spool assembly consists of a valve spool (10) and a  spring  (4). Fluid under main pressure enters the inlet port (11)   and   under   all   conditions   is   free   to   flow through the valve and the outlet port (5). (Either port  5  or  port  11  maybe  used  as  the  high-pressure port.) Figure  6-19,  view  A,  shows  the  valve  in  the open  position.  In  this  position,  the  pressure  in  the reduced-pressure outlet port (6) has not reached the  preset  operating  pressure  of  the  valve.  The fluid also flows through passage 8, through smaller passage 9 in the center of the valve spool, and into chamber  12.  The  fluid  pressure  at  outlet  port  6 is therefore distributed to both ends of the spool. When these pressures are equal the spool is hydrau- lically balanced. Spring 4 is a low-tension spring and applies only a slight downward force on the spool.  Its  main  purpose  is  to  position  the  spool and to maintain opening 7 at its maximum size. As the pressure increases in outlet port 6 (fig. 16, view B), this pressure is transmitted through passages 8 and 9 to chamber 12. This pressure also acts  on  the  pilot  valve  poppet  (1).  When  this pressure  increases  above  the  preset  operating pressure  of  the  valve,  it  overcomes  the  force  of pilot valve spring 2 and unseats the poppet. This allows fluid to flow through the drain port (15). Because the small passage (9) restricts flow into chamber  12,  the  fluid  pressure  in  the  chamber drops.  This  causes  a  momentary  difference  in pressure across the valve spool (10) which allows fluid pressure acting against the bottom area of the valve spool to overcome the downward force of spring 4. The spool is then forced upward until the pressures across its ends are equalized. As the spool moves upward, it restricts the flow through opening 7 and causes the pressure to decrease in the  reduced  pressure  outlet  port  6.  If  the  pressure in the outlet port continues to increase to a value above the preset pressure, the pilot valve will open again and the cycle will repeat. This allows the spool valve to move up higher into chamber 12; thus   further   reducing   the   size   of   opening   7. These cycles repeat until the desired pressure is maintained in outlet 6. When the pressure in outlet 6 decreases to a value below the preset pressure, spring 4 forces the spool downward, allowing more fluid to flow through  opening  7. COUNTERBALANCE   VALVE The counterbalance valve is normally located in  the  line  between  a  directional  control  valve  and the  outlet  of  a  vertically  mounted  actuating cylinder which supports weight or must be held 6-14 in position for a period of time. This valve serves as a hydraulic resistance to the actuating cylinder. For  example,  counterbalance  valves  are  used  in some  hydraulically  operated  forklifts.  The  valve offers a resistance to the flow from the actuating cylinder  when  the  fork  is  lowered.  It  also  helps to  support  the  fork  in  the  UP  position. Counterbalance  valves  are  also  used  in  air- launched weapons loaders. In this case the valve is located in the top of the lift cylinder. The valve requires a specific pressure to lower the load. If adequate pressure is not available, the load cannot be lowered. This prevents collapse of the load due to  any  malfunction  of  the  hydraulic  system. One  type  of  counterbalance  valve  is  illustrated in  figure  6-20.  The  valve  element  is  a  balanced spool  (4).  The  spool  consists  of  two  pistons permanently fixed on either end of a shaft. The inner  surface  areas  of  the  pistons  are  equal; therefore,  pressure  acts  equally  on  both  areas regardless of the position of the valve and has no effect on the movement of the valve—hence, the term balanced.  The shaft area between the two pistons  provides  the  area  for  the  fluid  to  flow Figure 6-20.—Counterbalance valve.