Directional Control Valves (DCV) Why Directional Control Valves are Necessary Directional Control Valves are fundamental components of any hydraulic or pneumatic system. Other terms used to describe them are bang-bang or discreet valves. They determine the path of fluid flow through a circuit and are used primarily to start, stop, accelerate, decelerate, or change the direction of motion in an actuator. Other benefits for using DCVs include: To isolate a certain branch of a circuit Bypass valves or return-line filters Deny flow in one direction Protect hydraulic components against surges in pressures Most directional control valves are of a spool-type design. A spool is used to control fluid flow and to connect internal passages and port. The spool has lands and undercuts, housed within precision-machined casing. As the spool shifts, the lands and undercuts open and close flow paths. Spool-type valves are widely used because they can be shifted to two, three, or more positions for routing fluid between different combinations of inlet and outlet ports. Ports refer to the number of lines into and out of the valve. They are sometimes referred to as “ways”. For example, 4-way, 3 position valves. The actuator is a mechanism used to move the position of the spool in a DCV. The four basic methods of actuation include: Manually-operated: The spool is shifted manually by moving a handle, pushing a button, or stepping on a foot pedal. Mechanically-operated: The spool is shifted by mechanical linkages such as a cam and rollers. Solenoid-operated: When an electric coil or solenoid is energized, it creates a magnetic force that pulls the armature into the coil. This causes the armature to push the spool of the valve. It should be noted that solenoids alone are not strong enough to move a DCV spool when fluid flow is greater than 25 gpm and cannot generate large forces unless supplied with large amounts of electrical power. However, they are used extensively. Pilot-operated: Pilot pressure is initiated when the fluid pressure in a fluid power system shifts the spool into its desired position. By applying a pilot signal (for either hydraulic or pneumatic systems) against a piston at either end of the valve spool, pilot pressure is introduced which pushes the piston to shift the spool. Another important consideration is the center position. There are four types: open, closed, tandem, and float. Open-center position DCVs are typically used in applications where there is only one cylinder in the hydraulic system. Conversely, a closed-center position DCV allows a hydraulic system to operate with more than one actuator and each actuator operates independently of the other. A tandem-center position DCV is used to hold the piston in a cylinder in the desired position while allowing fluid flow to be directed back to the reservoir without the need to activate a relief valve. Most directional control valves used in industrial applications have two, three, or four positions, but some can have five or six positions. Using directional control valves is absolutely essential to any industry that relies on hydraulic circuits. Anything with a motor is going to use directional control valves. Machine function can also be totally automated by interfacing the directional control components with hydraulic, pneumatic, electrical, or electronic control circuits.
