is, all pumps are either nonpositive displacement or  positive  displacement. Basically,  pumps  that  discharge  liquid  in  a continuous  flow  are  referred  to  as  nonpositive displacement,  and  those  that  discharge  volumes separated  by  a  period  of  no  discharge  are  referred to  as  positive  displacement. Although the nonpositive-displacement pump normally produces a continuous flow, it does not provide  a  positive  seal  against  slippage;  therefore, the output of the pump varies as system pressure varies.   In   other   words,   the   volume   of   fluid delivered for each cycle depends on the resistance to  the  flow.  This  type  of  pump  produces  a  force on  the  fluid  that  is  constant  for  each  particular speed  of  the  pump.  Resistance  in  the  discharge line  produces  a  force  in  a  direction  opposite  the direction  of  the  force  produced  by  the  pump. When these forces are equal, the fluid is in a state of  equilibrium  and  does  not  flow. If  the  outlet  of  a  nonpositive-displacement pump  is  completely  closed,  the  discharge  pressure will increase to the maximum for that particular pump   at   a   specific   speed.   Nothing   more   will happen except that the pump will churn the fluid and  produce  heat. In  contrast  to  the  nonpositive-displacement pump,  the  positive-displacement  pump  provides a positive internal seal against slippage. Therefore, this  type  of  pump  delivers  a  definite  volume  of fluid for each cycle of pump operation, regardless of  the  resistance  offered,  provided  the  capacity of  the  power  unit  driving  the  pump  is  not exceeded. If the outlet of a positive-displacement pump were completely closed, the pressure would instantaneously  increase  to  the  point  at  which  the unit driving the pump would stall or something would  break. Positive-displacement   pumps   are   further classified   as   fixed   displacement   or   variable displacement.   The   fixed-displacement   pump delivers the same amount of fluid on each cycle. The  output  volume  can  be  changed  only  by changing  the  speed  of  the  pump.  When  a  pump of  this  type  is  used  in  a  hydraulic  system,  a pressure   regulator   (unloading   valve)   must   be incorporated in the system. A pressure regulator or unloading valve is used in a hydraulic system to control the amount of pressure in the system and  to  unload  or  relieve  the  pump  when  the desired  pressure  is  reached.  This  action  of  a pressure regulator keeps the pump from working against  a  load  when  the  hydraulic  system  is  at maximum  pressure  and  not  functioning.  During this time the pressure regulator bypasses the fluid from  the  pump  back  to  the  reservoir.  (See  chapter 6   for   more   detailed   information   concerning pressure   regulators.)   The   pump   continues   to deliver a fixed volume of fluid during each cycle. Such terms as  fixed  delivery,  constant  delivery, and constant  volume  are all used to identify the fixed-displacement   pump. The   variable-displacement   pump   is   con- structed  so  that  the  displacement  per  cycle  can  be varied. The displacement is varied through the use of  an  internal  controlling  device.  Some  of  these controlling  devices  are  described  later  in  this chapter. Pumps may also be classified according to the specific  design  used  to  create  the  flow  of  fluid. Practically all hydraulic pumps fall within three design  classifications-centrifugal,  rotary,  and reciprocating.  The  use  of  centrifugal  pumps  in hydraulics is limited and will not be discussed in this text. ROTARY  PUMPS All  rotary  pumps  have  rotating  parts  which trap the fluid at the inlet (suction) port and force it  through  the  discharge  port  into  the  system. Gears,  screws,  lobes,  and  vanes  are  commonly used  to  move  the  fluid.  Rotary  pumps  are  positive displacement  of  the  fixed  displacement  type. Rotary  pumps  are  designed  with  very  small clearances between rotating parts and stationary parts  to  minimize  slippage  from  the  discharge side back to the suction side. They are designed to   operate   at   relatively   moderate   speeds. Operating  at  high  speeds  causes  erosion  and excessive  wear  which  results  in  increased clearances. There  are  numerous  types  of  rotary  pumps and various methods of classification. They may be  classified  by  the  shaft  position—either vertically  or  horizontally  mounted;  the  type  of drive—electric  motor,  gasoline  engine,  and  so forth; their manufacturer’s name; or their service application.   However,   classification   of   rotary pumps is generally made according to the type of rotating  element.  A  few  of  the  most  common types  of  rotary  pumps  are  discussed  in  the following   paragraphs. GEAR  PUMPS Gear pumps are classified as either external or internal gear pumps. In external gear pumps the  teeth  of  both  gears  project  outward  from  their 4-2