- Buyer's Guide
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.Contaminants
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
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
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
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.