Hydraulic fluid From Wikipedia, the free encyclopedia

Hydraulic fluid being poured.

Hydraulic fluids, also called hydraulic liquids, are 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 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.

Contents

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1 Functions and properties

2 Composition

o 2.1 Base stock

o 2.2 Other components

o 2.3 Biodegradable hydraulic fluids

3 Brake fluid

4 Safety

5 Trade names

6 Aircraft hydraulic systems

o 6.1 Specifications

o 6.2 Contamination

7 See also

8 References

9 External links

Functions and properties[edit]

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]

Function Property

Medium for power transfer and control

Non compressible (high bulk modulus)

Fast air release

Low foaming tendency

Low volatility

Medium for heat transfer Good thermal capacity and conductivity

Sealing Medium Adequate viscosity and viscosity index

Shear stability

Lubricant

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 Low toxicity when new or decomposed

Biodegradability

Functioning life Material compatibility

Composition[edit]

Base stock[edit]

The original hydraulic 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.

Natural oils such as rapeseed (also called canola oil) 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, esters, organophosphate ester, polyalphaolefin, propylene glycol, and silicone oils.

Other components[edit]

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[edit]

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[edit]

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. Silicone based fluids are not hygroscopic.

Safety[edit]

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.

Trade names[edit]

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.

Aircraft hydraulic systems[edit]

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.[3] 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.

Specifications[edit]

Aircraft hydraulic fluids fall under various specifications:

Common petroleum-based:

Mil-H-5606: Mineral base, flammable, fairly low flashpoint, usable from 65 F (54 C) to 275 F (135 C), red color

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.

Phosphate-ester based:

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

Contamination[edit]

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.

See also[edit]

Dexron

Hydraulic brake

Osmosis

Skydrol

Hydraulics International, INC.

References[edit]

1. Jump up^ Givens W. and Michael P., Fuels and Lubricants Handbook, G. Totten ed., ASTM International, 2003, p. 373 ISBN 0-8031-2096-6

2. Jump up^ Placek, D., Synthetics, Mineral Oils and Bio-based Lubricants, L. Rudnick ed., CRC Press, 2006, p. 519 ISBN 1-57444-723-8

3. Jump up^ Discovery channel-'seconds from disaster'

External links[edit]

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

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