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System operating pressure may vary from a ________ (psi) in small aircraft and rotorcraft, to _____ _ (psi) in large transports.
couple hundred pounds per inch
5000
Pascal’s Law states that:
pressure applied to any part of a confined liquid is transmitted with undiminished intensity to every other part. Thus, if a number of passages exist in a system, pressure can be distributed through all of them by means of the liquid.
Some of the properties and characteristics that must be considered when selecting a satisfactory liquid for a particular system are:
Viscosity- Viscosity is internal resistance to flow
Chemical stability - It is the liquid’s ability to resist oxidation and deterioration for long periods.
Flash point - is the temperature at which a liquid gives off vapor in sufficient quantity to ignite momentarily or flash when a flame is applied.
Fire point - is the temperature at which a substance gives off vapor in sufficient quantity to ignite and continue to burn when exposed to a spark or flame
Viscosity increases as temperature_ ________
It must not be so thick that it offers ______ _
A fluid that is too thin also leads to _____ _ of moving parts.
It is the liquid’s ability to resist _______ and _____ _ for long periods
decrease
resistance to flow
rapid wear
oxidation
deterioration
As they are formed, certain changes in the physical and chemical properties of the liquid take place. The liquid usually becomes ______in colour, _____ in viscosity, and _____ _are formed.
darker
higher
acids
Flash point is the temperature at which a liquid gives off __ in sufficient quantity to ignite momentarily or flash when a __ _ is applied.
vapor
flame
A __ _ flash point is desirable for hydraulic liquids because it indicates good resistance to combustion and a low degree of evaporation at normal temperatures.
high
When adding fluid to a system, use the type specified in the __ _ manual.
aircraft manufacturer’s maintenance
he three principal categories of hydraulic fluids are:
1.Minerals
2.Polyalphaolefins
3.Phosphate esters
What is the Mil spec number for Mineral oil-based hydraulic fluid?
MIL-H-5606
What colour is it dyed?(MIL-H-5606)
Red
What type of seals are used with petroleum-based fluids?
synthetic rubber seals
These fluids are used in most commercial transport category aircraft and are extremely ___ . However, they are not ____ under certain conditions
fire resistant
fireproof
Petroleum-based and phosphate ester-based fluids __ ; neither are the _
will not mix
seals
Should an aircraft hydraulic system be serviced with the wrong type fluid?
no
What are the two general contaminants?
• Abrasives, including such particles as core sand, weld spatter, machining chips, and rust.
• Nonabrasives, including those resulting from oil oxidation and soft particles worn or shredded from seals and other organic components.
When should a check of the system be made?
Whenever it is suspected that a hydraulic system has become contaminated or the system has been operated at temperatures in excess of the specified maximum, a check of the system should be made.
Hydraulic liquid that appears to the naked eye may be contaminated to the point that it is ____ for use. Thus, ___ _ of the hydraulic liquid does not determine the total amount of contamination in the system.
clean
unfit
visual inspection
Place a waste container under the reservoir drain valve and open valve so that a ___ _ .
Allow approximately ___ _ of fluid to drain. This purges any settled particles from the sampling port.
Insert a sample bottle under the fluid stream and fill, leaving an __ _ at the top. Withdraw the bottle and cap immediately.
Close drain valve.
teady, but not forceful, stream is running.
1 pint (250 ml)
precleaned
air space
Fill out sample identification label supplied in sample kit, making sure to include customer name
aircraft type, aircraft tail number, hydraulic system sampled, and date sampled
What provides adequate control of the contamination during hydraulic system operations?
Filters
Maintain all tools and the work area in a ____ _
in a clean, dirt-free condition
All hydraulic servicing equipment should be kept ___ and in ____
clean
good operating condition
Where are filters placed in the system to control particulate contamination?
in the pressure line, in the return line, and in the pump case drain line of each system.
What hydraulic fluid can dissolve away some of the fatty materials of the skin?
Phosphate esters
What PPE should be worn when handling any type of hydraulic fluid?
Use of proper gloves and eye protection
A basic hydraulic system consists of a:
Pump
Reservoir
Directional valve
Check valve
Pressure relieve valve
Selector valve
Actuator
filter
Describe an open centre Hydraulic system:
An open center system is one having fluid flow, but no pressure in the system when the actuating mechanisms are idle.
Describe a closed-centre system.
In the closed-center system, the fluid is under pressure whenever the power pump is operating.
An advantage of the open-centre system over the closed-centre system is:
is that the continuous pressurization of the system is eliminated.
An advantage of the closed-centre system over the open-centre system is:
The operation in closed-centre system is faster than the open-centre system.
What are Power boost systems?
It assist the pilot in overcoming high control forces,
What are power supply systems and fly-by wire flight controls?
The pilot input is electronically sent to the flight control servos. Cables or push rods are not used.
Advantages of Small power packs
Reduced weight
The reservoir is a tank in which an adequate supply of fluid for the system is stored. Fluid flows from the reservoir to the where it is forced through the system and eventually returned to the ______
pump
reservoir
What is the purposes of a reservoir:
The reservoir not only supplies the operating needs of the system, but it also replenishes fluid lost through leakage.
What types of gauges are used to attain reservoir level/quantities?
Gauges incorporated on or the reservoir may be a direct reading glass tube-type or afloat-type rod that is visible through a transparent dome.
What does the filler strainer (finger strainer) do?
To prevent the entry of foreign matter during servicing. To strain out impurities as the fluid enters the reservoir.
What is the advantage of using s quick disconnect service port?
It reduces the chances of contamination of the reservoir
How do you get the correct results when the hydraulic fluid quantities are checked or the reservoirs are to be filled?
The airplane should be in the correct configuration
Describe a filter.
A filter is a screening or straining device used to clean the hydraulic fluid, preventing foreign particles and contaminating substances from remaining in the system.
What happens if the filter becomes blocked?
the entire hydraulic system of the aircraft could fail through the breakdown or malfunctioning of a single unit of the system.
What types of filter element can you get?
The element may be a micron, porous metal, or magnetic type
What is the micron element designed to prevent the passage of solids greater than?
prevent the passage of solids greater than 10 microns (0.000394 inch) in size
When replacing filter elements, what should you be sure of?
Be sure that there is no pressure on the filter bowl.
What is the purpose of the bypass relief valve in a filter?
The bypass relief valve opens if the filter clogs, permitting continued hydraulic flow and operation of aircraft systems.
How can you monitor the condition of installed filter elements?
by measuring the drop in hydraulic pressure across the element under rated flow conditions
What does the red indicator popping out signify?
signifying that the element must be cleaned.
Power-driven pumps are the primary source of energy and may be either:
engine driven, electric motor driven, or air driven
What are Hand pumps generally installed for?
Hand pumps are generally installed for testing purposes, as well as for use in emergencies.
Modern aircraft use a combination of:
engine-driven power pumps, electrical-driven power pumps, air-driven power pumps, power transfer units (PTU), and pumps driven by a RAT.
What component is required when a constant displacement pump is used in a hydraulic system in which the pressure must be kept at a constant value?
A Pressure regulator
What type of pump is a gear-type power pump?
Constant-displacement pump
Draw a picture of a Gerotor pump
What 2 types of pumps can Piston pumps be?
constant-displacement pump
Variable displacement pump
What is the purpose of Pump drive couplings?
Pump drive couplings are designed to serve as safety devices.
How does the shear section of the drive coupling work?
If the pump becomes unusually hard to turn or becomes jammed, this section shears, preventing damage to the pump or driving unit
What type of pump is a vane-type power pump?
Constant-displacement pump.
Describe a variable-displacement pump
A variable-displacement pump has a fluid output that is varied to meet the pressure demands of the system.
What is the function of the flow control valves?
Flow control valves control the speed and/or direction of fluid flow in the hydraulic system
What is the purpose of a selector valve?
A selector valve is used to control the direction of movement of a hydraulic actuating cylinder or similar device. . It provides for the simultaneous flow of hydraulic fluid both into and out of the unit.
What are the two main types of selector valves?
open-center
closed-center
How are most selector valves controlled?
Most selector valves are mechanically controlled by a lever or electrically controlled by solenoid or servo.
Describe a check valve:
A check valve allows fluid to flow unimpeded in one direction, but prevents or restricts fluid flow in the opposite direction.
What is an integral check valve?
When part of a component, the check valve is said to be an integral check valve.
What does an arrow on the outside of the housing indicated?
An arrow on the outside of the housing indicated the direction in which fluid flow is permitted.
What does orifice check valves allow?
Allow full fluid flow in one direction and restricted flow in the opposite direction.
A priority valve gives priority to the ___ hydraulic subsystems over __ systems when system pressure is _
critical
noncritical
low
What do quick disconnect valves installed in hydraulic lines prevent?
Quick disconnect valves are installed in hydraulic lines to prevent loss of fluid when units are removed.
They detect a sudden __ , such as a burst downstream, and _ _ the fluid flow.
increase in flow
shut off
List the types/purpose of automatic pressure control valves:
There are many types of automatic pressure control valves.
-Some of them are an escape for pressure that exceeds a set pressure;
-some only reduce the pressure to a lower pressure system or subsystem;
-and some keep the pressure in a system within a required range.
Why is a pressure relief valve necessary?
This is necessary to prevent failure of components or rupture of hydraulic lines under excessive pressures.
The pressure relief valve is, in effect, a __
system safety valve.
The two purposes of a pressure regulator:
-One purpose of the pressure regulator is to manage the output of the pump to maintain system operating pressure within a predetermined range.
-The other purpose is to permit the pump to turn without resistance (termed unloading the pump) at times when pressure in the system is within normal operating range.
Why are pressure reducing valves used in a hydraulic system?
it lowers the normal system operating pressure by a specified amount. Pressure reducing valves provide a steady pressure into a system that operates at a lower pressure than the supply system.
What is the main purpose of the shuttle valve?
The main purpose of the shuttle valve is to isolate the normal system from an alternate or emergency system.
What are shutoff valves used for
-Shutoff valves are used to shutoff the flow of fluid to a particular system or component
-Shutoff valves are also used to create a priority in a hydraulic system and are controlled by pressure switches.
The function of an accumulator is to:
•Dampen pressure surges in the hydraulic system caused by actuation of a unit and the effort of the pump to maintain pressure at a preset level.
• Aid or supplement the power pump when several units are operating at once by supplying extra power from its accumulated, or stored, power.
• Store power for the limited operation of a hydraulic unit when the pump is not operating.
• Supply fluid under pressure to compensate for small internal or external (not desired) leaks that would cause the system to cycle continuously by action of the pressure switches continually kicking in.
List the two general types of accumulators:
Spherical
Cylindrical
What type of valve is installed on the nitrogen side of the accumulator?
A gas servicing valve, such as a Schrader valve
What separates the nitrogen and hydraulic fluid in an accumulator?
The internal piston
What is the purpose of the heat exchanger in a hydraulic system?
To cool the hydraulic fluid from the hydraulic pumps.
An actuating cylinder transforms energy in the form of into __, _ to perform work.
fluid pressure
mechanical force, or action
What are of two major types of Actuating cylinders?
Single action and Double action.
-The single-action (single port) actuating cylinder is capable of producing powered movement in one direction only.
-The double-action (two ports) actuating cylinder is capable of producing powered movement in two directions
What happens to the piston with respect to the housing, when a double acting actuator EXTENDS?
This results in the piston being forced toward the right.
Linear Actuator
Piston-type motors are basically the same as __ except they are used to convert energy into _ energy.
hydraulic pumps
hydraulic
mechanical (rotary)
Hydraulic motors are either __ or _ type
of the axial inline
bent-axis
Seals are used to prevent fluid from passing a certain point, and to keep __ and _ out of the system in which they are used.
air
dirt
Seals are divided into three main classes:
1. packings
2.gaskets
3.wipers
Describe Packings.
seals used internally on a sliding or moving assembly are normally called Packings.
Describe gaskets.
seals used between nonmoving fittings and bosses are normally called gaskets.
V-ring packings are __ _ seals and are always installed with the _ of the V facing __ _ _
one-way
open end
the pressure
The U-ring and U-cup seals pressure in __ direction; therefore, the lip of the packing's must face _
only one
toward the pressure
The cross-section is truly round and has been moulded and trimmed to _______
extremely close tolerances
The O-ring packing seals effectively in . This sealing is done by ___ _ of its elastic compound.
both directions
distortion
Describe Air Force-Navy (AN) Hydraulic O-rings.
Hydraulic O-rings were originally established under Air Force-Navy (AN) specification numbers 6227, 6230, and 6290 for use in fluid at operating temperatures ranging from –65 °F to +160 °F.
Describe Military Standard (MS) Hydraulic O-rings
Recently, newer compounds were developed under Military Standard (MS) specifications that offered improved low-temperature performance without sacrificing high-temperature performance. These O-rings are now standard for systems where the operating temperatures may vary from –65 °F to +275 °F.
How are O-rings packaged?
Manufacturers provide color coding on some O-rings. O-rings are available in individual hermetically sealed envelopes labeled with all pertinent data
When selecting an O-ring for installation, what is the most reliable way to identify O-ring?
The basic part number on the sealed envelope provides the most reliable compound identification
Note
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