What is in this article?:
- Engineering Essentials: Directional-Control Valves
- Spool valves
One of the most fundamental components of any fluid power system is the directional-control valve. Here's a summary of the different types, configurations, and uses.
The most common sliding-action valve is the spool-type valve, Figure 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, Figure 5, in which a number of 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.
Traditional valves are built in a configuration for inline mounting. But this arrangement is labor intensive, requires assembly of multiple components, and is prone to leakage. These factors add to installation cost and can have an even greater impact on maintenance costs. Therefore, most designers choose subbase or manifold mounting to avoid these problems.
In the subbase mounting system, all conducting lines are connected to the subbase and the valve attaches onto the base face with matching port pattern, Figure 6. The static subbase does not wear out, so it does not have to be removed. The valve can be lifted off the subbase after loosening a few screws without disturbing the plumbing. Hydraulic integrated circuits extend the subbase concept even further by mounting several valves onto and into a common manifold, Figure 7.
Removing and replacing pneumatic valves is even easier; many are held securely in place by a latch that can be locked or unlocked in seconds. When centralized control is needed, smart valve manifolds incorporating connections for fieldbus networks and integral programmable logic controllers are available, Figure 8.
Directional 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 ofactuators 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 (air or oil) 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.
Electrical 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.
Many valves have combinations of these operators so that the valve can be shifted in response to more than one type of signal. For instance, the solenoid of a 4/3 valve can shift the valve spool in one direction, and a spring would shift it back to the neutral position when the electric signal ceases. Because many valves use more than one type of operator, it is important to determine the role of each. For example, a pilot-solenoid valve may require pilot flow and electrical power to operate. Or it may use either one: solenoid power if an electrical source is available, or pilot operation is the environment must be explosion proof.
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