hydraulic systems Flashcards
what is the SI unit for forces?
the newton (N)
what is the SI unit for area?
the square millimetre (mm2)
despite the SI units for force and area, what force unit do most current aircraft tend to use? (3)
- pounds per square inch
- kilograms per square centimetre
- bar
what instruments are used to measure the viscosity of a liquid?
viscosimeters
what is cavitation described as?
dissolved gas coming out of solution to form free bubbles
in cavitation, the suction in the supply pipeline causes a ________ pressure, which induces gas to form in _____ _______ in the fluid to fill this area of reduced pressure
in cavitation, the suction in the supply pipeline causes a NEGATIVE pressure, which induces gas to form in LARGE BUBBLES in the fluid to fill this area of reduced pressure
when entering high pressure areas, cavitation bubbles that _______ on a metal surface cause ______ stress through repeated implosion
when entering high pressure areas, cavitation bubbles that IMPLODE on a metal surface cause CYCLIC stress through repeated implosion
cavitation is a significant cause of…
wear
a pump in a hydraulic system provides a ____, not ________
a pump in a hydraulic system provides a FLOW, not PRESSURE
in a compressed bleed air reservoir, the system will incorporate a pressure regulator which will maintain a reservoir pressurisation of about…
40-45 psi
what is an advantage of a bootstrap hydraulic reservoir?
no bleed air required - system is self pressurising using system high pressure to create inlet pressure
what is the advantage of a double acting type hand pump?
fluid is delivered with each stroke (up and down)
what is an example of a safety device in hydraulic pump drive couplings?
- quill drives
- if the pump becomes unusually hard to turn or jams, the shear section (quill) shears, preventing damage to pump or driving unit
in spur gear pumps, why is a small amount of fluid allowed to leak past the gears and around the shafts?
- lubrication and cooling purposes
- drains into hollow shafts of gears and picked up by low pressure at inlet side of pump
why is ‘case’ pressure maintained in a spur gear pump?
if shaft or seal becomes scored, fluid will be forced out rather than air being drawn into pump
how does a variable volume pump cater for varying system requirements?
- pistons attached to shoes which rotate against a stationary yoke
- angle between yoke and cylinder block varied to increase or decrease pump stroke
in a variable volume pump, what happens when system pressure is low after selection of a service?
- spring pressure on control piston turns yoke to max angle
- pistons are at full stroke
- deliver max output to system
when a variable volume pump is in minimum stroke position, a small flow is maintained through the pump. why?
- lubricate working parts
- overcome internal leakage
- dissipate heat
to reduce drag on the engine, many variable volume pumps have the facility to be…
‘off-loaded’
what is one of the more important requirements for hydraulic fluid?
cleanliness
why must solid particle contaminants be removed from hydraulic fluid?
they can damage components
what unit is filtering capability measured in?
- microns
- 1 micron = 1 millionth of a metre (approx one twenty five thousandth of an inch)
adequate filtering for a hydraulic system requires filter to remove all contamination greater than?
10 microns
hydraulic filters used in high pressure systems are usually made of?
- sintered metal
- very fine metallic mesh
- wire wound
a pressure differential of clogging indicator filter is set to operate at a…
predetermined pressure
under low temperature conditions, what prevents actuation of indicator button in a DPI filter?
bi-metallic element
under normal operation, where does hydraulic fluid flow in a twin element filter?
through both inner and outer filtering elements
in a twin element filter, should the outer element clog, the _______ bypass valve will open, directing the fluid to the inner element. In the event of inner element clogging also, the ___-_____ will open, allowing fluid to bypass both elements
in a twin element filter, should the outer element clog, the PRIMARY bypass valve will open, directing the fluid to the inner element. In the event of inner element clogging also, the BEL-VALVE will open, allowing fluid to bypass both elements
what are four reasons for accumulators being fitted to hydraulic circuits?
- when automatic cut out valve used with pump, ensures smooth operation
- store of hydraulic pressure for emergency system operation
- provides initial system impetus until pump provides flow
- damps out pump pulsations preventing fatigue damage to pipelines, clamps and aircraft structure
what are the three types of accumulator?
- separator piston
- diaphragm
- bladder
what is the purpose of an actuator?
transform fluid flow into linear or rotary motion
what type of actuator is used in most aircraft systems?
double acting actuator
in a double acting actuator, because of the presence of the piston rod, the area of the ___ of the piston is greater than the area _____ it
in a double acting actuator, because of the presence of the piston rod, the area of the TOP of the piston is greater than the area UNDER it
in a balanced actuator, _____ force can be applied to both sides of the piston
in a balance actuator, EQUAL force can be applied to both sides of the piston
the four common components used in hydraulic systems to achieve pressure control are:
- automatic cut out valves (ACOV)
- pressure relief valves (PRV)
- thermal relief valves (TRV)
- pressure reducing valve (PRV)
a closed centre hydraulic system where the pressure is supplied by a fixed volume hydraulic pump requires a ___-___ valve to maintain the pressure within a specified range
a closed centre hydraulic system where the pressure is supplied by a fixed volume hydraulic pump requires a CUT-OUT valve to maintain the pressure within a specified range
the automatic cut out valve (ACOV) also provides an ______ _______ for the pump output when the pre determined system pressure is reached
the automatic cut out valve (ACOV) also provides an IDLING CIRCUIT for the pump output when the pre determined system pressure is reached
an accumulator is essential when a cut-out is fitted to prevent…
hammering
in a pressure relief valve, the valve spring setting can usually be adjusted to set the pressure at which ___ _____ ____ ________
in a pressure relief valve, the valve spring setting can usually be adjusted to set the pressure at which THE VALVE WILL OPERATE
in a pressure relief valve, once the pressure exceeds the predetermined value, the valve will open and…
relieve any excess pressure to the return line
pressure relief valves may be used in both the power circuit and user systems, for example if an aircrafts flaps were left extended and the speed increased, fitting of a pressure relief valve in the ____ pipeline would allow the flaps to ________ and avoid airframe over stress
pressure relief valves may be used in both the power circuit and user systems, for example if an aircrafts flaps were left extended and the speed increased, fitting of a pressure relief valve in the DOWN pipeline would allow the flaps to BLOWBACK and avoid airframe over stress
thermal relief valves are fitted into parts of circuits where
no other form of pressure relief exists
thermal relief valves will only nee to release fluid in very small quantities to maintain system pressure; therefore they are fitted with a __________ device that prevents major loss should the valve jam in the open position
thermal relief valves will only nee to release fluid in very small quantities to maintain system pressure; therefore they are fitted with a RESTRICTOR device that prevents major loss should the valve jam in the open position
what are the three types of mechanical selectors?
- rotary
- poppet
- spool
the double solenoid selector is the most common as it provides _________ 2 way operation of systems, with a ______ _______
the double solenoid selector is the most common as it provides POSITIVE 2 way operation of systems, with a NEUTRAL SETTING
in a double solenoid selector, when operated in a system, an electrical input can be used to cancel power to the solenoid and the _____ _____ will return automatically to its centre, neutral position
this will provide a positive hydraulic lock within the operating system, and will prevent ant leaks within that system from using total fluid loss
in a double solenoid selector, when operated in a system, an electrical input can be used to cancel power to the solenoid and the SLIDE VALVE will return automatically to its centre, neutral position
this will provide a positive hydraulic lock within the operating system, and will prevent ant leaks within that system from using total fluid loss
restrictor valves are used to control the _____ ____ of fluid
restrictor valves are used to control the FLOW RATE of fluid
what are the two methods of restriction in restrictor valves?
- metered orifice
- variable orifice (throttling valve)
shuttle valves are incorporated into systems that have an alternative method of operation for ________ _______
this emergency operation may use hydraulic pressure from an alternate source or compressed gas, usually _______, from a storage cylinder
shuttle valves are incorporated into systems that have an alternative method of operation for EMERGENCY PURPOSES
this emergency operation may use hydraulic pressure from an alternate source or compressed gas, usually NITROGEN, from a storage cylinder
priority valves are sometimes referred to as pressure ___________ or pressure __________ valves. their purpose is to ensure flow/pressure to essential of primary operating systems, whenever any is available
priority valves are sometimes referred to as pressure MAINTAINING or pressure REGULATING valves. their purpose is to ensure flow/pressure to essential of primary operating systems, whenever any is available
a hydraulic ____ may be used in pipelines to seal them off if a serious leak occurs
a hydraulic FUSE may be used in pipelines to seal them off if a serious leak occurs
pressure relay valves, similar to the hydraulic fuse my be used in pipelines connecting to direct reading gauges of pressure transmitters, and prevents _____ ____ if a leak occurs
in the case of direct reading gauges, they will also prevent gauge faults that allow hydraulic fluid into the flight deck area
pressure relay valves, similar to the hydraulic fuse my be used in pipelines connecting to direct reading gauges of pressure transmitters, and prevents FLUID LOSS if a leak occurs
in the case of direct reading gauges, they will also prevent gauge faults that allow hydraulic fluid into the flight deck area
system isolation valves are fitted to allow shutting off of parts of the system for _________ purposes or in the event that a fire should occur in certain areas
system isolation valves are fitted to allow shutting off of parts of the system for MAINTENANCE purposes or in the event that a fire should occur in certain areas
non-return valves are fitted _________ of the hand pump and engine driven pump
non-return valves are fitted DOWNSTREAM of the hand pump and engine driven pump
hydraulic pipelines are marked using the international symbol for hydraulics, a _____ ____ on a white background
hydraulic pipelines are marked using the international symbol for hydraulics, a BLACK DISC on a white background
there are many different types of hydraulic hose, the most common being the ________ type
there are many different types of hydraulic hose, the most common being the AEROQUIP type
fluid sampling points are positioned to allow easy access, and will often be positioned in the _______ and _________ lines of the hydraulic pump
this component is usually in the form of a ___ connector, allowing fluid to pass through it in normal operation, the third connection being the sampling point which is normally fitted with a ________ ____ _____
fluid sampling points are positioned to allow easy access, and will often be positioned in the SUCTION and PRESSURE lines of the hydraulic pump
this component is usually in the form of a TEE connector, allowing fluid to pass through it in normal operation, the third connection being the sampling point which is normally fitted with a PRESSURE TIGHT BLANK
ground test connections are usually found in the aircrafts hydraulic ______ system in the suction, pressure and by-pass lines
ground test connections are usually found in the aircrafts hydraulic SUPPLY system in the suction, pressure and by-pass lines
correct seal type to be used with castor based fluid DTD900/4081 is…
natural rubber
correct seal type to be used with mineral based fluid MIL-H-5606 is…
synthetic rubber
correct seal type to be used with phosphate ester based fluid (skydrol) is…
butyl, ethylene, propylene or teflon
the combination of moving surfaces and high pressures may cause the o-ring seal to be extruded (squeezed) through any small gap. to eliminate this possibility, back up rings of _____, ______ or _______ are often used
the combination of moving surfaces and high pressures may cause the o-ring seal to be extruded (squeezed) through any small gap. to eliminate this possibility, back up rings of TEFLON, TUFNOL or MICARTA are often used
U and V section seals provide both ______ and _______ sealing but only in one direction. therefore moving components such as pistons will require 2 seals fitted back to back
U and V section seals provide both STATIC and DYNAMIC sealing but only in one direction. therefore moving components such as pistons will require 2 seals fitted back to back
standby or auxiliary systems are _______ power systems which have their own dedicated reservoir/pump etc…
standby or auxiliary systems are SEPERATE power systems which have their own dedicated reservoir/pump etc…
examples of emergency systems are: (5)
- electrical pumps
- air pumps
- RAT
- PTU
- accumulators
the RAT must be capable of operation over a broad speed range, maintaining normal hydraulic _______, whether the aeroplane is at maximum speed and height or slowing for landing
to accomplish this, the propeller is of a _______ ____ design, controlled by a _________ ________
the RAT must be capable of operation over a broad speed range, maintaining normal hydraulic PRESSURE, whether the aeroplane is at maximum speed and height or slowing for landing
to accomplish this, the propeller is of a VARIABLE PITCH design, controlled by a MECHANICAL GOVERNOR
a volumetric fuse is usually fitted to the RAT allowing the propeller to attain operating speed before being loaded by the pump. this allows full system pressure to be achieved approximately - seconds after deployment into the airstream
a volumetric fuse is usually fitted to the RAT allowing the propeller to attain operating speed before being loaded by the pump. this allows full system pressure to be achieved approximately 4-5 seconds after deployment into the airstream
reservoir standpipe (higher). a supply of fluid for the _________ pump is ensured by drawing the hydraulic fluid from the bottom of the reservoir
reservoir standpipe (higher). a supply of fluid for the EMERGENCY pump is ensured by drawing the hydraulic fluid from the bottom of the reservoir
a low level warning will usually be given by a light on the flight deck ________ panel and the ______ ______ panel
a low level warning will usually be given by a light on the flight deck HYDRAULIC panel and the MASTER WARNING panel
temperature sensors are normally fitted to the reservoirs, overheat indication will usually take form of __________ warning on the flight deck hydraulic panel, although some systems may provide __________ _______
temperature sensors are normally fitted to the reservoirs, overheat indication will usually take form of ILLUMINATED warning on the flight deck hydraulic panel, although some systems may provide TEMPERATURE GAUGES
pressure indication. electrically signalled systems use a pressure _________, which is a bourdon tube connected to a variable resister, and a voltage __________ to pressure is sent to a moving coil type gauge
pressure indication. electrically signalled systems use a pressure TRANSMITTER, which is a bourdon tube connected to a variable resister, and a voltage PROPORTIONAL to pressure is sent to a moving coil type gauge
ground test connections are fitted to all pipelines of the EDP to allow use of a _____ ____ _______ for full flow testing of systems when the engines cannot be operated
ground test connections are fitted to all pipelines of the EDP to allow use of a GROUND TEST TROLLEY for full flow testing of systems when the engines cannot be operated
samples of system fluid can be used to determine _________ contamination, ______, _______ gravity, _________ and water content
samples of system fluid can be used to determine PARTICULATE contamination, ACIDITY, SPECIFIC gravity, VISCOSITY and water content
