Topic 2: Basic Hydraulics Flashcards
the science relating to the behavior of liquids under various conditions and in aircraft.
Hydraulics
in aircraft, this provides a means of operating large and remote components that it would not be possible to operate satisfactorily by other means.
hydraulic system
provide a means of power transmission through the medium of hydraulics through an incompressible fluid via pipelines and actuators.
Aircraft systems
provide the power for the operation of components such as landing gear, flaps, flight controls, wheel brakes, windshield wipers, and other systems that require high power, accurate control, and rapid response rates.
Hydraulic systems
a 17th century mathematician who stated that:
“If a force is applied to a liquid in a confined space, then this force will be felt equally in all directions”.
Blaise Pascal
states that, “If a force is applied to a liquid in a confined space, then this force will be felt equally in all directions”.
PASCAL’S LAW
Principle where all hydraulic systems operate.
PASCAL’S LAW
The force employed when a hydraulic system is operated is caused by this.
pressure
This is created only when an attempt is made to compress fluids.
Hydraulic pressure
if a flow of oil is pumped through an open-ended tube there will be _________.
no pressure
if the end of the tube is blocked and the oil cannot escape, this will at once build up.
pressure
Without some form of _________ there can be no pressure.
restriction
This is required to deliver a flow of fluid into the system and some form of restriction is required to obtain pressure.
pump
In hydraulic systems, restriction is provided by movable pistons which travel backwards and forwards in cylinders. These assemblies being known as __________.
hydraulic jacks or actuators
Formula of Pressure:
Pressure = Force per Unit Area = FORCE / AREA
Formula of Force:
Force = Pressure x Area
Formula of Work:
Work = Force x Distance
He invented a hydraulic press and, in doing so, observed two facts:
a) The smaller the area under load, the greater the pressure generated.
b) The larger the area under pressure, the greater will be the load available.
Joseph Bramah (1749 – 1814)
a hydraulic press that observed two facts:
a) The smaller the area under load, the greater the pressure generated.
b) The larger the area under pressure, the greater will be the load available.
BRAMAH’S PRESS
2 facts observed in the brahmah’s press:
a) The smaller the area under load, the greater the pressure generated.
b) The larger the area under pressure, the greater will be the load available.
For a given system pressure, the ________ produced will be directly proportional to the piston area and the ________ moved by the piston will be inversely proportional to the piston area.
force; distance
It is a Passive Hydraulic System where a force is applied to a piston (piston A) only when it is desired to move the load (piston B).
Bramah Press
The Bramah Press is a ________ where a force is applied to a piston (piston A) only when it is desired to move the load (piston B).
Passive Hydraulic System
It describes a system only generating pressure when it is required rather than generating and maintaining pressure all of the time and only using it when something needs to be moved.
Passive Hydraulic System
for operating different services, this varies according to its size and loading.
Power
Power required for operating different services varies according to their _________.
size and loading
Power required for operating different services varies according to their size and loading. These can be catered for by using _________.
actuators with pistons of different areas
HYDRAULIC FLUID PROPERTIES:
▪ Incompressible
▪ Good Lubricating Properties
▪ Good Viscosity
▪ High Boiling Point/Low Freezing Point
▪ High Flash Point/Non-Flammable
▪ Chemically Inert/Non-Corrosive
▪ Resistant to Evaporation
▪ Free from Sludging and Foaming
▪ Reasonably Priced
▪ Readily Available
HYDRAULIC FLUIDS:
1) DEF STAN 91-48
2) SKYDROL
Equivalent to H515 NATO, OM15, and MIL-H-5606F
DEF STAN 91-48
Clean mineral hydraulic fluid
DEF STAN 91-48
Colored red hydraulic fluid
DEF STAN 91-48
Hydraulic fluid used with synthetic neoprene rubber seals
DEF STAN 91-48
Recommended for all hydraulic systems under high pressures and low or very low temperatures (-54 C° to 90 C°)
DEF STAN 91-48
Phosphate ester based synthetic hydraulic fluid
SKYDROL
a hydraulic fluid that is purple in color
SKYDROL
a hydraulic fluid used with Synthetic Butyl Rubber Seals
SKYDROL
Prolonged exposure to this will damage skin and eyes
SKYDROL
a hydraulic fluid with a Very high flashpoint (170 C°)
SKYDROL
a hydraulic fluid that is less prone to cavitation
SKYDROL
It is of major importance that only the specified _________ or its approved alternative is used in a hydraulic system.
hydraulic oil
If the ________ is added to a system, breakdown of the seals is likely causing fluid leakage, both internally within components and externally from the actuators.
incorrect fluid
It is governed by the resistance to motion encountered by the fluid.
efficiency of a hydraulic system
The efficiency of a hydraulic system is governed by this.
resistance to motion encountered by the fluid
This is expended in overcoming static resistance (friction) which are:
• Friction in cylinders caused by moving parts
• Piston rods rubbing against bearings and seals
• Fluid rubbing against the pipe walls
Force
Force is expended in overcoming static resistance (friction) which are:
• Friction in cylinders caused by moving parts
• Piston rods rubbing against bearings and seals
• Fluid rubbing against the pipe walls
This would allow nearly 100% of the force to be utilized but would incur large weight and cost penalties.
Large bore pipes and frictionless pistons
Large bore pipes and frictionless pistons would allow nearly 100% of the force to be utilized but would incur _________.
large weight and cost penalties
_________ between pistons and cylinders and piston rods and bearings cannot be completely eliminated, it can only be lessened by good design and workmanship.
Friction
Friction between pistons and cylinders and piston rods and bearings cannot be completely eliminated, it can only be lessened by _________.
good design and workmanship
The friction between the walls of the pipes and the fluid depends upon:
a) Velocity of the fluid in the pipes.
b) Length, bore, and the internal finish of the pipes.
c) Number of bends.
d) Viscosity of the oil.
This depends upon:
a) Velocity of the fluid in the pipes.
b) Length, bore, and the internal finish of the pipes.
c) Number of bends.
d) Viscosity of the oil.
friction between the walls of the pipes and the fluid
perform a very important function in a hydraulic system, which is preventing leakage of fluid.
Seals
a very important function seals perform in a hydraulic system
preventing leakage of fluid
these prevent fluid leaks by being squeezed between two non-moving surfaces.
Static seals, gaskets, and packing
fitted between sliding surfaces, these may be of many different shapes, depending on their use and on the pressure involved.
Dynamic seals
these ring seals are effective in one direction only
“U“ and “V”
these seals are often used where pressure is applied in either direction.
“O” rings and square section seals
Where high pressures are used, an O-ring is normally fitted with __________.
stiff backing ring
it retains the shape of the seal and prevents it from being squeezed between the two moving surfaces.
stiff backing ring
Where high pressures are used, it is normally fitted with stiff backing ring which retains the shape of the seal and prevents it from being squeezed between the two moving surfaces.
O-ring
these require lubrication to remain effective.
Dynamic seals
True or False. Wetting of the bearing surface or a slight seepage from the seals is normally acceptable.
True
these are made in a variety of materials, depending on the type of fluid with which they are to be used.
Seals
If a seal of an incorrect material is used in a system, the sealing quality will be seriously degraded, and this may lead to _________.
failure of the component
these are easily damaged by grit
Seals
this is often installed on actuators to prevent any grit that may be deposited on the piston rod from contaminating the seals.
wiper ring
COMPONENTS OF A BASIC HYDRAULIC SYSTEM:
1) Reservoir
2) Pump
3) Filter
4) Relief Valve
5) Selector Valve
6) Actuator
it delivers oil to the pump and receives oil from the actuators.
reservoir
a hydraulic system component that is either hand, engine, or electrically driven.
pump
3 types of pump of a hydraulic system:
hand, engine, or electrically driven
it enables the operator to select the direction of the flow of fluid to the required service and providing a return path for the oil to the reservoir.
selector or control valve
it actuates the component.
jack, or set of jacks, or actuators
it keeps the fluid clean.
filter
a safety device to relieve excess pressure.
relief valve
Apart from light aircraft breaks, it has little practical use.
Bramah Press / Passive Hydraulic System
Most systems are __________ where a pump supplies the pressure.
Active Hydraulic Systems
Two types of Active Hydraulic System:
1) Open Center System
2) Closed System
The main advantage of this system is that it is simple
OPEN CENTER SYSTEM
the main disadvantage of this system is that only one service can be operated at a time.
OPEN CENTER SYSTEM
Fluid is passed directly to the reservoir when no services are being operated; this allows the engine driven pump to run in an “offloaded condition” as little pressure is generated but there is still a flow through the pump to cool and lubricate it.
OPEN CENTER SYSTEM
On selection of a user system, the fluid is directed to the actuator which will move.
OPEN CENTER SYSTEM
The fluid from the other side of the actuator will return to the reservoir. When the actuator reaches the end of its travel, selector is returned to neutral in order to offload the pump and allow other services to be selected.
OPEN CENTER SYSTEM
The relief valve will relieve excess pressure if the selector is not returned to its neutral position.
OPEN CENTER SYSTEM
Its great advantage is that multiple services can be operated at the same time.
CLOSED SYSTEM
Their use is only limited by the volume flow capacity of the pump.
CLOSED SYSTEM
With this type of system, operating pressure is maintained in that part of the system which leads to the selector valves, and some method is used to prevent over-loading the pump.
CLOSED SYSTEM
In systems which employ a fixed volume pump (constant delivery), an automatic cut-out valve is fitted, to divert pump output to the reservoir when pressure has built up to normal operating pressure.
CLOSED SYSTEM
Basically, CLOSED SYSTEM operates the same way as open center system. Selector valve is moved opening up pressure and lines to the required service. However, there are two important differences:
▪ Two or more services can be operated at the same time.
▪ It is not important to return the selector valve to neutral once operation is complete; designed to deal with the consequent pressure increase.
The main advantage of OPEN CENTER SYSTEM
it is simple
main disadvantage of OPEN CENTER SYSTEM
one service can be operated at a time
In OPEN CENTER SYSTEM, fluid is passed directly to the reservoir when no services are being operated, this allows the engine driven pump to run in an __________ as little pressure is generated but there is still a flow through the pump to cool and lubricate it.
“offloaded condition”
great advantage of CLOSED SYSTEM
multiple services can be operated at the same time
In systems which employ a fixed volume pump (constant delivery), an _________ is fitted, to divert pump output to the reservoir when pressure has built up to normal operating pressure.
automatic cut-out valve
it provides storage space for the system fluid.
reservoir
Extra fluid is stored in the reservoir due to the following reasons:
• No system is leak free.
• Fluid expands and contracts as heated
or cooled.
• Actuators need more fluid when they are extended than contracted.
• Cavitation
This is stored in the reservoir due to the following reasons:
• No system is leak free.
• Fluid expands and contracts as heated
or cooled.
• Actuator need more fluid when they are extended than contracted.
• Cavitation
Extra fluid
Pressurized with air from engine compressor to further increase pressure at pump inlet to reduce possibility of cavitation.
RESERVOIR
This usually has the following:
• Connections for suction pipes to the pumps and return pipes from the system.
• A quantity transmitter unit to monitor the system.
• A filling inlet normally sealed with filler cap preventing debris entering the reservoir.
• In some cases, a temperature sensing probe.
reservoir
A reservoir usually has the following:
• Connections for suction pipes to the pumps and return pipes from the system.
• A quantity transmitter unit to monitor the system.
• A filling inlet normally sealed with filler cap preventing debris entering the reservoir.
• In some cases, a temperature sensing probe.
These are for suction pipes to the pumps and return pipes from the system.
Connections
used to monitor the system in the reservoir
quantity transmitter unit
normally sealed with filler cap preventing debris entering the reservoir
filling inlet
They remove foreign particles from the fluid.
FILTERS
These are fitted in both suction and pressure lines that is at both sides of the pump.
Filters
This filter is used to protect the pump.
Suction filter
This is used to ensure the cleanliness of fluid during use.
pressure filter
Sometimes, this is also present in the fluid return line to the reservoir to remove particles picked up during operation.
filter
Sometimes, there is also a filter in the _________ to remove particles picked up during operation.
fluid return line to the reservoir
Some of these are fitted with a device which senses the pressure difference across the filter element, and releases a visual indicator, in the form of a button or illuminates a warning lamp when the pressure difference increases as a result of the filter becoming clogged.
filters
Some of these are fitted with a relief or bypass valve which allows unfiltered fluid to pass to the system when the element becomes clogged.
filters
Other filters are fitted with a _________ which allows unfiltered fluid to pass to the system when the element becomes clogged.
relief or bypass valve
This type of filter element must be changed at regular intervals before clogging occurs.
filters fitted with a relief or bypass valve
Draw oil from the reservoir and deliver a supply of fluid to the system.
PUMPS
In most cases, these are used to provide alternate supply as part of the redundancy provision for the safe operation of the aircraft.
air pump, ram air turbine, and power transfer unit
these may be the only source of power in a small, light aircraft hydraulic system
Hand Pumps
These may also be used on larger aircraft:
a) To allow ground servicing to take place without the need for engine running.
b) So that lines and joints can be pressure tested.
c) So that cargo doors, etc., can be operated without power.
Hand Pumps
Reasons why Hand Pumps may also be used on larger aircraft:
a) To allow ground servicing to take place without the need for engine running.
b) So that lines and joints can be pressure tested.
c) So that cargo doors, etc., can be operated without power.
Two types of powered or engine driven pumps may be classified as follows:
▪ Constant Delivery (Fixed Volume) Pump
▪ Constant Pressure (Variable Volume) Pump
One example is spur gear displacement pump.
CONSTANT DELIVERY PUMP
One of the gears is driven by the power source normally engine.
CONSTANT DELIVERY PUMP
Supplies fluid at constant rate irrespective of demand and therefore needs an automatic cut out or relief valve to return the fluid to the reservoir.
CONSTANT DELIVERY PUMP
It is fitted to a system employing a constant delivery (fixed volume) pump, to control system pressure and to return fluid to the reservoir.
AUTOMATIC CUT OUT VALVES (ACOV)
This pump supplies fluid at a variable volume and controls its own pressure.
CONSTANT PRESSURE PUMP
This type of pump is typically fitted in modern aircraft whose systems operate at 3,000 – 4,000 psi
CONSTANT PRESSURE PUMP
This is fitted:
• a) To store hydraulic fluid under pressure
• b) To dampen pressure fluctuations
• c) To allow for thermal expansion
• d) To provide an emergency supply of fluid to the system in then event of pump failure
• e) To prolong the period between cut out and cut in time of the ACOV and so reduce the wear on the pump
• f) Provides the initial fluid when a selection is made, and the pump is cut out
accumulator / HYDRAULIC ACCUMULATORS
Used to convert fluid flow into linear or rotary motion
HYDRAULIC JACKS (ACTUATORS)
They vary in size and construction depending on the operating loads, but all consist of an outer cylinder within which slides a piston and seal assembly.
HYDRAULIC JACKS (ACTUATORS)
In a hydraulic jack (actuator), attached to the piston is a _________ which passes through a seal and wiper ring assembly fitted into the end of the cylinder.
piston rod (or ram)
3 Types of HYDRAULIC JACKS (ACTUATORS):
1) Single Acting Actuator
2) Double Acting Unbalanced Actuator
3) Double Acting Balanced Actuator
Hydraulically operated in one direction only.
SINGLE ACTING ACTUATOR
normally used as a locking device, the lock being engaged by spring pressure and released by hydraulic pressure.
SINGLE ACTING ACTUATOR
This has more fluid in the cylinder when the rod is extended.
SINGLE ACTING ACTUATOR
actuator used in most aircraft systems.
DOUBLE ACTING UNBALANCED ACTUATOR
Because of the presence of the piston rod in this actuator, the piston face to which the rod is attached has a smaller area on which the fluid can act than the other face.
DOUBLE ACTING UNBALANCED ACTUATOR
In this actuator, the area of the top of the piston is greater than the area under it. Consequently, more force can be applied during extension of the piston rod.
DOUBLE ACTING UNBALANCED ACTUATOR
using this, bigger force can be exerted during extension than retraction.
Force = Pressure x Area
In this actuator, equal force can be applied to both sides of the piston.
DOUBLE ACTING BALANCED ACTUATOR
It is often used in applications such as nose wheel steering.
DOUBLE ACTING BALANCED ACTUATOR
In this actuator, either one or both sides of the piston rod may be connected to a mechanism.
DOUBLE ACTING BALANCED ACTUATOR
When fluid is trapped between the piston of the actuator and a non-return valve or a selector valve at its neutral position, this is said to be formed
HYDRAULIC LOCK
Because the fluid is incompressible, and is unable to flow through the system, the piston cannot move even if an external load is applied to it and is therefore locked in its position called _________.
HYDRAULIC LOCK
Maximum system pressure is often controlled by adjustment of the main engine driven pump, but a number of other components are used to maintain or limit fluid pressures in various parts of a hydraulic system.
PRESSURE CONTROL
Other components used to maintain or limit fluid pressures in various parts of a hydraulic system:
▪ Relief valves
▪ Pressure Maintaining Valves
▪ Pressure Reducing Valves
These are components used to control the flow of fluid to the various services operated by the hydraulic system.
FLOW CONTROL COMPONENTS
components used to control the flow of fluid to the various services operated by the hydraulic system:
▪ Non-Return Valves
▪ Restrictor Valves (or choke)
▪ Selectors
▪ Electrically Operated Selectors
▪ Shuttle Valves
▪ Sequence Valves
▪ Modulators
▪ Flow Control Valves
▪ Fuses
Advantages of Pneumatics:
• Air is universally available and it is free.
• The system is lighter because the air can be dumped overboard after use so no return lines are required, and air is lighter than hydraulic fluid.
• There is no fire hazard, nor is air toxic.
• There are no viscosity problems with changes of temperature.
Disadvantage of Pneumatics:
• Air is extremely compressible.
