referred to as particulate contamination. Con-tamination is always present to some degree, evenin new, unused fluid, but must be kept below alevel that will adversely affect system operation.Hydraulic contamination control consists ofrequirements, techniques, and practices necessaryto minimize and control fluid contamination.CLASSIFICATIONThere are many types of contaminants whichare harmful to hydraulic systems and liquids.These contaminants may be divided into twodifferent classes—particulate and fluid.Particulate ContaminationThis class of contaminants includes organic,metallic solid, and inorganic solid contaminants.These contaminants are discussed in the followingparagraphs.ORGANIC CONTAMINATION.— Organicsolids or semisolids found in hydraulic systemsare produced by wear, oxidation, or polymeriza-tion. Minute particles of O-rings, seals, gaskets,and hoses are present, due to wear or chemicalreactions. Synthetic products, such as neoprene,silicones, and hypalon, though resistant tochemical reaction with hydraulic fluids, producesmall wear particles. Oxidation of hydraulic fluidsincreases with pressure and temperature, althoughantioxidants are blended into hydraulic fluids tominimize such oxidation. The ability of ahydraulic fluid to resist oxidation or poly-merization in service is defined as its oxidationstability. Oxidation products appear as organicacids, asphaltics, gums, and varnishes. Theseproducts combine with particles in the hydraulicfluid to form sludge. Some oxidation products areoil soluble and cause the hydraulic fluid toincrease in viscosity; other oxidation products arenot oil soluble and form sediment.METALLIC SOLID CONTAMINATION.—Metallic contaminants are almost always presentin a hydraulic system and will range in size frommicroscopic particles to particles readily visibleto the naked eye. These particles are the result ofwearing and scoring of bare metal parts andplating materials, such as silver and chromium.Although practically all metals commonly usedfor parts fabrication and plating may be foundin hydraulic fluids, the major metallic materialsfound are ferrous, aluminum, and chromiumparticles. Because of their continuous high-speedinternal movement, hydraulic pumps usuallycontribute most of the metallic particulatecontamination present in hydraulic systems. Metalparticles are also produced by other hydraulicsystem components, such as valves and actuators,due to body wear and the chipping and wearingaway of small pieces of metal plating materials.INORGANIC SOLID CONTAMINA-TION.— This contaminant group includes dust,paint particles, dirt, and silicates. Glass particlesfrom glass bead peening and blasting may alsobe found as contaminants. Glass particles are veryundesirable contaminants due to their abrasiveeffect on synthetic rubber seals and the very finesurfaces of critical moving parts. Atmosphericdust, dirt, paint particles, and other materials areoften drawn into hydraulic systems from externalsources. For example, the wet piston shaft of ahydraulic actuator may draw some of theseforeign materials into the cylinder past the wiperand dynamic seals, and the contaminant materialsare then dispersed in the hydraulic fluid.Contaminants may also enter the hydraulic fluidduring maintenance when tubing, hoses, fittings,and components are disconnected or replaced. Itis therefore important that all exposed fluid portsbe sealed with approved protective closures tominimize such contamination.Fluid ContaminationAir, water, solvent, and other foreign fluidsare in the class of fluid contaminants.AIR CONTAMINATION.— Hydraulic fluidsare adversely affected by dissolved, entrained, orfree air. Air may be introduced through impropermaintenance or as a result of system design. Anymaintenance operation that involves breaking intothe hydraulic system, such as disconnecting orremoving a line or component will invariablyresult in some air being introduced into thesystem. This source of air can and must beminimized by prebilling replacement componentswith new filtered fluid prior to their installation.Failing to prefill a filter element bowl with fluidis a good example of how air can be introducedinto the system. Although prebilling will minimizeintroduction of air, it is still important to vent thesystem where venting is possible.Most hydraulic systems have built-in sourcesof air. Leaky seals in gas-pressurized accumulatorsand reservoirs can feed gas into a system faster3-7
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