A directional-control valve directs or blocks airflow in a pneumatic circuit to control the speed or sequence of operations. A directional-control valve can be classified based on the flow paths under different operating conditions. The number of possible valve positions and the number of ports and flow paths is important factors. A few basic configurations are shown below.
In two-way, two-position valves, there are two ports that connect a passage that can be open or block control flow. Valve spools and poppets usually move direct flow by an electrically operated solenoid. In many systems, the valve is used to interlock, isolate, and connect various parts of the system by providing an easy on/off function.
A three-way, two-position valve consists of three ports connected by passages within the valve body. To control a single-acting cylinder or pilot another valve, the valves pressurize and exhaust one outlet port.
The three-way valve directs pressurized air to the cap-end of the cylinder. By shifting the spool to the opposite extreme, flow and pressure block the actuator. Since the actuator connects to the exhaust passage, spring force or gravity must return the rod to its original position.
A three-way valve can also use in pairs to operate a double-acting cylinder, replacing a four-way valve. When high cylinder speeds require, consider using paired three-way valves instead of four-ways. The close coupling of three-way valves to the cylinder ports reduces cylinder back pressure and pressure drop in the lines, permitting higher cylinder speeds. When compressed air require in intermediate positions or high-cyclic applications, the valves use to save compressed air.
The four-way, the two-position valve has four or five ports with two or three positions. A two-position valve has two distinct flow paths in each position for actuating and reversing cylinders, rotary actuators, or bidirectional motors. The spool directs flow from the pressure port to the actuator port, while the other port exhausts to the atmosphere.
The five-part version of the two-position, the four-way valve has five individual ports that allow for diverse valve configurations, including two different exhaust ports or pressure ports. This valve might supply high pressure to actuate a cylinder and supply high clamping force, but deliver lower pressure (via a regulator) to release the clamp. Energy can conserve by using lower pressure.
There are also five-port versions of four-way, three-position valves. For four-port, four-way valves, the most common center conditions. The two extreme positions are the power positions, which control actuator movement. Some system requirements require the center position, such as locking an actuator or letting it float.
As directional control valve use in industrial and mobile applications, the most straightforward circuit design uses individual components to perform individual functions. However, this exclusive use of individual components may result in extraordinarily cumbersome plumbing problems, many leakage points, increased pressure drops across the components, and additional heat loss. Using a building block approach, the circuit is more flexible for combining different options and functions. It is possible to combine hydraulic circuitry, operators, and flows into one assembly.
