CHAPTER 3HYDRAULIC FLUIDSDuring the design of equipment that requiresfluid power, many factors are considered inselecting the type of system to be used—hydraulic,pneumatic, or a combination of the two. Someof the factors are required speed and accuracy ofoperation, surrounding atmospheric conditions,economic conditions, availability of replacementfluid, required pressure level, operating tempera-ture range, contamination possibilities, cost oftransmission lines, limitations of the equipment,lubricity, safety to the operators, and expectedservice life of the equipment.After the type of system has been selected,many of these same factors must be consideredin selecting the fluid for the system. This chapteris devoted to hydraulic fluids. Included in it aresections on the properties and characteristicsdesired of hydraulic fluids; types of hydraulicfluids; hazards and safety precautions for workingwith, handling, and disposing of hydraulicliquids; types and control of contamination; andsampling.PROPERTIESIf fluidity (the physical property of a substancethat enables it to flow) and incompressibility werethe only properties required, any liquid not toothick might be used in a hydraulic system.However, a satisfactory liquid for a particularsystem must possess a number of other properties.The most important properties and some charac-teristics are discussed in the following paragraphs.VISCOSITYViscosity is one of the most importantproperties of hydraulic fluids. It is a measure ofa fluid’s resistance to flow. A liquid, such asgasoline, which flows easily has a low viscosity;and a liquid, such as tar, which flows slowly hasa high viscosity. The viscosity of a liquid isaffected by changes in temperature and pressure.As the temperature of a liquid increases, itsviscosity decreases. That is, a liquid flows moreeasily when it is hot than when it is cold. Theviscosity of a liquid increases as the pressure onthe liquid increases.A satisfactory liquid for a hydraulic systemmust be thick enough to give a good seal atpumps, motors, valves, and so on. These com-ponents depend on close fits for creating andmaintaining pressure. Any internal leakagethrough these clearances results in loss of pressure,instantaneous control, and pump efficiency.Leakage losses are greater with thinner liquids(low viscosity). A liquid that is too thin will alsoallow rapid wearing of moving parts, or of partsthat operate under heavy loads. On the otherhand, if the liquid is too thick (viscosity too high),the internal friction of the liquid will cause anincrease in the liquid’s flow resistance throughclearances of closely fitted parts, lines, andinternal passages. This results in pressure dropsthroughout the system, sluggish operationof the equipment, and an increase in powerconsumption.Measurement of ViscosityViscosity is normally determined by measuringthe time required for a fixed volume of a fluid(at a given temperature) to flow through acalibrated orifice or capillary tube. The instru-ments used to measure the viscosity of a liquidare known as viscometers or viscosimeters.Several types of viscosimeters are in use today.The Saybolt viscometer, shown in figure 3-1,measures the time required, in seconds, for 60milliliters of the tested fluid at 100°F to passthrough a standard orifice. The time measured is3-1
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