What Is A Poppet Valve
Poppet valves are similar to check valves in that the sealing element is a poppet but they are actuated by mechanical or electrical means. Advantages over convention spool-type directional controls are:
1. Virtually zero leakage in closed position.
2. Poppet elements do not stick even when left under pressure for long periods.
3. Fast, consistent response times (down to 15 ms).
a. Axial pressure balance is almost impossible and considerable force may be needed to open the poppet against the flow at high pressure. This limits valves which have direct mechanical actuation to low flow duties.
b. Generally individual poppets are required for each flow path which significantly increases the complexity of multiport valves.
Mechanical actuated valves are predominantly employed on presses and machine tool applications where they often form part of a dedicated control mechanism:
A. TWO-PORT SOLENOID OPERATED POPPET VALVES
These are available either as normally open or normally closed devices. In certain small models primarily intended for use as pilot valves the poppet is directly actuated by the solenoid. They are used as the control section of two-stage valves which are basically pilot operated check valves.
In one type of normally closed two-port solenoid controlled poppet valve shown in Figure 1, pressure at port A is applied to the back of the poppet via a small hole (orifice X) in the side-wall. Pressure keeps the poppet closed in the manner of a check valve. Energizing the solenoid lifts a plunger unblocking a hole (Y) in the center of the poppet. Imbalance occurs because of the pressure differential across orifice (X) and the poppet lifts permitting flow through the valve from A to B. Note that the solenoid armature (plunger) is permanently surrounded by the hydraulic fluid and hence balanced.
Relatively unrestricted reverse flow from B to A is possible when the solenoid is de-energized, the valve behaving as a normal check valve with only a small pressure differential needed to overcome the bias spring. If the solenoid is energized restricted reverse flow characteristics may be exhibited depending upon valve geometry.
Cartridge type construction is usually adopted, the valve fitting into a cavity.
Possible applications are similar to those for the pilot operated check valve (e.g. cylinder locking). A variation suitable for use as fast response, high flow rate, pump dump valves is the solenoid controlled logic element
B. THREE-AND FOUR-PORT SOLENOID OPERATED POPPET VALVES
In one design of three-port valve a solenoid assisted by an internally piloted piston is used to switch a double-coned poppet from one seat to another. Various configurations are available including four-port versions which incorporate a second double seated poppr section.
An established application for a three-port solenoid-poppet valve is as a safety unloader valve in an accumulator circuit. The valve symbol shows the solenoid energized which would be the normal operating condition and the circuit fails 'safe'. When the solenoid is de-energized, fluid in the accumulator depressurizes slowly through the fixed restrictor. The solenoid may be built into an accumulator safety block. In this situation a spool valve would be a constant source of leakage.