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Valve’s middle position configurations

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:

mid_pos_01

Configuration notes:

  • 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.

All ports are connected:

mid_pos_02

Configuration notes:

  • This spool configuration is used in single operations where no other operation is performed by the same source of power and where cylinders do not have to be held by pressure.
  • This configuration unloads the pump and allows the actuator to float. Avoid variable displacement pumps for this configuration. More info about close circuits you can find here.
  • Open center spools are also used to minimize shock in a system. Shock develops when a valve spool is shifted from one position to another across center position. The smoothest possible minimum shock condition is obtained when fluid under pressure is allowed to discharge to tank as the spool passes center condition. The ports are momentarily interconnected during spool crossover when the pilot valve solenoid is activated. This permits smooth rapid cycle operation.

Ports P and T are connected, ports A and B are blocked:

mid_pos_03

Configuration notes:

  • This configuration is used in hydraulic circuits where two or more hydraulic cylinders or motors are controlled from a single source of power. The valve’s spool is designed so that in center position, all cylinder connections are blocked and full pump delivery is connected to tank. The tank connection of one valve may be connected to the pressure connection of another valve and both valves operated simultaneously as long a the combined pressures developed by the two loads are within the capabilities of the power source.
  • This configuration unloads the pump. Avoid variable displacement pumps for this configuration. More info about close circuits you can find here.
  • The cylinder sits still unless there is an outside force trying to move it. Because spool-type valve is not a zero-leaking device valve never fully blocks flow. With external forces working on the cylinder, it may slowly creep with the valve centered.

Ports A, B and T are connected and separated from port P:

mid_pos_04

Configuration notes:

  • This configuration allows the actuator easy to float while blocking pump flow. Pump output is available for other valves and actuators with this center condition.
  • Avoid fixed displacement pumps for this configuration. More info about close circuits you can find here.
  • This configuration is a correct choice for holding applications where actuator can be hydraulically locked using pilot-operated check valves or counterbalance valves. The configuration allows pilot pressure to drop and the pilot-operated check valves (counterbalance valves) to close:
    mid_pos_05
    Using a directional valve with blocked A and B ports in center condition, may keep the pilot-operated check valves open and allow cylinder creep.
  • Also, this configuration is usually used for pilot of main directional valve with spring centering:
    mid_pos_06

Ports A and B are partially closed and connected with port T:

mid_pos_10

Configuration notes:

  • This configuration is ideal for restricted motor applications requiring meter-in/meter-out control for a hydraulic motor, where efficient control of deceleration is desired, or where symmetrical meter-in/meter-out flow control is needed during shift. It can also be used for cylinder applications with bleed-down.
  • Avoid fixed displacement pumps for this configuration. More info about close circuits you can find here.

Ports A, B and P are connected and separated from port T:

mid_pos_07

Configuration notes:

  • This configuration is called “regeneration center position”. It allows to connect both ports of a cylinder to each other in the regeneration mode and flow from rod port will follow to the blind port trough middle section of the valve.
  • Also, this configuration is usually used for pilot of main directional valve with hydraulic centering:
    mid_pos_08

Another cofigurations

 

mid_pos_09

Another configurations (the picture above is just example of some of them, but really there a huge quantuty of their combinations) allow the choice of different working characteristics while switching two directions. See manufacturer’s catalog to compare characteristics for each of them.

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Sources and an additional info for reading:

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