Directional Control Valves

Directional control valves are one of the most basic components in any hydraulic circuit. They direct the flow of fluid along different paths from one or more supplies. The most common form of directional control valve is a cylindrical body with a spool inside that is moved in and out by either mechanical, electrical or hydraulic means. The drawing in figure 1 shows a very basic directional control valve with three ports. The supply port, which is called the ‘P’ or pressure port, a tank or ‘T’ port which takes the spent fluid back to the hydraulic tank, and a working port, called the ‘A’ port, that delivers the fluid to the load. In the position shown, the pressure port is blocked and pressure fluid cannot do anything. The ‘A’ port is connected directly to the tank port so any fluid pressure from the load will be dissipated back to the hydraulic tank. The drawing on the right of figure 1 shows the hydraulic symbol for a three port directional control valve. This symbol depicts the blocked ‘P’ port and the ‘A’ port going directly to the ‘T’ port. This is the common way to show a valve in a hydraulic circuit drawing and is called the ‘at rest’ position. When you read a hydraulic circuit diagram you have to understand that when the valve is activated, the lever is pushed or the controling electrical coil is energised, then the symbol will switch over and the vertical arrow will connect ‘P’ to ‘A’ and ‘T’ will be blocked. This very situation is shown in figure 2 below. The lever has been activated, pushed in, and the spool has moved in to block the ‘T’ port from port ‘A’ while joining port ‘P’ to port ‘A’. This allows pressure fluid from port ‘P’ to flow through port ‘A’ to the load, which may be a cylinder or a motor or even another valve. The hydraulic symbol for this is shown on the right in figure 2. The symbol has swapped over to show the ‘P’ port connected to the ‘A’ port and the ‘T’ port is blocked. The ball with the spring on the left hand side of the valve spool is called a de-tent and is used to hold the lever in that position until it is pulled back to neutral. Other valves will not have this de-tent but will have a spring at the end of the spool to return it to the neutral, or centre position in a four port valve, when you let the handle go. The circuit in figure 3 shows a more complicated directional control valve set-up. It also shows how one directional control valve can control another directional control valve that then controls the load. In this configuration the two DC valves at the top are electrically operated proportional valves which can supply an infinitely variable, or proportional, hydraulic pressure to the pilot operated directional control valve at the bottom. The bottom valve is the one that does the actual lifting and lowering of the hydraulic cylinder above. The cylinder uses gravity to lower itself however the check valve underneath prevents this from happening until the bottom directional control valve opens it. Using a carefully controlled current from a PLC to control the top valve, the bottom valve is gradually moved in and out to give very precise movement of the cylinder. This circuit is used to move things like chutes and gates to very precise positions. In figure 4 we have a similar circuit, only the top controling valve is a direct operated, solenoid actuated directional spool valve of high performance. It controls the start, stop and direction of the cylinder through the larger pilot operated directional valve at the bottom. Both of these circuits make use of pilot pressures. This is somewhat similar to how an electronic circuit amplifies a signal. The smaller valves at the top control a large load, these cylinders are quite big, by controlling the large directional valve at the bottom that is very capable of handling the large cylinder. It is much easier to electrically control the smaller valves at the top. The circuit in figure 4 is used to open and close heavy gates very quickly.

Directional Control Valves