Basic Hydraulics Flashcards
science relating to the behavior of liquids under various conditions and in aircraft
hydraulics
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
hydraulic systems
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
17th century mathematician concerned with Pascal’s Law
Blaise Pascal
Principle where all hydraulic systems operate
Pascal’s Law
force employed when a hydraulic system is operated
pressure
created only when an attempt is made to compress fluids
hydraulic pressure
Without some form of _____, there can be no pressure
restriction
required to deliver a flow of fluid into the system
pump
In hydraulic systems, this restriction is provided by movable pistons which travel backwards and forwards in cylinders.
hydraulic jacks or actuators
invented a hydraulic press called the Bramah Press
Joseph Bramah
The _____ the area under load, the greater the pressure generated. The _____ the area under pressure, the greater will be the load available.
smaller, larger
Name at least 3 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
DEF STAN 91-48 is also known as
H515 NATO, OM15, MIL-H-5606F
Clean mineral hydraulic fluid
DEF STAN 91-48
hydraulic fluid colored red
DEF STAN 91-48
Used with synthetic neoprene rubber seals
DEF STAN 91-48
Recommended for all hydraulic systems under high pressures and low or very low temperatures
DEF STAN 91-48
temperature for DEF STAN 91-48
-54 deg cel to 90 deg cel
Phosphate ester based synthetic hydraulic fluid
Skydrol
hydraulic fluid purple in color
Skydrol
Used with Synthetic Butyl Rubber Seals
Skydrol
Prolonged exposure will damage skin and eyes
Skydrol
Very high flashpoint
Skydrol
flashpoint of Skydrol
170 deg cel
less prone to cavitation
Skydrol
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
Large bore pipes and frictionless pistons would allow ______ of the force to be utilized but would incur large weight and cost penalties.
nearly 100%
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:
- velocity of the fluid in the pipes
- Length, bore, and the internal finish of the pipes
- number of bends
- viscosity of the oil
prevents leakage of fluid in hydraulic systems
seals
prevent fluid leaks by being squeezed between two non-moving surfaces
static seals, gaskets, packing
these seals are effective in one direction only
U and V ring seals
these seals are often used where pressure is applied in either direction
O ring seal, square section seal
it is usually fitted with O-ring, where high pressures are used
stiff backing ring
retains the shape of the seal and prevents it from being squeezed between the two moving surfaces
stiff backing ring
these seals require lubrication to remain effective
dynamic seals
_____ of the bearing surface or a slight seepage from the seals is normally acceptable.
wetting
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
Reservoir, Pump, Filter, Relief Valve, Selector Valve, Actuator
delivers oil to the pump and receives oil from the actuators
reservoir of oil
either hand, engine, or electrically driven
pump
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 valve or control valve
actuate the component
jack or set of jacks or actuators
keep the fluid clean
filter
as a safety device to relieve excess pressure
relief valve
Apart from light aircraft breaks, it has little practical use
Passive Hydraulic System
in such a system, the pump supplies the pressure
Active Hydraulic System
Two Types of Active Hydraulic System
Open Center System, Closed System
main advantage of this system is that it is simple, the main disadvantage 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
offloaded condition
When the actuator reaches the end of its travel, selector is ________ in order to offload the pump and allow other services to be selected.
returned to neutral
The relief valve will relieve excess pressure if the selector is ______ to its neutral position.
not returned
Its great advantage is that multiple services can be operated at the same time. However, its 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
fitted in systems that employ a fixed volume pump (constant delivery) to divert pump output to the reservoir when pressure has built up to normal operating pressure
automatic cut-out valve
Difference of a Closed System and an Open Center System
- two or more services can be operated at the same time
- not important to return the selector valve to neutral once operation is complete
Why is it not important to return the selector valve to neutral once operation is complete in a closed system?
A closed system is designed to deal with the consequent pressure increase.
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.
- Actuator need more fluid when they are extended than contracted.
- Cavitation
monitor the system in a reservoir
quantity transmitter unit
prevents debris in entering the reservoir
filling inlet (normally sealed with filler cap)
fitted in both suction and pressure lines that is at both sides of the pump
filters
protects the pump
suction filter
ensures the cleanliness of fluid during use
pressure filter
filter in the fluid return line to the reservoir to remove particles picked up during operation
return filter
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
relief or bypass valve
draws oil from the reservoir and deliver a supply of fluid to the system
pumps
used to provide alternate supply as part of the redundancy provision for the safe operation of the aircraft
air pump, ram air turbine, power transfer unit (PTU)
may be the only source of power in a small, light aircraft hydraulic system; may also be used on larger aircraft in some cases
hand pumps
allows ground servicing to take place without the need for engine running; so that the lines and joints can be pressure tested; so that cargo doors can be operated without power
hand pumps
Classification of Pumps
- Constant Delivery (Fixed Volume) Pump
- Constant Pressure (Variable Volume) 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 (Fixed Volume) Pump
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 Valve
This pump supplies fluid at a variable volume and controls its own pressure
Constant Pressure (Variable Volume) Pump
Constant Pressure (Variable Volume) Pump is typically fitted in modern aircraft whose systems operate at ________.
3,000 to 4,000 psi
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
hydraulic accumulator
Used to convert fluid flow into linear or rotary motion
hydraulic jacks or actuators
consist of an outer cylinder within which slides a piston and seal assembly
hydraulic jacks or actuators
passes through a seal and wiper ring assembly fitted into the end of the cylinder
piston rod (or ram)
Hydraulically operated in one direction only; normally used as a locking device; has more fluid in the cylinder when the rod is extended
Single Acting Actuator
actuator used in most aircraft systems
double acting unbalanced actuator
equal force can be applied to both sides of the piston on this actuator; often used in applications such as nose wheel steering; Either one or both sides of the piston rod may be connected to a mechanism
double acting balanced actuator
formed when fluid is trapped between the piston of the actuator and a non-return valve or a selector valve at its neutral position
hydraulic lock
used to maintain or limit fluid pressures in various parts of a hydraulic system
relief valves, pressure maintaining valves, pressure reducing valves
components used to control the flow of fluid to the various services operated by the hydraulic system
flow control components
Give at least three flow control components
- non-return valves
- restrictor valves (or choke)
- selectors
- electrically operated selectors
- shuttle valves
- sequence valves
- modulators
- flow control valves
- fuses
disadvantage of pneumatics
air is extremely compressible
