The following characteristics and properties are important for hydraulic oils:
Hydraulic systems are widely used in industrial machinery, construction equipment, automotive, aircraft and marine applications.
Mineral based oils are the most common and low cost hydraulic fluids. They possess most of the characteristics important for hydraulic oils. The disadvantages of mineral (petroleum) based oils are their low fire resistance (low flash point), toxicity and very low biodegradability.
Phosphate esters are produced by the reaction of phosphoric acid with aromatic alcohols. Phosphate esters based hydraulic fluids possess excellent fire resistance, however they are not compatible with paints, adhesives, some polymers and sealant materials. They are also toxic.
Polyol esters are produced by the reaction of long-chain fatty acids and synthesized alcohols. Polyol ester based hydraulic fluids are fire resistant and possess very good lubrication properties. They are environmentally friendly but their use is limited by high cost.
Water glycol based fluids contain 35-60% of water in form of solution (not emulsion) and additives (anti-foam, anti-freeze, rust and corrosion inhibitors, anti-wear etc.). Water glycol based hydraulic fluids possess excellent fire resistance, they are non-toxic and biodegradable. However their temperature range is relatively low: 32°F - 120°F (0°C - 49°C). Water evaporation causes deterioration of the hydraulic fluids properties.
Vegetable hydraulic oils are produced mainly from Canola oil. Their chemical structure is similar to that of polyol esters. Vegetable hydraulic oils possess very good lubrication properties and high viscosity index (low temperature sensitivity of viscosity). They are non-toxic and biodegradable. The main disadvantage of vegetable hydraulic oils is their relatively low oxidation resistance.
Viscosity of a hydraulic fluid depends on its composition and the temperature. Low viscosity limit is determined by the lubrication properties of the oil and its resistance to cavitation. Upper viscosity value is limited by the ability of the oil to be pumped.
Common viscosity of hydraulic oils is in the range 16 - 100 centistokes. Optimum viscosity value is 16 - 36 centistokes.
The Society of Automotive Engineers (SAE) established a viscosity grading system for oils.
According to the SAE viscosity grading system all oils are divided into two classes: monograde and multigrade:
Monograde hydraulic oils are designated by one number (10, 20, 30, 40, etc.). The number indicates a level of the oil viscosity at a particular temperature. The higher the grade number, the higher the oil viscosity.
Viscosity of hydraulic oils designated with a number only without the letter “W” (SAE 10, SAE 20, SAE 30 etc.) was specified at the temperature 212°F (100°C). These oils are suitable for use at high ambient temperatures.
Viscosity of hydraulic oils designated with a number followed by the letter “W” (SAE 10W, SAE 20W, SAE 30W etc.) was specified at the temperature 0°F (-18°C). The letter “W” means winter. These grades are used at low ambient temperatures.
Viscosity of hydraulic oils may be stabilized by polymeric additives (viscosity index improvers). Viscosity of such oils is specified at both high and low temperature. These oils are called multigrades and they are designated by two numbers and the letter “W” (SAE 5W30, SAE 10W20, SAE 10W30 etc.). The first number of the designation specify the oil viscosity at cold temperature, the second number specifies the oil viscosity at high temperature.
For example: SAE 10W30 oil has a low temperature viscosity similar to that of SAE 10W, but it has a high temperature viscosity similar to that of SAE 30.
Multigrade hydraulic oils are used in a wide temperature range.
International Standardization Organization (ISO) established a viscosity grading (VG) system for industrial hydraulic oils. According to the system hydraulic oils are designated by the letters ISO followed by a number equal to the oil viscosity measured in centistokes at 40°C (104°F): ISO VG 32, ISO VG 46 etc.