Hydraulic oils are sub-divided into groups according to their additives.
-HL hydraulic oils
Are oils to which substances have been added to improve aging stability and corrosion protection. They are used in hydraulic systems, in which temperatures of around 50 °C and/or corrosion caused by moisture are to be expected.
-HLP hydraulic oils
offer better protection against wear than HL oils. They contain additives to slow down the aging process and to protect against corrosion and also additional substances to reduce wear during mixed friction operation, i.e. the area in which insufficient lubrication at the metal pairing can cause wear and and seizing up.
-HLP-D hydraulic oils
Contain detergent and dispergent (emulsifying) additives. These additives help to dissolve deposits and hold any impurities in the oil, (e.g. from aging or abrasion), and any water that has penetrated, in a suspension.
With hydraulic systems which are subjected either to large fluctuations in temperature or to low ambient temperatures, oils with a higher viscosity index, HV oils , are used.
Fire resistant fluid (type HFA)
A fire resistant hydraulic fluid which is oil-in-water-emulsion based. By the addition of concentrations of 2 to 5 % this fluid offers a minimum protection against wear and corrosion.
Due to its high water content the risk of combustion is minimal. However a disadvantage is the low viscosity which causes high leakage.
Fire resistant fluid (type HFB)
This fire resistant fluid, a water-in-oil-emulsion base, is made up of approx. 40 % water in mineral oil.
HFB fire resistant fluids are not used in all countries, as they do not always comply with the local mining fire regulations.
Fire resistant fluid (type HFC)
This fire resistant fluid is a water soluble polymer base with a water content of approx. 35 to 50 %. Although the temperature dependence of viscosity (VT)is more favourable than that of normal mineral oils, the pressure dependence of viscosity (VP) is considerable worse.
Fire resistant fluid (type HFD)
This fire resistant fluid contains no water and is manufactured synthetically using phosphate esters (HFD…R) with a relatively low combustibility, chlorinated hydrocarbons (HFD…S), HFDR and HFDS compounds amongst others (HFD…U).
Note: that all HFD fluids are incompatible with most sealing materials.