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Basic Hydraulics - Directional Control Valve

CHAPTER 5 - Directional Control Valve

Figure 5.1 - the internal workings of a directional control valve
OVERVIEW
The directional control valve is the component that starts, stops, and changes the direction of the fluid flowing through a hydraulic system. In addition to this, the directional control valve actually designates the type of hydraulic system design, either open or closed. The exercises in this section will give you a hands-on opportunity to see how these valves operate and learn the importance they play in a system.

Directional control valves are used to start, stop, and change the direction of flow in a hydraulic circuit. Although they may be designed as rotary or poppet style, the spool type directional control is the most common. This design consists of a body with internal passages that are connected or sealed by a sliding spool along the lands of the valve. Directional spool valves are sealed along the clearance between the moving spool, land, and the housing. The degree of sealing depends on the clearance, the viscosity of the fluid, and the pressure. Because of this slight leakage, spool type directional valves cannot alone hydraulically lock the actuator.
Figure 5.2 - the schematic symbols used to show the actuated position
Directional control valves are primarily designated by their number of possible positions, port connections or ways, and how they are actuated or energized. For example, the number of porting connections is designated as ways, or possible flow paths. A four-way valve would have four ports: P, T, A, and B. A three-position valve is indicated by three connected boxes. There are many ways to actuate or shift a valve: push button, hand lever, foot pedal, mechanical, hydraulic pilot, air pilot, solenoid, and spring.
Figure 5.3 - four examples of normally opened or closed 3-way
Directional control valves are also designated as normally opened or normally closed. These designations accompany two-position valves as follows: spring offset, solenoid operated, two-way valve normally closed; spring offset, solenoid operated, two-way valve normally open; spring offset, solenoid operated, three-way valve normally closed; spring offset, solenoid operated, three-way valve normally open.
Figure 5.4 - the industrial standard porting pattern for spool type
The spool type directional control valves in industrial applications are sub-plate or manifold mounted. The porting pattern is industry standard and designed by valve size. Directional control valve sizing is according to flow capacity, which is critical to the proper function of the valve. Port size along with pressure drop across the valve determines the flow capacity of the valve. This mounting pattern and size is designed as a D02 nominal flow 5 gpm, D03 nominal flow 10 gpm, D05 nominal flow 20 gpm, D05H nominal flow 25 gpm, D07 nominal flow 30 gpm, D08 nominal flow 60 gpm, or D10 nominal flow 100 gpm.



DIRECT ACTING
direct acting directional control valve may be either manual or solenoid actuated. Direct acting indicates that some method of force is applied directly to the spool, causing the spool to shift. In the illustration, energizing the solenoid or coil creates an electromagnetic force, which wants to pull the armature into the magnetic field. As this occurs, the connected pushpin moves the spool in the same direction while compressing the return spring. As the spool valve shifts, port P opens to port A, and port B opens to port T or tank. This allows the cylinder to extend. When the coil is de-energized, the return springs move the spool back to its center position.
Figure 5.5 - a cutaway view of a solenoid actuated

PILOT OPERATED
For control of systems requiring high flows, usually over 35 gpm, pilot operated directional control valves must be used due to the higher force required to shift the spool. The top valve, called the pilot valve, is used to hydraulically shift the bottom valve, or the main valve. To accomplish this, fluid is directed from either an internal or an external source to the pilot valve. When we energize the pilot valve, fluid is directed to one side of the main spool. This will shift the spool, opening the pressure port to the work port and directing return fluid back to the tank. It is often required to externally pilot or send fluid to the pilot valve from an external source.
Figure 5.6 - a cutaway view of pilot operated
The advantages to external piloting are constant pressure supply regardless of other influences in the main system, and the source may be filtered separately to prevent silting of the pilot valve. In addition to externally piloting, we may also externally or internally drain the valve. If the pilot valve is internally drained, oil flows directly into the tank chamber of the main valve. Pressure or flow surges occurring in the tank port when operating the main control spool may affect the unloaded side of the main spool, as well as the pilot valve. To avoid this, the pilot valve is externally drained by feeding pilot fluid flow back to the tank. Pilot operated directional control valves can be field changed from internal to external pilot and drain.


OPEN vs CLOSED CENTER
Most hydraulic circuits are categorized into two basic types: open center or closed center. The directional control valve actually designates the type of circuit. Open center circuits are defined as circuits which route pump flow back to the reservoir through the directional control valve during neutral or dwell time. This type of circuit typically uses a fixed volume pump, such as a gear pump. If flow were to be blocked in neutral or when the directional control valve is centered, it would force flow over the relief valve. This could possibly create an excessive amount of heat and would be an incorrect design. A closed center circuit blocks pump flow at the directional control valve in neutral or when centered. We must utilize a pressure compensated pump, such as a piston pump, which will de-stroke, or an unloading circuit used with a fixed volume pump.
Figure 5.7 - an example of a closed center directional control valve
A three-position directional control valve incorporates a neutral or center position which designates the circuit as open or closed, depending on the interconnection of the P and T ports, and designates the type of work application depending on the configuration of the A and B ports. The four most common types of three-position valves are: open type, closed type, flow type, and tandem type.
Figure 5.8 - a schematic illustration of the four most common 3 position
The open type configuration in Figure 5.9 connects P, T, A, and B together, giving us an open center and work force that drain to the tank. This configuration is often used in motor circuits to allow freewheeling in neutral.
Figure 5.9 - cutaway and schematic view of an open type
The closed type configuration in Figure 5.10 blocks P, T, A, and B in neutral, giving us a closed center. This center type is common in parallel circuits where stopping and holding a load in mid-cycle is desired.
Figure 5.10 - cutaway and schematic view of a closed type
The float type configuration in Figure 5.11 blocks P while interconnecting A and B ports to T. Because P is blocked, the circuit becomes closed center. This center type is commonly used in parallel circuits where freewheeling a hydraulic motor in neutral is required.
Figure 5.11 - cutaway and schematic view of a float type
The tandem type configuration in Figure 5.12 connects P to T while blocking work ports A and B. With P and T connected, the circuit is open center. This center type is used in connection with a fixed displacement pump. Because A and B are blocked, the load can be held in neutral.
Figure 5.12 –cutaway and schematic view of a tandem type
When specifying a directional control valve type, the type of circuit required and the work application must be taken into consideration.



SUMMARY
The directional control valve is the component that starts, stops, and changes the direction of the fluid flowing through a hydraulic system.

Directional control valves are primarily designated by:
  • Their number of possible positions.
  • Port connections or ways.
  • How they are actuated or energized.
The spool type directional control valves in industrial applications are sub-plate or manifold mounted.

A direct acting directional control valve may be either manual or solenoid actuated.

For control of systems requiring high flows, usually over 35 gpm, pilot operated directional control
valves must be used.

Most hydraulic circuits are categorized into two basic types: open center or closed center. The directional control valve actually designates the type of circuit.

The four most common types of three-position valves are: open type, closed type, float type, and
tandem type.


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