parts. Once a small amount of sludge or otherdeposits is formed, the rate of formation generallyincreases more rapidly. As these deposits areformed, certain changes in the physical andchemical properties of the liquid take place. Theliquid usually becomes darker, the viscosityincreases and damaging acids are formed.The extent to which changes occur in differentliquids depends on the type of liquid, type ofrefining, and whether it has been treated toprovide further resistance to oxidation. Thestability of liquids can be improved by theaddition of oxidation inhibitors. Inhibitorsselected to improve stability must be compatiblewith the other required properties of the liquid.FREEDOM FROM ACIDITYAn ideal hydraulic liquid should be free fromacids which cause corrosion of the metals in thesystem. Most liquids cannot be expected to remaincompletely noncorrosive under severe operatingconditions. The degree of acidity of a liquid, whennew, may be satisfactory; but after use, the liquidmay tend to become corrosive as it begins todeteriorate.Many systems are idle for long periods afteroperating at high temperatures. This permitsmoisture to condense in the system, resulting inrust formation.Certain corrosion- and rust-preventive addi-tives are added to hydraulic liquids. Some of theseadditives are effective only for a limited period.Therefore, the best procedure is to use the liquidspecified for the system for the time specified bythe system manufacturer and to protect the liquidand the system as much as possible fromcontamination by foreign matter, from abnormaltemperatures, and from misuse.FLASHPOINTFlashpoint is the temperature at which a liquidgives off vapor in sufficient quantity to ignitemomentarily or flash when a flame is applied. Ahigh flashpoint is desirable for hydraulic liquidsbecause it provides good resistance to combustionand a low degree of evaporation at normaltemperatures. Required flashpoint minimumsvary from 300°F for the lightest oils to 510°F forthe heaviest oils.FIRE POINTFire point is the temperature at which asubstance gives off vapor in sufficient quantityto ignite and continue to burn when exposed toa spark or flame. Like flashpoint, a high fire pointis required of desirable hydraulic liquids.MINIMUM TOXICITYToxicity is defined as the quality, state, ordegree of being toxic or poisonous. Some liquidscontain chemicals that are a serious toxic hazard.These toxic or poisonous chemicals may enter thebody through inhalation, by absorption throughthe skin, or through the eyes or the mouth. Theresult is sickness and, in some cases, death.Manufacturers of hydraulic liquids strive toproduce suitable liquids that contain no toxicchemicals and, as a result, most hydraulic liquidsare free of harmful chemicals. Some fire-resistantliquids are toxic, and suitable protection and carein handling must be provided.DENSITY AND COMPRESSIBILITYA fluid with a specific gravity of less than 1.0is desired when weight is critical, although withproper system design, a fluid with a specificgravity greater than one can be tolerated. Whereavoidance of detection by military units is desired,a fluid which sinks rather than rises to the surfaceof the water is desirable. Fluids having a specificgravity greater than 1.0 are desired, as leakingfluid will sink, allowing the vessel with the leakto remain undetected.Recall from chapter 2 that under extremepressure a fluid may be compressed up to 7percent of its original volume. Highly com-pressible fluids produce sluggish system operation.This does not present a serious problem in small,low-speed operations, but it must be consideredin the operating instructions.FOAMING TENDENCIESFoam is an emulsion of gas bubbles in thefluid. Foam in a hydraulic system results fromcompressed gases in the hydraulic fluid. A fluidunder high pressure can contain a large volumeof air bubbles. When this fluid is depressurized,as when it reaches the reservoir, the gas bubblesin the fluid expand and produce foam. Anyamount of foaming may cause pump cavitationand produce poor system response and spongy3-4
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