Notice that figure 12-3 does not indicate thephysical location of the individual componentswith respect to each other in the system. Forexample, the 3/4-inch, solenoid-operated, 4-wayvalve (10) is not necessarily located directly abovethe relief valve (26). The diagram does indicate,however, that the 4-way valve is located in theworking line, between the variable-displacementpump and the 1-inch rotary selector valve, andthat the valve directs fluid to and from the rotaryactuator.Combination DiagramsA combination drawing uses a combinationof graphic, cutaway, and pictorial symbols. Thisdrawing also includes all interconnecting piping.FLUID POWER SYSTEMSA fluid power system in which the fluid in thesystem remains pressurized from the pump (orregulator) to the directional control valve whilethe pump is operating is referred to as a closed-center system. In this type of system, any numberof subsystems may be incorporated, with aseparate directional control valve for eachsubsystem. The directional control valves arearranged in parallel so that system pressure actsequally on all control valves.Another type of system that is sometimes usedin hydraulically operated equipment is the open-center system. An open-center system has fluidflow but no internal pressure when the actuatingmechanisms are idle. The pump circulates the fluidfrom the reservoir, through the directional controlvalves, 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 directionalcontrol valves of an open-center system are alwaysconnected in series with each other, an arrange-ment in which the system pressure line goesthrough each directional control valve. Fluid isalways allowed free passage through each controlvalve 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 ispositioned to operate an actuating device, asshown in view B of figure 12-4, fluid is directedfrom the pump through one of the working linesto the actuator. With the control valve in thisposition, the flow of fluid through the valve tothe reservoir is blocked. Thus, the pressure buildsup in the system and moves the piston of theFigure 12-4.—Open-center hydraulic system.actuating cylinder. The fluid from the other endof the actuator returns to the control valvethrough the opposite working line and flows backto the reservoir.Several different types of directional controlvalves are used in the open-center system. Onetype is the manually engaged and manuallydisengaged. After this type of valve is manuallymoved to the operating position and the actuatingmechanism reaches the end of its operating cycle,pump output continues until the system reliefvalve setting is reached. The relief valve thenunseats and allows the fluid to flow back to thereservoir. The system pressure remains at thepressure setting of the relief valve until thedirectional control valve is manually returned tothe neutral position. This action reopens theopen-center flow and allows the system pressureto drop to line resistance pressure.Another type of open-center directionalcontrol valve is manually engaged and pressuredisengaged. This type of valve is similar to thevalve discussed in the preceding paragraph;however, when the actuating mechanism reachesthe end of its cycle and the pressure continues to12-5
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