Whether they're used in mobile or stationary equipment, hydraulic fluids give new meaning to the term multitasking. Hydraulic fluids must transmit power as fluid energy, lubricate components, cool the system by transferring heat, offer little resistance to flow, provide a viscous seal, withstand wide and frequent pressure cycles, minimize wear, reduce friction, prevent rust and corrosion, and keep system components deposit free.

That's a lot to ask, and no two fluids offer the same level of performance for every parameter across the board. Therefore, don't expect optimum performance and longevity from a hydraulic fluid unless you have discussed details of the specific application with your fluid supplier. Only then can the supplier recommend a fluid best suited to the application.

The big three
When discussing your specific needs with a supplier, many factors must be considered. However, three have the greatest impact on your selection.

Base oil used — Traditionally, hydraulic fluids have been produced from solvent neutral mineral oils (called Group I base oils). But the trend is toward the use of hydroprocessed and hydrocraked base oils (called Group II and Group III). As the group number increases, so does the cost. However, the higher base oil group number also provides longer life for equipment due to lower volatility, better oxidation and thermal resistance, and, sometimes, better demulsibility. In addition, the higher numbers contain less sulfur and have higher saturate levels, both of which are better for your equipment.

Additive package — Hydraulic fluids are made up of about 99% base oil and 1% or less of additives. The components of that 1% are critical to your operation, particularly when the trend is to use longer-life fluids. A variety of hydraulic fluid additive packages can be used in fluids, and using the right additive combination is critical. A fluid supplier can help identify what mix will provide the best performance for your equipment.

Factors that influence additive selection include its performance, compatibility, color, odor and economics. With the trend toward smaller hydraulic systems, the fluid stays in the system's reservoir for a shorter period of time. This reduces the time to release air from and cool the fluid, so the equipment tends to run hotter. In smaller systems, the additives in the fluid must work harder because they have less time to perform in the presence of contaminants, such as dirt, metal particles, and water.

Less time in the reservoir means that additives may be less effective at demulsifying the water that is inevitably contained in hydraulic fluid through condensation or leakage. Water in the fluid impacts fluid performance by plugging filters and resulting in corrosion and pump wear. Demulsifiers separate water from the fluid so the water can be drained from the system. Furthermore, corrosion inhibitors and anti-wear additives protect surfaces from contamination that can harm the equipment.

The additive package also needs to be concentrated enough to remain effective in case operating conditions become more severe over time, maintenance becomes overdue, or a mechanical problem occurs.

Viscosity — Assuming a fluid with the appropriate type of base oil and additive package is used, viscosity is the most important property to consider when choosing a specific fluid for an application. Viscosity is the strength of the cohesive force in the fluid. It determines the amount of fluid friction and the drag exerted by moving parts, which draws the fluid between metal surfaces. The viscosity of the fluid at the equipment's operating temperature determines the bearing friction, the rate at which the fluid will flow through a bearing and the load-carrying capacity of a bearing.

Fluid of the correct viscosity is readily distributed to moving surfaces — a key to long pump life. Fluid with lower than recommended viscosity can result in high internal pump leakage, which increases temperature. Conversely, fluid with higher than recommended viscosity will result in excessive pressure drop and, again, higher temperature. Following fluid recommendations of component manufacturers helps prevent these problems.