According to the book "Fuels and Lubricants Handbook: Technology, Properties, Performance, and Testing," hydraulic fluids are used to transfer power, heat and lubricate a hydraulic system. Hydraulic fluids are based in water, vegetable oil, synthetics or petroleum—with each type exhibiting unique characteristics. They are used in industrial machinery, garbage trucks, power steering systems, brakes and excavators.
Mineral-Oil-Based Hydraulic Fluids
The majority of hydraulic fluids currently in use have a mineral oil base. They provide enhanced performance at a low cost. The International Organization for Standardization (ISO) has established a system of classification for hydraulic fluids in its standard ISO 6743-4: 1999, "Lubricants, industrial oils and related products (class L)—Classification—Part 4: Family H (Hydraulic systems)" According to this standard, hydraulic fluids with a mineral oil base are categorized as HH, HL and HM fluids.
Type HH hydraulic fluids are refined mineral-oil-based fluids that do not have any additives. They are used in air-over-oil hydraulic systems, in low-pressure hydraulic systems and manual hydraulic jacks and pumps. HH fluids are able to transmit power but have limited lubricating capabilities and are unable to effectively withstand high temperatures.
HL type oils are refined mineral oils that contain oxidation and rust inhibitors to protect equipment from chemical deterioration and water contamination. HL fluids are used in piston pump applications.
HM are a version of HL-type fluids with improved antiwear additives. They are the most common mineral-oil-based hydraulic fluids due to their improved high-pressure hydraulic application performance. HL fluids use phosphorous, zinc and sulfur compounds to provide antiwear properties.
Fire Resistant Fluids
Fire resistant hydraulic fluids are used in foundry applications, military applications, die casting and in the basic metals industry. They are made with materials that have lower BTU (British thermal unit) content than mineral-oil-based fluids, such as water-glycol solutions, phosphate esters and polyol esters. They generate less heat when burnt than mineral-oil-based fluids. The ISO has classified fire-resistant hydraulic fluids as HFAE (soluble oils), HFAS (high water-based fluids), HFB (invert emulsions), HFC (water glycols), HFDR (phosphate esters) and HRDU (polyol esters).
Environmentally Acceptable Hydraulic Fluids
Environmentally acceptable hydraulic fluids are used in applications where there is a risk of spills and fluid leaks entering the environment and affecting terrestrial and aquatic life. They are used in offshore drilling, lawn equipment, dams, forestry and maritime industries. Environmentally acceptable hydraulic fluids are nontoxic to aquatic life and are completely biodegradable. They are classified by the ISO as HETG fluids (those that are based on natural vegetable oils), HEES fluids (fluids based on synthetic esters), HEPG (polyglycol type fluids) and HEPR (polyalphaolefin types).
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