valve,  around  the  main  valve  spring,  and  down to the return line. There is also a narrow passage (piston passage) through   the   main   valve   piston.   This   passage connects  the  high-pressure  line  to  the  valve chamber. The  pilot  valve  is  a  small,  ball-type,  spring- loaded check valve, which connects the top of the passage from the valve chamber with the passage through the main valve stem. The pilot valve is the  control  unit  of  the  relief  valve  because  the pressure   at   which   the   relief   valve   will   open depends on the tension of the pilot valve spring. The  pilot  valve  spring  tension  is  adjusted  by turning the adjusting screw so that the ball will unseat when system pressure reaches the preset limit. Fluid  at  line  pressure  flows  through  the narrow   piston   passage   to   fill   the   chamber. Because the line and the chamber are connected, the  pressure  in  both  are  equal.  The  top  and bottom  of  the  main  piston  have  equal  areas; therefore,   the   hydraulic   forces   acting   upward and downward are equal, and there is no tendency for   the   piston   to   move   in   either   direction. The  only  other  force  acting  on  the  main  valve is  that  of  the  main  valve  spring,  which  holds  it closed. When the pressure in the high-pressure line increases  to  the  point  at  which  the  pilot  valve is  set,  the  ball  unseats  (fig.  6-13,  view  B). This   opens   the   valve   chamber   through   the valve  stem  passage  to  the  low-pressure  return line. Fluid immediately begins to flow out of the chamber,  much  faster  than  it  can  flow  through the  narrow  piston  passage.  As  a  result  the chamber  pressure  immediately  drops,  and  the pilot  valve  begins  to  close  again,  restricting the   outward   flow   of   fluid.   Chamber   pressure therefore  increases,  the  valve  opens,  and  the  cycle repeats. So  far,  the  only  part  of  the  valve  that  has moved  appreciably  is  the  pilot,  which  functions just  like  any  other  simple  spring-loaded  relief valve.  Because  of  the  small  size  of  the  piston passage,  there  is  a  severe  limit  on  the  amount of  overpressure  protection  the  pilot  can  provide the  system.  All  the  pilot  valve  can  do  is  limit fluid  pressure  in  the  valve  chamber  above  the main  piston  to  a  preset  maximum  pressure, by   allowing   excess   fluid   to   flow   through   the piston  passage,  through  the  stem  passage,  and into the return line. When pressure in the system increases to a value that is above the flow capacity of   the   pilot   valve,   the   main   valve   opens, permitting  excess  fluid  to  flow  directly  to  the return line. This is accomplished in the following manner. As system pressure increases, the upward force on  the  main  piston  overcomes  the  downward force,  which  consists  of  the  tension  of  the  main piston spring and the pressure of the fluid in the valve chamber (fig. 6-13, view C). The piston then rises, unseating the stem, and allows the fluid to flow  from  the  system  pressure  line  directly  into the  return  line.  This  causes  system  pressure  to decrease  rapidly,  since  the  main  valve  is  designed to handle the complete output of the pump. When the  pressure  returns  to  normal,  the  pilot  spring forces the ball onto the seat. Pressures are equal above and below the main piston, and the main spring  forces  the  valve  to  seat. As  you  can  see,  the  compound  valve  over- comes  the  greatest  limitation  of  a  simple  relief valve by limiting the flow through the pilot valve to the quantity it can satisfactorily handle. This limits  the  pressure  above  the  main  valve  and enables the main line pressure to open the main valve. In this way, the system is relieved when an overload  exists. PRESSURE  REGULATORS Pressure   regulators,   often   referred   to   as unloading  valves,  are  used  in  fluid  power  systems to  regulate  pressure.  In  pneumatic  systems,  the valve,   commonly   referred   to   as   a   pressure regulator,  simply  reduces  pressure.  This  type  of valve  is  discussed  later  in  this  chapter  under pressure-reducing  valves.  In  hydraulic  systems  the pressure regulator is used to unload the pump and to maintain and regulate system pressure at the desired   values.   All   hydraulic   systems   do   not require  pressure  regulators.  The  open-center system (discussed in chapter 12) does not require a pressure regulator. Many systems are equipped with  variable-displacement  pumps  (discussed  in chapter  4),  which  contain  a  pressure-regulating device. Pressure regulators are made in a variety of types  and  by  various  manufacturers;  however,  the 6-9