Hydraulic fluids are subdivided into the following basic categories:
- Mineral-oil based hydraulic fluids
- Flame retardant hydraulic fluids
- Quickly biodegradable hydraulic fluids
Mineral-oil based hydraulic fluids are designated in ISO 6743/4 as HL, HM, HV, and in DIN 51524 with HL, HLP, HVLP.
- HL stands for hydraulic oils made of mineral oils with active ingredients that improve their corrosion protection and aging resistance.
- HLP oils improve the corrosion resistance, the aging resistance and the wear due to scoring in the mixed friction range.
- HVLPs additionally improve the viscosity/temperature behavior.
- There are also HLP-D hydraulic fluids that include cleaning additives (detergents).
There are certain additives in mineral oils that can accelerate the aging of elastomer seals at high temperatures. This results in subsequent vulcanization, which causes hardening and a loss of elasticity.
Flame retardant hydraulic fluids are classified in VDMA 24317. They are available as HFAE, HFAS, HFB, HFC and HFD oils:
- HFAEs are oil-in-water emulsions with a water content of greater than 80% and a mineral-oil or soluble polyglycol-based concentrate. In the mineral-oil based option it is necessary to watch out for decomposition and microbe growth. The fluid can be used at temperatures from +5 °C to +60 °C (+41 °F to +140 °F.
- For HFASs with synthetic concentrates there is no danger of decomposition. However, attention should be paid to the significantly increased tendency for corrosion.
- HFBs are water-in-mineral oil emulsions with a water content greater than 40%. These hydraulic oils can also be used from +5 °C to +60 °C (+41 °F to +140 °F), but are not approved for use in Germany due to their inadequate fire safety characteristics.
- HFCs are so-called aqueous glycols, quasi aqueous monomer or polymer solutions (frequently polyglycols). Their water content is generally between 35% and 65%. These flame resistant hydraulic fluids can be used at pressures of up to 250 bar (3625 PSI) and temperatures between -20 °C and +60 °C (between -4 °F and +140 °F.
When using HFC fluids, it must be clarified whether the seal materials being used are suitable. While fluororubber (FKM) is not suitable in every case, seals made of acrylnitrile-butadiene rubber (NBR) are unproblematic.
- HFDs are water-free fluids that can be used in the temperature range from +20 °C to +150 °C (+68 °F to +302 °F). Their compositions differ greatly, which means that a further distinction is made among HFD-R, HFD-S, HFD-T, HFD-U. These fluids are flame retardant, but can cause problems in the suction performance of pumps, and they are corrosive to many seal materials.
If HFC fluids are used, it must be clarified whether the sealing materials used are suitable. Fluorocarbon rubber (FKM) is not suitable in every case. Seals made of acrylonitrile-butadiene rubber (NBR) are unproblematic..
Quickly biodegradable hydraulic fluids are created using vegetable-based materials. Their abbreviation HE stands for Hydraulic Environmental, and also appears in the following designations:
- HETG (based on triglycerides / vegetable oils)
- HEES (based on synthetic ester), HEPG (polyglycols)
- HEPR (based on other fluids / primarily poly-alpha-olefins)
Pure water as a hydraulic fluid appears in very few applications, because its physical properties are hard to control.