Hydraulic system liquids are used primarily to transmit and distribute forces to various units to be actuated. Liquids are able to do this because they are almost incompressible. Pascal’s Law states that pressure applied to any part of a confined liquid is transmitted with undiminished intensity to every other part. Thus if a number of passages exist in a system pressure can be distributed through all of them by means of the liquid.
Manufactures of hydraulic devices usually specify the type of liquid best suited for use with their equipment, in view of the working conditions, the service required, temperature expected inside and outside the systems, pressure the liquid must withstand, the possibilities of corrosion and other conditions that must be considered. Some of the properties and characteristics that must be considered when selecting a satisfactory liquid for a particular system are discussed below.
One of the most important properties of any hydraulic fluid is its viscosity. Viscosity is internal resistance to flow. A satisfactory liquid for a given hydraulic system must have enough body to give a good seal at pumps, valves and pistons; but it must not be so that it offers resistance to flow, leading to power loss and higher operating temperature. A fluid that is too thin will also lead to rapid wear of moving parts, or of parts which have heavy loads
Chemical stability is another property which is exceedingly important in selecting a hydraulic liquid. It is the liquid ability to resist oxidation and deterioration for long periods. All liquids tend to undergo unfavorable chemical change under severe operating conditions.
Liquids may break down if exposed to air, water, salt, or other impurities especially if they are in constant motion or subject to heat. It should be noted that the temperature of the liquid in the reservoir of an operating hydraulic system does not always represent a true state of operating conditions. Localized hot spots occur on bearings, gear teeth, or at the point where liquid under pressure is forced through a small orifice. Continuous passage of a liquid through these points may produce local temperatures high enough to carbonize or sludge the liquid, yet the liquid in the reservoir may not indicate an excessively high temperature.
Some metals such as zinc, lead, brass and copper have an undesirable chemical reaction on certain liquids. These chemical process result in the formation of sludge, gum and carbon or other deposits which clog openings, cause valves and pistons to stick or leak, and give poor lubrication to moving parts. It may cause changes in the physical and chemical properties of the liquid. The liquid may become darker in colour, higher in viscosity and acids are formed.
Flash point is the temperature at which a liquid gives off vapour in sufficient quantity to ignite momentarily or flash when a flame applied. A high flash point is desirable for hydraulic liquids because it indicates good resistance to combustion and a low degree of evaporation at normal temperatures.
Fire point is the temperature at which a substance gives off vapour in sufficient quantity to ignite and continue to burn when exposed to a spark or flame. Like flash point a high fire point is required for the hydraulic liquids.