Directional Control Valves
(DCV)
Section 1.4
Section 1.4 Hydraulic Fundamentals
Introduction
Directional Control Valves.
Directional Control Valves provide links between various parts of the
hydraulic system, by connecting of disconnecting and/or changing the
oil flow direction.
Section 1.4 Hydraulic Fundamentals
DCV Designation
P Pressure Port
T Return (tank) Port
A&B Actuator Ports
Designation.
The number of ports and the number of valve positions that a DCV has
will be the basic to describe the type of valve designation
Section 1.4 Hydraulic Fundamentals
Actuation of Spools.
The spools are moved and controlled by different methods
of actuation.
Manually
operated Pneumatic
Hand lever Solenoid
with detent Spring return,
Roller solenoid
tappet operated
Spring
centered
Pedal
Pilot
operated,
Hydraulic electrical
controlled
Section 1.4 Hydraulic Fundamentals
Spools
Grooves Metering Notches
Grooves
1. Hold a small amount of oil for lubrication between body and spool.
2. Assists with sealing.
3. Retains small particles of contamination.
Metering Notches
1. Minimises the possibility of hydraulic lock up.
2. Better control of flow rates with fine operation and cushioning.
Section 1.4 Hydraulic Fundamentals
DCV Construction
There are various types of DCV, in the mobile hydraulics, the spool
valve type is most common.
Spool Valve
It provides good control, metering and cushioning by notches
and Open Centre Load Sensing.
Open Centre is when the spools are in the neutral position the
flow of oil from the pump is directed back to tank at low pressure.
Section 1.4 Hydraulic Fundamentals
Directional Control Valve - CLSS
Closed centre Load Sensing System.
(CLSS)
When all the spools are in the neutral
position, the oil pumped to the control
valve doesn’t return to tank.
The CLSS will normally have a variable
displacement pump in the system, so the
pump de-strokes in neutral and increases
the flow of oil when a hydraulic circuit is
in use.
Section 1.4 Hydraulic Fundamentals
DCV Construction
Seated Valves.
Has the advantage of obtaining
the perfect seal.
Used extensively in PPC systems
Orbital Valve
This valve is actuated by rotary
movement and is most commonly
used for hydraulic steering
systems
Section 1.4 Hydraulic Fundamentals
Spool Changes
Control Valve Functions
There are a wide variety of port connections which depends on the
particular requirement.
The function of this control valve from the hoist circuit of a dump truck
has 6 ports and 4 positions (6/4 DCV).
Spool Positions
HOLD
RAISE
LOWER
FLOAT
Section 1.4 Hydraulic Fundamentals
Spool Changes
Hold Position
The oil from the pump is
directed back to the hydraulic
tank.
The two lines connected to the
hydraulic cylinder are closed
off by the spool, this will lock
the cylinder piston in place
hydraulically.
Section 1.4 Hydraulic Fundamentals
Spool Changes
Raise Position
The spool is pushed to the left, by
pilot pressure. Closing off the return
to tank port for the pump pressure
and opening the raise circuit.
Pump pressure builds up to opening
the check valve, allowing oil to flows
through to raise the work equipment.
The lower port is now opened to
return to tank.
Section 1.4 Hydraulic Fundamentals
Spool Changes
Lower Position
The spool is pushed to the right,
by pilot pressure. Closing off the
return to tank port for the pump
pressure and opening the lower
circuit.
Pump pressure builds up to
opening the check valve, allowing
oil to flows through to lower the
work equipment. The raise port is
now opened to return to tank.
Section 1.4 Hydraulic Fundamentals
Spool Changes
Float Position
The pilot pressure is
increase to push the spool
completely to the right.
This exposes pump
pressure and both raise &
lower ports to return to
tank.
This takes all pressure off
the hydraulic circuit, and is
used for back blading.
Section 1.4 Hydraulic Fundamentals
Main Relief Valves
Two Stage Relief Valve
Main Relief Valve.
The Main Relief Valve is designed to maintain a
maximum working pressure. When the hydraulic
oil reaches maximum pressure the relief valve
will allow the flow to return to tank, protecting all
Single Stage Relief Valve hydraulic components.
Section 1.4 Hydraulic Fundamentals
Safety Suction Valve
The safety suction valve is to
protect the hydraulic circuit when
the actuator generates its own From
pressure. Actuator
(eg. When a hydraulic cylinder is
pushed by an external force)
Relief of high pressure
The Safety Suction Valve is to Tank Line
relief positive pressure.
When negative pressure is
generated the Safety Suction
Valve will open up and allow oil to
flow in from the hydraulic To
reservoir stopping excessive Actuator
negative pressure.
Section 1.4 Hydraulic Fundamentals
Safety Suction Valves
Safety Valve
Positive Pressure
Negative Pressure
Suction Valve
If the Hydraulic Cylinder is moved accidentally with the spool closed two things will occur.
Positive Pressure: this pressure is unable to blow out through the main relief valve so
when the pressure raises above 230 Kg/cm2 will blow out through safety valve.
Negative Pressure: this pressure goes below tank pressure, lifting the check valve off and
allowing oil to flow from the tank into the circuit.
Section 1.4 Hydraulic Fundamentals
Directional Control Valve
Complete Circuit.
1. Pilot pressure from boom
raise has energized the
spool pushing it to the left.
2. Oil from the hydraulic
pump has entered the
control valve, past the
main relief valve and load
check valve.
3. Gone to the spool and via
the hydraulic pipes to the
bottom of the hydraulic
cylinder, pushing the boom
up.
4. Oil on the top side of the
cylinder moves back to the
control valve, past the
spool and returns to tank.
Section 1.4 Hydraulic Fundamentals
Directional Control Valve
END OF
Directional Control Valves
Section 1.4 Hydraulic Fundamentals
