Directional Control Valves

Directional control valves perform only three functions:

• stop fluid flow
• allow fluid flow, and
• change direction of fluid flow.

These three functions usually operate in combination.

The simplest directional control valve is the 2-way valve. A 2-way valve stops flow or allows flow. A water faucet is a good example of a 2-way valve. A water faucet allows flow or stops flow by manual control.

A single-acting cylinder needs supply to and exhaust from its port to operate. This requires a 3-way valve. A 3-way valve allows fluid flow to an actuator in one position and exhausts the fluid from it in the other position. Some 3-way valves have a third position that blocks flow at all ports.

A double-acting actuator requires a 4-way valve. A 4-way valve pressurizes and exhausts two ports interdependently. A 3-position, 4-way valve stops an actuator or allows it to float. The 4-way function is a common type of directional control valve for both air and hydraulic circuits. A 3-position, 4-way valve is more common in hydraulic circuits.

The 5-way valve is found most frequently in air circuits. A 5-way valve performs the same function as a 4-way valve. The only difference is an extra tank or exhaust port. (Some suppliers call their 5-way valves, “5-ported 4-ways.") All spool valves are five ported, but hydraulic valves have internally connected exhaust ports going to a common outlet. Because oil must return to tank, it is convenient to connect the dual tank ports to a single return port. For air valves, atmosphere is the tank, so exhaust piping is usually unimportant. Using two exhaust ports makes the valve smaller and less expensive. As will be explained later, dual exhausts used for speed-control mufflers or as dual-pressure inlets make this configuration versatile.

Following are schematic symbols for commonly used directional control valves.

2-way directional control valves
A 2-way directional valve has two ports normally called inlet and outlet. When the inlet is blocked in the at-rest condition, as shown in Figure 8-1, it is referred to as "normally closed" (NC). The at-rest box or the normal condition is the one with the flow lines going to and from it.

The boxes or enclosures represent the valve’s positions. In Figure 8-1, the active box shows blocked ports, or a closed condition, while the upper box shows a flow path. When an operator shifts the valve, it is the same as sliding the upper box down to take the place of the lower box. In the shifted condition there is flow from inlet to outlet. Releasing the palm button in Figure 8-1 allows the valve spring to return to the normal stop flow condition. A 2-way valve makes a blow-off device or runs a fluid motor in one direction. By itself, a 2-way valve cannot cycle even a single acting cylinder.

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.

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 01584 876 033.

Alternatively, you can email any questions or concerns to sales@flowfitonline.com and we’ll get back to you as soon as we can!

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