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Hydraulic Fluid Sampling - 14105_45
Rotary Pumps

Fluid Power - Intro to Hydraulics, Pneumatics, and how it all works
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CHAPTER  4 PUMPS Pumps are used for some essential services in the Navy. Pumps supply water to the boilers, draw condensation  from  the  condensers,  supply  sea water to the firemain, circulate cooling water for coolers and condensers, pump out bilges, transfer fuel,  supply  water  to  the  distilling  plants,  and serve many other purposes. Although the pumps discussed  in  this  chapter  are  used  primarily  in hydraulic  systems,  the  principles  of  operation apply  as  well  to  the  pumps  used  in  other  systems. PURPOSE The purpose of a hydraulic pump is to supply a flow of fluid to a hydraulic system. The pump does not create system pressure, since pressure can be created only by a resistance to the flow. As the pump  provides  flow,  it  transmits  a  force  to  the fluid.  As  the  fluid  flow  encounters  resistance,  this force  is  changed  into  a  pressure.  Resistance  to flow  is  the  result  of  a  restriction  or  obstruction in the path of the flow. This restriction is normally the work accomplished by the hydraulic system, but can also be restrictions of lines, fittings, and valves  within  the  system.  Thus,  the  pressure  is controlled by the load imposed on the system or the  action  of  a  pressure-regulating  device. OPERATION A  pump  must  have  a  continuous  supply  of fluid available to the inlet port to supply fluid to the system. As the pump forces fluid through the outlet  port,  a  partial  vacuum  or  low-pressure  area is created at the inlet port. When the pressure at the inlet port of the pump is lower than the local atmospheric  pressure,  atmospheric  pressure  acting on the fluid in the reservoir forces the fluid into the  pump’s  inlet.  If  the  pump  is  located  at a  level  lower  than  the  reservoir,  the  force  of gravity supplements atmospheric pressure on the reservoir.  Aircraft  and  missiles  that  operate  at high   altitudes   are   equipped   with   pressurized hydraulic   reservoirs   to   compensate   for   low atmospheric   pressure   encountered   at   high altitudes. PERFORMANCE Pumps are normally rated by their volumetric output  and  pressure.  Volumetric  output  is  the amount of fluid a pump can deliver to its outlet port in a certain period of time at a given speed. Volumetric output is usually expressed in gallons per minute (gpm). Since changes in pump speed affect  volumetric  output,  some  pumps  are  rated by their displacement. Pump displacement is the amount of fluid the pump can deliver per cycle. Since most pumps use a rotary drive, displacement is usually expressed in terms of cubic inches per revolution. As  we  stated  previously,  a  pump  does  not create  pressure.  However,  the  pressure  developed by the restrictions in the system is a factor that affects the volumetric output of the pump. As the system pressure increases, the volumetric output decreases. This drop in volumetric output is the result  of  an  increase  in  the  amount  of  internal leakage  from  the  outlet  side  to  the  inlet  side  of the  pump.  This  leakage  is  referred  to  as  pump slippage and is a factor that must be considered in all pumps. This explains why most pumps are rated  in  terms  of  volumetric  output  at  a  given pressure. CLASSIFICATION  OF  PUMPS Many  different  methods  are  used  to  classify pumps. Terms such as nonpositive  displacement, positive   displacement,   fixed   displacement, variable   displacement,   fixed   delivery,   variable delivery, constant volume,  and others are used to describe  pumps.  The  first  two  of  these  terms describe the fundamental division of pumps; that 4-1







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