Hydraulic Fluid Classifications 

Hydraulic fluids continue to expand their critical role and work multi-functionality within the marketplace. Considering the incompressible nature of oils, the work performed by hydraulic fluids s essential to proper functioning of hydraulically actuated equipment of all types. A basic understanding of the classifications of Hydraulic fluids will go a long way in helping you make informed decisions regarding what to use in your operation’s equipment.

Biodegradable hydraulic fluids continue an increasing trend of replacing conventional petroleum and synthetic hydraulic fluids. Advances in the science & technology behind bio-based hydraulic fluid offerings have evolved to the point where many bio-hydraulic fluids offer the same or better performance than conventional petroleum or synthetic hydraulic fluids, while offering a decisive environmental advantage. Due in part to the expanding role of biodegradable hydraulic fluids, and because the diversity of hydraulic fluid types has become so broad, the need to further classify hydraulic fluid types was evident.

The International Organization for Standardization (ISO) established a classification system for hydraulic fluids that is designated; ISO 6743-4: 1999 (now updated to 2015), Lubricants, Industrial Oils and Related Products (Class L – Classification – Part 4: Family H (Hydraulic Systems).

  • Lubricants, industrial oils and related products (class L)
  • Classifications-Part 4L Family H (hydraulic systems)

The ISO 6743-4 classification system can be generally applied to the three primary classes of hydraulic fluids:

  • Biodegradable Hydraulic Fluids
  • Mineral (i.e. petroleum) Hydraulic Fluids
  • Fire Resistant Hydraulic Fluids

For the purpose of providing generalized descriptions to aid you in understanding the many different hydraulic fluid classifications, this BioBlend blog provides a generalized summary of the ISO 6743-4 classifications that you may  find useful in understanding some of the most common hydraulic fluid classifications. It also provides insights into the ISO classification descriptions of the BioFlo Biodegradable Hydraulic Fluid technologies.

The ISO 6743-4 classification of biodegradable hydraulic fluids:

 ISO-L-HETG: Triglyceride Base (vegetable oils, i.e. plant oils)

HETG = Hydraulic Environmental Triglycerides = A biodegradable (vegetable oil based) hydraulic fluid which uses triglyceride esters as the base fluid. Vegetable oils are all classified as triglyceride esters.

****BioBlend’s BioFlo AW – BioFlow AW | BioBlend – is an excellent example of an HETG hydraulic fluid

ISO-L-HEES: Synthetic Ester Base (similar to triglyceride but with a non-vegetable based acid & alcohol)

HEES = Hydraulic Environmental Ester oil Synthetic = A biodegradable hydraulic fluid which uses  synthetic esters as the base fluid. Synthetic esters share a similar chemical structure to HETG types – but are synthesized.

****BioBlend’s BioFlo Synthetic AW –  BioFlo Synthetic AW | BioBlend – is the perfect fit for anyone in need of a HEES synthetic hydraulic fluid!

ISO-L-HEPG: Polyalkyleneglycol Base (i.e. polyglycol)

HEPG = Hydraulic Environmental Polyalkylene Glycols = A biodegradable hydraulic fluid which uses polyalkylene glycols (PAGS) as the base fluid. PAGS are synthesized from hydrocarbon oxides. HEPG’s are typically used in areas where fire is a hazard and may require special seals and/or seal materials.

ISO-L-HEPR: Other Base Liquid – primarily Polyalphaolefins

HEPR = Hydraulic Environmental PAO and Related products = A biodegradable hydraulic fluid which uses polyalphaolefins and related hydrocarbons as the base fluid. These base stocks are synthesized from crude oil derivatives.

The ISO 6743-4 classification of mineral oils (i.e. petroleum) hydraulic fluids:

 ISO-L-HH – Mineral lubricants without corrosion inhibitors

ISO-L-HL – HH lubricants with oxidation reduction and anticorrosive additives

ISO-L-HM – HL lubricants with wear reducing additives

ISO-L-HR – HL lubricants with a high VI

ISO-L-HV – HM lubricants with a high VI

ISO-L-HG – HM lubricants with shock resistant features

ISO-L-HS – Synthetic liquids

NOTE on Global Classifications: Because we operate in a global marketplace with equipment deployed throughout North America that is manufactured in other countries, it’s likely you may see hydraulic fluid classifications beyond the ISO 6743-4 classification designations. For example, while the classifications of hydraulic fluid are set out in ISO 6743-4 with the designations HL, HM, HV … in Germany the designations HL, HLP, HVLP are standard and frequently used, in accordance with DIN 51524.

H and HH – Mineral oil with no active ingredients – is no longer used in practice.

HL – With active ingredients to increase the corrosion protection and resistance to aging

HM – With active ingredients to increase the corrosion protection and resistance to aging and to reduce wear due to scoring in the mixed friction area

HLP – Further active ingredients in addition to HL oil to reduce wear and increase resistance in the mixed friction area – widest application in practice

HV and HVLP – Like HLP, but with increased resistance to aging, as well as an improved temperature-viscosity relationship

HLPD – Like HLP, but with additives to improve particle transport (detergent effect) and dispersion capacity (water carrying capacity) and active ingredients to increase the corrosion protection (German designation, not standardised) 

The ISO 6743-4 classification of fire resistant hydraulic fluids:

 There are several types of fire-resistant fluids and they are generally classified as follows:

  • Oil and water emulsions
  • Water Polymer solutions
  • Anhydrous Synthetics

ISO-L-HFAE – Emulsions – ‘oil-in-water’ (usually >80% water content)

ISO-L-HFAS – Synthetic aqueous fluids – chemical combination in water (usually >80% water content)

ISO-L-HFB – Emulsions – ‘water-in-oil’ (usually >40% water)

ISO-L-HFC – Glycol solutions, polyalkylene glycol solutions or water glycols (usually >35% water content)

ISO-L-HFDR – Synthetic phosphate esters (without water) ISO-L-HFDS Chlorinated hydrocarbons (without water)


ISO-L-HFDU – Synthetic anhydrous liquids (without water) – other than phosphate ester … Polyol Ester or PAG (more resistant than HFDR, HFDS, or HFDT)

****Check out BioBlend’s BioFlo HFDU – Fire-Resistant Hydraulic Fluid | BioBlend – for the top FM Approved Bio-based Fire Resistant Hydraulic fluid on the market today!

As always you can visit our site to learn more about BioBlend bioblend.com and call us directly at 630-227-1800. Yu can also fill out a contact request form Contact BioBlend and we would be happy to follow up with you regarding your needs.

In part 2 next week we will follow up to this post with a Q&A about BioBlend’s products as they relate to these classifications. Please be sure to follow us on LinkedIn to stay current on renewable and biodegradable lubricants!

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