Bang-bang is the term often used to describe basic directional-control valves. It refers to how the valves shift - from fully open to fully closed. This usually occurs in an instant, causing fluid to rapidly accelerate and decelerate. Under certain conditions, this can cause fluid hammer, which sounds like a hammer striking the hydraulic system from inside. Hence, shifting the valve from one position to another can produce a bang-bang sound.

A less informal term to describe these components is discrete valves. This term refers to how the valves operate: they shift from one discrete position to another, such as extend, retract, and neutral. Proportional valves, on the other hand, control direction and speed. In addition to shifting into discrete positions, they can shift into intermediate positions to control actuator direction, speed, acceleration, and deceleration.

Even more basic than the discrete directional-control valve is the digital valve. As in digital electronics, digital valves operate either on or off. Whereas discrete valves generally use a spool to achieve two, three, or more positions, discrete valves use a plunger, poppet, or ball that seals against a seat. The advantage to this type of operation is that it provides a positive seal to prevent cross-port leakage.fig. 1. basic check valve allows fluid to flow in one direction, in this case from bottom to top. shown are iso symbol and cross-sectional photo of spring-loaded check valve. the spring keeps fluid from flowing unless downstream pressure acting on the poppet overcomes spring force.

Perhaps the simplest of all directional-control valves is the check valve, a specific type of digital valve. Basic check valves allow fluid to flow in one direction, but prevent fluid from flowing in the opposite direction. As with all fluid power components, directional-control valves can be represented by standard symbols published in ISO 1219. Figure 1 shows a cross-section of a spring-loaded check valve and its ISO 1219 representation.

Ports and positions

The two primary characteristics for selecting a directional-control valve are the number of fluid ports and the number of directional states, or positions, the valve can achieve. Valve ports provide a passageway for fluid (air or hydraulic fluid) to flow to or from other components. The number of positions refers to the number of distinct flow paths a valve can provide.

A 4-port, 3-position spool valve serves as a convenient illustration, Figure 2. One port receives pressurized fluid from the pump, and one routes fluid back to the reservoir (or to the atmosphere or exhaust muffler in a pneumatic systems). The other two ports are generally referred to as work ports and route fluid to or from the actuator. In this case, one work port routes fluid to or from the rod end of the cylinder, the other routes fluid to or from the cap end.

The valve represented in Figure 2 can be shifted to any of three discrete positions. As shown, in the neutral position, all ports are blocked, so no fluid will flow. Shifting the valve to the right routes fluid from the pump to the rod end of the cylinder, causing its piston rod to retract. As the piston rod retracts, fluid from the cylinder's cap end flows to the reservoir. Shifting the valve to the left routes fluid from the pump to the cap end of the cylinder, causing the piston rod to extend. As this occurs, fluid from the rod end of the cylinder flows to the reservoir. Returning the valve spool to the center position again blocks all flow. (In reality, a relief valve would be provided between the pump and directional valve. It is omitted here for simplicity.)fig. 2. schematic shows simple circuit to control cylinder extension and retraction using a  4-port, 3-position spool valve.

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. They are used extensively for directional control of actuators because a single valve can produce extension, retraction, and neutral. However, these same functions can be accomplished with digital valves. Figure 3 shows four normally closed (NC) digital valves grouped into a hydraulic integrated circuit to provide the same functionality as the spool valve represented in Figure 2. With all valves in the neutral condition, as shown, fluid flow to and from the pump, reservoir, and actuator is blocked. Energizing valve A routes pressurized fluid to the cap end of the cylinder, causing the rod to extend. Simultaneously energizing valve D routes fluid from the cylinder's rod end to the reservoir. In similar manner, energizing only valves B and C causes the rod to retract and routes fluid from the cylinder's cap end to the reservoir.fig. 3. arranging digital valves into a hydraulic integrated circuit allows them to accomplish the same functions as discrete spool-type valves while retaining the advantages of digital valves.

The valves in Figure 3 are arranged to match the closed-center spool condition of the valve in Figure 2. An open-center condition, Figure 4, could be achieved simply by making all the digital valves normally open (NO) instead of normally closed. Likewise, tandem- and float-center configurations can be accomplished by using NO and NC digital valves.

These and other common center-position configurations can be quite specialized, depending on the application of the valve. Most manufacturers offer a variety of center-position configurations as standard, off-the shelf items. Although the vast majority of directional-control valves for industrial applications are 2- and 3-position, many valves used in mobile equipment come in 4-position configurations to accommodate special needs.

When specifying the specific type of valve needed for an application, it has become common practice in North America to refer to the number of ports on a valve as the way, such as 2-way, 3-way, or 4-way. However, international standards use the word ports. Thus, what is known as 2-way, 2-position directional valve in the U.S. is called a 2-port, 2-position valve internationally and can be abbreviated 2/2. The number before the slash identifies the number of ports, and the second number refers to the number of positions.