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

HYDRAULICS & PNEUMATICS

Presented by: Dr. Abootorabi

Directional Control Valves

1

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

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Directional control devices can be grouped in four general

classifications:

Shut-off or two-way valves

Check valves

Three-way valves

Four-way valves

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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

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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

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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.

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Typical restriction check

valve:

Directional Control DevicesPilot-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

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Pilot pressure to open check valve


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 actuator17


Typical three-way directional control valve

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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.

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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

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Four-way valve powers double-acting cylinder during extension

and retraction.

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Four-way directional control valves are typically manufactured as two- or three-

position valves. This provides several operating options when designing

circuits.

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Typical two-position, four-

way valve (4/2-way valve ):


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:

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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

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Flow actuation uses internal fluid movement to actuate

the valve. No external mechanism or force is used.


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.

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Circuit containing a mechanically actuated directional control valve

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Directional Control DevicesPilot 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

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Directional Control DevicesElectrical 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

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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

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Typical detent operation

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The end.

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