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HOW DO HYDRAULIC SERVO VALVES WORK?


We have already discussed the "What is a hydraulic proportional valve?”. We have discussed the basic of hydraulic proportional valve and its applications in hydraulic systems.

Today we are interested to see one more very important type of hydraulic valve which is now in huge demand in hydraulic machinery or system i.e. Servo valve.

We will discuss here hydraulic servo valve in this article. So let us come to the main topic i.e. hydraulic servo valves.

Servo valves are controlled by electrical signals hence we normally say it as electro hydraulic servo valve and such valves will be used in a hydraulic circuit where we need to secure very accurate control over positioning as such valves are quite costly and complex also. Position control will be secured by a closed loop control system with provision of feedback sensor, command sensor, analog or digital controller and servo valve. We can use low power electrical signal for accurate controlling over positioning of actuators i.e. cylinders and hydraulic motors.

Let us see the main components of servo valves here
  • Flapper
  • Torque motor
  • Flapper nozzle

Servo valve will have normally two stage i.e. Flapper nozzle system and servo spool system. Servo valve will have two inlets; first one will be for hydraulic inlet and second will be for electrical input for torque motor. Electrical current will position the flapper and flapper position will control the pressure in chamber A & B.

Torque developed by torque motor will be directionally proportional to the applied current. The applied current will be small in a range of milliamp. As shown in figure, there will be two inlet orifices O1, O2 and two output nozzles as shown in figure i.e. n1 and n2 and one more nozzle called as back pressure nozzle i.e. n3. There will also be one feedback spring fixed between spool and flapper as shown in figure.

When flapper will be in neutral position, flow area of nozzle n1and n2 will be equal and hence pressure inside both nozzle Pn1 and Pn2 will be equal.

When flapper comes towards nozzle n1, flow area of nozzle n1 decreases and pressure Pn1 increased and as we are discussing that flapper is closed to nozzle n1 hence flow area of nozzle n2 increased and hence pressure Pn2 will be decreased and therefore there will be a pressure difference i.e. ΔP = Pn1-Pn2 across the servo spool. 

Servo spool will move towards right direction due to this pressure difference across the servo spool. Oil, which will be under high pressure, will flow through chamber A and return oil will flow through chamber B.

Similarly when flapper will come towards nozzle n2, flow area of nozzle n2 will be decreased and pressure Pn2 will be increased and flow area through nozzle n1 will be increased and pressure Pn1 will be decreased hence there will be pressure difference i.e. ΔP = Pn2-Pn1 across the servo spool and hence servo spool will move towards left direction and oil under high pressure will flow from B and return oil will flow through chamber A.

Feedback spring will pull back the servo spool to its original position.

We have seen here how one servo valve will work in a hydraulic unit.

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