Due to the unique nature of hydraulic oils, they are formulated differently from other oils. Typically, it follows the regular oil formulation of Base oil + additive to give the finished product. However, many various combinations occur depending on the application for which it is being formulated.
Base Oils Used in Hydraulic Oil
Similar to other oils, any base oil can be used to create hydraulic oils. However, depending on the application in which it is being used, the type of base oil will differ. The following table gives a summary of the types of base oils used for various hydraulic oils.
Additives in Hydraulic Oil
Generally, hydraulic oils are composed of 95-98% base oil and roughly 2-5% additives. One major distinction of hydraulic oils is that they can be either zinc-containing or zinc-free (also known as ashless). The ZnDTP molecule is responsible for antiwear properties, but this does not mean that zinc-free oils do not contain some form of antiwear additive.
Zinc-free oils are formulated with no zinc and minimal concentrations of phosphorus and sulphur. These zinc-free oils are formulated for special applications where the presence of zinc could react negatively with the environment, such as equipment containing mixed metals or silver.
There are other additives used for hydraulic oils, which are classified as either surface-active additives or base-oil-active additives.
Surface-active additives include steel/iron corrosion inhibitors, rust inhibitors, metal deactivators, wear inhibitors, friction modifiers, and detergents or dispersants.
Base active additives include: antioxidants, defoamers, VI Improvers, and pour point improvers.
Common Contaminants in Hydraulic Oil
Hydraulic systems are known for very tight clearances. As such, any form of physical contaminant can easily clog the valves or lines, leading to system failure. Keeping hydraulic oils clean is of paramount importance. Figure 6 illustrates typical clearances for hydraulic components, as well as the film thickness for various components.
The contaminants that exist in hydraulic systems can be either internally generated or externally consumed. Typically, dirt from external sources or metal wear (internally generated) form the major contaminants for hydraulic oils.
However, they are also susceptible to gaseous or liquid contaminants that can enter the system through system processes or external factors during oil handling before it enters the system. Figure 7 shows some cleanliness categories for various components.
