Fluid Power Systems

Notice that figure 12-3 does not indicate the physical  location  of  the  individual  components with  respect  to  each  other  in  the  system.  For example,  the  3/4-inch,  solenoid-operated,  4-way valve  (10)  is  not  necessarily  located  directly  above the relief valve (26). The diagram does indicate, however,  that  the  4-way  valve  is  located  in  the working line, between the variable-displacement pump  and  the  1-inch  rotary  selector  valve,  and that the valve directs fluid to and from the rotary actuator. Combination  Diagrams A  combination  drawing  uses  a  combination of  graphic,  cutaway,  and  pictorial  symbols.  This drawing also includes all interconnecting piping. FLUID  POWER  SYSTEMS A fluid power system in which the fluid in the system  remains  pressurized  from  the  pump  (or regulator)  to  the  directional  control  valve  while the pump is operating is referred to as a closed- center  system.  In  this  type  of  system,  any  number of   subsystems   may   be   incorporated,   with   a separate  directional  control  valve  for  each subsystem.   The   directional   control   valves   are arranged in parallel so that system pressure acts equally  on  all  control  valves. Another  type  of  system  that  is  sometimes  used in hydraulically operated equipment is the open- center  system.  An  open-center  system  has  fluid flow but no internal pressure when the actuating mechanisms are idle. The pump circulates the fluid from  the  reservoir,  through  the  directional  control valves, and back to the reservoir. (See fig. 12-4, view A.) Like the closed-center system, the open- center  system  may  have  any  number  of  subsystems, with a directional control valve for each subsystem. Unlike  the  closed-center  system,  the  directional control valves of an open-center system are always connected in series with each other, an arrange- ment  in  which  the  system  pressure  line  goes through  each  directional  control  valve.  Fluid  is always allowed free passage through each control valve and back to the reservoir until one of the con- trol valves is positioned to operate a mechanism. When one of the directional control valves is positioned  to  operate  an  actuating  device,  as shown in view B of figure 12-4, fluid is directed from the pump through one of the working lines to  the  actuator.  With  the  control  valve  in  this position,  the  flow  of  fluid  through  the  valve  to the  reservoir  is  blocked.  Thus,  the  pressure  builds up  in  the  system  and  moves  the  piston  of  the Figure 12-4.—Open-center hydraulic system. actuating cylinder. The fluid from the other end of  the  actuator  returns  to  the  control  valve through the opposite working line and flows back to  the  reservoir. Several  different  types  of  directional  control valves  are  used  in  the  open-center  system.  One type   is   the   manually   engaged   and   manually disengaged. After this type of valve is manually moved  to  the  operating  position  and  the  actuating mechanism reaches the end of its operating cycle, pump  output  continues  until  the  system  relief valve  setting  is  reached.  The  relief  valve  then unseats and allows the fluid to flow back to the reservoir.  The  system  pressure  remains  at  the pressure   setting   of   the   relief   valve   until   the directional control valve is manually returned to the   neutral   position.   This   action   reopens   the open-center flow and allows the system pressure to  drop  to  line  resistance  pressure. Another  type  of  open-center  directional control valve is manually engaged and pressure disengaged.  This  type  of  valve  is  similar  to  the valve  discussed  in  the  preceding  paragraph; however, when the actuating mechanism reaches the end of its cycle and the pressure continues to 12-5


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