The original hydraulics fluid, dating back to the time of ancient Egypt, was water. Beginning in the 1920s, mineral oil began to be used more than water as a base stock due to its inherent lubrication properties and ability to be used at temperatures above the boiling point of water. Today most hydraulic fluids are based on mineral oil base stocks. Hydraulic oil is also known as hydraulic fluid.
Other base stocks are used for specialty applications, such as for fire resistance and extreme temperature applications. Some examples include: glycol, esters, organophosphate ester, polyalphaolefin, propylene glycol, and silicone oils.
Hydraulic fluids can contain a wide range of chemical compounds, including: oils, butanol, esters (e.g. phthalates, like DEHP, and adipates, like bis(2-ethylhexyl) adipate), polyalkylene glycols (PAG), organophosphate (e.g. tributylphosphate), silicones, alkylated aromatic hydrocarbons, polyalphaolefins (PAO) (e.g. polyisobutenes), corrosion inhibitors (incl acid scavengers), anti-erosion additives, etc.
Biodegradable hydraulic fluids:
Natural oils such as rapeseed (also called canola oil) are used as base stocks for fluids where biodegradability and renewable sources are considered important. Environmentally sensitive applications (e.g. farm tractors and marine dredging) may benefit from using biodegradable hydraulic fluids based upon rapeseed (Canola) vegetable oil when there is the risk of an oil spill from a ruptured oil line.
Typically these oils are available as ISO 32, ISO 46, and ISO 68 specification oils. Silverhook Biodegradable Hydraulic Oil
How do you know if you're using the right hydraulic oil?
For most lubricated machines, there are plenty of options when it comes to lubricant selection. Just because a machine will run with a particular product doesn't mean that product is optimal for the application. Most lubricant mis-specifications don't lead to sudden and catastrophic failure, but rather they shorten the average life of the lubricated components and, thus, go unnoticed.
With hydraulics, there are two primary considerations - the viscosity grade and the hydraulic oil type. These specifications are typically determined by the type of hydraulic pump employed in the system, operating temperature and the system's operating pressure.