Directional control valves

The basic function of directional control valves is to change the flow direction of the working medium, release the working medium, or block the working medium in a hydraulic system. In directional control valves, compressed air or a hydraulic fluid is generally used as the working medium.

Basics of directional control valves

The change in the direction of the volumetric flow is determined by the cylinder’s extension direction. The stroke length of the cylinder’s lifting movement is controlled by start/stop commands.

The various ports of a directional control valve are the pressure source, the control port, the vent port, the tank port, the drain, the working port and the leakage oil port.

Differences and components of directional control valves

Directional control valves are generally defined by the flow path, the number of switching positions and the number of ports. Directional control valves differ in several key characteristics. A fundamental key characteristic is the number of switching positions. They are also distinguished by the design of the control element, which may be, for example, a poppet or a spool. The actuation type of a directional control valve is classified as electrical, pneumatic, mechanical, hydraulic or manual. Likewise, the valve ports are differentiated by number, size and type. The final key characteristic to mention is the number of flow paths of a directional control valve.

The devices used to influence the start, stop and flow direction of a flow can also be differentiated by their operating principle. A distinction is made between an on/off directional control valve (with negative or positive overlap) and a proportional directional control valve (with negative, zero or positive overlap). Various designs of a directional control valve include piston or spool valves and seat valves. The distinction according to the valve’s function is reflected in the conventional designation of directional control valves. A 3/2-way valve is a valve with three controlled ways (or ports) and two simple switching positions for controlling the actuator in the system. This results in the classic configurations of 2/2-, 3/2-, 3/3-, 4/2-, 4/3- and 5/3-way valves. In addition to these versions, special versions with more switching positions and/or ports are manufactured, for example a 4/4- or 6/4-way valve.

Control types of directional control valves

A directly actuated directional control valve switches only when movement occurs within the valve. In the simplest cases, the movement is performed by a seal. In many of these applications, several seals are located on a valve piston. The movement of the seal ensures that the air is diverted from one channel to another, or released or blocked. In directly actuated valves, this movement is triggered directly by an actuating element. Common elements include a solenoid coil, a foot switch or a simple lever. This circuit is particularly suitable for use in vacuum applications, as the switching function of the directional control valve is independent of the operating pressure. However, the force required for directly actuated valves increases proportionally with the size of the valve. For example, with complex electrically actuated directional control valves, the solenoid coil required to initiate the movement of the sealing ring may be larger than the directional control valve itself.

Indirectly actuated directional control valves are also commonly referred to as pilot-operated valves. Unlike directly actuated valves, the actuating element here does not move a seal; it merely opens or closes a small pilot orifice. In indirectly actuated directional control valves, the movement of the valve piston is performed by the working medium. This creates the key advantage of pilot-operated directional control valves: only a very small force is required to switch the valve. This means that even a small solenoid coil with low power consumption can actuate a large valve. However, this only works if a certain pressure is present at the valve.