hydraulic pumps, alternators, starters, water injection pumps, etc.
(4) Operating emergency devices.
Both pneumatic and hydraulic systems are similar units and use confined fluids. The word "confined" means trapped or completely enclosed. The word "fluid" implies such liquids as water, oil, or anything that flows. Since both liquids and gases will flow, they are considered as fluids; however, there is a great deal of difference in the characteristics of the two. Liquids are practically incompressible; a quart of water still occupies about a quart of space regardless of how hard it is compressed. But gases are highly compressible; a quart of air can be compressed into a thimbleful of space, In spite of this difference, gases and liquids are both fluids and can be confined and made to transmit power.
The type of unit used to provide pressurized air for pneumatic systems is determined by the system's air pressure requirements.
High Pressure System For high-pressure systems, air is usually stored in metal bottles (figure 8-31) at pressures ranging from 1,000 to 3,000 p.s.i., depending on the particular system. This type of air bottle has two valves, one of which is a charging valve. A ground-operated compressor can be connected to this valve to add air to the bottle. The other valve is a control valve. It acts as a shutoff valve, keeping air trapped inside the bottle until the system is operated.
Although the high-pressure storage cylinder is light in weight, it has a definite disadvantage. Since the system cannot be re-charged during flight, operation is limited by the small supply of bottled air.
Such an arrangement can not be used for the continuous operation of a system. Instead, the supply of bottled air is reserved for emergency operation of such systems as the landing gear or brakes. The usefulness of this type of system is increased, however, if other air-pressurizing units are added to the aircraft.
On some aircraft, permanently installed air compressors have been added to re-charge air bottles whenever pressure is used for operating a unit.
Several types of compressors are used for this purpose. Some have two stages of compression, while others have three. Figure 8-32 shows a simplified schematic of a two-stage compressor; the pressure of the incoming air is boosted first by cylinder No. 1 and again by cylinder No. 2.
The compressor in figure 8-32 has three check valves. Like the check valves in a hydraulic hand pump, these units allow fluid to flow in only one direction. Some source of power, such as an electric 331
Press, Spool (pilot valve) spool. Used Extension of for control.
CyL2 CyL,2 Ret CyL1 Cy. 2 C FIGURE 8-30. Typical spool-type, closed-center selector valve.
motor or aircraft engine, operates a drive shaft. As the shaft turns, it drives the pistons in and out of their cylinders. When piston No. 1 moves to the right, the chamber in cylinder No. 1 becomes larger, and outside air flows through the filter and check valve into the cylinder. As the drive shaft continues to turn, it reverses the direction of piston movement. Piston No. 1 now moves deeper into its cylinder, forcing air through the pressure line and into cylinder No. 2. Meanwhile piston No. 2 is moving out of cylinder No. 2 so that cylinder No. 2 can receive the incoming air. But cylinder No. 2 is smaller than cylinder No. 1; thus, the air must be highly compressed to fit into cylinder No. 2.
Control valve Drive shatt turned by motor or engine Charging valve Filter Check valve To air storage bottle Cylinder Cvlinder 2 Pressure line Check valves FIGURE 8-32. Schematic of two-stage air compressor.
FICURE 8-31. Steel cylinder for high-pressure air storage.
Because of the difference in cylinder size, piston No. 1 gives the air its first stage of compression.
The second stage occurs as piston No. 2 moves deeper into its cylinder, forcing high-pressure air to flow through the pressure line and into the air storage bottle.
Medium Pressure System A medium-pressure pneumatic system (100 - 150 p.s.i.) usually does not include an air bottle.
Instead, it generally draws air from a jet engine compressor section. In this case, air leaves the engine through a takeoff and flows into tubing, carry.
ing air first to the pressure-controlling units and then to the operating units. Figure 8-33 shows a jet engine compressor with a pneumatic system takeoff.
Low Pressure System Many aircraft equipped with reciprocating engines obtain a supply of low-pressure air from vane-type pumps. These pumps are driven by electric motors or by the aircraft engine. Figure 8-34 shows a schematic view of one of these pumps, which consists of a housing with two ports, a drive shaft, and two vanes. The drive shaft and the vanes contain slots so the vanes can slide back and forth through the drive shaft. The shaft is eccentrically mounted in the housing, causing the vanes to forr four different sizes of chambers (A, B, C, and D). In the position shown, B is the largest chamber and is connected to the supply port.
As depicted in the illustration, outside air can enter chamber B of the pump.
Pressure controller unit To Air takeoff operating units FIGURF. 8-33. Jet engine compressor with pneumatic system takeoff.
Supply port Vane Chamber Vane 2 Drive Housing shaft Pressure port FIcURE 8-34. Schematic of vane-type air pump.
When the pump begins to operate, the drive shaft rotates and changes positions of the vanes and sizes of the chambers. Vane No. 1 then moves to the right (figure 8-34), separating chamber B from the supply port. Chamber B now contains trapped air.
As the shaft continues to turn, chamber B moves downward and becomes increasingly smaller, gradually compressing its air. Near the bottom of the pump, chamber B connects to the pressure port and sends compressed air into the pressure line. Then chamber B moves upward again becoming increasingly larger in area. At the supply port it receives another supply of air. There are four such chambers in this pump, and each goes through this same cycle of operation. Thus, the pump delivers to the pneumatic system a continuous supply of compressed air at from 1 to 10 p.s.i.
Relief Valves Relief valves are used in pneumatic systems to prevent damage. They act as pressure-limiting units and prevent excessive pressures from bursting lines and blowing out seals. Figure 8-35 illustrates a cutaway view of a pneumatic system relief valve.
At normal pressures, a spring holds the valve closed (figure 8-35), and air remains in the pressure line. If pressure grows too high, the force it creates on the disk overcomes spring tension and opens the relief valve. Then, excess air flows through the valve and is exhausted as surplus air into the atmosphere. The valve remains open until the pressure drops to normal.
Control Valves Control valves are also a necessary part of a typical pneumatic system. Figure 8-36 illustrates how a valve is used to control emergency air brakes. The control valve consists of a three-port housing, two poppet valves, and a control lever with two lobes.
In figure 8-36A, the control valve is shown in the "off" position. A spring holds the left poppet closed so that compressed air entering the pressure port cannot flow to the brakes. In figure 8-36B, the control valve has been placed in the "on" position.
One lobe of the lever holds the left poppet open, To Spring- outside air PNEUMATIC SYSTEM COMPONENTS — Disk Pneumatic systems are often compared to hy.
draulic systems, but such comparisons can only hold true in general terms. Pneumatic systems do From not utilize reservoirs, hand pumps, accumulators, pressure regulators, or engine-driven or electrically-driven line power pumps for building normal pressure. But similarities do exist in some components. FIcURE 8-35. Pneumatic system relief valve.
Control lever Check Valves Check valves are used in both hydraulic and pneumatic systems. Figure 8-37 illustrates a flaptype pneumatic check valve. Air enters the left port
• Vent port of the check valve, compresses a light spring, forcing the check valve open and allowing air to flow Poppet out the right port. But if air enters from the right, valve - Spring air pressure closes the valve, preventing a flow of air out the left port. Thus, a pneumatic check valve is a one-direction flow control valve.
essure port Brake port Drilled Restrictors passage Restrictors are a type of control valve used in pneumatic systems. Figure 8-38 illustrates an ori- A. Control valve "off" fice type restrictor with a large inlet port and a small outlet port. The small outlet port reduces the rate of airflow and the speed of operation of an actuating unit.
from power source aB passage Drilled To emergency @ brakes B. Control valve "on" Compressed air Atmospheric pressure FICURE 8-36. Flow diagram of a pneumatic control volve.
and a spring closes the right poppet. Compressed air now flows around the opened left poppet, through a drilled passage, and into a chamber below the right poppet. Since the right poppet is closed, the high-pressure air flows out of the brake port and into the brake line to apply the brakes.
To release the brakes, the control valve is returned to the off" position (figure 8-36A). The left poppet now closes, stopping the flow of highpressure air to the brakes. At the same time, the right poppet is opened, allowing compressed air in the brake line to exhaust through the vent port and into the atmosphere.
Variable Restrictor Another type of speed-regulating unit is the variable restrictor shown in figure 8-39. It contains an adjustable needle valve, which has threads around the top and a point on the lower end. Depending on the direction turned, the needle valve moves the sharp point either into or out of a small opening to decrease or increase the size of the opening. Since air entering the inlet port must pass through this opening before reaching the outlet port, this adjustment also determines the rate of airflow through the restrictor.
Filters Pneumatic systems are protected against dirt by means of various types of filters. A micronic filter (figure 8-40) consists of a housing with two ports, a replaceable cartridge, and a relief valve. Normally, air enters the inlet, circulates around the cellulose cartridge, then flows to the center of the cartridge and out the outlet port. If the cartridge becomes clogged with dirt, pressure forces the relief valve open and allows unfiltered air to flow out the outlet port.
A screen-type filter (figure 8-41) is similar to the micronic filter but contains a permanent wire screen instead of a replaceable cartridge. In the screen filter a handle extends through the top of the hous ing and can be used to clean the screen by rotating it against metal scrapers.
Spring Housing Relief valve Inlet FICURE 8-37. Pneumatic system check valve.
Outlet Inlet Outlet port port FIGURE 8-38. Orifice restrictor.
Adjustable Cartridge needle valve Sediment bowl FIGURE 8-40. Micronic filter.
Inlet port FIGURE 8-39. Variable pneumatic restrictor.
If the main hydraulic braking system fails, power brakes are usually equipped with some type of emergency pressurizing system for stopping the aircraft. In many instances, these emergency systems are compressed air systems. Figure 8-42 illustrates one type of system which uses compressed air.
Air Bottie The air bottle usually stores enough compressed air for several applications of the brakes. A highpressure air fine connects the bottle to an air valve which controls operation of the emergency brakes.
If the normal brake system fails, place the control handle for the air valve in the "on" position.
The valve then directs high-pressure air into lines leading to the brake assemblies. But before air enters the brake assemblies, it must first flow through a shuttle valve.
Handle ing. The shuttle is a sort of floating piston that can move up or down in the hollow housing. Normally, the shuttle is down, and in this position it seals off the lower air port and directs hydraulic fluid from Relief valve the upper port into the two side ports, each of which leads to a brake assembly. But when the emergency pneumatic brakes are applied, high-pres- Outlet • Inlet sure air raises the shuttle, seals off the hydraulic line, and connects air pressure to the side ports of the shuttle valve. This action sends high-pressure air into the brake cylinder to apply the brakes.
After application and when the emergency brakes are released, the air valve closes, trapping pressure in the air bottle. At the same time, the air valve Scraper vents the pneumatic brake line to outside air pressure. Then as air pressure in the brake line drops, the shuttle valve moves to the lower end of the housing, again connecting the brake cylinders to the hydraulic line. Air pressure remaining in the Wire screen Housing brake cylinders then flows out the upper port of the shuttle valve and into the hydraulic return line.
FIGURE 8-41. Screen-type filter. Lines and Tubing Brake Shuttle Valve The circled inset at the upper right of figure 8-42 shows one type of shuttle valve. The valve consists of a shuttle enclosed by a four-port hous- Lines for pneumatic systems consist of rigid metal tubing and flexible rubber hose. Fluid lines and fittings are covered in detail in Chapter 5 of the Airframe and Powerplant Mechanics General Handbook, AC 65-9A.
Air valve Control handle Air bottle - Hydraulic pressure line Shuttle valve • Air to Hydraulic pressure brake line cyl.
Shuttle valve Air pressuré line Brakes - I Brakes FIcURE 8-42. Simplified emergency brake system.
TYPICAL PNEUMATIC POWER SYSTEM A typical turbine-engine pneumatic power system supplies compressed air for various normal and emergency actuating systems. The compressed air is stored in storage cylinders in the actuating systems until required by actuation of the system. These cylinders and the power system manifold are initially charged with compressed air or nitrogen from an external source through a single air-charge valve. In flight, the air compressor replaces the air pressure and volume lost through leakage, thermal contraction, and actuating system operation.
System control Hydraulic access doors limit switches The air compressor is supplied with supercharged air from the engine bleed air system. This ensures an adequate air supply to the compressor at all altitudes. The air compressor may be driven either by an electric motor or a hydraulic motor. The system described here is hydraulically driven. The following description is illustrated by the pneumatic power system shown in figure 8-43.
The compressor inlet air is filtered through a high-temperature, 10-micron filter and the air pressure is regulated by an absolute pressure regulator Main Door Door 28V d.c. bus switch switch Pneumatic system Electronic cockpit Compressor and motor Mold Line equipment pressurization indicator case seal drain Access door Absolute system Equip aux y Pneumatic system pressure ind.
air pressure Air compressor regulator air supply 28V a.c.
Vent bus Filter and relief valve Access Pressure door sensing Hydraulic switch transmitter Pressure motor 3,000
p.s.i.
Low How ref press To Motor pneumatic Check valve drain case blanket Heater manifold system Chemical drier
- Manifold air LEGEND pressure gage co: Pneumatic pressure Air charge Zzzz Equipment auxiliary air supply Filter valve * Hydraulic pressure, utility Thermostat co Hydraulic return, utility • Dump* Electrical (energized) Safetv Electrical (deenergized) fitting - Dump valve Tsar Moisture Moisture separator sole noid (blow down condition) FIcURE 8-13. Pneumatic power system.