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Solenoid valves with piston, servo-assisted

A plunger-type solenoid is used as the pilot valve with this principle as well, but the main valve function is implemented by a piston. The function of this valve (e.g. Types 5404 and 6221) results from the fact that fluid is also able to flow from the supply P through a control bore in the piston or bypass channel of the main valve. If the chamber above the piston is closed off with the pilot valve de-energized, the fluid system pressure builds up there as well and the piston, which simultaneously forms the valve seal, closes the valve seat in the main valve.

It is assumed that the fluid pressure can be discharged in outlet A. For as long as a pressure differential exists between the inlet port and the outlet port, the main valve remains closed as the result of the closing forces above the piston. After activating the pilot valve, the pressure in the chamber above the piston is able to drop. The force acting on the lower side of the piston as the result of the fluid system pressure is greater than that on the upper side of the piston, it lifts the piston and opens the main valve. Servoassisted solenoid valves operating on the basis of this design require a minimum pressure differential for correct opening and closing. Solenoid valves with a coupling (forced valve lifting) between the solenoid core and piston (e.g. Type 407) require no pressure differential for switching. Servo-assisted piston-operated valves are used chiefly in applications with high pressures, gaseous media and steam up to 180 °C.


  • More robust than servo-assisted solenoid valves with diaphragm
  • Higher pressure range
  • Suitable for gases and steam.


Servo-assisted 2/2-way solenoid valve, normally closed, with piston. Shown closed at left and open at right