Here are the practical advises to select the configuration of the middle position of 4-ways 3 positions directional valves. There are more than 250 configurations of the flow ways usually used in the practice, but in this quick review let see the generic configurations only.
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All ports are separated from each other:
- This spool configuration is used where two or more operations are performed by a single pump or an accumulator. Pump output is available for other valves and actuators with this center condition.
- Closed port P prevents the loss of fluid from the pump or accumulator when the spool crosses center. Avoid fixed displacement pumps for this configuration. More info about close circuits you can find here.
- The spool-type valves are not zero-leaking devices. In actual use, leakage oil across the spool lands pressurizes A and B ports, possibly causing a single rod cylinder to extend trough piston/annular area difference. By the same reason this configuration is not for holding applications.