Chapter 2 Pneumatic

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CHAPTER 2.0

PNEUMATICS

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2.1 Learning Outcomes

At the end of this course, the students would be able to apply and appreciate

the knowledge to:

(1) Perform the safety and regulations apply in engineering workshop

and/or laboratory.

(2) Describe briefly about pneumatic control system.

(3) Design the pneumatic circuit diagram and elements.

(4) Correctly install components in the pneumatic, electro pneumatic,

hydraulic and control system according to the given circuit diagrams.(5) Test runs the assembled system components base on the given circuit

diagrams.

2.2 Pneumatics Theory

A fluid power system is one that transmits and control energy through the

use of pressurized liquid or gas. In Pneumatics, this power is air. This of

course from the atmosphere and is reduced in volume by compression, thus

increasing its pressure. Compressed air mainly used to do work by acting on

a piston or vane. While this energy can be used in many facets of industry,

the field of industrial pneumatics is considered here. The correct use of pneumatic control requires an adequate knowledge of pneumatic

components and their function to ensure their integration into an efficient

working system. Although electronic control using a programmable

sequencer or other logic controller is currently specified, it is still necessary

to know the function of the pneumatic components in this type of system.

2.2.1 What Can Pneumatics do?

Operation of heavy or hot doors.

Unloading of hoppers in building, steel making, mining and chemical

industries.

Forming operations of bending, drawing and flattening.

Bottling and filling machines.

Pneumatic robots.

Component and material conveyor transfer.

Air separation and vacuum lifting of thin sheets.

Figure 2.1 show the application of pneumatic component at chip bonding

process which consists of a vacuum unit.

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Figure 2.1: Chip Bonding.

2.2.2 The Advantages and Limitations of Pneumatic

It has several operation modes; can be fully automated, manuallyoperated and semi automated mode.

It is clean, suitable for several processes which are very sensitive,

especially for hazardous chemicals and electronic components.

The equipments are very cheap compared to other methods.

The accuracy of the end results is moderate.

Reduce production time and cost mass production.

There is no other waste produced from the operating system except air.

Compress is relatively insensitive to temperature fluctuation. This

ensures reliable operation, even under extreme condition (depend on

protection material).

However, pneumatic technology also has their own limitations. Listed

below are the limitations of pneumatic technology:

Only clean air with lowest humidity is allowed to be in pneumatic

system. So, compress air need good preparation to filter any kind of

dirt and condensate.

It is seldom to get uniform and constant piston speed with

compressed air.

It has the limitation in producing the output force. It is suitable fornon-heavy duty purpose.

The exhaust air is very noisy.

Compressed air is very expensive in terms of conveying the power

but the high price is remunerated by the cheap pneumatic

equipments, fast and efficient production.

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2.2.3 The Basic of Pneumatic System

Pneumatic cylinders, rotary actuators and air motors provide the force and

movement of most pneumatic control systems to hold, move, form and

process material. To operate and control these actuators, other pneumaticcomponents are required i.e. air service units to prepare the compressed air

and valves to control the pressure, flow and direction of movement of the

actuators.

A basic pneumatic system, shown in Fig 2.2, consists of two main sections:

Figure 2.2: Basic Pneumatic System

The Air Production and Distribution System

(1) Compressor: Air taken in at atmospheric pressure is compressed and

delivered at higher pressure to the pneumatic system. It thus transforms

mechanical energy into pneumatic energy.

(2) Electric Motor: Transforms electrical energy into mechanical energy

(3) Pressure Switch: Controls the electric motor by sensing the pressure

in the tank.

(4) Check valve: Lets the compressed air from the compressor into the

tank. It is set to a maximum pressure at which it stops the motor and a

minimum pressure at which it restarts it.

(5) Tank: Stores the compressed air.(6) Pressure Gauge: Indicates the Tank Pressure.

(7) Auto Drain: Drains all the water condensing in the tank without

supervision.

(8) Safety Valve: Blows compressed air off if the pressure in the tank

should rise above the allowed pressure.

(9) Refrigerated Air Dryer: Cools the compressed air to a few degrees

above freezing point and condenses most of the air humidity.

(10) Line Filter: It helps to keep the line free from dust, water and oil.

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The Air Consuming System

(1) Air Take Off.

(2) Auto Drain.

(3) Air Service Unit.

(4) Directional Valve: Alternatively pressurizes and exhaust the cylinderconnections to control the direction of movement.

(5) Actuator.

(6) Speed Controllers: Allows easy speed adjustment of the actuator

movement.

2.2.4 Components and Symbol of Pneumatic System

The primary levels in a pneumatic system are:

Energy supply/Air generation and distribution.

Input elements (sensors).

Processing elements (processors).

Actuating devices (actuators)

The elements in the system are represented by symbols which indicate the

function of the element. The symbols can be combined to represent a

solution for a particular control task using the circuit diagram. The circuit is

drawn with the same structure as the signal flow diagram above. At the

actuator level the addition of the control element completes the structure.

The control element controls the action of the actuator after receiving signals

sent by the processor elements.

Figure 2.3: Pneumatic elements

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The directional control valve (DCV) may have a sensing, a processing or

an actuating control function. It the DCV is used to control a cylinder

motion, then it is a control element for the actuator group. If it is used in

the function of processing signals, then it is defined as a processor

element. If it is used to sense motions, then it is defined as a sensor. The

distinguishing feature between each of these roles is normally the method

of operating the valve and where the valve is situated in the circuit

diagram.

The development of pneumatic systems is assisted by a uniform approach

to the representation of the elements and the circuits. The symbols used for

the individual elements must display the following characteristics:

Function

Actuation and return actuation methods

Number of connections (all labeled for identification)

Number of switching positions

General operating principle

Simplified representation of the flow path

A symbol does not represent the following characteristics:

Size or dimensions of the component

Particular manufacturer, methods of construction or costs

Orientation of the ports

Any physical details of the element

Any unions or connections other than junctions

The symbols used in pneumatics are detailed in the standard DIN ISO

1219, "Circuit symbols for fluidic equipment and systems.

(a) Air generation and distribution

The air supply for a particular pneumatic application should be sufficient

and of adequate quality.

Air is compressed to approximately 1/7th of its volume by the air

compressor and delivered to an air distribution system in the factory. Toensure the quality of the air is acceptable, air service equipment is utilized

to prepare the air before being applied to the control system.

Malfunctions can be considerably reduced in the system if the compressed

air Is Correctly prepared. A number of aspects must be considered in the

preparation of the service air:

Quantity of air required to meet the demands of the system

Type of compressor to be used to produce the quantity required

Storage required

Requirements for air cleanliness

Acceptable humidity levels to reduce corrosion and sticky operation

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Lubrication requirements, if necessary

Low temperature of the air and effects on the system

Pressure requirements

Line sizes and valve sizes to meet demand

Material selection and system requirements for the environment Drainage points and exhaust outlets in the distribution system

Layout of the distribution system to meet demand.

As a rule pneumatic component is designed for a maximum operating

pressure of 8- 10 bar (800-7000 kPa) but in practice it is recommended to

operate at between 5 and 6 bar (500-600 kPa) for economic use. Due to the

pressure losses in the distribution system the compressor should deliver

between 6.5 and 7 bar (650-700 kPa) to attain these figures.

An air receiver should be fitted to reduce pressure fluctuations. In normaloperation the compressor fills the receiver when required and the receiver

is available as a reserve at all times. This reduces the switching cycles of

the compressor.

Figure 2.4: Air supply system

The symbols for the energy supply system can be represented as individual

elements or as combined elements. The choice between using simplified or

detailed symbols is dependent upon the purpose of the circuit and its com-

plexity. In general where specific technical details are to be given such as

requirements for non-lubricated air or micro-filtering, then the completedetailed symbol should be used. If a standard and common air supply is

used for all components, then the simplified symbols can be used.

Due to the high demand at certain stages of the air distribution system, a

ring main with cross-feed connections is recommended. In this way the

fluctuations are reduced. The ring main should be laid out with a 1-2%

gradient to allow drainage points for condensate from the compressor. If

there is a relatively high condensate level, then air drying equipment

should be fitted specifically to dry the air to the required quality.

Condensate is a common cause of failure in pneumatic controls.

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The air service unit is a combination of the following

Compressed air filter, Compressed air regulator and Compressed air

lubricator.

The correct combination, size and type of these elements are determined

by the application and the control system demand. An air service unit is

fitted at each control system in the network to ensure the quality of air for

each individual task.

ITEM SYMBOL MEANING

SUPPLY

Compressor

Pressure Source

Pneumatic Pressure

Source

Air Reservoir

SERVICE

EQUIPMENT

Filter : separation and

filtration of particles

Filter and separator(automatic)

Lubricator

Pressure regulator

COMBINEDSYMBOLS

Air Service Unit : Filter,Regulator, Gauge,

Lubricator

Simplified air service

unit with lubricator

Figure 2.5: Symbols use in energy conversion and preparation

The compressed air filterhas the job of removing all contaminants from

the compressed air flowing through it as well as water which has alreadycondensed. The compressed air enters the filter bowl through guide slots.

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Liquid particles and larger particles of dirt are separated centrifugally

collecting in the lower part of the filter bowl. The collected condensate

must be drained before the level exceeds the maximum condensate mark,

as it will otherwise be re-entrained in the air stream.

The purpose of the regulatoris to keep the operating pressure (secondary

pressure) virtually constant regardless of fluctuations in the line pressure

(primary pressure) and the air consumption.

The purpose of the lubricator is to deliver a metered quantity of oil mist

into a leg of the air distribution system when necessary for the use by

pneumatic control and working components.

(b) Input Elements

Valves can be divided into a number of groups according to their functionin relation to signal type, actuation method and construction. The primary

function of the valve is to alter, generate or cancel signals for the purpose

of sensing, processing and controlling. Additionally the valve is used as a

power valve for the supply of working air to the actuator. Therefore the

following categories are relevant:

Directional control valves: Signaling elements, Processing elements,

Power elements

Non-return valves

Flow control valves

Pressure control valves

Combinational valves

b.1) Directional control valves

The directional control valve controls the passage of air signals by

generating, canceling or redirecting signals. In the field of control

technology, the size and construction of the valve is of less importance

than the signal generation and the actuation method. Directional control

valves can be of the poppet or slide type, with the poppet utilized for small

flow rates and for the generation of input and process signals. The slide

valve is able to carry larger flow rates and hence lends itself to the powerand actuator control role.

The way valve is described by:

Number of ports or openings (ways): 2 way, 3 way, 4 way, 5 way, etc.

Number of positions: 2 positions, 3 positions, etc.

Methods of actuation of the valve: Manual, air pilot, solenoid, etc.

Methods of return actuation: Spring return, air return, etc.

Special features of operation: Manual overrides, etc.

Explanation Of Symbol Symbol Development

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Valve switching positions are represented as

squares

The number of squares shows how many

switching positions the valve has

Line indicate flow paths, arrows shows thedirection of flow

Shut off positions are identified in the boxes

by lines drawn at right angles

The connections ( inlet and outlet ports ) are

shown by lines on the outside of the box

Figure 2.6: Directional Control Valve Symbol Development

The directional control valve is represented by the number of controlled

connections, the number of positions and the flow path. In order to avoid

faulty connections, all the inputs and outputs of a valve are identified.

A numbering system is used to designate directional control valves and is

in accordance with ISO 5599 (Draft). Prior to this a lettering system was

utilized and both systems of designation are presented here:

Port or Connection

ISO

5599 Lettering System

Pressure port 1 PExhaust port 3 R (3/2 way valve)

Exhaust ports 5,3 R,S (3/2 way valve)

Signal outputs 2,4 B,A

Pilot line opens flow 1 to 2 12 Z (single pilot 3/2 way)

Pilot line opens flow 1 to 2 12 Y (5/2 way valve)

Pilot line opens flow 1 to 4 14 Z (5/2 way valve)

Pilot line flow closed 10 Z,Y

Auxiliary pilot air 81,91 Pz

SYMBOL EXPLANATION

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2/2 - way directional control valve,

normally open

3/2 way directional control valve

normally closed

3/2 way directional control valve,

normally open


Flow from 1 to 2 and from 4 to 3


Flow from 1 to 2 and 4 to 5


Mid position closed

Figure 2.7: Directional Control Valve, Ports and Positions (ways)

The methods of actuation of pneumatic directional control valves are

dependent upon the requirements of the task. The symbols for the methods

of actuation are detailed in ISO 1219. The types of actuation may vary e.g.

manually actuated, mechanically actuated, pneumatically actuated,electrical, combined actuation.

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Figure 2.8: Methods of Actuation

For example for manual actuation operated is generally obtained by

attaching an operator head, suitable for manual control, onto a mechanical

operated valve as indicated in Figure 2.9.

Figure 2.9: Manually of Actuation

b.2) Flow Control Valve

The flow control valve restricts or throttles the air in a particular direction

to reduce the flow rate of the air and hence control the signal flow. If the

flow control valve is left wide open then the flow should be almost the

same as if the restrictor is not fitted. In some cases it is possible to

infinitely vary the restrictor from fully open to completely close. If the

flow control valve is fitted with a non-return valve then the function of

flow-control is unidirectional with full free flow in one direction. A two

way restrictor restricts the air in both directions of flow and is not fitted

with the non-return valve. The flow control valve should be fitted as close

to the working element as is possible and must be adjusted to match the

requirements of the application.

SYMBOL EXPLANATION

Flow control valve,

adjustable

One-way flow control

valve

Figure 2.10: Flow Control Valves

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b.3) Pressure control valves

Pressure control valves are utilized in pneumatic systems. There are three

main groups:

Pressure regulating valves without relief port

Pressure limiting valves with relief port

Pressure sequence valves

The pressure regulating valve controls the operating pressure in a control

circuit and keeps the pressure constant irrespective of any pressure

fluctuations in the system.

The pressure limiting valves are utilized on the up-stream side of the

compressor to ensure the receiver pressure is limited, for safety, and that

the supply pressure to the system is set to the correct pressure.

The sequence valve senses the pressure of any external line and compares

the pressure of the line against a preset adjustable value, creating a signal

when the preset limit is reached.

SYMBOL EXPLANATION

Sequence valve-in line

Adjustable pressure

regulating valve, relieving

type

Figure 2.11: Pressure Valve

b.4) Auxiliary Symbols

There are a number of important symbols for accessories which are

utilized in conjunction with pneumatics. These include the exhaust air

symbols, visual indicators and the methods of connection of components.

SYMBOL EXPLANATION

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a

Pressure gauge

Silencer

Exhaust

Plug

0 Bar

Visual indicator

Figure 2.12: Auxiliary Symbol

c) Processing Elements

As a processing element the directional control valve redirects, generates

or cancels signals depending on the signal inputs received. The processing

element can be supplemented with additional elements, such as the AND-

function and OR-function valves to create the desired control conditions.

The non-return valve allows a signal to flow through the device in one

direction Non-return valves and in the other direction blocks the flow.

There are many variations in construction and size derived from the basic

non-return valve. Other derived valves utilize features of the non-return

valve by the incorporation of non-return elements. The non-return valve

can be found as an element of the one way flow control valve, quick

exhaust valve. Shuttle valve and the two-pressure valve.

SYMBOL EXPLANATION

Check valve

Shuttle valve

AND Valve

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Quick exhaust valve

Figure 2.13: Non-return valves and derivatives

(d) Actuator (Output)

As a power element the directional control valve must deliver the required

quantity of air to match the actuator requirements and hence there is a need

for larger volume flow rates and therefore larger sizes. This may result in a

larger supply port or manifold being used to deliver the air to the actuator.

The actuator group includes various types of linear and rotary actuators of.

The actuators are complemented by the final control element, whichtransfers the required quantity of air to drive the actuator. Normally this

valve will be directly connected to the main air supply and fitted close to

the actuator to minimize losses due to resistance.

Actuators can be further broken down into groups:

Linear actuators: Single acting cylinder, Double acting cylinder.

Rotary actuators: Vane type, Air motors.

SYMBOL EXPLANATION

Single acting cylinder with spring

return

Double acting cylinder

Double acting cylinder with double

ended piston rod

Double acting 2 cushion cylinder

Air motor rotation in one direction

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Rotary actuator

Figure 2.14: Actuators

2.2.5 Development of single actuator circuit

The simplest level of control for the single or double acting cylinder involves

direct control signals. With this, the cylinder is actuated directly via a

manually or mechanically actuated valve, without any intermediate switching

of additional directional control valves. If the port sizes and the flow values of

the valve are too large, the operating forces required may be too great for

direct manual operation.

Example 1:Direct control of a single-acting cylinder

A single acting cylinder of 25mm diameter is to advance a component

when a push button is pressed. As long as the push button is activated the

cylinder is to remain in the clamped position. If the push button released,

the clamp is to retract.

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Positional Sketch

Circuit Diagram

Example 2:Indirect control of a double acting cylinder

A double acting cylinder is to extend when a push button is operated. Upon

release of the push button the cylinder is to retract.

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Circuit Diagram

Example 3:The logic AND function

The piston rod of a double acting cylinder is to advance when both push

button of the 3/2 way valve is actuated. If either of these is released, then

the cylinder is to return to the initial position.

Circuit Diagram

Example 4:The logic OR function

A double-acting cylinder is to advance if one of two push buttons is

operated. If the push button is then released, the cylinder is to retract.

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Circuit Diagram

Example 5:Memory circuit and speed control of a cylinder

The piston rod of a double acting cylinder is to advance when a 3/2 way

push button valve is actuated manually. The cylinder is to remain advanced

until a second valve is actuated. The signal of the second valve can only

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take effect after the first valve has been released . The cylinder is to then

return to the initial position. Then cylinder is to remain in the initial

position until a new start signal is given. The speed of the cylinder is to be

adjustable in both directions.

Circuit Diagram

2.2.6 Combinational valves

The combined functions of various elements can produce a new function. The

new component can be constructed by the combination of individual elements

or manufactured in a combined configuration to reduce size and complexity.

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An example is the timer which is the combination of a one way flow control

valve, a reservoir and a 3/2 way directional control valve.

Figure 2.15: Time delay valve

Example 1:The timer delay valve

A double-acting cylinder is used to press together glued component. Uponoperation of a push button, the clamping cylinder extends. Once the fully

advanced position is reached, the cylinder is to remain a time of T= 6 seconds and

then immediately retract to the initial position. The cylinder retraction is to be

adjustable. A new start cycle is only possible after the cylinder has fully retracted.

Circuit Diagram

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2.2.7 Development of multiple actuators circuits

In case of multiple cylinder circuits, a clear definition of the problem isimportant. The representation of the desired motion of all actuators described

using the displacement-step diagram. The special condition for the start of the

sequence must also be defined.

Example 1:Coordinated motion

Two cylinders are used to transfer parts from a magazine onto a chute. When a

push button is pressed, the first cylinder extends, pushing part from the

magazine and positions it in preparation for transfer by the second cylinder onto

the out feed chute. Once the part is transfer, the first cylinder retracts, followed

by the second. Confirmation of all extended and retracted positions are required.

Displacement Step Diagram

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Circuit Diagram

2.3 Equipments

The important elements of pneumatic equipment:

Laboratory trolley or fixed workbench with drawers.

Compressor

Set of devices or individual components (e.g. cylinders, directional control

valves, logic elements, linear drive, pneumatic switch)

Table 1.0 shown the list of equipment which will be used for assembly at thepneumatic trainer.

Table 1.0: List of Equipments

Example EnergySignal

Input

Processing

Elements

Control

ElementsActuators

2.2.5 Exp. 1

Compressed air

production /

Distribution

Switch /

Push button

etc.

-3/2way

DCV

Single acting

cylinder

2.2.5 Exp. 2

Compressed air

production /

Distribution

Switch /

Push button

etc.

- 5/2wayDCV

Double actingcylinder

2.2.5 Exp. 3

Compressed air

production /

Distribution

Switch /

Push button

Limit switch

Dual

Pressure

valve

(AND)

5/2way

DCV

Double acting

cylinder

2.2.5 Exp. 4

Compressed air

production /

Distribution

Switch /

Push buttonShuttle

valve(OR)

5/2way

DCV

Double acting

cylinder

2.2.5 Exp. 5 Compressed air

production /Distribution

Switch /

Push button

- 5/2way

DCVFlow

Double acting

cylinder

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control

valve

2.2.6 Exp. 1

Compressed air

production /Distribution

Switch /

Push button

Limit switchTime delay

valve

Dual

Pressure

valve(AND)

5/2way

DCV

Flowcontrol

valve

Double acting

cylinder

2.2.6 Exp. 1

Compressed air

production /

Distribution

Switch /

Push button

Limit switch

-5/2way

DCV

Double acting

cylinder

2.4 Working Procedure

All components are to be fully plugged in the assembly board

Limit switches should be placed so that they contact only the side of the

trip cam never at the front.

Permissible operating pressure should not be exceeded

Pneumatics circuits are to be constructed with plastic tubes. Fully insert

the plastic tubes into the plug-in coupling or quick push connectors and

pull the locking ring over the tube connection

Before the circuit is disconnected turn off the pressure supply In order to avoid damage to the locating pegs carefully unplug the

components from the assembly board by pulling them upwards.

2.5 Tasks

a) A pneumatic press is to be operated by two push buttons PB1 and PB2. In

order to meet safety requirements, both of the push buttons must be pressed

together to start the press. The pneumatic press is retracted immediately when

one or both push buttons are released. Draw the circuit diagram for the

problem. Designate the valves and indicate the numbering system for the

connections.

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a.1) a single acting cylinder is to be used by 3/2way pilot valve.

a.2) a single acting cylinder is to be used and connected in series.

a.3) a double acting cylinder is to be used with used and gate.

a.4) a double acting cylinder is to be used by 3/2way pilot valve.

b) Operation of two identical valves by two push buttons causes the forming tool

of an edge folding device to thrust downwards and fold over the edge of a flat

sheet of sectional area. If both or even just one push button is released, double

acting cylinder slowly returns to the initial position.

b.1) a double pilot valve should be fitted for the control of the cylinder. If

the cylinder is to retract on reaching its fully extended position, rollerlever valves should be used as limit valves to confirm that this

position has been reached.

b.2) One of the push buttons need only be operated for a short duration

and the cylinder will fully extend, since the effect of the signal at

input at the 5/2way double pilot valve is maintained until a signal is

applied. As soon as the piston rod has reached the forward end

position, the limit switch generated a signal to 5/2way double pilot

valve and the valve is reversed. The retracted end position of the

piston rod can also be sense. These require an additional limit switch.

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c) A double acting cylinder is used to press together glued components. Upon

operation of a push button, the clamping cylinder slowly advances. Once the

fully extended position is reached, the cylinder is to remain for a time of 6 sec.

and then immediately retract to the initial position. A new start cycle is only

possible after the cylinder has fully retracted and after a delay of 5 sec. During

the delay the finished part is manually removed and replaced with new parts

for gluing. The retracting speed is to be rapid, but adjustable. Draw the

displacement step diagram and circuit diagram for the problem. Designate the

valves and indicate the numbering system for the connections.

d) Turned parts for spark plugs are fed in pairs on a rail to multi spindle

machining stations. In order to achieve separation, two double acting cylinders

are triggered by one actuator in alternating push-pull rhythm. In the initial

position, the upper cylinder is retracted, the lower cylinder in the forward

position. Turned parts are resting against the second cylinder.

A starting signal causes cylinder A to advance and cylinder B to retract. Two

sparking plug blanks roll onto the machining station. After and adjustable time

of 1 sec, cylinder A returns and cylinder B advances at the same time. A

further cycle can be started only when time interval 2 sec. has elapsed. Draw

the displacement step diagram and circuit diagram for the problem. Designate

the valves and indicate the numbering system for the connections.

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Chapter 2 Pneumatic

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Transcript of Chapter 2 Pneumatic