Types of hydraulic fluids
The various materials in use as hydraulic fluids range from water to inorganic salt solutions to water oil emulsions, synthetic and naturally occurring organic materials. Water was the first hydraulic fluid used during the early stages of industrial revolution. Presently, petroleum based hydrocarbon type fluids are in wide use. A good hydraulic fluid comprises of:
A hydraulic system has various components that contain surfaces that are in close contact and move in relation to each other. A good hydraulic fluid must protect against wear and separate & lubricate such surfaces.
Viscosity is a vital fluid property that varies with temperature and pressure. Fluids having large changes of viscosity with temperature are commonly referred as low viscosity index fluids and those having small changes of viscosity with temperature are known as high viscosity index fluids.
Chemical and Physical Stability
The characteristics of a fluid should remain unchanged during an extended useful life. Since many aspects of stability are chemical in nature, the temperatures to which the fluid will be exposed is an important criterion in the selection of a hydraulic fluid.
The hydraulic fluid should be inert to materials used in or near the hydraulic equipment. If the fluid in anyway attacks, destroys, dissolves or changes parts of the hydraulic system, the system may lose its functional efficiency and may start malfunctioning.
Good Heat Dissipation
Pressure drops, mechanical friction, fluid friction, leakages, all generate heat. The fluid must carry the generated heat away and readily dissipate it to the atmosphere or coolers.
The flash point of a hydraulic oil is defined as the temperature at which flashes will be generated when the oil is brought into contact with any heated matter.
The hydraulic liquids are petroleum derivatives and thus for critical applications, artificial or synthetic hydraulic fluids are used which have high fire resistances. Various grades of fluids with high water content are also available nowadays for oil hydraulic systems.
Prevent Rust Formation
Moisture and oxygen cause rusting of iron parts in the system that can lead to abrasive wear of system components and also act as catalyst to increase the rate of oxidation of the fluid. Fluids with rust inhibitors minimize rust formation in the system.
Low in Volatility
The fluid should posses low vapor pressure or high boiling point characteristic. The vapor pressure of a fluid varies with temperature and hence the operating temperature range of the system is important in determining the suitability of the fluid.
Low Coefficient of Expansion
The hydraulic fluid should have a low coefficient of expansion to minimize the total volume of the system required at the operating temperature.