Why is Directional Control Valve Better?

A directional control valve is used in hydraulic systems to precisely control the flow of hydraulic fluid. The valves allow directional control of the fluid by diverting the flow in different directions.

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Detour from a source to an actuator or from an actuator to the return line are possible. This allows hydraulic cylinders or hydraulic motors to be controlled to perform movements in different directions.

In this article, we take a closer look at the functions, features and designs of directional control valves. Be sure to read on and learn more about ARGO-HYTOS solutions for hydraulics.

How do directional control valves actually work?

Directional control valves provide precise control of flow in hydraulic lines. They provide several functions that are critical to the efficient operation of a hydraulic system.

The first function of a directional control valve is to control the direction of flow. It can divert flow from a source to an actuator or from an actuator to the return line. This allows movement in different directions, such as forward and reverse movement of a hydraulic cylinder or rotation of a hydraulic motor.

Another important function of the directional control valve is to block the flow. In certain situations, it is necessary to stop the flow. This applies, for example, when a certain function is not needed or when maintenance work is being performed on a part of the system.

The directional control valve can block the flow and prevent the hydraulic fluid from entering a certain area. This allows maintenance work to be carried out without paralyzing the entire system.

What are the characteristics of directional control valves?

A characteristic feature of directional control valves is the number of connections and switching positions. Depending on the application and requirements, directional control valves with different connections and switching positions can be selected.

The number of connections indicates how many hydraulic lines can be connected to the valve. The number of switching positions indicates how many different positions the valve can take to control the flow accordingly.

By combining different connections and switching positions, directional control valves can be used in a variety of ways and allow for complex hydraulic control systems.

Another aspect is the different spool shapes of directional control valves and their functions. ARGO-HYTOS offers a variety of spool shapes for directional control valves, including:

• 4/3 directional control valve with Y spool for unpressurized float position
• Hydraulically releasable check valves
• Overcenter valves

Each spool shape has its specific function and is suitable for certain applications. Choosing the right spool shape enables optimal control of the hydraulic system and ensures reliable and precise operation.

Directional control valves with different characteristics from ARGO-HYTOS

Directional control valves are available from ARGO-HYTOS in a wide range of designs and variants. The range includes over 30 different spool shapes as standard, which are suitable for different applications and functions. In addition, we always offer our customers the option of having special spools developed and manufactured by our experts.

When selecting a directional control valve, various options are available to customize the function and actuation of the valve. We have developed ten different manual overrides to allow manual control of the valve in an emergency.

Furthermore, seven different nominal voltages of the solenoids at the coil connection are available for selection. They ensure optimum adaptation to electrical systems. Five different solenoid coils are also available for electromagnetic actuation.

The plug variants also play an important role in the installation and connection of the directional control valves. We offer nine different plug variants to enable easy and safe connection with other components of the hydraulic system.

We place great emphasis on the quality and durability of our products. Therefore, directional control valves from ARGO-HYTOS are subjected to a salt spray test according to DIN EN ISO . This ensures reliable surface protection for up to 1,000 hours.

Types of directional control valves

A common design of directional control valves are the screw-in cartridge valves. These valves are characterized by their simple assembly, as they can be screwed directly into the corresponding threaded connections. The screw-in cartridge valves allow a flexible design of the hydraulic system and can be easily adapted and extended.

Spool type design with housing

Another design of directional control valves is the spool design with housing, also known as a surface mounted valve with uniform hole patterns according to ISO 440. In this design, a spool made of hardened steel is used as a control element.

The spool is slid as a whole in the cast iron body. It can connect or close ports to change the function of the valve. The spool design allows precise control of flow. It is often used in applications where extremely precise control is required.

Modular design: Directional control valves RPEK

The modular design with directional control valves is also known as RPEK valve design. In this design, the spool is integrated into the module plates for horizontal interlinking. The valve consists of an inlet section with pressure relief valve and/or switchable pressureless circulation as well as one to eight flangeable directional control valves. The P (pressure) and T (tank) connections are looped through all sections.

The modular design offers the advantage of small dimensions, flexibility and compactness. It is particularly suitable for applications where limited installation space is available. By integrating several valves into one module, efficient and reliable control of the flow is made possible.

More designs

Another type of directional control valve is the screw-in cartridge valve in spool design. Here, a spool in a steel sleeve opens or closes radial bores. Also widely used are screw-in cartridge valves with a poppet seat, in which the spool is replaced by a hardened poppet. These designs each offer specific advantages such as simple assembly or technical leakage-free operation.

Significance of the operation and control of directional control valves

Depending on the requirements and application areas, various actuation and control options of directional control valves are available.

Actuation options for directional control valves

• Electromagnetic: Directional control valves can be operated electromagnetically. Here, the force is generated by an actuating element such as a pole tube, coil, armature or plunger. This method of actuation provides precise and reliable control of the valve.
• Manual: Manual actuation methods such as hand force or foot force are used in certain applications. They provide simple and direct control over the directional control valve.
• Mechanical: In mechanical actuation, the directional control valve is controlled by mechanical forces, for example, levers or push buttons.
• Pneumatic: Directional valves can be pneumatically actuated, using compressed air to control flow. This allows for quick and efficient control of the valve.
• Hydraulic: Hydraulic actuation uses hydraulic control pressure to move the spool of the directional control valve. This method provides precise and reliable control of the valve.

Meaning of spring return or pulse control with detent

Spring return or pulse control with detent is an important function in the actuation of directional control valves. It allows the valve to return to a specific position when the actuating force is removed. This ensures defined control of the flow and safe operation of the valve.

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The detent ensures that the valve remains in the desired position even when no actuating force is applied. This is particularly important in applications where the position of the valve must remain stable over a long period of time to ensure reliable operation of the system.

Conclusion: The importance of directional control valves

Directional control valves enable precise control of flow in hydraulic systems. The valves provide directional control or block flow for specific functions or maintenance. They can be optimally used to ensure efficient operation and reliable control.

Would you like to learn more about our product portfolio? Or are you interested in an individual solution for your application? Then do not hesitate to contact the experts at ARGO-HYTOS. We offer comprehensive advice and support in selecting and implementing the most suitable valve technology for your needs.

Bang-bang is the term often used to describe basic directional-control valves. It refers to how the valves shift'from completely open to completely 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 binary valve. As in digital electronics, binary 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.

Perhaps the simplest of all directional-control valves is the check valve, a specific type of binary 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 . Figure 1 shows a cross-section of a spring-loaded check valve and its ISO representation.

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.

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 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 (Fig. 2). One port receives pressurized fluid from the pump, and one routes fluid back to the reservoir. 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 Fig. 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.)

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).

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