Characteristics of a Good Hydraulic Fluid

Characteristics of a Good Hydraulic Fluid

Nov 30, 2015

Topics: Manufacturing


Viscosity is a measure of a hydraulic fluid's resistance to flow. It is a hydraulic fluid's most important characteristic and has a significant impact on the operation of the system.

When a hydraulic oil is too thin (low viscosity), it does not seal sufficiently. This leads to leakage and wear of parts. When a hydraulic oil is too thick (high viscosity), the fluid will be more difficult to pump through the system and may reduce operating efficiency.

All hydraulic fluids must be able to retain optimum viscosity during operation in cold or hot temperatures, in order to consistently and effectively transmit power.


Compressibility is a measure of the amount of volume reduction due to pressure. Although hydraulic oils are basically incompressible, slight volume reductions can occur under certain pressure ranges.

Compressibility increases with pressure and temperature and has significant effects on high-pressure fluid systems. It causes servo failure, efficiency loss, and cavitation; therefore, it is important for a hydraulic oil to have low compressibility.

Wear Resistance

Wear resistance is a hydraulic fluid's ability to reduce the wear rate in frictional boundary contacts. Antiwear hydraulic fluids contain antiwear components that can form a protective film on metal surfaces to prevent abrasion, scuffing, and contact fatigue. Antiwear additives enhance lubricant performance and extend equipment life.

Oxidation Stability

Oxidation stability is a hydraulic oil's resistance to heat-induced degradation caused by a chemical reaction with oxygen. Hydraulic oils must contain additives that counteract the process of oxidation, improve the stability and extend the life of the fluid. Without these additives, the quality of the hydraulic oil will deteriorate quickly.

Thermal Stability

Thermal stability is the ability to resist breakdown at elevated temperatures.  Antiwear additives naturally degrade over time and this process can be accelerated at higher temperatures.  The result of poor thermal stability is the formation of sludge and varnish which can clog filters, minimize flow and increase downtime.  In addition, as these antiwear agents decompose at high temperatures, acids are formed which attack bronze and yellow metals in piston pumps and other hydraulic system components.  Hydraulic oils can be formulated with very high levels of thermal stability to minimize these issues and help extend the life of the hydraulic fluid and the components of the hydraulic system.


Water can react with additives in hydraulic fluids forming oil insoluble material.  These contaminants can precipitate from the lubricant and block filters, valves and other components resulting in decreased oil flow or the system going on bypass.  Blockage can eventually result in unplanned downtime.  Hydraulic fluids are designed to be filtered with modern filtration systems without fear of the additive being depleted or removed from the system.  This enables systems to stay clean without sacrificing critical performance requirements such as antiwear, rust protection or foam inhibition.

Rust and Corrosion Protection

In many systems, water can enter as condensation or contamination, and mix with the hydraulic oil. Water can cause rusting of hydraulic components.  In addition, water can react with some additives to form chemical species which can be aggressive to yellow metals.  Hydraulic oil formulations contain rust and corrosion inhibitors which prevent the interaction of water or other chemical species from attacking metal surfaces.

Foam Resistance

Foam results from air or other gases becoming entrained in the hydraulic fluid. Air enters a hydraulic system through the reservoir or through air leaks within the system.

A hydraulic fluid under high pressure can contain a large volume of dissolved or dispersed air bubbles. When this fluid is depressurized, the air bubbles expand and produce foam. Because of its compressibility and poor lubricating properties, foam can seriously affect the operation and lubrication of machinery.

Proper foam inhibitors modify the surface tension on air bubbles so they more easily break up.


Water that enters a hydraulic system can mix or emulsify with the hydraulic oil. If this 'wet' fluid is circulated through the system, it can promote rust and corrosion. Highly refined mineral oils permit water to separate or demulsify quickly. However, some of the additives used in hydraulic oils promote emulsion formation, preventing the water from separating and settling out of the fluid. Demulsifier additives are incorporated to promote water separation from hydraulic fluids.

Hydrolytic Stability

When hydraulic fluids come into contact with water, the water can interact with the additive system of the hydraulic oil resulting in the formation of acids.  Hydraulic fluids that lack hydrolytic stability hydrolyze in the presence of water to form oil insoluble inorganic salts that can block filters and valves inhibiting oil flow. This can result in hydraulic system failure.   Properly formulated hydraulic fluids are designed to contain additives that are resistant to interactions with water, helping to extend the life of the equipment.

Seal Compatibility

Leaking hydraulic fluids can cause many issues from simple housekeeping problems to more serious safety concerns and lubrication failures.  Most hydraulics systems utilize rubber seals and other elastomers to minimize or prevent hydraulic oil leakage.  Exposure of the elastomer to the lubricant under high temperature conditions can cause the rubber seals to harden, crack and eventually leak.  On the other hand, hydraulic oil exposure can seals to swell excessively preventing hydraulic valves and pistons from moving freely.  Hydraulic oils are tested against a variety of seal materials to ensure that the hydraulic fluid will be compatible with seals under various conditions.

For more information on hydraulic fluid, contact your Lubrizol representative.

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