Directional Control

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Chapter 5

Directional Control

Check Valves Directional ControlValves Directional Control Valve

Actuators Logic Valves
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Chapter 5Directional Control

A restriction check valve

has an orifice placed in

the center of the poppet

to permit restricted fluid

flow through the valve inthe NC position.


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A pilot-to-open check

valve allows fluid flow in

one direction and stops

fluid flow in the opposite

direction until it receivesa pressure signal through

a pilot line applied to the

piston port.


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A pilot-to-open check valve is often used to hold a

double-acting cylinder in place when the cylinder has a

load suspended on it.


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A pilot-to-close check valve allows fluid flow in

one direction until the pilot signal is sent to close

the valve.


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The primary function of a directional control valve is

to direct or prevent fluid flow to specific piping or

system actuators.


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A schematic diagram can be used to determine the

operating characteristics of a directional control valve.


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A way is the number

of valve ports, and

position is the

number of different

positions that a spoolof a directional

control valve is

capable of moving

into in a directionalcontrol valve.


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In a schematic diagram,

internal arrows inside the

position boxes depict the

direction of fluid flow for

each spool position.


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Naming standards include the number of positions,

the number of ways, the types of directional control

valve actuators, and the normal spool center position.


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Two-position,

two-way

directional

control valves

can turn on andoff separate

hydraulic

systems that are

powered by thesame hydraulic

power unit.


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A two-position,

three-way

directional control

valve has two

positions, one toallow fluid flow from

the cylinder port to

the tank port and

the other to allowfluid flow from the

pump to the

cylinder port.


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Two-position, four-way

directional control valves

are typically used to

control cylinders that lift

heavy loads.


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An advantage of three-position, four-way directional

control valves is that they have four different center

positions that can perform four different operations.


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Open-center positions are typically used in

applications where there is only one cylinder in the

hydraulic system.


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When using a closed-

center position, fluid flow

created by a variable-

displacement, pressure-

compensated pumpeither goes through a

relief valve or is

automatically reduced to

no flow.


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Tandem-center positions are used to hold a cylinder in

the desired position while allowing fluid flow to be

directed back to the tank without the need to activate a

relief valve.


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Float-center positions allow a the piston in a cylinder to

move while it is in the center position and can still

direct fluid flow back to the tank.


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Subplate mounting

allows for a directional

control valve to be

mounted to a plate that

attaches directly tosystem piping.


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Spring actuators are

used to shift the spool of

a directional control valve

to normal position.


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A pilot pressure actuator

uses fluid pressure to

actuate a directional

control valve spool.


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Directional control valves

can be single- or double-

solenoid actuated.


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When shifting is required, fluid flow from a piggyback

valve is directed as pilot pressure to either side of the

main directional control valve spool.


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A pilot choke is mounted between a main valve and a

pilot valve that is used to slow or briefly delay

reversals in spool movement.


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A pilot piston allows a directional control valve spool

to shift quickly by placing the pilot piston in a pilot-

pressure chamber near the spool in the main valve.


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A detent operates by locking the spool in a desiredposition and not allowing it to move back into its

normal position until it is manually repositioned.


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In an ANDhydraulic circuit,

pressure must

be present from

the hydraulicsystem (input 1)

and through the

pilot line (input

2) before the

cylinder (output)

can operate.


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Directional Control