CHAPTER 10HYDROSTATIC AND HYDRAULIC MACHINESCHAPTER LEARNING OBJECTIVESUpon completion of this chapter, you will be able to do the following:lExplain the difference between hydrostatic and hydraulic liquids.lDiscuss the uses of hydrostatic machines.lDiscuss the uses of hydraulic machines.In this chapter we will discuss briefly the pressureof liquids: (1) hydrostatic (liquids at rest) and (2)hydraulic (liquids in motion). We will discuss theoperation of hydrostatic and hydraulic machines andgive applications for both types.HYDROSTATIC PRESSUREYou know that liquids exert pressure. The pressureexerted by seawater, or by any liquid at rest, is knownas hydrostatic pressure.If you are billeted on a submarine, you are moreconscious of the hydrostatic pressure of seawater. Whensubmerged, your submarine is squeezed from all sidesby this pressure. A deep-sea diving submarine must beable to withstand the terrific force of water at greatdepths. Therefore, the air pressure within it must beequal to the hydrostatic pressure surrounding it.PRINCIPLES OF HYDROSTATICPRESSUREIn chapter 9 you found out that all fluids exertpressure in all directions. That’s simple enough. Howgreat is the pressure? Try a little experiment. Place a pileof blocks in front of you on the table. Stick the tip ofyour finger under the first block from the top. Not muchpressure on your finger, is there? Stick it between thethird and fourth blocks. The pressure on your finger hasincreased. Now slide your finger under the bottom blockin the pile. There you will find the pressure is greatest.The pressure increases as you go lower in the pile. Youmight say that pressure increases with depth. The sameis true in liquids. The deeper you go, the greater thepressure becomes. However, depth isn’t the whole story.Suppose the blocks in the preceding paragraph weremade of lead. The pressure at any level in the pile wouldbe considerably greater. Or suppose they were blocks ofbalsa wood-then the pressure at each level wouldn’tbe as great. Pressure, then, depends not only on thedepth, but also on the weight of the material. Since youare dealing with pressure—force per unit of area, youwill also be dealing with weight per unit of volume-ordensity.When you talk about the density of a substance, youare talking about its weight per cubic foot or per cubicinch. For example, the density of water is 62.5 poundsper cubic foot; the density of lead is 710 pounds percubic foot. However, to say that lead is heavier thanwater isn’t a true statement. For instance, a 22-caliberbullet is the same density as a pail of water, but the pailof water is much heavier. It is true, however, that a cubicfoot of lead is much heavier than a cubic foot of water.Pressure depends on two principles-depth anddensity. You can easily find the pressure at any depth inany liquid by using the following formula:P = H x Din whichP = pressure, in lb per sq in. or lb per sq ftH = depth of the point, measured in feet or inchesandD = density in lb per cu in. or lb per cu ft10-1
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