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.

Contents

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]

FunctionProperty
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
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
Functioning life
  • Material compatibility

Composition

Base stock

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.

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]

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. [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]

Uses

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.

Specifications

Mineral oil base:

  • 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 [11]
  • 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." [12]

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. [11]

  • 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 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

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 (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. [13] [14]

See also

Related Research Articles

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<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 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">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>

Gear oil is a lubricant made specifically for transmissions, transfer cases, and differentials in automobiles, trucks, and other machinery. It is of a 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.

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 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.

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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.

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

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<span class="mw-page-title-main">Defoamer</span> Chemical additive that reduces and hinders the formation of foam in liquids

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References

  1. Givens W. and Michael P., Fuels and Lubricants Handbook, G. Totten ed., ASTM International, 2003, p. 373 ISBN   0-8031-2096-6
  2. Placek, D., Synthetics, Mineral Oils and Bio-based Lubricants, L. ed., CRC Press, 2006, p. 519 ISBN   1-57444-723-8
  3. Schmitt, Vernon R. (1 January 2002). Controlled Bombs and Guided Missiles of the World War II and Cold War Eras: An Inside Story of Research and Development Programs. Society of Automotive Engineers. ISBN   9780768009132 . Retrieved 3 May 2017 via Google Books.
  4. What Does AW Stand For? - Petroleum Service Company
  5. Labor, Carl (21 July 2022). "Is Hydraulic Fluid Flammable?". Safetyfirst. Retrieved 30 October 2022.
  6. Peter, Skoog. "The Changing Economics of Fire-resistant Hydraulic Fluids" (PDF). Quaker Chemical Corporation. Retrieved 12 December 2014.
  7. "DOT Brake Fluid vs. Mineral Oil - and the Winner is." EpicBleedSolutions.com. Retrieved 3 May 2017.
  8. 1 2 "Power Steering Fluid". www.AA1car.com. Retrieved 3 May 2017.
  9. "Archived copy" (PDF). Archived from the original (PDF) on 2016-03-03. Retrieved 2015-05-27.{{cite web}}: CS1 maint: archived copy as title (link)
  10. Discovery channel-'seconds from disaster'
  11. 1 2 "Archived copy" (PDF). Archived from the original (PDF) on 2016-03-04. Retrieved 2017-02-25.{{cite web}}: CS1 maint: archived copy as title (link)
  12. "MIL-PRF-6083". QCLubricants.com. Retrieved 3 May 2017.
  13. "Oil You Need To Know About Hydraulic Oils - Crown Oil". www.crownoil.co.uk.
  14. "Machine-tool Lubrication". www.lathes.co.uk.