Pressure Control
Chapter 3
Material taken from Fluid Power Circuits and
Controls, John S. Cundiff, 2001
Introduction
Pressure control is a key element in the
design of any circuit.
Used correctly, it can achieve a given
functional objective, as well as safe
operation.
In circuit design the pressure must be
limited to a level below the working
pressure of the lowest-rated component
in the circuit.
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Introduction
Six pressure-control valves are
discussed:
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„
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Relief valves
Unloading valves
Sequence valves
Pressure-reducing valves
Counterbalance valves
Brake valves
Introduction
Each of these valves works on the same
principle; a spring force balances a
hydraulic force.
The hydraulic force is produced by fluid
pressure acting on a given area.
When hydraulic force exceeds the
spring force, the valve spool moves.
2
Primary Flow Control Valves Introduced
Review of Needed Symbols
An open-center (float)
valve allows flow between
all four ports when the
valve is in the center
(nonactuated) position.
The actuator (downstream
from the valve) is not held
in position but is free to
float.
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Review of Needed Symbols
The open-center valve
also allows free flow from
the inlet port to the
return (or tank) port, but
it blocks the actuator
ports.
The actuator can not
move (neglecting
leakage) when the opencenter valve is in the
center position.
Review of Needed Symbols
The closed- center
valve has all four ports
blocked when it is in
the center position.
There is no pathway
through the valve
between any of the
four ports.
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Relief Valve
The relief valve is used to limit
pressure in an entire circuit.
It is generally the first
component downstream from
the pump.
Relief valves can be direct
acting or pilot operated.
Direct-Acting Relief Valve
Pressure acts on the annular area of the
valve spool.
The hydraulic force is given by Fh = Paa
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„
„
Where Fh = hydraulic force (lbf)
P = pressure (psi)
Aa = annular area (in2)
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Direct-Acting Relief Valve
The notation Fs will be used for the
spring force.
When Fh equals Fs, the valve cracks
open, meaning that the spool lifts off its
seat and allows fluid to flow to the
reservoir.
As pressure increases, the spool lifts
higher, allowing more flow to bypass to
the reservoir.
At some pressure level, the total flow
bypasses to the reservoir.
Direct-Acting Relief Valve
A typical flow vs.
pressure curve for a
direct-acting relief
valve is shown.
The valve is set to
open at 1500 psi.
This pressure is
known as the
cracking pressure.
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Direct-Acting Relief Valve
At 2000 psi, the valve is
fully open, and all flow is
bypassed to the reservoir.
The 500 psi differential
between cracking and full
bypass is needed in
direct-acting valve when it
is used for flow control, as
well as pressure limiting.
Direct-Acting Relief Valve
In this circuit, the flow
control valve is an
adjustable orifice.
When the flow control
valve is partly closed, a
pressure drop is
created across the
valve.
Relief valve pressure =
pressure drop across
the flow control valve
plus the pressure drop
across the motor.
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Direct-Acting Relief Valve
Motor speed is
controlled by closing
the flow control valve
enough to crack open
the relief valve.
Part of the pump
output now bypasses
to the reservoir; thus,
flow to the motor is
reduced, and the
speed decreases.
Pilot-Operated Relief Valve
A pilot-operated relief valve has a
different pressure vs. flow curve then the
direct acting valve.
The pilot-operated valve opens
completely over a narrow pressure range.
This allows the circuit to operate over a
wider pressure range without loss of fluid
over the relief valve.
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Pilot-Operated Relief Valve
The main spool has a
small hole (orifice)
drilled in the skirt.
Because of this hole,
pressure is the same
on the top and bottom
of the skirt.
As long as there is
negligible flow
through the orifice,
there is no pressure
drop across the
orifice.
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Pilot-Operated Relief Valve
The pilot section is on
top of the valve.
A dart is held in place
by the pilot spring.
When the hydraulic
force on this dart
becomes greater than
the pilot spring force,
the dart is unseated,
and fluid flows from
the cavity above the
skirt, through an
internal drain to the
valve outlet.
Pilot-Operated Relief Valve
Flow through the
orifice replaces the
fluid lost from the
cavity above the
skirt.
The spool is still held
in position by the
main spool spring.
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Pilot-Operated Relief Valve
The key advantage of a pilot-operated
valve is that it allows the designer to
use pressure to within 100 psi of the
valve setting to meet the functional
objective of the circuit.
In comparison, the direct-acting valve
cracks open at 1500 psi, and pressure
must increase to 2000 psi before it is
fully open.
Pilot-Operated Relief Valve
A pilot-operated relief valve can be used
with a remote pilot.
The remote pilot functions like the pilot
built into the top of the main relief valve.
It allows the designer to set two pressure
levels with one main relief valve.
A pilot-operated relief valve can be used
to unload the pump at low pressure
during periods between work cycles.
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Example Circuits Using PilotOperated Relief Valves
A diagram of the
circuit that uses a
pilot-operated relief
valve to unload the
pump at low pressure
is shown.
The relief valve
symbol designated
“A” refers to the main
spool of the pilotoperated relief valve.
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Example Circuits Using PilotOperated Relief Valves
The orifice in the skirt is
is orifice B, and the
symbol designated with
letter “C” is a symbol that
shows that the valve is
held closed with spring
pressure and a pilot
pressure.
The relief valve symbol
designated with a letter
“D” refers to the pilot
stage of the valve (dart
held in place with the pilot
spring).
Example Circuits Using PilotOperated Relief Valves
Using the pilot-operated relief valve to
unload the pump between work cycles
is shown on the following slide.
Here, a special directional control valve
is used with a fifth port.
This port provides a pathway for the
pilot line to be connected to the
reservoir when the directional control
valve is centered.
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Example Circuits Using PilotOperated Relief Valves
When the directional control valve is
shifted, the pilot line is blocked, and the
pilot-operated relief valve will not open
until the pressure equals the pilot
spring pressure plus the main spring
pressure.
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Example Circuits Using PilotOperated Relief Valves
This circuit is designed to provide highpressure relief during extension and lowpressure relief during retraction.
Unloading Valve
The symbol for an
unloading value is
similar to the
symbol for a relief
valve except that
the pilot line is
not connected to
sense pressure at
the valve inlet.
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Unloading Valve Example
The accumulator is
a key component in
the circuit.
There are three
types of
accumulators:
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„
„
Bladder
Diaphragm
Piston
Unloading Valve
The diaphragm accumulator is a
pressure vessel divided into two
compartments by a flexible diaphragm.
The top half is precharged with a gas,
generally nitrogen, and sealed.
The bottom half is connected to the
hydraulic circuit.
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Unloading Valve
The accumulator is designed for some
rated pressure, and pressure must be
controlled to ensure that it does not
exceed this rating.
The unloading valve accomplishes this
task.
An accumulator provides pressure to
the actuator at the moment the
directional control valve is shifted.
Unloading Valve
Pressure does not have to build from a low
pressure as it does in a circuit with an opencenter directional control valve.
If a large load is being moved, pressure must
build to achieve enough force to overcome
static friction and provide the inertial force to
accelerate the mass.
It takes an interval of time to build this
pressure.
This time delay can be eliminated if pressure
is already available at the moment the
directional control valve is shifted.
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Unloading Valve
In this circuit, the pump
builds pressure in the
accumulator until the
setting of the unloading
valve is reached.
At this point, the
unloading valve opens,
and flow bypasses to the
reservoir.
The pressurized fluid is
trapped in the
accumulator by the check
valve and the closedcenter directional control
valve.
Unloading Valve
A functional diagram of an
unloading valve is shown next.
Two features are added to a pilotoperated relief valve to create the
unloading valve.
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Unloading Valve
A check valve is builtin, and a small piston
is included in the top
section in line with
the dart and pilot
spring.
When the unloading
valve is closed, fluid
flows through the
check valve to charge
the accumulator.
Unloading Valve
When the directional control valve is
shifted, fluid drains from the
accumulator and the pressure drops.
The hydraulic force on the piston drops
and, when the pilot spring force
becomes greater, the dart reseats.
At this point, pressure equalizes on both
sides of the spool skirt.
The spool spring reseats the spool and
the pump begins to build pressure.
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Unloading Valve
When the accumulator
is charged to the
desired pressure, the
unloading valve dumps
flow to the reservoir.
Unloading Valve
Pressure drops at the
directional control
valve as the
accumulator empties.
The minimum
pressure is a function
of the load, the
characteristics of the
accumulator, and the
characteristics of the
pump.
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Sequence and Pressure Reducing Valves
The sequence valve and press-reducing
valve have some similar features.
Both valves are externally drained,
meaning that there is a separate line
from the valve back to the reservoir.
The relief valve and unloading valve are
both internally drained.
Sequence and Pressure Reducing Valves
The outlets of the sequence and
pressure-reducing valves are not
connected to the reservoirs, so they can
not be internally drained.
The check valve built into both valves is
there to provide free flow in the reverse
direction.
In effect, it takes the valve out of the
circuit when the flow is reversed.
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Sequence and Pressure Reducing Valves
As shown on the next slide, the
sequence valve and pressure-reducing
valve symbols are similar.
The sequence valve is a normally closed
valve with a pilot line to sense inlet
pressure, and the pressure-reducing
valve is a normally open valve with pilot
line to sense outlet pressure.
Sequence and Pressure Reducing Valves
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Sequence Valve
The sequence valve is used to ensure
that a certain pressure level is achieved
in one branch of the circuit before a
second branch is activated.
Consider a machining operation where
the workpiece must be clamped with a
certain force before it is extended to
make contact with the cutting tool.
If the piece is not securely fastened, it
can slip and damage both the tool and
the piece.
Sequence Valve
In this circuit, the
sequence valve is set
on 600 psi, meaning
that the pressure must
build to 600 psi before
the valve opens.
This setting ensures
that the clamp cylinder
exerts a 600-psi clamp
force before the extend
cylinder moves.
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Sequence Valve
When the directional control valve is
shifted for reverse flow, the check valve
provides free flow, and there is no
sequencing of the cylinders.
Either one can retract before the other,
depending on the pressure required for
retraction.
The cylinder with the lowest pressure
requirement always retracts first.
Pressure Reducing Valve
A pressure-reducing valve does not
allow pressure downstream of the valve
to exceed the set point.
Suppose the workpiece must be
clamped with two clamps.
The second clamp is placed at a point
where too much clamping force will
damage the workpiece.
A pressure-reducing valve is used to
limit clamping pressure.
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Figure 3.23
Pressure-reducing valve used
to limit clamping force.
The valve is a second type of pressurereducing valve, the reducing/relieving
valve.
This valve operates like the pressurereducing valve except that it bypasses
fluid to the reservoir when the spool is
shifted upward by the hydraulic force.
Pressure Reducing Valve
The orifice between the inlet and
bypass (to reservoir) opens as pressure
increases, so the valve functions like a
relief valve.
It combines the functions of the
pressure-reducing valve and the relief
valve, thus the name reducing/relieving
valve.
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Counterbalance and Brake Valve
The counterbalance valve and brake
valve have similar symbols.
The single difference is that the brake
valve has a remote pilot line in addition
to an upstream pilot line.
As with the sequence and pressurereducing valves, a check valve is built in
to allow free flow in the reverse
direction.
Figure 3.26
Symbols for counterbalance and brake valves.
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Counterbalance and Brake
Valve
Resistive load: a load that acts in the
opposite direction to actuator motion.
Overrunning load: a load that acts in
the same direction as actuator motion.
Counterbalance Valve
The counterbalance valve, also called a
holding valve, is used to prevent a
weight from falling uncontrollably.
When the directional control is shifted,
the platen will fall unless there is a
means for creating an opposing
hydraulic force.
A counterbalance valve accomplishes
this task.
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Counterbalance Valve
Generally, the valve is set about 50 psi
higher than the pressure required to hold
the load.
Often it is desirable to use the weight of the
platen to help build the total pressing force.
In this case, a counterbalance valve with
remote pilot line connection is used.
If pressure at the cap end, and thus the
pilot line, drops below the set point, the
counterbalance valve partly closes to slow
the platen.
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Counterbalance Valve
There is an additional reason for using a
counterbalance valve other than to prevent
uncontrolled motion of the actuator.
When the platen is moving down too fast, the
pump can not keep the cap end filled with
fluid, and a negative pressure can develop.
It is possible to suck the fluid out of the
pump fast enough to cause void spaces in
the pump.
The condition when the pump is not
completely filled with fluid is called cavitation.
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Brake Valve
A brake valve performs the same
function as a counterbalance valve, but
it is designed to overcome a key
disadvantage.
The pressure drop across the
counterbalance valve is converted to
heat; consequently, half the hydraulic
power is wasted.
A brake valve overcomes this.
Brake Valve
A brake valve has an internal pilot
passage and a remote pilot passage.
Suppose the spring is set for 1000 psi.
When the pressure at the internal pilot
reaches 1000 psi, the piston pushes the
spool upward to open the valve.
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Brake Valve
The area in the piston is much less than
the area of the bottom of the spool.
A typical area ratio might be 10:1.
The remote pilot applies pressure
directly to the bottom of the spool;
consequently, only 100 psi is required
to compress the spring and open the
valve.
Pressure required to open the valve is
1000 psi at the internal pilot and 100
psi at the remote pilot.
Brake Valve
A brake valve is used in
this circuit.
It requires 100 psi at the
motor inlet to keep the
valve open.
As long as the load on the
motor requires more than
100 psi, the brake valve
does not affect circuit
efficiency.
If the load starts to
overrun, and the pressure
drops below 100 psi, the
brake valve closes.
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Brake Valve
It requires 1000 psi at the direct, or
internal, pilot to open the valve.
This 1000-psi pressure drop across the
brake valve converts the mechanical
energy of the overrunning load to heat
energy and slows the load.
When pressure at the inlet builds to 100
psi again, the brake valve opens.
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