Have you ever driven through a deep puddle of water only to have your hapless engine lock solid after ingesting a cylinder full? This is known as hydraulic lock and is an expensive illustration of hydraulic power which hydraulic oils convey.

In this article I’ll be focusing on hydraulic oils as opposed to brake fluids, which are a very specialised sub category.

Functions and properties
The primary function of hydraulic oils is to convey power. However, there are other important functions of modern hydraulic oils. These the major functions of hydraulic oil and the properties of the fluid that affect its ability to perform that function:

Hydraulic Performance properties
• Low compressibility (high bulk modulus)
• Fast air release

• Low foaming tendency
• Low volatility

Thermal performance
• Good thermal capacity and conductivity

Sealing properties
• Adequate viscosity and viscosity index
• Shear stability

Lubrication
• Sufficient viscosity for film maintenance
• Low temperature fluidity
• Thermal and oxidative stability
• Hydrolytic stability / water tolerance
• Cleanliness and filterability
• Demulsibility
• Wear reduction characteristics
• Corrosion control

Pump efficiency
• Proper viscosity to minimize internal leakage
• High viscosity index

Special requirements
• Fire resistance
• Friction modifiers
• Radiation resistance

Environmental impact properties
• Low toxicity when new or decomposed
• Biodegradability

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

Modern Hydraulic oils can contain a wide range of chemical compounds, including: mineral oils, butanol, esters (e.g. phthalates, like DEHP, and adipates), polyalkylene glycols (PAG), phosphate esters (e.g. tributylphosphate), silicones, alkylated aromatic hydrocarbons, polyalphaolefins (PAO) (e.g. polyisobutenes), corrosion inhibitors, etc.

For environmentally sensitive applications, such as farm tractors and marine dredging (where there is the risk of an oil spill from a ruptured oil line), biodegradable hydraulic oils based on rapeseed (Canola) vegetable oil can be used (often blended with customized synthetic esters). Typically these oils are available as ISO 32, ISO 46, and ISO 68 specification oils.

Other base stocks are used for speciality synthetic hydraulic oils, such as for fire resistance and extreme temperature applications. Some examples include: glycol, esters, organophosphate ester, polyalphaolefin, propylene glycol, and silicone oils (For Brake fluids).

Aircraft hydraulic oils
As aircraft performance increased, so the force required to operate mechanical flight controls increased, and hydraulic systems were introduced to reduce pilot effort. Hydraulic power is also used to start the auxiliary power unit (APU) for self-starting the aircraft's main engines. While many aircraft equipped with the M61 family of cannon, use hydraulic power to drive the gun system, permitting reliable high rates of fire.

Below are some of the more common aircraft Phosphate-ester based hydraulic oils.
• 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 optimise the product for specific conditions – such as Skydrol.

In future articles we’ll look at other aspects of hydraulic oils and their applications, so please subscribe to our RSS feed so as not to miss any of the information.