Hydraulic oils drive the mechanisms of hydraulic systems by transmitting the force from its source to a required unit, changing the amount or direction of the applied force. They are used as a working fluid in the hydraulic clutch actuators, hydraulic roof in cabriolets, for operation of ABS, ASR, ASC, in the hydraulic power steering or hydro-pneumatic suspension.
Classification of hydraulic oils
International Organization for Standardization (ISO) lists the following classes of this type materials in its 11158 standard:
- HH. These are mineral-based products without additional additives. They can be used in hydraulic systems without special requirements, retain operational properties at temperature from −10 to 90 °С.
- HL. This category includes purified mineral products with enhanced anticorrosion and anti-oxidative properties. They are used in systems with strict requirements for water separation. They withstand temperature of −10–90 °С range.
- HM. They also contain components which prevent oxidation and corrosion, but have improved anti-wear properties compared to the products of the above class. They are designated for high-loaded systems with high requirements for water separation. They can be used in -20 – 90°С temperature range.
- HR. They are made on the basis of HL, but better retain the optimum viscosity for mechanisms operation at various temperatures: from −35 to 120 °С.
- HV. Lubricants of HM category with enhanced viscosity-temperature characteristics are in their basis. They are also designated for use at temperature of about −35 – 120 °С range.
- HG. They have anti-wear properties, contain additives which prevent stick-slip friction. They can be used, for instance, in pilot bearings at temperature of −30–120 °С.
Often, hydraulic oil suppliers indicate the specification according to DIN 51524 standard on their products packaging. According to it, all the products are divided into:
- HL (part 1). It indicates the lubricants which contain anti-oxidizing and anti-corrosive additives. They have the viscosity no higher than 80 and are used at the pressure below 100 bar.
- HLP (part 2). They contain additives preventing corrosion, oxidation and wear-out. Products of this category have viscosity index from 80 to 100 and can be used in systems with the pressure above 100 bar.
- HVLP (part 3). Apart from anti-oxidizing, antifriction and anticorrosive additives, they contain components which increase the viscosity to 140. These products are also effective at the pressure above 100 bar.
The viscosity of the working fluids used in hydraulic actuators is defined by ISO 3448 standard. Depending on their kinematic viscosity at 40 °С temperature, they are divided into 18 classes: 2, 3, 5, 7, 10, 15, 22, 32, 46, 68, 100, 150, 220, 320, 460, 680, 1000 and 1500.
Types of hydraulic oils by their formulation
Depending on the base formulation and production technology, there are three types of the fluids:
- Mineral. They are made on the basis of paraffin and naphthenic compounds. They are the most common ones. They are inexpensive. They have well-balanced operational characteristics.
- Semi-synthetic. Their composition includes both synthetic and well purified mineral components in various proportions. They are distinguished by enhanced properties and higher price.
- Synthetic. They are made on the basis of polyalphaolefins, polyglycol and polyethers, as well as petroleum fractions processed by hydrocracking. They are the least common. They are used for manufacturing of special, often, fire-resistant fluids. They cost higher than other products.
Regardless of the base, the basic composition of all products is added with additives preventing corrosion, oxidation, foaming, as well as with friction modifiers, thickeners, metal deactivators and other compounds.
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