Basics of Directional-Control Valves

4. Above are common center-spool arrangements for matching neutral-position fluid routes to the application.

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.

Spool Valves

The most common sliding-action valve is the spool-type valve (Fig. 5). Fluid is routed to or from the work ports as the spool slides between passages to open and close flow paths, depending on spool position. Spool valves readily adapt to many different spool-shifting schemes, which broadens their use over a wide variety of applications.

Many mobile applications require metering or throttling to enable the operator to slowly or gently accelerate or decelerate a load. In these instances, the spool may be modified with V notches, for example, so that a small displacement of the spool gradually permits increasing or decreasing fluid flow to gradually speed or slow actuator and load movement. This technique is also used in valves for industrial equipment. A beveled or notched edge on the spool is commonly referred to as a soft-shifting feature.

A variation of the single- or multiple-spool valve is the stack valve, in which multiple 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.

Valve Operators

Valve operators are the parts that apply force to shift a valve’s flow-directing elements, such as spools, poppets, and plungers. The sequence, timing, and frequency of valve shifting is a key factor in fluid power system performance. As long as the operator produces enough force to shift the valve, the system designer can select any appropriate operator for the conditions and type of control under which the system will operate.

Operators for directional-control valves are either mechanical, pilot, electrical, and electronic, or a combination of these. Different types of actuators can all be installed on the same basic valve design. A common directional valve often is used that makes provision for mounting a variety of different operators on its body.

With a mechanical operator, a machine element or person applies force on the valve’s flow-directing element to move or shift it to another position. Manual operators include levers, palm buttons, push buttons, and pedals. Purely mechanical operators include cams, rollers, levers, springs, stems, and screws. Springs are used in most directional valves to hold the flow-directing element in a neutral position. In 2-position valves, for example, springs hold the non-actuated valve in one position until an actuating force great enough to compress the spring shifts the valve. When the actuating force is removed, the spring returns the valve to its original position. In 3-position valves, two springs hold the non-actuated valve in its center position until an actuating force shifts it. When the actuating force is removed, the springs re-center the valve, leading to the common identification, spring-centered valve. Detents are locks that hold a valve in its last position after the actuating force is removed until a stronger force is applied to shift the valve to another position. The detents may then hold this new position after the actuating force again is removed.

Mechanical operation is probably the most positive way to control industrial fluid power equipment. If a valve must shift only when a machine element is in a certain position, the equipment can be designed so that the machine element physically shifts the valve through a mechanical operator when the element reaches the correct position. This arrangement virtually eliminates any possibility of false or phantom signals from shifting the valve at the wrong time.

However, mounting mechanically operated valves on a machine requires some special cautions. The valve and actuator may be exposed to a wet or dirty environment that requires special sealing. The actuator will probably be subjected to impact loads, which must be limited to avoid physical damage. Valve alignment with the operating element also is important, so the valve must be mounted accurately and securely for long service life.

Pilot-actuated valves are shifted by pressurized fluid (often about 50 psig) that applies force to a piston that shifts the valve’s flow-directing elements. An important advantage of pilot operation is that large shifting forces can be developed without the impact and wear that affects mechanically actuated valves. Pilot-operated valves can be mounted in any convenient or remote location to which pressure fluid can be piped. The absence of sparks and heat buildup makes pilot-actuated valves attractive for applications in flammable or explosive environments.

Electric or electronic valve operation involves energizing a solenoid. The force generated at the solenoid plunger then shifts the valve’s flow-directing element. Solenoid-actuated valves are particularly popular for industrial machines because of the ready availability of electric power in industrial plants. However, mobile equipment makes extensive use of solenoid-operated valves as well. The selection of ac or dc solenoids depends on the form of electrical power available. At one time dc solenoids offered longer service life, but improvements in ac solenoid designs have eliminated that advantage.

There is a practical limit to the force that solenoids can generate. This means they cannot directly shift valves requiring high shifting forces. Furthermore, valves using large solenoids also consume substantial electrical power when valves must remain actuated for long intervals. Heat buildup can also pose problems in these situations. The solution is to use small, low-power solenoids in combination with pilot pressure. The solenoid starts and stops pilot flow, and pilot pressure provides the high force to shift the valve’s flow-directing mechanism (Fig 5).