NAVAIR 01-1A-17 and NSTM, chapter 556,provides information on low-lint wiping cloths.Rust or corrosion initially present in ahydraulic system can usually be traced toimproper storage of materials and componentparts. Particles can range in size from large flakesto abrasives of microscopic dimensions. Properpreservation of stored parts is helpful in elimi-nating corrosion.2. Particles introduced from outside sources.Particles can be introduced into hydraulic systemsat points where either the liquid or certain workingparts of the system (for example, piston rods) areat least in temporary contact with the atmosphere.The most common contaminant introductionareas are at the refill and breather openings,cylinder rod packings, and open lines wherecomponents are removed for repair or replace-ment. Contamination arising from carelessnessduring servicing operations is minimized by theuse of filters in the system fill lines and fingerstrainers in the filler adapter of hydraulicreservoirs.Hydraulic cylinder piston rodsincorporate wiper rings and dust seals to preventthe dust that settles on the piston rod during itsoutward stroke from entering the system when thepiston rod retracts. Caps and plugs are availableand should be used to seal off the open lines whena component is removed for repair orreplacement.3. Particles created within the system duringoperation. Contaminants created during systemoperation are of two general types—mechanicaland chemical. Particles of a mechanical nature areformed by wearing of parts in frictional contact,such as pumps, cylinders, and packing glandcomponents. These wear particles can vary fromlarge chunks of packings down to steel shavingsthat are too small to be trapped by filters.The major source of chemical contami-nants in hydraulic liquid is oxidation. Thesecontaminants are formed under high pressure andtemperatures and are promoted by the chemicalaction of water and air and of metals like copperand iron oxides. Liquid-oxidation products appearinitially as organic acids, asphaltines, gums,and varnishes—sometimes combined with dustparticles as sludge. Liquid-soluble oxidationproducts tend to increase liquid viscosity, whileinsoluble types separate and form sediments,especially on colder elements such as heatexchanger coils.Liquids containing antioxidants have littletendency to form gums and sludge under normaloperating conditions. However, as the tempera-ture increases, resistance to oxidation diminishes.Hydraulic liquids that have been subjected toexcessively high temperatures (above 250°F formost liquids) will break down, leaving minuteparticles of asphaltines suspended in the liquids.The liquid changes to brown in color and isreferred to as decomposed liquid. This explainsthe importance of keeping the hydraulic liquidtemperature below specific levels.The second contaminant-producing chemi-cal action in hydraulic liquids is one that permitsthese liquids to react with certain types of rubber.This reaction causes structural changes in therubber, turning it brittle, and finally causing itscomplete disintegration. For this reason, thecompatibility of system liquid with seals and hosematerial is a very important factor.4. Particles introduced by foreign liquids. Oneof the most common foreign-fluid contaminantsis water, especially in hydraulic systems thatrequire petroleum-based liquids. Water, whichenters even the most carefully designed system bycondensation of atmospheric moisture, normallysettles to the bottom of the reservoir. Oilmovement in the reservoir disperses the water intofine droplets, and agitation of the liquid inthe pump and in high-speed passages forms anoil-water-air emulsion. This emulsion normallyseparates during the rest period in the systemreservoir; but when fine dust and corrosionparticles are present, the emulsion is chemicallychanged by high pressures into sludge. Thedamaging action of sludge explains the need foreffective filtration, as well as the need for waterseparation qualities in hydraulic liquids.CONTAMINATION CONTROLMaintaining hydraulic fluid within allowablecontamination limits for both water and particu-late matter is crucial to the care and protectionof hydraulic equipment.Filters (discussed in chapter 9) will provideadequate control of the particular contaminationproblem during all normal hydraulic systemoperations if the filtration system is installedproperly and filter maintenance is performedproperly. Filter maintenance includes changingelements at proper intervals. Control of the sizeand amount of contamination entering the systemfrom any other source is the responsibility3-9
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