Hydraulic fluid

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Hydraulic fluid being poured into a storage container US Navy 090316-N-6597H-004 Boatswain's Mate 2nd Class George Cabeen empties used hydraulic fluid into a storage container.jpg
Hydraulic fluid being poured into a storage container

A hydraulic fluid or hydraulic liquid is the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water. [1] Examples of equipment that might use hydraulic fluids are excavators and backhoes, hydraulic brakes, power steering systems, automatic transmissions, garbage trucks, aircraft flight control systems, lifts, and industrial machinery.


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

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: [2]

Power transmission and control medium
  • Non compressible (high bulk modulus)
  • Fast air release
  • Low foaming tendency
  • Low volatility
Medium for heat transfer
  • Good thermal capacity and conductivity
Sealing medium
  • 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
Functioning life
  • Material compatibility


Base stock

The original hydraulics fluid, dating back to the time of ancient Egypt, was water.[ citation needed ] 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 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 ethers, organophosphate ester, polyalphaolefin, propylene glycol, and silicone oils.

NaK-77, a eutectic alloy of sodium and potassium, 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 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 pumps are unsuitable and centrifugal pumps have to be used. The addition of caesium 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. [3]

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), organophosphate (e.g. tributylphosphate), silicones, alkylated aromatic hydrocarbons, polyalphaolefins (PAO) (e.g. polyisobutenes), corrosion inhibitors (incl acid scavengers), anti-erosion additives, etc.

Biodegradable hydraulic fluids

Environmentally sensitive applications (e.g. farm tractors and marine dredging) may benefit from using biodegradable hydraulic fluids based upon rapeseed 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.

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. [4]


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. [5]

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. [6]


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. [7] Glycol-ether based fluids are hygroscopic, 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 fluid is a sub type of hydraulic fluid. Most are mineral oil or silicone based fluids, while some use automatic transmission fluid, made from synthetic base oil. [8] [9] Automatic transmissions use fluids for their lubrication, cooling and hydraulic properties for viscous couplings.

Use of the wrong type of fluid can lead to failure of the power steering pump. [8]

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 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 called a Ram-Air Turbine (RAT) which is concealed under the fuselage. [10] 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.


Mineral oil base
According to DIN 51502

Source: [11] [12]

  • 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: [11]

  • Mil-PRF-5606 (originally Mil-H-5606): Mineral base, flammable, fairly low flashpoint, usable from −65 °F (−54 °C) to 275 °F (135 °C), red color, developed in the 1940s [13]
  • 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." [14]

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. [13]

  • Mil-H-83282: Synthetic hydrocarbon base, higher flashpoint, self-extinguishing, backward compatible to -5606, red color, rated to −40 °F (−40 °C) degrees.
  • Mil-H-87257: A development of -83282 fluid to improve its low temperature viscosity.
Multi purpose oil UTTO/STOU

Source: [11] [15]

  • 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


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 (primarily C and D checks) to check contamination. [16]

Military Spec 1246C is one fluid contamination specification.

The ISO fluid contamination scale assigns a contamination category based on particle size count and distribution. [17]

Other uses

The properties of HLP 32 hydraulic oil make it ideal for lubricating machine tools. [18] [19]

See also

Related Research Articles

A lubricant is a substance that helps to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, transporting foreign particles, or heating or cooling the surfaces. The property of reducing friction is known as lubricity.

<span class="mw-page-title-main">Silicone</span> Family of polymers of the repeating form [R2Si–O–SiR2]

In organosilicon and polymer chemistry, a silicone or polysiloxane is a polymer composed of repeating units of siloxane. They are typically colorless oils or rubber-like substances. Silicones are used in sealants, adhesives, lubricants, medicine, cooking utensils, thermal insulation, and electrical insulation. Some common forms include silicone oil, grease, rubber, resin, and caulk.

<span class="mw-page-title-main">Motor oil</span> Lubricant used for lubrication of internal combustion engines

Motor oil, engine oil, or engine lubricant is any one of various substances used for the lubrication of internal combustion engines. They typically consist of base oils enhanced with various additives, particularly antiwear additives, detergents, dispersants, and, for multi-grade oils, viscosity index improvers. The main function of motor oil is to reduce friction and wear on moving parts and to clean the engine from sludge and varnish (detergents). It also neutralizes acids that originate from fuel and from oxidation of the lubricant (detergents), improves the sealing of piston rings, and cools the engine by carrying heat away from moving parts.

<span class="mw-page-title-main">Cutting fluid</span> Coolants and lubricants used in metalworking

Cutting fluid is a type of coolant and lubricant designed specifically for metalworking processes, such as machining and stamping. There are various kinds of cutting fluids, which include oils, oil-water emulsions, pastes, gels, aerosols (mists), and air or other gases. Cutting fluids are made from petroleum distillates, animal fats, plant oils, water and air, or other raw ingredients. Depending on context and on which type of cutting fluid is being considered, it may be referred to as cutting fluid, cutting oil, cutting compound, coolant, or lubricant.

<span class="mw-page-title-main">Synthetic oil</span> Lubricant consisting of artificially made chemical compounds

Synthetic oil is a lubricant consisting of chemical compounds that are artificially modified or synthesised. Synthetic lubricants can be manufactured using chemically modified petroleum components rather than whole crude oil, but can also be synthesized from other raw materials. The base material, however, is still overwhelmingly crude oil that is distilled and then modified physically and chemically. The actual synthesis process and composition of additives is generally a commercial trade secret and will vary among producers.

<span class="mw-page-title-main">Hydropneumatic suspension</span> Pneumatics

Hydropneumatic suspension is a type of motor vehicle suspension system, designed by Paul Magès, invented by Citroën, and fitted to Citroën cars, as well as being used under licence by other car manufacturers. Similar systems are also widely used on modern tanks and other large military vehicles. The suspension was referred to as Suspension oléopneumatique in early literature, pointing to oil and air as its main components.

An antifreeze is an additive which lowers the freezing point of a water-based liquid. An antifreeze mixture is used to achieve freezing-point depression for cold environments. Common antifreezes also increase the boiling point of the liquid, allowing higher coolant temperature. However, all common antifreeze additives also have lower heat capacities than water, and do reduce water's ability to act as a coolant when added to it.

<span class="mw-page-title-main">O-ring</span> Mechanical, toroid gasket that seals an interface

An O-ring, also known as a packing or a toric joint, is a mechanical gasket in the shape of a torus; it is a loop of elastomer with a round cross-section, designed to be seated in a groove and compressed during assembly between two or more parts, forming a seal at the interface.

<span class="mw-page-title-main">Gear oil</span> Lubricant used in vehicles and machinery

Gear oil is a lubricant made specifically for transmissions, transfer cases, and differentials in automobiles, trucks, and other machinery. It has high viscosity and usually contains organosulfur compounds. Some modern automatic transaxles do not use a heavy oil at all but lubricate with the lower viscosity hydraulic fluid, which is available at pressure within the automatic transmission. Gear oils account for about 20% of the lubricant market.

<span class="mw-page-title-main">Brake fluid</span> Hydraulic fluid used for braking applications

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 because liquids are not appreciably compressible.

A coolant is a substance, typically liquid, that is used to reduce or regulate the temperature of a system. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, chemically inert and neither causes nor promotes corrosion of the cooling system. Some applications also require the coolant to be an electrical insulator.

Transformer oil or insulating oil is an oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled wet transformers, some types of high-voltage capacitors, fluorescent lamp ballasts, and some types of high-voltage switches and circuit breakers. Its functions are to insulate, suppress corona discharge and arcing, and to serve as a coolant.

Grease is a solid or semisolid lubricant formed as a dispersion of thickening agents in a liquid lubricant. Grease generally consists of a soap emulsified with mineral or vegetable oil.

<span class="mw-page-title-main">Hydraulic brake</span> Arrangement of braking mechanism

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.

<span class="mw-page-title-main">Polyphenyl ether</span> Class of polymers

Phenyl ether polymers are a class of polymers that contain a phenoxy or a thiophenoxy group as the repeating group in ether linkages. Commercial phenyl ether polymers belong to two chemical classes: polyphenyl ethers (PPEs) and polyphenylene oxides (PPOs). The phenoxy groups in the former class of polymers do not contain any substituents whereas those in the latter class contain 2 to 4 alkyl groups on the phenyl ring. The structure of an oxygen-containing PPE is provided in Figure 1 and that of a 2, 6-xylenol derived PPO is shown in Figure 2. Either class can have the oxygen atoms attached at various positions around the rings.

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 conditions. It is manufactured by Solutia, and formerly manufactured by Monsanto. There are various lines of Skydrol including Skydrol 500B-4, Skydrol LD-4, and Skydrol 5.

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<span class="mw-page-title-main">Radiator (engine cooling)</span> Heat exchangers used for cooling internal combustion engines

Radiators are heat exchangers used for cooling internal combustion engines, mainly in automobiles but also in piston-engined aircraft, railway locomotives, motorcycles, stationary generating plants or any similar use of such an engine.

In fluid thermodynamics, a heat transfer fluid is a gas or liquid that takes part in heat transfer by serving as an intermediary in cooling on one side of a process, transporting and storing thermal energy, and heating on another side of a process. Heat transfer fluids are used in countless applications and industrial processes requiring heating or cooling, typically in a closed circuit and in continuous cycles. Cooling water, for instance, cools an engine, while heating water in a hydronic heating system heats the radiator in a room.

<span class="mw-page-title-main">DEG monobutyl ether</span> Chemical compound

Diethylene glycol butyl ether is an organic compound, one of several glycol ether solvents. It is a colorless liquid with a low odour and high boiling point. It is mainly used as a solvent for paints and varnishes in the chemical industry, household detergents, brewing chemicals and textile processing.


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  10. Discovery channel-'seconds from disaster'
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