Objectives

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Chapter 8 Basic Hydraulic Circuit
Objectives Upon completing this chapter, you
will be able to 1. Be familiar with the main
hydraulic components and standard symbols used
for their representation on circuit diagrams.
2. Explain the operation principle and
performance characteristics of some usually used
circuits. 3. Identify transmission of power
and flow path through the circuits 4.
Describe the function of each basic element in a
practical hydraulic circuit. 5. Evaluate the
safety of operation. 6. Design a circuit to
perform a desired function.
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8.1 Rapid Motion Circuit
The purpose for a rapid motion circuit is
to obtain a speed as high as possible so as to
raise productivity and to reasonably utilize the
power of the prime mover.
There are some methods to accomplish rapid
motion, here we only introduce a regenerative
circuit and a high-low pressure pump circuit.
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8.1.1 Regenerative Circuit
The return line from the cylinder is
connected back to the supply line.
Although both end are under the same pressure,
there is a difference in areas on opposite sides
of the piston. Thus there is a net unbalanced
force which will cause the piston to move to the
right. The oil leaving the rod end
combines with the pump flow entering the blank
end of the cylinder. A regenerate circuit
does increase the rod extension speed but it also
reduces the output force of the cylinder.
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8.1.2 Two-pump Rapid Motion Circuit
When the load is very small and the systematic
pressure is lower than the preset level of the
unload valve (element 3), the two pumps
delivering pressure oil into the cylinder and
the cylinder extending rapidly.
Setting the lower output pressure
Setting the systematic pressure
The power resource consists of a
high-pressure low-flow rate pump and a
low-pressure high flow rate pump.
Fig. 8-2 two-pump output circuit.
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8.1.2 Two-pump Rapid Motion Circuit
When the systematic pressure rises to the
preset level set by valve 3, valve 3 opens,
permitting pump1 to unload. Valve 4 closes. The
cylinder extends at a low speed.

Pump 1 unloading decreases power loss
greatly. With high efficiency the circuit is
commonly used in the case where rapid motion is
much faster than feed motion. Particularly, the
circuit is widely used in machine tools
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8.1.1 Speed control circuit by using a throttle
valve
8.1.1.1 meter-in circuit 1 Introduction
A variable flow control valve is placed between
the pump and the actuator to control the flow
rate of the oil into the cylinder. Speed control
can be achieved simply by varying the flow rate
of fluid into the cylinder.
Because the pressure relief valve is opening
the discharge of fluid from the pump is held at a
approximately constant pressure by the action of
the pressure relief valve.
Note
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8.1.1 Speed control circuit by using a throttle
valve
2 Velocity versus load characteristic
When regardless of the leakage and
compressibility of oil, the extension speed of
the cylinder is
The balance equation of the piston is
Thus
The flow rate of the oil through the throttle
valve into the cylinder is
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8.1.1 Speed control circuit by using a throttle
valve
Here, C flow rate coefficient
AT the flow area of the throttle
?pT the pressure drop of the throttle valve
m exponent, when a throttle orifice is a
thin wall orifice, m 0.5
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8.1.1 Speed control circuit by using a throttle
valve
With other conditions unaltered the speed of
piston is proportional to the flow area of the
throttle valve e.g., when area AT1, AT2 and AT3
given we can obtain speed v1,v2 and v3. When
the area is not varied, the piston speed will
vary with the load resistance in a approximate
parabola. That is, the load fluctuation will have
great influence on the speed stability.
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8.1.1 Speed control circuit by using a throttle
valve
3 Power and efficiency characteristic
The output power of the pump is
The effective output power of the cylinder is
The power loss of the circuit is
The efficiency of the circuit is
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8.1.1 Speed control circuit by using a throttle
valve
4 Meter-out Circuit
The flow control valve is placed in the return
line of the actuator limiting the flow rate of
oil from the cylinder.
Apply similarly reasoning to the circuit we can
obtain the speed versus load characteristic
formula of the meter-out circuit.
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8.1.1 Speed control circuit by using a throttle
valve
5 Comparison of the two kind circuits
(1) The capacity against overrunning load
Meter-out circuits are the most effective in
systems that act against overrunning loads.
(2)Speed stability Because of the back
pressure caused with the throttle valve, the
speed stability is better than that in the above
meter-in circuit.
(3)Heat influence on the system In a
meter-in circuit the heat degenerated due to
throttle is fed to the actuator which will
increase the leakage of the actuator. In a
meter-out circuit the heat is fed to the oil
reservoir where hot oil is cooled, hence the heat
will have little influence on the leakage of the
system.
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8.1.1 Speed control circuit by using a throttle
valve
(4)Actuating characteristic In a meter-out
circuit the system having stopped working for
long time the oil in the cylinder has flowed to
the reservoir. Therefor, when the system is
actuated again, the piston of the cylinder will
rush abruptly because the back pressure can not
be built up immediately. The piston of the
cylinder in a meter-in circuit will not due to
throttle effect of the flow control valve.
6 advantages and disadvantages The advantages
of meter-in and meter-out circuit is a wide speed
control range and finer speed control feature,
additionally low in initial cost as well as
convenient operation. The disadvantages of
throttle speed control circuits is low
efficiency, additionally generating substantial
amount of heat. The two kinds circuit are
suit to be used in the case where the speed and
horsepower are low.
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8.1.2.3 By-pass Control Circuit
The flow control valve is connected in parallel
to the actuator controlling the flow rate by
bleeding-off a portion of the oil back to the
tank. It is called bleed-off or by-pass.

The relief valve is closing in normal work
condition. The sole function of the pressure
relief valve is to limit the system pressures in
this case. Its setting is 1.25 times as high as
maximum load pressure.
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8.1.2.3 By-pass Control Circuit
From formula (8-9) we can see that throttle loss
is sole energy loss, hence, the efficiency of the
circuit is higher than that in previous speed
control circuits. By-pass circuit is suit to be
used in the case where the speed stability is not
a principal consideration

Note that to ensure the speed stability
(meter-in, meter-out and by-pass) the throttle
valve can be replaced with a pressure compensated
flow control valve. When the efficiency is
predominating factor a speed control circuit
using the variable volume can be considered. ?
8.2.2 Speed Control by Varying Volume
Speed control using variable volume is
accomplished by varying displacement of a
hydraulic pump or a hydraulic motor, therefore
there are three ways to control speed one is to
utilize a variable displacement pump, another to
utilize a variable displacement motor, the other
to utilize both a variable displacement pump and
a variable displacement motor.
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8.2.3.1 Speed Control Circuit Using Variable
Displacement Pump
valve 6 relieves the pressure of pump 1. pump 1
supplies make-up oil for leakage and improves the
suction lift condition of pump 3, The circuit
returns a portion of oil to the tank via valve 6
that the closed loop is cooled
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8.2.3.2 Speed Control Circuit Using Variable
Displacement motor
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8.3 Synchronous circuit
Synchronous circuits allow two or more actuators
to operate in unison regardless of differences in
the magnitude of the load resistance.
There are two ways of controlling the
synchronization of cylinders or motors one
method is to mechinically link both cylinders or
motors together, another is to meter an equal
amont of fluid to earch circuit by using either
synchronous valves or flow dividers or series
cylinders. The former is called mechanical
sychronization, the later is called hydraulic
synchronization.
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8.3.1 Mechanically Synchronous Circuit
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8.3.2 Synchronous Circuit Using Compensated Flow
Valves
By the two valves being adjusted carefully
the flow rate entering each cylinder can ensure
to be basically equal.
In spite of being simple the two valves are
adjusted inconveniently. The circuit is
not suit to be used in the case where resistive
forces vary frequently or a resistive force of a
cylinder is very different from that of another
although the two resistive forces are all
constant loads
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8.3.3 Synchronous Circuit using Synchronous Valve
The output of each segment of the synchronous
valve is equal and respective cylinder receives
equal fluid. Deviation from synchronization can
be attributed primarily to leakage and the
manufacturing deviation of the synchronous valve
and cylinders.
The synchronous valve which meters equal
amounts into cylinder 1 and cylinder 2 causing
them to extension in synchronization.
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8.3.4 Synchronous Circuit Using Flow Divider
Hydraulic oil is fed to the flow divider which
meters equal amounts into two cylinders causing
them to extension in synchronization.
The throttle valve functions to eliminate
cumulative deviation.
Please note
The deviation from synchronization can be
attributed primarily to leakage and the
displacement deviation of the motors and the
cylinders.
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8.4 Sequencing circuit
Sequencing circuits order cycle events,
such as the two cylinders, one after another.
There are principally two types of sequencing
circuit. One is operated mechanically, another
is operated by pressure step.
The mechanically operated sequencing circuits
conclude the sequencing circuit by using
mechanically operated sequence valve the
sequencing circuit by using electric limit switch
The sequencing circuits operated by pressure
step conclude the sequencing circuit by using
pressure operated switches the sequencing
circuit by using sequence valves
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8.4.1 Mechanically Operated Sequencing Circuit
When valve 3 shifted to its left position the
pump delivery flows through valve 3 into the
blank end of cylinder 1 and discharge from the
rod end of cylinder 1 through valve 3 to the
tank. Thus cylinder 1 extends. At its advance
stroke end actuates valve 4 to start the
extension of cylinders 2. When valve 3 returns
to its right position cylinder 1 retracts.
Cylinder 1 on its back stroke de-actuates valve
4, cylinder 2 retracting.
A advantage of the circuit is that sequential
operation is accomplished smoothly, reliably, and
with the degree of position accuracy. But it is
difficult for strokes and sequences to be
modified.
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When solenoid 1YA is energized, valve 3 is
shifted to its left position. The delivery of the
hydraulic pump flows through valve 3 into the
blank end of cylinder 1 and discharge from the
rod end of cylinder 1 through valve 3 flow freely
to the tank. Thus cylinder 1 extends.
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At the end of the extension stroke of cylinder 1
it comes in the contact with the limit switch S2
which causes 1YA to be de-energized and 3YA
energized, valve 4 shifted to its left position.
The delivery of the hydraulic pump flows through
valve 4 into the blank end of cylinder 2 and
discharge from the rod end of cylinder 2 through
valve 4 flow freely to the tank. Thus cylinder 2
extends.
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At the end of the extension stroke of cylinder
2 it comes in the contact with the limit switch
S4 which causes 3YA to be de-energized and 2YA
energized valve 3 to be shifted to its right
position. The pump delivery flows through valve 3
into the rod end of cylinder 1 and discharge from
the blank end of cylinder 1 through valve 3 flow
freely to the tank. Thus cylinder 1 retracts.
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At the end of the retraction stroke of cylinder
1 it comes in the contact with the limit switch
S1 which causes 2YA to be de-energized and 4YA
energized, valve 4 shifted to its right position.
The pump delivery flows through valve 4 into the
rod end of cylinder 2 and discharge from the
blank end of cylinder 2 through valve 4 flow
freely to the tank. Thus cylinder 2 retracts.
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At the end of the retraction stroke of cylinder
2 it comes in the contact with the limit switch
S3 which causes 3YA to be de-energized. At this
moment, a work cycle is completed.
The advantages of using electric limit switches
are that it is convenient to modify sequences and
strokes. This circuit is suitable to be used in
the case where strokes and sequences need
adjusting frequently.
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8.4.2 Pressure Step Operated Sequencing Circuit
When the start button is pressed, it energizes
the 1YA and valve 1 is shifted to its left
position cylinder 7 extending? After cylinder 7
is fully extended, the systematic pressure
increases and reaches to the setting of pressure
switch 3, 1YA is de-energized and 3YA is
energized which causes valve 1 to returns to its
neutral position and valve 2 to its left
position. Thus cylinder 8 extends.
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When cylinder 8 is fully extended the pressure
reaches the setting of pressure switch 5, 3YA
de-energized and 4YA energized which causes
valve 2 to be shifted to its right position. Thus
cylinder 8 retracts. When cylinder 8 is fully
retracted the pressure increases to the setting
of pressure switch 6, 4YA is de-energized and 2YA
is energized which cause valve 1 to be shifted to
its right position. Thus cylinder 7 retracts.
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At the end of the retraction stroke of cylinder
7, pressure switches 4 actuate next work cycle
unless a stop button is pressed.
Note
To ensure sequential operations to be reliable
the setting of each pressure switch should be
0.30.5MPa higher than the maximum working
pressure during the previous stroke but
0.30.5MPa lower than the pressure relief valve
at least.
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When 1YA is energized valve 5 is to its left
position, which causes the pump delivery to flows
through valve 5 into the blank end of cylinder 1
rather than into the blank end of cylinder 2, and
the oil from the rod end of cylinder 1 flows
freely to the tank. Thus cylinder extends?
Cylinder 1 having finished its extension stroke
the systematic pressure increase and reaches to
the setting of valve 4, valve 4 opens. The pump
delivery flows through valve 5 and 4 into the
blank end of cylinder 2, and the oil from the rod
end of cylinder 2 flows freely to the tank. Thus
cylinder 2 extends.
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When 1YA is de-energized and 2YA energized
which cause valve 5 to its right position. The
pump delivery is directed through valve 5 into
the rod end of cylinder 2 rather than cylinder
1,and the oil from blank end of cylinder 2 flow
freely through the integral check valve of valve
3 and valve 5 to the tank. Thus cylinder 2
retracts? Cylinder 2 having finished its
retraction stroke the systematic pressure
increase to the setting of the valve 3 which
opens. The oil from pump 6 flows through valve 5
and 3 into the rod end of cylinder 1 and the oil
from the blank end of cylinder 1 flows freely to
the tank. Thus cylinder 1 retracts.
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Please note
To ensure the sequential operation to be
reliable the setting pressure of the sequence
valve 3 should be higher 0.30.5MPa than the
maximum load pressure of cylinder 2 during the
retraction stroke but lower 0.30.5MPa than the
pressure relief valve 7.
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8.5Counterbalance circuit
Many operations involve heavy elevated loads
that poses a constant threat. When the mechanisms
are released from their top-most position they
would descend faster and faster if the velocity
unlimited. Protection against such hazards is
often achieved through the use of counterbalance
circuits. The principle of counterbalance circuit
is to maintain a back pressure in the cylinder or
motor return line.
The principle of counterbalance circuit is to
maintain a back pressure in the cylinder or motor
return line.
This section principally introduces two kinds
counterbalance circuits one is the
counterbalance circuit by using counterbalance
valve, another is the counterbalance circuits by
using flow control valve.
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8.5.1 Counterbalance Circuit Using Counterbalance
Valve
Once the descent velocity of the piston exceeds
above theoretical value, the pump can not supply
hydraulic oil sufficiently to the cylinder, which
causes the pump output pressure or pilot pressure
of the counterbalance valve drop down and the
counterbalance valve closes. As a result, the
descent velocity of the piston will decrease
until the velocity reaches theoretical value.
The circuit can limit the descent velocity of
the piston smoothly and is widely used in the
hydraulic crane systems.
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8.5.2 Counterbalance Circuit Using flow Control
Valve
During retraction, the hydraulic oil from the
pump flows into the rod end of the cylinder, at
the same time, and the hydraulic oil from the
blank end of the cylinder flows through the flow
control valve to tank. Because of the back
pressure the flow control valve sets the descent
velocity of the piston is limited.
Note
The descent velocity of the piston is
related with the magnitude of load and the flow
area of the flow control valve.
The circuit is used in the case where the
requirement to a descent velocity is not very
accurate, such as in fork truck systems and
excavator systems. .
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8.6 Pump-Unloading Circuit
A pump unloading means that the pump is
developing a minimum amount of horsepower.
The purpose for pump-unloading is to prolong
expected pump life and to avoid wasting power and
generating heat during idle periods.
Since the power of a pump is equal to pressure
times flow rate, the pump will be unload if
either pressure or flow rate is close to zero.
There are two kinds of unload cases----one in
which when the pump is unloading the actuators
need holding pressure, another in which do not.
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8.6.1 Unloading Circuits Using Neutral Position
of Directional Valve
In its neutral position a open center
directional control valve routes the constant
pump output back to tank at very low pressure.
The open center directional control valve is
the simplest method used to unload a pump.
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8.6.2Unloading Circuit Using A Relief Valve With
a Vent valve
The unloading circuit using a
pilot-operated relief valve with a vent valve.
When the solenoid is not actuated the pressure
relief sets system pressure when the solenoid is
actuated the pumps outlet is diverted through
the pilot-operated relief valve to tank the pump
is unloaded.
Fig.8-24 The unloading circuit using a
pilot-operated relief valve with a vent valve
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8.6.4 Unloading Circuit Using Pressure-Compensated
Variable displacement Pump
When the system pressure increases to
predetermined levels the pressure-compensation
mechanism causes the pump to reduce flow rate to
minimum. The pump supplies only make-up oil. The
power loss is also very small. The use of
this kind pump eliminates the need for both
relief and unloading valves, since pump
compensates to limit system pressure. Relief
valves or hydraulic fuses are usually installed,
however, to protect the system against hydraulic
shock, thermal expansion of liquid, and possible
malfunction of the pressure-compensation
mechanism.
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8.6.3 Unloading Circuit Using Unloading Valve
When 1YA is actuated valve 3 allows oil from
the pump to flow into both the cylinder and the
accumulator, the cylinder extending. The piston
rod having contacted the work piece system
pressure rises and reach a preset level, valve 1
opens, so that fluid from the pump passes freely
through valve 1 to the tank. Subsequently
maintaining pressure is first satisfied by liquid
stored in the accumulator. Eventually, as the
accumulator empties, system pressure drops
sufficiently to close valve 1, bring the pump
directly back on-line.
Fig. 8-25 The unloading circuit using an
unloading valve
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Summary
The circuits introduced in the chapter are
all typical and commonly used circuits. The
readers who are interested in the other basic
circuits can look over the references list in the
book. The purpose for studying basic circuits is
grasping the basic principle and features of the
circuits, and being able to organically configure
them for designs of various hydraulic systems.