In most air and hydraulic fluid powercircuits, valves are used to control the direction rate and pressure in fluidlines. Control valves for these functions, can be purchased with either manualor solenoid actuators.

In a manually actuated valve, the internal cylinder is shifted by hand,using a lever, push-button, plunger, or other manual actuated device.

By contrast, valves with a solenoid actuator, respond to an electricalsignal for shifting. With electrical control, machine cycles can be set up forautomatic sequencing. And operator control can be exercised from a remotelocation. Solenoid valves are said to be either direct or pilot operated. Adirect operated valve is shown here. 

A pilot operated valve has a pilot bleed orifice and utilize linepressure for operation. This is pilot operated relief valve. When the solenoidis energized, the core opens the pilot orifice and relieves pressure from thetop of the valve piston, or diaphragm to the outlet side of the valve. Thisresults in an unbalanced pressure lifting the piston or diaphragm off the mainorifice. When the solenoid is de-energized, the pilot orifice is closed, andfull line pressure is applied to the top of the piston or diaphragm, closingthe valve. Most directly operated valves also come in pilot operated versions,and are designed by a triangular graphic symbol. 

A single solenoid valve has one solenoid to assist in valve operation.The valve spool shifts when the solenoid receives an electrical signal and isenergized .The valve will remain shifted as long as electrical current isapplied to the solenoid. Once the electrical current is removed and thesolenoid is de-energized, the valve returns to its normal position by springforce. Fluid circuits designed to use single solenoid valves, must maintainelectrical current in order to keep the valve in its shifted position.

Double solenoid valves have twosolenoids, typically mounted on opposite ends of the valve body. This type ofto position double solenoid valve does not have a spring return. When the firstsolenoid is energized, the valve spool shifts into the first position, even ifthe first solenoid is de-energized, the spool remain shifted. This is becausethere is no spring to return the spool to its original position. Energizing thesecond solenoid will send the valve to the second position. Even if the secondsolenoid is de-energized, the valve will remain shifted until the firstsolenoid is re-energized. Since there is nothing holding the valve in theshifted position, other than friction, these types of valves should be mountedhorizontally to avoid shift, due to excessive airflow or vibration. If bothsolenoids are energized at the same time, the solenoids will work against eachother, and may cause the spool to become stuck. Electrical burn out of thesolenoid, or an overload of inrush current to the circuit. Any of which willcause severe damage to the system or valve.

A three-position valve has a spring centered neutral position for itsinternal spool. Because of the centering springs, it’s necessary to holdcurrent on one solenoid or the other to keep the spool in one of its sidepositions. When each solenoid is energized, the valve shifts to the appropriateposition. Anytime, both solenoids are de-energized, the valve spool will springto Center, and stop the fluid floW. Therefore, a three-position valverequires a maintained electrical signal for its operation. As before,energizing both solenoids at once may cause damage to the valve or to thesystem. To prevent this from occuring, electrical circuits should be designedmake it possible to have current on bothe solenoids at the same time. 

Solenoid valves are used in both hydraulic fluid and air systems tocontrol direction on cylinders from deceleration or speed control, and pumpunloading or pressure control. The physical apperance of each component will vary greatly,depending on brand, size and type. So care should be taken to reference toappropriate manufactures guidance when choosing a solenoid valve.