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Sorting through pneumatic directional control valves

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A staff report

A traditional, digital, pneumatic directional control valve consists of a body with external ports that lead to internal flow passages. One or more elements inside the valve move to open or block these passages, controlling the direction of air flow through the valve.

One method of classifying directional control valves is by the number of flow paths or ways within the valve in its various operating conditions. Other factors to be considered are the number of individual ports, the number of flow paths for which the valve is designed, and the internal connection of the ports by the movable element.

Let me count the ways
Simple 2-way directional valves, Figure 1, have only two ports, connected with a passage that can be opened or closed by an internal element. In one extreme position, the passage through the valve is open and air can flow from one port to the other. In the other extreme, the passage is closed to block flow between ports. This valve thus provides an on-off function.

Figure 1
Figure 1. ISO symbols for a sampling of basic digital valves (drawn with unspecified actuators): A is a normally closed 2-way valve; B is a normally open 2-way valve; C is a normally closed 3-way valve; D is a normally open 3-way valve; and E is a 2-position 4-way valve shown with a detent.

A 3-way directional valve has three ports connected through passages within the valve body. If port A is connected to an actuator, port P is connected to a pressure source, and port T is open to exhaust, the valve will control the flow of air to (and exhaust from) port A. The function of this valve is either to pressurize an actuator port or exhaust it. When the moving element of a 3-way valve is in one extreme position, the pressure passage is connected to the actuator passage. When in the other extreme position, the element connects the same actuator passage with the exhaust passage. This valve may be used to perform other circuit functions — e.g. it can select the supply pressure to a device when two different pressures are connected to ports P and T.

Perhaps the most common directional valve in simple pneumatic systems consists of a pressure port, two actuator ports, and one or more exhaust ports. These valves are designated as 4-way valves because they have four distinct flow paths within their bodies.

A typical application for a 4-ported, 4-way directional valve is to cause reversible motion of a cylinder. To perform this function, the movable element connects the pressure port with one cylinder port. Simultaneously, the element connects the other cylinder port with the exhaust port. When the valve shifts, the flow-path orientation reverses.

Four-way valves also are built with five external ports. They are referred to as 5-ported, 4-way valves. Such valves provide the same basic control of flow paths as the 4-ported version to control the motion of double-acting cylinders. With five ports, 4-way valves may provide dual-pressure control; for example, one pressure can be supplied to a cylinder when it is extending, and a different pressure during retraction. Another way to connect a 5-ported, 4-way valve is with a common inlet pressure source and two exhaust ports. This allows individual control of the exhaust flow from each of the valve’s outlet ports. This arrangement can provide inexpensive velocity control of pneumatic cylinders by appropriate restriction at the individual exhaust ports.

Springs, passing, and non-passing
The 2-position directional valves shown as A through D in Figure 1 use an actuator to shift their movable element to one extreme position. Then the element is returned to its original position by means of a spring. Two-position valves of this nature are known as spring-returned or spring-offset valves in pneumatic systems. (Other designs return the element with air pressure.)

Spring-returned 2-way valves can be designated as either normally passing or normally non-passing when the actuator is not energized. In the normally passing mode, fluid may flow through the valve; in the normally non-passing type, fluid may not pass. Because there is always a passage open through 2-position, 3-way valves, normally nonpassing usually indicates that the pressure passage is blocked when the valve actuator is not energized.

Three-position valves
The 4-way valves mentioned so far are 2-position devices providing alternate flow paths — one in the normal position, the other in the actuated position. A family of 3-position industrial pneumatic 4-way valves (in both 4- and 5-ported versions) also is readily available. In these valves, the two extreme positions are directly related to the cylinder’s direction of motion. They are the power positions of the valve, controlling the cylinder’s movement, first in one direction, then in the other — just as a 2-position, 4-way valve does.

The center position of a 3-position valve satisfies other system requirements. (The center position also is commonly referred to as the neutral condition.) There are a variety of center conditions available with 4-way directional valves, Figure 2. The most common center conditions are: pressure center (with both cylinder ports open to pressure), exhaust center (with both cylinder ports open to exhaust and pressure blocked), and blocked center (with all ports blocked).

Figure 2
Figure 2. Graphic symbols for most common center positions of 5-ported, 4-way directional valves.

The pressure-center type of valve might be used to alternately control the motion of two or three groups of singleacting cylinders. When the valve is in its center or neutral position, pressure is directed to both cylinders, causing both to extend. If the valve is shifted to either extreme position, one cylinder group retracts while the other remains extended. When the valve is shifted in the opposite direction, the motion of the cylinder groups is reversed.

With the exhaust-center condition, the double-acting cylinder is free to float with the valve in its neutral state, because both ports of the cylinder are opened to exhaust. The cylinder rod can be moved manually (subject to internal friction and external loading).

The blocked-center condition blocks all working ports and often is called closed center. Depending on the circuit design and cylinder loading conditions, this center condition may provide a holding action on the device to which the valve is connected. With suitable controls — and taking into account the compressibility of air — this type of valve can stop a cylinder at intermediate points along its stroke as it travels in either direction.

Basic valve designs
Basic valve design should be considered when selecting pneumatic directional control valves. The valve’s sealing function may be accomplished by shear action or poppet-type elements.

Shear-action valves control flow with an element that slides across the flow paths. There are four basic types: sliding plate or rotary disc, packed spool, packed bore, and lapped spool.

The sliding plate or rotary disc uses pressure unbalance to force the sealing mechanism against a mating surface. The effect is to control the flow of air to and from desired ports and seal the flow from others. This design can provide 2-, 3-, or 4-way action.

The other types of shear-action valves incorporate spools as their moving members. Typically, the spool strokes within a metal bore. Packedspool valves have resilient seals fitted around the spool lands to effect almost leak-proof sealing. The packed-bore valve is another very common directional valve. It’s metal spool works in a bore fitted with several stationary seals that provide isolation between ports. The lapped-spool valve is also classified as a shear-action type. This design depends on a close fit between the spool and bore — without seals —to control the flow of air from one port to another. This design does not give bubble- tight sealing because of the clearance between the spool and bore.

Operating characteristics
These characteristics make slidingplate valves attractive to designers: They can operate for many millions of cycles, even under adverse conditions, because they wear in during use. Wear tends to keep mating parts in contact, thus controlling leakage over long periods of use. The valve action tends to prevent foreign material from lodging between the mating surfaces. Temperature extremes do not seriously affect the valve. This design can function as both a directional control and flow control. Finally, this type of valve has good air-flow throttling in proportion to the movement of the control valve .

Some other considerations are important to minimize circuit problems with sliding-plate valves: Large forces may be required to shift the valve slide, especially at high pressure, when using designs with large pressure-sealed areas which are not at least partially pressure balanced. Units also may require long actuator stroke lengths to obtain full flow capabilities.

Sliding-plate valves must be continuously lubricated for maximum life, and this design may not provide bubbletight sealing. The sealing member can be forced from its mating surface if the pressure under the plate exceeds line pressure. These valves may not be suitable for positioning or stopping cylinders where such conditions can occur.

Packed-spool valves offer the circuit designer these characteristics: The design maintains the seal shape and size for long life. Packed-spool valves are less affected by improper torquing during installation. The valve’s spool is balanced. Packed-spool valves are relatively insensitive to contamination. Maintenance is less costly from the standpoint of parts replacement.

Other considerations are important to minimize circuit problems with packed-spool valves. Compatibility of seals and airborne liquid contaminants must be checked. Swollen seals require excessive shifting force on the spool; shrinking seals will increase leakage. Also, extreme temperatures may cause the seals to change size or harden.

Packed-bore valves offer the circuit designer these characteristics: The valve is available in a variety of flowpath patterns in most porting and actuating configurations. The design results in a balanced spool, so shifting forces are only slightly greater than for metalto- metal designs. Sudden pressure surges should not cause the valve to lose its sealing capabilities. The resilient seals make the valve less vulnerable to abrasion by foreign material than metal-to-metal designs. Limited contact areas reduce any tendency for the spool to varnish in place. The spool will not bind under ambient temperatures found in most industrial applications. Seals may be changed without changing mating parts.

Consider these factors to minimize potential circuit problems when designing with packed-bore valves: Compatibility of seals and airborne liquids must be checked (for the reasons cited under packed-spool valves). Lubrication of the air supply is suggested for longer valve life. Extreme temperatures may cause the seals to change size or harden.

Operating characteristics of lappedspool valves include: The design can provide almost any flow-path pattern desired, in most porting and actuating configurations. External forces required to shift a balanced spool are low; this is important when direct actuation of the spool is required. The force needed to position the spool tends to remain constant during a shifting stroke. This helps the spool complete its stroke once stiction has been overcome.

The lapped-spool design can be configured to prevent interconnection of pressure, outlet, and exhaust ports while the valve is being shifted. The elimination of crossover flow while the spool is in transit reduces the chance of shift failure when operating at low pressures as a pilot-operated valve, and can eliminate spurious signal pulses. Because the spool is balanced, sudden pressure surges resulting from external forces on cylinders cannot cause the valve to lose its sealing capabilities.

Other factors must be considered to minimize potential problems when designing with metal-to-metal lappedspool valves. Long-stroke requirements for spools may call for excessive travel of mechanical actuators to complete the shifting of the valve. Closely fitted parts are vulnerable to the ingress of foreign matter between mating parts, which can produce rapid wear and leakage, or cause the parts to stick.

Lapped-spool valves require good filtration and consistent lubrication. (It often may be best to run on filtered, dry, unlubricated air.) Oxidized airborne lubricant from a compressor or other material carried down an air line may cause the closely fitted parts to varnish in place. This is more likely to occur when valves must remain idle for long periods of time — such as over weekends. Initial cost is higher and maintenance costs may be also. Improper torquing during installation may cause the valve spool to fail to shift due to valve-body distortion. If the valve is subjected to vibration, detents may be necessary.

Poppet valves
Directional valves which incorporate poppets as their movable parts are suited for applications that need high flows. Because poppets open relatively large ports with short strokes, they have an inherent characteristic of fast response with minimum wear. Poppet valves for pneumatic service usually have resilient seals to provide tight sealing. Most poppet valves found in industrial applications are 3-way configuration. A 4-way poppet valve usually consists of two 3-way units.

Operating characteristics of poppet valves include rapid cycling capabilities; their lightweight moving members require only a short stroke to provide maximum flow opening. The design is reliable and has been proven over time. The valve’s short stroke generates minimum wear for maximum service life.

Resilient seals shut off flow paths tightly and help absorb the kinetic energy of the moving members. The design resists damage from foreign matter carried through the air line because the seats are self-cleaning. These valves are readily maintained — with inexpensive parts. The valve’s performance is not sensitive to air-line lubricants or to other materials carried in the air stream.

To minimize circuit problems with poppet type valves, consider these points: Poppet designs with direct-operated actuators may be bulky in large flow-port sizes. It is difficult to obtain some flow-path configurations with poppet valves. Also, their design inherently allows flow from the pressure port to escape to atmosphere as the valve shifts. Crossover flow may drop pressure at valve’s the inlet port below the rated minimum operating level, causing the valve to malfunction.

Directional-valve actuators
Directional valves’ movable members can be shifted to their positions by manual, mechanical, electrical, or pneumatic actuators, Figure 3. When directional valves must be controlled at an operator’s discretion, his or her muscle power is used on a manual actuator. Mechanical actuation is chosen when the shifting of a directional valve must occur at the time a cylinder physically reaches a specific position.

Figure 3
Figure 3. Schematics of common valve actuators.

Directional valves also can be remotely shifted with air or electricity. In air-operated valves, pilot pressure is applied to the spool ends or to separate pilot pistons. In electrically operated valves, a solenoid responds to an electrical signal (AC or DC, depending on design) and physically displaces the valve’s moving member.

Solenoid actuation is perhaps the most common way to shift pneumatic directional valves because electrical and electronic controls for so many other machine actions are readily available and popular. These controls can easily integrate the air-valve solenoids for sequencing and automation.

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