Hydraulic Fluids

The hydraulic fluid is the essential element of all hydraulic system and it must be selected according to actual operating conditions, safety-environmental requirements and applicable regulations. Performance, quality and cleanliness of the hydraulic fluid are important factors what are going to determine the operational efficiency, reliability, and service life of the entire hydraulic system.

Fluids in the hydraulic systems provide several functions that have influence on the system performance:

  • Transmit energy by flow of pressurized fluid

  • Lubrication of the moving, and sliding components and surfaces in contact (hydraulic cylinder, and valve, pump components including seals)

  • Heat removal, transferring through the entire system to the reservoir tank and cooler

  • Cleaning the system by carrying wear particles and contaminants to the filters

  • Corrosion protection

Demands toward the hydraulic systems have been increased constantly. Higher operating speed, pressure and temperature in combination with higher efficiency, which are expected by the hydraulic system operators. That is why correct selection of the hydraulic fluid requires an understanding of the operating parameters of the system and characteristics of the fluids as well. When selecting a hydraulic fluid, the following features are the most important for consideration:

  • Viscosity and viscosity index (viscosity-temperature behavior of the fluid) – fluid must maintain a minimum viscosity required even at the highest operating temperature and viscosity what can be pumped at the lowest temperature as well.

  • Anti-wear – wear is caused by metal-to-metal contact in the system. Special additives help to reduce wear, minimizing metal breakdown

  • Friction reduction – the hydraulic fluid or its additive creates protective film between moving metal parts, reducing friction

  • Aging resistance – longer service life of the fluid will help to reduce maintenance, fluid replacement and disposal costs

  • Corrosion protection – modern hydraulic fluids contain inhibitors against system rusting caused by water contamination by the fluid and chemical corrosion caused by acids (fluids are oxidized over time, forming acids)

  • Filterability – more and more fine filtration is required in the hydraulic systems. Fine filters exhibit premature filter plugging due to water contamination, fluid degradation and inappropriate formulations.

  • Air separation ability – when hydraulic fluid carries air, the risk of foaming is increasing (surface foam or entrained air). Properly selected base oil and anti-foam additive will help to prevent foaming, which degrades the system performance

  • Demulsifying ability – water should be separated from the hydraulic fluid without taking the fluid itself or any additive with it. In order to accelerate the possible water contamination of the fluid, normally demulsifiers added to the fluid.

  • Material compatibility – fluid should be compatible with all other materials in the system, including seal components

  • Disposability – increasing demand on environmental protection and changing regulations make the disposal of hydraulic fluids more challenging. Local environmental protection and anti-pollution regulations should be considered.

Different operating, environmental, and safety factors require different hydraulic fluids; this is why a broad range of fluids with various chemical composition and viscosity grades are available today. The hydraulic fluids can be classified according to ISO 6743-4 standard as follows:

Mineral (petroleum based) Hydraulic Fluids

The most frequently used hydraulic liquid is mineral oil-based, with suitable additives. It is also known as hydraulic oil.
HH -mineral oil-based without corrosion inhibitors (no longer used in practice)
HL – HH miner oil-based with active additives to reduce oxidation and increase corrosion protection
HM – HL mineral oil-based with active ingredients to reduce wear
HR – HL mineral oil-based with an improved temperature-viscosity relationship
HV – HM miner oil-based with a high viscosity index (improved viscosity-temperature behaviour)
HG – HM mineral oil-based with improved shock resistant features
HS – Synthetic hydraulic oils

Biodegradable Hydraulic Fluids

Biodegradable hydraulic liquids are manufactured using plant oils (e.g. rapeseed oil) and used in biologically critical environments
HETG – Triglyceride hydraulic fluids with very good viscosity-temperature behaviour and good corrosion protection. HETG fluids have high biological degradability and the water hazard class is WGK 0.
HEPG – Polyglycol hydraulic fluids with very good viscosity-temperature behaviour and good corrosion protection. HEPG fluids are biologically degradable and the water hazard class is WGK 0.
HEES – Synthetic ester based hydraulic fluids with very good viscosity-temperature behaviour. These fluids are biologically well degradable and the water hazard class is WGK 0. Important feature of the HEES fluids is the good lubricating characteristics and good resistance against aging.
HEPR – Polyalphaolefins and related hydrocarbon hydraulic fluids with very good viscosity–temperature behaviour and good corrosion protection. Typical characteristic of the HEPR fluids is the reduced biologically degradability (especially at higher viscosities) and the water hazard class WGK 1 – 2. These fluids have good lubricating properties and good resistance against aging.

Fire-Resistant Hydraulic Fluids

When the possibility exists that a hydraulic fluid may come in contact with a source of ignition or the surface of very hot equipment, fire-resistant fluids may be used.

HFA fluids – oil in water emulsions. Lack of the bacterial control and the corrosion could cause problems with the usage of these fluids. pH value of the fluid can become unstable, causing wear and chemical reaction with aluminium. HFA fluids are divided into two subgroups:
HFAE Oil-in-Water emulsions with low emulsion oil content (normally 1 to 5% emulsion oil related to the volume)
HFAS Solutions with typically not more than 10% fluid concentrate in water
HFB fluids – water in oil emulsions. The water content is above 40% and is mixed with a mineral oil. This emulsion is rarely used. Flame resistant, can be used at temperatures between +5°C and +60°C. Monitoring of fluid water content is necessary and frequent additions might needed in order to compensate evaporation losses.
HFC fluids – water polymers and water glycols. The water content is more than 35% in a polymer solution, flame resistant, can be used at temperatures between -20°C and +60°C.
They can react with zinc and cadmium, and produces solvent action on some paints. Water content and the pH value might be a problem, continuous monitoring is might required.
HFD fluids – water free, synthetic fluids. Synthetic liquids have a higher density than mineral oil or water (not HFD-U), they can cause problems with the suction performance of pumps and affect a lot of gasket materials. Flame resistant, can be used at temperatures between -20°C and +150°C.
These fluids can attack some plastics, zinc and cadmium. HFD fluids are divided into four subgroups:

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