Hydraulic Fluid

Hydraulic fluids are liquids which are used in hydraulic machinery and equipment. Composed of a variety of chemicals, water to water oil emulsions to inorganic salt solutions to synthetic and naturally occurring organic materials, hydraulic fluids are the motive medium in hydraulic machines. Water was the first hydraulic fluid used. Presently, petroleum based hydrocarbon type fluids are in wide use. Biodegradable hydraulic fluid is also becoming a sought after product in industries today. For more information on hydraulic fluids, check out our article Biodrgradable Hydraulic Fluid: The Emerging Trend

Hydraulic fluids are widely used in automobiles. Car transmissions, power steering systems, and power brakes all require hydraulic fluids. Hydraulic fluids are also used in excavators, forklifts, tractors, backhoes, bulldozers, garbage trucks, and other industrial machinery. Most aircraft flight control systems also require hydraulic fluid.

Choosing the Right Hydraulic Fluid
Viscosity is the single most important factor to be considerd when selecting a hydraulic fluid. It is of no importance on how good the other properties of the oil are if the viscosity grade is not correct. The viscocity grade should be correctly matched to the operating temperature range of the hydraulic system. Otherwise the maximum component life will not be derived. Several factors need to be considered to define the correct fluid viscosity grade for a particular hydraulic system. These are as follows:
»Starting viscosity at minimum ambient temperature.
»Maximum expected operating temperature. This depends on maximum ambient temperature.
»Permissible and optimum viscosity range for the components of the hydraulic system.

Features of hydraulic fluids
A good hydraulic fluid comprises of:
Good Lubricity: 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.
Stable Viscosity: 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.
System Compatibility: 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. Flash Point: 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.
Fire Resistant : 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.