Hydraulic fluids

The most commonly used hydraulic fluid is based on mineral oil. They are particularly inexpensive and widespread. Their properties can be adjusted using additives that make them resistant to oxidation and corrosion.

The requirements for these hydraulic oils are generally specified in the ISO 6743-4 with the Designations HL, HM and HV. In Germany, on the other hand, the Designations HL, HLP und HVLP according to DIN 51524 are commonly used.

• H and HH: Mineral oil without active ingredients – is no longer used in practice.
• HL: With active ingredients to increase corrosion protection and ageing resistance.
• HM: With active ingredients to increase corrosion protection, aging resistance and to reduce scuffing wear in the mixed friction area.
• HLP: In addition to HL oils, other active ingredients to reduce wear and increase load-bearing capacity in the mixed friction area – broadest application in practice.
• HV and HVLP: Identical to HLP, but with increased ageing resistance and improved viscosity-temperature behavior.
• HLPD: Identical to HLP, but with additives to improve particle transport (detergent effect) and dispersion capacity (water carrying capacity) with active ingredients to increase corrosion protection (German designation, not standardized).

These fluids are based on synthetic compounds and offer a higher resistance to extreme temperatures and oxidation. They are mainly used in applications that require high thermal stability and long operating times. These include:

HFAE: Oil in water emulsions

• The water content is over 80 % and is mixed with a concentrate based on mineral oil or soluble polyglycols.
• With a mineral oil-based concentrate, there is a risk of segregation and microbial growth.
• Flame retardant, suitable for temperatures between +5°C and +55°C.

HFAS: synthetic concentrates dissolved in water

• No risk of segregation as these are genuine solutions. However, the hydraulic components are significantly more susceptible to corrosion.

HFB: Water in oil emulsions

• The water content is over 40 % and is mixed with mineral oil. This emulsion is rarely used.
• Flame retardant, suitable for temperatures between +5°C and +60°C.
• HFB fluids are not approved in Germany due to their poor fire safety properties.

HFC: water glycols

• The water content is more than 35% in a polymer solution,
• Flame retardant, suitable for temperatures between -20°C and +60°C.
• Can be used up to pressures of 250 bar.

HFD: Synthetic fluids

• 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).
• Synthetic fluids have a higher density than mineral oil or water (not HFD-U), they can cause problems with the suction behavior of pumps and attack many sealing materials.
• Flame retardant, suitable for temperatures between -20°C and +150°C.

These environmentally friendly options are based on vegetable oils (e.g. rapeseed) or synthetic esters and are ideal for applications where there is a risk of the liquid entering the environment – for example construction machinery used in water protection areas, snow groomers or in the mountains.

Marking: HE = Hydraulic Environmental

Classification :

• HETG (based on triglycerides = vegetable oils),
• HEES (based on synthetic esters),
• HEPG (Basis Polyglycole),
• HEPR (other base fluids, primarily poly-alpha-olefins).

As a hydraulic fluid, water is harmless in every respect (but without corrosion protection). Pure water is not used in power hydraulics; it is mixed with oil to form an emulsion, similar to the cutting oil used in metal-cutting machines (this sometimes results in the problem of segregation). The first technical use of hydraulics was with water as a fluid. Water has a practically constant low viscosity.

Classification:

• Tap water (filtered),
• Technical water (water-oil emulsion),
• Sea or salt water (filtered, not suitable due to aggressiveness).