back-and-forth   motion   of   pistons   inside   of cylinders that provides the flow of fluid. Recipro- cating   pumps,   like   rotary   pumps,   operate   on the   positive   principle—that   is,   each   stroke delivers   a   definite   volume   of   liquid   to   the system. The master cylinder of the automobile brake system,   which   is   described   and   illustrated   in chapter 2, is an example of a simple reciprocating pump. Several types of power-operated hydraulic pumps,  such  as  the  radial  piston  and  axial  piston, are also classified as reciprocating pumps. These pumps are sometimes classified as rotary pumps, because a rotary motion is imparted to the pumps by   the   source   of   power.   However,   the   actual pumping is performed by sets of pistons recipro- cating  inside  sets  of  cylinders. HAND  PUMPS There  are  two  types  of  manually  operated reciprocating   pumps—the   single-action   and the   double-action.   The   single-action   pump provides flow during every other stroke, while the double-action  provides  flow  during  each  stroke. Single-action   pumps   are   frequently   used   in hydraulic  jacks. A  double-action  hand  pump  is  illustrated  in figure  4-10.  This  type  of  pump  is  used  in  some aircraft  hydraulic  systems  as  a  source  of  hydraulic power   for   emergencies,   for   testing   certain subsystems  during  preventive  maintenance inspections,  and  for  determining  the  causes  of malfunctions  in  these  subsystems. This  pump  (fig.  4-10)  consists  of  a  cylinder, a  piston  containing  a  built-in  check  valve  (A),  a piston  rod,  an  operating  handle,  and  a  check  valve (B)  at  the  inlet  port.  When  the  piston  is  moved Figure 4-10.—Hydraulic hand pump. to  the  left,  the  force  of  the  liquid  in  the  outlet chamber and spring tension cause valve A to close. This  movement  causes  the  piston  to  force  the liquid  in  the  outlet  chamber  through  the  outlet port  and  into  the  system.  This  same  piston movement causes a low-pressure area in the inlet chamber. The difference in pressure between the inlet  chamber  and  the  liquid  (at  atmospheric pressure)  in  the  reservior  acting  on  check  valve B causes its spring to compress; thus, opening the check valve. This allows liquid to enter the inlet chamber. When the piston completes this stroke to the left,   the   inlet   chamber   is   full   of   liquid.   This eliminates  the  pressure  difference  between  the  inlet chamber  and  the  reservior,  thereby  allowing spring  tension  to  close  check  valve  B. When  the  piston  is  moved  to  the  right,  the force of the confined liquid in the inlet chamber acts  on  check  valve  A.  This  action  compresses the   spring   and   opens   check   valve   A   which allows   the   liquid   to   flow   from   the   intake chamber  to  the  outlet  chamber.  Because  of  the area  occupied  by  the  piston  rod,  the  outlet chamber cannot contain all the liquid discharged from   the   inlet   chamber.   Since   liquids   do   not compress,  the  extra  liquid  is  forced  out  of  the outlet  port  into  the  system. PISTON  PUMPS Piston   pumps   are   made   in   a   variety   of types   and   configurations.   A   basic   distinction is  made  between  axial  and  radial  pumps.  The axial   piston   pump   has   the   cylinders   parallel to  each  other  and  the  drive  shaft.  The  radial piston   design   has   the   cylinders   extending radially   outward   from   the   drive   shaft   like the  spokes  of  a  wheel.  A  further  distinction is  made  between  pumps  that  provide  a  fixed delivery  and  those  able  to  vary  the  flow  of  the fluid.  Variable  delivery  pumps  can  be  further divided  into  those  able  to  pump  fluid  from  zero to full delivery in one direction of flow and those able to pump from zero the full delivery in either direction. All  piston  pumps  used  in  Navy  shipboard systems  have  the  cylinders  bored  in  a  cylinder block  that  is  mounted  on  bearings  within  a housing.  This  cylinder  block  assembly  rotates  with the  pump  drive  shaft. 4-9