What are Hydraulic Fluids?choose the right engine oil for your car

Hydraulic fluid is the medium by which energy is transferred to all hydraulic systems. However, the tasks performed by hydraulic fluid go much further than the transfer of power. Although it is the core purpose of hydraulic fluid, it is also valuable in four secondary functions: transferring heat, removing contaminants, sealing and lubrication.

Transferring Heat

Hydraulic machines produce a huge amount of additional heat during regular operations, often caused by insufficiencies of the components themselves, such as pumps and motors. Minus a way to carry heat away from these components, they could easily overheat with resulting damage to seals and internal components, especially as a result of low local viscosity. As engine oil makes its way back to the reservoir, it often travels through a cooler to help maintain optimum temperature ranges before it is pumped back out to the system. Equally, hydraulic fluid can carry heat into a system during cold starts, when needed.

Removing ContaminantsOil Analysis

If hydraulic systems with closed-loops didn’t bleed off fluid at a precise rate, contamination would quickly build to critical and harmful levels. Due to the damage heat can cause, it can be considered a form of contamination, but as well as transferring heat, hydraulic fluid also carries particles and water away from sensitive components through filters or other conditioning devices. This can then be cleaned up and returned to the circuit. Oil without a driving force would remain within sensitive components, allowing essentially trapped contamination to slowly destroy its surroundings.


Although most would consider hydraulic oil as being a fluid that the pieces of hardware seal against, hydraulic fluid (especially oil) actually provides sealing within areas such as pumps, valves and motors. For example, a spool valve has a seal at each end that stops oil from escaping the valve, but each notch on the spool is sealed from the adjacent cavities by only the tight metal-to-metal tolerances and the oil’s surface tension and resistance to shearing.



Hydraulic OilsLubrication is needed in the majority of hydraulic components to protect internal parts from wearing or damage as a result of metal-to-metal friction. Oil provides full-film lubrication between moving parts, such as the slippers and lens plate of a piston pump. Without the lubricating properties of oil, hydraulic systems would be incredibly inefficient and unreliable.

How Common are these functions?

These functions are common to all types, excluding some water-based fluids that require unique design attention during engineering. However, the majority of machines use refined or synthetic oil, formulated to detailed levels and tested for important properties like viscosity, pour point and viscosity index. But these three properties are often considered when choosing a fluid for a specific application, based on maintaining an exact viscosity throughout a particular set of machine operating conditions. For example, if ambient temperatures are low, you would need an oil with lower rated viscosity and a low pour point. If your machine sees varying temperature ranges,  a high viscosity index is very important. Viscosity Index describes an oil’s ability to maintain its viscosity over a wide temperature range.

It is also extremely important to consider the viscosity requirement of the mechanisms in your hydraulic system. A piston pump, for example, may require between 16 and 40 centistokes, which is a description of kinematic viscosity.

Hydraulic oil is made with other important properties, but these tend to be common regardless of brand, viscosity or application. Hydraulic oil has a package of chemical additives for helping the performance of both the oil and the components of the hydraulic system. These additives can improve a number of functions including the foaming resistance of the oil, corrosion/rust resistance and the water-retention properties. The additive package separates low and high-quality fluids, and additives will also improve the viscous properties of the oil.

Phoenix HydraulicTotal Red Oil Barrel

Phoenix Hydraulic is the latest high performance anti-wear hydraulic oil. This general purpose hydraulic is oil designed for use in a broad range of applications. including vane and piston pumps. This high-quality hydraulic oil is manufactured from solvent refined, high viscosity index base oils, incorporating modern additive technology. The key benefits of this range are its good Viscosity Index and antiwear properties, anticorrosion and antifoam abilities and its low air retention. To find out more about this cutting-edge product, you can call Finol today at 01-4555484. Alternatively, click here to be put in contact with one of our expert team members.


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Finol Oils, 3 Stannaway Drive Crumlin Dublin 12 | Tel:+353 01 4555484