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

By Joji Parambath

The textbook presents information on the functions, types, characteristics, and selection of hydraulic fluids. The book also offers useful information on fluid contamination, the effect of contamination on fluids, fluid analysis, and the quality standards of fluids.

A complete range of 32 textbooks, in Paperback & Kindle eBooks Editions, on Pneumatics and Hydraulics, under Fluid Power Educational Series, authored by Joji Parambath has been released. For more details please visit:


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.

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.

Viscosity Index

Certain hydraulic systems are subjected to wide variations in temperatures. Such systems require high viscosity index (VI) fluid to maintain its viscosity at a constant value irrespective of variations in the temperature.

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.


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.

Wear Resistance

A fluid intended to be used under normal operating conditions should be formulated with anti-wear additives to improve its wear resistance.

Oxidation Resistance

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.

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.

Air Release Property

An essential characteristic needed of a fluid is its good air release feature.

Foam Resistance

A hydraulic fluid should 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.


It refers to the fluid’s ability to resist its degradation in the presence of extreme temperatures or increased chemical activities or 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.

Fire-resistant Fluids

Synthetic Fluids (HFD type)

Biodegradable Fluids

Food-grade Fluids

Joji Parambath


Fluidsys Training Centre Pvt Ltd

Bangalore, India


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