1. HYDRAULICS & PNEUMATICS
Directional Control Valves
Presented by: Dr. Abootorabi

2. Directional Control Devices
Directional control devices allow a system operator to control the direction of fluid flow in the system
Starting and stopping of actuators
Control of actuator movement direction

3. Directional Control Devices
Directional control devices can be grouped in four general classifications:
Shut-off or two-way valves
Check valves
Three-way valves
Four-way valves

4. Directional Control Devices
The primary purpose of shut-off valves is to block fluid flow through a hydraulic system line:
Globe valve
Gate valve
Ball valve
Spool valve
Needle valve

5. Directional Control Devices
Typical globe valve

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Typical gate valve

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Typical ball valve

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Typical spool valve

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Typical needle valve

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The primary purpose of check valves is to allow free flow in one direction while preventing reverse flow.
Other functions include:
Bypassing components during the return cycle of the system
Providing flow resistance to maintain a minimum system pressure required for pilot operations

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Typical inline check valve

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Typical right-angle check valve

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A standard check valve consists of a valve body containing a one-way valve located between inlet and outlet ports:
The one-way valve allows fluid flow through the valve in only one direction
Some designs contain a spring that seats the valve poppet or ball
In other designs, the poppet is seated only by fluid flow

14. Directional Control Devices
Restriction check valves allow free flow in one direction and restricted flow when flow direction is reversed.
This is accomplished via a metering orifice machined into the poppet.
Typical restriction check valve:

15. Directional Control Devices
Pilot-operated check valves can allow reverse flow through the valve:
Typically, pilot pressure opens the valve
In some designs, pilot pressure may also hold the valve shut to block flow in both directions
Pilot pressure to open check valve

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Pilot pressure to block flow through valve

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Three-way directional control valves provide a means to extend rams and single-acting cylinders.
The actuator is returned to its original position by an external force:
System load
Spring built into the actuator

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Typical three-way directional control valve

19. Directional Control Devices
During extension, the three-way valve connects the actuator inlet line to a system supply line, allowing fluid to enter and extend the unit.
During retraction, the valve blocks the supply line and connects the actuator line to a system return line, allowing external force to return the actuator to its original position while directing displaced fluid to the reservoir.

20. Directional Control Devices
Four-way directional control valves provide a means to power actuators in either direction:
Valve has four external ports for connection to system supply line, reservoir, and inlet and outlet of the actuator
Internal structure of the valve allows the ports to be alternately connected when a change in actuator direction is necessary

21. Directional Control Devices
Four-way valve powers double-acting cylinder during extension and retraction.

22. Directional Control Devices
Four-way directional control valves are typically manufactured as two- or three-position valves. This provides several operating options when designing circuits.
Typical two-position, four-way valve (4/2-way valve ):

23. Directional Control Devices
In two-position valves, the first position operates the actuator in one direction, while the second position reverses the direction.
In three-position valves, a center position is added that provides additional circuit operating characteristics.

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Typical three-position, four-way valve:

25. Directional Control Devices
A number of center position configurations are available:
Closed
Open
Tandem
Floating
Regenerative

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Symbols for four-way valve center position

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The center position affects directional control characteristics and overall system efficiency.
Each style provides distinct operating characteristics that allow hydraulic system designers to obtain maximum performance from a system.

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A number of activation methods are used to shift the internal components of directional control valves.
Five general categories:
Flow actuation
Manual operation
Mechanical operation
Pilot operation
Electrical operation
Flow actuation uses internal fluid movement to actuate the valve. No external mechanism or force is used.

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Manual operation methods include:
Handwheels
Levers
Push buttons
Foot pedals
These devices require constant operator presence and are typically found in less-complex systems.

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Mechanical operation methods include:
Rollers
Cams
Levers
Rams
Mechanical operation is often used when the opening and closing of the valve must occur at a specific position in actuator travel.

31. Directional Control Devices
Circuit containing a mechanically actuated directional control valve

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Pilot operation uses system pressure to activate the valve, rather than physical labor.
This method is effective when:
Larger forces are need to shift the valve
Remote operation is required because of safety or tight physical factors

33. Directional Control Devices
Electrical control of hydraulic systems is common in many types of equipment:
Simple solenoid devices to shift basic valves
Electronic controllers operate proportional solenoid valves to produce extreme accuracy and repeatability

34. Directional Control Devices
Multiple-position directional control valve may be held in a desired position using springs or detents.
Springs are located on the ends of the valve spool to return the valve to its normal operating position.

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Symbols for spring-return valves

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Detents are locking devices that hold the spool in a selected position:
The spool may be held until the operator manually shifts the valve
Increased system pressure at the end of an operation may automatically shift detent valves back to the normal position

37. Directional Control Devices
Typical detent operation

38. The end.