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.
1
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.
2
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.
3
8.1.2 Two-pump Rapid Motion Circuit
Setting the
lower output
pressure
6
7
3
5
4
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 systematic
pressure
1
2
Fig. 8-2 two-pump output circuit.
The power resource
consists of a high-pressure
low-flow rate pump and a
low-pressure high flow rate
4
pump.
8.1.2 Two-pump Rapid Motion Circuit
6
7
3
5
4
1
2
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 circuit5 is
widely used in machine tools
8.1.1 Speed control circuit by using a throttle valve
8.1.1.1 meter-in circuit
1 Introduction
1
2
1
1
2
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.
y
p
p
Note:
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
6
valve.
8.1.1 Speed control circuit by using a throttle valve
2 Velocity versus load characteristic
When regardless of the leakage and
1
compressibility of oil, the extension
2
speed of the cylinder is：
q1
1
v
(8 -1)
1
2
A1
The balance equation of the piston is
p1 A1  p2 A2  F
(8 - 2)
y
F
Thus
p1 
A1
p
p
The flow rate of the oil through the
throttle valve into the cylinder is：
q1  CAT (pT ) m  CAT ( p p  p1 ) m
F m
 CAT ( p p  )
(8  3)
A1
7
8.1.1 Speed control circuit by using a throttle valve
q1 CAT
F m
v

( pp  )
A1
A1
A1
(8-3)
Here, C = flow rate coefficient；
AT = the flow area of the throttle；
ΔpT= the pressure drop of the throttle valve;
m = exponent, when a throttle orifice is a thin wall orifice,
m= 0.5
8
8.1.1 Speed control circuit by using a throttle valve
With other conditions unaltered the
q1 CAT
F m speed of piston is proportional to the
v

( pp  )
A1
A1
A1
flow area of the throttle valve
e.g., when area AT1, AT2 and AT3
T3
given we can obtain speed v1,v2 and v3.
When the area is not varied, the
3
T2
piston speed will vary with the load
2
T1
1
resistance in a approximate parabola.
That is, the load fluctuation will have
great influence on the speed stability.
The maximum load capacity is
Fmax  p p A1
When F=Fmax , the pressure difference of both sides of the throttle
is equal to zero, no fluid passing through the throttle valve and
overall flow rate discharged through the relief valve to the
9
reservoir, the speed of the piston being equal to zero.
8.1.1 Speed control circuit by using a throttle valve
3 Power and efficiency characteristic
The output power of the pump is：
Pp  p p q p
The effective output power of the cylinder is
P1  Fv  p1q1
The power loss of the circuit is
P  Pp  P1  p p q p  p1q1
 p p ( q1  q )  ( p p  pT )q1
 p p q  pq1
overflow power loss
throttle power loss
The efficiency of the circuit is

Pp  P
Pp

p1q1
p pq p
10
8.1.1 Speed control circuit by using a throttle valve
4 Meter-out Circuit
A1 p1
Δq
qp
q1
F
p2
A2
q2
Δp
AT
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.

CAT
A21 m
( p p A1  F ) m
(8 -6)
11
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.
12
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.
13
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 bleedoff 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.
14
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
15
displacement pump and a variable displacement motor.
8.2.3.1 Speed Control Circuit Using Variable Displacement Pump
Prevent the
system from
overloading
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
Relieve the
pressure of
pump 1
4
5
M
TM
P
M
n
3
2
1
T
M
P
M
n
M
6
supply
make-up oil
for leakage
V
p
16
8.2.3.2 Speed Control Circuit Using Variable Displacement motor
4
5
nM
3
2
TM
PM
nM
PM
TM
6
1
Fig. 8-9 speed control circuit
using fixed displacement pump
and a variable displacement motor
VM
Fig. 8-10 speed control characteristic
using fixed displacement pump
and a variable displacement motor
The advantage of the speed control circuit is to keep a constant
power at various rotational speed, while disadvantage is a slightly
narrow speed adjusting range ( Rc  3 ).
17
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.
18
8.3.1 Mechanically Synchronous Circuit
Fig 8-13 Mechanically Synchronous circuit
19
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
20
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.
21
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. 22
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
23
8.4.1 Mechanically Operated Sequencing Circuit
①
③
②
④
1
3
2
4
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.
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.
24
①
2
1
S3 S4
S1 S2
3
1YA
4
2YA
3YA
4YA
Fig. 8-18Mechanically operated sequencing
circuit by using electric limit switches
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.
25
②
2
1
S3 S4
S1 S2
3
1YA
4
2YA
3YA
4YA
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 deenergized
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.
26
③
1
S3 S4
S1 S2
3
1YA
4
2YA
3YA
4YA
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 deenergized 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.
27
④
1
S3 S4
S1 S2
3
1YA
4
2YA
3YA
4YA
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.
28
1
S3 S4
S1 S2
3
1YA
4
2YA
3YA
4YA
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
29
adjusting frequently.
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.
30
Thus cylinder 8 extends.
Fig.8-19 The sequencing circuit
using pressure switches.
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. 31
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
0.3~0.5MPa higher than the
maximum working pressure
during the previous stroke but
0.3~0.5MPa lower than the
32
pressure relief valve at least.
①
④
②
③
1
2
3
4
5
1YA
2YA
7
6
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
33
cylinder 2 extends.
①
④
②
③
1
2
3
4
5
1YA
2YA
7
6
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.
34
Thus cylinder 1 retracts.
①
④
②
③
1
Please note:
2
3
4
5
1YA
2YA
7
6
To
ensure
the
sequential operation to
be reliable the setting
pressure of the sequence
valve 3 should be higher
0.3~0.5MPa than the
maximum load pressure
of cylinder 2 during the
retraction stroke but
lower 0.3~0.5MPa than
the pressure relief valve
7.
35
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.
36
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
37
systems.
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
38
systems and excavator systems. .
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.
39
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.
Fig.8-23 The unloading circuit
using the neutral position of a
directional control valve
The
open
center
directional control valve is
the simplest method used
to unload a pump.
40
8.6.2Unloading Circuit Using A Relief Valve With a Vent valve
1
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 pump’s outlet
is diverted through the
pilot-operated relief valve
to tank the pump is
Fig.8-24 The unloading circuit using a pilot- unloaded.
operated relief valve with a vent valve
41
8.6.4 Unloading Circuit Using Pressure-Compensated Variable
displacement Pump
When the system pressure increases
to predetermined levels the pressurecompensation 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.
42
8.6.3 Unloading Circuit Using Unloading Valve
1YA
2YA
3
4
2
1
Fig. 8-25 The unloading circuit
using an 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. 43
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.
44