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 sub­ systems may be incorporated, with a separate directional control valve for each sub­ system. 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 by 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 (Figure 9-4, 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 con­ nected in series with each other, an arrangement in which the system pressure line goes through each of the directional control valves. Fluid is always allowed free pas­ sage through each valve and back to the reservoir until one of the control valves is positioned to operate a mechanism.

When one of the directional control valves is positioned to operate an actuating device, as shown in Figure 9-4, view B, 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 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 sys­ tem. One type is the manually engaged and manually disengaged directional control valve. 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 pres­ sure 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 sys­ tem pressure to drop to line resistance pressure.

Another type of open-center directional control valve is manually engaged and pres­ sure disengaged. This type of valve is similar to the valve discussed in the preceding paragraph, but when the actuating mechanism reaches the end of its cycle and the pressure continues to rise to a predetermined pressure, the valve automatically returns to the neutral, open-flow, position.

One advantage of the open-center system is that the continuous pressurization of the system is eliminated. Since the pressure is gradually built up after the directional con­ trol valve is moved to an operating position, there is very little shock from pressure surges. This provides a smooth operation of the actuating mechanisms. However, the operation is slower than in a closed-center system, where pressure is always available the moment the directional control valve is positioned. Since most applications require instantaneous operation, closed-center systems are the most widely used.

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