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Fundamentals of
Electrohydraulics
Collection of
Transparencies
T A P B
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Order no.: 095051
Designation: E.-HYDR-FOLIEN.
Description: D:OT-TP601-GB
Edition: 07/2001
Author: Dieter Scholz
Graphics: Doris Schwarzenberger
L t 18 07 2001 B t i H b
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Foreword
This set of overhead transparencies has been designed for use with the basic level
of the technology package TP600 “Electrohydraulics”. The set of overhead
transparencies and the technology package form part of the Learning System for
Automation from Festo Didactic GmbH & Co.
The 30 transparencies in this set have been optimised from the didactic and
methodological points of view. Each transparency is accompanied by a short text
which provides the trainer with a quick overview of the training contents of the
transparency concerned.
Basic principles of electrohydraulics
Function and use of electrohydraulic components
Basic principles of electrical engineering
Production and interpretation of standard hydraulic and electrical circuit
diagrams
The text sheet includes a complete illustration of the transparency, in certain cases
with additional explanations and legends which the trainer can enter on the
transparency in the course of a training session.
The advantages of this concept are:
The trainer can add to the transparencies step by step in the course of a training
session.
The trainer is thus able to create a lively and interesting atmosphere.
The provision of accompanying text means less preparation work for the trainer.
The enclosed CD-ROM contains the entire overhead transparencies and
accompanying text of this edition in an electronically presentable form in the files
„Electrohydraulics _transparencies.pdf“ and „Electrohydraulics _text.pdf“. In
addition to the screen presentation, which can be made in any order, the contents
can be printed out and text and graphics can be used for your own training
preparations, insofar as the functionality of the required Adobe®
Acrobat®
Reader
it thi Thi f l di t ib t bl ft i il bl th CD ROM i th
Training contents
New!
Electronic presentation
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Contents
Electrohydraulic press _______________________________________ Transparency 1
Schematic layout of an electrohydraulic installation ______________ Transparency 2
Electrical power supply unit __________________________________ Transparency 3
Electrical circuit ____________________________________________ Transparency 4
Measuring circuits __________________________________________ Transparency 5Ohm’s law _________________________________________________ Transparency 6
Rectifiers __________________________________________________ Transparency 7
Analogue, digital and binary __________________________________ Transparency 8
Momentary and sustained contact switches _____________________ Transparency 9
N/O, N/C and changeover contacts ___________________________ Transparency 10
Limit switches_____________________________________________ Transparency 11
Proximity switches _________________________________________ Transparency 12
Solenoid (electromagnet) ___________________________________ Transparency 13
Electrohydraulic converters (solenoid valves)___________________ Transparency 14
Dry solenoids – wet solenoids________________________________ Transparency 15
Solenoid valve connections__________________________________ Transparency 16
Relays and contactors ______________________________________ Transparency 17
Direct activation – indirect activation __________________________ Transparency 18
Signal inversion ___________________________________________ Transparency 19
Latching__________________________________________________ Transparency 20Circuit diagram ____________________________________________ Transparency 21
Hydraulic power pack_______________________________________ Transparency 22
Hydraulic circuit ___________________________________________ Transparency 23
Electrical power – hydraulic power ____________________________ Transparency 24
Power losses______________________________________________ Transparency 25
Pressure switches__________________________________________ Transparency 26
4/3-way solenoid valve _____________________________________ Transparency 27
4/3-way solenoid valve, piloted ______________________________ Transparency 28
Electrical safety____________________________________________ Transparency 29
Hydraulic safety ___________________________________________ Transparency 30
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1
Electrohydraulic press
Motor with hydraulic pump
Control panel
Valve manifold Cylinder
The hydraulic press is controlled via the electrical control panel.
Electrical signals are used to activate the valves in the hydraulic installation.
The electrohydraulic press is used to form rectangular troughs.
Control pane
Motor withhydraulic pump
Valve manifold
Actuator(cylinder)
Function units
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© Festo Didactic GmbH & Co. TP 601, 01Transparency
Electrohydraulic press
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2
Schematic layout of an electrohydraulic installation
Division of installation into signal control section and hydraulic power section.
Input and processing of electrical signals.
Solenoid valves form the interface between the electrical signal control section and
the hydraulic power section.
Controls the flow of hydraulic fluid by means of solenoid valves. Actuators convert
hydraulic energy into motions.
Signal control section
Signalinput
Hydrauliccylinder
Solenoidvalve
Hydraulicpowersupply(conversion)
Powersupply
Signalprocessing
Power section
Signal control section
Hydraulic power section
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© Festo Didactic GmbH & Co. TP 601, 02Transparency
Schematic layout of an electrohydraulic installation
Signal control section
Power section
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3
Electrical power supply unit
Connection to mains supply.
Supplies signal control section with specified or maximum voltage and currentvalues.
Voltage transformation
Rectification
Smoothing
Stabilisation
Fuse protection
In mobile hydraulic systems, rechargeable battery systems or generators are used
as a power supply for the signal control section.
230VAC
24V
DC
TR
50 °C
G C T T
Transformer Rectifier Smoothing Stabilisation Fuse
Functions in
power supply unit
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© Festo Didactic GmbH & Co. TP 601, 03Transparency
Electrical power supply unit
230VAC
24VDC
TR G C T T
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4
Electrical circuit
A circuit consists of at least a power source, a load device and connecting lines.
Electrical current will flow only in a closed circuit.
Electrical current flows from the positive terminal of the power supply to the
negative terminal.
Closed circuit without power supply.
Introduction of a power supply to complete the circuit.
A B
+
Technical direction
of current flow
Figure A
Figure B
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© Festo Didactic GmbH & Co. TP 601, 04Transparency
Electrical circuit
A B
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5
Measuring circuits
The voltage drop across a load device is measured in parallel with the device.
Electrical current is measured in series.
++
Voltage measurement Current measurement
Voltage measurement
Current measurement
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© Festo Didactic GmbH & Co. TP 601, 05Transparency
Measuring circuits
++
--
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6
Ohm’s law
Describes the relationship between current (I), voltage (V) and resistance (R).
With ohmic resistances, the current rises in linear proportion to the voltage.
The value pairs for a constant resistance lie on a straight line. The current I is
proportional to the voltage V.
Mathematical formula for Ohm’s law: V = R • I
I = V : R
+
R
A
2.0
1.5
1.0
0.5
0100 20
V
I
30 40V
10 V
0.5 A
0.33 A
Voltage V
20 V
1.0 A
0.66 A
30 V
1.5 A
1.0 A
40 V
2.0 A
1.33 A
A
V
V = R • I
I =RV
Resistance R
Relay
Solenoid coil
Ohm’s law
Graph
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© Festo Didactic GmbH & Co. TP 601, 06Transparency
Ohm’s law
+R
-
A
2.0
1.5
1.0
0.5
0100 20
V
I
30 40V
Resistance R 10 V
Voltage V
20 V 30 V 40 V
A
V
Relay
Solenoid coil
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7
Rectifiers
Rectifiers convert AC to DC.
Diodes are conductive in only one direction.
The polarity of the negative half-wave of the alternating voltage is reversed.
The pulsating voltage is smoothed using a capacitor and filters (reactance coil,
resistor).
~
=
V V
t
V
tt
C R
L
Diodes
Voltage
retification
Voltage
smoothing
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© Festo Didactic GmbH & Co. TP 601, 07Transparency
Rectifiers
~
=
V V
t
V
tt
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8
Analogue, digital and binary
Analogue values describe continuous functions, such as the continuous filling of a
bucket of water.
Digital values describe integer parts of a maximum value. A dice, for example, shows
one of six possible numbers.
Binary values are a special form of digital values. They can assume only two values,
such as in the case of a switch, which can only be on or off.
On 1
V a l u e
+
1
6
5
4
3
2
0 V o l u m e V
Filling time t Experiments with dice
L i g h t
Time tSwitchingtime point
Off 0
full
half-full
empty
Analogue values
Digital values
Binary values
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© Festo Didactic GmbH & Co. TP 601, 08Transparency
Analogue, digital and binary
+
-
Experiments with dice Time t
V o
l u m e
V
V a
l u e
L i g h t
Filling time t
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9
Momentary and sustained contact switches
Actuation of contacts.
A momentary contact switch has a “rest position” and an “actuated position”.
A sustained contact switch has two detents. It thus has two rest positions.
Momentary and sustained switches can close or open current paths or switch from
one current path to another.
3 3
4 4
Momentary contact switch
Sustained contact switch
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© Festo Didactic GmbH & Co. TP 601, Transparency 09
Momentary and sustained contact switches
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10
N/O, N/C and changeover contacts
Contact normally open in rest position.
Example: Momentary-contact switch
N/O and N/C contacts in a single housing; one contact is open while the other is
closed.
The designations of the switch contacts are governed by the European standard
EN 50012.
Connection(N/C contact)
42
1
3 1
4 2
Connection(N/O contact)
Switch elements
N/O
N/C
Changeover contact
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© Festo Didactic GmbH & Co. TP 601, 10Transparency
N/O, N/C and changeover contacts
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11
Limit switches
Determining the position of a positioning slide.
Electrical contacts are actuated when a defined intermediate or end position is
reached.
Limit switches can be connected up as N/O, N/C or changeover contacts.
Switch leverStem (insulated)
Contact
Snap-actionmechanism
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© Festo Didactic GmbH & Co. TP 601, 11Transparency
Limit switches
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12
Proximity switches
Signal is generated by the presence of any material with good conductivity in an
oscillating magnetic field.
Signal is also generated by all insulators with high dielectric constants in an
electrical field.
Signal is generated when light barriers are interrupted or when light is reflected
back to an optical sensor.
Signal is generated by magnets whose fields close the built-in contacts in the switch.
Magnet
Metal
MetalorInsulator
reflectivecompo-nent
inductive
capacitive
optical magnetic
Reed switch
Inductive
Capacitive
Optical
Reed switches
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© Festo Didactic GmbH & Co. TP 601, 12Transparency
Proximity switches
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13
Solenoid (electromagnet)
A magnetic field with concentric field lines is created around a conductor carrying an
electrical current.
A coil carrying an electrical current develops a uniform magnetic field in its axial
direction due to the overlapping of field lines.
By appropriate alignment of elementary magnets, an iron core can be made into a
switchable electromagnet.
+
Field around
a conductor
Coil
Coil
Iron core
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© Festo Didactic GmbH & Co. TP 601, 13Transparency
Solenoid (electromagnet)
+
-
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14
Electrohydraulic converters (solenoid valves)
Convert electrical signals into hydraulic switching operations.
The switching armature is drawn into the coil, which acts as a magnet.
This produces a displacement of the actuating piston in the hydraulic 4/2-way valve.
A B
P T
Switching armature
4/2-way solenoid valve
BT A P L
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© Festo Didactic GmbH & Co. TP 601, 14Transparency
Electrohydraulic converters (solenoid valves)
A B
P T
15
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15
Dry solenoids – wet solenoids
The solenoid is separated from the hydraulic fluid; a seal is required on the stem.
Simple construction
The armature chamber of the solenoid is filled with hydraulic fluid, the solenoid
operates in oil.
Low friction
Little abrasion
Good heat dissipation Long service life
seal
Dry solenoid Wet solenoid
Dry solenoid
Advantage
Wet solenoid
Advantages
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© Festo Didactic GmbH & Co. TP 601, 15Transparency
Dry solenoid Wet solenoid
16
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16
Solenoid valve connections
Solenoid with plug base
Cable with connector socket
Electrical signals are transmitted via standard connectors.
Reliable alignment of connections
Connections can be attached and detached quickly
Protected against dust and water
2
1
3
Protective earth connection
Plug base
Electricalconnections
Connector components
Advantages
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© Festo Didactic GmbH & Co. TP 601, 16Transparency
Solenoid valve connections
2
1
3
17
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17
Relays and contactors
Switches which are actuated and held in their actuated positions
electromagnetically.
Used to separate control and power circuits.
Hinged-armature design
Single-break contacts
Used for logic gating of signals
(generally) changeover contacts
Each main contact is of double-break type (contacts used as main contacts),suitable for
all power ratings from small to very large
only N/O and N/C contacts
+
+
Contacts
Return spring
Return spring
Contacts Coil
Coil
Iron core
Armature
Hinged-armature solenoid Plunger-armature solenoid
Definition
Special feature
of relays
Special featureof contactors
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© Festo Didactic GmbH & Co. TP 601, 17Transparency
Relays and contactors
+
+-
-
18
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8
Direct activation – indirect activation
Connection of a solenoid valve via a switch (Fig. A).
Connection of a solenoid valve via a relay (Fig. B).
Separation of control and power circuits
The current passing through switch S1 (Fig. B) is lower, and the service life of the
switch is significantly greater.
A B
24 V
0 V
1 224 V
0 V
S1 S1 K1
Y1 K1 Y1
1
3 3 3
4 4 4
Direct activation
Indirect activation
Advantage of
indirect activation
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24 V
0 V
1 2
© Festo Didactic GmbH & Co. TP 601, 18Transparency
Indirect activationDirect activation
24 V
0 V
1
19
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Signal inversion
The output signal is inversely proportional to the input signal.
Signal inversion at relay K1.
Signal inversion at switch S1.
The switching elements are shown under zero-voltage conditions.
24 V
0 V
1 2
S1 K1
0
10
1
Y1
0
1
B
S1 K1
K1 Y1
31
42
24 V
0 V
1 2
S1 K1
1
00
1
Y1
0
1
A
S1 K1
K1 Y1
3 1
4 2
Figure A
Figure B
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24 V
0 V
1 2
S1 K1
0
1
Y1
B
S1 K1
K1 Y1
31
42
24 V
0 V
1 2
© Festo Didactic GmbH & Co. TP 601, 19Transparency
Signal inversion
S1 K1
0
1
Y1
A
S1 K1
K1 Y1
3 1
4 2
20
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Latching
Latching only with indirect activation.
Actuation of switch S1: Relay K1 reverses holding the current path of K1 closed.
The actuation of switch S2 interrupts the latched current path.
K1 K13 3
4 4
24 V
0 V
1 2 3
S1
K1 Y1
3
4
S21
2
S1 S2
0 0
K1 & Y1
0
1
0
1
0 0
1 1
1
dominant on
1
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© Festo Didactic GmbH & Co. TP 601, 20Transparency
Latching
K1 K13 3
4 4
24 V
0 V
1 2 3
S1
K1 Y1
3
4
S21
2
S1 S2
0
K1 & Y1
0
1
0
1
0
1
1
21
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Circuit diagram
Lines of potential with constant voltage are drawn horizontally.
Current paths are drawn vertically and numbered consecutively.
Circuit symbols are shown in the direction of current flow.
Switching elements are generally shown under zero-voltage conditions.
Latching to guard against sticking of relay contacts in current paths 6 and 7.
K2
3
4S2
K2
3
4
A2
A1
K1
3
4S1
S0
K1 Y1 Y2
3
3
4
4
K2 K11 1
2 2
A2
A1
K1 K2
3 3
4 4
24 V
0 V
1
2 3 4 5 6 7
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© Festo Didactic GmbH & Co. TP 601, 21Transparency
Circuit diagram
K2 3
4
S2
K2
3
4
A2
A1
K1 3
4
S1
S0
K1 Y1 Y2
3
3
4
4
K2 K11 1
2 2
A2
A1
K1 K23 3
4 4
22
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Hydraulic power pack
Hydraulic power pack for use in stationary hydraulic systems.
Power is delivered via the pressure medium using a pump and drive motor.
The pack incorporates devices to remove air, water and solid particles from the
hydraulic fluid.
Baffle plate
Pressure relief valve(safety valve)
Hydraulic pump
Suction pipe
Suction filter
Return line
Air filter
Filler filter
Fluid level indicator
Cleaning aperture
Drain screw
Pressure lineElectric motor n = 1500 rpm
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© Festo Didactic GmbH & Co. TP 601, 22Transparency
Hydraulic power pack
23
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Hydraulic circuit
Hydraulic systems consist of open or closed circuits.
Hydraulic fluid is used as an energy carrier.
This energy is output via cylinders and motors.
Relieved hydraulic fluid is returned to the tank or hydraulic pump.
Hydraulicmotor
Motor
Filter
Motor
Hydraulic cylinder
Electrohydraulicpower valve
Safetyvalve
Safetyvalve
Hydraulicpump
Returnline
Safety valve
Feed pump(top-up forleakage losses)
Hydraulic pump
open closed
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© Festo Didactic GmbH & Co. TP 601, 23Transparency
Hydraulic circuit
open closed
24
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Electrical power – hydraulic power
Driven by electric current on the basis of a potential difference.
Electrical power is the product of voltage and current
Pelectr. = V • I (unit: watts).
Driven via fluid flow on the basis of a pressure difference.
Hydraulic power is the product of pressure and flow rate
Phydr. = p • Q (unit: watts).
Electric motor
+
P = • Ielectr. P = p • qhydr.
Current I
V o l t a g e
V
P r e s s u r e
p
Flowrate q
Gear motor
Electric motor
Hydraulic motor
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© Festo Didactic GmbH & Co. TP 601, 24Transparency
Electrical power Hydraulic power
+
-
25
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Power losses
Frictional losses
Magnetic losses
Frictional losses Volumetric losses
Pressure losses dp
Power losses expressed by P = dp • Q
Frictional losses
Volumetric losses
P = p • qhydr. loss
Frictional andvolumetric losses
Frictional andpressure losses
Frictional,volumetric andmagnetic losses
P o w e r l o s s e s
Efficiency of electric motors
Efficiency of hydraulic pump
Valves, piping, tubing
Hydraulic motors
and cylinders
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© Festo Didactic GmbH & Co. TP 601, 25Transparency
Power losses
26
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Pressure switches
Actuation of electrical contacts when a defined minimum pressure is reached.
The minimum pressure (switching point) can be varied by means of an adjustable
spring.
Example shows switching at approx. 4 bar with rising or falling pressure.
Generally connected up in practice as a changeover contact.
Hysteresis2
3
1
1
2 3
4 bar
pp
a
a/c
b
c
b
Switching point with rising pressure
Switching point with falling pressure
Symbolic representationin electrical circuit diagram
Symbolic representationin hydraulic circuit diagram
Pressure
p
Time t
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© Festo Didactic GmbH & Co. TP 601, 26Transparency
Pressure switches
2
31
1
2 3
Pressurep
Time t
4 bar
pp
27
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4/3-way solenoid valve
Valve spool is directly actuated by solenoids.
The piston spool is displaced to the right and creates the connections P – A
and B – T.
Rest position produced by spring centring of piston spool. All inputs of the 4/3-way
valve are closed in this position.
T A P B
T A P B
A
a b
B
P T
a
a
b
b
Actuation on left
Rest position
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© Festo Didactic GmbH & Co. TP 601, 27Transparency
4/3-way solenoid valve
A
a b
B
P T
T A P B
T A P B
28
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4/3-way solenoid valve, piloted
Direct activation of pilot spool via solenoids.
The pilot piston spool is displaced to the right, and hydraulic fluid is directed to the
right-hand side of the main control spool, creating the connections P – A and B – T.
Rest position produced by spring centring of pilot and main piston spools. All ports
of the 4/3-way valve are closed at the main valve.
Simplified symbol
P T
A
A
B
B
P T
PB AT
Detailed symbol
Actuation on left
Rest position
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P T
© Festo Didactic GmbH & Co. TP 601, 28Transparency
4/3-way solenoid valve, piloted
A
A
B
B
P T
PB AT
29
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Electrical safety
EMERGENCY STOP button placed in an easily accessible position
Protection against voltages
Protection by means of insulation
Protection by means of earthed conductive housings Safety isolation by means of transformers
L1
24 V
PE
N
1) Switch off 2) Check
3) Carry out workEMERGENCY
STOP
Basic insulation
Additionalinsulation
220 V AC
Safety insulation Protective earth Safety isolation
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© Festo Didactic GmbH & Co. TP 601, 29Transparency
Electrical safety
Safety insulation Protective earth Safety isolation
30
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Hydraulic safety
Safety regulations must be observed.
Switch off motor/pump
Depressurise accumulator
Lower load
Check pressure
Work can now be carried out by authorised trained personnel.
m
1) Switch offmotor/pump
2) Depressurise accumulator
4) Check pressure
3) Lower load
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© Festo Didactic GmbH & Co. TP 601, 30Transparency
Hydraulic safety
m