BASIC
HYDRAULICS
LEARNING
ACTIVITY
PACKET
pressure control
circuits
TM
BB831-XA05XEN
LEARNING ACTIVITY PACKET 5
PRESSURE CONTROL CIRCUITS
INTRODUCTION
The relief valve covered in an earlier LAP is part of a family of hydraulic valves
called pressure control valves. This LAP will introduce two more in this family:
sequence valves and pressure reducing valves. Both of these valves are commonly used.
ITEMS NEEDED
Amatrol Supplied
1 85-BH Basic Hydraulic Training System
1 85-HPS Hydraulic Power Unit
FIRST EDITION, LAP 5, REV. A
Amatrol, AMNET, CIMSOFT, MCL, MINI-CIM, IST, ITC, VEST, and Technovate are trademarks or registered
trademarks of Amatrol, Inc. All other brand and product names are trademarks or registered trademarks of their
respective companies.
Copyright © 2009 by AMATROL, INC.
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Amatrol,Inc., P.O. Box 2697, Jeffersonville, IN 47131 USA, Ph 812-288-8285, FAX 812-283-1584 www.amatrol.com
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TABLE OF CONTENTS
SEGMENT
1 SEQUENCE VALVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
OBJECTIVE
OBJECTIVE
SKILL
SKILL
1
2
1
2
SEGMENT
2 SEQUENCE VALVE APPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Describe the function of a pressure sequence valve and give an application
Describe the operation of a direct-acting sequence valve and give its schematic symbol
Connect and adjust the pressure setting of a sequence valve
Connect and operate a pressure sequence circuit
OBJECTIVE 3 Describe the function of a bypass check valve in a sequence valve circuit
Activity 1 Sequence valve operation with a check valve
OBJECTIVE 4 Describe the operation of an integral check valve and give its schematic symbol
SKILL 3 Design a pressure sequence circuit
OBJECTIVE 5 Describe the function of a two-sequence valve control circuit
SKILL 4 Design a two-sequence valve control circuit
OBJECTIVE 6 Explain why a sequence valve is externally drained
Activity 2 Sequence valve drain operation
SEGMENT
3 PRESSURE REDUCING VALVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
OBJECTIVE
OBJECTIVE
SKILL
SKILL
7
8
5
6
SEGMENT
4 PRV APPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Describe the function of a pressure reducing valve and give an application
Describe the operation of a direct-acting PRV and give its schematic symbol
Connect and adjust the pressure setting of a PRV
Connect and operate a reduced pressure circuit
OBJECTIVE 9 Describe the function of a PRV’s bypass check valve
Activity 3 PRV operation with a check valve
SKILL 7 Design a hydraulic circuit that uses a pressure reducing valve
OBJECTIVE 10 Explain why a PRV is externally drained
Activity 4 PRV drain operation
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SEGMENT 1
SEQUENCE VALVES
OBJECTIVE 1
DESCRIBE THE FUNCTION OF A PRESSURE SEQUENCE
VALVE AND GIVE AN APPLICATION
Many machines require a series of steps in order to do their jobs.
Often these machines depend on one step being completed before
another step begins. This series of steps is commonly called a sequence
of operation. One way to make the machine perform this sequence is to
use one or more hydraulic sequence valves.
A hydraulic sequence valve operates on the principle that the system
pressure will usually rise when the first actuator reaches the end of its
stroke. The sequence valve senses this rise in pressure and opens to
allow flow to the second actuator. This creates a two-step sequence.
A sequence valve looks just like a relief valve, as shown in figure 1.
In fact, it actually is a relief valve with an external drain line.
Figure 1. Sequence Valve
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Sequence valves are most often used as a simple method to sequence
two actuators to avoid the cost of an electrical control circuit. A typical
example is the clamp and grind circuit shown in figure 2.
CLAMP
CYLINDER
500 PSI
LOAD
EXTENDED
GRINDER
MOTOR
SEQUENCE
VALVE
200 PSI
LOAD
TURNING
Figure 2. Hydraulic Cylinder / Motor Circuit
OBJECTIVE 2
DESCRIBE THE OPERATION OF A DIRECT-ACTING
SEQUENCE VALVE AND GIVE ITS SCHEMATIC SYMBOL
One reason to use a sequence valve in a clamping application in a
machine tool is the sequence valve will not open until the clamp cylinder
is clamped.
Sequence valves are most often used for simple applications that
involve two actuators. Common applications include a clamping
operation followed by another operation such as drilling, machining,
pressing, or grinding. In each case, the operation of the circuit depends
on the first actuator to stop and block the flow after it completes its step
so that pressure will rise and open the sequence valve to start the next
step.
Sequence valves are available in both direct-operated and
pilot-operated designs. They both perform the same function but the
direct-operated valve’s operation is simpler.
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The direct-operated sequence valve consists of a body, spool, spring
and a method of adjustment, as shown in figure 3.
The body has two main ports and a drain port. One of the main ports
is the inlet, or primary port. It is connected to the end of the spool,
opposite the spring, by a passage in the body, and receives oil from the
main supply. The other main port is the outlet, or secondary port, which
supplies oil to the downstream part of the circuit when the valve opens.
The drain port is connected to the spring chamber and keeps pressure
out of the spring chamber by feeding any internal leakage directly to the
tank.
ADJUSTMENT
KNOB
ADJUSTMENT
SCREW
DRAIN PORT
OUTLET PORT
SPRING
SPOOL
INLET PORT
BODY
Figure 3. Basic Construction of a Direct-Operated Sequence Valve
The pressure sequence valve is normally closed. This means the path
from the inlet port to the outlet port is normally blocked. The spring
holds the spool in a certain position to block these ports. Spring force
holding the spool closed can be adjusted by the adjustment screw and
knob.
To open the valve, the pressure must rise to a high enough level to
overcome the spring force and push the spool against the spring. Oil then
flows through from the inlet to the outlet. This operation will be
discussed in more detail in Skill 2.
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The schematic symbol for a sequence valve is shown in figure 4.
Notice that the main ports are blocked from one another in the normal
condition. This is shown by the position of the arrow inside the valve
block. The symbol also shows that the valve is held closed by a spring
and the pilot line senses upstream (inlet) pressure. The symbol is the
same as a relief valve symbol except for the addition of the external
drain line.
SYMBOL
EXPLANATION
EXTERNAL DRAIN LINE
CONNECTED TO RESERVOIR
ADJUSTABLE
PRESSURE
SETTING
INLET
OUTLET
PILOT LINE
SENSES
PRESSURE
UPSTREAM
NORMALLY
CLOSED
Figure 4. Symbol for a Pressure Sequence Valve with Explanation
NOTE
Drain lines are always shown on a drawing as dotted lines.
Pilot lines (lines to perform control functions) are always shown
as dashed lines.
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Knowing the schematic symbol for a sequence valve makes it
possible to properly draw the valve in the hydraulic cylinder/motor
circuit, as shown in figure 5.
To understand how this concept works, figure 5 shows a circuit for
clamping a part and running a motor. Because the sequence valve is
normally closed, the clamp cylinder will extend first when the DCV is
shifted. The motor does not run when the clamp cylinder is extending
because the pressure to extend the cylinder is lower than the pressure
setting of the sequence valve.
When the clamp cylinder becomes extended, the pressure rises and
causes the sequence valve to open. The motor then begins to run. Here,
the sequence valve would be adjusted to open at a pressure above 500
psi. Flow would be blocked to the motor until circuit pressure exceeds
500 psi. This would occur only when the clamp cylinder has clamped,
stopping the extend movement of the cylinder.
Also, notice that the grinder motor pressure is lower than the
sequence valve pressure. The downstream pressure of the sequence valve
only depends on the load; it is not affected by upstream pressure.
CLAMP
CYLINDER
500 PSI
LOAD
EXTENDED
GRINDER
MOTOR
SEQUENCE
VALVE
200 PSI
LOAD
TURNING
Figure 5. Hydraulic Cylinder / Motor Circuit Schematic
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SKILL 1
CONNECT AND ADJUST THE PRESSURE SETTING
OF A SEQUENCE VALVE
Procedure Overview
In this procedure you will connect a hydraulic
sequence valve in a test circuit to demonstrate its basic
operation. You will show that the sequence valve will open
at a preset pressure setting and that the pressure
downstream of the sequence valve is independent of the
sequence valve’s setting. It only depends on downstream
resistance.
❑ 1. Set up the sequence valve circuit shown in figures 6 and 7.
Notice that the port markings on this valve are numbered. This is
commonly done by the manufacturer when the bodies are made
from bar stock.
For this valve, the inlet port is 1, the outlet port is 2, and the
drain port is 3.
DIRECTIONAL
CONTROL
VALVE
SUPPLY
MANIFOLD
A
IN
GAUGE
A
GAUGE B
NEEDLE
VALVE
3
1
RETURN
MANIFOLD
2
SEQUENCE
VALVE
OUT
IN
FLOW METER
Figure 6. Schematic of Sequence Valve Test Circuit
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HYDRAULIC INSTRUMENTATION PANEL
GAUGE A
GAUGE B
FLOW
METER
GAUGE C
SUPPLY
MANIFOLD
RELIEF \ SEQUENCE
VALVE
PRESSURE REDUCING
VALVE
1
1
2
2
3
3
NEEDLE
VALVE
A
B
RETURN
MANIFOLD
IN
D.C.V.
#1
CHECK VALVE #1
A
B
OUT
B
A
CHECK VALVE #2
B
A
BASIC HYDRAULIC VALVE MODULE
Figure 7. Pictorial of Sequence Valve Test Circuit
❑ 2. Perform the power unit checkout procedures before starting the
power unit.
A. Check the oil level. Fill if necessary.
B. Press the stop push button on the motor starter to make sure the
starter is in the Off position.
C. Plug in the power cord to a wall outlet.
D. Reduce the relief valve to its minimum pressure setting (turn
CCW fully).
❑ 3. Perform the following substeps to set up the circuit for testing.
A. Open the needle valve completely.
B. Turn the pressure sequence valve’s adjustment knob fully
CCW.
This is the valve’s minimum pressure setting and will cause it
to open at the lowest pressure.
C. Turn on the hydraulic power unit.
D. Increase the power unit’s relief valve setting until the pressure
at Gauge S reads 500 psi / 3447 kPa.
You are now ready to test the ability of the pressure sequence
valve to be adjusted to the various pressure settings. It will
block flow at pressures below its adjustment setting and open
to allow flow at pressures above its adjustment setting.
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❑ 4. Open the shutoff valve and observe the readings on the flowmeter
and Gauge A.
Flowmeter ___________________________________(gpm / lpm)
Gauge A ______________________________________(psi / kPa)
You should observe that all of the pump flow is now going
through the sequence valve. The pressure at Gauge A should be
lower than the relief valve setting because the sequence valve’s
setting is at a minimum. The pressure at Gauge A may seem to be
high. This is due to the high back pressure caused by the
components and hoses downstream.
❑ 5. Turn the sequence valve adjustment knob CW until Gauge A reads
350 psi / 2415 kPa. Observe the flow rate through the sequence
valve.
UPSTREAM
GAUGE A
PRESSURE
(psi/kPa)
FLOW RATE
(gpm/lpm)
350/2415
/
400/2760
/
440/3036
/
You should observe that the sequence valve adjusted the pressure
easily and pump flow through the sequence valve remains about
the same as that obtained in step 4.
❑ 6. Turn the adjustment knob CW until Gauge A reads each of the
other pressures listed in the chart in step 5. As you do this, observe
the corresponding flow rate at the flowmeter for each pressure
setting.
You should observe that the sequence valve easily operates over a
range of adjustment settings.
❑ 7. Repeat step 6 several times to experiment with your ability to
adjust the pressure setting of the sequence valve.
❑ 8. Adjust the pressure setting of the sequence valve until Gauge A
reads 330 psi / 2277 kPa. Now you are ready to observe the
operation of the sequence valve under simulated circuit conditions.
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❑ 9. Simulate a cylinder extending under a minimum load by pushing
in on the lever of the DCV and holding it.
Record below the readings of pressure Gauge A and the
flowmeter.
Gauge A ______________________________________(psi / kPa)
Flow Rate ___________________________________(gpm / lpm)
You should observe that the sequence valve closes when the load
pressure is below its pressure setting. This is indicated by zero
flow through the flowmeter.
Figure 8 shows the valve operation when the inlet pressure is
lower than the sequence valve setting. Notice that the valve stays
closed because the spring force is greater than the force from the
oil pressure at the inlet. This condition would occur when the flow
demand in the circuit was high enough to prevent pressures from
going above 300 psi / 2070 kPa.
VALVE ADJUSTMENT
SET AT
330 psi/2277kPa
DRAIN
TO
TANK
GAUGE
(0 PSI/KPa)
OIL FLOW
BLOCKED
GAUGE
(300 psi/2070 kPa)
FROM OIL
SUPPLY
Figure 8. Sequence Valve Adjusted to 330 psi / 2277 kPa and Pressure
at Inlet is 300 psi / 2070 kPa
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❑ 10. Now simulate the cylinder bottoming out by releasing the lever of
the DCV to block the flow. Record below the flowmeter and
pressure gauge readings that result.
Gauge A ______________________________________(psi / kPa)
Gauge B ______________________________________(psi / kPa)
Flowmeter ___________________________________(gpm / lpm)
You should observe that the sequence valve opens at its pressure
setting and allows oil to flow through it.
As shown in figure 9, pressure at the inlet is also applied to the end
of the spool opposite the spring. At pressures above 330 psi / 2277
kPa, the forces move the spool to a position where the spring force
is equal to the pressure against the end of the spool opening the
valve. This allows oil to flow and perform the next machine
operation. The outlet pressure will depend only on the downstream
resistance.
VALVE ADJUSTMENT
SET AT
330 psi/2277kPa
DRAIN
TO
TANK
GAUGE
(0 TO 500 psi)
DEPENDING ON
DEMAND
OIL
FLOWING
GAUGE
(330 psi/2277 kPa)
FROM OIL
SUPPLY
Figure 9. Sequence Valve Adjusted to 330 psi / 2277 kPa and Pressure
at Inlet is 330 psi / 2277 kPa or Greater
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❑ 11. Now test the effect that downstream pressure has on sequence
valve operation. To do this, close off the needle valve until the
pressure at Gauge B reads 250 psi / 1725 kPa.
You should observe that this causes little change in the flow
through the sequence valve or upstream pressure at Gauge A.
❑ 12. Now continue to increase the downstream pressure to 350 psi /
2415 kPa and observe whether the sequence valve stays open or
closes. This simulates a downstream load that is higher than the
sequence valve setting.
You should observe that the sequence valve remains open with
little change in flow rate and Gauge A will be above 350 psi/2415
kPa. As long as the pressure is greater than the valve’s pressure
setting, it will stay open.
❑ 13. Perform the following substeps to shut down the hydraulic power
supply.
A. Reduce the relief valve to its minimum setting.
B. Close the shutoff valve.
C. Turn off the power unit.
Do not change the sequence valve setting. This same setting
will be used in skill 2.
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SKILL 2
CONNECT AND OPERATE A PRESSURE SEQUENCE CIRCUIT
Procedure Overview
In this procedure, you will test the ability of a sequence
valve to be used to run a grinder motor after a cylinder has
clamped the part. In this circuit, the oil flows in only one
direction through the sequence valve. Therefore, a bypass
check valve is not needed.
❑ 1. Connect the sequence circuit shown in figure 10.
In this circuit, the grinder motor will not run until the clamp
cylinder is clamped. Also, notice that the normally-closed
sequence valve prevents the grinder motor from running
backwards when the clamp cylinder is retracted.
CLAMP CYLINDER
(SMALL BORE CYLINDER)
SUPPLY
MANIFOLD
NEEDLE
VALVE
IN
A
OUT
B
TEE
TEE
GAUGE B
RETURN
MANIFOLD
3
GAUGE
A
1
MOTOR
2
SEQUENCE
VALVE
Figure 10. Schematic of Clamp and Grind Sequence Circuit
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❑ 2. Perform the power unit checkout procedures.
NOTE
The sequence valve should still be set at 330 psi / 2277 kPa
from Skill 1. Continue to use this setting.
❑ 3. To keep actuator speeds low, close the needle valve fully (CW)
❑ 4.
❑ 5.
❑ 6.
❑ 7.
and then open it one full turn (CCW).
Increase the power unit’s relief valve setting until the pressure at
Gauge S reads 500 psi/3447 kPa and then open the shutoff valve.
Operate the circuit by pushing in on the lever of the DCV and
holding it. Observe the action of the two actuators.
You should observe that the cylinder extends. When the cylinder
becomes extended, the motor will run.
Now retract the clamp cylinder by pulling out on the lever of the
DCV.
The motor should be stopped while the clamp is being retracted.
Repeat steps 5 and 6 several times to become more familiar with
the operation of the sequencing circuit. As you do this, record the
readings of pressure Gauges A and B when the cylinder is
extending and when the motor is running.
You should observe that while the cylinder is extending, Gauge A
is less than 330 psi / 2277 kPa, and since the sequence valve is
closed, Gauge B drops to a low pressure. With the motor running,
Gauge A must be at or above 330 psi / 2277 kPa to open the
sequence valve even though the pressure to operate the motor
(Gauge B) is less than 330 psi / 2277 kPa.
PRESSURE
GAUGE A
(psi/kPa)
PRESSURE
GAUGE B
(psi/kPa)
Cylinder Extending
/
/
Motor Running
/
/
OPERATION
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❑ 8. Now change the setting of the sequence valve to 400 psi / 2760
kPa. To do this, perform the following substeps.
A. Push the lever of the DCV in to extend the cylinder.
B. Continue to hold the lever of the DCV actuated while the motor
runs.
C. With the motor running, adjust the sequence valve’s setting
until the pressure at Gauge A reads 400 psi / 2760 kPa. The
sequence valve is now set.
NOTE
To adjust a sequence valve in an industrial application, you
must fully extend or retract the first actuator and then adjust the
sequence valve’s setting. This is the procedure you just
performed.
❑ 9.
❑ 10.
❑ 11.
❑ 12.
❑ 13.
D. Pull out on the DCV’s lever to retract the cylinder.
Operate the sequence valve circuit with the new pressure setting.
You should observe that the circuit operation is the same as before.
Motor speed will drop off because the pressure drop across the
needle valve is less.
Repeat step 8 to set the sequence valve to 200 psi / 1380 kPa.
Operate the sequence valve circuit with the new pressure setting.
Again you should observe that the circuit operation is the same as
before. Motor speed will increase because the pressure drop across
the needle valve has increased.
Perform the following substeps to shut down the hydraulic power
supply.
A. Reduce the relief valve to its minimum setting.
B. Close the shutoff valve.
C. Turn off the power unit.
Move the handle of the DCV back and forth to remove any
pressure in the circuit.
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SEGMENT 1
SELF REVIEW
1. The sequence valve depends on an actuator to stop and block
flow so that ___________ rises to open it.
2. Sequence valves are used to sequence two actuators to avoid
the cost of a(n) _____________ control circuit.
3. The downstream pressure of a sequence valve depends on
the ______________.
4. The drain is connected to the sequence valve’s spring
chamber to keep the ________ out.
5. A sequence valve is actually a relief valve with a(n)
________.
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SEGMENT 2
SEQUENCE VALVE APPLICATIONS
OBJECTIVE 3
DESCRIBE THE FUNCTION OF A BYPASS CHECK VALVE
IN A SEQUENCE VALVE CIRCUIT
In the previous segment, the circuit sequenced a motor which ran in
only one direction. In many other applications the circuit must sequence
a cylinder or motor which operates in both directions. This presents a
problem because the sequence valve is normally closed and oil will not
flow in reverse through the valve. To deal with this, a check valve can be
used to bypass the flow in the reverse direction.
A typical circuit application for a bypass check valve is a clamp and
press machine, as shown in figure 11. In this circuit, the check valve is
plumbed around the sequence valve to allow the press cylinder to retract.
PRESS
CYLINDER
PART
CLAMP
CYLINDER
Figure 11. Sequence Valve Circuit Used for a Clamp and Press Application
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The operation of this circuit is explained as follows: When the DCV
is shifted, the clamp cylinder extends at low pressure. When it clamps
the part, the pressure rises and opens the sequence valve. This causes the
press cylinder to extend. At this point, the check valve is closed.
Once the press cylinder extends and performs the operation, the
DCV can be shifted to retract both cylinders. The bypass check valve
then opens and allows the oil being pushed out of the cap end of the
press cylinder to flow freely around the sequence valve to the reservoir.
Activity 1. Sequence Valve Operation with a Check Valve
Procedure Overview
In this procedure, you will demonstrate that a check
valve is needed with a sequence valve for free reverse
flow.
❑ 1. Connect the sequence valve with a bypass check valve, as shown
in the circuit in figure 12.
In this circuit, the sequence valve will be used to extend a cylinder
and the bypass check valve will allow the cylinder to retract. The
needle valve will allow the simulation of an increasing load which
will allow the sequence valve to operate.
CHECK
VALVE
GAUGE
A
HOSE "B"
SEQUENCE
VALVE
SUPPLY
MANIFOLD
IN
A
OUT
B
1
2
3
RETURN
MANIFOLD
HOSE "A"
SMALL
BORE
CYLINDER
NEEDLE
VALVE
Figure 12. Circuit to Test the Operation of a Bypass Check Valve
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❑ 2. Perform the power unit checkout procedures.
❑ 3. Perform the following substeps with the hydraulic supply.
❑ 4.
❑ 5.
❑ 6.
❑ 7.
❑ 8.
❑ 9.
❑ 10.
❑ 11.
A. Turn on the hydraulic power unit.
B. Adjust the relief valve pressure to 500 psi / 3447 kPa.
C. Open the shutoff valve.
Perform the following substeps to set the sequence valve pressure
setting.
A. Move the hose labeled “A” from the cap end of the cylinder to
the return manifold.
B. Push in on the handle of the DCV and hold it shifted.
C. Adjust the sequence valve so that Gauge A reads 300 psi / 2070
kPa.
D. Release the handle of the DCV.
E. Return the hose labeled “A” back to the cylinder.
You are now ready to cycle the cylinder with a bypass check
valve around the sequence valve.
Perform the following substeps to operate the circuit.
A. Open the needle valve fully.
B. Push in on the handle of the DCV and hold it shifted.
C. Slowly close the needle valve and observe the pressure at
Gauge A and the cylinder.
You should observe that when the pressure at Gauge A reaches
the setpoint (300 psi / 2070 kPa), the cylinder will extend.
Now retract the cylinder by pulling out on the handle of the DCV.
Repeat steps 5 and 6 until you are familiar with the operation of
the circuit.
Disconnect the hose labeled “B” at the check valve and repeat
steps 5 and 6. Observe the circuit’s operation without a bypass
check valve. Does it operate properly?
You should observe that the cylinder does not retract or retracts a
small amount because the sequence valve blocks the free flow of
oil from the cap end of the cylinder.
Push in momentarily on the handle of the DCV to relieve any
pressure and reconnect the check valve.
Retract the cylinder.
Perform the following substeps to shut down the hydraulic power
supply.
A. Reduce the relief valve to its minimum setting.
B. Turn off the power unit.
C. Close the shutoff valve.
D. Move the handle of the DCV back and forth to remove any
pressure in the circuit.
Leave this circuit set up. It will be used in Activity 2.
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OBJECTIVE 4 DESCRIBE THE OPERATION OF AN INTEGRAL CHECK VALVE
AND GIVE ITS SCHEMATIC SYMBOL
A bypass check can be plumbed externally to the sequence valve as
discussed in the previous activity. Another option offered by valve
manufacturers is a built-in check valve called an integral check valve. A
cross-section drawing of a direct-operated sequence valve with integral
check valve and the symbol are shown in figure 13.
SECTION DRAWING
SYMBOL
DENOTES ALL
IN ONE BODY
OUTLET
PORT
IN
OUT
INTEGRAL
CHECK
INLET
PORT
DENOTES FREE-FLOW
IN THIS DIRECTION
Figure 13. Section Drawing and Symbol for a Sequence Valve with
Integral Check
SKILL 3
DESIGN A PRESSURE SEQUENCE CIRCUIT
Procedure Overview
In this procedure, you will design a sequence circuit
that uses a bypass check valve. This will give you a
chance to complete your understanding of the operation of
an integral check valve and test your creativity.
❑ 1. Design a fluid power circuit to power the clamp and press machine
shown in figure 14. Draw all of the power unit components. The
sequence is as follows:
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• When the operator pulls the lever of a directional control valve
(crossed arrows condition), the clamp cylinder extends.
• When the clamp cylinder is clamped, the press cylinder then
extends.
• When the press cylinder is extended, the operator pushes the lever
of the directional control valve forward (straight arrows condition).
This causes the clamp cylinder and the press cylinder to retract.
• When the cylinders are both retracted, the operator releases the
DCV lever and the cycle stops.
Special Conditions
The extend speed of the clamp cylinder should be adjustable. The
press cylinder extend speed should be independently adjustable (at
a speed less than the clamp speed). The retract speeds should use
full pump flow.
HINT
Be very careful where you place the flow control valve for
clamping speed. Incorrect placement will cause back pressure at
the sequence valve which can cause it to open prematurely.
Show your design to your instructor and explain its operation. This
is part of your skills assessment.
PRESS
CYLINDER
CLAMP
CYLINDER
Figure 14. Clamp and Press Circuit
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OBJECTIVE 5
DESCRIBE THE FUNCTION OF A TWO-SEQUENCE VALVE
CONTROL CIRCUIT
In some sequence circuit applications, it is not acceptable to retract
the clamp cylinder at the same time as the other cylinder performing the
work. The circuit designed in the previous skill does this. A typical
application where the cylinders must retract in sequence as well as
extend in sequence is the clamp and drill circuit. If the clamp retracted at
the same time as the drill cylinder, the part could be thrown out. To
avoid this, the cylinders are sequenced in the reverse direction so that the
work cylinder retracts before the clamp cylinder. To do this, a second
sequence valve is used.
SKILL 4
DESIGN A TWO-SEQUENCE VALVE CONTROL CIRCUIT
Procedure Overview
In this procedure, you will design a control circuit that
sequences a clamp and drill operation in both directions.
Your design will build on the design of the clamp and press
circuit.
❑ 1. Design a fluid power circuit to control the clamp and drill machine
shown in figure 15. Start your drawing from the supply and return
manifolds. The sequence is as follows:
• When the operator pushes the lever of the directional control
valve (straight arrows condition), the clamp cylinder extends.
• When the clamp pressure rises above 300 psi / 2070 kPa, the
drill cylinder extends.
• When the drill cylinder is extended, the operator pulls the DCV
lever to the crossed arrows condition. This causes only the drill
cylinder to retract. The clamp cylinder stays clamped.
• When the drill cylinder is retracted, system pressure builds to
400 psi / 2760 kPa. This causes the clamp cylinder to retract.
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• When the clamp cylinder is retracted, the operator releases the
DCV lever and the cycle stops.
HINT
You will need two sequence valves, both with bypass check
valves, to accomplish this task.
Show your design to your instructor and explain its operation. This
is part of your skills assessment. You will not hook this up on the
trainer because there aren’t two sequence valves.
DRILL
CYLINDER
CLAMP
CYLINDER
Figure 15. Design Sheet for a Two-Sequence Valve Circuit
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OBJECTIVE 6
EXPLAIN WHY A SEQUENCE VALVE
IS EXTERNALLY DRAINED
The sequence valve is externally drained because its outlet is
pressurized. This pressure causes oil to seep into the spring chamber. If
oil fills this chamber and becomes trapped, it will prevent the spool from
opening the valve. The valve then is locked in a closed position, as
shown in figure 16.
TRAPPED OIL
PREVENTS VALVE
FROM OPENING
DRAIN
BLOCKED
FROM OIL
SUPPLY
Figure 16. Blocked Drain Port
The external drain prevents any trapping of oil and allows the spool
to move freely. This drain must always be connected to the tank and
never blocked.
In contrast, the operation of the relief valve is the same as the
sequence valve except that it is drained internally instead of externally.
The relief valve does not need to be externally drained, because the
outlet is connected to a tank and this tank line is not pressurized. This
allows the oil seepage to drain naturally.
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Activity 2. Sequence Valve Drain Operation
Procedure Overview
In this procedure, you will demonstrate what happens
when a sequence valve drain is blocked or not connected
to tank.
❑ 1. Use the same setup as used in Activity 1.
❑ 2. Perform the power unit checkout procedures.
❑ 3. Perform the following substeps with the hydraulic supply.
A. Turn on the hydraulic power unit.
B. Adjust the relief valve pressure to 500 psi / 3447 kPa.
C. Open the shutoff valve.
❑ 4. Cycle the DCV several times to verify that the operation is correct.
You should observe that the cylinder extends and retracts as it did
in Activity 1.
❑ 5. Now disconnect the sequence valve’s drain line at the sequence
valve.
The quick-connect at the sequence valve will block the drain line.
❑ 6. After a few cycles, cycle the DCV to test the operation of the
sequence valve without the drain line. Does it operate properly?
Record your observations.
_____________________________________________________
❑ 7.
❑ 8.
❑ 9.
❑ 10.
You should observe that the sequence valve will not open to
extend the cylinder.
Reconnect the drain line.
Test the circuit again to see if the sequence valve now works.
You should observe that it now works.
Perform the following substeps to shut down the hydraulic power
supply.
A. Reduce the relief valve to its minimum setting.
B. Turn off the power unit.
C. Close the shutoff valve.
Move the handle of the DCV back and forth to remove any
pressure still in the circuit.
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SEGMENT 2
SELF REVIEW
1. A blocked drain line of a sequence valve will cause the valve
to _________________.
2. Flowing backwards through a sequence valve without a
bypass check is not possible because the valve
is_____________________.
3. A bypass check built into the same body of a sequence valve
is called a(n)__________ check valve.
4. A sequence valve may be used as a(n) ____________ valve.
5. An example of a two-sequence valve circuit would be a(n)
____________ and ____________ circuit.
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SEGMENT 3
PRESSURE REDUCING VALVES
OBJECTIVE 7
DESCRIBE THE FUNCTION OF A PRESSURE REDUCING
VALVE AND GIVE AN APPLICATION
A pressure reducing valve (PRV) is used in multiple actuator circuits
to reduce system pressure in one branch of the circuit. This is necessary
when a delicate item would be damaged by the high forces placed on it
by the actuator. The clamp and drill circuit shown in figure 17 is an
example of this type of application.
To understand why the PRV is needed in order to accomplish this
function, remember from Pascal’s Law and previous activities, that the
pressure will be the same at both of the cylinders during part of the cycle. In
figure 17, with the clamp holding a thin-walled tube, a high pressure would
cause the tube to collapse. To prevent this, a pressure reducing valve can be
placed in one branch to limit the pressure and thus the clamping force on the
tube. The tube would not be damaged by the clamp.
LOW
PRESSURE
HIGH
PRESSURE
SUPPLY
CLAMP
D.C.V
PRESSURE
REDUCING
VALVE
DRILL
CYLINDER
RETURN
DRILL
CLAMP
CYLINDER
THIN
WALLED
TUBE
DRILL
D.C.V
HIGH
PRESSURE
Figure 17. Hydraulic Clamp-Drill Circuit with PRV
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The pressure reducing valve used in the 850 Series trainer is shown
in figure 18. You may notice that it looks exactly like the relief valve.
The only difference is in how the internal ports are arranged. This valve
senses pressure downstream instead of upstream.
Figure 18. 850 System Pressure Reducing Valve
OBJECTIVE 8 DESCRIBE THE OPERATION OF A DIRECT-ACTING PRV AND
GIVE ITS SCHEMATIC SYMBOL
The pressure reducing valve is also available in both a
direct-operated and a pilot-operated design. Both designs provide the
same function but the direct-operated is simpler to understand.
The direct-operated pressure reducing valve consists of a body,
spool, spring, and an adjustment, as shown in figure 19.
ADJUSTMENT
DRAIN
PORT
INLET
PORT
SPRING
SPOOL
BODY
OUTLET
PORT
Figure 19. Basic Construction of a Direct-Operated PRV
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The body has two main ports and a drain port. One of the main ports
is the inlet, which receives oil from the main system. The other main port
is the outlet, which is connected to the end of the spool, opposite the
spring, by a passage in the body and feeds oil to the branch circuit at
reduced pressure.
The drain port is connected to the spring chamber and keeps pressure
from building in the spring chamber by feeding any internal leakage
directly to the tank.
The schematic symbol for the pressure reducing valve is shown in
figure 20. Notice that the valve is shown normally open with a pilot line
SYMBOL
EXPLANATION
EXTERNAL DRAIN LINE
CONNECTED TO RESERVOIR
ADJUSTABLE
PRESSURE
SETTING
INLET
NORMALLY
OPEN
OUTLET
SENSES
PRESSURE
DOWNSTREAM
Figure 20. Symbol for a Pressure Reducing Valve with Explanation
sensing downstream pressure.
The pressure reducing valve is a normally open valve. This means
the inlet port is normally connected to the outlet. The spring holds the
spool in a certain position to connect these ports together. The spring
force holding the spool open can be adjusted by the adjustment screw
and knob. To close the valve, the spool must overcome the spring force
and move to close off the outlet port. It does this with oil pressure acting
against the spool end that is opposite the spring. With this end of the
spool connected to the outlet, as soon as outlet pressure produces more
force on the spool than the spring, the spool moves to close off the
outlet.
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Knowing the schematic symbol for a PRV makes it possible to
properly draw in the valve for the hydraulic clamp-drill circuit, as shown
in figure 21.
MEANS THAT DRAINLINE
IS CONNECTED TO
THE RESERVOIR
PRESSURE
REDUCING
VALVE
DRILL
CYLINDER
DRILL
CLAMP
CYLINDER
THIN
WALLED
TUBE
Figure 21. Schematic of Hydraulic Clamp-Drill Circuit
Notice that the drain line on the pressure reducing valve is shown
connected to the reservoir in a short-hand method. This means it is
connected to the return line.
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SKILL 5
CONNECT AND ADJUST THE PRESSURE SETTING OF A PRV
Procedure Overview
In this procedure, you will connect a hydraulic PRV in a
test circuit to demonstrate its basic operation. You will then
show that the PRV restricts flow to limit downstream
pressure.
❑ 1. Connect the pressure reducing valve circuit shown in figures 22 and
23.
In this circuit, the upstream pressure will be measured by Gauge A
and the downstream pressure by Gauge B. The needle valve will
be used to create a load downstream to test the operation of the
PRV.
Notice that the ports on this valve are numbered. The outlet port is
1, the inlet port is 2, and the drain port is 3.
SUPPLY
MANIFOLD
GAUGE A
PRESSURE
REDUCING
VALVE
3
2
SHUTOFF
VALVE
GAUGE B
NEEDLE
VALVE
1
FLOW
METER
RETURN
MANIFOLD
Figure 22. Diagram of Pressure Reducing Test Circuit
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HYDRAULIC INSTRUMENTATION PANEL
GAUGE A
GAUGE B
FLOW
METER
GAUGE C
SUPPLY
MANIFOLD
PRESSURE REDUCING
VALVE
RELIEF \ SEQUENCE
VALVE
1
1
2
2
3
3
NEEDLE
VALVE
A
B
RETURN
MANIFOLD
IN
D.C.V.
#1
CHECK VALVE #1
A
B
OUT
B
A
CHECK VALVE #2
B
A
BASIC HYDRAULIC VALVE MODULE
Figure 23. Pictorial of Pressure Reducing Test Circuit
❑ 2. Perform the power unit checkout procedures.
❑ 3. Perform the following substeps to set up the circuit for testing.
A. Close the needle valve completely.
B. Turn the pressure reducing valve’s adjustment knob fully
CCW.
This is the valve’s minimum allowable downstream pressure
setting and will cause it to close at the lowest downstream
pressure.
C. Turn on the hydraulic power unit.
D. Increase the power unit’s relief valve setting until the pressure
at Gauge A reads 500 psi / 3447 kPa.
You are now ready to test the ability of the pressure reducing
valve to reduce the downstream pressure in a blocked line.
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❑ 4. Open the shutoff valve and observe the pressure readings at
Gauges A and B.
Gauge A _______________________________________(psi/kPa)
Gauge B _______________________________________(psi/kPa)
Since the PRV adjustment knob is set at minimum and the outlet
flow is blocked, the pressure at Gauge B is showing the minimum
pressure setting of the valve.
You should also observe that the pressure at Gauge A is
approximately 500 psi / 3447 kPa.
If the flow to the circuit is blocked, pressure builds behind the
PRV spool. The spool moves to close off the outlet port, allowing
only enough oil to keep the outlet pressure at the PRV’s pressure
setting. Figure 24 shows this condition with the spool blocking the
outlet port.
VALVE ADJUSTMENT
SET AT MINIMUM
DRAIN PORT
CONNECTED
TO TANK
GAUGE A
(500 PSI)
FROM
OIL
SUPPLY
SPRING
COMPRESSED
GAUGE B
(30 PSI)
SPOOL MOVED UP
TO CLOSE OUTLET
TO
CIRCUIT
PRESSURE HOLDS
SPOOL TO
CLOSE OUTLET
BLEED
LINE
Figure 24. Outlet to PRV Blocked
It is also important to note that a pressure reducing valve is
self-relieving. This means it will drain off oil from its outlet
through the drain line if outlet pressure becomes greater than its
pressure setting. This is needed in order to reduce the pressure
downstream in a blocked line when the valve’s setting is reduced.
This self-relieving feature is provided by a continuous bleed to the
drain when the spool closes off the outlet port. This bleed
connection between the outlet and drain is made by the bleed line
which is drilled through the center of the spool, as shown in figure
24.
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❑ 5. Turn the PRV adjustment knob CW until Gauge B reads 100 psi /
690 kPa. Observe the pressure at Gauge A (upstream pressure) and
record below.
You should observe that the upstream pressure remains near 500
psi / 3447 kPa.
PRESSURE DOWNSTREAM
GAUGE B
(psi/kPa)
PRESSURE UPSTREAM
GAUGE A
(psi/kPa)
100/690
/
200/1380
/
300/2070
/
400/2760
/
❑ 6. Turn the adjustment knob CW until Gauge B reads each of the
other pressures listed in the chart in step 5. Observe and record the
corresponding upstream pressure at Gauge A for each downstream
pressure reading.
You should observe that the upstream pressure stays at the relief
valve pressure of 500 psi / 3447 kPa as the downstream pressure is
changed.
You should also observe that with blocked flow the PRV easily
reduces the downstream pressure from its minimum to any
pressure up to upstream pressure.
❑ 7. Repeat step 6 several times to experiment with your ability to
adjust the downstream pressure setting of the pressure reducing
valve.
❑ 8. Now, open the needle valve exactly 1/2 turn to allow flow through
the circuit.
You are now ready to test the ability of the PRV to provide a
reduced downstream pressure while it is open.
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❑ 9. Adjust the pressure reducing valve until Gauge B (downstream
pressure) reads 100 psi / 690 kPa. Then observe the reading at
Gauge A and the flow rate. Record below the upstream pressure
(Gauge A) and the flow rate.
You should observe that the PRV has opened far enough to allow
flow through the circuit, but is still able to maintain a reduced
pressure lower than the system pressure of 500 psi / 3447 kPa.
PRESSURE DOWNSTREAM
GAUGE B
(psi/kPa)
PRESSURE UPSTREAM
GAUGE A
(psi/kPa)
FLOW
RATE
(gpm/lpm)
100/690
/
/
200/1380
/
/
300/2070
/
/
400/2760
/
/
To understand how the valve works, look at figure 25. This figure
shows that the valve opens to allow oil to flow through it. When
the flow increases to a point that the needle valve creates a back
pressure of 100 psi / 690 kPa, the pressure downstream moves the
valve’s spool to a position where the spring force is equal to the
pressure against the end of the spool. The spool will be held in the
exact position (partially open) to provide 100 psi / 690 kPa
pressure as the oil flows through the valve.
VALVE
ADJUSTMENT
SET AT 100 PSI
GAUGE A
(500 psi)
FROM
OIL SUPPLY
GAUGE B
(100 psi)
0 psi/kPa
PRESSURE
MOVES SPOOL
Figure 25. Reducing Valve Adjusted to 100 psi / 690 kPa and Pressure
at Inlet of 500 psi / 3500 kPa
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If the needle valve were to be opened more, the downstream
pressure would start to drop. This is automatically corrected
because this slight drop in pressure allows the valve spring to
move the spool’s position to open the valve further and allow more
flow to go through the valve and the needle valve. This larger flow
causes more back pressure from the needle valve which increases
until the pressure is stable again at 100 psi / 690 kPa. In this case,
the flow rate will now be higher than before but the downstream
pressure holds constant.
If the pressure setting of the valve were increased to 200 psi / 1390
kPa, for example, the valve’s spring would be compressed more
and would require more pressure to move the spool. The valve will
then open further until the flow increases to the point where the
needle valve’s back pressure rises to 200 psi / 1390 kPa. At this
point, the valve will hold this position to maintain a flow rate that
creates 200 psi / 1390 kPa while flowing through the needle valve.
❑ 10. Repeat step 9 for each of the other Gauge B pressures listed in the
table.
You should observe that with oil flowing through the PRV, the
valve still reduces pressure downstream from minimum to the inlet
pressure setting. However, notice what happens to the flow rate
when the downstream pressure setting is changed. You should
observe that the flow rate is lower at lower pressure settings. This
occurs because the pressure drop across the needle valve
decreases, decreasing the flow rate in the circuit.
❑ 11. Open the needle valve fully and observe the pressure upstream and
downstream.
You should see that the PRV opens completely to try to provide
enough flow to create the back pressure needed. However, it is not
enough. In this case, the application needs more flow than the
circuit can give and still maintain the reduced pressure.
The last case to explore is where the inlet pressure drops below the
PRV’s pressure setting. When this happens, the valve stays open
because the spring force is greater than the force from the oil
pressure at the outlet. This causes downstream pressure to be the
same as upstream pressure.
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❑ 12. Perform the following substeps to test the PRV.
A. Close the needle valve fully.
B. Adjust the PRV setting (Gauge B) to 200 psi / 1390 kPa.
C. Now lower the relief valve setting (Gauge A) to 100 psi / 690
kPa and read Gauge B.
Gauge B ____________________________________(psi/kPa)
You should observe that the PRV’s downstream pressure is the
same as the relief valve pressure (100 psi / 690 kPa). In this
case, the valve is wide open because it is trying to allow
enough flow through to create a greater downstream pressure,
as shown in figure 26.
VALVE ADJUSTMENT
SET AT 200 PSI
DRAIN PORT
TO TANK
SPRING
SLIGHTLY
COMPRESSED
SPOOL
WIDE OPEN
GAUGE A
(100 PSI)
FROM
OIL
SUPPLY
GAUGE B
(100 PSI)
TO
CIRCUIT
Figure 26. Reducing Valve Adjusted to 200 psi / 1390 kPa and Pressure
at Inlet is 100 psi / 690 kPa
D. Open the needle valve 1 full turn and observe the pressure at
Gauges A and B.
Gauge A ____________________________________(psi/kPa)
Gauge B ____________________________________(psi/kPa)
You should observe that this had no effect on either pressure.
The PRV is still wide open and always will be if the inlet
pressure is less than the downstream pressure setting.
❑ 13. Perform the following substeps to shut down the hydraulic power
supply.
A. Reduce the relief valve to its minimum setting.
B. Close the shutoff valve.
C. Turn off the power unit.
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SKILL 6
CONNECT AND OPERATE A REDUCED PRESSURE CIRCUIT
Procedure Overview
In this procedure, you will set up and test the operation
of a circuit that uses a pressure reducing valve to control
the pressure of a clamp cylinder.
❑ 1. Connect the clamp circuit shown in figure 27.
In this circuit, the reducing valve will provide reduced system
pressure at the cap end of the cylinder only during extend and when
held extended. A bypass check valve is used so that you can retract
the cylinder. Although the PRV is normally open, the check valve is
needed because it can close if the pressure is high enough.
GAUGE
A
SUPPLY
MANIFOLD
GAUGE
B
SMALL BORE
CYLINDER
TEES
3
IN
OUT
A
2
1
B
CHECK
VALVE
PRESSURE
REDUCING
VALVE
NEEDLE
VALVE
RETURN
MANIFOLD
NOTE: CONNECT TEE'S AT
THE CHECK VALVE
Figure 27. Schematic of Clamp Circuit with a Pressure Reducing Valve
❑ 2. Perform the power unit checkout procedures.
❑ 3. Turn on the hydraulic power unit and adjust the relief valve
pressure to 500 psi / 3447 kPa.
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❑ 4. Open the shutoff valve.
❑ 5. Close the needle valve (turn fully CW). Then open it 1/4 turn.
This will put back pressure on the cylinder during movement and
allow the PRV to control the downstream pressure when the
cylinder is extending.
❑ 6. Perform the following substeps to set the clamp pressure on the
pressure reducing valve to 100 psi / 690 kPa.
A. Turn the pressure reducing valve’s adjustment knob fully CCW
to its minimum adjustment setting.
B. Shift the DCV to extend the cylinder.
C. When the clamp cylinder stops, continue to hold the DCV
shifted. Then adjust the pressure reducing valve until Gauge B
reads 100 psi / 690 kPa.
You have now set the pressure reducing valve to a specific
pressure.
NOTE
To set a pressure reducing valve in an industrial application,
use this same method: deadhead (fully extend or retract) the
actuator and then adjust the PRV setting.
D. Retract the cylinder using the DCV.
❑ 7. Now extend the cylinder by shifting and holding the DCV.
Observe the pressure at Gauge B while the cylinder extends and
after it becomes extended.
You should observe that reduced pressure remains near 100 psi /
690 kPa during extend and when held extended. Gauge A remains
at 500 psi / 3447 kPa.
❑ 8. Shift the DCV in the opposite direction to retract the cylinder.
❑ 9. Repeat steps 7 and 8 several times to become more familiar with
the operation of the circuit. As you do this, observe and record the
pressure readings of Gauges A and B for each of the conditions
listed in the chart.
CIRCUIT CONDITION
PRESSURE
GAUGE A
(psi/kPa)
PRESSURE
GAUGE B
(psi/kPa))
Clamp cylinder extending
Clamp cylinder extended
Clamp cylinder retracting
Clamp cylinder retracted
You should observe that the clamp cylinder will be approaching
500 psi / 3447 kPa during retraction because the needle valve is
metering out. While the clamp cylinder is being held retracted,
both gauges will reach 0 psi/0 kPa because they are connected to
the tank.
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❑ 10. Now perform the following substeps to set the PRV to a new
❑ 11.
❑ 12.
❑ 13.
❑ 14.
❑ 15.
pressure setting.
A. Shift the DCV to extend the cylinder.
B. When the cylinder becomes extended, continue to hold the
DCV shifted. Then adjust the pressure reducing valve until
Gauge B reads 250 psi / 1725 kPa.
C. Retract the cylinder using the DCV.
Cycle the cylinder several times and observe the pressure at Gauge
B while extending.
The pressure should be rising to 250 psi/1725 kPa while extending.
Repeat steps 10 and 11 for a reduced pressure setting of 150 psi /
1035 kPa. Observe the pressure at Gauge B while extending.
This pressure should be approximately 150 psi/1035 kPa.
Now reset the PRV setting to 250 psi / 1725 kPa.
Open the needle valve fully (turn CCW).
Extend the cylinder and observe the pressure at Gauge B while the
cylinder is extending and after it becomes extended. Record below:
CIRCUIT CONDITION
PRESSURE
GAUGE B
(psi/kPa)
Cylinder extending
Cylinder extended
You should observe that the pressure at Gauge B is lower than the
pressure reducing valve setting while the cylinder is extending.
When it becomes extended, it rises to the PRV setting but does not
exceed it.
❑ 16. Perform the following substeps to shut down the hydraulic power
supply.
A. Reduce the relief valve to its minimum setting.
B. Close the shutoff valve.
C. Turn off the power unit.
D. Move the handle of the DCV back and forth to remove any
pressure in the circuit.
Leave the circuit set up. It will be used in Activity 3.
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SEGMENT 3
SELF REVIEW
1. The pressure-reducing valve senses __________ pressure.
2. To increase the downstream pressure of a PRV, you turn the
adjustment knob________.
3. A spring holds the PRV in a normally ____________
position.
4. A PRV is used in multiple actuator circuits to reduce the
__________ in one branch of the circuit.
5. A PRV is in the __________ position when the adjustment
pressure setting is higher than system pressure.
6. The __________ relieving feature of a PRV allows excess
downstream pressure to bleed off through the drain port.
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SEGMENT 4
PRV APPLICATIONS
OBJECTIVE 9
DESCRIBE THE FUNCTION OF A PRV’S
BYPASS CHECK VALVE
Being a normally open valve, the PRV would appear to cause no
problems when oil flows backwards through the valve. Unfortunately,
this may not always be true. Any pressure pulse on the valve’s outlet that
is higher than the adjusted setting will cause the valve to close. This
blocks flow and prevents the actuator from moving. Fast cycling circuits
tend to produce these pressure pulses and cause the valve to do this.
To keep the valve from blocking reverse flow, a check valve is
placed in parallel with the PRV to allow bypass flow. To do this, a
separate check valve can be connected in the circuit or a pressure
reducing valve can be supplied from the manufacturer with a check valve
built into the body. A cross-section of a direct-operated PRV with
integral check valve, along with the symbol, are shown in figure 28.
ENCLOSURE
INLET
PORT
CHECK
VALVE
OUTLET
PORT
BYPASS
CHECK
VALVE
REDUCING VALVE
WITH BY-PASS
CHECK VALVE
Figure 28. Basic Construction and Schematic Symbol of a Direct
Operated PRV with Integral Bypass Check Valve
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Activity 3. PRV Operation with a Check Valve
Procedure Overview
In this activity, you will demonstrate the need for a
bypass check valve with a PRV in a fast cycling circuit. To
do this, you will cause the PRV without a bypass check to
close and block the circuit.
❑ 1. Using the same setup as used in Skill 6 (reduced pressure circuit in
❑ 2.
❑ 3.
❑ 4.
❑ 5.
❑ 6.
❑ 7.
❑ 8.
figure 27), remove the bypass check valve from the circuit by
disconnecting both hoses at the check valve.
Perform the power unit checkout procedures.
Turn on the hydraulic power unit and adjust the relief valve
pressure to 500 psi / 3447 kPa.
Close the needle valve. Then open it 1/4 turn.
Open the shutoff valve.
Extend the cylinder and hold the DCV’s lever shifted until the
cylinder becomes extended.
While holding the DCV shifted, adjust the PRV’s setting to 100
psi / 690 kPa.
From the cylinder being held extended, rapidly shift the DCV to
the other end to retract the cylinder. Observe the operation of the
cylinder. Does it operate properly?
______________________________________________(Yes/No)
❑ 9. Repeat steps 6 and 8 several times to test the operation of the
circuit without a bypass check valve.
_____________________________________________________
_____________________________________________________
You should observe that sometimes the PRV will close during
retraction, blocking flow and causing the cylinder to retract very
slowly or stop.
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❑ 10. Perform the following substeps to shut down the hydraulic power
supply.
A. Reduce the relief valve to its minimum setting.
B. Turn off the power unit.
C. Move the handle of the DCV back and forth to remove any
pressure in the circuit.
❑ 11. Connect the check valve into the circuit to bypass the pressure
reducing valve, as shown in figure 27.
❑ 12. Turn on the hydraulic power supply and adjust the pressure to 500
psi / 3447 kPa.
❑ 13. Repeat steps 6 and 8 several times and observe the operation of the
cylinder. Does it operate properly?
_____________________________________________________
_____________________________________________________
You should observe that the cylinder retracts at a normal speed
each time because the bypass check valve is allowing a free flow
of oil during retraction.
❑ 14. Perform the following substeps to shut down the hydraulic power
supply.
A. Reduce the relief valve to its minimum setting.
B. Close the shutoff valve.
C. Turn off the power unit.
D. Move the handle of the DCV back and forth to remove any
pressure in the circuit.
Leave the circuit set up. It will be used in Activity 4.
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SKILL 7
DESIGN A HYDRAULIC CIRCUIT THAT USES
A PRESSURE REDUCING VALVE
Procedure Overview
Pressure reducing valves can be used in any
application where the process requires one or more
actuators to operate at pressures lower than the main
circuit pressure. The metal expander machine shown in
figure 29 is an example of a clamp and press circuit that
uses a pressure reducing valve to control the clamp
pressure. This machine is commonly used to make tubes
such as rocket engine casings very smooth and concentric
(circular).
In this procedure, you will combine your knowledge of
sequence valves and pressure reducing valves to design a
control circuit for the metal expander machine.
❑ 1. Design and draw a hydraulic control circuit to control the metal
expander machine shown in figure 30. Draw in the power unit
components. The sequence is as follows:
• When the operator pushes the lever of the DCV forward (straight
arrows condition), the two clamp cylinders extend at a controlled
speed.
• When the clamp cylinders contact the metal tube they clamp at a
pressure of 200 psi / 1380 kPa.
The mandrel cylinder then begins to extend. The trigger to
extend is a rise in system pressure which opens a sequence
valve. The mandrel cylinder extends at a controlled speed (less
than that of the clamp cylinders).
• When the mandrel cylinder is extended, the operator pulls the
DCV lever to the crossed arrows condition. This causes the
mandrel cylinder to retract. Speed is not controlled.
• When the mandrel cylinder is retracted, a sequence valve opens
causing the clamp cylinders to retract.
• When the clamp cylinders are retracted, the operator releases the
DCV lever and the cycle stops.
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Show your design to your instructor and explain its operation. This
is part of your skills assessment.
Figure 29. Metal Expander Machine
CLAMP 1
CLAMP 2
TUBE
MANDREL
MANDREL
CYLINDER
Figure 30. Metal Expander Machine Circuit
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OBJECTIVE 10
EXPLAIN WHY A PRV IS EXTERNALLY DRAINED
Because pressure is on the downstream side of a PRV, oil will
naturally seep into the spring chamber. If the oil is trapped in this
chamber, it will prevent the spool from closing the valve to reduce the
pressure. The external drain prevents this trapping of oil and allows the
spool to move freely. The drain must always be connected to the tank
and never blocked.
Activity 4. PRV Drain Operation
Procedure Overview
In this activity, you will demonstrate what happens to
the operation of a PRV when the drain is blocked. To block
the PRV drain, you will remove the hose to the PRV drain
port.
❑ 1. Connect or use the same circuit setup used in Skill 6 (reduced
❑ 2.
❑ 3.
❑ 4.
❑ 5.
❑ 6.
❑ 7.
pressure circuit from figure 27) and Activity 3.
Perform the power unit checkout procedures.
Turn on the hydraulic power unit and adjust the relief valve
pressure to 500 psi / 3447 kPa.
Open the shutoff valve.
Cycle the cylinder several times to verify that the PRV is working
properly.
Disconnect the pressure reducing valve’s drain line from the return
manifold.
Cycle the cylinder several times to test the operation of the PRV
without a drain line. Does it operate properly? Record your
observations.
_____________________________________________________
You should observe that the PRV stops reducing pressure after a
few cycles. It then opens and puts full system pressure downstream
while extending and being held extended.
❑ 8. Perform the following substeps to shut down the hydraulic power
supply.
A. Reduce the relief valve to its minimum setting.
B. Close the shutoff valve.
C. Turn off the power unit.
❑ 9. Move the handle of the DCV back and forth to remove any
pressure still in the circuit.
❑ 10. Disconnect your hoses and store them.
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SEGMENT 4
SELF REVIEW
1. Downstream pressure will ________ if the external drain of
a PRV is blocked.
2. The drain port of a PRV must always be connected directly
to the 1 __________.
3. To prevent a PRV from blocking flow when flowing
backwards through the valve, use a(n) __________ valve.
4. The external drain of a PRV keeps oil pressure from
building in the _________ chamber.
5. Pressure-reducing valves can be used in any application
where the process requires one or more ____________ to
operate at pressures lower than the main circuit pressure.
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