Hydraulic System Components - Hydraulics - Airframes & Aircraft Systems #8
Summary
TLDRThis lesson delves into the intricacies of an aircraft's hydraulic system, focusing on the reservoir's role in storing fluid and preventing leaks. It discusses the necessity of reservoirs despite the system's sealed nature, due to fluid expansion and contraction, and the presence of small, controlled leaks for actuator lubrication. The lesson also covers accumulators, their types, and functions, including pressure stabilization and emergency fluid supply. Additionally, it touches on the importance of filters for protecting system components from debris, and the automatic cutout valve's role in controlling system pressure and reducing pump wear.
Takeaways
- 💧 A reservoir is essential for an aircraft's hydraulic system, even though the system is designed to be leak-free, as it compensates for fluid expansion and contraction, and lubricates actuators.
- 🔄 Accumulators store variable amounts of fluid in hydraulic systems, which is crucial for the system's operation and is detailed further in the lesson.
- 💨 The hydraulic pump's operation involves both supplying pressure and creating suction, which can lead to cavitation if not managed properly by the reservoir.
- 🛑 Cavitation occurs when bubbles in the fluid implode with force, potentially causing severe pump damage, and it can be identified by specific sounds.
- 📡 Reservoirs help prevent cavitation by maintaining a positive pressure at the pump inlet, often achieved by being located higher than the pump or by pressurization with air.
- 🔗 The reservoir includes various connections for the hydraulic system, such as suction pipes to pumps, return pipes, and a quantity transmitter unit for monitoring.
- 🚫 To prevent debris from entering the reservoir, a strainer is used in the filling inlet, which is crucial for maintaining fluid cleanliness.
- 🛠️ Filters are used throughout the hydraulic system to protect components from damage caused by foreign particles and to ensure system efficiency.
- 🔄 An accumulator's function includes dampening pressure fluctuations and providing an emergency fluid supply in case of pump failure, highlighting its importance in system reliability.
- 🔩 The automatic cutout valve (ACOV) manages system pressure in constant delivery fixed volume pump systems, reducing pump wear by controlling the pump's operation.
Q & A
Why is a reservoir necessary in an aircraft's hydraulic system even though it is a sealed system?
-A reservoir is necessary because no system can be completely leak-free. It allows for small amounts of fluid to leak across the seals of actuators for lubrication, accommodates fluid expansion and contraction due to temperature changes, and stores extra fluid needed by actuators depending on their extension.
What is the role of an accumulator in a hydraulic system?
-An accumulator stores hydraulic fluid under pressure and serves to dampen pressure fluctuations, provide a small emergency supply of fluid in case of pump failure, and allow for thermal expansion and contraction of fluid.
How does a reservoir help prevent cavitation in a hydraulic system?
-A reservoir helps prevent cavitation by providing a head of fluid to the pump, ensuring a positive pressure at the pump inlet. It is often located higher in the system than the pump, and in some installations, it is also pressurized with air from the engine compressor.
What is the purpose of the baffles and fins in a reservoir?
-Baffles and fins in a reservoir are fitted to prevent sloshing and foaming of the fluid during in-flight maneuvers, ensuring the stability of the fluid within the reservoir.
What is the function of a suction filter in a hydraulic system?
-A suction filter protects the pump and other system components by removing foreign particles from the fluid before it enters the pump, preventing damage that could be caused by debris.
How does the bootstrap method work in pressurizing a reservoir?
-In the bootstrap method, a piston sits on top of the fluid in the reservoir and is pushed down by an actuator powered from its own hydraulic system, pressurizing the fluid. When the system is not operating, the pressure is held by a non-return valve.
What is the purpose of a filling inlet and strainer in a reservoir?
-A filling inlet, normally sealed with a filler cap, allows for the addition of fluid to the reservoir. The strainer in the filling inlet prevents debris from entering the reservoir, helping to keep the hydraulic fluid clean.
What is the role of an automatic cutout valve (ACOV) in a hydraulic system?
-An automatic cutout valve (ACOV) controls system pressure and provides the pump with an idling circuit when no services have been selected. It operates in two modes, cut in and cut out, to regulate the flow of fluid and reduce pump wear.
How does an accumulator provide the initial fluid when a selection is made and the pump is cut out?
-When a service is selected and the pump is cut out, the accumulator provides the initial fluid under pressure until the system pressure drops efficiently, signaling the pump to come online.
What can be inferred about the condition of a hydraulic system based on the time between cut out and cut in of the ACOV?
-The time between cut out and cut in of the ACOV can indicate the condition of the system. External leakage may cause frequent loading and unloading of the pump, while internal leakage, often caused by piston seal failure, can also lead to increased pump activity and potentially higher fluid temperatures.
Outlines
🛫 Hydraulic System Components: The Role of the Reservoir
This paragraph delves into the intricacies of an aircraft's hydraulic system, emphasizing the critical role of the reservoir. Despite the system's design for minimal leakage, a reservoir is essential due to the inherent impossibility of a perfectly sealed system. It serves to accommodate fluid expansion and contraction with temperature changes and to provide extra fluid for actuators that require more when extended. The reservoir also integrates accumulators, which store variable fluid amounts. A key function of the reservoir is to prevent cavitation in the hydraulic pump by maintaining a positive pressure at the pump inlet, often aided by pressurization with air from the engine compressor. The reservoir's design includes connections for suction and return pipes, a quantity transmitter for monitoring, and a filling inlet with a strainer to prevent debris entry. Additionally, baffles and fins are installed to mitigate fluid sloshing during flight maneuvers. The importance of keeping hydraulic fluid free from contaminants to avoid damage to system components is highlighted.
💧 Hydraulic Filters and Accumulators: Ensuring System Integrity
The second paragraph focuses on the importance of filters within the hydraulic system, which are crucial for protecting pumps and other components from damage caused by foreign particles. Filters are installed in both suction and pressure lines to maintain fluid cleanliness. Some filters are equipped with a device that detects pressure differential, signaling when they become clogged. Filters can be made from various materials, and while most are disposable, wire elements can be cleaned, often using ultrasonic processes. The paragraph also introduces accumulators, devices that store hydraulic fluid under pressure. Two common types are described: cylindrical with a floating piston and spherical with a flexible diaphragm. Accumulators help stabilize system pressure, provide emergency fluid supply in case of pump failure, and assist in thermal expansion management. They are charged through a charging valve and can be pressurized with air or nitrogen. The discussion also touches on the operation of an automatic cutout valve (ACOV) and its role in controlling system pressure and pump idling.
🔧 Functions of the Accumulator and Automatic Cutout Valve
This section further elaborates on the functions of the accumulator in an aircraft's hydraulic system. The accumulator not only stores hydraulic fluid under pressure but also plays a vital role in dampening pressure fluctuations and providing an emergency fluid supply in the event of pump failure. It ensures that there is sufficient fluid available for system operation, even when the pump is not actively supplying fluid. The paragraph also discusses the automatic cutout valve (ACOV), which is used in systems with constant delivery fixed volume pumps. The ACOV controls system pressure and provides an idling circuit for the pump when no services are selected. It operates in two modes: cut in, where it allows the pump to operate the service, and cut out, where it returns the pump's output to the reservoir without load. The time between cut out and cut in indicates the system's condition, with external or internal leakages causing frequent valve operations and potentially leading to increased fluid temperature and pump wear.
🛢️ Reservoir and Accumulator: System Stability and Fluid Management
The final paragraph summarizes the key points about the reservoir and accumulator in the hydraulic system. The reservoir provides storage for system fluid and air space to accommodate fluid volume variations due to actuator displacement or thermal expansion. It compensates for small leaks and supplies fluid to the pump, often with pressurization to prevent cavitation. Filters are essential for protecting system components from damage caused by foreign particles in the fluid. The cutout valve is fitted to manage system pressure and to give the pump an idling circuit when no services are selected. The accumulator stores hydraulic fluid under pressure to stabilize pressure fluctuations, allow for thermal expansion, and provide an emergency fluid supply in case of pump failure. It also reduces pump wear by controlling the time between cut out and cut in of the ACOV and supplies initial fluid when a selection is made with the pump cut out.
Mindmap
Keywords
💡Reservoir
💡Cavitation
💡Accumulator
💡Hydraulic Pump
💡Seals
💡Filters
💡Automatic Cutout Valve (ACOV)
💡Thermal Expansion
💡Baffles and Fins
💡Pressure Relief Valves
💡Non-Return Valve
Highlights
The necessity of a reservoir in an aircraft's hydraulic system, even in a sealed system.
Fluid leakage across seals for lubrication and the expansion and contraction of fluid due to temperature changes.
The role of accumulators in storing variable amounts of fluid depending on the system configuration.
Prevention of cavitation in hydraulic pumps by maintaining a positive pressure at the pump inlet.
Pressurization of reservoirs with air from the engine compressor to increase fluid pressure at the pump inlet.
Bootstrap method for pressurizing reservoirs using a piston and actuator.
Functions of reservoirs including connections for suction and return pipes, and monitoring system fluid levels.
Importance of keeping hydraulic fluid free from foreign bodies to prevent damage to system components.
Use of filters in both suction and pressure lines to protect pumps and maintain fluid cleanliness.
The purpose of accumulators in storing hydraulic fluid under pressure and their two common types.
How accumulators help to dampen pressure fluctuations and provide emergency fluid supply in case of pump failure.
The function of an automatic cutout valve (ACOV) in controlling system pressure and providing an idling circuit for the pump.
The significance of the time between cut out and cut in of the ACOV as an indicator of system condition.
The role of filters in protecting system components from damage caused by foreign particles in the fluid.
Maintenance of filter elements and the use of ultrasonic cleaning as a recommended method.
The importance of correct pre-charge pressure in accumulators to prevent rapid fluctuations and potential system damage.
Transcripts
in this lesson we all now look in
greater detail at some of the other
components that make up an aircraft's
hydraulic system
the first component we are going to
discuss is the reservoir you may think
that in a sealed leak free system a
reservoir would not be necessary however
that is not true
firstly no system can ever be leak free
in fact we have already said that a very
small amount of fluid is allowed to leak
across the seals of actuators in order
to lubricate them
also the fluid expands and contracts as
it is heated and cooled
as we shall see later some types of
hydraulic actuator need more fluid when
they are extended than when contracted
the extra fluid is stored in the
reservoir when not required
most hydraulic systems also have
accumulators fitted and these will
contain variable amounts of fluid
depending on the system configuration
again we will discuss these in greater
detail shortly
when a hydraulic pump is operating as
well as supplying pressure at its outlet
it also of course sucks fluid in at its
Inlet
this suction causes a big drop in the
pressure of the fluid and can lead to
low temperature boiling and the
formation of bubbles
when these bubbles pass through to the
pressure side of the pump
they then implode with great force
this is known as cavitation and if
allowed to happen it can cause severe
damage to the pump
the sound of pumps operating while
cavitating can range from a low pitch to
steady knocking sound like on a door to
a high-pitched and random crackling
similar to a metallic impact
the reservoir helps to prevent
cavitation by providing a head of fluid
for the pump to do this the reservoir is
located higher in the system than the
pump to produce a positive pressure at
the pump Inlet
in many installations the reservoir is
also pressurized with air from the
engine compressor to further increase
the fluid pressure at the pump Inlet
thus reducing the possibility of
cavitation at high altitude
another way that reservoirs are
pressurized is by the bootstrap method
a piston sits on top of the fluid in the
reservoir it is pushed down by an
actuator powered from its own hydraulic
system pressurizing the fluid
when the system is not operating the
pressure is held by the non-return valve
you
the reservoir also contains connections
for suction pipes to the pumps and
return pipes from the system
a quantity transmitter unit to allow the
flight crew to monitor the system for
correct servicing and in-flight leaks
a filling Inlet normally sealed with the
filler cap note the strainer in the
filler Inlet preventing debris entering
the reservoir
and in some cases a temperature sensing
probe which may be used to operate a
fluid temperature gauge and or an over
temperature warning light
the baffles and fins are fitted to
prevent sloshing and foaming of the
fluid during in-flight maneuvering
in systems which are fitted with more
than one pump usually the main pump is
engine driven whilst the backup may be
electrically or pneumatically driven or
may even be a hand pump
the main pump will draw its fluids
through a stack pipe in the reservoir
we'll collect its fluid from the bottom
of the reservoir
this ensures that if fluid is lost from
that part of the system supplying the
main pump or supplied solely by the main
pump a reserve of fluid with a backup
pump will still be available
it is very important that hydraulic
fluid is kept free of foreign bodies any
debris would quickly damage the pump and
components it would also cause problems
in things such as pressure relief valves
as they can have very narrow passages
which are easily blocked by foreign
particles
Builders are fitted in both suction and
pressure lines that is on both sides of
the pump
the suction filter protects the pump and
the pressure filter ensures the
cleanliness of the hydraulic fluid
during use
there is also sometimes a filter fitted
in the fluid return line to the
reservoir to remove particles picked up
during component operation
filters remove foreign particles from
the fluid thus protecting the seals and
working surfaces of the components
some filters are fitted with a device
which senses the pressure differential
across the filter element and releases a
visual indicator in the form of a button
or illuminates a warning lamp when the
pressure differential increases as a
result of the filter becoming clogged
false indication of element clogging as
a result of high fluid viscosity at low
temperature is prevented by a bi-metal
spring which inhibits indicator button
movement and low temperatures
other filters are fitted with a relief
or bypass valve which allows unfiltered
fluid to pass to the system when the
element becomes cloaked in this type of
filter the element must be changed at
regular intervals before clocking occurs
some filters combine both systems with
the warning indicator set to operate
shortly before the bypass valve opens
individual components often have a small
filter fitted to their Inlet connection
and constant pressure pumps will have a
case drain filter to help monitor pump
condition we will cover the term case
drain when discussing pumps
filter elements may be manufactured from
paper felt or gauze from metal wire or
from a combination of these materials
all elements except those made from wire
are usually discarded when removed but
wire elements may usually be cleaned
cleaning by an ultrasonic process is
normally recommended but if a new or
cleaned element is not available when
the filter becomes due for check
maybe cleaned in a suitable solvent as a
temporary measure
an accumulator is a device used to store
hydraulic fluid under pressure
two different types of accumulator are
illustrated here but many other types
are used however the two shown are those
most commonly in use in aircraft systems
whatever its shape the chamber of the
accumulator is split into two parts by a
separator in the case of the cylinder a
floating piston is used whilst the
spherical accumulator has a flexible
diaphragm
for the purpose of this explanation we
will concentrate on the cylindrical type
as this is the most common of all
the volume on one side of the floating
piston is pressurized with air or
nitrogen and the other is connected to
the hydraulic system pressure line
accumulator is normally inflated through
a charging valve which may be attached
directly to the accumulator or installed
on a remote ground servicing panel and
connected to the accumulator by means of
a pipeline
the charging valve usually takes the
form of a non-return valve which may be
opened by means of a plunger in order to
relieve excessive pressure
to pre-charge or check the gas pressure
the system hydraulic pressure should be
released this will allow the gas
pressure to move the floating piston to
the bottom of the accumulator
when a pump is operating and hydraulic
pressure builds up in the system the gas
is compressed until fluid and gas
pressure equalized at normal system
pressure at this point the pump
commences to idle and system pressure is
maintained by the accumulator the
accumulator gas pressure gauge will now
read system pressure
turn valve is fitted upstream of an
accumulator in order to prevent fluid
from the accumulator being discharged
back through the pump to the reservoir
the accumulator carries out a number of
functions in the system it helps to
dampen out pressure fluctuations
thermal expansion and contraction of
fluid trapped in the pipes
can provide a small emergency supply of
fluid to the system in the event of pump
failure the initial gas charge of the
accumulator is greater than the pressure
required to operate any service and the
fluid volume is usually sufficiently
large to operate any service once except
that brake accumulators normally permit
a guaranteed number of brake
applications or the ability to stop the
aircraft during a rejected takeoff
if the system is using a constant
delivery fixed volume type pump with an
automatic cutout valve it will prolong
the period between cutout and cut-in of
the automatic cutout valve and so reduce
the wear on the pump operation of the
automatic cutout valve will be explained
shortly
the accumulator will also provide the
initial fluid when a selection is made
and the pump is cut out or at minimum
stroke
if a service is selected a supplier
fluid under pressure is available until
pressure drops efficiently to bring the
pump online
incorrect pre-charge pressure of the
main accumulator can cause the automatic
cutoff valve to cut in and out too
frequently
this may cause rapid fluctuations of
system pressure which can be felt and
heard as hammering in the system this
can rapidly cause damage to the system
if it is not dealt with
an automatic cutout valve a CoV is
fitted to a system employing a constant
delivery fixed volume pump to control
system pressure and to provide the pump
with an idling circuit when no services
have been selected
the a COV consists of a piston which is
sensing system pressure
this is opposed by a spring
the piston is controlling a poppet valve
the a COV operates in two modes these
are known as cut in and cut out
the valve is in the cut in position when
spring pressure is greater than pump
output pressure the piston is fully down
and the poppet valve is closed
cut out is when hydraulic pressure is
greater than spring pressure the piston
moves up and the poppet valve is open
this allows the pump output to return to
the reservoir
you
with the system not operating and the
pressure zero the ACO V will be in the
cut in position
when the pump starts operating pressure
will build up until when normal system
operating pressure is reached
the pressure on the bottom of the piston
will overcome the spring
the piston will move up opening the
poppet valve
and allowing the pump to return its
fluid output to the reservoir without
any load there will still be fluid
flowing through the pump to cool and
lubricate it this is the cut out
position
system pressure will be
held on the bottom of the piston by the
non-return valve
when a surface is operated
system pressure will fall
the spring force will now be greater
than the hydraulic pressure so the
piston will move down closing the poppet
valve
this will cut off the pump return to the
reservoir and allow it to operate the
service the a CoV will remain in the cut
in position until the service reaches
the end of its travel
and pressure builds up again
it will then return to the cutout
position
this type of system needs a reservoir of
fluid energy downstream of the automatic
cutout valve otherwise any slight
leakage through components or from the
system would result in frequent
operation of the valve and frequent
loading and unloading of the pump
an accumulator fulfills this as well as
a number of other functions
the time between cut out off load and
cut in on load of the a CoV is a good
indication of the condition of the
system
external leakage will cause a reduction
in the operating period with frequent
loading and unloading of the pump
internal leakage usually caused by a
piston seal failure will similarly cause
frequent loading and unloading of the
pump this could also cause an increase
in fluid temperature due to the extra
work being done by the pump
that is the end of the lesson on
components to summarize the main points
a reservoir provides both storage space
for the system fluid and sufficient air
space to allow for any variations of
fluid volume in the system
which may be caused by actuator RAM
displacement
or by thermal expansion
it compensates for small leaks
and it provides ahead of fluid for the
pump most reservoirs are pressurized to
provide a positive fluid pressure at the
pump Inlet in order to help prevent
cavitation
filters are used to protect pumps and
other system components from damage
caused by foreign particles in the fluid
cutout valve is fitted to a system
employing a constant delivery fixed
volume pump to control system pressure
and to provide the pump with an idling
circuit when no services have been
selected
finally an accumulator is fitted to
store hydraulic fluid under pressure in
order to dampen pressure fluctuations
to allow for thermal expansion
an emergency supply of fluid to the
system in the event of pump failure
period between cut out and cut in time
at the AC o V and so reduce the wear on
the pump
it also provides the initial fluid when
a selection is made and the pump is cut
out
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