Hydraulic Fluid
Functions and properties
Functions and properties
The primary function of a hydraulic fluid is to convey power. In use, however, there are other important functions of hydraulic fluid such as protection of the hydraulic machine components. The table below lists the major functions of a hydraulic fluid and the properties of a fluid that affect its ability to perform that function:
Function
Property
Medium for power transfer and control
Low compressibility (high bulk modulus)
Fast air release
Low foaming tendency
Low volatility
Medium for heat transfer
Good thermal capacity and conductivity
Sealing Medium
Adequate viscosity and viscosity index
Shear stability
Lubricant
Viscosity for film maintenance
Low temperature fluidity
Thermal and oxidative stability
Hydrolytic stability / water tolerance
Cleanliness and filterability
Demulsibility
Antiwear characteristics
Corrosion control
Pump efficiency
Proper viscosity to minimize internal leakage
High viscosity index
Special function
Fire resistance
Friction modifications
Radiation resistance
Environmental impact
Low toxicity when new or decomposed
Biodegradability
Functioning life
Material compatibility
Composition
Base stock
The original hydraulic fluid, dating back to the time of ancient Egypt, was water. Beginning in the 1920s, mineral oil began to be used more than water as a base stock due to its inherent lubrication properties and ability to be used at temperatures above the boiling point of water. Today most hydraulic fluids are based on mineral oil base stocks.
Natural oils such as rapeseed (also called canola oil) are used as base stocks for fluids where biodegradability and renewable sources are considered important.
Other base stocks are used for specialty applications, such as for fire resistance and extreme temperature applications. Some examples include: glycol, esters, organophosphate ester, polyalphaolefin, propylene glycol, and silicone oils.
Other components
Hydraulic fluids can contain a wide range of chemical compounds, including: oils, butanol, esters (e.g. phthalates, like DEHP, and adipates, like bis(2-ethylhexyl) adipate), polyalkylene glycols (PAG), phosphate esters (e.g. tributylphosphate), silicones, alkylated aromatic hydrocarbons, polyalphaolefins (PAO) (e.g. polyisobutenes), corrosion inhibitors, etc.
Biodegradable hydraulic fluids
Environmentally sensitive applications (e.g. farm tractors and marine dredging) may benefit from using biodegradable hydraulic fluids based upon rapeseed (Canola) vegetable oil when there is the risk of an oil spill from a ruptured oil line. Typically these oils are available as ISO 32, ISO 46, and ISO 68 specification oils. ASTM standards ASTM-D-6006, Guide for Assessing Biodegradability of Hydraulic Fluids and ASTM-D-6046, Standard Classification of Hydraulic Fluids for Environmental Impact are relevant.
Brake fluid
Brake fluid is a subtype of hydraulic fluid with high boiling point, both when new (specified by the equilibrium boiling point) and after absorption of water vapor (specified by wet boiling point). Under the heat of braking, both free water and water vapor in a braking system can boil into a compressible vapor, resulting in brake failure. Glycol-ether based fluids are hygroscopic, and absorbed moisture will greatly reduce the boiling point over time. Silicone based fluids are not hygroscopic, but their inferior lubrication is not suitable for all braking systems.
Safety
Because industrial hydraulic systems operate at hundreds to thousands of PSI and temperatures reaching hundreds of degrees Celsius, severe injuries and death can result from component failures and care must always be taken when performing maintenance on hydraulic systems.
Fire resistance is a property available with specialized fluids.
Trade names
Some of the trade names for hydraulic fluids include Tellus, Durad, Fyrquel, Houghto-Safe, Hydraunycoil, Lubritherm Enviro-Safe, Pydraul, Quintolubric, Reofos, Reolube, and Skydrol.
Aircraft hydraulic systems
The use of hydraulic systems in aircraft almost certainly began with braking systems.[citation needed] As aircraft performance increased in mid-20th century, the amount of force required to operate mechanical flight controls became excessive, and hydraulic systems were introduced to reduce pilot effort. The hydraulic actuators are controlled by valves; these in turn are operated directly by input from the aircrew (hydro-mechanical) or by computers obeying control laws (fly by wire). See flight controls.
Hydraulic power is used for other purposes. It can be stored in accumulators to start an auxiliary power unit (APU) for self-starting the aircraft's main engines. Many aircraft equipped with the M61 family of cannon use hydraulic power to drive the gun system, permitting reliable high rates of fire.
The hydraulic power itself comes from pumps driven by the engines directly, or by electrically driven pumps. In modern commercial aircraft these are electrically driven pumps; should all the engines fail in flight the pilot will deploy a propeller-driven electric generator which is concealed under the fuselage. This provides electrical power for the hydraulic pumps and control systems as power is no longer available from the engines. In that system and others electric pumps can provide both redundancy and the means of operating hydraulic systems without the engines operating, which can be very useful during maintenance.
Specifications
Aircraft hydraulic fluids fall under various specifications:
Common petroleum-based:
Mil-H-5606: Mineral base, flammable, fairly low flashpoint, usable from 65F (54C) to 275F (135C), red color
Mil-H-83282: Synthetic hydrocarbon base, higher flashpoint, self-extinguishing, backward compatible to -5606, red color, rated to 40F (40C) degrees.
Mil-H-87257: A development of -83282 fluid to improve its low temperature viscosity.
Phosphate-ester based:
BMS 3-11: Skydrol 500B-4, Skydrol LD-4, Skydrol 5 and Exxon HyJetIV-A plus Typically light purple, not compatible with petroleum-based fluids, will not support combustion.
Contamination
Special, stringent care is required when handling aircraft hydraulic fluid as it is critical to flight safety that it stay free from contamination. It is also necessary to strictly adhere to authorized references when servicing or repairing any aircraft system. Samples from aircraft hydraulic systems are taken during heavy aircraft maintenance checks to check contamination.
See also
Dexron
Hydraulic brake
Osmosis
Skydrol
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Hydraulics
Concepts
Hydraulics Hydraulic fluid Fluid power Hydraulic engineering
Technologies
Machinery Accumulator Brake Circuit Cylinder Drive system Manifold Motor Power network Press Pump Ram Rescue tools
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Aircraft components and systems
Airframe structure
Cabane strut Canopy Cruciform tail Empennage Fairing Fabric covering Flying wires Former Fuselage Interplane strut Horizontal stabilizer Jury strut Leading edge Longeron Nacelle Rear pressure bulkhead Rib Spar Stabilizer Stressed skin Strut Tailplane Trailing edge T-tail Twin tail Vertical stabilizer V-tail Wing root Wing tip
Flight controls
Aileron Airbrake Artificial feel Autopilot Canard Centre stick Deceleron Elevator Elevon Electro-hydrostatic actuator Flaperon Flight control modes Gust lock Rudder Servo tab Side-stick Spoiler Spoileron Stabilator Stick pusher Stick shaker Trim tab Yaw damper Wing warping Yoke
High-lift and aerodynamic
devices
Blown flap Dog-tooth Flap Gouge flap Gurney flap Krueger flaps Leading edge cuff LEX Slats Slot Stall strips Strake Vortex generator Wing fence Winglet
Avionic and flight
instrument systems
ACAS Air data computer Airspeed indicator Altimeter Annunciator panel Attitude indicator Compass Course Deviation Indicator EFIS EICAS Flight data recorder Flight management system Glass cockpit GPS Heading indicator Horizontal situation indicator INAS TCAS Transponder Turn and bank indicator Pitot-static system Radar altimeter Vertical Speed Indicator Yaw string
Propulsion controls, devices and
fuel systems
Autothrottle Drop tank FADEC Fuel tank Gascolator Inlet cone Intake ramp NACA cowling Self-sealing fuel tank Throttle Thrust lever Thrust reversal Townend ring Wet wing
Landing and arresting gear
Autobrake Conventional landing gear Arrestor hook Drogue parachute Landing gear extender Tricycle gear Tundra tire Undercarriage
Escape systems
Ejection seat Escape crew capsule
Other systems
Aircraft lavatory Auxiliary power unit Bleed air system Deicing boot Emergency oxygen system Environmental Control System Hydraulic system Ice protection system Landing lights Navigation light Passenger service unit Ram air turbine
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Aircraft piston engine components, systems and terminology
Piston engines
Mechanical components
Camshaft Connecting rod Crankpin Crankshaft Cylinder Cylinder head Gudgeon pin Hydraulic tappet Main bearing Obturator ring Oil pump Piston Piston ring Poppet valve Pushrod Rocker arm Sleeve valve Tappet
Electrical components
Alternator Capacitor discharge ignition Generator Electronic fuel injection Ignition system Magneto Spark plug Starter motor
Terminology
Air-cooled Bore Compression ratio Dead centre Engine displacement Four-stroke engine Horsepower Ignition timing Manifold pressure Mean effective pressure Naturally-aspirated Monosoupape Overhead camshaft Overhead valve Shock-cooling Stroke Time between overhaul Two-stroke engine Valve timing Volumetric efficiency
Propellers
Components
Propeller speed reduction unit Propeller governor
Terminology
Autofeather Blade pitch Contra-rotating Constant speed Counter-rotating Scimitar propeller Single-blade propeller Variable pitch
Engine instruments
Tachometer Hobbs meter Annunciator panel EFIS EICAS Flight data recorder Glass cockpit
Engine controls
Carburetor heat Throttle
Fuel and induction system
Avgas Carburetor Fuel injection Gascolator Inlet manifold Intercooler Pressure carburetor Supercharger Turbocharger
Other systems
Auxiliary power unit Coffman starter Hydraulic system Ice protection system Recoil start
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Aircraft gas turbine engine components, systems and terminology
Gas turbines
Mechanical components
Axial compressor Centrifugal compressor Combustor Constant Speed Drive Propelling nozzle
Terminology
Afterburner (reheat) Bypass ratio Compressor stall Engine Pressure Ratio (EPR) Flameout Turbofan Turbojet Turboprop Turboshaft Windmill restart
Propellers
Components
Propeller speed reduction unit Propeller governor
Terminology
Autofeather Blade pitch Contra-rotating Constant speed Counter-rotating Proprotor Scimitar propeller Variable pitch
Engine instruments
Annunciator panel ECAM EFIS EICAS Flight data recorder Glass cockpit
Engine controls
Autothrottle FADEC Thrust lever Thrust reversal
Fuel and induction system
Jet fuel
Other systems
Air start system Auxiliary power unit Bleed air system Hydraulic system Ice protection system
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Lists relating to aviation
General
Timeline of aviation Aircraft (manufacturers) Aircraft engines (manufacturers) Rotorcraft (manufacturers) Airports Airlines (defunct) Civil authorities Museums
Military
Air forces Aircraft weapons Missiles Unmanned aerial vehicles (UAVs) Experimental aircraft
Accidents/incidents
General Military Commercial (airliners) Deaths
Records
Airspeed Distance Altitude Endurance Most-produced aircraft
References
^ Givens W. and Michael P., Fuels and Lubricants Handbook, G. Totten ed., ASTM International, 2003, p. 373 ISBN 0-8031-2096-6
^ Placek, D., Synthetics, Mineral Oils and Bio-based Lubricants, L. Rudnick ed., CRC Press, 2006, p. 519 ISBN 1-57444-723-8
^ Bosch Automotive Handbook, 4th Edition, Robert Bosch GmbH, 1996, pp. 241 - 243. ISBN 0-8376-0333-1
^ Discovery channel-'seconds from disaster'
External links
Information about Fluid Power is also available on the National Fluid Power Association web-site nfpa.com
USDA Research. Biodegradable Plant-Based Hydraulic Fluid
Industrial Hydraulic Oils
Aviation hydraulic fluids
Categories: Fluid dynamics | Gear oilsHidden categories: All articles with unsourced statements | Articles with unsourced statements from February 2008
by: gaga
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