In automation systems, valves are equivalent to relays that control the flow of electricity. Instead of distributing electricity to motors, drives, and other devices, Directional Control Valves distribute air to cylinders, actuators, and nozzles.
A pneumatic valve, also known as a directional control valve, can be operated manually, by the solenoid, or by air. In their simplest form, 2-way and 3-way valves can be considered normally open (NO) or normally closed (NC), terms that refer to the state of the valve without power. Another very common valve is a four-way valve, which switches supply and exhaust between two ports.
Typically, manually operated valves are controlled by a foot pedal, toggle actuator, handle, knob, or push button. When the Directional Control Valves are activated, a spring or the operator returns the valve to its home position.
In solenoid-operated valves, an electrical coil controls the position of a poppet, plunger, or spool to open or close a valve. Solenoid control voltages are typically 12VDC, 24VAC/DC, 120VAC, or 240VAC.
Air piloted valves are operated by an external air source, such as a solenoid in a remote location. Additionally, the valve can be internally air piloted, allowing the use of a small integrated electric solenoid to provide the air pilot signal to control the larger valve spool.
Directional Control Valve Types
Pneumatic valve configurations or valve types describe how air is connected to and switched through the device. Understanding how this configuration impacts the device the valve controls is essential to selecting the right valve for the job.
Pneumatic symbols for Directional Control Valves have three parts: actuation (how the valve is actuated), position (the number of positions and ports), and flow (how air flows through the device). As a result of pushing the boxes left or right, there are actuation methods on both sides of the symbol.
A number of boxes indicate a number of positions, usually two or three. The information in each box defines the flow of supply air or exhaust for each position.
There are one or more flow paths in each valve position, and the arrows indicate air and exhaust flow. In order to determine the number of ports, one must count a single box of the symbol at the points where each path touches a box. A “T” symbol indicates that a flow path can also be blocked.
A valve’s number of ports and positions determine the work it is designed for, so selecting these options is an important choice. In this valve, there is one inlet port and one outlet port on a 2-port or 2-way, 2-position valve. There is no way to vent air pressure through this type of valve unless that is its only purpose.
There are a number of different ways for air to enter or leave a valve. while the different states are called “positions”. Industrial valves are typically 2-, 3- or 4-way configurations. 2- and 3-way valves have two positions while 4-way valves can either have two or three positions.
Common Valve types :
- 2-port (2-way), 2-position
- 3-port (3-way), 2-position
- 5-port (4-way), 2-position
- 5-port (4-way), 3-position
A 3-port or 3-way, 2-position valve can supply and exhaust pressure by adding a third port. These ports are air in, air out, and exhaust. This type of valve only works well in applications such as single-acting cylinders with a spring return, or in air blow off processes such as blowing chips in a machining process.
By adding two more ports, the valve becomes a 5-port (4-way), 2-position valve. Technically, a 5-port valve is a 4-way valve since two ports are open to ‘Exhaust’. This is done primarily to simplify valve construction.
This is the most common directional valve because it can extend and retract double-acting cylinders, providing a wide range of control options. There are two inlet ports, two outlet ports, and two exhaust ports on this type of valve.
In a 2-position configuration, one output flows air from the inlet, and the other from the exhaust port. When the valve is switched, the two outputs are in opposing modes. This is the most common method of extending and retracting a double-acting pneumatic actuator, pressurizing one side of the cylinder and exhausting the other.
Two-position, single solenoid Directional Control Valves have a spring return. When an energized valve is connected to a double-acting cylinder if electrical power is lost (such as when an emergency stop is pressed) but the air remains, then the cylinder will retract. The cylinder will retract once pressure is restored unless the valve is re-energized if the emergency stop also dumps air pressure in the system.
3 Position Directional Control Valves
When a 2-position, double solenoid valve has a detent feature, the valve spool is held at the position it was at the moment the emergency stop was pressed.
Regardless of whether the cylinder was at mid-stroke when the emergency stop button was pressed, when air is reapplied, the valve commands the cylinder to continue its motion to the original energized position, even with both solenoids de-energized. Due to the maintained valve position, this motion can cause issues.
Unintended cylinder motion after an emergency stop, for example, can damage tooling and should be avoided in the design process.
The 5-port or 4-way, 3-position valve has a center position that can be chosen to exhaust or block pressure when neither solenoid is actuated.
The valves are typically used in applications where it is necessary to stop a cylinder in mid-stroke. The reset and start buttons are also used to an inch or jog cylinders or when air must exhaust during an emergency stop and no movement is permitted after air is reapplied until the reset or start button is pressed.
Using these valves requires caution due to their additional complexity. Under emergency stop conditions, center block 3-position valves can trap air and cause unexpected movement, especially if tooling is jammed.
In this case, all energy, including trapped air, should be removed when an emergency stop is pressed. It is also possible for air to leak out, causing the cylinder to drift or drop.
The 3-position center exhaust valve dumps all pressure into a cylinder in an emergency stop or when both solenoids are de-energized.
The lack of air during startup will result in very high and potentially damaging cylinder speeds during the first cycle. At startup, both sides of the cylinder must be charged with air pressure.
Valve Form Factor
The form factor of a valve is often determined by its use. This includes both its internal configuration and its external design. Poppet, diaphragm, and spool are common internal configurations. Usually, poppet valves use direct solenoid operation, similar to a gate valve in a two-way, two-position application.
Through a pilot port, a piston moves the valve stem, opening the valve. A diaphragm valve is similar to a poppet valve, but the operator solenoid is physically isolated from the valve and the working fluid by the diaphragm.
On 4-way, 2- and 3-position body ported valves, spool valves are usually direct-acting or pilot-acting. Spool valves have pistons with seals that move along a bore when shifted, opening or closing ports depending on their position. Flow paths can be changed more easily with them, they are easy to actuate, and they are not affected by pressure.
Many valves are stackable due to their external design, which allows more Directional Control Valves to fit into a smaller space. It is easier to mount some valves than others, and some can be specified to mount individually or in a manifold. Applications with high pneumatic valve counts may benefit from compact, modular, manifold-mounted valves.
Generally, valves can be connected electrically in three ways: hardwired, modular wired, or digitally. Most Directional Control Valves have a connector built-in with removable flying leads, or a DIN style wiring connector.
In manifold mounted valve configurations, modular wiring typically utilize. A D-sub connector is usually embedded in the manifold base. For large pneumatic systems, this is an efficient and clean integration option.
Ethernet/IP and other digital communication protocols are becoming increasingly popular. As a way to replace individual discrete wires with a single cable. It is especially useful when a large number of valves need to operate in a small space. In addition, this can reduce costs on the controller side by using a single communication port. Rather than multiple output modules.
To connect pneumatic tubing to the Directional Control Valves, threaded ports or push-to-connect fittings are also available.
For pneumatic directional control applications, a 5-port (4-way), 2-position valve is often the best choice. Adding a manual operator and an indicator light on the electrical connection makes maintenance easier. So these options will consider.