Hydraulic Fluids – Functions, Characteristics, & Categories

Modern hydraulic applications demand compact machines designed with tighter tolerances and that run at faster cycle times. They are designed to work with small amounts of fluids. They operate at higher pressures, temperatures, and speeds. Under these circumstances, fluids are subjected to severe stresses.

Hydraulic fluids must be carefully maintained throughout their service life. Three important points concerned with the proper maintenance of fluids are  (1) Knowing the type of contaminants,   (2) means for controlling contamination, (3) assessing the health of the fluids. Analysis of fluid can help detect an emerging problem in the system.

Functions of Hydraulic Fluids

  • to transmit power
  • to provide lubrication to moving parts
  • to provide sealing between clearances
  • to assist in the removal of contaminants and heat

Preparation of Hydraulic Fluids

Hydraulic fluids are prepared from base stocks and additives. The base stock possesses all the essential characteristics to perform well in a particular class of hydraulic systems. Some examples of the base stock are petroleum oils, high-water-based fluids, synthetic fluids, and vegetable oils.

Many types of fluids can be formulated by adding the base stock with varieties of additives, to meet the exacting requirements of complex systems. Blending the base fluid with suitable additives can improve fluid’s physical and chemical properties, and make the properties more stable even in the presence of heat, oxygen, and water.

Fluid Characteristics


If the fluid is exposed to cold temp, then its viscosity tends to be high and more energy is required to pump the fluid. A thick fluid produces higher pressure drop and generates excessive heat. It may lead to the sluggish operation, higher power consumption, and lower mechanical efficiency of the system. It may also produce cavitation and damage filters.

If the fluid is exposed to hot temp, then its viscosity tends to be low. A fluid that is too thin tends to rupture the fluid film between sliding surfaces, produces leakages and a higher rate of friction, and wear of precision parts. It also produces a higher rate of oxidation and reduction in its service life.

The fluid must be thin enough to make it flow smoothly but thick enough to maintain sufficient lubricating film between sliding surfaces and to provide a proper sealing. For most applications, the viscosity may be kept at about 30 cSt at the operating temperature.

Viscosity Index

Certain hydraulic systems are subjected to wide variations in temperatures. A high-pressure, high-precision hydraulic system is sensitive to changes in the viscosity of its fluid medium at low temperatures. Mobile hydraulic systems are exposed to the outside environment. Such systems require high viscosity index (VI) fluid to maintain its viscosity at a constant value irrespective of variations in the temperature.

Some fluids have large VIs, to begin with. Some other fluids have their VIs reinforced through VI improvers. Such fluids are sometimes called as ‘multi-viscosity’ fluids or ‘multi-grades. VI additives are expensive and lose their effectiveness under high shear stress.

Fluid Compressibility

A good hydraulic fluid should have very low compressibility (high bulk modulus) so that it remains ‘stiff’, and that helps to get a fast response from the associated system. However, the compressibility of the fluid increases with an increase in the temperature and the pressure to which it is subjected. A typical mineral-based fluid undergoes about 0.5% reduction in volume for every 70 bar of pressure exerted upon, up to the pressure of 300 bar. Water-based fluids and synthetic fluids have higher bulk modulus as compared to that of mineral-based fluids.

The effect of compressibility shows up as a loss of fluid volume. This volume loss represents a power loss, as no downstream actuator is capable of recapturing the compressive energy. Therefore, the compressibility of fluids for special systems such as precision machines or servo systems must be considered while selecting them.


A fluid provides a load-carrying film in the clearance between two moving surfaces. The film prevents metal-to-metal contact and thus minimizes friction. Under modest load conditions, petroleum fluids satisfy the lubrication requirements of systems. With high loads, it is hard to maintain a sufficiently thick fluid film. Fluids for such application should be formulated with EP additive to improve its load-carrying properties.

Wear Resistance

A fluid intended to be used under normal operating conditions should be formulated with anti-wear additives to improve its wear resistance. An anti-wear additive is the stabilized zinc dithiophosphate (ZDP). The ZDP under highly stressed condition may produce undesirable ash. Fluid manufacturers look for environmentally-safe ashless additive alternatives to zinc-based additives.

Oxidation Resistance

Over a period, fluid passes through various components and naturally oxidizes and forms reaction products, such as acids, sludge, gum, and varnish. The exposure of the fluid to heat, metal catalysts, air, and water accelerates the natural process of oxidation. The signs of the oxidation process appear as changes in its colour, odour, and acidity level.

A superior hydraulic fluid should resist any reaction with oxygen. Better oxidation resistance can be achieved by selecting a base fluid having good chemical stability. Antioxidants can be used for the excellent oxidation resistance and the effective neutralization of acids.

Corrosion Resistance

Corrosion occurs due to the reaction of moisture and oxygen in the fluid with metal surfaces. It leads to abrasive wear of the parts and increases the leakage by opening up tolerances of close-fitting parts. System rusting occurs when oxygen and moisture attack ferrous parts. Chemical corrosion occurs when acids tend to attack copper and brass parts. A suitable rust inhibitor added to the fluid protects the fluid against system rusting and chemical corrosion.

Air Release Property

Leaks on the suction side of the pump can cause entrainment of excess air in the associated hydraulic system. The presence of a large amount of air tends to promote oxidation. It also assists in the generation of excessive heat due to the compression and decompression of the air. An essential characteristic needed of a fluid is its good air release feature.

Foam Resistance

The form is a mass of air bubbles that collect at the air-liquid surface in the reservoir. The foam may be generated by the excessive churning or impingement of return fluid at the bottom part of the reservoir. If the process of foaming is excessive, the foam is likely to be drawn into the fluid again. A hydraulic fluid should, therefore, have the property of low foaming.


Water promotes oxidation, impairs lubrication, and supports corrosion. A desirable feature of a high-quality fluid is its excellent demulsibility property. This feature allows water to be readily separated from the fluid. Highly refined mineral oils have inherently good demulsibility property. Alternatively, the fluid can be blended with an appropriate demulsifier that allows for easy separation of water.


It refers to the fluid’s ability to resist its degradation in the presence of extreme temperatures or increased chemical activities or water. That is, the fluid should have excellent thermal, chemical, and hydrolytic stabilities. Thermal Stability refers to the ability to resist degradation when subjected to high temperatures and extreme shear. Chemical Stability refers to the ability to resist degradation when subjected to increased chemical activities. Hydrolytic stability refers to the ability to resist chemical decomposition in the presence of water.


The basic parameters of a fire-resistant fluid are its resistance to ignition and resistance to the propagation of the flame from its source of ignition.

Flash Point

Flash Point refers to the lowest temperature at which a fluid gives off enough vapours to form an ignitable mixture that may generate flashes when it is brought into contact with a heated matter.

Fire Point

Fire Point refers to the lowest temperature at which a fluid gives off an adequate amount of vapours to its surrounding air, which is capable of supporting combustion continuously after ignition its surface.

Pour Point

Pour Point refers to the lowest temperature at which a fluid can flow when cooled under the specified test conditions.

The categories of Hydraulic Fluids

As modern hydraulic systems require high-performance hydraulic fluids to meet the stringent requirements of the systems, manufacturers prepare varieties of hydraulic fluids.

Petroleum-based Fluids

Petroleum oil has been the preferred energy transfer medium for hydraulic systems for many years. They have good lubricating and corrosion-inhibiting properties. They are low-cost fluids. They are available in a broad range of viscosities. But, they are flammable. They are also toxic and not very much bio-degradable. They must be compatible with the materials of construction (seals).

Fire-resistant Fluids

They are needed for high-temperature or hazardous hydraulic applications. Two basic types of fire-resistant fluids are (1) High-water-based-fluids (HWBF) and (2) Synthetic fluids. HWBFs are very much fire-resistant due to their high water content. Synthetic fluids have an exceptional fire-resistant property but they are costly. ISO 6743-4 divides fire-resistant fluids into HFA, HFB, HFC, and HFD.

  • HFA: Oil-in-water emulsions with a combustible proportion of 20% maximum
  • HFB: Water-in-oil emulsions with a combustible proportion of 60% maximum
  • HFC: Water glycol solutions with a water proportion of at least 35%
  • HFD: Water-free fluids on a synthetic base

Synthetic Fluids (HFD type)

Synthetic fluids are prepared from alkaline compounds that are blended with additives. Synthetic fire-resistant fluids are (1) Phosphate esters, (2) Polyol esters, (3) Halogenated hydrocarbons. They have good fire resistance and excellent lubrication characteristics. But, they are expensive and often not compatible with many seal materials. They may also give off toxic vapours, and require special disposal plan.

Biodegradable Fluids

For ecologically-sensitive applications, where fluid leakage could have an adverse impact on the environment, there is a demand for ecologically safe ‘green’ fluids. The best choice of fluids for such applications is the biodegradable fluid. On the occurrence of spillage, a readily biodegradable fluid breaks 60% of the fluid into harmless products, as a result of the reaction with naturally occurring bacteria, when exposed to the atmosphere for twenty-eight  days in a standard test. The most important base fluids of biodegradable hydraulic fluids are (1) Synthetic esters and (2) Vegetable oils.

Food-grade Fluids

Devices used for manufacturing and packaging of beverages, food, cosmetics, and medicines must be hygiene-specific. These devices must be designed with food-grade fluids to provide protection from the risk of incidental contacts of products with the fluids. Fluid manufacturers prepare the food-grade fluids from highly refined, non-toxic Polyalphaolefin (PAO) synthetic base fluids, a highly specialized non-toxic food-grade additive package, and a food-grade Anti microbicide. They need to be maintained to a high level of cleanliness, sanitation, and quality.

Authored by JOJI Parambath, Founder/Director, Fluidsys Training Centre, Bangalore



JOJI PARAMBATH, Industrial Hydraulic Systems – Theory and Practice, Universal Publishers, Boca Raton, USA, 2016.


Note: A comprehensive account of the topic is given in the textbook on ‘Industrial Hydraulic Systems-Theory and Practice’ by Joji Parambath.

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Please contact Fluidsys Training Centre, Bangalore for training in the field of Pneumatics, Electro-pneumatics, Hydraulics, Electro-hydraulics, etc.

 email: info@fluidsys.in | website: https://fluidsys.org



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