Additional information on the various typesof viscometers and their operation can be foundin the Physical Measurements Training Manual,NAVAIR 17-35QAL-2.Viscosity IndexThe viscosity index (V.I.) of an oil is a numberthat indicates the effect of temperature changeson the viscosity of the oil. A low V.I. signifiesa relatively large change of viscosity with changesof temperature. In other words, the oil becomesextremely thin at high temperatures and extremelythick at low temperatures. On the other hand, ahigh V.I. signifies relatively little change inviscosity over a wide temperature range.An ideal oil for most purposes is onethat maintains a constant viscosity throughouttemperature changes. The importance of the V.I.can be shown easily by considering automotivelubricants. An oil having a high V.I. resistsexcessive thickening when the engine is cold and,consequently, promotes rapid starting and promptcirculation; it resists excessive thinning when themotor is hot and thus provides full lubrication andprevents excessive oil consumption.Another example of the importance of the V.I.is the need for a high V.I. hydraulic oil for militaryaircraft, since hydraulic control systems may beexposed to temperatures ranging from below–65°F at high altitudes to over 100°F on theground. For the proper operation of the hydrauliccontrol system, the hydraulic fluid must have asufficiently high V.I. to perform its functions atthe extremes of the expected temperature range.Liquids with a high viscosity have a greaterresistance to heat than low viscosity liquids whichhave been derived from the same source. Theaverage hydraulic liquid has a relatively lowviscosity. Fortunately, there is a wide choice ofliquids available for use in the viscosity rangerequired of hydraulic liquids.The V.I. of an oil may be determined if itsviscosity at any two temperatures is known.Tables, based on a large number of tests, areissued by the American Society for Testingand Materials (ASTM). These tables permitcalculation of the V.I. from known viscosities.LUBRICATING POWERIf motion takes place between surfaces incontact, friction tends to oppose the motion.When pressure forces the liquid of a hydraulicsystem between the surfaces of moving parts, theliquid spreads out into a thin film which enablesthe parts to move more freely. Different liquids,including oils, vary greatly not only in theirlubricating ability but also in film strength. Filmstrength is the capability of a liquid to resist beingwiped or squeezed out from between the surfaceswhen spread out in an extremely thin layer. Aliquid will no longer lubricate if the film breaksdown, since the motion of part against part wipesthe metal clean of liquid.Lubricating power varies with temperaturechanges; therefore, the climatic and workingconditions must enter into the determination ofthe lubricating qualities of a liquid. Unlikeviscosity, which is a physical property, thelubricating power and film strength of a liquidis directly related to its chemical nature.Lubricating qualities and film strength can beimproved by the addition of certain chemicalagents.CHEMICAL STABILITYChemical stability is another property whichis exceedingly important in the selection of ahydraulic liquid. It is defined as the liquid’s abilityto resist oxidation and deterioration for longperiods. All liquids tend to undergo unfavorablechanges under severe operating conditions. Thisis the case, for example, when a system operatesfor a considerable period of time at hightemperatures.Excessive temperatures, especially extremelyhigh temperatures, have a great effect on the lifeof a liquid. The temperature of the liquid in thereservoir of an operating hydraulic system doesnot always indicate the operating conditionsthroughout the system. Localized hot spots occuron bearings, gear teeth, or at other points wherethe liquid under pressure is forced through smallorifices. Continuous passage of the liquid throughthese points may produce local temperatures highenough to carbonize the liquid or turn it intosludge, yet the liquid in the reservoir may notindicate an excessively high temperature.Liquids may break down if exposed to air,water, salt, or other impurities, especially if theyare in constant motion or subjected to heat. Somemetals, such as zinc, lead, brass, and copper, haveundesirable chemical reactions with certainliquids.These chemical reactions result in the forma-tion of sludge, gums, carbon, or other depositswhich clog openings, cause valves and pistons tostick or leak, and give poor lubrication to moving3-3
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