Edited by Alan L. Hitchcox, editor

The demands placed on hydraulic systems constantly change as industry requires higher efficiency, speed, operating temperatures, and pressures. Selecting the best hydraulic fluid requires understanding each of particular fluid’s characteristics in comparison with an ideal fluid. An ideal fluid would have these characteristics:
• high thermal stability,
• high 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 single fluid has all of these ideal characteristics, it is possible to select one that is the best compromise for a particular hydraulic system. When making this selection, you should know such basic characteristics of the system as:
• maximum and minimum operating and ambient temperatures,
• type of pump or pumps used,
• operating pressures,
• operating cycle,
• loads encountered by various components, and
• type of control and power valves.

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, the hydraulic fluid must not be so viscous at low temperature that it does not readily flow.

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. The designer’s goal is to minimize metal breakdown through an additive that protects the metal. By comparison, friction is reduced by preventing metal-tometal contact through the use of fluids that create a thin protective film between moving metal parts.

Excessive wear may not be the fault of the fluid. It may be caused by poor system design, such as excessive pressure or inadequate cooling.

Anti-wear additive — A compound frequently added to hydraulic fluid to reduce wear is zinc dithiophosphate (ZDP), but today, ashless anti-wear hydraulic fluids have become popular with some companies and in certain states to reduce loads on waste treatment plants. No ZDP or other type heavy metals have been used in the formulation of ashless anti-wear fluids.

The pump is the critical dynamic element in a hydraulic system, and each pump type has different wear protection needs. Vane and gear pumps need anti-wear protection. Rust and oxidation (R&O) protection is more important in piston pumps. This is because gear and vane pumps operate with inherent metal-to-metal contact, whereas pistons ride on an oil film.

When two or more types of pumps are used in the same system, it is impractical to have a separate fluid for each, even though their operating requirements differ. The common fluid selected must meet the operating requirements of all pump types.

Foaming — When foam is carried by a fluid, it degrades system performance and, therefore, should be eliminated. Foam usually can be prevented by eliminating air leaks within the system. However, 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 the easiest to eliminate, with defoaming additives or by proper reservoir design so that foam entering the reservoir dissipates before it is drawn into the pump.

Entrained air can cause more serious problems because it is drawn into the system. In worst cases, it causes cavitation. Certain anti-foam agents, when used at a high concentration to reduce surface foam, can increase entrained air. Fluid viscosity is also linked to foam problems, because it influences how easily air bubbles can migrate through the fluid and escape.

R&O inhibitors — Most fluids need rust and oxidation inhibitors. These additives both protect the metal and contain anti-oxidation chemicals that help prolong fluid life. Two potential corrosion problems must be considered: system rusting and acidic chemical corrosion. System rusting occurs when water carried by the fluid attacks ferrous metal parts. Most hydraulic fluids contain rust inhibitors to protect against system rusting. The tests used to measure this capability are ASTM D 665 A and B. To protect against chemical corrosion, consider additives that exhibit good stability in the presence of water (hydrolytic stability) to prevent breakdown and acidic attack on system metals.

Oxidation and thermal stability — Over time, fluids oxidize and form decomposition products — acids, sludge, and varnish. Acids can attack system parts, particularly soft metals. Extended high-temperature operation and thermal cycling also encourage the formation of decomposition products. The system should be designed to minimize these thermal problems, and the fluid should contain additives that exhibit good thermal stability, resist oxidation, and neutralize acids as they form. Although not always practical or easy to attain, constant moderate temperature and steadystate operation are best for system and fluid life.

Water retention — Large quantities of water in a hydraulic fluid system can be removed by periodically draining the reservoir. However, small amounts of water can become entrained, particularly if the reservoir is small. Demulsifiers are often added to the fluid to speed the separation of water. Filters can then physically remove any remaining water from the fluid without taking fluid or additives with it.

Temperature — System operating temperature varies with job requirements. Here are a few general rules: maximum recommended operating temperature usually is 150°F. Operating temperatures of 180° to 200°F are practical, but fluid will have to be changed more often. Systems can operate at temperatures as high as 250°F, this can cause fairly rapid fluid decomposition and especially rapid additive decomposition — sometimes within days.

Seal compatibility — In most systems, seals are selected so that when they encounter the fluid they will not change size and expand only slightly, thus ensuring tight fits. The fluid selected should be checked to ensure compatibility with seal materials, so it will not interfere with proper seal operation.

Fluid life, disposability Fluid life and disposability are two important final considerations that do not directly relate to fluid performance in the hydraulic system, but have a great influence on total cost. Fluids with a long operating life bring added savings through reduced maintenance and replacement-fluid costs. The cost of changing a fluid can be substantial in a large system. Component life should also be longer with a higher-quality, longer-life fluid.

Longer fluid life also reduces disposal problems. With greater demands to keep the environment clean, and everchanging definitions of what is toxic, the problem of fluid disposal increases. Fluids and local anti-pollution laws should both be evaluated to determine any potential problems.

Synthesized hydrocarbon (synthetic) hydraulic fluids contain no waxes that congeal at low temperatures nor compounds that readily oxidize at high temperatures. Synthetic hydraulic fluids are being used for applications with very low, very high, or a very wide range of temperatures.

Much of this information was excerpted from the 2010/2011 Fluid Power Handbook & Directory, which will be published in January 2010.