The oil in a hydraulic system must first and foremost transfer energy, but the moving parts in components must also be lubricated to reduce friction and consequent heat generation. Additionally, the oil must lead dirt particles and friction heat away from the system and protect against corrosion. Oil
requirements include:
1.Good lubricating properties.
2.Good wear properties.
3.Suitable viscosity.
4.Good corrosion inhibitor.
5.Good anti-aeration properties.
6.Reliable air separation.
7.Good water separation motor oil.
Liquids being used include mineral oil, water, phosphate ester, waterbased ethylene glycol compounds, and silicone fluids. The three most common types of hydraulic liquids are petroleum-based, synthetic fireresistant, and water-based fire-resistant.
Petroleum-Based Fluids
The most common hydraulic fluids used are the petroleum-based oils.These fluids contain additives to protect the fluid from oxidation
(antioxidant), to protect system metals from corrosion (anticorrosion), to reduce tendency of the fluid to foam (foam suppressant), and to improve viscosity.
Synthetic Fire-Resistant Fluids
Petroleum based oils contain most of the desired properties of a hydraulic liquid. However, they are flammable under normal conditions and can become explosive when subjected to high pressures and a source of
flame or high temperatures. Nonflammable synthetic liquids have been developed for use in hydraulic systems where fire hazards exist.
Water-Based Fire-Resistant Fluids
The most widely used water-based hydraulic fluids may be classified as water-glycol mixtures and water-synthetic base mixtures. The water-glycol mixture contains additives to protect it from oxidation, corrosion, and biological growth and to enhance its load-carrying capacity. Fire resistance of the water mixture fluids depends on the vaporization and smothering effect of steam generated from the water. The water in water-based fluids is
constantly being driven off while the system is operating. Therefore frequent checks to maintain the correct ratio of water are important.