What Are Directional Control Valves, And Why Do You Need One?

Directional control valves are used to control the direction and movement of hydraulic fluid through a system. They are often referred to as switching valves, and come in three main categories: hydraulic check valves, directional spool valves and poppet valves that make up the different types of control valves.

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Things To Consider When Choosing Directional Control Valves

There are five major points to consider when it comes to analysing the performance and suitability of directional control valves:

• Dynamic Power Limits;
• Static Power Limits;
• Resistance To Flow;
• Switching Time;
• Leakage;

What Is A Hydraulic Check Valve And How Can They Be Used?

Check valves are the simplest and most common form of directional control valve which are regularly used in hydraulic systems. These valves can be used to stop the flow of liquid in one direction, whilst still allowing the free flow of fluid in the opposite direction. These models are also commonly known as non-return valves.

Hydraulic check valves can also fulfil a range of other roles within a hydraulic system, including:

• Prefill-valves for anti-cavitation;
• Bypass valves or return-line filters;
• Denying flow in one direction;
• Pre-tensioning by creating backpressure within the system;
• Protect hydraulic components against surges in pressures;

Most check valves are spring-loaded, and rely on a ball or plate to seal the flow in a single direction. Check valves are designed to be able to reliably isolate circuits without running the risk of leakage. A range of different elements, including poppets with soft seals can also be used within these valves to isolate circuits.

What Is A Directional Spool Valve And How Can They Be Used?

These kinds of directional control valves are composed of a moving spool which is situated inside the housing of a valve. An actuating force then moves the control spool, which allows the channels within the housing to be connected or separated. These types of directional control valves have a range of unique features which makes them suitable for different conditions, including:

• Their simplicity makes them cheap to produce;
• A low actuating force;
• A high switching power;
• Low-level losses, despite consistent oil leakage;
• A wide range of control functions for operator ease;

These types of control valves can be either directly-operated or pilot-operated. These valves can be connected with solenoids or mechanically controlled via levers and rollers, or via hydraulic or pneumatic systems.

What Are Directional Poppet Valves?

These types of control valves are fitted into housing bores with a threaded connection, which is why they are commonly referred to as cartridge valves. These valves are suitable for operating situations of up to 1,000bar and can contain a range of seating elements, including balls, poppets and plates.

Just some of the key features which make these models extremely useful can include:

• No leakage;
• A long and reliable product lifetime;
• High maximum operating capacities;
• Very good sealant characteristics;

Their design allows these valves to become more tightly sealed when the operating pressure increased. Compared to other kinds of control valves, their maximum flow is often limited, making them unsuitable for systems which require high flow rates.

For High-Quality Directional Control Valves, Contact Flowfit Today

Here at Flowfit, we can provide a diverse range of valves, including hydraulic check valves for a diverse range of systems and applications. For more information, please don’t hesitate to get in touch with our professional team of hydraulic specialists today on 876 033.

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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.

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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 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).