Have you ever walked through a bad puddle of water only to have your miserable engine completely stall after swallowing a full cylinder? This is known as a hydraulic lock and is an expensive illustration of the hydraulic power that hydraulic oils transmit.

In this comment I will focus on hydraulic fluids as opposed to brake fluids, which are a very specialized subgroup.

Functions and properties

The crucial function of hydraulic oils is to transmit energy. On the other hand, there are other important functions of current 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 volumetric modulus)

• Quick air release

• Low tendency to foaming

• Low volatility

Thermal performance

• Good conductivity and thermal function

Sealing properties

• Suitable viscosity and viscosity index

• Shear stability

Lubrication

• 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 leakage

• Superior viscosity index

Special requirements

• Fire resistant

• Friction modifiers

• Radiation resistance

Environmental impact properties

• Low toxicity when new or decomposed

• Biodegradability

Makeup

Dating back to ancient Egypt, the mode for hydraulic fluid 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 oils and, more recently, synthetic alternatives.

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

For environmentally hypersensitive applications, such as agricultural tractors and marine dredges (where there is the consequence of an oil spill from a broken oil line), biodegradable hydraulic oils based on rapeseed (canola) vegetable oil can be used (often mixed with custom synthetic esters). ). These oils are generally available as ISO 32, ISO 46, and ISO 68 specification oils.

Other raw materials 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 activate mechanical flight controls increased and hydraulic systems were introduced to facilitate pilot effort. Hydraulic power is also used to start the auxiliary power unit (APU) for self-starting the aircraft’s main engines. While several aircraft equipped with the M61 gun family exploit hydraulic power to propel the gun system, allowing for reliable high rates of fire.

Below are some of the most common phosphate ester 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 in precise circumstances, such as Skydrol.

With the rising costs of crude oils, synthetic hydraulic oils are gaining more and more followers for their superior performance. 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 that last longer than mineral equivalents, real cost savings are achieved.