Explanations: Hydraulic and Pneumatic Power Systems

Question 1

What is a selector valve?

Answer 1

A fluid power system component that directs the flow of pressurized fluid to one end of an actuating cylinder, and simultaneously, directs return fluid to the reservoir from the other end of the cylinder.

A closed center selector valve has a variety of ports. Whether a given port is an input or output port depends upon the configuration of the valve’s interior which rotates into different positions, either closing off or providing a path for fluid to flow either into or out of a given port.

Question 2

what is the difference between a pneumatic system and a hydraulic system?

Answer 2

Pneumatics provides fluid power by means of pressurized air or gases. Hydraulics provides fluid power by means of pressurized liquids, such as oil or water

Question 3

What is a compressor in a pneumatic system?

Answer 3

A machine that takes ambient air from the surroundings and discharges it at a higher pressure.

Question 4

What is the difference in installation between a close center and open center selector valve?

Answer 4

Open center selector valves are installed in series with each other, while close center valves are installed in parallel with each other.

Question 5

Why are relief valves installed?

Answer 5

to protect a system from excessive pressure buildup that can be caused by thermal expansion or by a compressor power system malfunction.

Question 6

Why does an aircraft pneumatic system require a moisture separator?

Answer 6

Because if moisture were allowed to remain in the air, the temperature drop that occurs when the air expands would cause it to freeze and block the system.

Question 7

What type of components require bleeding of the fluid to remove the air and what type only require the operation of several cycles to remove the air?

Answer 7

The cycles remove air from double-acting hydraulic components, whereas bleeding the fluid removes air from single-acting components such as brakes.

Question 8

What does a pressure regulator in a hydraulic system do?

Answer 8

It maintains a system operating pressure within a predetermined range. The regulator unloads the pump when the upper limit of this pressure is reached.

The pump remains unloaded, circulating the fluid through the system with no opposition until the lower limit of the pressure is reached, at which time the pump again forces fluid into the system.

Question 9

What is the difference between and gasket and a packing?

Answer 9

a gasket is not subject to motion, and a packing is.

Question 10

What role does the backup ring perform in a high pressure system?

Answer 10

It prevents the fluid from forcing the o-ring into the space between the piston and cylinder wall.

Question 11

Can an accumulator be removed from an aircraft with an air preload still in it?

Answer 11

Yes, but the preload must be discharged before the accumulator itself is disassembled.

Question 12

Why have synthetic fluids been developed?

Answer 12

because of the flammability of petroleum base fluids.

Question 13

Why is skydrol adversely affected by atmospheric humidity?

Answer 13

it is susceptible to contamination from moisture.

Question 14

What is a flash point?

Answer 14

The lowest temperature at which there will be enough flammable vapor to support combustion when an ignition source is applied.

Question 15

What number is skydrol?

Answer 15

MIL-H-8446

Question 16

Systems using skydrol should be flushed with what? Components using skydrol should be flushed with what?

Answer 16

systems flushed with trichloroethylene. components with stoddard solvent, MEK, or isopropyl alcohol.

Question 17

What two systems is a shuttle valve connected to in a landing gear actuating cylinder?

Answer 17

both the normal operating system and the emergency system, but it can’t access both at the same time.

Question 18

Why should you always adjust pressure regulating valves with the highest pressure setting first?

Answer 18

If a valve having a lower pressure setting is adjusted first, you will never be able to reach the pressure required for the higher setting.

Question 19

What is the issue caused by a low accumulator preload?

Answer 19

the accumulator is not holding the pressure on the system as long as it would if the preload pressure were correct.

Question 20

What is another name for an unloading valve?

Answer 20

a system pressure regulator.

Question 21

How does a unloading valve, aka system pressure regulator, work?

Answer 21

When the system pressure reaches the regulating range, a check valve in the unloading valve traps pressure in the system where it is held by the accumulator.

A valve then opens to allow fluid from the pump to circulate back to the reservoir with very little opposition.

The fluid flows with the pump unloaded until the system pressure drops to the kick-in pressure of the unloading valve. When the kick-in pressure is reached, the output of the pump is again directed into the system.

Question 22

If all pressure relief devices fail, what occurs?

Answer 22

the shaft will shear and prevent the pump being further damaged?

Question 23

What is the hydraulic pump drive shaft?

Answer 23

Hydraulic Pump - Gear category consists essentially of two intermeshing toothed gears enclosed in a common housing. A shaft, the drive shaft, extends beyond the housing. This shaft is attached to one of the gears which, when it is turned by the shaft, drives the second gear.

Question 24

Where are thermal relief valves installed?

Answer 24

In portions of a hydraulic system that normally hold fluid trapped by a selector valve. They relieve the pressure that is built up in the lines as the fluid is heated by the surrounding air.

Thermal relief valves are set to relieve a pressure higher than any of the other valves, so they will not interfere with normal system operation of protection.

Question 25

What function does a shear pin serve?

Answer 25

a shear pin or a shear section of the drive shaft is used with a constant-displacement, engine-driven, gear-type hydraulic pump to protect the pump from damage in case of an overload.

If all the regulating and relief valves fail, the shaft will shear and keep the pump from being seriously damaged.

Question 26

How does the pressure differ in different parts of an enclosed system?

Answer 26

According to Pascal’s law, pressure is the same in all parts of an enclosed system.

Question 27

Why do some hydraulic systems need heat exchangers?

Answer 27

because some systems have such high pressures and high rates of fluid flow that the fluid becomes too hot for efficient operation. The heat exchangers cool the fluid similar to engine oil coolers.

Question 28

Where is the outlet from the hydraulic reservoir to the main engine driven pump in relation to the hand pump?

Answer 28

the main engine driven pump outlet is above the outlet that does to the hand pump.

If a line breaks and the engine-driven pump forces all the fluid to which it has access overboard, there is still fluid left in the reservoir. There is enough fluid available to the auxiliary hand pump for such critical operations as landing gear extensions.

Question 29

What happens if the check valve is sticking open?

Answer 29

it allows system pressure stored in the accumulator to push back on the piston.

Question 30

What is a power control valve?

Answer 30

a hand-operated shutoff valve with an automatic turn-on feature. It must be closed manually, but it opens automatically.

When the unit it OPEN, it permits free circulation of fluid from the engine-driven pump to the reservoir. In this way, it relieves the pump of its load.

In order to adjust the system pressure relief valve, the power control valve must be pushed in to its CLOSED position and held until the system pressure builds up high enough to adjust the system pressure relief valve.

Question 31

What is the pressure of the air in a hydraulic system?

Answer 31

It is the same as the pressure of the fluid.

Question 32

How is air separated from the fluid in an accumulator?

Answer 32

By a diaphragm, a bladder, or a movable piston.

Question 33

Why must the pressure regulator action be eliminated before any pressure relief valves can be adjusted?

Answer 33

because otherwise the pressure will never rise high enough to adjust the relief valve.

Question 34

What component do pump shaft seals have in case there is a leak?

Answer 34

hydraulic pumps have a shaft seal vented into a compartment in the pump and drain overboard. If the pump shaft seal leaks, hydraulic fluid will drip out of the overboard drain line.

Question 35

What occurs if there is a restriction in the pump outlet?

Answer 35

if there is a restriction in the pump outlet, or between the pump outlet and the system pressure regulator, the system pressure will drop when some unit is actuated.

Question 36

How are some hydraulic reservoirs pressurized?

Answer 36

With bleed air from the engine compressor or from a venturi-tee.

All of the air pressure must be relieved before the reservoir is opened.

Question 37

Where are quick-disconnect fittings normally found?

Answer 37

at the hydraulic pump.

They allow the lines to be quickly disconnected without loss of fluid or air entering the system.

Question 38

How much does the amount of fluid change per cycle in a constant-displacement type hydraulic pump?

Answer 38

It doesn’t. It moves the same amount of fluid each time it rotates.

Question 39

Why does a landing gear have a crossflow valve?

Answer 39

The function of a crossflow valve is to bypass hydraulic fluid from the landing gear up line to the down line when the landing gear is being extended.

When the landing gear is released from its uplocks, its weight causes it to fall so rapidly that fluid cannot fill in behind the piston in the actuating cylinder, and excessive pressure builds up on the opposite side of the piston.

The crossflow valve permits fluid to flow from the up side of the piston to the down side and thus allows the gear to fall more easily and with an even motion.

Question 40

Why are reservoir pressurized?

Answer 40

To prevent pump cavitation, I’m assuming, because the change in air pressure of a plane at a high altitude would fuck that shit all the way up.

Question 41

Skydrol has an operating temperature of what?

Answer 41

-65 degrees F to greater than 225 degrees F

Question 42

The normal system power pump connects to the what?

Answer 42

The fluid outlet of the reservoir that is fed from the standpipe.

If the power pump forces all of the fluid available to it overboard through a broken line, there will still be fluid in the reservoir that can be used by the emergency hand pump.

Question 43

Where is the supply line from the standpipe in a hydraulic reservoir attached?

Answer 43

To the inlet of the main hydraulic system pump.

This standpipe ensures that there will be a supply of fluid available to the emergency pump if the main system pump should force overboard all of the fluid available to it.

Question 44

When is it possible to read the accumulator air preload on the accumulator gauge?

Answer 44

When all system pressure has been removed from the system.

Question 45

When the engine is inoperative, but the system still has hydraulic pressure, how do you determined the air pressure charge in the accumulator?

Answer 45

slowly operate some hydraulic unit, such as the power brakes, to bleed the pressure off of the system. Watch the system pressure gauge. The last pressure indicated on the gauge before it suddenly drops to zero is the pressure of the air in the accumulator.

Question 46

What will happen when the diaphragm is ruptured or a seal is leaking in an accumulator?

Answer 46

it will allow hydraulic fluid to get into the air side of the accumulator and be released with the air when the air valve core is depressed.

Question 47

How do restrictors control the rate of movement of hydraulically operating mechanisms?

Answer 47

By restricting the flow of fluid into the mechanism.

Question 48

Why does a loud hammering noise occur when there is too low or no preload in the accumulator?

Answer 48

The pressure regulator puts the pump on the line. Since there is no compressible fluid in the system, the cutout pressure is immediately reached. This causes a shock on the system. As soon as the regulator directs the pump outlet to the return manifold, the system pressure drops below the cut-in pressure and the pump is again put on the line.

Question 49

How do you prevent a teflon hose that has developed a set from being straightened or bent further when it is removed?

Answer 49

By supporting the ends.