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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.)

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.

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.

A variation of the single- or multiple-spool valve is the stack valve, Figure 5, in which a number of spool and envelope sections are bolted together between an inlet and outlet section to provide control of multiple flow paths. In addition to providing a central valve location for the machine operator, the valve grouping reduces the number of fluid connections involved and increases ease of sealing. The number of valves that can be stacked in this manner varies from one manufacturer to another.

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.

Mounting configurations

Traditional valves are built in a configuration for inline mounting. But this arrangement is labor intensive, requires assembly of multiple components, and is prone to leakage. These factors add to installation cost and can have an even greater impact on maintenance costs. Therefore, most designers choose subbase or manifold mounting to avoid these problems.

In the sub-base mounting system, all conducting lines are connected to the sub-base and the valve attaches onto the base face with matching port pattern. The static sub-base does not wear out, so it does not have to be removed. The valve can be lifted off the sub-base after loosening a few screws without disturbing the plumbing. Hydraulic integrated circuits extend the sub-base concept even further by mounting several valves onto and into a common manifold, Figure 6.

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