Have you ever driven through a bad puddle of water only to have your miserable engine seize after ingesting a full cylinder? This is known as hydraulic lock and is a costly illustration of the hydraulic power that hydraulic oils transmit.

In this review I will focus on hydraulic fluids rather than brake fluids, which are a very specialized subgroup.

functions and properties

The crucial function of hydraulic oils is to transmit power. On the other hand, there are other significant functions of today’s hydraulic fluids.

The following table lists the main functions of hydraulic oil and the properties of the fluid that shape its ability to perform that function:

Hydraulic Performance Properties

• Low compressibility (high bulk modulus)

• Quick air release

• Low tendency to foam formation

• Low volatility

thermal performance

• Good thermal function and conductivity

sealing properties

• Appropriate viscosity and viscosity index

• Shear stability


• Satisfactory viscosity for film maintenance

• Low temperature fluidity

• Thermal and oxidative stability

• Hydrolytic stability / water tolerance

• Purity and filterability

• Demulsibility

• Wear reduction characteristics

• Corrosion control

pump efficiency

• Adequate viscosity to reduce internal leaks

• Superior viscosity index

special requirements

• Fire resistant

• Friction modifiers

• Radiation resistance

Environmental impact properties

• Low toxicity when new or decomposes

• Biodegradability


Since ancient Egypt, the hydraulic fluid mode was water. It was only in the 1920s that mineral oil began to be used. This was due to the inherent lubricating properties of oils and performance at temperatures above the boiling point of water. Today, most hydraulic fluids are based on mineral base oils and, more recently, on synthetic alternatives.

Today’s hydraulic oils can contain a wide range of chemical compounds, including: mineral oils, butanol, esters (eg, phthalates such as DEHP and adipates), polyalkylene glycols (PAGs), phosphate esters (eg, tributylphosphate), silicones, alkylated aromatic hydrocarbons, polyalphaolefins (PAO) (eg polyisobutenes), corrosion inhibitors, etc.

For environmentally sensitive applications, such as farm tractors and marine dredging (where there is the consequence of an oil spill from a broken oil line), biodegradable rapeseed (canola) vegetable oil-based hydraulic oils (often blended with custom synthetic esters). ). These oils are generally available as ISO 32, ISO 46 and ISO 68 specification oils.

Other base oils are used for special applications such as fire resistance and extreme temperature applications. Some examples include: glycol, esters, organophosphate ester, polyalphaolefin, propylene glycol, and silicone oils (for brake fluids).

Aircraft hydraulic systems

As aircraft performance improved, the force required to actuate the mechanical flight controls increased, and hydraulic systems were introduced to ease the strain on the pilot. Hydraulic power is also used to start the auxiliary power unit (APU) for automatic starting of the aircraft’s main engines. While several aircraft equipped with the M61 family of cannons harness hydraulic power to drive the weapon system, allowing for reliable high rates of fire.

Below are some of the more common phosphate ester-based hydraulic oils for aircraft.

• Skydrol 500B-4 (Type IV class 2)

• Skydrol LD-4 (Type IV class 1)

• Skydrol 5 (Type V)

• Skydrol PE-5 (Type V)

As with other modern oils, new synthetic hydraulic oils are blended to optimize the product for precise circumstances, such as Skydrol.

With the rising cost of crude oils, synthetic hydraulic oils are gaining more and more favor for their superior performance. While there has always been a marked difference in price, the unit costs of mineral and synthetic hydraulic oils are now very similar, but with synthetic hydraulic oils lasting longer than mineral equivalents, real cost savings are achieved.


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