than it can be removed, even with the best ofmaintenance. Another lesser known but majorsource of air is air that is sucked into the systempast actuator piston rod seals. This usually occurswhen the piston rod is stroked by some externalmeans while the actuator itself is not pressurized.WATER CONTAMINATION.— Water is aserious contaminant of hydraulic systems.Hydraulic fluids are adversely affected bydissolved, emulsified, or free water. Watercontamination may result in the formation of ice,which impedes the operation of valves, actuators,and other moving parts. Water can also cause theformation of oxidation products and corrosionof metallic surfaces.SOLVENT CONTAMINATION.— Solventcontamination is a special form of foreign fluidcontamination in which the original contami-nating substance is a chlorinated solvent. Chlori-nated solvents or their residues may, whenintroduced into a hydraulic system, react with anywater present to form highly corrosive acids.Chlorinated solvents, when allowed to com-bine with minute amounts of water often foundin operating hydraulic systems, change chemicallyinto hydrochloric acids. These acids then attackinternal metallic surfaces in the system,particularly those that are ferrous, and producea severe rust-like corrosion. NAVAIR 01-1A-17and NSTM, chapter 556, contain tables ofsolvents for use in hydraulic maintenance.FOREIGN-FLUIDS CONTAMINATION.—Hydraulic systems can be seriously contaminatedby foreign fluids other than water and chlorinatedsolvents. This type of contamination is generallya result of lube oil, engine fuel, or incorrecthydraulic fluid being introduced inadvertently intothe system during servicing. The effects of suchcontamination depend on the contaminant, theamount in the system, and how long it has beenpresent.NOTE: It is extremely important that thedifferent types of hydraulic fluids are not mixedin one system. If different type hydraulic fluidsare mixed, the characteristics of the fluid requiredfor a specific purpose are lost. Mixing thedifferent types of fluids usually will result in aheavy, gummy deposit that will clog passages andrequire a major cleaning. In addition, seals andpacking installed for use with one fluid usuallyare not compatible with other fluids and damageto the seals will result.ORIGIN OF CONTAMINATIONRecall that contaminants are produced fromwear and chemical reactions, introduced byimproper maintenance, and inadvertently intro-duced during servicing. These methods of con-taminant introduction fall into one of the fourmajor areas of contaminant origin.1. Particles originally contained in the system.These particles originate during the fabricationand storage of system components. Weld spatterand slag may remain in welded system com-ponents, especially in reservoirs and pipeassemblies. The presence is minimized by properdesign. For example, seam-welded overlappingjoints are preferred, and arc welding of opensections is usually avoided. Hidden passages invalve bodies, inaccessible to sand blasting or othermethods of cleaning, are the main source ofintroduction of core sand. Even the most carefullydesigned and cleaned casting will almost invari-ably free some sand particles under the action ofhydraulic pressure. Rubber hose assemblies alwayscontain some loose particles. Most of theseparticles can be removed by flushing the hosebefore installation; however, some particleswithstand cleaning and are freed later by theaction of hydraulic pressure.Particles of lint from cleaning rags cancause abrasive damage in hydraulic systems,especially to closely fitted moving parts. Inaddition, lint in a hydraulic system packs easilyinto clearances between packing and contactingsurfaces, leading to component leakage anddecreased efficiency. Lint also helps clog filtersprematurely. The use of the proper wipingmaterials will reduce or eliminate lint contamina-tion. The wiping materials to be used for a givenapplication will be determined bya.b.c.substances being wiped or absorbed,the amount of absorbency required,and/orthe required degree of cleanliness.These wiping materials are categorized forcontamination control by the degree of lint ordebris that they may deposit during use. Forinternal hydraulic repairs, this factor itselfwill determine the choice of wiping material.3-8
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business