A hydraulic fluid or hydraulic liquid is the medium by which power is transferred in

hydraulic machinery Hydraulic machines use liquid fluid power to perform work. Heavy construction vehicles are a common example. In this type of machine, hydraulic fluid is pumped to various hydraulic motors and hydraulic cylinders throughout the machine a ...

. Common hydraulic fluids are based on

mineral oil Mineral oil is any of various colorless, odorless, light mixtures of higher alkanes from a mineral source, particularly a distillate of petroleum, as distinct from usually edible vegetable oils. The name 'mineral oil' by itself is imprecise, ...

or water. Examples of equipment that might use hydraulic fluids are

excavator Excavators are heavy equipment (construction), heavy construction equipment primarily consisting of a backhoe, boom, dipper (or stick), Bucket (machine part), bucket, and cab on a rotating platform known as the "house". The modern excavator's ...

s and

backhoe A backhoe is a type of excavating equipment, or excavator, consisting of a digging bucket on the end of a two-part articulated arm. It is typically mounted on the back of a tractor or loader (equipment), front loader, the latter forming a "backh ...

hydraulic brake A hydraulic brake is an arrangement of braking mechanism which uses brake fluid, typically containing glycol ethers or diethylene glycol, to transfer pressure from the controlling mechanism to the braking mechanism. History During 1904, Frederic ...

power steering Power steering is a system for reducing a driver's effort to turn a steering wheel of a motor vehicle, by using a power source to assist steering. Hydraulic or electric actuators add controlled energy to the steering mechanism, so the driver can ...

systems,

automatic transmissions An automatic transmission (AT) or automatic gearbox is a multi-speed transmission used in motor vehicles that does not require any input from the driver to change forward gears under normal driving conditions. The 1904 Sturtevant "horseless ca ...

garbage truck A garbage truck is a truck specially designed to collect municipal solid waste and transport it to a list of solid waste treatment technologies, solid waste treatment facility, such as a landfill, materials recovery facility, recycling center ...

aircraft flight control system A conventional Fixed-wing aircraft, fixed-wing aircraft flight control system (AFCS) consists of flight control surfaces, the respective cockpit controls, connecting linkages, and the necessary operating mechanisms to control an aircraft's di ...

s, lifts, and

industrial machinery The following Outline (list), outline is provided as an overview of and topical guide to industrial machinery: Essence of industrial machinery * Heavy equipment * Household hardware, Hardware * Industrial process * Machine * Machine tool * Too ...

. Hydraulic systems like the ones mentioned above will work most efficiently if the hydraulic fluid used has zero

compressibility In thermodynamics and fluid mechanics, the compressibility (also known as the coefficient of compressibility or, if the temperature is held constant, the isothermal compressibility) is a measure of the instantaneous relative volume change of a f ...

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:

Composition

Base stock

The original hydraulics fluid, dating back to the time of

ancient Egypt Ancient Egypt () was a cradle of civilization concentrated along the lower reaches of the Nile River in Northeast Africa. It emerged from prehistoric Egypt around 3150BC (according to conventional Egyptian chronology), when Upper and Lower E ...

, was

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

. Beginning in the 1920s,

began to be used more than water as a base stock due to its inherent

lubrication Lubrication is the process or technique of using a lubricant to reduce friction and wear and tear in a contact between two surfaces. The study of lubrication is a discipline in the field of tribology. Lubrication mechanisms such as fluid-lubr ...

properties and ability to be used at temperatures above the

boiling point The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding envi ...

of water. Today most hydraulic fluids are based on mineral oil base stocks. Natural oils such as

rapeseed Rapeseed (''Brassica napus'' subsp. ''napus''), also known as rape and oilseed rape and canola, is a bright-yellow flowering member of the family Brassicaceae (mustard or cabbage family), cultivated mainly for its oil-rich seed, which naturall ...

are used as base stocks for fluids where

biodegradability Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegrada ...

and

renewable source Renewable energy (also called green energy) is energy made from renewable resource, renewable natural resources that are replenished on a human lifetime, human timescale. The most widely used renewable energy types are solar energy, wind pow ...

s are considered important. Other base stocks are used for specialty applications, such as for fire resistance and extreme temperature applications. Some examples include:

glycol ether Glycol ethers are a class of chemical compounds consisting of alkyl ethers that are based on glycols such as ethylene glycol or propylene glycol. They are commonly used as solvents in paints and cleaners. They have good solvent properties while ha ...

organophosphate ester In organic chemistry, organophosphates (also known as phosphate esters, or OPEs) are a class of organophosphorus compounds with the general structure , a central phosphate molecule with alkyl or aromatic substituents. They can be considered ...

polyalphaolefin A polyolefin is a type of polymer with the general formula (CH2CHR)n where R is an alkyl group. They are usually derived from a small set of simple olefins ( alkenes). Dominant in a commercial sense are polyethylene and polypropylene. More special ...

propylene glycol Propylene glycol ( IUPAC name: propane-1,2-diol) is a viscous, colorless liquid. It is almost odorless and has a faintly sweet taste. Its chemical formula is CH3CH(OH)CH2OH. As it contains two alcohol groups, it is classified as a diol. An al ...

, and

silicone oil A silicone oil is any liquid polymerized siloxane with organic side chains. The most important member is polydimethylsiloxane. These polymers are of commercial interest because of their relatively high thermal stability and their lubricating prop ...

s. NaK-77, a eutectic alloy of

sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...

and

potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...

, can be used as a hydraulic fluid in high-temperature and high-radiation environments, for temperature ranges of . Its

bulk modulus The bulk modulus (K or B or k) of a substance is a measure of the resistance of a substance to bulk compression. It is defined as the ratio of the infinitesimal pressure increase to the resulting ''relative'' decrease of the volume. Other mo ...

at is 310,000

psi Psi, PSI or Ψ may refer to: Alphabetic letters * Psi (Greek) (Ψ or ψ), the twenty-third letter of the Greek alphabet * Psi (Cyrillic), letter of the early Cyrillic alphabet, adopted from Greek Arts and entertainment * "Psi" as an abbreviat ...

(2.14 GPa), higher than of a hydraulic oil at room temperature. Its

lubricity Lubricity is the measure of the reduction in friction and/or wear by a lubricant. The study of lubrication and wear mechanisms is called tribology. Measurement of lubricity The lubricity of a substance is not a material property, and cannot be m ...

is poor, so positive-displacement

pump A pump is a device that moves fluids (liquids or gases), or sometimes Slurry, slurries, by mechanical action, typically converted from electrical energy into hydraulic or pneumatic energy. Mechanical pumps serve in a wide range of application ...

s are unsuitable and centrifugal pumps have to be used. The addition of

caesium Caesium (IUPAC spelling; also spelled cesium in American English) is a chemical element; it has Symbol (chemistry), symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only f ...

shifts the useful temperature range to . The NaK-77 alloy was tested in hydraulic and fluidic systems for the

Supersonic Low Altitude Missile The Supersonic Low Altitude Missile or SLAM was a U.S. Air Force nuclear weapons project conceived around 1955, and cancelled in 1964. SLAMs were conceived of as unmanned nuclear-powered ramjets capable of delivering thermonuclear warheads deep ...

Other components

Hydraulic fluids can contain a wide range of chemical compounds, including:

oil An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) and lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturate ...

butanol Butanol (also called butyl alcohol) is a four-carbon alcohol with a formula of C4 H9 OH, which occurs in five isomeric structures (four structural isomers), from a straight-chain primary alcohol to a branched-chain tertiary alcohol; all are a bu ...

, esters (e.g.

phthalate Phthalates ( ), or phthalate esters, are esters of phthalic acid. They are mainly used as plasticizers, i.e., substances added to plastics to increase their flexibility, transparency, durability, and longevity. They are used primarily to soften ...

s, like DEHP, and

adipates Adipic acid or hexanedioic acid is the organic compound with the chemical formula, formula C6H10O4. It a white crystalline powder at standard temperature and pressure. From an industrial perspective, it is the most important dicarboxylic acid at ...

, like

bis(2-ethylhexyl) adipate Bis(2-ethylhexyl) adipate or DEHA or DOA is an organic compound with the formula (CH2CH2CO2C8H17)2. It is the diester of 2-ethylhexanol and adipic acid. It is a colorless oily liquid. DEHA is sometimes called " dioctyl adipate", incorrectly. ...

polyalkylene glycol Synthetic oil is a lubricant consisting of chemical compounds that are artificially modified or synthesised. Synthetic oil is used as a substitute for petroleum-refined oils when operating in extreme temperature, in metal stamping to provide en ...

s (PAG),

organophosphate In organic chemistry, organophosphates (also known as phosphate esters, or OPEs) are a class of organophosphorus compounds with the general structure , a central phosphate molecule with alkyl or aromatic substituents. They can be considered ...

(e.g.

tributylphosphate Tributyl phosphate, known commonly as TBP, is an organophosphorus compound with the chemical formula (CH3CH2CH2CH2O)3PO. This colourless, odorless liquid finds some applications as an extractant and a plasticizer. It is an ester of phosphoric a ...

), silicones, alkylated aromatic hydrocarbons, polyalphaolefins (PAO) (e.g.

polyisobutene Polyisobutene (polyisobutylene) is a class of organic polymers prepared by polymerization of isobutene. The polymers often have the formula Me3C H2CMe2sub>nH (Me = CH3). They are typically colorless gummy solids. Cationic polymerization, initi ...

s),

corrosion inhibitors A corrosion inhibitor or anti-corrosive is a chemical compound added to a liquid or gas to decrease the corrosion rate of a metal that comes into contact with the fluid. The effectiveness of a corrosion inhibitor depends on fluid composition and ...

(incl acid scavengers), anti-

erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...

additives, etc.

Biodegradable hydraulic fluids

Environmentally sensitive applications (e.g. farm tractors and marine

dredging Dredging is the excavation of material from a water environment. Possible reasons for dredging include improving existing water features; reshaping land and water features to alter drainage, navigability, and commercial use; constructing d ...

) may benefit from using biodegradable hydraulic fluids based upon

vegetable oil Vegetable oils, or vegetable fats, are oils extracted from seeds or from other parts of edible plants. Like animal fats, vegetable fats are ''mixtures'' of triglycerides. Soybean oil, grape seed oil, and cocoa butter are examples of seed ...

when there is the risk of an

oil spill An oil spill is the release of a liquid petroleum hydrocarbon into the environment, especially the marine ecosystem, due to human activity, and is a form of pollution. The term is usually given to marine oil spills, where oil is released into th ...

from a ruptured oil line. Typically these oils are available as

ISO The International Organization for Standardization (ISO ; ; ) is an independent, non-governmental, international standard development organization composed of representatives from the national standards organizations of member countries. Me ...

32, ISO 46, and ISO 68 specification oils.

ASTM ASTM International, formerly known as American Society for Testing and Materials, is a standards organization that develops and publishes voluntary consensus technical international standards for a wide range of materials, products, systems and s ...

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.

Anti-wear hydraulic fluids

Anti-wear (AW) hydraulic oils are made from a petroleum base fluid and commonly contain the anti-wear additive Zinc dialkyldithiophosphate (ZDDP). This additive works to protect the hydraulic pump. They come in multiple viscosity grades that have varying applications. For example, AW 46 hydraulic oils can be used to operate the hydraulic systems in off-road equipment such as dump trucks, excavators, and backhoes, while AW 32 hydraulic oils may be more suitable for colder weather applications like in a snow plow's pump.

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. Water-glycol and polyol-ester are some of these specialized fluids that contain excellent thermal and hydrolitic properties, which aid in fire resistance.

Uses

Brake fluid

Brake fluid Brake fluid is a type of hydraulic fluid used in hydraulic brake and hydraulic clutch applications in automobiles, motorcycles, light trucks, and some bicycles. It is used to transfer force into pressure, and to amplify braking force. It wo ...

is a subtype of hydraulic fluid with high

, both when new (specified by the equilibrium boiling point) and after absorption of

water vapor Water vapor, water vapour, or aqueous vapor is the gaseous phase of Properties of water, water. It is one Phase (matter), state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from th ...

(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 In physics, a vapor (American English) or vapour (Commonwealth English; American and British English spelling differences#-our, -or, see spelling differences) is a substance in the gas phase at a temperature lower than its critical temperature,R ...

, resulting in brake failure. Glycol-ether based fluids are

hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption (chemistry), absorption or adsorption from the surrounding Natural environment, environment, which is usually at normal or room temperature. If water mol ...

, and absorbed moisture will greatly reduce the boiling point over time. Mineral oil and silicone based fluids are not hygroscopic.

Power steering fluid

Power steering Power steering is a system for reducing a driver's effort to turn a steering wheel of a motor vehicle, by using a power source to assist steering. Hydraulic or electric actuators add controlled energy to the steering mechanism, so the driver can ...

fluid is a sub type of hydraulic fluid. Most are mineral oil or silicone based fluids, while some use

automatic transmission fluid Automatic transmission fluid (ATF) is a hydraulic fluid that is essential for the proper functioning of vehicles equipped with Automatic transmission, automatic transmissions. Usually, it is coloured red or green to differentiate it from motor ...

, made from synthetic base oil. Automatic transmissions use fluids for their lubrication, cooling and hydraulic properties for

viscous coupling A viscous coupling is a mechanical device which transfers torque and rotation by the medium of a viscous fluid. Design Rotary viscous couplings with interleaved, perforated plates and filled with viscous fluids are used in automotive syste ...

s. Use of the wrong type of fluid can lead to failure of the power steering pump.

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. 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). Hydraulic power is used for other purposes. It can be stored in accumulators to start an

auxiliary power unit An auxiliary power unit (APU) is a device on a vehicle that provides energy for functions other than propulsion. They are commonly found on large aircraft and naval ships as well as some large land vehicles. Aircraft APUs generally produce 115&n ...

(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 Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...

s. 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 In electricity generation, a generator, also called an ''electric generator'', ''electrical generator'', and ''electromagnetic generator'' is an electromechanical device that converts mechanical energy to electrical energy for use in an externa ...

called a '' Ram-Air Turbine'' (RAT) which is concealed under the fuselage. This provides

electrical power Electric power is the rate of transfer of electrical energy within a electric circuit, circuit. Its SI unit is the watt, the general unit of power (physics), power, defined as one joule per second. Standard prefixes apply to watts as with oth ...

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.

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 Aircraft maintenance checks are periodic inspections that have to be done on all commercial and civil aircraft after a certain amount of time or usage. Military aircraft normally follow specific maintenance programmes which may, or may not, be si ...

(primarily C and D checks) to check contamination.
Military Spec 1246C
is one fluid contamination specification. The ISO fluid contamination scale assigns a contamination category based on particle size count and distribution.

Other uses

The properties of HLP 32 hydraulic oil make it ideal for lubricating machine tools.

Specifications

According to DIN 51502

Source: * HL: With agents to enhance corrosion protection and age resistance. Used in hydraulic systems that do not pose any requirements as to wear protection. * HLP: With agents to enhance corrosion protection and age resistance. Suitable for most fields of application and components. * HLPD: As HLP but also used in systems where solid or liquid contamination need to be kept temporarily suspended. * HVLP: With agents to enhance corrosion protection, age resistance, to reduce scuffing wear in mixed friction areas, and to improve the viscosity-temperature behavior. Used in system operated over a wide temperature range. * HVLPD: As HVLP but also used in systems where solid or liquid contamination need to be kept temporarily suspended.

According to MIL for military applications

Source: * Mil-PRF-5606 (originally Mil-H-5606): Mineral base, flammable, fairly low flashpoint, usable from , red color, developed in the 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 aiming devices of tactical and support ordnance equipment, except combat armored vehicles/equipment which require FRH. The hydraulic fluid is also used as a preservative fluid for aircraft hydraulic systems and components where MIL-H-5606 (OHA) or MIL-PRF-87257 is used as an 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 back of mineral based hydraulic fluids. * Mil-H-83282: Synthetic hydrocarbon base, higher flashpoint, self-extinguishing, backward compatible to -5606, red color, rated to degrees. * Mil-H-87257: A development of -83282 fluid to improve its low temperature viscosity.

Multi purpose oil UTTO/STOU

Source: * UTTO: Is an abbreviation for "Universal Tractor Transmission Oil". It is a hydraulic oil that can also be used in the transmissions and wet brakes of machines like tractors. * STOU: Is an abbreviation for "Super Tractor Oil Universal". It is a hydraulic oil that can also be used in the motor, the transmissions and wet brakes. So its main difference to the UTTO oil is that it can be used also as an motor oil.

Phosphate-ester base

* 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 Industrie - NSA307110 * British Aerospace - BAC M.333.B * Bombardier - BAMS 564–003 * SAE - Ac974 * SAE - AS1241 *

Skydrol Skydrol is a brand name of fire-resistant hydraulic fluid used in aviation and aerospace applications. It is a phosphate ester-based fluid that is known for its excellent fire resistance and ability to withstand extreme temperature and pressure con ...

Viscosity

Source: Commonly used hydraulic oil viscosities fall under the ISO VG (Viscosity Grade) classification system, which is based on the oil's kinematic viscosity at . The most prevalent grades for general industrial and mobile hydraulic systems are typically: * ISO VG 32 * ISO VG 46 * ISO VG 68 Additional viscosities such as the following, are also used, but less frequently or for specific low/high-temperature applications. * ISO VG 15 * ISO VG 22 * ISO VG 100

References

External links

Information and purchase of military specification (mil-spec) hydraulic fluid

Information about Fluid Power is also available on the National Fluid Power Association web-site nfpa.com
* USDA Research

Aviation hydraulic fluids
{{Navboxes , title = Aircraft-related templates , list = {{Aircraft components {{Aircraft piston engine components {{Aircraft gas turbine engine components Fluid dynamics Gear oils Hydraulics