in  a  constant,  uniform  flow  through  the  action of just three moving parts-a power rotor and two idler  rotors.  The  power  rotor  is  the  only  driven element,  extending  outside  the  pump  casing  for power  connections  to  an  electrical  motor.  The idler  rotors  are  turned  by  the  power  rotor  through the   action   of   the   meshing   threads.   The   fluid pumped  between  the  meshing  helical  threads  of the idler and power rotors provides a protective film to prevent metal-to-metal contact. The idler rotors  perform  no  work;  therefore,  they  do  not need to be connected by gears to transmit power. The  enclosures  formed  by  the  meshing  of  the rotors inside the close clearance housing contain the fluid being pumped. As the rotors turn, these enclosures  move  axially,  providing  a  continuous flow.  Effective  performance  is  based  on  the following   factors: 1. The rolling action obtained with the thread design  of  the  rotors  is  responsible  for  the  very quiet pump operation. The symmetrical pressure loading  around  the  power  rotor  eliminates  the need  for  radial  bearings  because  there  are  no radial  loads.  The  cartridge-type  ball  bearing  in  the pump  positions  the  power  rotor  for  proper  seal operation.  The  axial  loads  on  the  rotors  created by discharge pressure are hydraulically balanced. 2. The key to screw pump performance is the operation  of  the  idler  rotors  in  their  housing bores. The idler rotors generate a hydrodynamic film  to  support  themselves  in  their  bores  like journal bearings. Since this film is self-generated, it  depends  on  three  operating  characteristics  of the  pump—speed,  discharge  pressure,  and  fluid viscosity. The strength of the film is increased by increasing  the  operating  speed,  by  decreasing pressure, or by increasing the fluid viscosity. This is why screw pump performance capabilities are based  on  pump  speed,  discharge  pressure,  and fluid  viscosity. The supply line is connected at the center of the pump housing in some pumps (fig. 4-8, view B).  Fluid  enters  into  the  pump’s  suction  port, which  opens  into  chambers  at  the  ends  of  the screw assembly. As the screws turn, the fluid flows between the threads at each end of the assembly. The  threads  carry  the  fluid  along  within  the housing  toward  the  center  of  the  pump  to  the discharge  port. VANE  PUMP Vane-type  hydraulic  pumps  generally  have circularly or elliptically shaped interior and flat end  plates.  (Figure  4-9  illustrates  a  vane  pump with  a  circular  interior.)  A  slotted  rotor  is  fixed to a shaft that enters the housing cavity through one  of  the  end  plates.  A  number  of  small rectangular plates or vanes are set into the slots of the rotor. As the rotor turns, centrifugal force causes the outer edge of each vane to slide along the surface of the housing cavity as the vanes slide in   and   out   of   the   rotor   slots.   The   numerous cavities, formed by the vanes, the end plates, the housing, and the rotor, enlarge and shrink as the rotor and vane assembly rotates. An inlet port is installed in the housing so fluid may flow into the cavities  as  they  enlarge.  An  outlet  port  is  provided to  allow  the  fluid  to  flow  out  of  the  cavities  as they  become  small. The pump shown in figure 4-9 is referred to as   an   unbalanced   pump   because   all   of   the pumping  action  takes  place  on  one  side  of  the rotor. This causes a side load on the rotor. Some vane  pumps  are  constructed  with  an  elliptically shaped housing that forms two separate pumping areas on opposite sides of the rotor. This cancels out the side loads; such pumps are referred to as balanced  vane. Usually  vane  pumps  are  fixed  displacement and  pump  only  in  one  direction.  There  are, however,   some   designs   of   vane   pumps   that provide variable flow. Vane pumps are generally restricted to service where pressure demand does not  exceed  2000  psi.  Wear  rates,  vibration,  and noise  levels  increase  rapidly  in  vane  pumps  as pressure demands exceed 2000 psi. RECIPROCATING  PUMPS The term  reciprocating  is  defined  as  back-and- forth motion. In the reciprocating pump it is this Figure  4-9.—Vane  pump. 4-8