hydraulic circuit design and analysis

1. CHAPTER 6
HYDRAULIC CIRCUIT
DESIGN AND
ANALYSIS
1

2. 2
6.1 Introduction
Contents
6.2 Control of single-acting hydraulic cylinder
6.3 Control of double-acting hydraulic cylinder
(Regenerative cylinder circuit, Pump unloading
circuit, Double pump hydraulic circuit, Hydraulic
cylinder sequencing circuit, Cylinder synchronizing
circuit, Fail safe circuit, various valve applications
in hydraulic circuit (counterbalance valve, pilot
check valve)
6.4 Speed control of hydraulic cylinder
6.5 Speed control of hydraulic motor
6.6 Hydraulic motor braking system
6.7 Air-over-oil circuit

3. 6.1 Introduction
A hydraulic circuit is a group of components such
as pumps, actuators, control valves, and the like
arranged so that they perform a specific useful task.
Three important considerations:
1. Safety of operation
2. Performance of desired function
3. Efficiency of operation
Hydraulic circuits are developed using graphic
symbols for all components. ANSI (American
National Standards Institute) symbols will be used
here.
Electrical control devices will not be included here.
3

4. 6.2 Control of a Single-Acting Hydraulic Cylinder
fig.6.1.pptx shows a two-position, three-way,
manually actuated, spring-offset directional control
valve (DCV) controlling the operation of a single
acting cylinder.
The valve shown is in un actuated condition (spring
off-set mode) which is the end of retraction by which
time the oil from the cylinder would have flown out
to the tank. When the valve is actuated manually ie.
the envelope is pushed to the right, oil flows to the
blank end, thus extending the piston rod. To control
the pressure of the oil, the relief valve is included.
4

5. `
6.3 Control of a Double-Acting Hydraulic Cylinder
fig.6.2.pptx shows the circuit used to control a
double-acting hydraulic cylinder. The operation is
described as follows:
1) As shown the four-way valve is in its spring-
centered position, cylinder is hydraulically locked,
pump unloaded back to the tank
2) When manually operated to the left envelop
configuration, oil flows to the blank end of the
cylinder extending the cylinder rod against the
external load. Also oil in the rod end flows to the
tank through valve B
5

6. 3) When the four-way valve is deactivated, the spring-
centered envelope prevails, and the cylinder is once
again hydraulically locked.
4) When the four-way valve is actuated into the right
envelope configuration, the cylinder retracts as oil
flows from port P through port B. Oil in the blank
end is returned to the tank via the flow path from port
A to port T.
5) At the end of the cycle if there is no demand of oil
by the system, the pump flow goes to the tank
through the relief valve. PRV protects from overload
too.
6

7. 6.3.1 Regenerative Cylinder Circuit
fig-chp6fig.6.3a.pptx shows a regenerative circuit
that is used to speed up the extending speed of a double-
acting cylinder.
Left envelope configuration has the pump flow and
the exhaust flow flowing to the blank end (volume flow
rate is large).
Right envelope configuration has the exhaust from the
blank end to the tank. The pump flow bypasses the DCV
directly to the rod compartment (since one of the ports is
closed).
During extension (left envelope configuration) the flow
from the rod end (QR) regenerates with the pump flow
(QP) to provide a total flow rate (QT).
7

8. 6.3.1(a) Cylinder Extending Circuit
Looking at Fig.6.3(b) Partial circuit showing flow
paths.pptx
QT = QP + QR or QP = QT - QR
For extension velocity VP,ext
QP =[AP - (AP –Ar)] VP,ext
Solving for the extending speed gives
The small rod area gives a large extending speed.
It can be greater than the retracting speed if the rod
area is made small enough
r
P
ext
P
A
Q
V 
,
8

9. 6.3.1(b) Ratio of Extending and Retracting Speeds
The retraction speed is given by
The ratio of the two speeds will be
The two speeds will be the same when AP = 2Ar.
r
P
P
ret
P
A
A
Q
V


,
1
)
/(
/
,
,






r
P
r
r
P
r
P
P
r
P
ret
P
ext
P
A
A
A
A
A
A
A
Q
A
Q
V
V
9

10. 6.3.1(c) Load-Carrying Capacity During Extension
As the pressure is the same in both faces during
extension, the load carrying capacity becomes
Fload,ext = pAP-p(AP-Ar) = pAr
The speed is increased at the expense of the
reduced force carrying capacity.
6.3.1(d) Drilling Machine Application
The circuit is shown in Fig.6.4 Drilling machine
application.pptx. Four-way valve is spring centered
and manually operated.
The sequence of operations are as follows:
• The spring-centered position gives rapid spindle
advance (extension) 10

11. • The left envelope mode gives slow feed(extension)
when the drill starts to cut into work piece
• The right envelope mode retracts the piston.
At the centre position oil from the rod end
regenerates with the pump flow thus giving rapid
extension.
6.3.2 Pump-Unloading Circuit
fig-chp6fig.6.5.pptx shows the pump-unloading
circuit. In the left envelope configuration the piston
extends. At the end of the extension stroke the
pressure will rise and is maintained at this high state
by the check valve. Through the pilot line the
unloading valve will open allowing pump to
discharge at a lower pressure. 11

12. In the right envelope configuration, the piston
retracts and the motion of the piston reduces the
pressure in the pilot line. This resets the unloading
valve until complete retraction where a pressure build
up may occur again.
6.3.3 Double-Pump Hydraulic System
Here two pumps are used (high pressure-low flow
pump and low pressure-high flow pump) fig-
chp6fig.6.6.pptx . When the valve is shifted to both
extreme envelopes, both pumps supply fluid at
medium pressure (high flow rate for high speed) until
the cylinder contacts sufficient load resistance to
raise the system pressure to a predetermined level.12

13. At this juncture the pilot line opens the unloading
valve to relieve the high flow pump. The check valve
protects the high flow pump from high pressure. The
PRV protects the high pressure pump from over
pressure. Usually the high speed extension is with no
load.
As this process requires both high flow rate and
high pressure, the above circuit eliminates the
necessity of having a very expensive high pressure,
high flow pump.
13

14. 6.3.4 Hydraulic Cylinder Sequencing Circuit
Sequence valves prioritizes the operation system.
fig-chp6fig.6.7.pptx shows two sequence valves to
operate two double acting cylinders. The DCV (spring-
centered) is of the closed centre option which locks the
cylinders at their prevailing conditions.
In the extreme left envelope configuration the left
cylinder extends. At the end of extension pressure builds
up and the right sequencing valve opens and the right
cylinder extends. In the extreme right envelope
configuration the right cylinder retracts first and then the
left retracts. One application could be the use of the left
cylinder for clamping and the right cylinder for drilling.
14

15. 6.3.5 Cylinder Synchronizing Circuits
6.3.5(a) Cylinders Connected in Parallel
fig-chp6fig.6.13.pptx shows two identical cylinders
expected to be synchronized. Synchronization requires
identical loads. If not, the lower load cylinder will
extend first since it requires less pressure. No two
cylinders are really identical. Packing friction will differ
from cylinder to cylinder.
6.3.5(b) Cylinders Connected in Series
This arrangement allows the synchronization of the two
cylinders. Looking at fig-chp6fig.6.14.pptx and for
the DCV at the top envelope oil is delivered to the blank
end of cylinder 1. As it extends fluid is delivered to the
blank end of cylinder2.
15

16. For the cylinders to be synchronized the piston area
of cylinder 2 must equal the effective area of the rod
end of cylinder 1 and this can easily be proved by
applying continuity equation.
For the same speed (synchronization) AP1 – AR1 = AP2
Force balance on cylinders 1 and 2 gives
p1AP1 –p2(AP1-AR1) = F1
p2AP2 – p3(AP2 –AR2) = F2
Since AP2 = AP1 – AR1 and p3 = 0 the above
equation will give
p1AP1 = F1 + F2 16
 
2
)
1
( cyl
in
cyl
out Q
Q 
2
1 )
(
)
( cyl
eff
cyl
eff v
A
v
A 

17. 6.4 Speed Control of a Hydraulic Cylinder
fig-chp6fig.6.15.pptx shows a circuit where speed
control of a hydraulic cylinder is accomplished
during the extension stroke using a flow control
valve. The operation is as follows:
a) When the directional control valve is actuated, oil
flows through the flow control valve to extend the
cylinder. The extending speed is dependent on the
setting of the FCV.
b)When the DCV is in the un actuated or spring-offset
mode, the cylinder retracts and there will be
uncontrolled flow (the oil flows through the check
valve as well as the flow control valve)
17

18. Analysis of extending speed control
Flow control reduces the flow rate which in turn
will raise the pressure of the fluid before the FCV.
Continued closing results in p1 reaching and
exceeding the cracking pressure of the PRV. This
results in slower speed since part of the pump flow
goes back to the tank through the PRV.
Qcyl = Q pump – QPRV
Where: flow rate through flow control valve, = capacity
coefficient of FCV, p1 = pPRV (PRV setting), p3 ≈ 0, SG = specific
gravity of oil.
SG
p
p
C
SG
p
C
Q 2
1
v
v
FCV




18

FCV
Q v
C

19. Force balance on the cylinder will give
p2Apiston = Fload or p2 = Fload/Apiston
Also the extending speed of the cylinder can be
represented as a function of the flow rate through the
FCV and given as :
vcyl = Qcyl/Apiston = QFCV/Apiston
And after substition of QFCV and p2 yields the result:
SG
A
F
p
A
c
v
piston
load
PRV
piston
v
cyl
/


19

20. 6.5 Speed Control of a Hydraulic Motor
Fig.6.16 .pptx shows circuit where speed control of
a hydraulic motor is accomplished using a pressure-
compensated flow control valve.
The operation is as follows:
a) In the spring-centered position, the motor is locked
b) In the left envelope configuration, the motor rotates
in one direction. It’s speed can be varied by adjusting
the setting of the throttle of the flow control valve.
c)When the DCV is deactivated, the motor stops
suddenly,
d) When the right envelope is in operation, the motor
turns in the opposite direction.
20

21. 6.6 Hydraulic motor braking system
If a hydraulic motor drives a machine with a large
inertia, it would create a flywheel effect on the motor
when the flow is stopped. Stopping the motor would
cause it to act as a pump. If such a case happens
Fig.6. 17 Hydraulic motor braking system.pptx
• The circuit must supply the fluid while it is working
as a pump, other wise it may pull in air
• The discharge must return directly to the tank or
through a PRV
This would stop the motor rapidly without damage to
the system
21

22. 6.7 Air-over-Oil Circuit
Pressurized air is sometimes used to pressurize the
oil to be used in the cylinder. Fig.6.18 Air-over-oil
circuit.pptx shows one arrangement of this type.
DCV in top envelope: Pressurized air is allowed
into the surge tank and the cylinder extends.
Uncontrolled flow through the FCV and free flow
through the check valve.
DCV in bottom envelope: Air exhausts and the
cylinder retracts with controlled speed.
22

23. Fig.6.1 Control of single acting hydraulic cylinder

24. Fig.6.2 Control of double acting hydraulic cylinder

25. Fig.6.3(a) Complete regenerative cylinder circuit

26. Fig.6.3(b) Partial circuit showing flow paths

27. Fig.6.4 Drilling machine application

28. Fig.6.5 Pump-unloading circuit

29. Fig.6. 6 Double pump hydraulic system

30. Fig. 6.7 Hydraulic cylinder sequence circuit

31. Fig.6.8 Counterbalance valve application

32. Fig.6. 9 Locked cylinder using pilot check valves

33. Fig.6.10 Cylinders hooked in parallel will not operate
in synchronization.

34. Fig.6.11 Cylinders hooked in series will operate in
synchronization

35. Fig.6.12 Fail-safe circuit

36. Fig.6.13 Fail-safe circuit with overload protection

37. Fig.6.14 Two-handed safety circuit

38. Fig.6.15 Meter-in speed control of hydraulic cylinder
during extending stroke using FCV.

39. Fig.6.16 Speed control of hydraulic motor using
pressure-compensated FCV

40. Fig.6. 17 Hydraulic motor braking system

41. Fig.6.18 Air-over-oil circuit

42. Example 6-1

43. ASSIGNMENT IV
1.

44. 2. Properly complete the circuit diagram of the figure given below. The
clamp cylinder is to extend first, and then the work cylinder extends by
the action of a directional control valve (DCV). By further action of
the DCV, the work cylinder retracts, and then the clamp cylinder
retracts. Also explain the purpose of the check valves.

45. 3. For the given circuit, give the sequence of operation of
cylinders 1 and 2 when the pump is turned on. Assume both
cylinders are initially retracted.