At least 75 percent of all hydraulic systems fail due to contaminated or aging hydraulic fluid.1 Contamination causes aging/degradation of fluids and hydraulic systems failure for numerous reasons. In addition to increasing internal leakage (which lowers the efficiency of pumps, motors and cylinders), contamination decreases the ability of valves to control flow and pressure, thus wasting horsepower and generating excess heat. Furthermore, it causes parts to stick due to sludge or silting, or seize when large amounts of contaminants accumulate in the clearances. Sources of contamination can include the manufacturing process, hydraulic fluids, environmental exposure, system wear and servicing.2

Many military systems rely on hydraulics; therefore these systems are used in every conceivable environment and are among the highest contributors to maintenance and service workloads. This ubiquity is a double-edged sword; the systems' continuous use makes them a common maintenance item, and the maintenance and diverse operational environments present many openings for contamination. This article presents some of the basics regarding hydraulic fluids and how they become contaminated, and expands upon the deleterious effects contamination has on hydraulic systems. Various causes of fluid contamination are also discussed, and the relatively simple measures that can be taken to resolve the problem.

While hydraulic system contaminants can be either solid particles or liquids, water is the most common. Solid particles contaminate by chemically reacting with the fluid, or fouling the system through accumulation. For instance, a new hydraulic fluid can contain 500,000 particles per 100 ml that are 5 µm or larger, which exceeds the level recommended by most manufacturers for normal operation.3 Unfortunately, 0.5 µm particles can be harmful to most systems. Due to the wide range of hydraulic fluids, many standards for cleanliness are available, which compounds the problem. For example, MIL-H- 5606F requires that in 100 ml of fluid there be no more than 10,000 particles ranging between five to 15 µm, no more than 1,000 particles between 16 to 25 µm, and no more than five particles over 100 µm.

The behavior of water as a contaminant varies from system to system. Water can form an emulsion with the fluid, or it may be partially immiscible, floating on the surface or settling to the bottom of the fluid depending on the relative density. The presence of water ultimately results in corrosion of the system components and resultant contamination of the fluid by corrosion products. Water ingress may be caused by design flaws, service environment, maintenance activities, internal generation and various methods of fluid servicing.

Improper storage of open fluid containers and inadequate transfer of fluid to the system may cause serious contamination problems. Contaminants also enter hydraulic systems through the reservoir breather (when used) and contaminated replacement components. Environmental contamination enters the system through ingestion, and can be prevented through the use of sealed reservoirs, vacuum breakers (relief valves) and/or high-efficiency breather filters (such as desiccators) in areas where humidity is high enough to become hazardous.5 Contamination from stored containers may be avoided through proper storage and handling procedures. Large containers are likely to include a high level of particles; therefore, fluid filtering is suggested prior to use.

In general, moisture is harmful to hydraulic systems, with the exception of aqueous-type hydraulic fluids and the systems designed to utilize them. Moisture enters waterproof containers through "breathing," which occurs when a container is exposed to wide temperature ranges. Therefore, containers stored outdoors should be placed on their sides to prevent water from accumulating on the tops. Furthermore, container lids should be periodically checked for tightness.

Hydraulic systems may even be degraded by the included additives designed to fight environmental attack and corrosion (through corrosion by galvanic action, or due to slime and ultimate malfunctioning). For example, an additive in certain MIL-H-5606 hydraulic fluids contains a contaminant that is soluble in the additive itself but is not soluble in the finished hydraulic fluids. Hydraulic fluids containing corrosion inhibitors will often form a slime when moisture is introduced. Some mahogany sulfonate rust inhibitors containing calcium chloride and calcium sulfate (inorganic water-soluble salts) cause degradation of hydraulic systems through staining and galvanic corrosion of steel.

Contamination with liquids, other than water, can occur with both miscible and immiscible fluids. This type of contamination takes place when one hydraulic fluid is replaced with another without thoroughly cleaning the system. Nevertheless, certain military specifications indicate the compatibility of two fluids and when they can be mixed. The two mixing fluids are compatible in the sense that together they do not form resinous gums, sludges or insoluble solid materials; however they are still contaminated because they do not preserve their individual performance properties in a mixture.

Contamination Effects
The effects of contamination typically begin with the hydraulic fluid, then are carried into the system, and can continuously worsen in a closed or sealed system. Additionally, contaminants such as water, other liquids, and even required chemicals (additives) can cause solids or slime to accumulate in the system, gradually reducing the performance of valves and other tight-tolerance components. These vicious cycles will continue to quietly rob a hydraulic system's performance, long after decontamination measures are taken.

Solid particles cause damage according to their size. High concentrations of small particles (£ 10 mm) form silt which erodes the interior mating surfaces of valves, rendering them inoperable. Contaminating solid particles that are equal in size to the clearance between two moving surfaces can cause both jamming and wear. Cycling the valve may clear this contamination, but often clearance can be achieved only by disassembly. Larger contaminating particles block ports and orifices, and can cause transient malfunction (coincident jamming) when trapped by a mechanism moving from one position to the next (for example, a particle trapped between a poppet and its seat prevents the closing of a relief valve).

System malfunctions caused by contamination may be classified into the following three categories:

  • Degradation (gradual changes over time in pump flow rates, valve leakage and wear of cylinder barrels causing cylinder speed decline, etc.)

  • Transient (intermittent failures)

  • Catastrophic (complete failure of a system or component)

All of these can be reduced through a well-planned contamination control program.

The sensitivity of the system to contamination is the basis of planning contaminant removal. The components are rated according to this sensitivity, also known as the omega rating. It is typically more efficient to use a filter dedicated to the entire system that meets the specifications of the most sensitive components.

Contamination Solutions
To solve the contamination problem, analysis is needed to identify the type and concentration of the contaminant. A comprehensive discussion on methods for contamination analysis is beyond the scope of this article. However, a few methods deserve to be recognized. Methods for gauging solid particle contamination involve counting, sizing and/or weighing the particles contained in a given volume of fluid. Microscopic particle counting is conducted according to standards such as ISO 4407, SAE ARP-5987 and ASTM F312. Automatic particle counting is conducted according to ISO 11500, ISO 4406:99 and ISO 11171, which calibrates and establishes reproducibility of optical particle counters.

Reducing Contamination
While contamination in hydraulic systems is a constant issue, there are measures that can be employed to lessen its impact. These measures include relatively simple handling and storage processes and paying careful attention to fluids and fluid combinations specified in systems. The long-term performance of critical systems depends on placing a renewed emphasis on maintenance and handling procedures that are often taken for granted.

Proper Fluid Selection
Maintainers must pay careful attention in selecting the proper fluid to use in a particular system, and it is critical to avoid mixing any fluids. Even when a system specifies that more than one fluid may be used, maintenance personnel should always try to use just one fluid, and not mix others. If a different fluid must be used, the system should be drained and flushed before adding the new fluid. Keep in mind that even if two fluids are miscible, the mixture may not perform with the ideal properties of either component separately. Also, the combination of some fluids and/or their additives may be harmful to the system.

Manufacturers and suppliers must be diligent in specifying appropriate fluids for a system. These specifications should be clear about potential incompatibilities if multiple fluids are used, and should recommend proper system maintenance procedures for all environmental conditions. Fluids should also be chosen with proper regard to environmental and hazardous materials regulations.

Proper Fluid Handling and Storage
Hydraulic fluids should be kept in their sealed containers until ready for use. Both new and partially used containers should be carefully handled and stored. Lids should be checked routinely and tightly secured, with drums being kept in a relatively benign environment. Large temperature swings cause the containers to "breathe" from expanding and contracting air and liquid inside. This, combined with moisture from rainfall or humidity, can cause the ingress of water into the container. Some fluids are hygroscopic, meaning they absorb moisture when exposed to humid air, thus requiring extra measures to be taken to reduce the chance of contamination.

System Maintenance
Hydraulic systems, because of their prevalence, are often in need of repair due to constant use. Standard operating procedures should be consulted for fluid contamination opportunities, and system maintenance procedures should be similarly diligent in keeping contaminants from systems. This can be accomplished through procedural changes identifying which fluids are appropriate for certain systems, and the proper handling of those fluids and systems. Additionally, procedures should be adjusted to indicate what actions should be taken to replace a fluid or change fluid types. In general, the system should be flushed of all old fluid and cleaned prior to the addition of the new or replacement fluid.

In the Future
Plans for future hydraulic fluids include research to reduce their fire risk and the development of more thermally stable hydraulic fluids. These will meet the challenges of increased pressures and electrohydrostatic actuation due to higher temperatures. Novel purification methods that would enable the reuse of hydraulic fluids are also under consideration. The continuation of laboratory simulations of the conditions that caused stuck valves in helicopters, hydraulic pump tests and toxicity are expected to be included in future programs.

Editor's Note:

This article originally appeared in the AMPTIAC Quarterly, Vol. 7 No. 1. The authors of the original article, Ms. Battat and Mr. Babcock, are former AMPTIAC staff members. Also, AMPTIAC (Advanced Materials and Processes Technology Information Analysis Center) is now part of AMMTIAC (Advanced Materials, Manufacturing, and Testing Information Analysis Center), which is operated by Alion Science and Technology.