Directional control valve – intelligent control of flow rates

A directional control valve is a hydraulic component that can precisely control a flow rate. The term refers to a group of valves that perform different functions. In combination, the various types of directional control valves make it possible to build logic circuits.

A directional control valve can perform the following functions:

• Blocking
• Diverting
• Returning
• Controlling pressure
• Influencing flow rate.

To achieve a logic circuit, several valves are connected in series. In some cases, combination valves are also used, which can perform multiple functions. These are particularly advantageous where installation space is limited. However, they also cost more than single-function directional control valves.

The shut-off valve as a directional control valve

Shut-off valves block a flow rate. Any actuator located downstream of the valve is thereby shut down. Shut-off valves have a short response time and therefore perform their function abruptly. This distinguishes them, for example, from throttle valves.

The directional valve as a directional control valve

A directional valve diverts a flow rate in another direction. This diversion serves to switch one actuator on and another off. Two-way valves are the most common. However, directional valves with a wider range of actuation are also available. Directional valves can also be used to return a flow rate.

The pressure valve as a directional control valve

Although a pressure valve does not “switch” in the strict sense—i.e., it does not abruptly change the operating state—it is still one of the logic elements of a hydraulic control system. With these valves, the pressure of the flow is converted. As a rule, this means reducing the pressure by means of a throttle. However, other designs are also available. Increasing pressure without an intermediate pump, however, always comes at the expense of the flow rate’s speed.

The flow control valve as a directional control valve

Flow control valves regulate the speed in a hydraulic line, not the pressure. They can, for example, control the extension speed of a linear motor without reducing its force.

Actuation of directional control valves

The simplest directional control valves are operated manually. The best-known of this type is the ball valve with lever. These are used to manually switch a hydraulic circuit on and off. Ball valves are manual additions when the shut-off in the hydraulic couplings is not sufficient to connect hydraulic circuits to one another. Hydraulic systems with high control requirements are now controlled electrically, pneumatically, or electro-hydraulically. For this purpose, each directional control valve has an additional control unit that actuates the valve when required.

Electric valves operate with small electromagnetic linear actuators. Pneumatically operated control units for directional control valves work similarly. With electro-hydraulically operated directional control valves, an upstream valve—similar to a relay in an electrical circuit—actuates the actual valve in the hydraulic circuit. This type of control is often found, for example, in the control levers of construction machinery.

Maintenance of directional control valves

One disadvantage of directional control valves is that they are often installed deep within a hydraulic system. Nevertheless, it is very important to comply with the specified maintenance intervals and to inspect the valve for proper function and leak-tightness.