Smc Pneumatics User Manual

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P N E U M A T I C T E C H N O L O G Y

Notes

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Notes

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Notes

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1 INTRODUCTION 1

What can Pneumatics do? 1 Properties of Compressed Air 2

2 THE BASIC PNEUMATIC SYSTEM 3

The Air Production and distribution System 3 The Air Consumption System 4

3 COMPRESSED AIR THEORY 6

Units 6

Pressure 9

Properties of Gases 10

Isothermic change (Boyle’s Law) 10 Isobaric change 11

Charles Law 11

Law of Gay Lussac 11 Isochoric change 11 Adiabatic (Isentropic) change 12 Standard Volume 12 Flow 12

Bernoulli's Equation 12 Air Humidity 12

Relative humidity 14 Pressure and Flow 15

Use of the diagram: 16

Formulae: 17

Sonic Conductance 19

Indication of flow Characteristics 19 ISO 6358-1989, JIS B 8390-2000 19 Sonic conductance C 19 Critical Pressure Ratio B 19 Measuring sonic conductance C and critical pressure ratio b 20 Formulas to calculating air flow rates Q 21 The standard flow Qn 22 Flow section S [mm2]: 23 4 AIR COMPRESSION AND DISTRIBUTION 24

Compressors 24

Reciprocating Compressors 24

Single stage Piston Compressor 24

Two stage Piston Compressor 25

Diaphragm compressor 25 Rotary compressors 26

Rotary sliding vane compressor 26

Screw compressor 26 Compressor rating 27

Volumetric Efficiency 27

Thermal and Overall Efficiency 27

Compressor Accessories 27

Air receiver 27

Sizing a receiver 28 Inlet filter 28

Air Dehydration 29

Aftercoolers 29

Air cooled 29

Water cooled 29 Air dryers 30

Absorption (deliquescent) Drying 30

Adsorption (desiccant) Drying 31

Refrigerant drying 32 Membrane Air Dryer 33 Water Removal Filter 33 Main line filter 34

Air Distribution 34

Dead End Line 35 Ring Main 35 Secondary Lines 36

Automatic Drains 36

Sizing Compressed Air Mains 37

Materials for Piping 40

Standard Gas Pipe (SGP) 40 Stainless steel pipes 40 Copper Tube 40 Rubber Tube (“Air Hose”) 41 Plastic tubing 41 Fittings in Systems 42

5 AIR TREATMENT 43

Filtering 43

Standard Filter 43

Pressure Regulation 47

Standard Regulator 47 Pilot Operated Regulator 49 Filter-Regulator 50

Characteristics 50

Sizing of Regulators and Filters 50

Compressed Air Lubrication 51

Proportional Lubricators 51

F.R.L. Units 53

Size and Installation 53

6 ACTUATORS 54

Linear Cylinders 54

Single Acting Cylinder 54 Double Acting Cylinder 54 Cylinder Construction 55

Cushioning 55

Special Cylinder Options 56

Double Rod 56 Non Rotating Rod 56 Twin Rod 56 Guided Cylinder 57 Slide Table 58 Flat Cylinder 58 Tandem Cylinder 58 Multi Position Cylinder 59

Cylinder Mounting 60

Floating Joints--- Rod Couplers 60 Column Strength 61

Cylinder Sizing 61

Cylinder Force 61

Theoretical Force 61

Required Force 63 Load Ratio 64 Speed Control 67 Air Flow and Consumption 67

Rotary Actuators 72

Rack and Pinion Type 72

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Vane Type Rotary Actuators: 72 Sizing Rotary Actuators 72

Torque and Inertia 72

Special Actuators 75

Locking Cylinder 75 Rodless cylinders 75

With magnetic coupling, unguided 75 Guided types, with magnetic coupling 76 Guided, with mechanical coupling 76

Slide Units 77 Hollow Rod Cylinder 77 Linear Rotating Cylinder 78 Air Chuck (Gripper) 78

7 DIRECTIONAL CONTROL VALVES 79

Valve Functions 79

Symbol 79 Port Identification 80 Monostable and bi-stable 80

Valve Types 80

Poppet Valves 81 Sliding Valves 82

Spool Valves 82

Elastomer seal 82 Metal Seal 82 Plane Slide Valve 83 Rotary Valves 84

Valve Operation 84

Mechanical operation 84

Care when using Roller Levers 85

Manual Operation 85 Air Operation. 86

Piloted Operation. 87

Solenoid Operation 88 Direct Piping 89 Manifolds 89 Sub Bases 89 Multiple Sub Bases 90 Ganged Sub Bases 90

Valve Sizing 90

Indications for Flow capacity 90

Orifices in series connection 92

Flow capacity of tubes 93

Valves with Cylinders 94

Auxiliary Valves 97 Non-Return Valves 97

Speed Controllers 97 Shuttle Valve 97 Quick Exhaust Valves 98

8 BASIC CIRCUITS 99

Introduction 99 Elementary Functions 99

Flow Amplification 99 Signal Inversion 99 Selection 101 Memory Function 100

Time functions 101

Delayed switching on 102 Delayed switching off 102 Pulse on switching off 103 Pulse on releasing a valve 103

Cylinder Control 104

Manual Control 104

Single Acting Cylinder 104

Direct Operation and Speed Control 104 Interlock: AND Function 105 Inverse Operation: NOT Function 105

Double acting Cylinder 106

Direct Control 106

Holding the end positions 106 Detecting Cylinder Positions 107 Automatic Return 107

Repeating Strokes 109

Sequence Control 109

How to describe a sequence 109

Nomenclature 109

Sequence of two Cylinders 110

Single Cycle / Repeating Cycle 111

Opposing Commands 112

Elimination with a Pulse 112

Clamping: Pressure Control 112

Cascade System 113 Appendix 116

Symbols 116

Air Treatment Equipment 117 Actuators 118 Valves 118 JIS Valve Symbols 120

Circuits 121

Basic Rules 121 Rest Position 121 Circuit Layout 122 Nomenclature 123

Sample Diagrams 125 Fluid Power Formulas 126

Notes 134 Summary of Formulas & Equivalents 135 Notes 141 Afterword 146

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11 IINNTTRROODDUUCCTTIIOONN

A fluid power system is one that transmits and controls energy through the use of pressurized liquid or gas. In Pneumatics, this media is air. This of course comes from the atmosphere and is reduced in volume by

compression, thus increasing its pressure. Compressed air is mainly used to do work by acting on a piston or vane

--- producing some useful motion for instance. While many facets of industry use compressed air, the general field of Industrial Pneumatics will be 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. It is always the responsibility of the designer to certify safety in all conditions --- including a failed condition. As with any other energy source, compressed air can cause harm if not properly applied.

Although electronic control using a programmable sequencer or other logic controller may be currently

specified, it is still necessary to know the basic function of the pneumatic components.

This book deals with the technology of the components in control systems, describing types and design

features of air treatment equipment, actuators and valves, methods of interconnection and introduces basic pneumatic circuits.

WHAT CAN P NEUM ATI CS DO?

The applications for compressed air are limitless, from the optician’s gentle use of low pressure air to test fluid

pressure in the human eyeball, the multiplicity of linear and rotary motions on robotic process machines, to the high forces required for pneumatic presses and concrete breaking pneumatic drills.

The short list below serves only to indicate the versatility and variety of pneumatic control at work, in a

continuously expanding industry.

• Operation of system valves for air, water or chemicals

• Operation of heavy or hot doors

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

• Ramming and tamping in concrete and asphalt laying

• Lifting and moving in slab molding machines

• Crop spraying; crop seeding, and the operation of other agricultural equipment

• Spray painting and electrostatic powder coating

• Holding and moving in woodworking and furniture making

• Holding in jigs and fixtures in assembly machinery and machine tools

• Holding for gluing, heat sealing or welding plastics

• Holding for brazing or welding

• Metal-forming operations such as bending, drawing and flattening

• Spot welding machines

• Riveting

• Operation of guillotine blades

• Bottling and filling machines

• Wood working machinery drives and feeds

• Test rigs

• Machine tool, work or tool feeding

• Component and material conveyor transfer

• Pneumatic robots

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• Auto gauging

• Air separation and vacuum lifting of thin sheets

• Dental drills

• And so much more… new applications are developed daily

PROPERT IES OF COMP RES SED AI R

Some important reasons for the wide use of compressed air in industry are:

Availability

Most factories and industrial plants have a compressed air supply in working areas, and portable

compressors can serve more remote situations.

Storage It is easily stored in large volumes if required.

Simplicity of Design, Installation, and Control

Pneumatic components are of simple design and are easily fitted to provide extensive automated systems

with comparatively simple control.

Choice of Movement Pneumatic components offer both linear movement and angular rotation with simple and continuously

variable operational speeds.

Economy Installation is of relatively low cost due to modest component cost. There is also a low maintenance cost

due to long life without service. Pneumatic actuators do not produce heat --- other than the small amount caused by internal friction, and do not consume energy unless motion occurs.

Reliability

Pneumatic components have a long working life resulting in high system reliability.

Resistance to Environment Pneumatic systems are largely unaffected by high temperature, dusty, and corrosive atmospheres in which

other systems may fail.

Environmentally Clean

It is clean, and with proper exhaust air treatment, can be installed to clean room standards.

Safety It is not a fire hazard in high-risk areas, and the system is unaffected by overload as actuators simply stall

or slip.

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2 3

22 TTHHEE BBAASSIICC PPNNEEUUMMAATTIICC SSYYSSTTEEMM

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 pneumatic components 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 2.1, consists of two main sections:

• The Air Production and Distribution System

• The Air Consumption System

Production

Consumption

Fig. 2.1 The Basic Pneumatic System

The component parts and their main functions are:

THE AIR P ROD UCTI ON AND DIS TRI BUTION S YSTE M

1. Compressor Air taken in at atmospheric pressure is compressed and delivered at a higher pressure to the

pneumatic system. It thus transforms mechanical energy into pneumatic energy.

2. Electric Motor Supplies the mechanical power to the compressor. It transforms electrical energy into mechanical

energy.

3. Pressure Switch Controls the electric motor by sensing the pressure in the tank. It is set to a maximum pressure at

which it stops the motor, and a minimum pressure at which it restarts it.

4. Check Valve Lets the compressed air from the compressor into the tank and prevents it leaking back when the

compressor is stopped.

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5. Tank Stores the compressed air. Its size is defined by the capacity of the compressor. The larger the

volume, the longer the interval between compressor runs. Most systems should be designed for a 50% duty cycle, providing at least 2x system demand in storage.

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 and After-cooler Cools the compressed air to a few degrees above freezing point and condenses most of the air

humidity. This avoids having water in the downstream system. There should (for maximum efficiency) be an aftercooler preceding the unit to start the cooling process or the refrigerated dryer will be overtaxed. Ideally, inlet air temp should be ambient or room temperature air.

10. Line Filters Being located in the main pipe, the mainline filter must have a minimal pressure drop and the

capability of oil mist removal. It helps to keep the line free from particulate, water, and oil. A mainline filter is typically used before the aftercooler and either a second mainline filter or a mist-separator is installed inline as the compressed air leaves the production area. These filters are the primary devices to insure clean, oil-free air.

THE AIR C ONS UMPT ION SYST EM

A. Air Take-off

For consumption, air is taken off from the top of the main pipe to allow occasional condensate to stay in the main pipe. When it reaches a low point, a water take-off from beneath the pipe will flow into an Automatic Drain and the condensate will be removed. Normally there would be a union in the pipe and a shut-off valve to allow maintenance of the downstream components.

B. Auto Drain

Every descending tube should have a drain at its lowest point. The most efficient method is an Auto Drain, which prevents water from remaining in the tube should manual draining be neglected. Directly above the Auto Drain is an expansion chamber, allowing the air to cool (through expansion) and remove more entrained liquid.

C. Air Service Unit

Traditionally known as an FRL, for filter, regulator, and lubricator. These components condition the compressed air to provide clean air at optimum pressure, and (where required by component or history --- most pneumatic devices are as a standard non-lube, meaning no additional lubricant is required) add lubricant to extend the life of those pneumatic system components (e.g. air motors) that need lubrication. 

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D. Directional Valve

Alternately pressurizes and exhausts the cylinder connections to control the direction of movement. Shown as an individual device, there may be a number of directional valves grouped on a manifold.

E. Actuator

Transforms the potential energy of the compressed air into mechanical work. Shown is a linear cylinder, it can also be a rotary actuator or an air tool etc.

F. Speed Controllers

Allow easy and step-less speed adjustment of the actuator movement.

We will discuss these components in more detail in sections 4 to 7, after a brief look at the theory of

compressed air. This is a must for understanding what happens in a pneumatic system.

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33 CCOOMMPPRREESSSSEEDD AAIIRR TTHHEEOORRYY

UNIT S

For the practical application of pneumatics, it is necessary to appreciate the natural laws relating to the

behavior of air as a compressed gas and the physical dimensions in common use.

The International System of Units (the “Metric” system) has been in acceptance worldwide since 1960, but the

USA and UK still use the Imperial System to a great extent.

It is extremely important, in this ever shrinking world, that all measurement systems become clearly understood. The definitive study of pneumatics on an international scale requires familiarity and competence with either set of units; therefore this document will employ both English and SI units.

Quantity Symbol SI Unit Name Remarks

1 . B A S I C U N I T S :

Mass m Kg Kilogram Length s m Meter Time t s Second Temperature, absolute T K Kelvin 0°C = 273.16 K Temperature (Celsius)

t, 

Radius r m Meter Angle



Area, Section A,S m2 Square meter Volume V m3 Cubic meter Speed (velocity) v m s Angular Speed



Acceleration a m s Inertia Force F N Newton

Weight G N Earth acceleration

Impulse

I

Work W J Joule = Newton meter = kg.m2 /s-2 Potential energy E, W J Joule Kinetic energy E, W J Joule 0.5·m··v2 Torque M J Joule Power P W Watt

3. RELATED TO COMPRESSED AIR

Pressure p Pa Pascal = N/m2 Standard volume Vn m

Volume flow Q m Energy, Work E, W

Power P W Watt

Table 3.1 SI Units used in pneumatics

°C Degree Celsius

2. COMPOSED UNITS:

1 Radian (m/m)

-1

Meter per second

s-1 Radians per second

-2

Meter per sec. per sec.

Kgm2 Kilogram meter squared

= kg · m/s2

9.806 m/s2

Ns Newton Second

= J.s-1

Standard Cubic Meter

Normal Liter (Nl)

s-1 Std. cubic meters / sec

N.m

Joule

at =760 mm Hg

Pa.m3 = N.m p .Q = N.m·s-1 = W



= 0°C and p

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To name units by powers of ten, smaller and larger than the above basic units, a number of prefixes have been agreed upon and are listed below.

Power Prefix Symbol Power Prefix Symbol

10-1 10-2 10-3 10-6

deci d

centi c

milli m

micro µ

101 102 103 106

Deka da

Hecto h

Kilo k

Mega M

Table 3.2 Prefixes for powers of ten

This leads us to a km (kilo-meter or 1000 meters), a mm. (milli-meter or .001 meters) and a µ m (micro-meter or a micron). Practice with these prefixes and pay attention to what the symbol represents in terms of powers of ten. Pay special attention to the difference between M and m (for about a trillion reasons).

Converting from one standard of units to another is well documented. Converting is easiest when dealing with an answer --- e.g. when dealing with a mathematical formula, use one standard only (for all terms) and then convert the answer. Be aware that formulae may change when expressed in different units or standards.

The tables following show a comparison between the Metric SI units and the Imperial units.

Magnitude Metric Unit (m) English (e)

Factor m e Factor e m

Mass kg Pound 2.205 0.4535

g Ounce 0.03527 28.3527

Length m Foot 3.281 0.3048

m Yard 1.094 0.914 mm Inch 0.03937 25.4

Temperature °C °F 1.8°C+32 (°F-32)/1.8

Area, Section m2 Sq. ft. 10.76 0.0929

cm2 Sq. inch 0.155 6.4516

Volume m3 Cu. yard 1.308 0.7645

Volume Flow m

dm3

/min scfm 35.31 0.02832

/min (l/min) scfm 0.03531 28.32

Cu. inch

Cu. ft.

0.06102

0.03531

16.388

28.32

Force N Pound force (lbf.) 0.2248 4.4484

Pressure Bar

kgf/cm2

kPa

MPa

Lbf./sq. inch (psi)

14.5

14.22

0.145 145

0.06897

0.0703

6.897

0.006897

Table 3.3a Conversion of Units

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Table 3.3b Conversion of Units

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20 0 k P a

P ne um at ic s

1 ba r

V ac uu m

T ec h no lo g y

M eteo ro lo g y

Ab solute

Va cuu m

Atm os phe ric

P h ys i c s

p =

pre s su r e

Atm os ph ere

0 T o r r

10 0 0 kP a

14 5 p sig

10 B ar

0.1 M Pa

1 M Pa

P R E S SU R E

It should be noted that the metric unit of pressure is the Pascal (Pa).

1 Pa = 1 N/m2 (Newton per square meter)

This unit is extremely small, needing 101325 Pascals to equal 1 Bar. To avoid huge numbers in practice, an agreement has been made to make the Bar equal to 100,000 Pa.

100,000 Pa = 100 kPa = 0.1 MPa = 14.5 psi = 1 Bar

It corresponds with sufficient accuracy for practical purposes with the old metric unit kgf/cm2. More precise

equivalents are 1 STD atm =14.696 psi =1.01325 bar =1.03323 kgf/cm2.

In English units pressure is expressed in psi (almost never referred to as p.s.i. as one would expect), or

pounds per square inch, also relating a force to an area.

1 MPa = 10 Bar = 145 psig

4 ba r

3 ba r

2 ba r

0 ba r

ab s

36 0 T orr,

= -5 33 m ba r

-1 5.8 in Hg

ab s olu te

50 0 k P a

40 0 k P a

30 0 k P a

10 0 k P a

Pre ssu re

}

10 5 0 m ba r 30 in H g

ov e r- p res s u re

45 p sig

30 p s i g

15 p sig

Sta n da rd

10 132 5 Pa

14 .6 96 ps ia

Fig. 3.4 the various systems of pressure indication

A pressure in the context of pneumatics is assumed as over-pressure i.e. above atmospheric pressure and is

commonly referred to as gauge (also gage) pressure (GA or psig).

A pressure can also be expressed as absolute pressure (ABS or psia) i.e. a pressure relative to a full

vacuum. In vacuum technology a pressure below atmospheric i.e. under pressure is used.

1013 mbar as a reference. Note that this is not exactly .1 MPa = 100 kPa, although for normal pneumatic calculations the difference can be ignored.

The various ways of indicating pressure are illustrated in Fig 3.4, using a standard atmospheric pressure of

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1013



PROPERT IES OF GASES

I S O T H E R M I C C H A N GE ( B O Y L E ’ S LA W )

"…with constant temperature, the pressure of a given mass of gas is inversely proportional to its volume”, or:

p · V = constant

= 1;

= 1

= 0.5;

= 2

= 0.2;

= 5

b c

Fig. 3.5 illustration of Boyle’s Law

If volume V1 = 1 m3at a standard absolute pressure of 101325 Pa is compressed at constant temperature to a

volume V2 = 0.5 m3 then:

V1 = p2 ·V2 p

i.e. p2 =

The ratio V1/V2 is the “Compression Ratio” Cr

With a gauge pressure of .4 MPa,

The table below shows the compression ratio for pressures from

.1 to 1 MPa

Note the difference between reducing a volume of atmospheric air to half, 1:2.026 and the pressure ratio at a

gauge pressure of .1 MPa (.2 use gauge pressure in MPa +.1!

101325Pa•1 m

0.5m

gauge

1 2 3 4 5 6 7 8 9 10

0.987 1.987 2.974 3.961 4.948 5.935 6.922 7.908 8.995 9.882

p1·V1

= 202650 Pa

), 1:1.987! But this is theory; – no adjustment is made in practice when we simply

abs

1013.4.

= 4.95

3 = =

If volume V1 = 1 ft3 at a standard absolute pressure of 14.7 psi is compressed at constant temperature to a

volume V2 = 0.5 ft3 then:

V1 = p2 ·V2 p

14.7 psi 1 ft

i.e. p2 =



0.5ft

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p1·V1

= 29.4 psia

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459

273

298

273

499

539

499

Calculating the compression ratio in Imperial or English units is done in the same way, p1 converted to absolute

pressure (add 14.7 psi) divided by 14.7 psi (one atmosphere) or Cr = p1 + 14.7 / 14.7

All compressed air is related at a standard condition (at some over-pressure). Thus, to convert from free air, CF, (air in the room, measured in Cubic Feet) to SCF (air under pressure, measured in Standard Cubic Feet) the reference pressure must be noted. CF multiplied by compression ratio will determine SCF. Mathematically, SCF = CF * CR or SCF = CF * (p1 + 14.7 / 14.7)

P (psig)

10 20 30 40 50 60 70 80 90 100

1.68 2.36 3.04 3.72 4.4 5.08 5.76 6.44 7.12 7.80

On the other hand it would be wrong to use Boyle’s Law in pneumatics. In the case of tools as well as cylinders the change is never Isothermic but always Adiabatic change. (See further below and pg. 74-76)

I S O B A R I C C H AN G E

Ch a rles Law

"…at constant pressure, a given mass of gas increases in volume by

Celsius rise in temperature ---

for every 0F rise in temperature"

Law of Gay Lu s sac

V / T = constant, so

and V2 =

V1 T

Example 1: V1 = 100 m3, T 1 = 0°C, T2 = 25°C, V2 = ?

We have to use the absolute temperatures in K, thus

100

, V2 =

298•100

= 109.157 m

Example 2: V1 = 100 ft3, T 1 = 40°F, T2 = 80°F, V2 = ?

We have to use the absolute temperatures in R (Rankin, for degrees F or Kelvin for degrees C), thus

100

, V2 =

100 5397

 .

= 108 ft

I S O C H OR I C C H AN G E

of its volume for every degree

273

p 1

p 2

"At constant volume, the pressure is proportional to the temperature"

(“Isochoric” comes from the Greek words read “chora”), for space, field etc., and -, “iso” = equal)

p1·p

and p2 = p1

T1·T2

Where T is the absolute temperature in K (Kelvin) or R (Rankin).

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p·V



p 1

p 2

v 1



The previous relationships are combined to provide the general gas equation:

p1 V

p2 V2

= Constant

This law provides one of the main theoretical bases for calculation to design or select pneumatic equipment when temperature changes have to be considered.

AD IA B AT IC ( I S E N T R O P I C ) C H A N GE

The previous Laws assume a slow change, so only the two considered magnitudes are changing. In practice, for example --- when air flows into a cylinder, this is not the case and “adiabatic change” occurs. Then Boyle’s

= c

Law “p·V is constant “ changes to p · V= constant.

It would take too much time to go into greater detail, the diagram illustrates the difference clearly enough: we see that there is a loss of

= c

volume when pressure builds up quickly. We will meet this law again when discussing the air consumption of cylinders.

V O L U M E

Due to these mutual relationships between volume, pressure and temperature, it is necessary to refer all data on air volume to a standardized volume, the normal cubic meter (m

). Defined as the air quantity of 1.293 kg

mass at a temperature of 320F and an absolute pressure of 760 mm Hg (101325 Pa) and a relative humidity of 0%

--- or the standard cubic foot (scf) that is one cubic foot of air at sea level (absolute pressure of 14.7 psi) having a temperature of 680F and a relative humidity of 36%.

F L O W

The basic unit for volume flow "Q" is the Normal Cubic Meter per second (m

/s). In pneumatic practice

volumes are expressed in terms of normal liters per minute (Nl / min) or normal cubic decimeters per minute (dm3/min). The usual non-metric unit for volume flow is the “standard cubic foot per minute”, (scfm).

Ber nou l li's Equ a tio n

Bernoulli states:

"If a liquid of specific gravity flows horizontally through a tube with varying diameters, the total energy at point 1 and 2 is the same"

or, p1 +

· v 12 = p2 +

 · v 2

The relationship between pressure, the velocity of the air, and the density of the air () applies to gases if the flow speed does not exceed 330 m/s approx. (1083 ft/sec). Velocity (ft/sec) can be calculated:

Fig. 3.6 illustration of Bernoulli's Law v = 0.054Q /D2 (Q is cfm, D is i.d. in inches) Applications of this equation are the venturi tube

and flow compensation in pressure regulators (see page 43).

AI R H U M I D I T Y

Atmospheric air always contains a percentage of water vapor. The amount of moisture present will depend on

the atmospheric humidity and temperature.

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



7.459







273



When atmospheric air cools it will reach a certain point at which it is saturated with moisture, this is known as

the dew point. If the air cools further it can no longer retain all the moisture and the surplus is expelled as miniature droplets to form a condensate (dew).

The actual quantity of water that can be retained depends entirely on temperature; 1m3 of compressed air is

only capable of holding the same quantity of water vapor as 1m3 of atmospheric air.

The table below shows the number of grams of water per cubic meter (and cubic feet) for a wide temperature

range from -40°C to +40°C and from –40 0F to 200 0F. The bold line refers to atmospheric air with the volume at the temperature in question. The thin line gives the amount of water per Normal Cubic dimension. All air consumption is normally expressed in standard volume; this makes calculation unnecessary.

For the temperature range of pneumatic applications the table below gives the exact values. The upper half

refers to temperatures above freezing, the lower to below freezing. The upper rows show the content of a standard cubic meter, the lower ones the volume at the given temperature.

Temperature °C 0 5 10 15 20 25 30 35 40

g/m

(Normal)

n *

g/m

(Atmospheric)

4.98 6.99 9.86 13.76 18.99 25.94 35.12 47.19 63.03

4.98 6.86 9.51 13.04 17.69 23.76 31.64 41.83 54.11

Temperature °C 0 –5 –10 –15 –20 –25 –30 –35 –40

g/m

(Normal)

g/m

(Atmospheric)

4.98 3.36 2.28 1.52 1.00 0.64 0.4 0.25 0.15

4.98 3.42 2.37 1.61 1.08 0.7 0.45 0.29 0.18

Temperature °F 40 60 68 80 100 120 140 160 180

g/ft

(Standard) .175 .369 .375 .726 1.368 2.438 4.170 6.834 10.836

g/ft

(Atmospheric) .137 .362 .375 .71 1.29 2.22 3.67 5.82 8.94

Temperature °F 32 30 20 10 0 -10 -20 -30 –40

g/ft

(Standard) .128 .118 .077 .050 .031 .020 .011 .007 .004

g/ft

(Atmospheric) .137 .127 .085 .056 .036 .023 .014 .009 .005

Table 3.7 Water Saturation of Air (Dew Point)

The term g/ft3 standard refers to a volume at 680F. At 800F its volume is extended to 1+ or 1.026 ft Consequently to have one standard cubic foot at 800F, 1.03 ft3 of atmospheric air at 800F are required with all its water content; so that makes 1.03 x 0.71 = 0.726 grams of water. In other words, in the table we are adjusting the volume of air present from standard, which assumes a temperature of 68oF, to the actual volume present at the higher temperature.

In addition, the term g/m

normal refers to a volume at 00C. At 300C its volume is extended to 1+ or 1.1

m3. To have one normal m3 at 300C, 1.1 m3 of atmospheric air at 300C are required with all its water content; so that makes 1.1 x 31.64 = 35.12 grams of water.

Definitions of the different standards for air, ISO and DIN:

ISO 8778 and JIS B8393 Temperature: 20 C Pressure: 0.1 MPa (100kPa) RH: 65%

DIN 1945 (Deutsche Industrie Norm -> German Industrial Standard) Temperature: 0 C Pressure: 101325 Pa RH: 0%

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P N E U M A T I C T E C H N O L O G Y

.1013

Rel ati v e h u mid i ty

With the exception of extreme weather conditions, such as a sudden temperature drop, atmospheric air is

never saturated. The ratio of the actual water content and that of the dew point is called relative humidity, and is indicated as a percentage.

Relative humidity (r.h.) =

actual water content

saturation quantity (dew point)

· 100%

Example 1: Temperature 25oC, r.h. 65%. How much water is contained in 1 m3?

Dew point 25oC = 24 g/ m3 · 0.65 = 15.6 g/ m3

When air is compressed, its capacity for holding moisture in vapor form is only that of its reduced volume.

Hence, unless the temperature rises substantially, water will condense out.

Example 2: 10 m3 of atmospheric air at 15oC and 65% r.h. is compressed to .6-MPa gauge pressure. The

temperature is allowed to rise to 25oC. How much water will condense out?

From Table 3.7: At 15oC, 10 m3 of air can hold a maximum of 13.04 g/m3 ·10 m3 = 130.4 g At 65% r.h. the air will contain 130.4 g ·0.65 = 84.9 g (a) The reduced volume of compressed air at .6-MPa pressure can be calculated:

p1·V1 = p2 V2 

= V

2 

MPA1013.



·10 m3 = 1.44 m3

From Table 3.7 1.44 m3 of air at 25oC can hold a maximum of 23.76 g ·1.44 = 34.2 g (b) Condensation equals the total amount of water in the air (a) minus the volume that the compressed air can

absorb (b), hence 84.9 – 34.2 = 50.6 g of water will condense out.

This condensate must be removed before the compressed air is distributed, to avoid harmful effects in the line

and the pneumatic components. Example 3: Temperature 800F, r.h. 65%. How much water is contained in 1 ft3?

Dew point 800F = 0.71 g/ ft3 · 0.65 = 0.46 g/ ft

Observe that the metric chart dimensions would exhibit identical relationships when converted to

Imperial units.

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g H O/m

500

100

150

1.5

0.5

0.1

-30 500

100 C

Fig. 3.8 Dew points for temperatures from –30 to about +80°C

The bold curve shows the saturation points of a cubic meter at the related temperature, the thin curve at

standard volume. All air consumption is normally expressed in normal volume; this makes calculation

unnecessary.

P R E S SU R E AN D F L O W

The most important relationship for pneumatics is that between pressure and flow. THEY ARE NOT THE SAME. DO NOT THINK THEY ARE INTERCHANGEABLE TERMS… e.g. a flow control

is not a pressure regulator (repeat as required until retained). It is the relationship between flow and pressure that we will now consider.

If there is no flow, the pressure in an entire system is the same at every point, but when there is flow from one

point to another, the pressure in the latter will always be lower that at the first. This difference is called pressure drop. It depends on three values:

• Initial pressure

• Volume of flow

• Flow resistance of the connection

The flow resistance for air has no unit; in electricity its equivalent is Ohm (). In pneumatics, the opposite of

resistance is used, which is conductance, or the ability to allow flow, expressed as the equivalent flow section S. The equivalent flow section S is expressed in mm2 and represents the area of an orifice in a thin plate (diaphragm) that creates the same relationship between pressures and flow as the element defined by it. Valves have complicated orifice shapes, therefore the flow rate through the device is measured first (along with pressure drop, temperature, etc), and then the device may be assigned the corresponding equivalent flow section. Other standards include the kv or Cv factor (a dimensionless number referring to a flow coefficient) --- Consider Cv as a conductance value. An easy approximation would be that:

Cv of 1 = 18 S mm2, e.g. an equivalent orifice size of 18 mm2 equals the flow of a Cv of 1. (Note: the original

definition of Cv stated that a Cv of 1 = a flow capacity of 1 gpm with a pressure drop of 1 psi)

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(54.44 l /

min)

S = 1 mm

20 40 80 100 120

(dm /min)

The relationship between pressure and flow is by definition the same as in electricity, where “voltage drop

equals current times resistance”. This can be transformed for pneumatics to “pressure drop equals flow divided by Flow Section”, only, while the electric units are directly proportional, the relationship for air is very complex and never simply proportional. In electricity, a current of 1 A (one Ampere) creates over a resistor of 1 Ohm a voltage drop of 1 Volt, regardless if this drop is from 100 to 99 or from 4 to 3 volts. The pressure drop over the same object and with the same standard volume flow varies with the initial pressure and also with the temperature. Reason: The compressibility of the air.

For defining one of the four interrelated data, mentioned previously, from the other three, we require a diagram.

.10

(MPa)

Sonic Flow

Fig. 3.9 Diagram showing the relationship between pressure and flow for an orifice with an equivalent Flow

Section of 1 mm2

The triangle in the lower right corner marks the range of “sonic flow speed”. When the airflow reaches a speed

close to the speed of sound flow can no longer increase --- whatever the difference of pressure between input and output might be. As you can see, all the curves drop vertically inside this triangle. This means that the flow no longer depends on the pressure drop, but only on the input pressure.

Use of the dia g ram :

The pressure scale at the left side indicates both input and output pressure. At the first vertical line on the left,

which represents a zero flow, input and output pressures are the same. The various curves, for input pressures from 1 to 10 bar, indicate how the output pressure decreases with increasing flow.

Example 1: Input pressure .6 MPa, pressure drop 1 bar = output pressure .5 MPa. We follow the curve “.6” to the

point where it cuts the horizontal line marked “.5”. From there we go vertically down to the Flow scale (dotted line) and find about 55 l/min. The 54.44 l/min written below that line is the exact value, calculated with the formula further below. These input and output pressures define the so-called “Standard Volume Flow Qn”, a figure found in valve catalogues for a quick comparison of the flow capacity of valves.

The Volume Flow of 54.44 l/min applies to an element (Valve, fitting, tube etc.) with an equivalent orifice “S” of

1 mm2. If an element has for example an “S” of 4.5 mm2, the flow would be 4.5 times higher, in this case 4.5 ·

54.44 l/min = 245 l/min



Sound is, after all, vibrating air molecules. Thus the “speed of sound” (sonic condition, Mach #) is the terminal velocity for air movement. For compressed air to flow there must be a pressure drop --- and maximum flow occurs at a certain % pressure drop. There can be a greater pressure drop (up to 100%) but maximum flow (for whatever size orifice) occurs at 47% of p1.

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P N E U M A T I C T E C H N O L O G Y

1.024

Example 2: Given an element with an “S” of 12 mm2, a working pressure of .7 MPa and an air consumption of 600

l/min. What output pressure will result?

A flow of 600 l/min through an “S” of 12 mm2 corresponds with a flow of

600

= 50 l/min through an

12 equivalent section of 1 mm2. We need this conversion for the use of the diagram of fig. 3.9. We now follow the curve starting at .7 MPa until it intersects with the vertical line for 50 l/min. A horizontal line towards the pressure scale indicates about .63 MPa.

Fo r mul a e:

When it is required to have a more exact value than that which can be estimated from the diagram, the flow

can be calculated with one of the two following formulae.

A glance at the diagram of Fig. 3.9 makes it clear, that there must be different formulae for the sonic flow range and the “subsonic” flow condition. The transient from subsonic to sonic flow is reached, when the pressure ratio of the absolute input and output pressures is less or equal to 1.896:

Sonic flow: p1 + .1013 <.1896 · (p2 +.1013)

Subsonic flow: p1 + .1013 > .1896 · (p2 +.1013)

The Volume flow Q for subsonic flow equals:

Q = 22.2 · S · (p2 + .1013) · (p1 –p2) (l/min)

And for sonic flow:

Q = 11.1 · S · (p1 + .1013) (l/min)

Where S in mm2 and p in bar; 22.2 is a constant with the equation

, which is liters per 60 seconds and per

60 N s

force (defined by the ruling pressure). Note that a pneumatic system can never operate satisfactorily under sonic flow conditions, as a supply

pressure of, for example, .6 MPa would give us less than .27 MPa for work.

Example 3: We calculate the flow; assumed in example 2, with an input pressure of .7 MPa, a total equivalent

flow section of 12 mm2 for valve and tubes and the calculated working pressure of .63 MPa:

Q = 22.2 · 12 · 7.313·0.7 = 602.74 l/min.

This shows that the accuracy of the diagram is sufficient for practical pneumatic use.

In Imperial units

The formula for subsonic flow:

P2pC





scfm

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And for sonic flow: Q=0.486 Cv (p2+14.7)

Fig. 3.10 Airflow curves for a device having a Cv of 1.0 (derived from the above two formulae) Flow at a certain pressure drop can be derived from Fig. 3.10. Select the p1 (upstream pressure) from the diagonal line and follow straight across to the vertical axis --- this is the

maximum flow at that pressure. Now select a pressure drop from either the bottom numbers (downstream pressure) or from the numbers on the outer arc of the graph (p in psi). Next, follow the curve of the selected p1 until it intersects your p2 or p selection and then follow straight across from that point to the vertical axis to find flow in scfm.

The results are linear, e.g. if the device in application has a Cv of 2.0 multiply your result from Fig. 3.10 by 2, Cv of

0.5 multiply by one half, etc. Observe that critical flow occurs at a certain pressure drop – to discover this for yourself, find 100 psig (which is

114.7 psia) on the diagonal critical flow line. Drop straight down to the p2 horizontal axis and note that p2 is approximately 46 psig (or 60.7 psia). This confirms that a pressure differential of (approximately) 47% produces maximum flow. There can be a greater drop in pressure but flow will not increase. Looked at another way, maximum flow occurs when p2

abs

= p1

* 0.53, or in our example 60.7 psia = 114.7 psia * 0.53. Although we

abs

state that for all practical purposes, flow does not increase once sonic velocity has been reached, in tests conducted with precision instruments, it has been observed that the flow rate does increase slightly. This increase is so very slight that it is discounted in actual practice.

Observe that use of Fig. 3.10 requires a known pressure drop. In real world applications (with so many variables) this knowledge is difficult to come by, so the cautious individual will rely on a safe estimate of what a desired pressure drop ought to be. Predicting a system’s actual pressure drop is very difficult. The NFPA (National Fluid Power Association, a U.S. standards group) recommends a maximum pressure drop of 15%.

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Example 1: How many scfm will flow through a valve with a Cv of 1.0 given a supply pressure of 80 psig and a

20-psi pressure drop?

From the chart Fig. 3.10 find 80 psig on the critical flow line. Next, find 60 psig (80 psig minus a 20 psi

pressure drop) on the horizontal axis at the bottom. Moving vertically from the 60 psig find the intersection of the 80 psig curve (from the critical flow line) and move straight across to the vertical axis where the answer of approximately 38 scfm will be found.

Example 2: A flow of 40 scfm is required for an application and supply is 60 psig. What size Cv must all

components exceed? From the chart Fig. 3.10 find the scfm of a Cv of 1.0. If the application flows to atmosphere (e.g. a “blow-

off”) the critical flow scfm will be used; if the application involves other devices (e.g. cylinders or actuators) use the rule of thumb 10 psi pressure drop. Note that some designers typically use a 5 psi or 2 psi pressure drop when speeds are critical or a more conservative sizing solution is required. Observe that at a supply pressure of 60 psig, an orifice with a Cv of 1.0 will flow approximately 36 scfm. With a 10 psi pressure drop (p2 is 50 psig) the flow is approximately 25 scfm --- and thus a Cv of more than 1.6 will provide 40 scfm (40 scfm / 25 scfm = 1.6).

For more information on Cv please refer to pages 92 and following regarding dealing with sizing of components

and systems.

Indication of flow characteristics:

The flow rate of a pneumatic valve is an important characteristic of its performance characteristic. In order to determine a meaningful flow rate for a valve, a standardized, repeatable test method is required. This test method is defined by the International Organization for Standardization ISO 6358-1989. Other national or customary pneumatics standards such as VDI 2173 (Germany) and JIS B8390-2000 (Japan) are based on this ISO standard.

ISO 6358-1989, JIS B 8390-2000:

Based on the sonic conductance C [dm3/s*bar] characteristic and the critical pressure ratio b [-], which is defined in ISO6358-1989 and JIS B 8390-2000, it is possible to determine the flow rate of a pneumatic component.

Sonic conductance C:

The air-passages of a pneumatic component (e.g. directional valve) consist of a number of flow resistances, caused by deflecting and/or redirecting air flow. These resistances directly affect the flow capacity of the component. The sonic conductance C, defines the maximum flow capacity in sonic flow conditions.

Critical pressure ratio b:

The Critical pressure ratio b constitutes the pressure ratio between the downstream pressure (p2) and the upstream pressure (p1). In conditions where the effective pressure ratio (p2 to p1) is less than the rated b-value; it is presumed that sonic flow or choked flow condition exists. If it is greater than the b-value; subsonic flow condition exists. Therefore, the pressure ratio inducing sonic conductance is for each component diverse, and depends on its construction. Directional valves have rated b-values from 0.16 to 0.5. Speed controls, fittings and tubing generally have a b-value of 0.5.

Note: The critical pressure ratio b of an ideal nozzle is 0.528.

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Measuring sonic conductance C and critical pressure ratio b:

Fig 3.11: Test circuit based on ISO 6358-1989, JIS B 8390-2000 (simplified circuit)

In order to obtain the flow capacity of a pneumatic resistance, it is necessary to determine the value for sonic conductance first. To determine this value a constant supply pressure (>0.3 MPa gauge) is needed. By opening the flow control valve, the pressure decreases and the flow increases. By continuing to open the control valve, flow will increase until sonic conductance (choked flow) on the narrowest cross section in the test item is reached. With the maximum flow, it is now possible to calculate the sonic conductance value C (Formula #1 below solved for C).

To reduce variations in the calculation of the respective b-values, four measuring points of approx. 80, 60, 40 and 20 % in the subsonic flow range are selected and used for the calculation of the b-values. The arithmetic mean of all calculated b-values will be the rated critical pressure ratio b (Formula #2 solved for b). These four points are also used to chart the graph.

Fig 3.12: Flow behavior in relationship to pressure ratio

If the b-value is known, use the following simple formula to calculate p2, which will be the downstream pressure when sonic flow occurs.

Example: A valve has a rated critical pressure ratio b of 0.36. The supply pressure (p1) is 0.6 MPa. At what output pressure (p2) will the flow in the valve change form subsonic to sonic flow?

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







1.06.





Formulas to calculate air flow rates Q:

If the pressure ratio is less or equal to the rated b-value:

1.01

A sonic condition exists.

In that case use the following formula #1:

If the pressure ratio is greater than the rated b-value:

1.01

A subsonic condition exists.

In that case use the following formula #2:

Q = Flow [dm3/min], [l/min] C = Sonic conductance [dm3/(s*bar)] b = Critical pressure ratio [-] p1 = Upstream pressure [MPa] p2 = Downstream pressure [MPa] t = Temperature [°C]

Example 1: C = 2dm3/(s*bar) b = 0.3 p1 = 0.6MPa p2 = 0.5MPa t = 20°C

1.02

1.01



pbp

1.02



pCQ

1.02



pCQ

1.05.0



1.0)1.01(2

293



)1.01(600

273



1)1.01(600

3.086.0

MPap 152.01.0)1.06.0(36.02



 

  



1.02



1.01







   



293



273



The pressure ratio of 0.83 is greater than 0.3, therefore use formula #2 for subsonic flow

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0.86

 

1)1.06.0(2600



 



1.06.0



3.01



1.05.0





Alternatively to the complex formula above, formula #1 in conjunction with the graph below may be used to determine the flow rate in subsonic conditions instead.

Example 2:

)1.06.0(2600 NlQ 

293



20273





3.0

  

293



20273



NlQ 

min/54.508



min/840

min/5046.0min/840 NlNlQ 

Fig.12: Flow rate ratio graph to correct for subsonic flow

The standard flow Qn :

Some pneumatic vendors provide, besides the C- and b-values, additional information on the flow capacity, indicated as Qn in min/

flow of free air min)/( pressure of 0.5 MPa.

It is recommended to calculate the standard flow Qn, using the formula #2 (p1 = 0.6MPa, p2 = 0.5MPa), or with the following rules of thumb:

Qn = 227 x C  if b = 0.10 Qn = 233 x C  if b = 0.15 Qn = 240 x C  if b = 0.20 Qn = 247 x C  if b = 0.25 Qn = 254 x C  if b = 0.30 Qn = 263 x C  if b = 0.35 Qn = 272 x C  if b = 0.40 Qn = 282 x C  if b = 0.45 Qn = 294 x C  if b = 0.50

l (normal or standard liter/min). This standard flow Qn is defined as follows: Standard

l through a pneumatic component at an upstream pressure of 0.6 MPa and a downstream

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



0.5

Flow section S [mm2]:

The Flow section S is commonly used for fittings and tubing. The relationship between flow section S and sonic conductance C is described in the formulas below.

0.5

CS or

S = Flow section [mm C = Sonic conductance [dm

Note: Fittings and tubing have generally a rated b-value of 0.5

C 

]

/(s x bar)]

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P N E U M A T I C T E C H N O L O G Y

Displacement Compressors

Reciprocating

Rotary

Piston

Diaphragm

Vane

Screw

44 AAIIRR CCOOMMPPRREESSSSIIOONN AANNDD DDIISSTTRRIIBBUUTTIIOONN

COMP RES SOR S

A compressor converts the mechanical energy of an electric or combustion motor into the potential energy of compressed air.

Air compressors fall into two main categories: Reciprocating and Rotary.

The principal types of compressors within these categories are shown in Fig 4.1.

Fig. 4.1 The Main Compressor types used for Pneumatic Systems

R E C I PR O C A T IN G C O M P R E S S O R S

Sin gle sta g e P i sto n Co mpre sso r

Air taken in at atmospheric pressure is compressed to the required pressure in a single stroke.

Downward movement of the piston increases volume to create a lower pressure than that of the atmosphere, causing air to enter the cylinder through the inlet valve.

At the end of the stroke, the piston moves upwards, the inlet valve closes as the air is compressed, forcing the outlet valve to open discharging air into a receiver tank.

This type of compressor is generally used in systems requiring air in the .3-.7 MPa range.

Fig. 4.2 Single Stage Piston Compressor

Tw o st age Pist o n C omp r ess o r

In a single-stage compressor, when air is compressed above .6 MPa, the excessive heat created greatly reduces the efficiency. Because of this, piston compressors used in industrial compressed air systems are usually two stages.

Air taken in at atmospheric pressure is compressed in two stages to the final pressure.

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If the final pressure

Intercooler

Intake

Output

Intake

Output

is .7 MPa, the first stage normally compresses the air to approximately .3 MPa, after which it is cooled. It is then fed into the second stage cylinder that compresses it to .7 MPa.

The compressed air enters the second stage cylinder at a greatly reduced temperature after passing through the inter-cooler, thus improving efficiency compared to that of a single stage unit. The final delivery temperature may be in the region of 1200C (2500 F).

P N E U M A T I C T E C H N O L O G Y

Fig. 4.3 Two Stage Piston Compressor

Di a phr a gm c omp ress o r

Diaphragm compressors provide compressed air in the .3-.5 MPa range totally free of oil and are therefore widely used by food, pharmaceutical and similar industries.

The diaphragm provides a change in chamber volume. This allows air intake in the down stroke and compression in the up stroke.

Smaller types, with a fractional HP electric motor and small reservoir make possible portable compressors, ideal for spray painting.

Fig. 4.4 Diaphragm Compressor

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increasing favor.

R O T A R Y C O M P R E S S O RS

Ro t ary sli d ing van e co m pre s sor

This has an eccentrically mounted rotor having a series of vanes sliding in radial slots.

As the rotor rotates, centrifugal force holds the vanes in contact with the stator wall and the space between the adjacent blades decreases from air inlet to outlet, so compressing the air.

Lubrication and sealing is achieved by injecting oil into the air stream near the inlet. The oil also acts as a coolant to limit the delivery temperature.

Scr e w c o mp r ess o r

Two meshing helical rotors rotate in opposite directions. The free space between them decreases axially in volume and this compresses the air trapped between the rotors (Fig 4.6.).

Fig. 4.5 Vane Compressor

Oil flooding provides lubrication and sealing

Drive

between the two rotating screws. Oil separators remove this oil from the outlet air.

Continuous high flow rates in excess of 400 m3/min are obtainable from these machines at pressures up to 1.0 MPa.

More so than the Vane Compressor, this type of compressor offers a continuous pulse-free delivery.

The most common industrial type of air compressor is still the reciprocating machine,

Fig 4.6 Screw Compressor Principle

IntakeOutput

although screw and vane types are finding

C O M P R E S S OR R A T IN G

A compressor capacity or output is stated as Standard Volume Flow, given in m

/s or /min, dm

/s or liters

/min. The capacity may also be described as displaced volume, or ''Theoretical Intake Volume'', a theoretical figure. For a piston compressor it is based on:

Q (l/min) = (piston area in dm2) x (stroke length in dm) x (# of first stage cylinders) x (rpm)

Q (cfm) = ((piston area in in2) x (stroke length in inches) x (# of first stage cylinders) x (rpm)) / 1728

In the case of a two-stage compressor, only the first stage cylinder should be considered.

The effective delivery is always less due to volumetric and thermal losses.

The volume loss is inevitable, as it is not possible to discharge all of the compressed air from the cylinder at the end of the compression stroke; there is some space left, the so-called “dead volume”.

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90%

70%

80%

60%

.12 .1 1 .1 0 .9 .8 .7 .6 .5 .4

To tal

E fficienc y

S ingle S tage

Tw o S ta ge

Thermal loss occurs due to the fact that during compression the air assumes a very high temperature; therefore its volume is increased and decreases when cooling down to ambient temperature (see Charles Law in section 3).

Vol ume t ric Eff i cie n cy

The ratio:

free air delivered

displacement

expressed as a percentage is known as the volumetric efficiency, and will vary with

the size, type and make of machine, number of stages and the final pressure. The volumetric efficiency of a twostage compressor is less than that of a single stage type as both the first and second stage cylinders have dead volumes.

Th e rma l an d Ov e ral l Ef f ici e ncy

Beside the losses described above, there are also thermal effects, which lower the efficiency of the air compression. These losses reduce the overall efficiency further depending on the compression ratio and load. A compressor working at almost full capacity accumulates great heat and loses efficiency. In a two-stage compressor, the compression ratio per stage is less and the air, partly compressed in a first stage cylinder, is cooled in an inter-cooler before compression to final pressure in a second stage cylinder.

Example: If the atmospheric air, taken in by a first stage cylinder, is compressed to a third of its volume, the

absolute pressure at its outlet is .3 MPa. The heat, developed by this relatively low compression, is correspondingly low. The compressed air is then led to a second stage cylinder, through the inter-cooler, and then again reduced to a third of its volume. The final pressure is then .9 MPa abs.

The heat developed by compressing the same air volume in a single stage directly from atmospheric

pressure to .9 MPa

The diagram in Fig. 4.7 compares the typical overall

, would be much higher and the overall efficiency severely reduced.

abs

efficiencies of single and two stage compressors with various final pressures.

For low final pressures, a single stage compressor is better, as its pure volumetric efficiency is higher. With increasing final pressure however, thermal losses become more and more

Fig. 4.7 Overall efficiency Diagram

Final P re s sure (M Pa )

important and two stage types, having a higher thermal efficiency become preferable.

The specific energy consumption is a measure of the overall efficiency and can be used to estimate the generating cost of compressed air. As an average figure, it can be assumed that one kW of electrical energy is

needed for the production of 120-150 l/min (= 0.12…0.15 m

/ min / kW), for a working pressure of .7 MPa or 1

HP of electrical energy is needed to produce 4~5 cfm at a working pressure of (.7 MPa) (31-39 scfm).

Exact figures have to be established according to the type and size of compressor.

COMP RES SOR AC CESS ORI ES

AI R R E C E I V E R

An air receiver is a pressure vessel of welded steel plate construction, installed horizontally or vertically directly downstream from the aftercooler to receive the compressed air, thereby damping the initial pulsations in the air flow.

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Its main functions are to store sufficient air to meet temporary heavy demands in excess of compressor capacity, and minimize frequent "loading" and "unloading" of the compressor, but it also provides additional cooling to precipitate oil and moisture carried over from the aftercooler, before the air is distributed further. To this end it is an advantage to place the air receiver in a cool location.

The vessel should be fitted with a safety valve, pressure gauge, automatic drain, and inspection covers for checking or cleaning inside --- and may need to be approved by some governing body (e.g. ASME in North America).

Siz ing a r e cei v e r

Air receivers are sized according to the compressor output, size of the system and whether the demand is relatively constant or variable.

Electrically driven compressors in industrial plants, supplying a network, are normally switched on and off between a minimum and a maximum pressure. This control is called “automatic”. This needs a certain minimum receiver volume to avoid over-frequent switching.

Mobile compressors with a combustion engine are not stopped when a maximum pressure is reached, but the suction valves are lifted so that the air can freely flow in and out of the cylinder without being compressed (often referred to as “unloading”). The pressure difference between compressing and running idle is quite small. In this case only a small receiver is needed.

For industrial plants, the rule of thumb for the size of the reservoir is:

Air receiver capacity  compressor output of compressed air per minute. (Not Free Air)

Some would suggest a factor of x1.5 when sizing a receiver for a large system, and as much as x3 for small compressors.

Example: compressor delivery 600 cfm (free air) and an output pressure of .7 MPa (100 psi). What size receiver

is required?

Where V = capacity of receiver

Q = compressor output (cfm)

Pa = atmospheric pressure

P1 = compressor output pressure

V = (600*14.7)/ (100+14.7) = 77 ft3 as a minimum number, a prudent suggestion might begin with 120 ft3.

I N L E T F I L T E R

A typical city atmosphere can contain 40 million solid particles, i.e. dust, dirt, pollen, etc. per m3. If this air were compressed to .7 MPa, the concentration would be 320 million parts/m3 or 7.8 million parts/ft3. An important condition for the reliability and durability of a compressor is that it must be provided with a suitable and efficient intake filter to prevent excessive wear of cylinders, piston rings, etc. which is caused mainly by the abrasive effect of these impurities.

The filter must not be too fine as the compressor efficiency decreases due to high resistance to airflow, and so very small particles (2-5 µ) cannot be removed.

The air intake should be sited so that, as far as possible, clean dry air is drawn in, with intake piping of sufficiently large diameter to avoid excessive pressure drops. When a silencer is used, it may be arranged to include the air filter, which will be located upstream of the silencer position, so that it is subjected to minimum pulsation effects.

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P N E U M A T I C T E C H N O L O G Y

Cooling Water IN

Cooling Water OUT

Air Input

Air Output

AI R DEHYDR AT ION

AF TE R C O O L E R S

After final compression, the air will be hot and when cooling, will deposit water in considerable quantities in the airline system, which should be avoided. The most effective way to remove the major part of this condensate is to subject the air to aftercooling, immediately after compression.

Aftercoolers are heat exchangers, being either air-cooled or water-cooled units.

Ai r co o led

Consisting of a nest of tubes through which the compressed air flows and over which a forced draft of cold air is passed by means of a fan assembly. A typical example is shown in Fig.4.8.

The outlet temperature of the cooled compressed air should be approximately 400C (105 0F), given an inlet air temperature of 70oC (158oF).

Fig. 4.8 Principle of an Air Cooled Aftercooler

Wat e r c o ole d

A water-cooled aftercooler is essentially a steel shell housing tubes with water circulating on one side and air on the other, usually arranged so that the air flows in the opposite direction of the water flowing through the cooler. The principle is shown in Fig. 4.9

Fig. 4.9 Principle of a Water Cooled Aftercooler

A water-cooled aftercooler should ensure that the air discharged would be approximately 40oC (105 0F) with 300C being the inlet temperature of the cooling water. The major benefit of the water-cooled aftercooler is that it will accommodate inlet air temperatures up to 180oC (3500 F).

An automatic drain attached to or integral with the aftercooler removes the accumulated condensation.

Aftercoolers should be equipped with a safety valve, pressure gauge, and it is recommended that thermometers or sensors to monitor air and water temperatures are included.

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AI R D R Y E R S

Aftercoolers cool the air to approximately 40oC (1050 F). The control and operating elements of the pneumatic system will normally be at ambient temperature [approx. 20CC (680 F)]. This may suggest that no further condensate will be precipitated, and that the remaining moisture passes out with the exhaust air released to atmosphere. However, the temperature of the air leaving the aftercooler may be higher than the surrounding temperature through which the pipeline passes, for example during nighttime. This situation cools the compressed air further, thus condensing more of the vapor into water.

The measure employed in the drying of air is lowering the dew point, which is the temperature at which the air is fully saturated with moisture (i.e.100% humidity). The lower the dew point, the less moisture remains in the compressed air.

There are three main types of air dryers available, which operate on an absorption, adsorption, or refrigeration process.

Ab s orp t ion (d e liq u esc e nt) Dry i ng

The compressed air is forced through a drying agent such as dehydrated chalk or magnesium chloride, which remains in solid form, or lithium chloride or calcium chloride, which reacts with the moisture to form a solution which is drained from the bottom of the vessel.

Output

The drying agent must be replenished at regular intervals (2-4 times/year) because the dew point will increase as the salt is consumed during operation. A pressure dew point of 50C at .7 MPa is possible (40 0F at 100 psi).

The main advantages of this method are that it is of low initial and operating cost. The disadvantages are that the inlet temperature must not exceed 300C (860 F), the chemicals involved are highly corrosive necessitating carefully monitored filtering to ensure that a fine corrosive mist is not carried over to the pneumatic system, and the spent chemical must be properly disposed of, which is becoming increasingly costly.

Input

Fig. 4.10 Principle of the Absorption Air Dryer

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Ad s orp t ion (d e sic c ant ) Dr y i ng

Exhaust

Output Dry Air

Input Wet Air

Column 1

(working)

Column 2

(regenerating)

A chemical such as silica gel or activated alumina in granular form is contained in a vertical chamber to physically adsorb moisture from the compressed air passing through it. Adsorption is a physical process of a liquid adhering to the surface of certain materials (a sponge absorbs, retaining moisture internally --- adsorb is a surface effect, think “adhere”). When the drying agent becomes saturated it is regenerated by drying, heating, or, by a flow of previously dried air as in Fig.

4.11. Wet compressed air is

supplied through a directional control valve and passes through desiccant column 1. The dried air flows to the outlet port. Between 10-20% of the dry air passes through orifice O2 and column 2 in reverse direction to re-adsorb moisture from the desiccant to regenerate it.

P N E U M A T I C T E C H N O L O G Y

Fig. 4.11 Principle of the Adsorption Air Dryer

The dry air enters the saturated chamber and expands (dropping the temperature further, making the dry air effectively even more dry to facilitate the regenerating process). The regenerating airflow goes then to exhaust. A timer or sensor periodically switches the directional control valve to alternately direct the supply air to one column while regenerating the desiccant in the other, thus providing continuous dry air.

Extremely low dew points are possible with this method, for example - 40oC (which is, oddly enough, -40 0F).

A color indicator may be incorporated in the desiccant to monitor the degree of saturation. Micro-filtering is essential before the dryer inlet to prevent contamination of the adsorbent, and recommended on the dryer outlet to prevent carry over of adsorbent mist. Initial and operating costs are comparatively high, but maintenance costs tend to be low.

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P N E U M A T I C T E C H N O L O G Y

















Ref rig e ran t dr y ing

This is a mechanical unit incorporating a refrigeration circuit and two heat exchangers.

Humid high temperature air is pre-cooled in the first heat exchanger  by transferring part of its heat to the cooled output air.

Dry Air OUT Hot Air IN



Heat Exchanger:



input air / output air

It is then cooled by the refrigerator principle of heat extraction as a result of evaporating Freon (not any more, however) gas in the refrigerator circuit, in heat

exchanger. At this time, moisture and oil mists condense and are automatically drained.

The cold dry air return pipe passes through air heat exchanger  and gains heat from the incoming high temperature air. This prevents dew forming on the discharge outlet, increases volume and lowers relative humidity.

An output temperature of usually 30C (370 F) is possible by modern methods, although an output air temperature of 50C is sufficient for most common applications of compressed air. Inlet temperatures may be up to 600C (1400 F) but it is more economical to pre-cool the air to run at lower inlet temperatures. Refrigerant dryers have a pressure dew point of 30 C at .7 MPa (-21 at P

As a general rule, the cost of drying compressed air may be 10-20% of the cost of compressing air.









Fig. 4.12 Principle of the Refrigerated Air Dryer

atm

Heat Exchanger or Evaporator:



input air / freon

Freon cooler or Condensor



Condensor Fan(for 3)

Compressor for Freon



Thermostatic valve

Air filter





Auto Drain

The cost of not drying compressed air is seen in increased maintenance of all pneumatic components used in the system, plus the associated increased downtime, far exceeding the costs of adding a drying system.

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P N E U M A T I C T E C H N O L O G Y

Mem b ran e Ai r Dry e r

The membrane air dryer uses hollow fibers composed of a macro molecular membrane through which moisture passes easily, but which is difficult for air (oxygen and nitrogen) to pass through.

When humid, compressed air is supplied to the inside of the hollow fibers, only moisture permeates the membrane and moves to the outside due to the pressure difference between the moisture inside and outside of the fibers. The compressed air becomes dry air and continues out of the dryer. Part of the dry air from the outlet side is passed through a very small orifice to reduce the pressure and purge the outside of the hollow fibers. The moisture, which permeated to the outside of the hollow fibers, is discharged to the atmosphere by this purge air. In this way, the partial pressure outside of the hollow fibers remains low and dehumidification is continuously

Fig. 4.13 A typical Membrane Air Dryer

Membrane air dryers are parts that require no electricity, so wiring is not necessary. They are also light-weight and compact. No coolant is used as in refrigerated units, and no heat or vibration is generated. A dew point indicator is provided in the membrane air dryer which indicates, by a change of color, that the dew point has been reached.

performed.

Note: The minimum dew point can be as low as -60C (-760 F). Micro-filtering on the inlet is essential to prevent clogging of the membrane.

Wat e r r e mov a l f ilt e r

A water removal filter will remove liquid water that is present in the airline. A water removal filter should be installed at the point of highest pressure as far from the compressor as possible. Particle and oil filters, plus pressure regulation if required, should be installed downstream of the water removal filter. Because of its open weave resin mesh element, pressure drop across a water removal filter is negligible and element replacement is rarely required.

Water removal filters are typically installed as a back-up filter in the event of a dryer malfunction, or in place of a drying system when humid air is acceptable as long as it contains no liquid water.

Fig. 4.14 A typical Water Removal Filter

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P N E U M A T I C T E C H N O L O G Y

Filter Cartridge

Metal Bowl

Viewing Glass

Drain Valve

Main li ne filte r

A large capacity filter should be installed after the air receiver to remove contamination, oil vapors from the compressor, and water from the air. Proper selection must be sized according to the system flow. In some cases there are two main line filters (one in reserve serving as backup during the filter element change --- which should be a regularly scheduled maintenance item).

This filter must have a minimum pressure drop and the capability to remove oil vapor from the compressor in order to avoid emulsification with condensation (seen as a white, milky liquid) in the line.

It has no deflector, which requires a certain minimum pressure drop to function properly as the “Standard Filter” discussed later in the section on Air Treatment. A built-in or an attached auto drain will ensure a regular discharge of accumulated water.

Fig. 4.15 Typical Line Filter

The filter is generally a quick-change cartridge type.

Note that many systems use one main-line filter after the compressor and a mist-separator (micro-filter) after

the drying system, as indicated on page 3.

AI R DISTRIBUT ION

The air main is a permanently installed distribution system carrying the air to the various consumers. Typically installed at the ceiling level (where the temperatures can be at their highest levels – which fosters entrained moisture), the air main can be a tremendous source of contamination in the installation process and during normal use.

During the installation process care must be taken to reduce the metal filings, pipe dope, Teflon tape, and other foreign materials that will be generated from assembly. The large size of most air mains makes contamination seem acceptable (a question of relativity at this point), yet when the contamination is seen relative to the extremely small tolerances in modern automation components (valves, actuators, grippers…) the effect can be disastrous.

If the air main comes in contact with outside air temperatures (connecting two buildings, perhaps being routed underground, etc.) it will serve as a moisture producer.

As many mains are iron pipe, rust is the eventual by-product. Careful examination should be made when reusing older pipes to create a new airline. If the opportunity presents itself and a new airline is to be created, consider the piping configuration as well.

There are two main layout configurations: DEAD END LINE and RING MAIN. After examining 4.16 and 4.17 it should become apparent that the Ring main configuration would be preferred for better supply flow. The additional cost is a one-time concern (for the additional pipe) but the advantages can be enjoyed every day of operation.

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D E AD E N D L I N E

P N E U M A T I C T E C H N O L O G Y

Fig. 4.16 Typical Dead End Line Mains

To assist drainage, the pipe should have a slope of about 1 in 100 in the direction of flow and it should be adequately drained. At suitable intervals the main can be brought back to its original height by using two long sweep right angle bends. A drain leg with automatic drain should be installed at each low point.

R I N G M AI N

Fig. 4.17 Typical Ring Main

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In a ring main system main air can be fed from two sides to a point of high consumption. This will reduce pressure drop. However this drives condensate in any direction and sufficient water take-off points with Auto Drains should be provided. Isolating valves can be installed to divide the air main into sections. This limits the area that will be shut down during periods of maintenance or repair.

S E C O ND A R Y L I N E S

Unless an efficient aftercooler and air dryer are installed, the compressed air distribution pipe acts as a cooling surface and water and oil will accumulate throughout its length.

Branch lines are taken off the top of the main to prevent water in the main pipe from running into them, instead of into drainage tubes that are taken from the bottom of the main pipe at each low point of it. These should be frequently drained or fitted with an automatic drain.

The Water remains

in the Pipe

The Water runs into the

Auto Drain

a b

Fig 4.18 Take-offs for air (a) and Water (b)

Auto drains are more expensive to install initially, but this is offset by the man-hours saved in the operation of the manual type. With manual draining neglect leads to compound problems due to contamination of the main.

Au t oma t ic D rai n s

Two types of automatic drains are shown in the Figures 4.19 and 4.20.

In the float type of drain. 4.19, the tube guides the float, and is internally connected to atmosphere via the filter, a relief valve, hole in the spring loaded piston and along the stem of the manual operator.

The condensate accumulates at the bottom of the housing and when it rises high enough to lift the float from its seat, the pressure in the housing is transmitted to the piston which moves to the right to open the drain valve seat and expel the water. The float then lowers to shut off the air supply to the piston.

Filter

Nozzle

Float

Drain Seat

Pressure

Relief Valve

Manual Operation

Fig. 4.19 Float Type Auto Drain

The relief valve limits the pressure behind the piston when the float shuts the nozzle. This pre-set value ensures a consistent piston re-setting time as the captured air bleeds off through a functional leak in the relief valve.

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P N E U M A T I C T E C H N O L O G Y

Fig 4.20 shows an electrically driven type, which periodically purges the condensate by a rotating cam wheel tripping a lever-operated poppet valve.

It offers the advantages of being able to

Synchronous

Motor

Cam Wheel

work in any orientation and is highly resistant to vibration, so lending itself to use in mobile compressors, and bus or truck pneumatic systems.

Manual

Operation

Fig. 4.20 Motorized Auto Drain

S I Z I N G C O M P R E S S E D A I R M A I N S

The cost of air mains represents a high proportion of the initial cost of a compressed air installation. A reduction in pipe diameter, although lowering the investment cost, will increase the air pressure drop in the system; potentially the operating costs will rise and will exceed the additional cost of the larger diameter piping.

Also, as labor charges constitute a large part of the overall cost, and, as this cost varies very little between pipe sizes, the cost of installing say a 25 mm diameter bore pipe is similar to that of a 50 mm Diameter pipe. But the flow capacity of the 50mm Diameter pipe will be four times that of 25 mm pipe. This additional volume may equal two or three (or more) receiver tank volumes, reducing compressor duty cycles.

In a closed loop ring main system, two pipe paths feed the supply for any particular take-off point. When determining pipe size, this dual feed should be ignored; assuming that at any time air will be supplied through one pipe only.

The size of the air main and branches is determined by the limitation of the air velocity, normally recommended at 6 m/s, while sub-circuits at a pressure of around 6 bar and a few meters in length may work at velocities up to 20m/s. The pressure drop from the compressor to the end of the branch pipe should not exceed 30 kPa. The nomogram (Fig 4.21) allows us to determine the required pipe diameter.

Bends and valves cause additional flow resistance, which can be expressed as additional (equivalent) pipe lengths in computing the overall pressure drop. Table 4.22 gives the equivalent lengths for the various fittings commonly used.

Example (a) To determine the size of pipe that will pass 16800 l/min of free air with a maximum pressure drop of

not more than 30 kPain 125 m of pipe. The 2 stage compressor switches on at .8 MPa and stops at 1.0 MPa; the average is .9 MPa.

30 kPa pressure drop in 125 m of pipe is equivalent to

30 kPa

125 m

=0.24 kPa / m.

Referring to Nomogram 4.21: Draw a line from .9 MPa on the pressure line through 0.24 kPa / m on the

pressure drop line to cut the reference line at X.

Join X to 0.28 m

/s and draw a line to intersect the pipe size lines at approximately 61 mm.

Pipe with a minimum bore of 61 mm can be used. A 65 mm nominal bore pipe (see Table 4.23) has a bore

of 68 mm and would satisfy the requirements with some margin.

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P N E U M A T I C T E C H N O L O G Y

Example (b) If the 125 m length of pipe in (a) above has a number of fittings in the line, e.g., two elbows, two 90°

bends, six standard tees and two gate valves, will a larger size pipe be necessary to limit the pressure drop

to 30 kPa? In Table 4.22, column “65 mm Diameter”, we find the following equivalent pipe length: Two elbows: 2· 1.4 m = 2.8 m Two 90° bends: 2 · 0.8 m = 1.6 m Six standard tees: 6 · 0.7 m = 4.2 m Two gate valves: 2 · 0.5 m = 1.0 m Total 9.6 m

The twelve fittings have a flow resistance equal to approximately 10 m additional pipe length. The “Effective Length” of the pipe is thus 125 + 9.6 135 m

And the allowed p / m:

Referring again to nomogram in fig 4.21: The pipe size line will now cut at almost the same diameter; a

nominal bore pipe of 65 mm, with an actual inner diameter of 68 mm will be satisfactory.

Note: The possibility of future air demands should be taken into account when determining the size of main lines for a

new installation. For that matter, compressors, after-coolers, receivers, filters, and dryers should also be sized with future increased demands in mind. Remember, cheaper only costs less once!

30 kPa

135 m

= 0.22 kPa / m

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P N E U M A T I C T E C H N O L O G Y

Line

Pressure

(

²p

kPa / m

= bar /100 m

Reference

3/8"

1.5

100

3.0

2.5

2.25

2.0

1.75

1.5

0.5

0.4

0.3

2.5"

0.2

.8 .9 .10

.11 .12

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.25

0.15

0.05

0.04

0.03

0.025

0.02

0.015

0.1

1.5"

1.25"

3/4"

0.2

0.01

0.15

1/2"

MPa)

Pipe Length

Line

Fig. 4.21 Nomogram for Sizing the Mains Pipe Diameter

(m /s

Inner Pipe Dia.

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P N E U M A T I C T E C H N O L O G Y

Type of Fitting Nominal pipe size (mm)

Elbow

90* Bend (long)

90* Elbow

180* Bend

Globe Valve

Gate Valve

Standard Tee

Side Tee

Table 4.22 Equivalent Pipe Lengths (meters) for the main fittings

Mate r ial s fo r Pi p in g

Traditionally, any conductor that is threaded is referred to as a pipe; others would be either tube or hose.

Sta n dar d Ga s Pi p e ( S GP)

The air main is usually a steel or malleable iron pipe. This is obtainable in black or galvanized form, which is

less liable to corrode. This type of piping can be screwed to accept the range of proprietary malleable fittings. For over 80 mm Diameter, welded flanges are often more economical to install rather than cut threads into large pipes. The specifications of the Carbon Steel Standard Gas Pipe (SGP) are:

100 4 114.3 4.5 12.100

15 20 25 30 40 50 65 80 100 125

0.3 0.4 0.5 0.7 0.8 1.1 1.4 1.8 2.4 3.2

0.1 0.2 0.3 0.4 0.5 0.6 0.8 0.9 1.2 1.5

1.0 1.2 1.6 1.8 2.2 2.6 3.0 3.9 5.4 7.1

0.5 0.6 0.8 1.1 1.2 1.7 2.0 2.6 3.7 4.1

0.8 1.1 1.4 2.0 2.4 3.4 4.0 5.2 7.3 9.4

0.1 0.1 0.2 0.3 0.3 0.4 0.5 0.6 0.9 1.2

0.1 0.2 0.2 0.4 0.4 0.5 0.7 0.9 1.2 1.5

0.5 0.7 0.9 1.4 1.6 2.1 2.7 3.7 4.1 6.4

Nominal Width Outside

Diameter.

A B mm mm kg/m

6 1/8 10.5 2.0 0.419

8 1/4 13.8 2.3 0.652 10 3/8 17.3 2.3 0.851 15 1/2 21.7 2.8 1.310 20 3/4 27.2 2.8 1.680 25 1 34.0 3.2 2.430 32 1 1/4 42.7 3.5 3.380 40 1 1/2 48.6 3.5 3.890 50 2 60.3 3.65 5.100 65 2 1/2 76.1 3.65 6.510 75 3 88.9 4.05 8.470

Thickness Mass

T a ble 4.2 3 Pi p e Si ze Spe cifi cat io n

Sta i nle s s s t eel pip e s

These are primarily used when very large diameters in long straight main lines are required.

Cop p er Tube

Where corrosion, heat resistance and high rigidity are required, copper tubing up to a nominal diameter of 40 mm can be used, but will be relatively costly over 28-mm. diameter. Compression fittings used with annealed quality tubing provide easy working for installation.

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P N E U M A T I C T E C H N O L O G Y

Rub b er Tube (“ A ir H ose ” )

Rubber hose or reinforced plastic is most suitable for air actuated hand tools as it offers flexibility for freedom of movement for the operator. The dimensions of Pneumatic Rubber Hose are:

Nominal Width,

inches

Outside

Diameter. Mm

Inside Dia.

Inner Sectional Area

mm2 1/8 9.2 3.2 8.04 1/4 10.3 6.3 31.2 3/8 18.5 9.5 70.9 1/2 21.7 12.7 127 5/8 24.10 15.9 199 3/4 29.0 19.0 284

1 35.4 25.4 507 1 1/4 45.8 31.8 794 1 1/2 52.1 38.1 1140 1 3/4 60.5 44.5 1560

2 66.8 50.8 2030

2 1/4 * 81.1 57.1 2560

2 1/2* 90.5 63.5 3170

Table 4.24 Rubber hose Specification* Cloth-wrapped hose

*Rubber hose is mainly recommended for tools and other applications where the tube is exposed to

mechanical wear.

Pla s tic tub i ng

Plastic tubing is commonly used for the interconnection of pneumatic components. Within its working temperature limitations it has obvious advantages for installation, allowing easy cutting to length, and rapid connection by either compression or quick-fit fittings.

If greater flexibility for tighter bends or constant movement is required, softer grades of nylon or polyurethane are available, but may have lower maximum safe working pressures. Be aware that plastic tubing is sized by its Outside Diameter (OD), not its Internal Diameter (ID). For example, a ¼” tube has a typical I.D. of only 0.125”.

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Fi t tin g s i n Sy s tem s

In systems, pneumatic components are connected by various methods.

The INSERT type provides a reliable retaining force inside and outside of the tube. The tube is pressed by the sleeve when screwing in the cap nut. The tube (insert) entering into the tube reduces its inner diameter and thus represents a considerable extra flow resistance.

Insert sleeves are not reusable.

The PUSH - IN connection has a large retaining force and the use of a special profile seal ensures positive sealing for pressure and vacuum. There is no additional flow restriction, as the connection has the same inner flow section as the inner diameter of the fitting tube.

Reusable for hundreds of insertions.

The SELF-SEALING fitting has a built in mechanism so that air does not exhaust after removal of the tube and is also applicable for copper free applications.

a. If no tube is pushed in, a check

valve shuts off the fitting.

When a tube is inserted, it opens

the airflow by pushing the check valve

from its seat. Note that the check valve

creates a restriction to flow, compared to

a standard fitting, so care must be taken

to properly size the fitting.

Fig. 4.25 Example of an Insert Fitting.

Fig. 4.26 Example of a Push-in Fitting, elbow type

a b

Fig. 4.27 Example of a Self-Seal Fitting.

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Page 49

P N E U M A T I C T E C H N O L O G Y

Deflector

Filter Element

Vortex

Baffle Plate

Quiet Zone

Bowl Guard

Drain Valve

Clean Air

Bowl

Pilot Valve

Drain Valve

Float

Symbol

Filter/Separat

Symbol Filter/Separator

with Auto Drain

55 AAIIRR TTRREEAATTMMEENNTT

As described previously, all atmospheric air carries both dust and moisture. After compression, moisture condenses out in the aftercooler and receiver but there will always be some that will be carried over. Moreover fine particles of carbonized oil, pipe scale and other foreign matter, such as worn sealing material, form gummy substances. All of this is likely to have injurious effects on pneumatic equipment by increased seal and component wear, seal expansion, corrosion and sticking valves.

To remove these contaminants, the air should be further cleaned (filtered) as near as possible to the point of use. Air treatment also includes Pressure Regulation and occasionally Lubrication.

FILTERI NG

S T AN D A R D F IL T E R

The standard filter is a combined water separator and filter. If the air has not been de-hydrated beforehand, a considerable quantity of water will be collected and the filter will hold back solid impurities such as dust and rust particles.

The water separation occurs mainly by a rapid rotation of the air, caused by the deflector at the inlet. The heavier particles of dirt, water and oil are thrown outwards to impact on the wall of the filter bowl before running down to collect at the bottom. The liquid can then be drained off through a manual drain cock or an automatic drain. The baffle plate creates a quiet zone beneath the swirling air, preventing the separated liquid from being reentrained into the air stream.

Fig. 5.1 Typical Filter/Water Separator and an Automatic Drain as option

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P N E U M A T I C T E C H N O L O G Y

The filter element removes the finer particles of dust, rust scale and carbonized oil as the air flows through to the outlet. The standard element will remove all contamination particles down to 5 microns in size. Some elements can be easily removed, cleaned and re-used a number of times before needing to be replaced because of excessive pressure drop.

The bowl is typically made from polycarbonate. For safety a metal bowl guard must protect it. For chemically hazardous environments special bowl materials must be used. If the bowl is exposed to heat, sparks, etc, a metal bowl should be used.

If the condensate accumulates at a high rate it is desirable to provide automatic draining.

The right hand side of Fig. 5.1 shows a float type of auto drain unit built-in for standard filters.

M I C R O F I L T E R S O R C O AL E S C E R S – (sometimes referred to as Mist Separators)

Where contamination by oil vapor is undesirable, a micro-filter is used. Being a pure filter it is not equipped with a deflector plate.

The air flows from the inlet to the center of the filter cartridge then outwards through the outlet.

Dust is trapped within the micro filter element. The oil vapor and water mist is converted into liquid by a coalescing action within the filter material, forming drops on the filter cartridge to collect at the bottom of the bowl. Approximately 99% of oil mist can be removed.

Cartridge

Perforated Stainless Steel Plate

Filtering Tissue

0.3 µm

PVC Sponge

Filter Paper, 4µm

ISO Symbol

Multistage Filter

S U B - M I CR O F I L T E R S

electrostatic spray painting, cleaning and drying of electronic assemblies etc --- the principle of operation is the same as a micro filter, but its filter element has additional layers with a higher filtration efficiency.

F I L T E R S EL E C T I O N

The size of air filter that is required for a particular application is dependent on two factors: -

a) The maximum flow of compressed air used by the pneumatic equipment.

b) The maximum acceptable pressure drop for the application. Manufacturers provide flow/pressure diagrams to enable correct sizing to be done. It should be noted that standardizing on one large filter for every application might result in water not being

separated as efficiently due to the lower flow velocity resulting from using a larger filter.

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P N E U M A T I C T E C H N O L O G Y

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P N E U M A T I C T E C H N O L O G Y

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P N E U M A T I C T E C H N O L O G Y

Relieving

p1p

PRE SSUR E RE GUL ATI ON

Regulation of pressure is necessary because at pressures above optimum, rapid wear will take place with little

or no increase in output. Air pressure that is too low is uneconomical because it results in poor efficiency.

S T AN D A R D R EG U L A T OR

Pressure regulators have a piston or

diaphragm to balance the output pressure against an adjustable spring force.

The secondary pressure is set by the

adjusting screw loading the setting spring to hold the main valve open, allowing flow from the primary pressure p1 inlet port to the secondary pressure p2 outlet port. Then the pressure in the circuit connected to the outlet rises and acts on the diaphragm, creating a lifting force against the spring load.

When consumption starts, p2 will initially

drop and the spring, momentarily stronger than the lifting force from p2 on the diaphragm, opens the valve.

If the consumption rate drops, p2 will slightly increase, this increases the force on the diaphragm against the

spring force --- diaphragm and valve will then lift until the spring force is equaled again. The airflow through the valve will be reduced until it matches the consumption rate and the output pressure is maintained.

Fig 5.5. Principle of the Pressure Regulator

Adjusting Knob

Adjusting Spindle

Setting Spring

Diaphragm Disc

Diaphragm

Valve

Valve Spring

If the consumption rate increases, p2 will slightly decrease. This decreases the force on the diaphragm against

the spring force, diaphragm and valve drop until the spring force is equaled again. This increases the airflow through the valve to match the consumption rate.

Without air consump-

tion the valve is closed. If the secondary pressure rises above the set value by virtue of:

• re-setting the

regulator to a lower outlet pressure, or

• an external reverse

thrust from an actuator,

the diaphragm will lift to

open the relieving seat so that excess pressure can be bled off through the vent hole in the regulator body.

Do NOT rely on this

orifice as an exhaust flow path.

Fig. 5.6 Relieving Function

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P N E U M A T I C T E C H N O L O G Y



Flow Compensated Regulator (Q-

Compensation)

With very high flow rates the valve is wide

open. The spring is therefore elongated and thus weaker and the equilibrium between p2 on the diaphragm area and the spring occurs at a lower level. This problem can be corrected by creating a third chamber with a connection to the output channel. In this output channel the flow velocity is high. As explained in section 3, the static pressure is then low (Bernoulli). As p3 is now at a lower static pressure, the balance against the weakened spring at high flow rates is compensated, thus allowing the valve to remain open longer.

The effect can be improved by inserting a tube

in the connection, cut at an angle with the opening oriented towards the outlet (Fig 5.8). The angled tube creates a venturi effect, thus lowering the pressure at p3 even more as a way to compensate for high flow rates.

p1 p2

Connection

Fig. 5.7 Principle of a Flow Compensated

Regulator

Pressure Compensation

There is still an inconvenience in the regulator

of Fig. 5.7: if the inlet pressure p1 increases, a higher force is acting on the bottom of the valve, trying to close it. That means that an increasing input pressure decreases the output pressure and vice versa. A valve having equal surface areas for both input and output pressure in both directions can eliminate this. This is realized in the regulator of Fig. 5.8

The most important parts are:

Adjusting Spindle Setting Spring Relieving Seat Diaphragm Flow Compensation Chamber Flow Compensation Connection Tube







 







Valve

O-Ring for Pressure Compensation Valve Spring O-Ring for Flow Compensation

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

Fig. 5.8 Fully compensated Pressure Regulator

Page 55

P N E U M A T I C T E C H N O L O G Y

Adjusting Knob

Pilot Pressure Relief

Setting Spring

Main Diaphragm

Pilot Valve

Main Valve

Main Secondary

Pressure Relief

Pilot

p 1 p 2

P I L O T O P ER A T E D R E G U L A T OR

The pilot operated regulator offers greater accuracy of pressure regulation across a large flow range. This accuracy is obtained by replacing the setting spring of a standard regulator with pilot pressure from a

small pilot regulator sited on the unit.

The pilot regulator on top of the unit supplies or exhausts pilot air only during corrections of the output

pressure. This enables the regulator to achieve very high flow rates but keeps the setting spring length to a minimum.

Main Valve

Fig 5.9 Pilot Pressure Regulator

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P N E U M A T I C T E C H N O L O G Y

further

2000

4000

6000

.1 MPa

(l /min)

(MPa)

F I L T E R - R EG U L A T O R

Air filtering and pressure regulation is combined in the

single filter regulator to provide a compact space saving unit.

Siz ing of R eg u lat o rs a nd F ilt ers

A regulator size is selected to give the flow required by the application with a minimum of pressure variation across the flow range of the unit.

Manufacturers provide graphical information regarding the flow characteristics of their equipment. The most important is the Flow / p2 diagram. It shows how p2 decreases with increasing flow. (Fig. 5.11). The curve has three distinct portions:

1. The inrush, with a small gap on the valve that does not yet allow real regulation

2. The regulation range and

3. The saturation range; the valve is wide open and

regulation is impossible

p 2

(MPa)

²p

Q (l/min)

.3 MPa

.5 MPa

0.05

Fig. 5.11 Typical Flow/Pressure Characteristics:

a: Regulator, b: Filter

.7 MPa

Fig 5.10 Typical Filter Regulator

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P N E U M A T I C T E C H N O L O G Y

FRL elements have to be sized in accordance with the required flow capacity. For Regulators, the average volume flow should be the one in the middle of the regulating range (II in fig.5.11 a). The pressure drop also defines the size of the filter. For a “Standard Filter/Separator" (not a Line Filter), a minimum pressure drop of about

0.02 MPa (3 psi) is required to ensure functioning. With maximum flow, Æp (allowable or desirable delta p) should however be kept between .02 – .06 MPa (3 – 7 psi).

The size is therefore defined by the required flow, not by the connection size of the component. Modular systems give the capability to adapt the connection thread to the available tube size.

COMP RES SED AI R LU BRICAT ION

Lubrication is no longer a necessity. The vast majority of modern pneumatic components is now available permanently lubricated, and will operate reliably with no additional lubrication.

The life and performance of these components is fully up to the requirements of modern high cycling process machinery.

The advantages of "non-lube" systems include:

a) Savings in the cost of lubrication equipment, lubricating oil and maintaining oil levels. b) Cleaner more hygienic systems; of particular importance in food and pharmaceutical industries. c) Oil free atmosphere, for a healthier, safer working environment.

Certain equipment still requires lubrication. To ensure they are continually lubricated, a certain quantity of oil is added to the compressed air by means of a lubricator.

P R O P O RT I O N A L L UB R I C A T OR S

In a (proportional) lubricator a pressure drop between inlet and outlet, directly proportional to the flow rate, is created and lifts oil from the bowl into the sight feed dome.

With a fixed size of restriction, a greatly increased flow rate would create an excessive pressure drop and produce an air/oil mixture that had too much oil, flooding the pneumatic system.

Conversely a decreased flow rate may not create sufficient pressure drop resulting in a mixture that is too lean.

To overcome this problem, lubricators must have self-adjusting cross sections to produce a constant mixture.

Air entering a lubricator (as shown in Fig 5.12) follows two paths: it flows over the damper vane to the outlet and also enters the lubricator bowl via a check valve.

When there is no flow, the same pressure exists above the surface of the oil in the bowl, in the oil tube and the sight feed dome. Consequently there is no movement of oil.

When air flows through the unit, the damper vane restrictor causes a pressure drop between the inlet and outlet. The higher the flow, the greater the pressure drop.

Since the sight feed dome is connected by the capillary hole to the low-pressure zone immediately after the damper vane, the pressure in the dome is lower than that in the bowl.

This pressure difference forces oil up the tube, through the oil check valve and flow regulator into the dome.

Once in the dome, the oil seeps through the capillary hole into the main air stream in the area of the highest air velocity. The oil is broken up into minuscule particles, atomized and mixed homogeneously with the air by the turbulence in the vortex created by the damper vane.

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P N E U M A T I C T E C H N O L O G Y

Oil Check Valve

Oil Tube

Sight Feed Dome

Refill Plug

Capillary

Connection

Air

Check

Valve

Damper Vane

Oil Throttle

Bowl

Bowl Guard

Sintered Bronze

Oil Filter

Fig 5.12 Proportional Lubricator

The damper vane is made from a flexible material to allow it to bend as flow increases, widening the flow path, to proportionally adjust the pressure drop and thus maintain a constant mixture throughout.

The oil throttle allows adjustment of the quantity of oil for a given pressure drop. The oil check valve retains the oil in the upper part of the tube when the airflow temporarily stops.

The air check valve allows the unit to be refilled under pressure, while work can normally go on.

The correct oil feed rate depends on operating conditions, but a general guide is to allow one or two drops per cycle of the machine.

A pure (no-additives) mineral oil of 32 centi-stokes viscosity is recommended (ISO standard VG32). Some oil companies have special oil for compressed air lubrication, with a high capacity to absorb moisture without loss of lubricating properties.

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F.R. L. UNIT S

P N E U M A T I C T E C H N O L O G Y

Lubricator

Regulator

Modular filter, pressure regulator and lubricator elements can be combined into a service unit by joining with spacers and clamps. Mounting brackets and other accessories can be easily fitted in more recent designs.

S I Z E AN D I N S T A L L AT IO N

The combination unit must again be sized for the maximum flow rate of the system. Manufacturers will generally provide this information.

Most systems require an approved shut-off or lock out valve. In addition, there are devices that allow an Emergency Stop function and a slow start option, where air is introduced to the system at a reduced rate.

For correct placement and operation of these devices consult the manufacturers’ instructions. For maintenance there should be a way to stop airflow after the F.R.L. unit and before the unit, isolating the F.R.L. for repair. In most cases, the Emergency Stop should be downstream of the F.R.L. to prevent backflowing (reverse flow) the filter (which could cause element collapse), the regulator (diaphragm could be damaged), and the lubricator (driving oil mist inside the filter element).

Fig. 5.13 Typical FRL Unit in a modular design

Filter

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P N E U M A T I C T E C H N O L O G Y

Stop

Spring

Sintered Bronze Filter

Piston

ISO Symbol :

66 AACCTTUUAATTOORRSS

The work done by pneumatic actuators can be linear or rotary. Piston cylinders provide linear movement. Vane type or rack and pinion type rotary actuators typically produce reciprocating rotary motion up to 270°. Air motors are used for continuous rotation.

LINE AR CY LIND ERS

Pneumatic cylinders of varying designs are the most common power components used in pneumatic automation. There are two basic types from which special constructions are derived:

• Single-acting cylinders with one air inlet to produce a power stroke in one direction

• Double-acting cylinders with two air inlets to produce extending and retracting power strokes

S I N G L E A C T IN G C Y L I N D E R

A single acting cylinder develops thrust in one direction only. The piston rod is returned by a fitted spring or by external force from the load or spring.

It may be a "push" or "pull" type (Fig 6.1)

Fig. 6.1 Typical Single Acting Cylinder, Spring Retracted or “Push” type

Single acting cylinders are used for clamping, marking, ejecting etc. They have a somewhat lower air consumption compared with the equivalent size of double acting cylinder. However there is a reduction in thrust due to the opposing spring force, and so a larger bore may be required. Designing the cylinder to accommodate the spring results in a longer overall length and a limited stroke length.

D O U B L E AC TI N G C Y L I N D E R

With this actuator, thrust is developed in both extending and retracting directions as air pressure is applied alternately to opposite sides of a piston. The thrust available on the retracting stroke is reduced due to the smaller effective piston area, but is only a consideration if the cylinder is to "pull" the same load in both directions.

ISO Symbol :Rod Seal /

Scraper

Rod Bearing

Fig. 6.2 Double Acting Cylinder

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P N E U M A T I C T E C H N O L O G Y

Front or Rod Cover

Port

Barrel Seal

Cylinder

Piston

Seal

Magnetic

Ring

Back or

Cover

Cushion Seal

Cushion

Barrel

Tie Rod

Tie Rod Nut

Piston Rod

Scraper Ring /

Rod-Seal

ISO Symbol

Cyl ind e r C o nst r uct i on

The construction of a double acting cylinder is shown. The barrel is normally made of seamless tube which may be hard coated and super-finished on the inner working surface to minimize wear and friction. The end caps may be aluminum alloy or malleable iron castings held in place by tie-rods, or in the case of smaller cylinders, fit into the barrel tube by screw thread or crimped on. Aluminum, brass, bronze, stainless steel or synthetic material may be used for the cylinder body. Stainless steel or composite fiber synthetics are often used in aggressive or unsafe environments.

Barrel or Tube

Guiding or Wear

Head

Rod End

Blind End

Fig. 6.3 Component parts of a double acting air cylinder with air cushions

Cu s hio n ing

Pneumatic cylinders are capable of very high speed and considerable shock forces can be developed at the end of the stroke. Smaller cylinders often have fixed cushioning, i.e. rubber bumpers, to absorb the shock and prevent internal damage to the cylinder. On larger cylinders, the impact effect can be absorbed by an air cushion that decelerates the piston over the last portion of the stroke. This cushion traps some of the exhausting air near the end of the stroke before allowing it to bleed off more slowly through an adjustable needle valve (Fig.6.4).

Adjusting Screw

Fig. 6.4 Principle of the Air Cushion

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P N E U M A T I C T E C H N O L O G Y

The normal escape of the exhausting air to the outlet port is closed off as the cushion piston enters the cushion seal, so that the air can only escape through the adjustable restriction port. The trapped air is compressed to a relatively high pressure, which reduces the inertia of the piston, acting as a brake.

When the piston reverses, the cushion seal acts as a check valve to allow airflow to the piston. It however restricts the airflow and delays the acceleration of the piston. The cushioning stroke should therefore be as short as possible.

To decelerate heavy loads or high piston speeds, an external shock absorber is required. If the piston speed exceeds about 500 mm/s an external mechanical stop must be provided, which is also the case with built-in cushioning.

S P E C IA L C YL I N D E R O P T IO N S

Do u ble Rod

ISO Symbol

Fig. 6.5 Principle of the double rod

A double rod makes a cylinder stronger against side load, as it has two bearings at the widest distance possible. This type of cylinder is often mounted with the rods fixed and the cylinder itself moving to displace a part.

No n Ro t ati ng R od

The piston rod of a standard cylinder rotates slightly as there is no guide to prevent this. Therefore it is not possible to directly mount a tool, e.g. a cutting blade.

For this kind of application, where no considerable torque is exercised on the tool, a cylinder with non-rotating rod can be used. The suppliers specify the maximum allowable torque. As Fig. 6.6 shows, two flat planes on the rod and a fitting guide prevent the rotation.

It shows also how a torque creates a high force on the edges of the rod profile, which will damage it in the long run.

Tw i n R o d

This type of cylinder has a high lateral load resistance and high non-rotating accuracy. These compact dual rod cylinders are of high precision and ideal for pick and place operations. Do not assume that the dual cylinders equal the theoretical force of one larger cylinder’s theoretical force, e.g. two 25 mm. bores in a dual rod cylinder produce half the force of one 50 mm bore cylinder (prove this).

Fig. 6. 6 Non-Rotating Rod

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P N E U M A T I C T E C H N O L O G Y

Symbol:

Unofficial:

ISO:AA

Section A-A

Fig. 6.7 Twin Rod Cylinder

Gu i ded Cyl i nd e r

Guided cylinders operate with one cylinder and a guide, also providing lateral load resistance and non-rotating accuracy. There are two types of guide cylinders:

1. Slide Bearing, which is used to resist lateral movement.

2. Ball Bushing, which is used for pushing and lifting where greater linearity is needed.

Fig. 6.8 Guided Cylinder

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Fig 6.9 Compact Guided Cylinder

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P N E U M A T I C T E C H N O L O G Y

ISO Symbol :

Sli d e T abl e

A precision guided work table and cylinder are combined to provide even greater linearity, lateral load resistance, non-rotating accuracy.

Fig 6.10 Examples of Slide Tables

Fl a t C y lin d er

A cylinder normally has square covers and, generally, a round cylinder. By stretching the piston to a relatively long rectangular shape with round ends, it achieves the same force as a conventional cylinder. The advantage, of course, is the saving in space achieved if they are to be stacked together. This is suitable for most non-rotating applications.

Section A-A

ISO Symbol :

Fig. 6.11 Principle of a Flat Cylinder

Tan dem Cyl i nde r

A tandem cylinder consists of two double acting cylinders joined together with a common piston rod to form a single unit.

Fig. 6.12 Principle of the Tandem Cylinder

By simultaneously pressurizing both cylinder chambers the output force is almost double that of a standard cylinder of the same diameter. It offers a higher force from a given diameter of cylinder; therefore it can be used where installation space restricts the diameter that can be fitted.

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P N E U M A T I C T E C H N O L O G Y

012

100

200

300

Stroke Lengths

Positions

ISO Symbols :

Mul t i P o sit i on C yli n de r

The two end positions of a standard cylinder provide two fixed positions. If more than two positions are required, a combination of two double acting cylinders may be used.

There are two principles: For three positions, the assembly on the left is required; it enables users to mount the cylinder body to the

machine. It is very suitable for vertical movements, e.g. in handling devices. The second is to mount two independent cylinders together back to back. This allows four different positions,

but one rod must be attached to the machine, and the cylinder body and other rod must be allowed to move. A combination with three cylinders of different stroke length gives 8 positions, one with four 16, but a rather exotic structure is required and the movement, when cylinders run in opposite directions, is very unstable.

200 100

Fig. 6.13 Three and four position cylinder

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P N E U M A T I C T E C H N O L O G Y

C Y L I N D ER M O U N T I N G

To ensure that cylinders are correctly mounted, manufacturers offer a selection of mountings to meet all requirements including pivoting movement using swivel type mountings.

Direct

Foot

Rear flange

Rear Clevis

Threaded neck

Front flange

Trunnion (mid)

Fig. 6.14 The various methods of Cylinder Mounting

Fl o ati n g J o in t s-- - Ro d A l ign m ent Co u ple r s

To accommodate unavoidable "misalignment" between the cylinder rod movement and the driven object, a floating joint must be fitted to the piston rod end.

The investment in these devices will insure longer cylinder life and more reliable operation --far exceeding the cost of the device itself.

Fig 6.15 “Floating joint”

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Co l umn Str e ng t h

on one side and



P N E U M A T I C T E C H N O L O G Y

When excess thrust is applied to a cylinder the buckling strength must be taken into consideration. This excess thrust can manifest itself when there is

1 -: Compressing Stress.

2 -: If the stressed part, i.e. a cylinder, is long and slender.

The buckling strength depends greatly upon the mounting method. There are four main cases:

1. Rigidly fixed

loose at the opposite end.

2. Pivoting on both ends.

3. Rigidly fixed on one side,

pivoting on the other.

4. Rigidly fixed at both ends.

The above-mentioned conditions apply if a cylinder lifts or pushes a load; it is then subjected to compressing stress. If a certain specified stroke length is exceeded, the cylinder can “break out’ sideways and seize thus rendering the cylinder useless. To avoid unnecessary loss of time and money, check with the “buckling length table” in the supplier’s catalogue. The general rule of thumb is if the stroke of cylinders above 50 mm bore is three times the diameter or, in the case of smaller cylinders, the stroke is five times the bore and the cylinder is pushing a load.

Fig. 6.16 The four mounting cases

CYLI NDER SI ZING

C Y L I N D ER F O R C E ( T H E OR E T I C A L F O R C E )

Linear cylinders have the following standard diameters as recommended in ISO:

8, 10, 12, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 140, 160, 200, 250, 320 mm

The force developed by a cylinder is a function of the piston diameter, the operating air pressure and the frictional resistance. For the theoretical force, the thrust on a stationary piston, the friction is neglected. This, theoretical force, is calculated using the formulae Force = Pressure * Area (F = PA):

Force (N) = Piston area (m2) · air pressure (N/m2), or

Force (lbf.) = Piston area (in2) · air pressure (lbf./in2)

Thus for a double acting cylinder:

Extending stroke: FE =

Where (D = piston diameter, pg = Working (gauge) pressure)

Retracting stroke: FR =

· D2 · pg

· (D2 - d2) · pg where (d = piston rod diameter)

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P N E U M A T I C T E C H N O L O G Y



25 30

50 63 80

4 6 8 10 12 20 16

And for a single acting cylinder:

E s

It may be quicker to use a diagram such as the one in Fig. 6.17, showing the theoretical force for 1.0, .7 and .5 MPa, or any similar supplier’s information to select a cylinder size.

2.5

1000

ø (mm)

· D2 · pg - Fs (Fs = Spring force at the end of stroke)

100000

500 400

(N)

300 250

200 150

125

100

25 20

12.5

2.5

Example: Determine the theoretical size of a cylinder operating at a pressure of 6 bar that would generate a

clamping force of 1600 N.

32 40

Fig. 6.17 Theoretical Force of pneumatic cylinders, from 2.5 to 30 mm (left and top scales)

and from 32 to 300 mm (right and bottom scales) for 1.0, .7 and .5 MPa working pressure

(MPa)

1.0 .7 .5

100

125 140 160

ø (mm)

200

250 300

50000 40000

25000 20000

15000

12500

10000

5000 4000

2500 2000 1500

1250

1000

500 400

250

(N)

Assuming an extending stroke: - FE =

4 F

Transposing: D =

A 63 mm diameter cylinder would be selected, the larger size providing extra force to overcome frictional

resistance.

By using the diagram, we look for 1600 N on the Force Scale at the right side and find 1500 as a dashed line. We

follow it to the left until we reach a point between the Pressure Lines for .5 and .7 MPa and find an intersection between 50 and 63 mm on the Diameter Scale on the bottom. There is no doubt that the same diameter is correct for 1600N as well as 1500 N.

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p 



D2 * p

4 *1600 N

* 600000 N / m

= 0.0583 m = 58.3 mm.

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P N E U M A T I C T E C H N O L O G Y



F=G

= µ ·

=m/

2 ·

F =G

· (sin



+ µ · cos





)

Req uir e d F o rce

The required force depends on the mass of the load, the angle of movement or elevation, the friction, the

working pressure and the effective piston area.

The load consists of the Weight of the mass (Fig. 6.18 a), the Force R represented by the friction factor times mass (Fig. 6.18 b) and the required acceleration (Fig. 6.18 c). The re–partition of these forces depends on the angle of the cylinder axis with the horizontal plane (elevation) as shown in Fig. 6.18 d.

A horizontal movement (elevation = 0°) has only friction to overcome. Friction is defined by the friction coefficient µ, which varies between about 0.1 to 0.4 for sliding metal parts, and about 0.005 for iron, rolling on iron (0.001 for balls on the ring in a ball bearing). This coefficient enters the formula as a cosine, which varies from 1 for horizontal to 0 for vertical.

The mass represents a load, equal to its weight, when the movement is vertical (90° elevation). The weight is the force created by the earth's acceleration on the mass. The earth's acceleration equals, on a latitude of 450 (Standard for Europe and N. America), 9.80629 m·s-2 or 32.17 ft sec2. With a horizontal movement the weight is a zero load as it is fully born by the construction. The entire cylinder thrust is then available for acceleration. The load of the mass varies therefore with the inclination from 0 to 100%. Its value as a factor is the sine of the inclination angle, 0 for horizontal, and 1 for vertical:

b c

Fig 6.18 The component forces of the LOAD

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L O AD R A T I O

P N E U M A T I C T E C H N O L O G Y

This ratio is generally referred to as “Lo” and equals

A cylinder should not have a higher load ratio than about 85%. If an accurate speed control is required or load forces vary widely, 60-70% should not be exceeded --- perhaps no more than 50% in vertical applications.

Table 6.19 gives the Load Ratio for cylinders from 25 to 100 mm diameter and various elevations and two friction coefficients for rolling (0.01) and sliding steel parts (0.2).

Bore Mass (kg)

50 – – – – – – – .22 4.0 25 – (8.72) (9.67) 7.15 8.49 5.09 6.74 .1 2.0

12.5 5.18 4.36 4.83 3.57 3.45 2.54 3.37 0.05 1.0

90 - - - - – 0.22 4.39 45 - (9.56)

22.5 5.49 4.78 5.3 3.92 4.66 2.79 3.7 0.055 1.1

200 - – – _ – 0.2 4.0 100 – (8.7) (9.65) 7.13 8.48 5.08 6.73 0.1 2.0 50 5.0 4.35 4.83 3.57 4.24 2.54 3.36 0.05 0.0

100

800 – – – – 0.2 4.0 400 – (8.7) (9.65) 7.14 8.44 5.08 6.73 0.1 2.0 200 5.0 4.35 4.83 3.57 4.22 2.54 3.36 0.05 1.0

100 – – – – – – – .4 8.0

180 - - - - - 0.44 8.78

250 – – – – – – – 0.39 7.8 125 – – – – – (9.92) – 0.2 3.9

65 – – – 7.24 (8.6) 5.16 6.83 0.1 2.03 35 5.46 4.76 5.28 3.9 4.63 2.78 3.68 0.05 1.09

400 -- - - - 0.4 7.99

650 – – – – 0.41 8.18 300 – – – – 0.19 3.78 150 (9.44) 8.23 (9.12) 6.74 8.01 4.8 6.36 0.09 1.89

75 4.72 4.11 4.56 3.37 4.01 2.4 3.18 0.05 0.94

1000 – – – – 0.39 7.81

500 – – – – 0.2 3.9 250 (9.76) 8.5 (9.43) 6.97 8.28 4.96 6.57 0.1 1.95 125 4.88 4.25 4.71 3.48 4.14 2.48 3.28 0.05 0.98

1600 – – – – 0.4 7.99



60° 45° 30°

0.01

Required force

Theoretical Force

µ 0.2 µ

0.01

- 7.84 (9.31) 5.58 7.39 0.11 2.2

· 100%

µ 0.2 µ

0.01



µ 0.2 µ

0.01

µ 0.2

Table 6.19 Load Ratios for 0.5 MPa working pressure and friction coefficients of 0.01 and 0.2

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P N E U M A T I C T E C H N O L O G Y

A more practical help for finding the correct cylinder diameter would be to know the allowed load under various conditions. Therefore, Table 6.20 shows the mass of the total load in kg that results in a Load Ratio of 85%. It is based on .5 MPa working pressure on the cylinder and again the two friction coefficients 0.01 for rolling (left column) and 0.2 for sliding (right column). These values are the maximum mass of the total load.



60° 45° 30°



CYL Bore µ: 0.01 0.2 0.01 0.2 0.01 0.2 0.01 0.2

25 32 40 50 63 80

100

2.12 2.45 2.2 3.0 2.5 4.25 3.15 212.3 10.6

3.92 4.5 4.05 5.48 4.62 7.7 5.82 392.0 19.6

5.45 6.25 5.64 7.63 6.42 10.7 8.09 545.0 2.7.5

8.5 9.77 8.8 11.9 10.02 16.73 12.64 850.0 42.5

13.5 15.5 13.98 18.9 15.92 26.55 20.05 1350.0 67.5

21.77 25.0 22.55 30.5 25.67 42.8 32.35 2177.5 108.9

34.02 39.05 39.08 35.2 47.62 66.92 50.55 3402.0 170.1

Table 6.20 Mass in kg for cylinders from 25 to 100 mm Diameter for a Load Ratio of 85% with 0.5 MPa working pressure.

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P N E U M A T I C T E C H N O L O G Y

Retracting Side Cylinder Force (Double acting cylinder)

Fig. 6.21 Extending Side Cylinder Force (Double acting

cylinder)

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Fig. 6.22

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P N E U M A T I C T E C H N O L O G Y

S P E E D C O N T R O L

The speed of a cylinder is defined by the extra force behind the piston, above the force opposed by the load. The load ratio should never exceed 85% approx. The lower the load ratio the better the speed control, especially when the load is subject to variations. A positive speed control is obtained by throttling the exhaust of the cylinder by means of a “Speed Controller”, which is a combination of a check valve, to allow free flow towards the cylinder, and an adjustable throttle (needle valve). An example of speed control is shown in the section on valves in the chapter on Auxiliary Valves. To get a constant speed, the Load Ratio should be approximately 75% or less.

Force is mass (W/g) times acceleration. The units are for force: kg · m · s-2 and for acceleration: m · s-2. In English units W = lbs and g = 32.17 ft/sec2.

Example: Mass of the load 100 kg, working pressure .5 MPa, Cylinder Diameter 32 mm, horizontal movement

with a friction coefficient of 0.2. The theoretical force is 401.2 N

Table 6.19 shows this case and 90 kg mass a load ratio of 43.9 %.

Thus for 100 kg: 43.9 ·

100

= 48.8 %.

The Force of the load is 48.8% of 401.92 N = 196 N. With a cylinder efficiency of 95%, 95 - 48.8% = 46.2%

of the force is left for the acceleration of the load. This is 185.7 N. The acceleration is therefore: 185.7 kg · m · s-2 / 100 kg = 1.857 m· s-2. Without control, the piston would theoretically approach 2 m/s after one

second. “Theoretically” means, if there is no limitation to the access of compressed air behind and no backpressure in front of the piston.

The limitation of the exhaust airflow creates a pneumatic load, which is defined by the piston speed and the volume flow through the restriction of the speed controller. Any increase of the piston speed increases the opposing force. This limits and stabilizes the piston speed. The higher the pneumatic part of the total load is, the stronger it can stabilize the piston speed.

With a load ratio of 85% and a cylinder efficiency of 95%, 10 percent of the force is stabilizing the pneumatic load. When the mechanical load shows a variation of ± 5% there is a compensation of half the influence. With a load ratio of for example 50%, these variations will no longer have any visible effect on the speed.

Note: For a subtle speed control, the flow capacity of the tube has to be much higher than that of the speed controller setting. With a tube that is too small in diameter the tube limits the flow and changing the needle position has little effect.

AI R F L O W A N D C O N S U M P T I O N

There are two kinds of air consumption for a cylinder or pneumatic system.

The first is the average consumption per hour, a figure used to calculate the energy cost as part of the total cost price of a product and to estimate the required capacity of compressor and air main.

The second is the peak consumption of a cylinder required to ascertain the correct size of its valve and connecting tubes, or for a whole system, to properly size the F.R.L. unit and supply tubes.

The Air Consumption of a cylinder is defined as:

Piston area · Stroke length · number of single strokes per minute · absolute pressure in bar,

Explanation: When the piston is against the cylinder cover (Fig. 6.23 a), the volume is zero. When we pull the

rod out until the piston is on the opposite end, the cylinder is filled with atmospheric pressure of 101325 Pa

(Fig. 6.23 b). When the pressure from the supply enters, the swept volume times the gauge pressure

abs

in bar is added, in addition to the atmospheric pressure of 101325 Pa.

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1.0

)1.0(P



1.0

)1.0(P



q = Air consumption required for one stroke of air cylinder

1.0

qqqqq





qqq







b c

Fig 6.23 Theoretical Air Consumption of a cylinder

The theoretical air consumption of a cylinder is conceptualized as indicated in Fig. 6.23. To further clarify, see the following formulae:

1. The air consumption of a cylinder equals:

LAq

1c1

qc = Air consumption of air cylinder [dm3(ANR)]*

qp = Air consumption of tubing or piping [dm3(ANR)]

[dm3(ANR)]

A = Piston area at extension side [mm2]*

L = Cylinder stroke [mm]

2. The consumption of each tube between valve and cylinder equals:

3. The total consumption equals: Double Acting Cylinder Single Acting Type Cylinder

11p1





 

p2c2p1c1





P = operating pressure [MPa]*

ℓ = Piping length [mm]

a = Piping internal sectional area [mm2] *Subscript 1: Extension side *Subscript 2: Retraction side

LAq

2c2



22p2

p1c1



 





4. Required flow [L/min(ANR)] equals:

p1c1





Q = Required flow [dm3/min (ANR)]

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p2c2





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P N E U M A T I C T E C H N O L O G Y

T = time for whole stroke Subscript 1: Extension side Subscript 2: Retraction side Components should be sized to the larger of the two required flows calculated above. For more precise calculation of air consumption and required air in accordance with specific conditions, please

make use of SMC’s “Model Selection Program” and “Energy Saving Program” @ smcusa.com

Table 6.24 gives the theoretical air consumption per 100 mm of stroke, for various cylinder diameters and working pressures:

Working Pressure in MPa

Piston dia. 3 4 5 6 7

20 25 32 40 50 63 80

100

0.0124 0.0155 0.0186 0.0217 0.0248

0.0194 0.0243 0.0291 0.0340 0.0388

0.0319 0.0398 0.0477 0.0557 0.0636

0.0498 0.0622 0.0746 0.0870 0.0993

0.0777 0.0971 0.1165 0.1359 0.1553

0.1235 0.1542 0.1850 0.2158 0.2465

0.1993 0.2487 0.2983 0.3479 0.3975

0.3111 0.3886 0.4661 0.5436 0.6211

Table 6.24 Theoretical Air Consumption of double acting cylinders from 20 to 100 mm diameter,

in liters ANR per 100 mm stroke

Example 1. Find the energy cost per hour of a double acting cylinder with an 80 mm diameter and a 400 mm.

stroke with 12 double strokes per minute and a working pressure of .6 MPa

In Table 6.24 we see that an 80 mm diameter cylinder consumes 3.5 liters (approx.) per 100 mm stroke

so:

Q /100 mm stroke x 400 mm stroke x number of strokes per min x forward and return stroke = 3.5 x 4

x 24 = 336 l/min ANR.

In the paragraph "Thermal and Overall Efficiency" in section #4, we find an electrical consumption of 1

kW for 0.12 - 0.15 m3/min with a working pressure of 7 bar. Therefore, to produce 1 m

/ min we require

approximately 8 Kw of electric power.

We are making the assumption that one kW-hr (kilowatt-hour) costs 5 cents.

The cost of producing a volume flow of 1 m

1 m

/min

· 40 cents / hr = 13.4 cents per hour.

/min

In our example:

0.336 m

/min is then:

5 ct · 8 kW

kW hr

= 40 cents / hr.

The sum of all the cylinders on a machine, calculated that way, represents the air consumption as energy cost.

It should however be noted that:

• the consumption figures in the above table do not include the “dead volume” at either end of the stroke, if any, or the air required to fill the connecting tubes for each stroke (see previous page)

• the transfer of energy is not without losses (see further below).

Peak Air Consumption of Actuators

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P N E U M A T I C T E C H N O L O G Y



The peak air consumption of an actuator is defined as follows:

Piston area x Piston speed x Absolute pressure



pvD

60)1.0(



000,100

Q = Peak flow [Nl/min] D = Piston diameter [mm] v = Piston speed [mm/s] p = Pressure [MPa]

Example: An actuator has a 63 mm bore, an average piston speed of 500 mm/s, and a supply pressure of 0.6 MPa. What is the peak consumption under these conditions?

)63(

Q 



MPaMPasmmmm

60)1.06.0(/500



min/654

000,100

The table 6.23 indicates the peak consumption of actuators at different speeds, at a supply pressure of 0.5MPa. If supply pressure differs from 0.5MPa; use the correction factor in table 6.26.

Average Piston Speed [mm/s] Bore size 100 200 300 400 500 600 700 800 900 1000 12 16 20 25 32 40 50 63 80 100

Table 6.25 : Peak air consumption of double acting actuators in Nl/min with 0.5MPa supply pressure

If system pressure is higher or lower than 0.5MPa, use the correction factor below and multiply with peak consumption value from the chart above.

Working Pressure [Mp a] Correction Factor

4 8 12 16 20 24 28 33 37 41

7 14 22 29 36 43 51 58 65 72 11 23 34 45 57 68 79 90 102 113 18 35 53 71 88 106 124 141 159 177 29 58 87 116 145 174 203 232 260 289 45 90 136 181 226 271 317 362 407 452 71 141 212 283 353 424 495 565 636 707

112 224 336 449 561 673 785 897 1009 1122 181 362 543 723 904 1085 1266 1447 1628 1809 283 565 848 1130 1413 1696 1978 2261 2543 2826

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0.33 0.5 0.67 0.83 1 1.17 1.33 1.5 1.67 1.83

Table 6.26 : Correction factor

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Cylinders Air Consumption (For one Cycle)

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P N E U M A T I C T E C H N O L O G Y

Rack

Pinion

Ball Bearings

ISO Symbol:

Vane

Elastomer

Damper

270°

180°

90°

ROTAR Y ACTU AT ORS

R AC K A N D P I N I O N T Y P E

The output shaft has an integral pinion gear driven by a rack attached to a double piston. Standard angles of

rotation are 90° or 180°.

Fig 6.27Rack and Pinion Rotary Actuator

V AN E T Y P E R O T AR Y A C T U A T O R S :

Air pressure acts on a vane, which is attached to the output shaft. A fitted rubber seal or elastomer coating seals the vane against leakage.

A special threedimensional seal seals the stopper against the shaft and the housing. The size of the stopper defines the rotation angle of 90, 180 or 270°.

Adjustable stops may be provided to adjust any angle of rotation of the unit.

Fig 6.28 Vane Type Rotary Actuator

S I Z I N G R O T A R Y A C T U AT OR S

To r que and Ine rti a

Linear cylinders have a cushion to reduce the impact when the piston hits the cover. The capacity of the cushioning is the kinetic energy it can absorb. This energy equals propelled with little friction and high speed.

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· v 2. It is most important when a load is

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P N E U M A T I C T E C H N O L O G Y

These dynamics are even more important to understand in the case of a rotary actuator. Stopping a rotating mass without cushioning risks breaking the pinion or vane. The allowable energy published by the manufacturer must be carefully respected.

r 1

J = m·r

J = m ·

ca b

r1 + r2

J = m ·

d e

g h i

J = m ·

m·ma=

=bm·

(

a+b

)

J = m ·

2 r

a + b

J = m

k l

J = m

4a + c

4b + c

Fig. 6.29 Formulae for the moment of inertia of various body shapes

To define this energy we need to know the inertia of the rotating mass. Think of its material being composed of extremely small parts; the sum of the mass of each individual part, multiplied by the square of its distance from the rotation axis gives the total inertia.

The basic case is a cylinder. Its inertia equals its mass times the square of the radius:

J = m · r2. (kg · m2)

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P N E U M A T I C T E C H N O L O G Y

Shock Absorbers

Stopper Lever on

square Shaft End

Reaction

Stops



srrad

s1 rad:





57.3°





The inertia of more complicated forms has to be calculated with the help of formula for specific shapes. Fig.

6.29 shows the formulae for a number of basic shapes.

A rotating construction has to be split up into basic elements and the partial inertia totaled. For example a chuck on an arm as in Fig. 6.29k is added to the inertia of the arm by multiplying its mass with the square of the distance of its center of gravity from the rotation axis.

Whenever possible, rotating masses have to be stopped against a mechanical stop, preferably a shock absorber. The shock absorber should be placed as far from the axis as possible as in Fig. 6.30a. Any closer to the center would create a reaction force; see Fig. 6.30b. If an external stop on the arm itself is not possible, it can be done with a stopper lever on the opposite end of the shaft. This method subjects the actuator to high reaction forces and should be done only with the consent of the supplier.

Fig. 6.30 Stopping a rotating arm

The inertia for rotating objects is what the moving mass is to a linear movement. The energy is defined by its speed. For a rotation, the speed is defined by the “Angular Speed ”. It is expressed in radians per second. Fig.

6.31 illustrates these expressions.

Fig. 6.31 Definitions of angular speed

As for the cushioning capacity for linear movements, for the maximum allowed energy to be stopped by a rotary actuator we have to consider the final speed. Acceleration by compressed air, if not limited by a stabilizing backpressure, may be considered to be almost constant. The movement starts at zero and reaches about double the average speed (Stroke per time) at the end of stroke.

For fast pneumatic movements, calculations have to be based on twice the average speed as Fig. 6.32

Low Speed High Speed



a b



Fig. 6.32 Average and final speed



Final Speed

Average Speed

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P N E U M A T I C T E C H N O L O G Y

Pilot Port

Clamping Lever

Brake Piston

Shoe

Brake

Magnetic Rings with

opposite polarity

Iron Discs

StainlessSteel

Cylinder Tube

Piston

Carriage

Load

SPE CIAL ACT UATOR S

L O C K I N G C Y L I N D E R

A cylinder can be fitted with a locking head in place of the standard end cover.

It will hold the piston rod in any position. The locking action is mechanical, so ensuring the piston rod is securely held, in the case of pressure breakdown or even if pressurized.

R O D L E SS C Y L I N D E RS

Wit h m a gne t ic c oup lin g , u n gu i ded

Fig. 6.33 Typical Locking Cylinder

Fig 6.34. Typical Rodless Cylinder with magnetic coupling between piston and carriage

A conventional cylinder of say 500-mm of stroke may have an overall extended dimension of 1100 mm. A rodless cylinder of the same stroke can be installed in a much shorter space of approximately 600 mm. It has particular advantages when very long strokes are required.

The magnetic retaining force limits the force available from a magnetically coupled type of rodless cylinder. It equals that of a normal rod cylinder, up to .7 MPa working pressure, but with dynamic shocks a separation of the carriage from the piston is possible. Vertical movements are therefore not recommended, unless a safety margin specified by the supplier is observed.

When the coupling between the carriage and the load cannot be done in the centerline of the cylinder, but at a certain distance (X in Fig. 6.35), the allowable force decreases drastically. The data specified by the supplier has to be respected to avoid damage to the cylinder.

Fig 6.35 Side Load X reduces the allowable load

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P N E U M A T I C T E C H N O L O G Y

Carrier

Piston

Cushioning Tube

Cushioning

Seal

Covering Strip

Seal Belt

Gu i ded typ es, wit h ma g net ic c oup l ing

Depending on the kind of guide used, the problem of side load can be solved or made worse. With ball bearings for the guide, a side load can be considerable and also the stroke length. Precision guides however have so little tolerance that the slightest deformation increases friction. For these types, the stroke length is a main factor for the allowable force. Suppliers give data for any possible mounting orientation and side load.

Fig. 6.36 shows a typical guided rodless cylinder with magnetic coupling between piston and carriage.

Fig. 6.36 Rodless cylinder with guides, Shock Absorbers and cylinder switches

It is recommended that the carriage is decelerated softly with shock absorbers on both ends; in Fig. 6.36 they are built in. A rail holds adjustable switches, operated by a magnet built-in to the carriage.

Gu i ded , wi th m ech a nic a l c o upl ing

Fig.6.37 Rodless Cylinder with mechanical coupling

For lifting or moving heavier loads, a "slotted cylinder" type excludes the risk of disconnection of the carrier from the piston under dynamic shocks, but it is not totally leak free unlike the magnetically coupled type.

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P N E U M A T I C T E C H N O L O G Y

B1AB

Pistons

Vacuum

Connection

(stationary)

Anti-Rotation Rod

Locking Nut for

Vacuum Pad

Switch

Sli d e U nit s

The slide unit is a precision linear actuator of compact dimensions, which can be used on robotic manufacturing and assembly machines.

b c

Fig. 6.38 Typical Slide Unit

Precisely machined work mounting surfaces and parallel piston guide rods ensure accurate straight-line movement when built in as part of the construction of a transfer and position machine.

In one position, the body can be fixed and the rods with end bars can move (b). Upside down, the end bars touch the mounting surface and the body can move (c). In both cases, the valve can be connected to the fixed part, either by the ports A and B, or A1 and B1 in Fig. 6.38 a.

H O L L O W R OD C Y L I N D E R

This actuator is specifically designed for "pick and place" applications.

The hollow rod provides a direct connection between a vacuum source and a vacuum pad, attached to the rods working end. The connecting tube at the rear of the cylinder remains static, while the rod extends and retracts.

Fig. 6.39 Hollow Rod Cylinder with a non moving vacuum connection

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Opened

Closed

Main Piston

Secondary Piston

Speed Control Screw

L I N E A R R O T A T IN G C Y L I N D E R

A so-called rotating cylinder is an assembly of a linear cylinder with a rotary actuator. A rotating arm can be attached to the shaft and be equipped with a gripper or vacuum pad to pick up work pieces and deposit them in another location after rotating the arm. This gives a complete “pick and place” unit for materials handling.

Fig. 6.40 Typical Rotating Cylinder

AI R C H U C K ( G R I PP E R )

An actuator designed to grip components in robotic type applications.

The type shown has two opposing pistons, to open and close the jaws.

Fig.6.41 Typical Pneumatic Fulcrum Type Gripper

Fig.6.42 shows three typical applications of the last two elements:

Fig. 6.42 Typical Applications of the Rotating Cylinder and Air Gripper

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P N E U M A T I C T E C H N O L O G Y

B A P EB EA

A P EB EA

EB EA

A P R1 R2

77 DDIIRREECCTTIIOONNAALL CCOONNTTRROOLL VVAALLVVEESS

VALVE FUNCTI ONS

A directional control valve determines the flow of air between its ports by opening, closing or changing its internal connections. The valves are described in terms of: the number of ports, the number of switching positions, its normal (not operated) position and the method of operation. The first two points are normally expressed in the terms 5/2, 3/2, 2/2 etc. The first figure relates to the number of ports (excluding pilot ports) and the second to the number of positions.

The main functions and their ISO symbols are:

Symbol Principal Construction Function Application

A B

P R

4/2 Switching

1 3

R P

A B

4 2

P R1

Table 7.1 Valve Symbols, Principles, description and main applications

2/2 ON/OFF without exhaust.

3/2 Normally closed (NC), pressurizing or exhausting the output A 3/2 Normally open (NO), pressurizing or exhausting the output A

between output A and B, with common exhaust

5/2: Switching between output A and B, with separate exhausts. 5/3, Open center: As 5/2 but with outputs open to exhaust in midposition 5/3 Closed center: As 5/2 but with midposition fully shut off

5/3 Pressurized center:

Air motors and pneumatic tools

Single acting cylinders (push type), pneumatic signals

Single acting cylinders (pull type), inverse pneumatic signals

Double acting cylinders

Double acting cylinders, with the possibility to depressurize the cylinder Double acting cylinders, with stopping possibility

Special applications, i.e. Locking or Rodless Cylinder

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P N E U M A T I C T E C H N O L O G Y

P O R T I D E NT I F I C A T IO N

The denominations the nomenclature used to identify of the various ports was not uniform until the 5/2 and 5/3. Until the 5/2 and 5/3 were invented, there was more tradition than any respected standard.

Originally, the codes previously used for older hydraulic equipment were adopted. “P” for the supply port comes from “pump”, the hydraulic source of fluid energy, and is understood to mean “pressure” in pneumatic systems.

The outlet of a 2/2 (two ports, two positions) or 3/2 valve has always been "A”, with the second, antivalent output port labeled “B”.

The exhaust port was originally labeled “R” from Return (to the oil tank). We can think of R as return to atmosphere in pneumatic systems. The second exhaust port in 5/2 valves was sometimes named S, or the former “R1” and the latter “R2”.

The pilot port initiating the power connection to port A has originally been coded “Z” (the two extreme letters in the alphabet belongs together) and the other “Y”.

After 20 years of bargaining about pneumatic and hydraulic symbols, one of the ISO work groups had the idea that ports should have numbers instead of letters, delaying the termination of the standard ISO 1219 by another 6 years. Supply should be “1”, the outputs “2“ and “4”, the pilot port connecting”1” with “2” is then “12” etc. Table 7.2 shows the four main sets of port identifications in use. Preferred are now the ISO numbers.

Standard Supply

Port

output

NO output Exhaust of

Exhaust of NO Pilot for NC Pilot for

Old JIS P A B R S Z Y

ISO 1219 P A B R S Z Y

JIS P A B R1 R2 Z Y JIS 1 4 2 5 3 14 12

NFPA P A B EA EB PA PB

ISO 5599 1 4 2 5 3 14 12

SMC P (1) A (4) B (2) EA (5) EB (3) PA (14) PB (12)

Table 7.2 Typical port identifications

M O N O S T A B LE A N D B I - S T A B LE

Spring returned valves are monostable (stable in one default or preferred condition). They have a defined preferred position to which they automatically return. A bi-stable valve has no preferred position and remains in either position until one of its two impulse signals are operated.

VALVE TYP ES

The two principal methods of construction are Poppet and Slide with either elastic (rubber) or metal seals. Fig 7.3 relates to the various combinations.

Directional

Control

Valves

Poppet Valves

Spool Valves

Sliding Valves

Rotary

Valves

Plane Slide

Valves

Fig. 7.3 The various types of valves and sealing methods

Elastomer

Seal

Metal Seal

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P N E U M A T I C T E C H N O L O G Y

A P

P O P P ET V A L VE S

Flow through a poppet valve is controlled by a disc or plug lifting at right angles to a seat, with an elastic seal.

Poppet valves can be two or three port valves, for a four or five port valve two or more poppet valves have to be integrated into one valve.

a b c

Fig. 7.4 The main types of poppets

In a) the inlet pressure tends to lift the seal off its seat requiring a sufficient force (spring) to keep the valve closed. In b) the inlet pressure assists the return spring holding the valve closed, but the operating force varies therefore with different pressures. These factors limit these designs to valves with 1/8" ports or smaller.

Fig.7.5 Mechanically operated poppet valve

Fig 7.5 a) shows a NC 3/2-poppet valve as shown in Fig. 7.4 b.

In its non-operated position (aa), the outlet exhausts through the plunger. When operated, (bb) the exhaust port closes and the airflow’s from the supply port P to the outlet A.

Design 7.6 c) is a balanced poppet valve. The inlet pressure acts on equal opposing piston areas.

N.C.

N.O.

N.C.

N.O.

a b

Fig 7.6 Balanced 3/2 Poppet Valve

This feature allows valves to be connected up normally closed (NC) or normally open (NO).

ISO

ISO Symbol

NONC

Normally open valves can be used to lower or return single acting cylinders and are more commonly used in safety or sequence circuits.

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P N E U M A T I C T E C H N O L O G Y

S L I D I NG V A L VE S

Spool, rotary and plane slide valves use a sliding action to open and close ports.

Spo ol V alve s

A cylindrical spool slides longitudinally in the valve body with the air flowing at right angles to the spool movement. Spools have equal sealing areas and are pressure balanced.

Ela s tom e r s eal

Common spool and seal arrangements are shown in Fig. 7.7 and 7.8. In Fig 7.7 O-rings are fitted in grooves on the spool and move in a metal sleeve. Two of them are crossing output ports, which are therefore divided in a great number of small holes in the sleeve.

Fig. 7.7 Spool Valve with O-Rings on the spool, crossing the cylinder ports

The valve in Fig. 7.8 has seals fitted in the valve body, which are kept in position by means of sectional spacers

Fig 7.9 shows a spool with oval rings. None of them have to cross a port, but just to open or close its own seat. This design provides a leakage free seal with minimum friction and therefore an extremely long life, typically up to 50 million cycles.

EBEA

Fig. 7.8 Spool Valve with seals in the housing

Fig. 7.9 Valve with oval ring spool

Meta l Se a l

Lapped and matched metal spool and sleeve valves have very low frictional resistance, rapid cycling and exceptionally long working life, up to 200 million cycles. Even with a minimal clearance of 0.003 mm, a small internal leakage rate of about one l/min occurs. This has no consequence as long as the cylinder does not need to be held in position by a 5/3 valve with closed center for a long period of time.

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4 2 1

4 2

1 5 3

B EA EB

A B P EA EB

A B

EA EB PA

ISO Symbols

ISO Labels

Fig. 7.10 Principle of the seal-less Spool and Sleeve Valve

Pla n e S lid e Val v e

Flow through the ports is controlled by the position of a slide made of metal, ceramic, nylon, or other plastic. The slide is moved by an elastomer (rubber) sealed, air operated spool.

ISO Symbols

NFPA label

Fig. 7.11 5/2 Plane Slide Valve

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P N E U M A T I C T E C H N O L O G Y

ISO

EX P A B EX P

A B

EX P

A B

EX P

Plunger

Straight Roller

Square Roller

Roller Lever

Ro t ary Valv e s

A metal-ported disc is manually rotated to interconnect the ports in the valve body. Pressure imbalance is employed to force the disc against its mating surface to minimize leakage. The pressure supply must be above the disc, or leakage will occur. In practice, this means that this type of valve must be plumbed as specified.

A B

P R

Fig 7.12 Section through a Rotary Disc Valve and a disc for a 4/3 function with closed center

VALVE OP ERATI ON

M E C H AN I C A L O P E R A T IO N

On an automated machine, mechanically operated valves can detect moving machine parts to provide signals for the automatic control of the working cycle.

The main direct mechanical operators are shown in Fig. 7.13

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Fig 7.13 The main Mechanical Operators

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P N E U M A T I C T E C H N O L O G Y

Yes

Car e wh en u sin g Ro ller Lev ers

Special care must be taken when using cams to operate roller lever valves. Fig. 7.14 illustrates this: the utilized portion of the rollers total travel should not go to the end of stroke. The slope of a cam should have an angle of about 30°; steeper slopes will produce mechanical stresses on the lever.

PT. OT. TT.

NO !

PT: Pre-travel OT.: Over Travel TT.:Total

Roller Stroke to be utilized

Fig. 7.14 Care with Roller Levers and Cams

The one-way roller (or idle return roller) will only operate when the control cam strikes the actuator when moving in one direction. In the reverse direction the roller collapses without operating the valve.

M A N UA L O PE R A T IO N

Manual operation is generally obtained by attaching an operator head, suitable for manual control, onto a mechanically operated valve.

Flush Raised Mushroom

Fig. 7.15 The main monostable Manual Operators

Manually operated, monostable (spring returned) valves are generally used for starting, stopping and otherwise controlling a pneumatic control unit.

For many applications it is more convenient if the valve maintains its position. Fig. 7.16 shows the more important types of bi-stable manual operators

Rotating Knob Toggle Key

Fig. 7.16 Bi-stable Manual Operators

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Air connection for

AAP

PB PA A B

P EA EB

NFPA

AI R O P E R AT IO N

Directional control valves, used as “Power Valves”, should be located as close as possible to its actuator and be switched by remote control with a pneumatic signal.

A monostable air operated valve is switched by air pressure acting directly on one side of the spool or on a piston and returned to its normal position by spring force. The spring is normally a mechanical spring, but it can also be an “air spring” by applying supply pressure to the spool end, opposite to the pilot port, or a combination of

Pilot Input

R P

spring assistance

Piston with twice the area of the spool at spring side

Fig. 7.17 3/2 Air operated Valve, with air assisted spring return Air assisted spring return gives more constant switching characteristics, and higher reliability. In Fig 7.18 an air spring is provided through an internal passage from the supply port to act on the smaller

diameter piston. Pressure applied through the pilot port onto the larger diameter piston actuates the valve.

This method of returning the spool is often used in miniature valves, as it requires very little space

Pilot Input Pilot Input A

R P

ISO Symbol

Fig 7.18 Air operated 3/2 Valve with air spring return

The air-operated valves discussed so far have been single pilot or monostable types, but the more common air

operated valve for cylinder control has a double pilot and is designed to rest in either position (bi-stable).

P EA

Fig. 7.19 Bi-stable, air operated 5/2 Valve

In Fig. 7.19, a short pressure pulse has last been applied to the pilot port "PB", shifting the spool to the right

and connecting the supply port "P" to the cylinder port "B". Port “A" is exhausted through "EA". The valve will remain in this operated position until a counter signal is received. This is referred to as a ‘memory function’.

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b c

Bi-stable valves hold their operated positions because of friction, but should be installed with the spool

horizontal, especially if the valve is subjected to vibration. In the case of metal seal construction, the positions may be locked by a detent.

Pil o te d Op e rat i on

A direct operation occurs when a force, applied to a push button, roller or plunger, moves the spool or poppet

directly. With indirect or “piloted” operation, the external operator acts on a small pilot valve that in turn switches the main valve pneumatically. The external operator can be mechanical, as shown below in Fig 7.20, or electrical, as shown in Fig. 7.23.

NFPA Symbol:

P EB

Fig 7.20 Indirect Mechanical Operation

Fig. 7.20 a shows a 5/2 Valve with indirect or “piloted” mechanical operation in its normal position. The

magnified details in b and c show the pilot part in normal (b) and in operated position (c).

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P N E U M A T I C T E C H N O L O G Y

ISO Symbol

2 4 1 3 5

A P R1 R2

S O L E N O ID O P E R AT I O N

Electro pneumatically and electronically controlled systems are typically discussed separately and it is sufficient

at this stage only to consider the electrical operation of directional control valves.

In small size solenoid valves, an iron armature moves inside an airtight tube. The armature is fitted with an

elastomer poppet and is lifted from a supply seat in the body by the magnetic force of the energized coil. See Figure 7.21a.

JIS Symbol

Fig 7.21 a: 2/2, b: 3/2 direct solenoid, spring return, poppet type valve.

A 3/2 valve has also an exhaust seat on top and the armature an elastomer poppet in its top end. (Fig. 7.21 b) Directly operated 5/2 solenoid valves rely on the electromagnetic force of the solenoid to move the spool (Fig

7.22). It can only be a seal-less lapped spool and sleeve type without friction.

Fig. 7.22 Direct solenoid operated 5/2 Valve with spring return

To limit the size of the solenoid, larger and elastomer sealed valves have indirect (piloted) solenoid operation.

JIS Symbol

Fig. 7.23 5/2 Monostable Solenoid Valve with elastomer bonded spool

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ISO Symbol

Common

Supply

Common

Exhausts for

A and B Ports

Cylinder Ports

A and B

The 5/3 valve has a third (center) position to which it will return, by means of springs, when both solenoids are

de-energized. (Fig 7.24)

2 4

Fig 7.24.Pilot operated 5/3 Solenoid Valve with closed center and spring centering valve mounting

D I R E C T P I P I N G

The most common method of connection to a valve is to screw fittings directly into the threaded ports of a so-

called body ported valve. This method requires one fitting for each cylinder, pilot and supply port and one silencer for each exhaust port. All the valves shown previously are body-ported types, except Fig. 7.22, which is sub base mounted.

M A N IF O L D S

Manifolds have common supply and exhaust

channels for a given number of body-ported valves. The outputs are connected separately to each valve.

Fig. 7.25 shows a manifold with four valves of

different functions: a 5/3, a bi-stable and two monostable types of the same series.

A manifold should be ordered to accommodate

the required number of valves, extension is not possible, but using a blanking kit can seal spare positions.

With 5 or more valves it is recommended that

air is supplied and silencers mounted at both ends.

Fig. 7.25 Typical Manifold

S U B BA S E S

Valves with all of their ports on one face are designed to be gasket mounted on a sub base, to which all the external connections are made. This allows quick removal and replacement of a valve without disturbing the tubing. Generally, a base mounted valve has a slightly better flow capacity than a bodyported valve of the same type. Fig.

7.22 shows a typical base mounted valve

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Fig. 7.26 Single Sub base

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P N E U M A T I C T E C H N O L O G Y

M U L T I P L E S U B B A S E S

In a similar way to the manifold,

multiple sub bases supply and exhaust a number of valves through common channels. Also the cylinder ports are provided in the sub base.

Multiple sub bases also have to be

ordered for the required number of valves and are able to be blanked off in the same way as manifolds.

Fig. 7.27 shows a manifold with four

base-mount types 3/2 Solenoid Valves. The common exhaust ports are to be equipped with Silencers, preferably on both ends to avoid backpressure. This is not only recommended for sound elimination but also for dust protection.

G A N G E D S U B B A S E S

Ganged Sub Bases are assemblies of

individual bases, which allow any reasonable number to be assembled into one unit. This system has the advantage of allowing extension or reduction of the unit if the system is altered, without disturbing the existing components. There is still the option to blank off positions, if required.

Fig. 7.28 shows a typical assembly,

equipped with one monostable and two bistable solenoid valves and a blanking plate. The individual sub bases are hold together with clamps. Other constructions may also have bolts or tie rods for the purpose. O Rings, inserted in grooves around the channels, provide a leakage free connection of supply and exhaust channels from end to end.

Common

Supply

Valve Outputs

(A Ports)

Common

Exhaust

Fig. 7.27 Multiple Sub Base with four 3/2 Valves

Blanking

Plate

Supply

ExhaustsClamps End Plate

Fig. 7.28 Ganged Sub Base with three valves and one blanked

position.

VALVE SI ZING

I N D I C AT I O N S FO R F L O W C AP A C I T Y

Port dimensions do not indicate the flow capacity of the valve. The selection of the valve size will depend on the

required flow rate and permissible pressure drop across the valve.

The manufacturers provide information on the flow capacity of valves. Flow capacity is usually indicated as the

so called “standard flow” Qn in liters of free air per minute at an inlet pressure of .6 MPa and an outlet pressure of .5 MPa, or with a flow factor, Cv or kv, or with the equivalent Flow Section “S”. These factors require formulae or

diagrams to define the flow under various pressure conditions.

The Cv factor of 1 is a flow capacity of one US Gallon of water per minute, with a pressure drop of 1 psi at

60F.

The kv factor of 1 is a flow capacity of one liter of water per minute with a pressure drop of .1 MPa at 20C.

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67.22

The equivalent Flow Section “S” of a valve is the flow section in mm2 of an orifice in a diaphragm, creating

the same relationship between pressure and flow.

All three methods require a formula to calculate the airflow under given pressure conditions. They are as

follows:

Q = 4000 · Cv · pp  •)1013.2( ·

Q = 279.4 · kv · pp  •)1013.2( ·

Q = 222 · S · pp  •)013.12( ·

273 +

273

273 +

273



Where Cv, kv =Coefficients of flow

S =Equivalent Flow Section in mm2 Q = Flow rate normal liters/min (nl/min) p2 = Outlet pressure needed to move load (MPa)

Ap or p = Permissible pressure drop (MPa)

= Air temperature in *C

With this, the dimension of “S” is

To find the flow capacity, these formulae are transformed as follows:

 •)1013.2(•4000

 •)1013.2(•4.279

 •)1013.2(•222

Cv =

Kv =

S =

In Imperial Units:

Cv =

scfm

)•2/(

ppTemp

psiarankine



1 Cv = 1 kv =

The normal flow Qnl for other various flow capacity units is: 981.5 68.85 54.44

The Relationship between these units is as follows:

0.07

0.055 0.794

1 S =

14.3 18

1.26

Note: The outcome of this calculation gives in fact not the required flow capacity of the valve, as we simply stated above, but

for the assembly of the valve and the connecting tubes and filling. To get as much flow capacity, that of the valve has to be higher. How much higher?

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Ori fic e s in se r i es c onn ect i on

Before we can determine the sizes of valve and tubing, we have to look at how pressure drops over a number

of subsequent orifices in series. The formula for the resulting “S” is (or substitute Cv for “S” in this equation):

total

+ …

To avoid unnecessarily dealing with such formulae we look for a thumb rule. Fig. 7.29 and Fig. 7.30.A show the

relationship between a number of orifices in series connection and the resulting flow.

Cv=1 Cv=1 Cv=1 Cv=1

Cv=1 Cv=1

vsys

=.70 C

vsys

=.58

vsys

=.50 C

vsys

=.45 C

vsys

=.41 C

vsys

=.38 C

vsys

=.35 C

vsys

=.33 C

vsys

=.32

Fig. 7.29 In Series circuit, all devices having a Cv of 1 and the resulting impact on the circuit’s overall C

Cv=1

Cv=1.4

vsys

Cv=1

Cv=1.4

=1.0

vsys

Cv=2

Cv=1.73

Cv=3

Cv=1.73 Cv=1.73 Cv=2 Cv=2

=1.0

vsys

Cv=4

Cv=5

Cv=6

Cv=2 Cv=2

=1.0

vsys

=0.89

vsys

=0.86

vsys

=0.84

vsys

=0.83

vsys

=0.82

vsys

Fig. 7.30 A, B Orifices in series connection and resulting flow

Returning to our topic, we can say that it is most obvious to have about the same flow capacity for the valve

and the connecting tube with its fittings. We consider these parts as two equal flow capacities in series connection and to have the required calculated flow through both parts, the required Cv has to be multiplied with 1.4 ( 2 ) to

allow for adiabatic change. It should also be noted that the controlling orifice for an air cylinder is not always defined by its port size. Many times a manufacturer will control a cylinder’s maximum velocity by using a restrictive orifice --- that will, in turn, define the flow capability of the cylinder.

Observe that in Fig. 7.30B, the smallest Cv determines the flow for the system just as the weakest link

determines the strength of a chain. Therefore, when trying to improve upon a system’s flow capability (to make a cylinder respond more quickly), any productive change must be made to the most restrictive orifice.

The smallest orifice determines the flow for the circuit; system flow is smaller than the flow allowed by the

smallest orifice.

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



F L O W C AP A C IT Y O F T U B E S

Still unknown is the flow capacity of tubes and fittings. The formula for the equivalent section of a tube is:



 



S = Equivalent Flow Section [mm] d = Diameter [mm] l = Lengths [m]

Use one of the following coefficient of friction depending on the piping material: Steel piping Plastic, rubber and copper piping

You can by-pass this calculation by reading the equivalent Section of nylon tubes, normally used for

pneumatics, from the diagram 7.31. The bold numbers above the curves indicate the ID of the tubes.







110



 

31.0

= =

0307.0

0197.0



 d

31.0



 d

Fig. 7.31 the equivalent Flow Section S in mm2 of the current tube sizes and length

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The Flow Section of fittings has to be specified in the catalogues. The total of a tube length with its two fittings

can be calculated with the formula above. To reduce the need of its use to exceptions, you can find the sections for the most current tube assemblies in Table 7.32.

Tube Material

Dia.

(mm)

4 x 2.5 N,U 1.86 3.87 1.6 1.6 1.48

5.6 4.2 3.18

6 x 4 N,U 6.12 7.78 6 6 3.72

13.1 11.4 5.96

8 x 5 U 10.65 13.41 11 (9.5) 11 6.73

18 14.9 9.23

8 x 6 N 16.64 20.28 17 (12) 16 10.00

26.1 21.6 13.65

10 x 6.5 U 20.19 24.50 35 (24) 30 12.70

29.5 25 15.88

10 x 7.5 N 28.64 33.38 30 (23) 26 19.97

41.5 35.2 22.17

12 x 8 U 33.18 39.16 35 (24) 30 20.92

46.1 39.7 25.05

12 x 9 N 43.79 51.00 45

58.3 50.2 32.06

1 m 0.5 m Insert type One Touch 0.5 m tube + Straight Elbow Straight Elbow 2 straight

Table 7.32 Equivalent Flow Section of current tube connections

Length

Fittings

(27) 35

Total S Value

fittings

29.45

Table 7.32 shows the flow capacity of current tubes and fittings, based on so called “push-in” or “One Touch”

fittings (Fig. 4.26), having the same inner diameter as the tube. Insert fittings (Fig. 4.25) reduce the flow considerably, especially in smaller sizes, and should be avoided for pneumatics.

Val v es w it h Cy l ind e rs

We now return to the cylinder consumption. This is first of all the peak flow, depending on speed. Secondly, we have to define the allowable pressure drop, a major figure in calculating the valve size. An

assumption of average velocity may be made, since maximum flow is achieved at a pressure drop of approximately 46% --- for our purposes 23% is the maximum allowable pressure drop (half of 46%) --- the NFPA states a 15% maximum pressure drop is desired.

The actual size of the valve has to be much higher than the theoretical value, to compensate for the additional

pressure drop in the connecting tubes and fittings, as discussed above. But if the maximum flow is determined (limited) by the fittings and tubing part of the circuit --- changing the valve for a larger flow capability will not have an effect. E.g. if the valve has a Cv of 2 and the tubing and fittings collectively have a Cv of 1 --- the system will not be improved by a valve with a Cv 4); note Fig. 7.29.2.

To make things easy, all the calculations mentioned before on this subject have been condensed into Table

7.33, which gives you the required equivalent section S for the valve and for the selection of a suitable tube and fittings assembly from Table 7.32. The table is based on a supply pressure of .6 MPa (approx. 90 psig) and a pressure drop of .1 MPa (15 psig) through the valve. It includes also the loss by adiabatic pressure change and the temperature coefficient for 20°C. Usually this will suffice for most real world applications. Note that the values shown on this chart have mathematically taken the calculated required system Cv, and multiplied by two in order to account for the valve, tubing, and two fittings.

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