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

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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).

Selecting the appropriate valve for any application is imperative to ensure proper direction of flow, flow rate, exhaust function, and even basic valve functionality. 

Valve Function

Valve function outlines the number of flow paths a valve has as well as the positions of a valve. The flow paths or ports are referred to as ways in the valve function. The number of positions is how many different positions that valve can be in.

2/2 Valves

A 2-way 2 position valve is known as a 2/2 valve. A basic example of a 2/2 valve is a ball valve. A basic ball valve has two ports (in and out) along with two positions (open or closed).

3/2 Valves

A 3-way 2 position valve is known as a 3/2 valve. A basic example of a 3/2 valve is a pilot valve like the E310A-1W012 10 mm valve from Clippard. A pilot valve is used to send a pressure signal when actuated and exhaust that signal when de-actuated. This valve has 3 ways (in or supply, output, and exhaust) along with 2 positions (closed&#;pressure blocked with the outlet vented to atmosphere, or open&#;pressure connected to outlet and the exhaust blocked).

4/2 Valves

A 4-way 2 position valve is known as a 4/2. These valves are common in double acting cylinder applications where they are always supplying pressure to one side of the cylinder while venting the opposite side to atmosphere.

5/2 Valves

A 5-way valve is essentially the same function as a 4/2, but with the added benefit of independent exhaust paths for each output port. This is most commonly not referred to as a 5-way, but a fully ported four way valve. This allows the designer to have specific controls on the exhaust paths for either the extend and the retract exhaust paths. Some manufacturers do not reference 5/2 or 4/2 when offering a 4-way valve and although the differences are subtle, it may be import to your specific application.

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6/2 Valves

Less common but still needed in specific applications, the 6-way valve is similar to having two 3-way valves mechanically tied within a single valve. Applications that require fail safe feature that assure that both valves will actuate at the same time are ideal for this function.

3 Position Valves

Some valves may have 3 positions in their function, which allows all ports to be blocked or open to atmosphere. These can be electronic valves or manual control valves that are spring centered with no power. These valves are not very common and are used for specific applications to either lock in a specific position of a cylinder or to open paths to atmosphere allowing the cylinder to be mechanically or manually positioned.

Valve Porting

Valve Porting pertains to the usability of the port and the direction of flow through the valve in different positions. There are two key points we will cover. First, the usage of the port&#;can we physically connect to the port? And second, the flow direction that the valve must accommodate for the application.

Fully Ported

A "fully ported" valve is any valve that you can physically capture or plumb to all the ports. This does not mean that all the ports are necessarily threaded, but this does mean that the flow path can be captured via a specific method opposed to a vent hole that goes to atmosphere.

Fully ported valves are ideal for applications where the media needs to be plumbed, for example: in clean rooms where all gases must be vented external to the room. Clippard's ET-2-12 with threads on the inlet port and threads on the outlet port could be referred to as fully ported since all the ports on the valve can be plumbed, but the ET-3-12 is not fully ported as the exhaust path is vented through small holes in the valves cap. Hence Clippard's offering of the ETO-3-12 fully ported version of the ET-3-12.

It is important to note that "fully ported" does not specify that all ports can be pressurized, only that they can be ported.

Bi-Directional

A bi-directional valve is a 2/2 valve that is not sensitive to which port is the inlet or the outlet. In Europe this is referred to as a bi-stable valve. Bi-directional valves are necessary for demanding applications where the higher pressure differential could be on either the inlet or the outlet.

A common ball valve is a simple example of a valve that doesn't care what side the pressure is coming from and works in either direction. This valve would be known as bi-directional.

The opposite valve to this would be a check valve, which flows in a single direction only.

Universally Ported

What about the possibility of a valve with more than two ports that is fully ported and bi-directional? This would be a universally ported valve. A 3-way universally ported valve can be configured for six functions, such as: 3/2 normally-closed, 3/2 normally-open, 2/2 normally-closed, 2/2 normally-open, selector, or even diverter.

It is important to note that not all fully ported 3/2 valves are universally ported.

Electronic Valve Actuation Types

The actuation is what shifts the valve from one position to another and there are two popular types that need to be considered in selecting the proper valve for any application. The actuation can be any of the functions or porting discussed above and is another valve characteristic that is important to understand.

Direct Acting

Direct acting valves are simple in design since the actuation of the valve directly translates into the change of the position. This is typically achieved by moving the valve element via a magnetic coil. All of the Clippard EV Series valves are direct acting since energizing the coil directly lifts the spider (a flat armature spring) to change the flow state. The benefits of direct acting valves are that they have no minimum pressure requirements, fast response time, and long life due to few moving parts. The lack of pressure requirements makes the valve design ideal for universally ported functions. Clippard's EVO-3-12 is a great example of a universally ported valve. The EVO-3-12 can be used with 80 psig on the normally-open inlet port, vacuum on the normally-open exhaust and select between them for an output.

Pressure Assisted

Pressure assisted valves are the most common valves used in automation applications. They rely on the pressure supplied to the valve to assist with actuation. The amount of force available from pressurized air to shift a valve is far greater than the force that can be generated from a magnetic coil on a direct acting valve with similar flow and pressure ratings. Once the internal pilot valve opens the pressure will assist the valve in shifting. This allows the ports to open and the valve to flow at high flow and/or pressures.

The benefit of pressure assisted valves is that they can take little electrical power because they are using the power of pressurized media. This allows for a smaller overall size for the given flow rates and port size. The drawbacks can be that they require a minimum pressure and don't allow universal porting options unless the pilot pressure is from an external force.

Pressure assisted valves are generally engineered for specific applications to achieve the maximum flow and pressure ranges while using the smallest amount of electrical power possible. Other pressure assisted valves that do not have internal pilots are balanced and tuned so that pressure works in conjunction with spring forces and the coil to lift the seat and allow for flow. An example of a pressure assisted valve is the Clippard Maximatic MME-41PES-D012. Energizing the coil on the MME-41PES-D012 applies pressure to a piston to shift the spool in the valve.

For help selecting the best valve for your application, call 877-245- or contact your local distributor.
 

By Mike Kettering &#; Technical Sales Specialist &#; Clippard