· Thermal stability
· Hydrolytic stability
· Low chemical corrosiveness
· High anti-wear characteristics
· Low tendency to cavitate
· Long life
· Total water rejection
· Constant viscosity that is unaffected by temperature
· Cost effective
Factors that influence the performance of the hydraulic fluid
Viscosity – is the most important characteristic of hydraulic fluids and it has the most impact on the hydraulic system. Viscosity is the measure of the fluid’s resistance to flow. In order to consistently and effectively transmit power, the fluid must be able to retain optimum viscosity when operating in cold or hot temperature.
Compressibility – increases with pressure and temperature which can have a significant effect on high pressure fluid systems. The amount of volume reduction during pressure can cause servo failure, loss of efficiency and cavitation which makes it important for hydraulic fluids to have low compressibility.
Wear resistance – wear is unavoidable because of metal-to-metal contact but this can be reduced through the use of hydraulic fluids with anti-wear components to form a protective film on metal surfaces to prevent abrasion, scuffing and contact fatigue. In some circumstances, wear is not due to fluid but poor system design such as excessive pressure and inadequate cooling.
Oxidation and thermal stability – is the fluids resistance to heat-induced degradation caused by a chemical reaction with oxygen. Fluids can oxidize and cause the formation of acids, sludge and varnish. Hydraulic oils must have additives that can counteract the process of oxidation for improved stability and longer life and should be formulated with very high levels of thermal stability to help minimize these issues.
Filterability – contaminants can precipitate from the lubricant and block filters, valves and other components that can decrease oil flow or the system can go on bypass. Hydraulic fluids are designed to be filtered with modern filtration systems to enable the system to stay clean without sacrificing critical performance that can lead to serious downtime.
Rust and corrosion protection – water enters the system in the form of condensation and it can cause rusting of the hydraulic components or react with some additives to form chemical species that can be aggressive to yellow metals. Hydraulic fluids must be formulated with rust and corrosion inhibitors that will prevent the interaction of water and prevent it from attacking metal surfaces.
Foaming – when foam is carried by a fluid, it can degrade system performance and should be eliminated. Foam is prevented by eliminating air leaks from the system. Surface foam can be eliminated with de-foaming additives or proper foam inhibitors that will modify surface tension on air bubbles so they can break more easily.
There are two other major considerations that include fluid life and disposability for added savings through reduced maintenance and replacement-fluid costs.