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Guiding  Torpedoes
Principles of Hydraulic Pressure

Basic Machines - Intro to machines and motion theories
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and form bubbles in the veins. Any sudden release in the pressure on a fluid results in the freeing of some gases that are dissolved in the fluid. You have seen this happen when  you  suddenly  relieve  the  pressure  on  a  bottle  of pop  by  removing  the  cap.  The  careful  matching  of hydrostatic pressure on the diver by air pressure in the diving suit is essential if diving is to be done at all. Determining  Ship’s  Speed Did you ever wonder how the skipper knows the speed the ship is making through water? The skipper can get  this  information  by  using  several  instruments-the patent  log,  the  engine  revolution  counter,  and  the pitometer (pit) log. The “pit log” operates, in part, by hydrostatic   pressure.   It  really  shows  the  difference between  hydrostatic  pressure  and  the  pressure  of  the water  flowing  past  the  ship-but  this  difference  can  be used to find ship’s speed. Figure  10-4  shows  a  schematic  drawing  of  a pitometer log. It consists of a double-wall tube that sticks out forward of the ship’s hull into water that is not disturbed by the ship’s motion. In the tip of the tube is an opening (A). When the ship is moving, two forces or pressures are acting on this opening: (1) the hydrostatic pressure caused by the depth of the water above the opening and (2) a pressure caused by the push of the ship through  the  water.  The  total  pressure  from  these  two forces transmits through the central tube (shown in white on the figure) to the left-hand arm of a manometer. In the side of the tube is a second opening (B) that does not face the direction in which the ship is moving. Opening  B  passes  through  the  outer  wall  of  the double-wall tube, but not through the inner wall. The only pressure affecting opening B is the hydrostatic figure 10-4.-A pitometer log. pressure. This pressure transmits through the outer tube (shaded  in  the  drawing)  to  the  right-hand  arm  of  the manometer. When  the  ship  is  dead  in  the  water,  the  pressure through both openings A and B is the same, and the mercury in each arm of the manometer stands at the same level. However, as soon as the ship begins to move, additional pressure develops at opening A, and the mercury pushes down in the left-hand arm and up into the right-hand arm of the tube. The faster the ship goes, the  greater  this  additional  pressure  becomes,  and  the greater the difference will be between the levels of the mercury in the two arms of the manometer. You can read the speed of the ship directly from the calibrated scale on the manometer. Since air is also a fluid, the airspeed of an aircraft can be found by a similar device. You have probably seen the thin tube sticking out from the nose or the leading edge of a wing of the plane. Flyers call this tube a pitot tube. Its basic principle is the same as that of the pitometer  log. HYDRAULIC PRESSURE Perhaps  your  earliest  contact  with  hydraulic pressure  was  when  you  got  your  first  haircut.  The hairdresser put a board across the arms of the chair, sat you on it, and began to pump the chair up to a convenient level. As you grew older, you probably discovered that the gas station attendant could put a car on the greasing rack  and-by  some  mysterious  arrangement-jack  it head high. The attendant may have told you that oil under pressure below the piston was doing the job. Come to think about it, you’ve probably known something  about  hydraulics  for  a  long  time. Automobiles and airplanes use hydraulic brakes. As a sailor, you’ll have to operate many hydraulic machines. You’ll want to understand the basic principles on which they  work. Primitive  man  used  simple  machines  such  as  the lever, the inclined plane, the pulley, the wedge, and the wheel  and  axle.  It  was  considerably  later  before someone discovered that you could use liquids and gases to exert forces at a distance. Then, a vast number of new machines appeared. A machine that transmits forces by a liquid is a hydraulic machine. A variation of the hydraulic machine is the type that operates with a compressed gas. This type is known as the pneumatic machine. This chapter deals only with basic hydraulic machines. 10-4







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