Similar to there being endless types of greases, there are also many types of hydraulic oils specifically designed for certain systems. Hydraulics comprise of lots of different operations as such, they will be called upon to perform in various applications. Some of these can include being fire resistant, biodegradable or even being able to also act as an engine oil. These properties can be influenced by the type of base oil used to produce these oils. For example, fire resistant or rapidly biodegradable fluids or even specialty hydraulic fluids can use PAOs (Polyalphaolefins), PAGs (Polyalkylglycols), POE (ester oils) or other synthetic oils as their base oil.
As per (Mang & Dresel, 2007), hydraulics require special types of additives for their applications. The most important additives for hydraulic oils are:
“Surface active additives” – For hydraulic oils these can be rust inhibitors, metal deactivators, wear inhibitors, friction modifiers, detergents / dispersants, etc.
“Base Oil active additives” – For hydraulic oils, these can be antioxidants, defoamers, VI Improvers, Pourpoint improvers, etc.
Typically, the additives for hydraulic oils can be broadly classed into those which contain zinc and ash and those which do not. Zinc and Ash free oils can represent 20-30% of hydraulic oils on the market and are used for specialty applications where the presence of zinc or ash can hamper the functionality of the equipment.
One such example is the use of these oils in the JCB Fastrac 3000 series for the hydraulic oils. These systems contain yellow metals which can be easily degraded with the presence of zinc or the filterability of the oil can be impacted due to the presence of water. Hence, zinc and ash free oils must be used in these instances.
The following shows a chart of the types of hydraulic fluids as per (Mang & Dresel, 2007) broken down by hydrokinetic applications, hydrostatic applications and mobile systems.
As seen above, there are many different classifications of hydraulic oils. To provide some clarification on the symbols used in DIN 51 502 and ISO 6743/4, (Mang & Dresel, 2007) produced this table.
When looking at hydraulic oil classifications, these categories will come up and it is important to be able to understand what each of these mean as well as how it translates to your system. Typically, the most common are the ISO HM and ISO HV.
The ISO HM refers to oils with improved anti-wear properties used in general hydraulic systems with highly loaded components and where there is a need for good water separation operating in the range of -20 to 90°C.
The ISO HV oils are HM oils with additives that improve viscosity-temperature behavior. Ideally, these are used in environments that experience significant changes in temperatures, such as construction or marine, between the ranges of -35 to 120°C.
