Types of Hydraulic Fluids Review

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Types of Hydraulic Fluids Review

Petroleum: Petroleum-based oils are the most commonly used stock for hydraulic applications where there is no danger of fire, no possibility of leakage that may cause contamination of other products, no wide temperature fluctuations, and no environmental impact.

Base stock

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 Hydrualic Oil 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 ( 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.

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:

Fluid 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
  • 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

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

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