A hydraulic fluid or hydraulic fluid is the medium by which power is transferred in hydraulic machinery . Common hydraulic fluids are based on mineral oil or water.  Examples of equipment that use hydraulic fluid are excavators and backhoes , hydraulic brakes , power steering systems, automatic transmissions , garbage trucks , aircraft flight control systems , elevators and industrial machinery .
The hydraulic systems mentioned above will operate most efficiently if the hydraulic fluid used has zero compressibility .
Functions and Properties
The primary function of hydraulic fluid is to transmit power. In use, however, hydraulic fluids have other important functions such as protecting hydraulic machine components. The table below lists the major functions of a hydraulic fluid and the properties of the fluid that affect its ability to perform that function:
|medium for power transfer and control||non compressible (high bulk modulus )strong wind releaselow foaming tendencylow volatility|
|medium for heat transfer||Good thermal efficiency and conductivity|
|sealing medium||Sufficient viscosity and viscosity indexshear stability|
|lube||Viscosity for film maintenancelow temperature fluidityThermal and oxidative stabilityHydrolytic Stability / Water Tolerancecleaning and filtering capacitydemonetisationAntiwear Featurescorrosion control|
|pump efficiency||Proper viscosity to reduce internal leakagehigh viscosity index|
|special function||Retardantfriction modificationradiation resistance|
|Environmental effect||Low toxicity when new or dismantledbiodegradability|
|working life||material compatibility|
The original hydraulics fluid in ancient Egyptian times was water . Beginning in the 1920s, mineral oil began to be used as a base stock rather than water, because of its inherent lubricating properties and the ability to be used at temperatures above the boiling point of water. Most hydraulic fluids today are based on a mineral oil base stock.
Natural oils such as rapeseed ( also known as canola oil) are used as a base stock for liquids where biodegradability and renewable sources are considered important.
Other base stock is used for special applications, such as for fire resistance and extreme temperature applications. Some examples include: glycol ethers , organophosphate esters , polyalphaolefins , propylene glycol and silicone oils.
Nak -77, a eutectic alloy of sodium and potassium , used as a hydraulic fluid in high temperature and high radiation environments, for temperature ranges of 10 to 1400 °F (-12 to 760 °C) can go. Its bulk modulus at 1000 °F (538 °C) is 310,000 psi (2.14 GPa), which is higher than hydraulic oil at room temperature. It has poor lubricity , so positive-displacement pumps are unsuitable and centrifugal pumps have to be used. The addition of cesium changes the useful temperature range from -95 to 1300 °F (-70 to 704 °C). supersonic low altitude missileThe NaK-77 alloy was tested in hydraulic and fluidic systems.
Hydraulic fluid can contain a wide range of chemical compounds, including: oils , butanol , esters (such as phthalates , such as DEHP , and adipates , such as bis(2-ethylhexyl) adipate), polyalkylene glycols (PAG), organophosphate (such as tributylphosphate) ), silicones, alkylated aromatic hydrocarbons, polyalphaolefins (PAOs) (such as polyisobutene), corrosion inhibitors (acid scavengers), anti-erosion additives, etc.
Biodegradable hydraulic fluid
Environmentally sensitive applications (such as farm tractors and marine dredging) may benefit from the use of biodegradable hydraulic fluids based on rapeseed (canola) vegetable oil when there is a risk of oil spills from a broken oil line. Typically these oils are available as ISO 32, ISO 46, and ISO 68 specification oils. ASTM standard ASTM-D-6006, Guide for the Assessment of Biodegradability of Hydraulic Fluids and ASTM-D-6046, Standard Classification of Hydraulic Fluids for Environmental Impact are relevant.
Anti wear hydraulic fluid
Anti-wear (AW) hydraulic oils are made from petroleum base fluids and typically contain the anti-wear additive zinc dialkyldithiophosphate (ZDDP). This additive acts to protect the hydraulic pump. They come in several viscosity grades that have varying applications. For example, AW 46 hydraulic oils can be used to operate hydraulic systems in off-road equipment such as dump trucks, excavators and backhoes, while AW 32 hydraulic oils can be used for cold weather applications such as snow plow pumps. may be more suitable. 
Because industrial hydraulic systems operate at hundreds to thousands of psi and temperatures reach hundreds of degrees Celsius, serious injuries and death can result from component failures and care should always be taken when performing maintenance on hydraulic systems.
Fire resistance is a property available with special liquids. Water-glycol and polyol-esters are some of these specialty liquids that have excellent thermal and hydrolytic properties, which aid in fire resistance.
Brake fluid is a subtype of hydraulic fluid with a high boiling point, both when new (specified by 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 the braking system can boil into a compressed vapor, resulting in brake failure.  Glycol-ether based liquids are hygroscopic, and the absorbed moisture will greatly reduce the boiling point over time. Mineral oil and silicone based liquids are not hygroscopic.
Power steering fluid
Power steering fluid is a subtype of hydraulic fluid. Most are mineral oil or silicone based fluids, while some use automatic transmission fluids made from synthetic base oils.   Automatic transmissions use fluids for their lubrication, cooling and hydraulic properties for viscous coupling.
Using the wrong type of fluid can result in power steering pump failure. 
Aircraft hydraulic systems
As aircraft performance increased in the mid-20th century, the amount of force required to operate mechanical flight controls became excessive, and hydraulic systems were introduced to reduce pilot effort. Hydraulic actuators are controlled by valves; These in turn are operated by input directly from the aircrew (hydro-mechanical) or by a computer (fly by wire) obeying the control laws.
Hydraulic power is used for other purposes. It can be stored in accumulators to start an auxiliary power unit (APU) to self-start the main engines of the aircraft. Many aircraft equipped with the M61 family of cannon use hydraulic power to drive the gun system, allowing for a reliable high rate of fire.
The hydraulic power itself comes directly from engine-powered pumps or electrically driven pumps. In modern commercial aircraft these are electrically driven pumps; If all engines fail in flight, the pilot will deploy a propeller-driven electric generator called a Ram-Air Turbine (RAT) which is hidden under the fuselage.  It provides electrical power for hydraulic pumps and control systems as power is no longer available from the engine. In that system and others, electric pumps can provide both redundancy and a means of operating hydraulic systems without engine operation, which can be very useful during maintenance.
- mil-prf-5606 (originally mil-h-5606): mineral base, flammable, fairly low flashpoint, -65 °F (-54 °C) to 275 °F (135 °C), reddish color, developed 1940s 
- MIL-PRF-6083: Usable from -54 °C to 135 °C “where corrosion protection is required and a determination has been made that MIL-PRF-46170 (FRH) hydraulic fluid cannot be used This includes use in recoil mechanisms and hydraulic systems for rotating weapons or targeting equipment of tactical and support armament equipment, excluding combat armored vehicles/equipment that require FRH. The use of hydraulic fluid in aircraft Also used as a preservative fluid for hydraulic systems and components where MIL-H-5606 (OHA) ) or MIL-PRF-87257 is used as the operational fluid.”
Synthetic Hydrocarbon Base: These synthetic fluids are compatible with mineral-base hydraulic fluids and were developed to address the low flash point draw backs of mineral-based hydraulic fluids. 
- Mil-H-83282: Synthetic hydrocarbon base, high flashpoint, self-extinguishing, backward compatible to -5606, red color, rated to −40 °F (−40 °C).
- Mil-H-87257: -83282 Development of a fluid to improve its low temperature viscosity.
- US/NATO Military Specification – MIL-H-8446
- Boeing Seattle – BMS3-11
- Boeing Long Beach – DMS2014
- Boeing Long Island – CDS5478
- Lockheed – LAC C-34-1224
- Airbus Industries – NSA307110
- British Aerospace – BAC M.333.B
- Bombardier – BAMS 564-003
- SAE – Ac974
- SAE – AS1241
Special, strict care is required when handling aircraft hydraulic fluid because it is vital to flight safety that it remain free of contamination. Strict adherence to authorized references is also required when servicing or repairing any aircraft system. Samples are taken from aircraft hydraulic systems during heavy aircraft maintenance checks (mainly C and D checks) to check for contamination.
Military Spec 1246C is a fluid contamination specification.
The ISO fluid contamination scale provides a contamination range based on particle size count and distribution.
The properties of HLP32 hydraulic oil make it ideal for lubricating machine tools.