Hydraulic fluids, also called hydraulic liquids, are the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on waste, mineral oil or water. Examples of equipment that might use hydraulic fluids include excavators and backhoes, hydraulic brakes, power steering systems, 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:
|Medium for power transfer and control||
|Medium for heat transfer||
The original hydraulics 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.
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.
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 (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 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. Mineral oil and silicone based fluids are not hygroscopic.
None of these fluids mix, so it is important to use the type the vehicle was designed to use.  The system contains rubber seals that will be damaged with use of the incorrect fluid.
Power Steering Fluid
Use of the wrong type of fluid can lead to failure of the power steering pump. 
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-gycol and polyol-ester are some of these specialized fluids that contain excellent thermal and hydrolitic properties, which aid in fire resistance.
Some of the trade names for hydraulic fluids include Arnica, Tellus, Durad, Fyrquel, Houghto-Safe, Hydraunycoil, Lubritherm Enviro-Safe, Pydraul, Quintolubric, Reofos, Reolube,Valvoline Ultramax and Skydrol. Gandhar
Aircraft hydraulic systems
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).
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. 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:
- 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 1940's
- MIL-PRF-6083: Useable 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 −40 °F (−40 °C) degrees.
- Mil-H-87257: A development of -83282 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 Industrie - NSA307110
- British Aerospace - BAC M.333.B
- Bombardier - BAMS 564-003
- SAE - Ac974
- SAE - AS1241
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.
- Hydraulic brake
- Hydraulic fuse
- Hydraulics International, INC.
- Hydropneumatic suspension - automobile application
- Oleo strut - aircraft application
- 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
- Peter, Skoog. "The Changing Economics of Fire-resistant Hydraulic Fluids" (PDF). Quaker Chemical Corporation. Retrieved 12 December 2014.
- Discovery channel-'seconds from disaster'
- 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. Biodegradable Plant-Based Hydraulic Fluid
- Biodegradable Hydraulic Oils
- Industrial Hydraulic Oils
- Aviation hydraulic fluids
- Blenders Of Hydraulic Oils