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Demands placed on hydraulic systems constantly change with higher efficiency and speed needs at higher operating temperatures and pressures. Selecting the best hydraulic fluid requires an understanding of each fluid’s characteristics, including thermal stability, hydrolytic stability, low chemical corrosiveness, high anti-wear characteristics, low tendency to cavitate, long life, total water rejection, constant viscosity, regardless of temperature, and low cost.

Although no fluid is ideal, it is possible to select one that is the best compromise for a particular system. This requires knowledge of the system in which a fluid will be used. The designer should know system characteristics such as maximum and minimum operating and ambient temperatures, pumps used, operating pressures and cycle, loads encountered by all components, and control and power valves types.

Influential factors
Each of the following factors influences hydraulic fluid performance:

Viscosity — Maximum and minimum operating temperatures, along with the system’s load, determine the fluid’s viscosity requirements. The fluid must maintain a minimum viscosity at the highest operating temperature. However, it can not be so viscous at low temperature that it cannot be pumped.

Wear — Of all hydraulic system problems, wear is most frequently misunderstood because wear and friction usually are considered together. Friction should be considered apart from wear.

Wear results from metal-to-metal contact. Minimizing metal breakdown through a protective additive is critical. In contrast, friction is reduced by preventing metal-to-metal contact through the use of fluids that create a thin protective oil or additive film between moving metal parts. Excessive wear may not be the fault of the fluid. It may be caused by poor system design.

Anti-wear — Zinc dithio phosphate (ZDP) is common in hydraulic fluids but ashless anti-wear fluids are being used where the metal content in any wastewater from manufacturing processes must be minimized. No ZDP or other type heavy metals have been used in the formulation of ashless anti-wear fluids.

Different style pumps need different protection. Vane and gear pumps need anti-wear protection. Rust and oxidation (R & O) protection is more important in piston pumps because pistons ride on an oil film. When two or more types of pumps are used in a system, it is necessary to pick a common fluid that fulfills the needs of all pump types.

Foaming — When foam is carried by a fluid, it degrades system performance so needs to be eliminated. It can be prevented by eliminating air leaks within the system but two general types of foam still occur frequently — surface foam (which usually collects on the fluid surface in a reservoir), and entrained air.

Surface foam is easy to eliminate via defoaming additives or by designing the system so it dissipates in the reservoir. Entrained air can cause more serious problems because it is drawn into the system. It can cause cavitation, a hammering action that can destroy parts. It is often prevented by properly selecting additive and base oils. However, high-concentration anti-foam agents can increase entrained air.

Fluid viscosity also influences foaming because the more viscous a fluid, the longer it can take for air bubbles to migrate through the fluid and escape.

R & O — Most fluids need R & O inhibitors, which protect the metal and contain anti-oxidation chemicals that help prolong fluid life.

Corrosion — Two potential corrosion problems exist: system rusting and acidic chemical corrosion. System rusting occurs when water carried by the fluid attacks ferrous metal parts. Most fluids contain rust inhibitors to protect against this. To protect against chemical corrosion, additives that offer stability in the presence of water to prevent breakdown and acidic attack on system metals should be considered.

Oxidation and thermal stability — Over time, fluids oxidize and form acids, sludge, and varnish, which can damage system parts, like soft metals. Extended high-temperature operation and thermal cycling worsen the formation of fluid decomposition products. System design should minimize these thermal problems, and the fluid should have additives that exhibit good thermal stability, inhibit oxidation, and neutralize acids.

Water retention — Large amounts of water in hydraulic oil can be removed by regularly draining the reservoir. Small amounts of water can become entrained, especially if the reservoir is small. Demulsifiers are often added to the fluid to speed the separation of water. Filters can remove any remaining water from the hydraulic oil. Water should leave the oil without taking fluid or additives with it.

Temperature — System operating temperature varies, with the suggested maximum generally being 150°F. Temperatures of 180° to 200°F are practical, but fluid will have to be changed more often. Systems can operate up to 250°F, but this can cause rapid decomposition of the fluid and its additives.

Fluid makeup
Most fluids are evaluated based on their ratings for rust and oxidation (R & O), thermal stability, and wear protection, plus other characteristics that must be considered for efficient operation:

Seal compatibility — In most systems, seals are selected so the hydraulic fluid will not change their size or shape, ensuring tight fits. The fluid selected should be checked to ensure that the fluid and seal materials are compatible.

Fluid life, disposability — Two important considerations don’t directly relate to fluid performance in the hydraulic system but have a great influence on total cost: fluid life and disposability.

Fluids that have long operating lives bring added savings through reduced maintenance and replacement costs. Part life should also be longer with higher-quality, longer-life fluid. Longer fluid life also reduces disposal problems.

Demand for environmentally friendly and non-toxic fluids increases disposability issues. Review local environmental laws to determine potential problems.

Synthetic hydraulic fluids contain no waxes that congeal at low temperatures nor compounds that oxidize at high temperatures, which is inevitable in natural mineral oils. Synthetic fluids are used for applications with very low, very high, or a wide range of temperatures.

This article appeared in print as "Hydraulic fluids: the lifeblood of any system," in the May 2011 issue of Hydraulics & Pneumatics magazine.

Click here to view an in-depth tutorial article on hydraulic fluids from our Fluid Power Handbook & Directory. The article covers basic terminology, fire resistance, environmental issues, and more.