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. Examples of equipment that might use hydraulic fluids include
excavators and
backhoes,
brakes,
power steering systems,
transmissions,
garbage trucks,
aircraft flight control systems,
elevators, and industrial machinery.
Hydraulic systems like the ones mentioned above will work most efficiently if the hydraulic fluid used has low 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:
{| class=wikitable
! Function
! Property
|-
| Medium for power transfer and control
|
Low compressibility (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
Base stock
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
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),
phosphate esters (e.g.
tributylphosphate), silicones, alkylated aromatic hydrocarbons, polyalphaolefins (PAO) (e.g.
polyisobutenes),
corrosion inhibitors, 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
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, but their inferior lubrication is not suitable for all braking systems.
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.
Trade names
Some of the trade names for hydraulic fluids include
Arnica,
Tellus,
Durad, Fyrquel, Houghto-Safe, Hydraunycoil, Lubritherm Enviro-Safe, Pydraul, Quintolubric, Reofos, Reolube, and
Skydrol.
Aircraft hydraulic systems
The use of hydraulic systems in aircraft almost certainly began with braking 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). See
flight controls.
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.
Specifications
Aircraft hydraulic fluids fall under various specifications:
Common petroleum-based:
* Mil-H-5606: Mineral base, flammable, fairly low flashpoint, usable from to , red color
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 based:
There are no military specifications for Skydrol hydraulic fluids. The list below contains most of the existing industry specifications and approvals:
*S.A.E. - AS1241
Boeing Seattle - BMS3-11
Boeing Long Beach - DMS2014
Lockheed - LAC C-34-1224
Airbus Industrie - NSA307110
British Aerospace - BAC M.333.B
Bombardier - BAMS 564-003
Below are some of the more common aircraft Phosphate-ester based hydraulic fluids.
* Skydrol 500B-4 (Type IV class 2): The Skydrol 500 series of fluids has the longest service history among phosphate ester products. The first version, Skydrol 500, was introduced in 1952. Steady improvements to the formulation led in 1978 to the current version, Skydrol 500B-4 which contains the same breakthrough anti erosion additive and acid scavenger found in Skydrol LD-4 . Skydrol 500B-4 is the most worker friendly of the aviation phosphate esters; it is least irritating to skin and less prone to form mists which can be irritating to the respiratory tract . This has given the product enormous popularity for use in work shops and indoor test stands.
* Skydrol LD-4 (Type IV class 1): Was also introduced in 1978, and is today the largest selling aviation phosphate ester fluid in the world. At the time of its introduction it was a breakthrough product, solving problems of valve erosion and thermal stability common in earlier fluids. Its excellent thermal stability under real world conditions has given it a reputation as the gold standard among Type IV fluids. In recent years it has received an additional qualification of 5000 psi approval under Boeing BMS 3-11 (Type V, Grade B and Grade C). Skydrol LD-4 features low density, excellent thermal stability, valve erosion prevention, and deposit control.
* Skydrol 5 (Type V): Introduced in 1996, Skydrol 5 was the first Type V fluid qualified under the Boeing BMS 3-11 specification. Skydrol 5 offers higher temperature capability than Type IV fluids, the lowest density, and better paint compatibility. Skydrol 5 does not have universal air frame manufacturer approval.
Skydrol PE-5 (Type V): Skydrol PE-5, introduced in 2010, has full approval from Airbus and Boeing for use in all of their aircraft models. Skydrol PE-5 was developed to meet and exceed the more demanding Type V fluid requirements. It features the longest fluid life of any commercially available fluid, low density and low viscosity at low temperature; an unbeatable combination of the best features for optimum fluid performance.
*Exxon HyJet IV-A plus (Type IV): Exxon HyJet IV-A plus is a fire-resistant phosphate ester hydraulic fluid designed for use in commercial aircraft. It is the best-performing Type IV fluid and approaches to a great extent many of the performance capabilities of Type V fluids, including high temperature stability,fluid life, low density, and rust protection. It is superior to all other Type IV fluids in these respects. Exxon HyJet IV-A plus meets the specifications of all major aircraft manufacturers and SAE AS1241.
* Exxon Hyjet V (Type V): Exxon HyJet V is a Type V fire-resistant phosphate ester hydraulic fluid, which is superior in thermal and hydrolytic stability to commercially available Type IV hydraulic fluids. Better stability means the extent of fluid degradation in aircraft systems will be less than Type IV fluids, in-service fluid life will be longer, and aircraft operator maintenance costs will be lower. HyJet V provides excellent high and low temperature flow properties (kinematic viscosities) and rust protection. HyJet V has also demonstrated an improvement over the erosion protection performance afforded by Type IV fluids.
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 to check contamination.
See also
Dexron
Hydraulic brake
Osmosis
Skydrol
Hydraulics International, INC.
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. Biodegradable Plant-Based Hydraulic Fluid
Industrial Hydraulic Oils
Aviation hydraulic fluids
Blenders Of Hydraulic Oils
Category:Fluid dynamics
Category:Gear oils
