ANSI symbols are introduced here as they are needed. The directional control valve symbol is the most intuitive and selfexplanatory of the symbols. Some experience with the directional control valve symbol has already been gained. At this point, it is necessary to review the three most common center configurations (Fig. 3.1) for spool-type directional control valves.
An open-center (float) valve allows flow between all four ports when the valve is in the center (nonactuated) position. The actuator (downstream from the valve) is not held in position but is free to float.
The open-center valve also allows free flow from the inlet port to the return (or tank) port, but it blocks the actuator ports. The actuator cannot move (neglecting leakage) when the open-center valve is in the center position.
The closed-center valve has all four ports blocked when it is in the center position. There is no pathway through the valve between any of the four ports.
There are many configurations for directional control valves. Two are shown in Figs. 3.2 and 3.3. The check valves ensure that flow can go only from the pump to the circuit. Thus, the pump is isolated from pressure spikes that may occur due to load dynamics. Both figures show two directional control valves stacked in the same housing. Ten or more valves can be stacked in this manner. (The reader may have observed a bank of handles on a machine for manual actuation of individual valves stacked in this manner.) As is often the case, the figures show a relief valve built into the valve housing, a feature which simplifies circuit assembly.
Figure 3.2 shows a valve stack where the flow passes directly through the valve and returns to the reservoir when neither spool is actuated. The bottom half of this figure shows both spools activated simultaneously. If the pressure required by both cylinders is approximately equal, the pump flow will divide, and some flow will go to each cylinder. However, if one cylinder requires more pressure, the flow will always take the path of least resistance and goes to the lower-pressure cylinder first. When this cylinder is fully extended or hits a stop, the pressure will rise to the level required to extend the second cylinder.
The valves in Fig. 3.3 are still open-center valves, but they are configured differently. Both spools are shown in the actuated position in the bottom half of the figure. Flow goes to Cylinder 1 and no flow to Cylinder 2. If Cylinder 1 is returned to the center position, then flow will go to Cylinder 2. This spool design ensures that only one circuit can be actuated at any time.