Have you ever driven through a bad puddle of water only to have your miserable engine lock solid after ingesting a cylinder full? This is known as hydraulic lock and is a costly illustration of hydraulic power which hydraulic oils convey.
In this commentary I’ll be focusing on hydraulic fluids as opposed to brake fluids, which are a very specialized sub grouping.
Functions and properties
The crucial function of hydraulic oils is to convey power. Then again, there are other significant functions of present hydraulic fluids.
The table below lists the chief 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 foaming tendency
• Low volatility
• Good thermal function and conductivity
• Adequate 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
• Wear reduction characteristics
• Corrosion control
• Appropriate viscosity to curtail internal leakage
• Superior viscosity index
• Fire resistance
• Friction modifiers
• Radiation resistance
Environmental impact properties
• Low toxicity when new or decomposed
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 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.
Current hydraulic oils can contain a wide range of chemical compounds, as well as: 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 hypersensitive applications, such as farm tractors and marine dredging (where there is the consequence of an oil spill from a ruptured oil line), biodegradable hydraulic oils based on rapeseed (Canola) vegetable oil can be used (often blended with tailored synthetic esters). Typically these oils are available as ISO 32, ISO 46, and ISO 68 specification oils
Other base stocks are used for specialty applications, such as for fire resistance and extreme temperature applications. Some examples embrace: glycol, esters, organophosphate ester, polyalphaolefin, propylene glycol, and silicone oils (For Brake fluids).
Aircraft hydraulic systems
As aircraft performance improved, so the force required to activate mechanical flight controls increased, and hydraulic systems were introduced to ease 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 family of cannon, exploit 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 optimize the product for precise circumstances – such as Skydrol.
With the rising costs of crude based oils synthetic hydraulic oils are increasingly gaining favor for their superior performance. Whereas there’s always been a marked difference in pricing the unit costs for mineral and synthetic hydraulic oils are now very similar, but with synthetic hydraulic oils outlasting the mineral equivalents real cost savings are achieved