Hydraulic fluid

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A hydraulic fluid is a fluid that is required to transfer energy ( volume flow , pressure ) in hydraulic systems in fluid technology . The overall market for hydraulic oils represents the second largest area of lubricants after motor oils . In Germany, around 150,000 t are consumed annually, of which around 60,000 t are used in mobile applications.


Hydraulic fluids must have good lubricating properties, high aging resistance and high wetting and adhesive properties. They also need a high flash point and a low pour point . For use in hydraulic systems, compatibility with seals and freedom from resins and acids are important. Further properties are a low temperature influence on the viscosity (both dynamic and kinematic viscosity), low compressibility and shear stability as well as low foaming.

The main task of hydraulic fluids is to transfer the hydraulic power from the pump to the motor or cylinder with as little loss as possible. In addition to this main task, the hydraulic fluid performs the lubrication and corrosion protection for the moving parts (piston, sliding surfaces, bearings, switching elements) and the metal surfaces of the hydraulic system. It also removes impurities (for example from abrasion ), water and air as well as waste heat .

Types of hydraulic fluids

Depending on the intended use and required property, hydraulic fluids consist of different types of fluids on different material bases.

Hydraulic fluids based on mineral oils

The most frequently used hydraulic fluids are made from mineral oil with appropriate additives . They are also known as hydraulic oil. The requirements for these hydraulic oils are specified in ISO 6743/4 with the designations HL, HM, HV. In Germany, the designations H, HL, HLP, HVLP according to DIN 51 524 are common.

H: without additives, correspond to the lubricating oils according to DIN 51 517. These hydraulic oils are hardly used today.

HL: with active ingredients to increase the corrosion protection and aging resistance (also HL according to DIN 51 524, part 1). They are used at pressures of up to 200 bar and meet the usual thermal loads.

HLP: with active ingredients to increase corrosion protection, with high pressure additives and aging resistance (also HLP according to DIN 51 524, part 2). They are used at pressures up to and above 200 bar and meet the usual thermal loads.

HM: with active ingredients to increase corrosion protection, resistance to aging and to reduce scuffing in the mixed friction area (also HLP according to DIN 51 524, part 2)

HV: with active ingredients to increase the corrosion protection, the aging resistance, to reduce the scuffing wear in the mixed friction area as well as to improve the viscosity-temperature behavior (also HVLP DIN 51 524, part 3)

HLPD: with active ingredients to increase corrosion protection, resistance to aging and detergent additives (German name, not standardized)

In addition to these standardized hydraulic oils, engine and gear oils can also be used for mobile hydraulic applications. Automatic transmission fluids (ATF → Automatic Transmission Fluid ) are used in hydrodynamic converters and are hydraulic oils whose lubricating and friction properties have been enhanced by adding auxiliary materials ( additives ) for the mechanics occurring in the transmissions. They are partly subject to manufacturer-specific standards and partly to standards issued by manufacturers that are also used elsewhere. These included the Dexron and Mercon series, among many others .

Hydraulic fluids for the food and feed industry

H1 hydraulic oils , registered by NSF International (formerly National Sanitation Foundation) in the NSF H1 category, may be used in the food and feed industry, for applications where occasional and technically unavoidable contact cannot be ruled out ("lubricant for incidential food contact "). NSF H2 hydraulic oils may be used if contact can be absolutely excluded. They are not to be confused with "rapidly biodegradable hydraulic fluids" (environmentally friendly hydraulic fluids). Food-compatible hydraulic fluids are often based on very pure petrochemical white oil , as is also used in pharmaceuticals and cosmetics, or on polyalphaolefins (PAO). All raw materials for H1 lubricants must meet the requirements of the Food and Drug Administration (USA). There are also few hydraulic fluids that are both NSF H1 certified and rapidly biodegradable.

Rapidly biodegradable hydraulic fluids

Hydraulic fluids that are biodegradable have been developed for use in biologically critical environments (construction machinery in water protection areas, forest machines in the forest, piste equipment in the mountains, etc.). These fluids can be produced from mineral oil, but they are often based on renewable raw materials, such as B. made of vegetable oils . Biodegradable fluids made from renewable raw materials are also known as bio-hydraulic fluids. Environmentally friendly hydraulic fluids are pollutants of pollution class  I, the marking is HE ( H ydraulic E nvironmental).

A distinction is made between the following types of environmentally friendly hydraulic fluids:

  • HETG (Basis T ri g lyceride = vegetable oils): These fluids are very easily biodegradable and generally not hazardous to water. Compared to mineral oils, they have a lower resistance to aging and can only be used to a limited extent under thermal stress.
  • HEPG (base P oly g lykole): polyglycols be prepared from mineral oil, they are easily biodegradable and non-hazardous to water. Their properties are comparable to those of mineral oils, they are water-soluble and cannot be mixed with mineral oils or vegetable oils.
  • HEES (based on synthetic esters ): Synthetic esters can be produced on the basis of renewable raw materials as well as on the basis of mineral oil. They are very easily biodegradable and not hazardous to water or meet the water hazard class  1. They are highly resistant to aging and are insensitive to extreme working temperatures.
  • HEPR (other base fluids, primarily poly-alpha-olefins ).

The proportion of bio-hydraulic oils in Germany has increased massively in recent years; in 2000 it was only around 3% and rose to 19% of the total market by 2005, particularly in mobile hydraulics . One of the main reasons for this is the bio-lubricants market launch program of the Federal Ministry for Food, Agriculture and Consumer Protection , which specifically promoted the switch to bio-lubricants between 2000 and 2008.

Flame retardant liquids

A second group is made up of flame-retardant liquids, which are mainly used where mineral oils cannot be used due to high fire risks. The use of flame-retardant liquids is mandatory, especially for use in hard coal mining and in civil aviation . Other main applications are systems in which the hydraulic fluid can come into contact with glowing or hot metal or open flames in the event of leaks (die casting, forging presses, power plant turbines , metallurgical plants and rolling mills).

The hardly inflammable liquids are divided into the following groups:

HFA: Oil-in-water emulsions or solution products with a water content of more than 80% and concentrate based on mineral oil or based on soluble polyglycols . With concentrate based on mineral oil, there is a risk of segregation and microbial growth. The liquid is flame-retardant and can be used for temperatures between +5 ° C to +55 ° C. The very low viscosity results in high leakage losses.

HFB: water-in-oil emulsions with a water content of more than 40% and mineral oil. The liquid is flame retardant and can be used for temperatures between +5 ° C and +60 ° C. In Germany, it is not approved due to its poor fire properties and is rarely used.

HFC: water glycols with a water content above 35% and polyglycol solution. The liquid is hardly inflammable and can be used for temperatures between −20 ° C to +60 ° C and pressures of up to 250 bar. It is the most common hydraulic fluid among the flame-retardant fluids. Upon contact with zinc in the pipeline system, zinc soaps are formed. B. pressure filters can be added.

HFD: Water-free synthetic liquids with a higher density than mineral oil or water (not HFD-U), can cause problems with the suction behavior of pumps and attack many sealing materials, as their compatibility with plastics is problematic. The liquid is flame retardant and can be used for temperatures between −20 ° C to +150 ° C. These are the following types, depending on the main component:

  • HFD-R: phosphoric acid ester
  • HFD-S: anhydrous chlorinated hydrocarbons
  • HFD-T: Mixture of HFD-R and HFD-S
  • HFD-U: anhydrous other composition (consisting of fatty acid esters or polyglycols)

HFD-U fluids are not classified as flame-retardant, as they do not achieve the RI value> 25 in the Buxton test, which is decisive for flame retardancy.


Pure water is not used in power hydraulics, instead it is mixed with oil to form an emulsion, similar to the cooling lubricant used in cutting machines. The first technical use of hydraulics was with water as the fluid. Water has a practically constant low viscosity. The compression factor is approx. 60% of the value of the hydraulic oil.

Properties and parameters

properties unit Mineral oils Polyglycol water solution Phosphate ester Chlorine aromatics Mixture of phosphate esters and chlorinated aromatics water
Density at 15 ° C g cm −3 0.87-0.9 1.01-1.09 1.1-1.3 1.3-1.45 1.2-1.4 1
Volume expansion coefficient 10 −3 K −1 0.65 0.7 0.7 0.7 0.7 0.206
Middle compression module G Pa 2 3.5-4 2.3-2.8 2.3-2.8 2.3-2.8 2.08
Bunsen absorption coefficient 0.08-0.1 0.03-0.04 0.08-0.09 0.08-0.09 0.08-0.09 0.02
Specific heat capacity J kg −1 K −1 1885 3350 1250-1650 1050 1250 4182
Thermal conductivity W m −1 K −1 6.97-13.95 25.18 8.37 8.37 8.37 38.38
Flash point ° C 210 - 210-240 200-220 210-230 -
Ignition temperature ° C 310-360 - 450-600 670 600-630 -
Maximum operating temperature ° C 90 65 150 150 150 65


Individual evidence

  1. Volker Lenz, Michael Weber: Lubricants and process materials . In: Market analysis of renewable raw materials . Fachagentur Nachwachsende Rohstoffe e. V., Gülzow 2006; Pp. 239–261, fnr-server.de (PDF)
  2. Fachagentur Nachwachsende Rohstoffe e. V. (Ed.): Data and facts on renewable raw materials. Gülzow 2007, p. 57, fnr-server.de (PDF)
  3. ^ William M. Haynes: CRC Handbook of Chemistry and Physics . 92nd edition. Taylor & Francis, 2011, ISBN 978-1-4398-5511-9 .