AMT Hydraulic and Pneuma2c Power Systems Chapter 12 Pascal's Law •  Pascal's law – when pressure is applied to a fluid in an enclosed container, the pressure is transmiBed equally throughout all of the fluid, and it acts at right angles to the walls that enclose the fluid !
In a hydraulic system or pressurized fluid system where the fluid is free to move about, the pressure is equal throughout the system Not Contained – Pressure Based on Height The pressure produced by a liquid in a container is caused by the height of the liquid above the point at which the pressure is measured. The higher the liquid above the gage, the greater the pressure.
Pascal's Law – Pressure is Equal For our calculation, the
weight of the fluid will be
almost nothing compare to
the pressure applied to the
fluid. For our calculations,
P1, P2 and P3 will be the
same.
When we apply pressure on the liquid in a closed container, the pressure rises the same amount in all parts of the container.
Fluid Pressure Formulas •  Force = Pressure X Area Pressure = Force / Area !  Area = Force / Pressure !
Force = Pressure X Area The amount of force produced by the piston in a hydraulic cylinder may be found by mul2plying the area of the piston by the amount of pressure inside the cylinder.
Area = Force / Pressure The area of a piston needed to produce a given amount of force with a certain amount of pressure may be found by dividing the amount of force by the pressure.
Pressure = Force / Area The amount of pressure produced in a hydraulic cylinder may be found by dividing the amount of force on the piston by the area of the piston.
Hydraulic System Hydraulic cylinders produce a mechanical advantage. A 1-­‐pound force (F1) can liV a 10-­‐pound weight (W2), but no work is gained. The work done by the small piston is the same as that done by the large piston.
Hydraulic System •  Hydraulic systems are used to create mechanical advantage like a lever or gear system !
Turn a small force into a larger force •  Brakes and hydraulic jacks !
Turn a small distance into a larger distance •  Trade distance for force !
Example: Car jack •  Lots of strokes of the jack at very liBle pressure •  Heavy car raises a short distance Simple Hydraulic System Hydraulic Jack Hydraulic Fluid •  Viscosity – internal resistance to flow Low viscosity – thinner !  High viscosity – thicker !  Will change with temperature !  Measured with the Saybolt viscometer !
•  Time require to pass 60 milliliters of fluid, at 100 °F, through a standard orifice !
Lower viscosity means less resistance in a hydraulic system •  Preferable •  Lower losses Hydraulic Fluid •  Chemical Stability – hydraulic fluid degrades over 2me and when exposed to air, water, salt or other impuri2es As the fluid degrades, it usually turns darker color and the viscosity gets higher (thicker) !  Manufacturer may give a replacement 2me to insure fresh fluid !
•  Flash Point – temperature at which a liquid gives off vapors in sufficient quan2ty to ignite momentarily Hydraulic Fluid •  Fire Point – temperature at which a liquid gives off vapor in sufficient quan2ty to ignite and con2nue to burn •  Higher flash and fire point are beBer Hydraulic Fluid Types •  Mineral Based !
MIL-­‐H-­‐5606 •  Call “5606” !
New version is MIL-­‐H-­‐6083 •  BeBer fire protec2on •  Interchangeable with MIL-­‐H-­‐5606 Dyed red !  Made from petroleum !  Can be flammable under normal condi2ons !
•  This is why new types of fluids are used Hydraulic Fluid Types •  Mineral Polyalphaolefin MIL-­‐H-­‐83282 !  New version is MIL-­‐H-­‐46170 !
•  Rust inhibited version Polyalphaolefin has beBer flammability characteris2cs than MIL-­‐H-­‐5606 !  High viscosity than MIL-­‐H-­‐5606 at colder temperature !  Can be used with same seals, gaskets and hoses as MIL-­‐H-­‐5606 !  Not always interchangeable with MIL-­‐H-­‐5606 because of viscosity issues !
Hydraulic Fluid Types •  Phosphate Ester Based Called Skydrol® !  MIL-­‐H-­‐8446 !  Light purple in color !  Synthe2c type !  BeBer fire resistant than MIL-­‐H-­‐5606 !
•  Higher flash point Comes in types – I through V !  Can be contaminated if exposed to humidity (water) !  Can destroy older paints and finishes !
•  Compa2ble with most aircraV metals Hydraulic Fluid Types •  Vegetable Oil Based Caster oil and alcohol !  MIL-­‐H-­‐7644 !  Blue in color !  Used with natural rubber seals !  Not used any more because it is very flammable !
Hydraulic Fluid Types •  Don’t intermix fluid types Follow the manufacturer’s recommenda2ons !  Check fluid color !  Not compa2ble with automo2ve DOT fluids !  If wrong fluid is used, the system must be completely flushed. New seals, gaskets and hoses must be installed. !
Hydraulic Fluid Contamina2on •  Hydraulic systems must be kept perfectly clean •  Contaminates will: Break down fluid !  Damage actuators and seals !
•  Contamina2on control: Clean work area and tools !  Seal fluid containers !  Clean components before disassembling !  Cap open fipngs and hoses !  Clean components before reassembling !  Lint-­‐free cloths !
Hydraulic Fluid Contamina2on •  Contamina2on control: Use correct seals and gaskets !  Maintain maintenance schedule: !
•  Clean filters •  Replace fluids !
Maintain fluid sampling schedule »  Drain 1 pint of fluid before sampling •  Replace seals and gaskets Flushing Hydraulic Fluid •  Follow the manufactures recommended procedures and schedule •  Required if wrong fluid used •  Maybe required if wrong seal or gasket used •  Required if system gets dirty •  Naphtha or varsol is used to flush MIL-­‐H-­‐5606 !
Maybe impossible to buy in California •  Phosphate ester-­‐base hydraulic fluid (Skydrol®) cleaned with Stoddard solvent (mineral spirits), methyl-­‐ethyl-­‐ketone (MEK), or isopropyl alcohol Basic Hydraulic System Hydraulic Valve Diagrams Open Center Hydraulic System •  No pressure in the system when actua2ng system is idle •  Selector values are in series •  Fluid is always allowed free passage through each selector valve and back to the reservoir un2l one of the selector valves is posi2oned to operate a mechanism. •  Fluid is flowing through the valve in the OFF posi2on Open Center Hydraulic System Open Center Hydraulic System •  Actuator response is slower than a closed center system !
System pressure must build up •  Very liBle shock from pressure surges Closed Center Hydraulic System •  As long as the pump is opera2ng, the system is under pressure !
Cau2on must be used because the system maybe under pressure even aVer the pump is off •  Selector valves are in parallel •  Fast actuator ac2on •  Most common type of system Closed Center Hydraulic System Hydraulic Power Pack Hydraulic Power Pack Non Pressurized Reservoir •
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Reservoir Supplies fluid to the pumps Replenishes fluids lost by leaks Handles overflow Place to purge air bubbles Baffles and fins are used to stop fluid swirling (vortexing) Reservoir Emergency Reserve •  Part of reservoir is reserved for emergency use only •  Outlet for emergency pump is located at the lowest part of the tank •  Main outlet is connected through a standpipe Main pump draws fluid from a higher point than the emergency pump !  The extra height provides a reserve that is used for the emergency pump only !
•  If only the emergency pump works, check for low fluid level Non Pressurized Reservoir •  Used on aircraV that don’t fly at high al2tudes •  Used on aircraV where the reservoir is in the pressurized por2on of the aircraV •  Can be refilled while hydraulic system is s2ll running •  Heat can build up pressure in the system !
Release pressure before opening Cavita2on Pressurized Reservoir •  Prevents pumps from cavita2ng or running dry in lower pressure environments •  Air-­‐pressurized Pressure provided by engine bleed air !  Air pressure regulator maintains the correct air pressure !
•  Releases extra pressure to maintain the correct pressure •  Fluid pressurized !
Gets pressure from the hydraulic system •  Always release pressure before servicing Pressurized Reservoir Filters •  Removes dirt from fluid •  Most systems have more than one filter •  Type !
Micron •  Paper filter •  Must be replaced !
Porous metal •  Cleaned and reinstalled !
Magne2c •  Cleaned and reinstalled •  Maybe part of a failure detec2on system Filters •  Filter bypass valves allows fluid to flow past clogged filter !
BeBer to have dirty fluid than no fluid •  Differen2al pressure indicator measures the pressure across the filter Low pressure, clean filter !  High pressure, dirty or clogged filter !
Filters Filters Hydraulic Hand Pump Hydraulic Pump •  Hydraulic pumps pressurize the fluid •  Power sources: Hand !  Engine driven !
•  Could include the APU Electric motor !  Bleed air !  Air – Ram Air Turbine (RAT) !
•  Shear pins, couplings or shaVs are used to isolate failed pump from drive motor/engine Shear ShaV Hydraulic Pump Classifica2ons •  Posi2ve Displacement – internally sealed so that it is always producing pressure Used for all hydraulic applica2ons !  Doesn’t like air in fluid !
•  Air will cause chaBering, pulsing, low pressure •  Non-­‐Posi2ve Displacement – lack of internal seals means volume and pressure will change based on input and output pressures Centrifugal and propeller pumps !  Commonly used for fuel boost pumps !
•  Can separate air from fluid Hydraulic Pump Classifica2ons •  Constant Displacement – force a fixed amount of fluid through the outlet port for each revolu2on of the pump !
Requires either an internal or external pressure regulator/relief valve •  “Dumps” extra fluid to: !
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Pump inlet Back to reservoir •  The volume of fluid pumped is the same, the pressure is reduced because some of the fluid is “dumped” Pressure Regulator (Relief) Valve Hydraulic Pump Classifica2ons •  Constant Displacement Type: Gear type !  Gerotor !  Vane !  Piston !
•  Some2me a variable displacement Gear Type Pump Gear Type Pump Gerotor Pump Vain Pump Inline Piston Pump Bent Piston Pump Hydraulic Pump Classifica2ons •  Variable Displacement Type – varies the amount of fluid output to meet pressure demand !
Automa2cally controls the pressure with built-­‐in regulator Variable Displacement Pump Variable Displacement Pump Valves •  Control the direc2on and/or speed of the fluid •  Selector valve type: Open-­‐center – allows con2nues flow of fluid !  Close-­‐center – blocks flow when in Neutral or Off posi2on !
•  Most common valve is a four-­‐port, closed-­‐center valve !
Servo control – electric solenoids are used to control valve posi2on Open-­‐Center Valve Open-­‐Center Valve Close-­‐Center Valve Power Assist Valves •  Check Valve – allows flow in only one direc2on Valves •  Orifice type check valve – Unrestricted flow in one direc2on, restricted/limited flow in the other direc2on !
Example: landing gear •  Unrestricted flow used to liV gear •  Restricted flow used to lower gear !
Gravity is helping •  Sequence valve – controls a sequence of events Example: landing gear – open doors, then lower gear !  Sequence can be controlled mechanically or electrically !
Orifice Type Check Valve Valves •  Priority valve – gives priority to cri2cal subsystems over non-­‐cri2cal •  Quick disconnect valves (coupling) – prevents loss of fluid when hydraulic lines are disconnected •  Hydraulic fuse – shuts down fluid flow if flow is too high !
Protects system from burst downstream Valves •  Cross flow valve – bypasses fluid from one side of an actuator to the other side !
Example: Landing gear actuator – move fluid from the liVing side to lowering side to smooth out lowering Quick Disconnect Valve Pressure Control Valves •  Relief valve – releases excess pressure System relief valve – monitors the whole system !  Thermal relief valve – releases excess pressure caused by thermal expansion (too much heat) !
•  Has the highest release pressure !
Individual component relief valve – protect component from overstresses •  Flap overload valve – relieves the pressure on the flaps if the pilot tries to lower flaps at too high a speed Pressure Control Valves •  Pressure regulator – type of relief valve that is used to regulate pressure Releases excess pressure to maintain a regulated pressure !  Used with constant displacement pumps !
•  Returns excess fluid to the pump’s inlet or the system reservoir !
When adjus2ng the pressure, adjust the valve with the highest sepng first Pressure Regulator (Relief) Valve Pressure Control Valves •  Pressure reducer – reduces fluid pressure for systems that need lower pressure •  ShuBle valve – switches between normal and emergency pressure •  Shutoff valve – used to isolate high priority systems !
Shuts off low priority systems •  Unloading valves – unloads pump during engine startup Accumulator •  Stores hydraulic energy !
Hydraulic fluid is not compressible •  Can’t store energy !
Accumulator uses the hydraulic pressure to compress air (nitrogen) •  Acts like a spring •  The compressed air can absorb/store energy •  Releases stored energy when needed •  Func2ons: !
Accumulator Dampen pressure surges •  Reduces hydraulic “hammering” !
Aids/supplements pump when several units are opera2on at once •  Reduces the cycling of constant-­‐pressure hydraulic systems !
Supplies power when pump not opera2ng •  If no pressure aVer the pump stops, the accumulator in not opera2ng correctly !
Compensates for small leaks Spherical Accumulator Cylindrical (Piston) Accumulator •  Type: !
Accumulator Spherical •  Diaphragm between air and fluid !
Cylindrical •  Piston between air and fluid •  When hydraulic system is running, the air pressure will equal the hydraulic pressure •  When servicing, check air charge !
Check pressure before opera2on •  Air charge checked with no hydraulic pressure !
Release pressure when opening or removing •  If fluid found in air side, diaphragm is leaking Heat Exchanger Actuators •  Convert hydraulic pressure into mo2on or ac2on •  Linear -­‐ back and forth !
Single or double ac2on •  One side of a double ac2on is oVen stronger than the other !
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Piston rod reduces the piston area of the weaker side Rotary – creates circular/angular mo2on •  Example: rack and pinion Linear Actuators Hydraulic Rack and Pinion Hydraulic Units •  Hydraulic motor – converts hydraulic pressure to rotary mo2on Easy to adjust speed !  Smooth !  Cushions shock !  No fire hazard if the motor is stalled !
•  A stalled electric motor can overheat and short out !
Hydraulic Motor-­‐Driven Generator (HMDG) •  Also called Constant-­‐Speed Drive (CSD) •  Uses hydraulic pressure to turn 400 Hz generator at a constant RPM by varying the hydraulic pressure Hydraulic Units •  Ram Air Turbine (RAT) – provides electrical and hydraulic power when engine(s) fails Gaskets/Seals Keep hydraulic fluid in and dirt out Packing – seals moving parts Gasket – seals sta2c flat surfaces V-­‐ring (Chevron) & U-­‐ring – open end faces the pressure •  O-­‐ring – circular in shape •
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Most common type •  Backup ring – Teflon rings used to backup O-­‐
rings in high pressure systems !
On the low pressure side Gaskets/Seals •  Wiper – used to clean and lubricate the exposed por2on of a piston shaV •  Use exact part-­‐number when replacing !
Defines material and size •  Material: MIL-­‐H-­‐5606 – neoprene or Buna-­‐N (synthe2c rubber) !  Skydrol – butyl rubber, ethylene-­‐propylene, nylon and natural fibers !
•  When installing, coat with clean hydraulic fluid and don’t scratch or twist !
Keep mold line constant Packings V-­‐Ring/Chevron O-­‐Ring Packing Backup Ring Installa2on of Teflon Backup Ring O-­‐Ring Color Coding O-­‐Ring Installa2on O-­‐Ring Installa2on O-­‐Ring Installa2on No Air in Hydraulic Systems •  Air is compressible and cause problems with hydraulics Spongy or slow response !  Cavita2on of pumps !  ChaBering, pulsing and low pressure !
•  “Bleed” all air out of system Cycle systems to force air into reservoir !  Closed systems like brakes need to have fluid flushed through them !
Pneuma2c Systems •  Compress air or nitrogen systems •  Power provided by pressure boBles, pumps, compressors or bleed air •  Pressure regulators maintain correct system pressure •  Use relief valves to protect against damage from over pressure •  Uses check valve just like hydraulic systems •  No return lines needed !
Air is just “dumped” overboard Pneuma2c Systems •  High pressure systems 1000 to 3000 PSI !  Older type system !  Engine driven compressors need a moisture separator !
•  At shutdown, a solenoid operated dump valve releases the pressure and dumps the moisture (water) •  Emergency systems – high pressure “boBles” are used to operate things like landing gears if hydraulics fail !
High pressure “boBle” is filled on the ground Pneuma2c Systems •  Medium pressure systems Usually driven by bleed air !  Start engines, pressuriza2on systems, deicing !
•  Can drive hydraulic pumps •  Low pressure systems Engine driven or electric pumps !  Deicing boots !  Naviga2on equipment !
High Pressure Pneuma2c Systems Low Pressure Pneuma2c Systems FAA Ques2on •  How would the air pressure charge in the accumulator be determined if the engine is inopera2ve, but the system s2ll has hydraulic pressure? a. Read it directly from the main system pressure gauge with all actuators inopera2ve. !  b. Build up system pressure with the emergency pump and then read the pressure on a gauge aBached to the air side of the accumulator. !  c. Operate a hydraulic unit slowly and note the pressure at which a rapid pressure drop begins as it goes toward zero. !  Answer: c !