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physical properties that strongly depend on the operating conditions, particularly on pressure and temperature.

      Significant advancements in the formulation of hydraulic fluids have been made in recent years. This has led to the development of hydraulic fluids whose properties are very close to the ideal behavior. The features usually desired in a hydraulic fluid are:

       limited influence of pressure and temperature on fluid viscosity;

       high lubrication capacity;

       high bulk modulus (meaning low compressibility);

       long‐life resistance and chemical stability;

       high flash point value (limiting risks of explosions and flammability);

       limited toxicity;

       high compatibility with the component material; and

       limited tendency to induce material corrosion.

2.2 Classification of Hydraulic Fluids

      This section provides a high‐level classification of the hydraulic fluids according to the current international standards. Details like formulation and chemical properties of each hydraulic fluid are out of the scope for this textbook. Nevertheless, it is important for the reader to understand that the panorama of the possible options for the working fluid is vast. Progress in various areas is constantly made by the industry to enhance the behavior of the commercially available hydraulic fluids. More details can be found in literature. Suggested readings are the book by Totten and De Negri [7] or by Zarotti [8].

Several standards have been formulated to classify the fluids currently available for hydraulic systems. The most important one is the ISO 6473 [9]. From this standard, a hydraulic fluid belongs to one of the following categories:

       mineral oil (the fluid identifier starts with H);

       fire‐resistant fluid (the fluid identifier starts with HS);

       synthetic fluid.

      2.2.1 Mineral Oils (H)

The most common hydraulic fluids in hydraulic systems for off‐road (agricultural vehicles, construction, and mining machines) and industrial applications are mineral‐based oils. These fluids have good operating features (according to the parameters described in the previous paragraphs) and are particularly economical in comparison with the other categories of hydraulic fluids. A good subclassification of the mineral oils can be represented by the schematic of Figure 2.2. From a base oil (HH), several other hydraulic fluids can be derived by using additives: HL implies an HH oil with anti‐oxidation and anti‐rust additives. From an HL oil, the use of additives that reduce the dependency of viscosity with temperature (VI improvers) leads to HR oils. If anti‐wear additives are added to an HL oil, the fluid is then indicated with the letters HM (or HLP). HM oils are probably the most common fluids in today's hydraulic systems. More variants can be derived from the HM oils by adding VI improvers (HV oils) or with the use of additives that reduce the stick‐slip effect that can occur inside hydraulic control valves.

The International Organization for Standardization (ISO) classification of Figure 2.2 is similar to the alternative DIN standard, which is still in use, especially in European countries. Table 2.1 shows the designations of the two standards for the most common oils.

Figure 2.2 Classification of mineral oils according to ISO 6473‐4.

      Source: Adapted from Zarotti [8].

Table 2.1 Correspondence between the DIN and the ISO standard for mineral oils.

      Source: Adapted from Assofluid [11].

DIN (code)	Additives	ISO
H	None	HH
HL	Anti‐rust + antioxidant + antifoam	HL
HLP	Anti‐rust + antioxidant + antifoam + anti‐wear	HM
HLP‐D	HLP + detergent + dispersant	Not envisaged
Not envisaged	HM + additives to improve viscosity index	HV

2.2.2 Fire‐Resistant Fluids (HF)

As the name of the category suggests, fire‐resistant fluids are specifically used in applications where fire or explosion hazards must be limited. These fluids are in most cases water based. A basic classification of these fluids can be done according to ISO 7745 [10], summarized by the tree diagram of Figure 2.3.

      HFA fluids contain at least 80% of water, and they can be further divided into HFAE, which are oil‐in‐water emulsions with anti‐wear additives and HFAS, which contains other chemical solutions in water. HFB are emulsions of water in oil, with a minimum of 40% of water content. HFC are glycol‐in‐water solutions (from 35% to 60% of water content), and they also include additives to improve fluid viscosity. HFD fluids are synthetic products without water. In particular, the HFDR fluids are based on phosphate esters; the HFDS fluids are based on chlorinated hydrocarbons; the HFDT fluids are a base of a mixture of HFDR–HFDS fluids; finally, HFDU are fluids with other synthetic products (not further specified).

      For HF fluids, especially those containing water, the maintenance or replacement of the working fluid is essential. It is also important to periodically monitor the composition of the fluid to ensure it has not been altered. Hydraulic fluids are often referred to with their commercial name rather than with the correct ISO denomination. A significant example in the fire‐resistant fluid category is the Skydrol, which is a fire‐resistant fluid used in aviation. This fluid belongs to the HFDR category.

Figure 2.3 Classification of fire‐resistant fluids according to ISO 7745.

      Source: Adapted from Zarotti [8].

      2.2.3 Synthetic Fluids (HS)

      ISO 6473 specifies the acronym for HS synthetics fluids, without further specifying their composition. Usually, these fluids are developed for specific applications. A typical example is the case of extreme temperature conditions. The technology for HS fluids is continuously evolving, and the number of options today available is too high to be summarized in a short list. Nevertheless, many current HS fluids are based on silicate esters and polyol esters.

      2.2.4

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