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 ...

. 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 construction equipment consisting of a boom, dipper (or stick), bucket and cab on a rotating platform known as the "house". The house sits atop an undercarriage with tracks or wheels. They are a natural progression fr ...

s and

backhoe A backhoe—also called rear actor or back actor—is a type of excavating equipment, or digger, 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 front loader, the latt ...

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, Frederi ...

power steering A power steering is a mechanical device equipped on a motor vehicle that helps drivers steer the vehicle by reducing steering effort needed to turn the steering wheel, making it easier for the vehicle to turn or maneuver at lower speeds. Hydra ...

systems, automatic transmissions, garbage trucks,

aircraft flight control system A conventional fixed-wing aircraft flight control system consists of flight control surfaces, the respective cockpit controls, connecting linkages, and the necessary operating mechanisms to control an aircraft's direction in flight. Aircraft ...

s, lifts, and industrial machinery. 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, was

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

. 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-lubric ...

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 env ...

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, or oilseed rape, is a bright-yellow flowering member of the family Brassicaceae (mustard or cabbage family), cultivated mainly for its oil-rich seed, which naturally contains a ...

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 biodegradati ...

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 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 h ...

s, organophosphate ester,

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 speciali ...

propylene glycol Propylene glycol ( IUPAC name: propane-1,2-diol) is a viscous, colorless liquid, which is nearly odorless but possesses a faintly sweet taste. Its chemical formula is CH3CH(OH)CH2OH. Containing two alcohol groups, it is classed as a diol. It ...

, 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, lubricating, and dielect ...

NaK In data networking, telecommunications, and computer buses, an acknowledgment (ACK) is a signal that is passed between communicating processes, computers, or devices to signify acknowledgment, or receipt of message, as part of a communications ...

-77, a eutectic alloy of

sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...

and

potassium Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmos ...

, can be used as a hydraulic fluid in high-temperature and high-radiation environments, for temperature ranges of 10 to 1400 °F (-12 to 760 °C). Its

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

at 1000 °F (538 °C) is 310,000 psi (2.14 GPa), higher than of a hydraulic oil at room temperature. Its lubricity is poor, so positive-displacement

pump A pump is a device that moves fluids (liquids or gases), or sometimes slurries, by mechanical action, typically converted from electrical energy into hydraulic energy. Pumps can be classified into three major groups according to the method they ...

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

caesium Caesium (IUPAC spelling) (or cesium in American English) is a chemical element with the 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 five elemental metals that a ...

shifts the useful temperature range to -95 to 1300 °F (−70 to 704 °C). The NaK-77 alloy was tested in hydraulic and fluidic systems for the Supersonic Low Altitude Missile.

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) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...

butanol Butanol (also called butyl alcohol) is a four-carbon alcohol with a formula of C4 H9 O H, 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 Bis(2-ethylhexyl) phthalate (di-2-ethylhexyl phthalate, diethylhexyl phthalate, diisooctyl phthalate, DEHP; incorrectly — dioctyl phthalate, DIOP) is an organic compound with the formula C6H4(CO2C8H17)2. DEHP is the most common member of the cl ...

, and adipates, like

bis(2-ethylhexyl) adipate Bis(2-ethylhexyl) adipate or DEHA 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. Another ...

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

corrosion inhibitors In chemistry, a corrosion inhibitor or anti-corrosive is a chemical compound that, when added to a liquid or gas, decreases the corrosion rate of a material, typically a metal or an alloy, that comes into contact with the fluid. The effectiveness ...

(incl acid scavengers), anti-

erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is d ...

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 fruits. Like animal fats, vegetable fats are ''mixtures'' of triglycerides. Soybean oil, grape seed oil, and cocoa butter are examples of seed oils, or f ...

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 t ...

from a ruptured oil line. Typically these oils are available as ISO 32, ISO 46, and ISO 68 specification oils.

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

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 works ...

is a subtype of hydraulic fluid with high

, 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 Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance' ...

, 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 A power steering is a mechanical device equipped on a motor vehicle that helps drivers steer the vehicle by reducing steering effort needed to turn the steering wheel, making it easier for the vehicle to turn or maneuver at lower speeds. Hydra ...

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 type of hydraulic fluid used in vehicles with automatic transmissions. It is typically coloured red or green to distinguish it from motor oil and other fluids in the vehicle. The fluid is optimized for th ...

, 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 system ...

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 (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

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 is a device that converts motive power ( mechanical energy) or fuel-based power (chemical energy) into electric power for use in an external circuit. Sources of mechanical energy include steam turbines, g ...

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

electrical power Electric power is the rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt, one joule per second. Standard prefixes apply to watts as with other SI units: thousands, millions and billion ...

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

Mineral oil base: * Mil-PRF-5606 (originally Mil-H-5606): Mineral base, flammable, fairly low flashpoint, usable from to , 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. 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 fire-resistant aviation hydraulic fluid manufactured by Solutia (now part of Eastman Chemical Company), and formerly manufactured by Monsanto. There are various lines of Skydrol including Skydrol 500B-4, Skydrol LD-4, and Skydrol 5. Sk ...

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 sim ...

(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.

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
{{DEFAULTSORT:Hydraulic Fluid Fluid dynamics Gear oils Hydraulics