when  the  valve  is  open.  A  small  piston  (9)  is attached  to  the  bottom  of  the  spool  valve. When the valve is in the closed position, the top piston of the spool valve blocks the discharge port  (8).  With  the  valve  in  this  position,  fluid flowing from the actuating unit enters the inlet port (5). The fluid cannot flow through the valve because  discharge  port  8  is  blocked.  However, fluid will flow through the pilot passage (6) to the small pilot piston. As the pressure increases, it acts on the pilot piston until it overcomes the preset pressure of spring 3. This forces the valve spool (4)  up  and  allows  the  fluid  to  flow  around  the shaft  of  the  valve  spool  and  out  discharge  port 8.  Figure  6-20  shows  the  valve  in  this  position. During reverse flow, the fluid enters port 8. The spring   (3)   forces   valve   spool   4   to   the   closed position. The fluid pressure overcomes the spring tension  of  the  check  valve  (7).  The  check  valve opens  and  allows  free  flow  around  the  shaft  of the  valve  spool  and  out  through  port  5. The  operating  pressure  of  the  valve  can  be adjusted  by  turning  the  adjustment  screw  (1), which  increases  or  decreases  the  tension  of  the spring.  This  adjustment  depends  on  the  weight that  the  valve  must  support. It is normal for a small amount of fluid to leak around the top piston of the spool valve and into the  area  around  the  spring.  An  accumulation would  cause  additional  pressure  on  top  of  the spool   valve.    This   would   require   additional pressure  to  open  the  valve.  The  drain  (2)  provides a  passage  for  this  fluid  to  flow  to  port  8. DIRECTIONAL  CONTROL  VALVES Directional   control   valves   are   designed   to direct the flow of fluid, at the desired time, to the point  in  a  fluid  power  system  where  it  will  do work. The driving of a ram back and forth in its cylinder  is  an  example  of  when  a  directional control valve is used. Various other terms are used to  identify  directional  valves,  such  as  selector valve,  transfer  valve,  and  control  valve.  This manual  will  use  the  term  directional  control  valve to  identify  these  valves. Directional  control  valves  for  hydraulic and  pneumatic  systems  are  similar  in  design and  operation.  However,  there  is  one  major difference.  The  return  port  of  a  hydraulic  valve is ported through a return line to the reservoir, while  the  similar  port  of  a  pneumatic  valve, commonly  referred  to  as  the  exhaust  port,  is usually  vented  to  the  atmosphere.  Any  other differences  are  pointed  out  in  the  discussion  of the valves. Directional control valves may be operated by differences  in  pressure  acting  on  opposite  sides of the valving element, or they maybe positioned manually,  mechanically,  or  electrically.  Often  two or more methods of operating the same valve will be  used  in  different  phases  of  its  action. CLASSIFICATION Directional control valves may be classified in several ways. Some of the different ways are by the  type  of  control,  the  number  of  ports  in  the valve  housing,  and  the  specific  function  of  the valve.  The  most  common  method  is  by  the  type of valving element used in the construction of the valve.   The   most   common   types   of   valving elements  are  the  ball,  cone  or  sleeve,  poppet, rotary  spool,  and  sliding  spool.  The  basic operating  principles  of  the  poppet,  rotary  spool, and sliding spool valving elements are discussed in this text. Poppet The poppet fits into the center bore of the seat (fig. 6-21). The seating surfaces of the poppet and the  seat  are  lapped  or  closely  machined  so  that the center bore will be sealed when the poppet is Figure 6-21.—Operation of a simple poppet valve. 6-15