An open-center system is one having fluid flow, but no pressure in the system whenever the actuating mechanisms are idle. Fluid circulates from the reservoir, through the pump, through the selector valves, and back to the reservoir. Pressure developed in the system of an open-center system is controlled by open-center selector valves and is limited by a system relief valve. Figure 10-36 shows an open-center system. Note the position of the selector valves and the fact that the valves are connected in series. In this type of system, there is no pressure in the system until one of the subsystems is actuated by the positioning of the selector valve. When in the neutral position (fig. 10-36, view A), the open-center selector valve directs the fluid to the return line. When the selector valve is positioned out of neutral, pressure builds up in the actuating section and operates the selected mechanism (fig. 10-36, view B). When an open-center system is not being used (no actuating mechanisms), the pump is said to be idling because there is no pressure buildup in the system; therefore, there is no load on the pump. Constant volume pumps are used in open-center systems and normally do not require a pressure regulator.
The closed-center system always has fluid stored under pressure whenever the pump is operating; however, when pressure is built up to predetermined value, the load is automatically removed from the pump by a pressure regulator or the integral control valve of the variable volume pump.
The representative hydraulic system discussed earlier is a closed-center system, but all closed-center systems are basically the same. Any number of subsystems may be incorporated into the closed-center system. This system differs from the open-center system in that the selector valves are arranged in parallel rather than in series.
Every hydraulic system has two major parts or sections: the power section and the actuating section. A power section develops, limits, and directs the fluid pressures that actuate various mechanisms on the equipment. The actuating section is the section
Figure 10-36.-Basic open-center hydraulic system.Continue Reading