Physical Properties of air
The surface of the globe is entirely covered by a mantle of air.
Air is a mixture out of the following gases:
The following terms and units are required for definitions in pneumatics:
Derived Quantities
Since everything on earth is subjected to the absolute atmospheric pressure (pat), this pressure cannot be felt. The prevailing atmospheric pressure is therefore regarded as the base and any deviation is termed pg (gauge pressure).
A pressure of pg = 3 bar is a pressure which is 3 bar above the atmospheric pressure.
A pressure of pg = -0.5 bar is a pressure which is 0.5 below the atmospheric pressure.
Is the pressure compared to the zero point (a room with absolutely no air inside is without any pressure) it is termed as absolute pressure pabs.
The atmospheric pressure does not have a constant value. It varies with the geographic location and the weather. Within the international measuring system pat is therefore defined as pat = 1,013 bar (standardised atmospheric pressure).
The international unit for pressure is Pascal (Pa).
1 Pa = 1 N/m2
In pneumatics the more common unit for pressure is bar.
1 bar = 10 N/cm2
Thus the conversion between bar and Pascal is as following:
1 Pa = 10-5 bar
1 bar = 105 Pa
What is force?
Law of Newton:
F Force (Newton)
m Mass (Kg)
a Acceleration (m/s2)
The acceleration on earth due to the gravity is g = 9.81 m/s2 » 10 m/s2.
Example: On a box with a mass of 50 Kg acts a force of 490.5 N.
Characteristics of Pneumatic Systems
Pneumatics has for some considerable time been used for carrying out the simple mechanical tasks. Linear, swivel and rotary motions can be easily performed with pneumatic components. Especially pneumatic cylinders have a significant role as linear drive units due to their relatively low cost, ease of installation, simplicity and robust construction.
Pneumatics is competing with two other working media:
- Electrics
- Hydraulics
Advantages of pneumatic systems:
Availability: Air is available practically everywhere in unlimited quantities
Transport: Air can be easily transported in pipelines, even over large distances
Storage: Air can be stored in a reservoir and removed as required
Temperature: Compressed air is insensitive to temperature fluctuations
Explosion proof: Compressed air offers minimal risk of explosion of fire
Cleanness: Unlubricated exhaust air is clean. Leaking pipes or components cause no contamination to the environment.
Components: The operating components are of simple construction and relatively inexpensive
Speed: Compressed air is a very fast working medium.
Adjustable: Speed and forces are infinitely adjustable
Overload safe: Pneumatic tools and components can be loaded to the point of stopping without damage.
Disadvantages of pneumatic systems:
Preparation: Compressed air requires good preparation. Dirt and condensate has to be removed.
Compressibility: Air is compressible and therefore it is not always possible to achieve uniform motions or constant speeds.
Force: Compressed air is most economical at a working pressure of 6-7 bar. This limits the achievable forces. (Festo’s largest cylinder DGP-320 has a thrust of ~50.000 N at 6 bar).
Noise level: Exhausting air is loud. Using sound absorbing silencer can solve the problem.
Cost: Compressed air is a relatively expensive medium. The high energy costs are partially compensated by inexpensive components.
Air Generation and Distribution
Equipment to be considered in the generation and preparation of compressed air
1 Air Compressor
2 Recooler
3 Water and oil separator
4 Air Reservoir
5 Drainage Point
6 Air Dryer
7 Air filter and pressure regulator
Note: The location of compressor influences to a greater or lesser degree the amount of contamination
· water
· oil
· water combines with oil to form emulsions
· valves and machinery damage: rapid wear of seals and moving parts
· contamination of silencer: Reduced flow through control valve, slowing down of actuators
· icing of valves: expanded air is cooling down - moisture freezes
· moisture precipitates: corrosion and wash out or initial grease, may hardening seals
· oiled-up valves: oil blocks bore for pilot air, switching function is impaired causes valve malfunction, may cause seal swelling, contamination of process e.g. food industry, paint spraying, etc.
The atmospheric air taken in by the compressor always contains a proportion of moisture. The higher the air temperature, the greater the quantity of water vapour which can be take up by the air. If the saturation point of 100% is reached, the water is precipitated in the form of droplets
How much water can be carried by air?
100% of saturation at 1 bar:
Compressing the air means reduction in volume. With less volume, compressed air cannot carry as much water as uncompressed air!
Result: During the compression of air a considerable amount of water is condesating. Water separator and drainage points are essential components after the compressor stage.
Air compressor
There are a large number of different types of compressors available, with most of the compressors on the market employing one of two operating principles: The piston compressor and the screw compressor.
How does it work?
Powered by an electric motor, the pistons of the compressor are moved back and forth. The movement of the pistons creates an underpressure at the inlet end and air is sucked in. The air is forced into the second chamber via a cooling coil. The pressure is intensified in accordance with the ratio between the surface areas and the air compressed correspondingly.
How does it work?
A pair of meshing screws transports sucked-in air from the inlet end to the discharge end. The air is compressed by the tapering of the gap between the spindles. At the inlet end the spindles are wider and this enlargement of the gap causes the air to be sucked in.
Water and oil separator
Cooling of the compressed air leads to condensate forming which of course has to be removed from the air stream. The so-called cyclone separator is used for this purpose and is installed downstream of the recooler. The compressed air carrying droplets of moisture and aerosols flows over a swirler, which creates a rotating air stream. Centrifugal force presses the heavier droplets against the sides of the cyclone after which they flow down to collect in the condensate trap at the bottom.
Air Dryers
Low temperature drying
Compressed air with a temperature of about 30°C (compressor with re-cooler) gets into inlet A. Over an heat-exchanger B the air comes into the cooling-unit C. There the Air is cooled down to about 0°C. Water falls out D. Then the cold air goes back to the heat-exchanger an is warmed up by incoming warm air. Outlet temperature is about 20°C; Pressure dew point normally +3° to +5°C.
Adsorption drying
Compressed air with a temperature of about 30°C (compressor with re-cooler) gets into inlet A. First the air have to pass a switching valve B. Then the air gets into drying chamber 1. By outlet point C a small part of dry air (about 15-20%) as taken to dry chamber 2.(Cold regeneration). In chamber 1 vapour is taken by an granulate.
This can be:
- Molecular sieve NaAlO2 SiO2 (used by FESTO-dryer)
- Active aluminium oxideAl2O3
- Silica gel SiO2
When the switching valve B changes the ports from chamber 1 to chamber 2 the volume from chamber 1 goes quickly out over an air-silencer. 90% of the humidity goes to the surrounding area. Rest of 10% humidity is dried by dry air. Regenerating time about 2 minutes.
Membrane drying
Wet air flows trough the microfilter 1 (separate to order) and gets clean into the entry A of the drying unit 2. Further the air flows through the fibre membranes 3 and the water vapour will pass through the membrane wall due the differences of partial pressures. After the water vapour passes through it is necessary to remove the water vapour from the outside area to keep the vapour pressure low. This is done by taking 10% to 30% of the dry air (B-4-5-C) and purging it back over the outside of the membrane. The wet air goes out over a silencer 6 to the surrounding area.
Air Filter
Available types:
Function of service unit with sinter or micro filter element:
The compressed air passes through a baffle plate in the filter bowl. The air is rotated, and the heavier dust particles and water droplets are spun by centrifugal force against the inner wall of the filter bowl and run down the wall of the housing. The air which has been precleaned then passes through the filter element.
Function of service unit with charcoal filter element:
The filter element consists out of a highly porous charcoal element. The charcoal itself has a enormous effective surface. Even gaseous air contaminants like oil vapour and fumes are absorbed within by the charcoal.
Quality classes of compressed air
Air Lubricator
Today’s modern pneumatic components come with a lifetime grease package and are designed to run without additional lubrication. Nevertheless in some cases lubrication of the compressed air is desirable or even necessary.
Lubrication of compressed air is necessary for:
- fast cylinder speeds (v > 1m/s)
- rapid oscillating motions
- cylinder of large diameter (d> 125mm)
- when lubrication has been used before
Mist lubricator
How does it work?
Compressed air is passing through a venturi nozzle in the lubricator head, creating a vacuum in the drip chamber. At the same time the oil reservoir is pressured through a check valve. The resulting pressure difference forces the oil upwards through the rise into the viaduct where it drips into a nozzle. The orifice of the nozzle is adjustable and the number of oil drops per minute can be easily counted. The drops fall through the duct into the air stream where the oil is atomised resulting in a fine oil mist.
A piece of white cardboard can be used to check the level of lubrication. At the point in the system, which is the furthest from the lubricator the cardboard is held at a distance of approximately 20 cm form a exhaust port. After the system has operated for some time, the oil mist should be seen as a pale yellow colour on the cardboard. Dripping oil is a clear sign of over-lubrication.
Pressure Regulator
Pressure regulators ensure a constant supply pressure (secondary pressure) irrespective of the pressure fluctuations in the main loop (primary pressure).
How does it work?
The operation of a pressure regulator can be divided in three different stages:
1. The output pressure acts on one side of a diaphragm and a spring acts on the other side. The spring force can be adjusted by means of a knurled screw. When output pressure P2 is less than input pressure P1 the spring pushes the diaphragm up an the valve stem opens. Compressed air is flowing in to the system.
2. When the output pressure increases, the diaphragm moves against the spring force causing the valve to close. Further rise in output pressure P2 is now stopped. P2 remains steady until air is exhausted from the secondary side (e.g. cylinder is moving) of the regulator. In this case the valve opens again and new air flows into the system (see No. 1)
3. If the pressure on the secondary side increases considerably, for example during cylinder load changes, the diaphragm is pressed against the spring. The centrepiece of the diaphragm opens and compressed air can exhaust to the atmosphere through vent holes in the regulator housing. After the overpressure is relieved, the orifice in the diaphragm is closed again.
The pressure regulator can be adjusted between the limits zero an the supply pressure. However, for best regulation characteristics, the input pressure should be always 1 bar above the desired output pressure.
Air filter, pressure regulator and lubricator are often combined in a single unit, the so-called air service unit. A service unit should be located as close as possible to the compressed air consuming
device.
The following recommendations regarding air quality are given for standard pneumatic components:
Humidity: Pressure dew point up to 10 °C
Temperature: Higher than the dew point of the compressed air, lower than the maximum
permitted ambient temperature.
Solids: Particle size up to 40 mm
Lubrication: Festo pneumatic cylinders have been already given a basic lubrication from the
factory, thus they can be operated with unlubricated as well as lubricated air.
Directional control valves
Directional control valves influence the path taken by an air stream. The directional control valve is characterised by its number of controlled ports and by the number of switching positions. Additional information is given to define the methods of actuation to achieve the different switching positions. The construction of the valve is important when analysing the flow characteristics like flow rate, maximum pressure and switching times for a particular application.
The following design principles are utilised:
Poppet valves:
- Ball valve
- Disc seat valve
Slide valves:
- Plate slide valve
- Longitudinal slide valve
2/2-way valve
A 2/2-way valve has two ports, an inlet port (1) and an outlet port (2). In one switching position the valve is closed and no air can flow from inlet to outlet port. In the other switching position the valve is open, allowing a flow of air through the valve.
2/2-way valves are simple in design and mostly used as on/off valves.
On the right side a hand lever operated 2/2-way ball valve is shown.
3/2-way valve
A 3/2-way valve has three ports and two switching positions. The valve is available in normally closed and normally open configuration.
Normally closed 3/2-way ball seat valve
In the not actuated position the valve exhaust the working port (2) to the atmosphere, while the supply port (1) is closed. When the valve is actuated, the supply air is connected to the output port (2) and the exhaust port is closed.
Normally open 3/2-way disc seat valve
In the not actuated position the air supply at port (1) is connected to the working outlet port (2). Once actuated the air supply (1) is closed and the air from working port (2) empties through exhaust port (3).
Application for 3/2-way valves:
- Pneumatic push button
- Pneumatic limit switch
- Control of a single acting cylinder
4/2-way valve
A 4/2-way valve has 4 ports: One supply port (1), one exhaust port (3) and two working ports (4) and (2). Furthermore the valve has two switching positions.
The 4/2-way disc seat valve is not actuated. The supply air is connected to the working port (2) while the air from working port (4) is exhausting to the atmosphere trough port (3).
When the two plungers are actuated simultaneously, the connections (1) to (2) and (4) to (3) are closed by the first movement. By pressing the valve plungers further against the discs, the passages between (1) and (4) and (2) to (3) are opened.
The valve has no overlapping between the two switching positions and is returned to its start position by the springs. 4/2-way valves are used to control double acting cylinders.
5/2-way valve
5/2-way valves have 5 connecting ports and two working positions. Basically a 5/2-way valve has the same functionality as a 4/2-way valve. The difference between the two valves is that the 5/2-way valve has a separat exhaust port for each working port. Working port (4) exhausts through port (5) while working port (2) exhausts through port (3). Today 4/2-way valves are generally replaced by 5/2-way valves.
5/2-way suspended disc seat valve, pilot air operated
The 5/2-way valve is not actuated. The supply air is connected to the working port (2) while the air from working port (4) is exhausting to the atmosphere trough port (5).
When the valve stem is switch to the working position, the connections (1) to (2) and (4) to (5) are closed and the passages between (1) and (4) and (2) to (3) are opened. The valve has small overlapping between the two switching positions where all five ports are shortly connected together. In normal operation this overlapping is not recognized.
The ports (12) and (14) are pilot air signals, which switch the valve stem. When pilot signal (12) is applied, the valve switches to the position where (1) is connected to (2), while pilot signal (14) would switch the valve to the position where (1) is connected to (4). When both pilots are applied, the valve remains in its current switching position.
5/2-way valve with applied pilot signal (14) 5/2-way valve with applied pilot signal (12)
5/3-way valve
A 5/3-way valve has 5 ports and three switching positions. Beside the two switching positions of a standard 5/2-way valve, the 5/3-way valve has an additional mid-position. This position is reached when both pilot signals (12) and (14) are not present.
5/3-way valves come in three different designs for the mid-position:
5/3-way longitudinal slide valve mid-position closed
Mid-position: All valve ports are closed
Pilot (12) applied: Port (1) is connected to port (2) while port (4) exhausts through port (5)
Pilot (14) applied: Port (1) is connected to port (4) while port (2) exhausts through port (3)
Pilot operated valves
To avoid high actuating force, mechanically controlled valves can be equipped with an internal pilot valve to assist valve switching. This allows for example large bore valves to be operated with small actuating forces. This increases the sensitivity of the system.
3/2-way roller lever valve, internal pilot, normally closed
How does it work?
A small hole connects the pressure connection (1) and the pilot valve. If the roller lever is operated, the pilot valve opens. Compressed air flows to the servo piston and actuates the main valve disc. First the connection from port (2) to port (3) is closed. Second the disc seat of the main valve opens, allowing the air to flow from pressure supply port (1) to working port (2).
The air supply for the pilot valve is either internally connected to the supply port (1) or supplied through a separate port (mostly numbered 12 or 14). External pilot air supply has to be chosen in the following cases:
- Valve is operated with a pressure less than approximately 3 bar
- Valve is operated with vacuum
- Flow path is used bi-directional
External pilot is recommended in the following cases:
- System incorporates a slow pressure build-up valve (soft-start)
- Compressed air is heavily lubricated (tap-off of dry air before lubricator for control purpose)
For proper valve operation the pilot air has to be correctly exhausted. Pilot exhaust ports use the numbers (82) or (84).
Symbols and description of components
The following characteristics have to be displayed:
· Function
· number of connections
· number of switching positions
· Actuation and return actuation
· simplified representation of the flow path
A symbol does not present the following characteristics:
· Size or dimension of component
· construction and design
· orientation of ports
· any physical detail of the element
The symbols used in pneumatics are detailed in the standard DIN ISO 1219 ‘’Circuit symbols for fluidic equipment and systems’’.
Valve symbols are comprised out of the following basics
From these basic the symbol for a valve is comprised. The connections are always drawn to the box, which shows the initial position.
Examples:
Actuation methods
The method of actuation of pneumatic directional control valves is dependent upon requirements of the task. The types of actuation vary in mechanical, pneumatic, electrical and combined actuation. The symbols for the methods of actuation are specified in ISO 1219.
Beside the method of actuation, the method of return actuation must also be taken in consideration. Both are shown in the symbol on either side of the position boxes.
Supply and exhaust ports
Supply of compressed air is indicated by a triangle. The exhaust of a valve can be ducted (exhaust port with thread) or unducted (exhaust is just a bore)
A numbering system is used to designate the ports of directional control valves.
Examples:
Logic and flow control valves
Non return valve (Check valve)
Non return valves can stop the flow completely in one direction. In the opposite direction the flow is free with a minimal pressure drop due to the resistance of the valve. The one-way blocking action can be effected by cones, balls, plates or diaphragms.
Pilot operated non return valve
Unlike the standard non return valve, which allows flow in only one direction, the pilot operate non return valve allows an flow opposite to the blocking direction when a pilot signal (21) is applied.
If the entering pressure at connection P is higher that the outgoing pressure at A, the check valve allows the flow to pass freely. Otherwise, the valve stops the flow. Additionally, the check valve can be opened via the control line (21). This action allows the flow to pass freely in both directions.
Two pressure valve (AND)
Compressed air flows through the valve only if signals are applied at both inlets (X) and (Y).
The two pressure valve is switched based on the compressed air entering into both input connections (X) and (Y). Only one input signal blocks the flow through the valve. If signals are applied to both (X) an (Y), the signal which is last applied passes on to the outlet (A).
Shuttle valve (OR)
Compressed air flows through the valve if a signal is applied at one of its inlets (X) and (Y).
If compressed air is applied to the first inlet, the ball inside the valve seals the opposing inlet and a signal is generated at the outlet (A). Should both input connections begin receiving compressed air, the connection with the higher pressure takes precedence passes on to the outlet. When the air flow is reversed (e.g. cylinder is exhausting), the ball remains in its previously assumed position.
Quick exhaust valve
Quick exhaust valves are used to increase piston speed of cylinders. The valve allows a cylinder to extend or retract at its maximum speed by reducing the low resistance in the exhaust path. To reduce flow resistance the air is expelled to atmosphere close to the cylinder via a large orifice.
The valve has supply connection (P), an outlet (A) and an exhaust port (R). When flow direction through the valve is from (P) to (A) the air passes freely to (A) while the disc blocks exhaust port (R). However if air flow is in the opposite direction the disc seals port (P) and the air is quickly expelled to atmosphere.
Flow control valve
Flow control valves influence the volumetric flow of compressed air. The valves are normally adjustable and the setting can be locked in position. Flow control valves are used to regualte the speed of actuators. They should be fitted as close as possible to the cylinder.
Throttle valve
A throttle valve regulates the speed equally in both flow directions.
One way flow control valve
The one-way flow control valve is made up of a throttle valve and a check valve. With this type of valve the air flow is throttled in only one direction. The check valve stops the flow of air in the bypass an air can flow only through the throttle orifice. In the opposite direction, air can pass freely through the opened check valve.
Throttling circuits for double acting cylinders
For double acting cylinders, two throttling principles can be distinguished:
Supply air throttling
The air entering the cylinder is throttled. Exhaust air from the cylinder by-passes the throttle and can escape freely. Main disadvantage of this throttling system is that the slightest fluctuations in load on the piston rod lead to irregularities in the speed (stick-slip effect). Supply air throttling is only useful for single acting or small volume cylinders.
Exhaust air throttling
With exhaust air throttling, the supply air flows freely to the cylinder and the exhaust air from the cylinder is throttled. This creates a state where the piston is loaded between two cushion of air - on one side the supply pressure and on the other side the exhaust air being restricted by the throttle. The exhaust air has to pass the throttling orifice since the check valve closes the bypass.
Exhaust air throttling improves the motion behaviour of cylinders at low speeds. It is the only recommended way of throttling for double acting cylinders.
Pneumatic actuators
A pneumatic actuator is designed for converting pressure energy into useful work (motion). They can be divided into the following groups:
Single acting cylinder
With single acting cylinders compressed air is applied on only one side of the piston face. Therefore the cylinder can generate work in only one direction. The return movement of the piston is effected by a built-in spring or by the application of an external force.
For single acting cylinders with built-in spring, the stroke is limited by the natural length of the spring. The construction and simplicity of operation makes single acting cylinders particular suitable for compact, short stroke cylinders in the following types of applications:
- Clamping of work-pieces
- Cutting operation
- Ejecting parts
- Pressing operation
- Feeding and lifting
Advantages of single acting cylinders
- simple design
- less air consumption than double acting cylinder
- no lip seal at piston rod required
Disadvantages of single acting cylinders
- limited stroke due to natural length of the spring
- work force only in one direction
- work force gets less the more the rod is advancing (problem for long strokes)
Double acting
A double acting cylinder is operated by the reciprocal input of compressed air. When compressed air is applied to the rear port of the cylinder while the other side is open to the atmosphere, the cylinder starts to advance. To return the piston to its initial position the air supply has to be connected to the front port while the rear chamber of the cylinder has to be exhausted. The switching of air is done by means of a directional control valve.
Pneumatic end cushioning
If large masses are moved by a cylinder, cushioning is used in the end positions. Before reaching the end position, a cushioning piston interrupts the direct flow of air to the exhaust port. The remaining air is forced to flow through a flow control valve. Therefore the speed of the piston is slowed down for the last part of the stroke to reduce impact on the cylinder cap.
The amount of cushioning has to be adjusted by means of the flow control valve according to the maximum load the cylinder will carry.
Position sensing
Cylinders can be fitted with a magnet in the piston. This allows a contactless sensing of the position of the piston with proximity switches.
Further special designs of double acting cylinders includes:
- Square piston rod
- Double ended piston rod
- Heat resistant seals
- Flat cylinder (square piston)
- Stainless steel cylinder
- Multi position cylinder
- Tandem cylinder
- Rodless cylinder
F Force (Newton)
m Mass (Kg)
a Acceleration (m/s2)
The acceleration on earth due to the gravity is g = 9.81 m/s2 » 10 m/s2.
Example: On a box with a mass of 50 Kg acts a force of 490.5 N.
The theoretical piston force is calculated as the following:
A Effective piston area (m2)
p Operation pressure (Pa)
h Efficiency factor
Example:
Cylinder diameter d1 = 100 mm
Piston rod diameter d2 = 25 mm
Pressure pe = 6 bar
Efficiency factor h = 0.85
1. Effective area for advance stroke:
Aa = d12 x p /4 = (100 mm)2 x p /4 = 7853 mm2 = 78.5 cm2
2. Force for advance stroke:
Fa = p x Aa x h = 60 N/cm2 x 78.5 cm2 x 0.85 = 4003.5 N
3. Effective area for return stroke:
Ar = (d12 x p/4) - (d22 x p/4) = ((100 mm)2 x p/4) - ((25mm)2 x p/4) = 7363.1 mm2 = 73.6 cm2
4. Force for return stroke
Fr = p x Ar x h = 60 N/cm2 x 73.6 cm2 x 0.85 = 3753.6 N
Note: A double acting cylinder has on its advance stroke a greater force than on its return stroke. This is due to different effective piston areas for. For the return stroke the area of the rod has to be subtracted form the area of the piston to get the effective area.
Exercise:
Cylinder diameter d1 = 63 mm
Piston rod diameter d2 = 20 mm
Pressure pe = 6 bar
Efficiency factor h = 0.9
What is the piston force for advance and return stroke?
Designation of symbols in control diagrams:
The design of pneumatic control circuits is defined in VDI 3226. All elements are drawn in their initial positions. The actuating devices with their final control elements are located on top of the drawing. The processing elements and the input elements are located below. All Elements are designated from the top to the bottom.
Designation of Elements
For further signal or processing elements the numbering is continued in the same order.
Some elements e.g. roller lever valves may be actuated in their initial position. This is indicated by a cam, which actuates the roller lever of the valve.
Note: The position of limit switches is marked in the control diagram.
Introduction to the STEP-Diagram
For a quick overview of the functional steps of a control circuit, a displacement step diagram is used. It displays the cylinder movements in a graphical manner. It also includes information about the input signals, which trigger a certain movement.
Example I
Extraction of the double acting cylinder is started by pressing the push button 1.2. After the cylinder has reached its front position the roller leaver valve 1.3 switches the main control valve 1.1 back to its initial position and the cylinder is reversing. The cycle can be restarted, when the cylinder has been completely retracted.
STEP-Diagram
Example II
The extraction of the cylinder can be started by the actuation of the push button 2.2 or the foot lever valve 2.4. The movement can only be started, when the cylinder is in its initial position, which is sensed by the roller lever valve 2.6. After reaching its front position the roller leaver valve 2.3 switches the final control valve 2.1 back to its initial position and the cylinder is retracting
STEP-Diagram
The surface of the globe is entirely covered by a mantle of air.
Air is a mixture out of the following gases:
| Gas | Volume % | Weight % |
| nitrogen | 78,08 | 75,51 |
| oxygen | 20,95 | 23,01 |
| carbon dioxide | 0,03 | 0,04 |
| hydrogen | 0,01 | 0,001 |
| argon | 0,93 | 1,286 |
| helium | 0,0005 | 0,00007 |
| neon | 0,0018 | 0,0012 |
| krypton | 0,0001 | 0,0003 |
| xenon | 0,0000 | 0,00004 |
| Unit | Symbol | Units and unit symbol |
| Length | L | Meter (m) |
| Mass | m | Kilogram (kg) |
| Time | t | Second (s) |
| Temperature | T | Kelvin (K) |
| Unit | Symbol | Units and unit symbol |
| Force | F | Newton (N) = 1 Kg*m/s2 |
| Area | A | Square meter (m2) |
| Volume | V | Cubic meter (m3) |
| Flowrate | Q | (m3/s) |
| Pressure | p | Pascal (Pa) 1 Pa = 1 N/m2 1 bar = 105 Pa |
A pressure of pg = 3 bar is a pressure which is 3 bar above the atmospheric pressure.
A pressure of pg = -0.5 bar is a pressure which is 0.5 below the atmospheric pressure.
Is the pressure compared to the zero point (a room with absolutely no air inside is without any pressure) it is termed as absolute pressure pabs.
 |
The international unit for pressure is Pascal (Pa).
1 Pa = 1 N/m2
In pneumatics the more common unit for pressure is bar.
1 bar = 10 N/cm2
Thus the conversion between bar and Pascal is as following:
1 Pa = 10-5 bar
1 bar = 105 Pa
What is force?
Law of Newton:
Force = Mass x Acceleration  |
m Mass (Kg)
a Acceleration (m/s2)
The acceleration on earth due to the gravity is g = 9.81 m/s2 » 10 m/s2.
Example: On a box with a mass of 50 Kg acts a force of 490.5 N.
Characteristics of Pneumatic Systems
Pneumatics has for some considerable time been used for carrying out the simple mechanical tasks. Linear, swivel and rotary motions can be easily performed with pneumatic components. Especially pneumatic cylinders have a significant role as linear drive units due to their relatively low cost, ease of installation, simplicity and robust construction.
Pneumatics is competing with two other working media:
- Electrics
- Hydraulics
Advantages of pneumatic systems:
Availability: Air is available practically everywhere in unlimited quantities
Transport: Air can be easily transported in pipelines, even over large distances
Storage: Air can be stored in a reservoir and removed as required
Temperature: Compressed air is insensitive to temperature fluctuations
Explosion proof: Compressed air offers minimal risk of explosion of fire
Cleanness: Unlubricated exhaust air is clean. Leaking pipes or components cause no contamination to the environment.
Components: The operating components are of simple construction and relatively inexpensive
Speed: Compressed air is a very fast working medium.
Adjustable: Speed and forces are infinitely adjustable
Overload safe: Pneumatic tools and components can be loaded to the point of stopping without damage.
Disadvantages of pneumatic systems:
Preparation: Compressed air requires good preparation. Dirt and condensate has to be removed.
Compressibility: Air is compressible and therefore it is not always possible to achieve uniform motions or constant speeds.
Force: Compressed air is most economical at a working pressure of 6-7 bar. This limits the achievable forces. (Festo’s largest cylinder DGP-320 has a thrust of ~50.000 N at 6 bar).
Noise level: Exhausting air is loud. Using sound absorbing silencer can solve the problem.
Cost: Compressed air is a relatively expensive medium. The high energy costs are partially compensated by inexpensive components.
Air Generation and Distribution
Equipment to be considered in the generation and preparation of compressed air
 |
2 Recooler
3 Water and oil separator
4 Air Reservoir
5 Drainage Point
6 Air Dryer
7 Air filter and pressure regulator
Note: The location of compressor influences to a greater or lesser degree the amount of contamination
Types of main contamination in compressed air
· solids (dust)· water
· oil
Interaction of contaminants
· dust combine with water or oil to form lager particles· water combines with oil to form emulsions
| Compressed air should be properly prepared to prevent malfunction |
Results of poorly-prepared air:
· pipe-work contamination and damage: flow-rate decreases, rust of iron, oxidation of copper· valves and machinery damage: rapid wear of seals and moving parts
· contamination of silencer: Reduced flow through control valve, slowing down of actuators
· icing of valves: expanded air is cooling down - moisture freezes
· moisture precipitates: corrosion and wash out or initial grease, may hardening seals
· oiled-up valves: oil blocks bore for pilot air, switching function is impaired causes valve malfunction, may cause seal swelling, contamination of process e.g. food industry, paint spraying, etc.
The atmospheric air taken in by the compressor always contains a proportion of moisture. The higher the air temperature, the greater the quantity of water vapour which can be take up by the air. If the saturation point of 100% is reached, the water is precipitated in the form of droplets
How much water can be carried by air?
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| °C | g/m3 | °C | g/m3 |
| 1 | 5,15 | 26 | 24,38 |
| 2 | 5,52 | 27 | 25,78 |
| 3 | 5,92 | 28 | 27,22 |
| 4 | 6,35 | 29 | 28,77 |
| 5 | 6,80 | 30 | 30,36 |
| 6 | 7,28 | 31 | 32,02 |
| 7 | 7,77 | 32 | 33,78 |
| 8 | 8,29 | 33 | 35,64 |
| 9 | 8,84 | 34 | 37,57 |
| 10 | 9,40 | 35 | 39,60 |
| 11 | 9,99 | 36 | 41,72 |
| 12 | 10,65 | 37 | 43,91 |
| 13 | 11,35 | 38 | 46,20 |
| 14 | 12,07 | 39 | 48,62 |
| 15 | 12,82 | 40 | 51,14 |
| 16 | 13,63 | 41 | 53,76 |
| 17 | 14,48 | 42 | 56,49 |
| 18 | 15,37 | 43 | 59,35 |
| 19 | 16,32 | 44 | 62,34 |
| 20 | 17,29 | 45 | 65,44 |
| 21 | 18,31 | 46 | 68,63 |
| 22 | 19,38 | 47 | 71,92 |
| 23 | 20,53 | 48 | 75,40 |
| 24 | 21,74 | 49 | 79,08 |
| 25 | 23,04 | 50 | 82,98 |
Result: During the compression of air a considerable amount of water is condesating. Water separator and drainage points are essential components after the compressor stage.
Air compressor
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How does it work?
Powered by an electric motor, the pistons of the compressor are moved back and forth. The movement of the pistons creates an underpressure at the inlet end and air is sucked in. The air is forced into the second chamber via a cooling coil. The pressure is intensified in accordance with the ratio between the surface areas and the air compressed correspondingly.
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A pair of meshing screws transports sucked-in air from the inlet end to the discharge end. The air is compressed by the tapering of the gap between the spindles. At the inlet end the spindles are wider and this enlargement of the gap causes the air to be sucked in.
| Type | Advantages | Disadvantages |
| Reciprocating piston compressor Single- and double-acting, single- and multi-stage, oil-free and oil-lubricated | Very suitable for high pressures Large pressure control range Good regulation characteristics Very familiar, accordingly repairs by own personnel are often possible | Not smooth-running due to oscillating mass forces High maintenance costs due to wear on pistons and cylinders High compression end temperature |
| Screw compressor Single- and multi-stage, air- and water-cooled oil-free and oil-lubricated | Smooth-running Low maintenance costs Low wear Low residual oil content with oil-injected versions | High power consumption Limited pressure and control range Compressor elements require maintenance and overhaul by manufacturer |
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Air Dryers
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Low temperature drying
Compressed air with a temperature of about 30°C (compressor with re-cooler) gets into inlet A. Over an heat-exchanger B the air comes into the cooling-unit C. There the Air is cooled down to about 0°C. Water falls out D. Then the cold air goes back to the heat-exchanger an is warmed up by incoming warm air. Outlet temperature is about 20°C; Pressure dew point normally +3° to +5°C.
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Adsorption drying
Compressed air with a temperature of about 30°C (compressor with re-cooler) gets into inlet A. First the air have to pass a switching valve B. Then the air gets into drying chamber 1. By outlet point C a small part of dry air (about 15-20%) as taken to dry chamber 2.(Cold regeneration). In chamber 1 vapour is taken by an granulate.
This can be:
- Molecular sieve NaAlO2 SiO2 (used by FESTO-dryer)
- Active aluminium oxideAl2O3
- Silica gel SiO2
When the switching valve B changes the ports from chamber 1 to chamber 2 the volume from chamber 1 goes quickly out over an air-silencer. 90% of the humidity goes to the surrounding area. Rest of 10% humidity is dried by dry air. Regenerating time about 2 minutes.
Membrane drying
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Air Filter
Available types:
| Filter element | Filter size (mm) | Residual oil in air (mg/m3) |
| Sinter cartridge | 40 | - |
| Sinter cartridge | 5 | - |
| Micro filter | 1 | < 0,5 |
| Fine filter | 0,01 | < 0,01 |
| Charcoal filter | 0,003 | < 0,003 |
The compressed air passes through a baffle plate in the filter bowl. The air is rotated, and the heavier dust particles and water droplets are spun by centrifugal force against the inner wall of the filter bowl and run down the wall of the housing. The air which has been precleaned then passes through the filter element.
Function of service unit with charcoal filter element:
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The filter element consists out of a highly porous charcoal element. The charcoal itself has a enormous effective surface. Even gaseous air contaminants like oil vapour and fumes are absorbed within by the charcoal.
Quality classes of compressed air
| Class | Particle size mm | Particle density mg/m3 | pressure dew point °C | maximal oil content mg/m3 |
| 1 | 0.1 | 0.1 | -70 | 0.01 |
| 2 | 1 | 1 | -40 | 0.1 |
| 3 | 5 | 5 | -20 | 1.0 |
| 4 | 15 | 8 | +3 | 5 |
| 5 | 40 | 10 | +7 | 25 |
| 6 | not specified | not specified | +10 | not specified |
| 7 | not specified | not specified | not defined | not specified |
Today’s modern pneumatic components come with a lifetime grease package and are designed to run without additional lubrication. Nevertheless in some cases lubrication of the compressed air is desirable or even necessary.
Lubrication of compressed air is necessary for:
- fast cylinder speeds (v > 1m/s)
- rapid oscillating motions
- cylinder of large diameter (d> 125mm)
- when lubrication has been used before
| Note: If a cylinder has once been operated with lubricated air, lubrication is always needed, as additional oil flushes out the basic lubrication. |
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Mist lubricator
How does it work?
Compressed air is passing through a venturi nozzle in the lubricator head, creating a vacuum in the drip chamber. At the same time the oil reservoir is pressured through a check valve. The resulting pressure difference forces the oil upwards through the rise into the viaduct where it drips into a nozzle. The orifice of the nozzle is adjustable and the number of oil drops per minute can be easily counted. The drops fall through the duct into the air stream where the oil is atomised resulting in a fine oil mist.
A piece of white cardboard can be used to check the level of lubrication. At the point in the system, which is the furthest from the lubricator the cardboard is held at a distance of approximately 20 cm form a exhaust port. After the system has operated for some time, the oil mist should be seen as a pale yellow colour on the cardboard. Dripping oil is a clear sign of over-lubrication.
Pressure Regulator
Pressure regulators ensure a constant supply pressure (secondary pressure) irrespective of the pressure fluctuations in the main loop (primary pressure).
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How does it work?
The operation of a pressure regulator can be divided in three different stages:
1. The output pressure acts on one side of a diaphragm and a spring acts on the other side. The spring force can be adjusted by means of a knurled screw. When output pressure P2 is less than input pressure P1 the spring pushes the diaphragm up an the valve stem opens. Compressed air is flowing in to the system.
2. When the output pressure increases, the diaphragm moves against the spring force causing the valve to close. Further rise in output pressure P2 is now stopped. P2 remains steady until air is exhausted from the secondary side (e.g. cylinder is moving) of the regulator. In this case the valve opens again and new air flows into the system (see No. 1)
3. If the pressure on the secondary side increases considerably, for example during cylinder load changes, the diaphragm is pressed against the spring. The centrepiece of the diaphragm opens and compressed air can exhaust to the atmosphere through vent holes in the regulator housing. After the overpressure is relieved, the orifice in the diaphragm is closed again.
The pressure regulator can be adjusted between the limits zero an the supply pressure. However, for best regulation characteristics, the input pressure should be always 1 bar above the desired output pressure.
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Air filter, pressure regulator and lubricator are often combined in a single unit, the so-called air service unit. A service unit should be located as close as possible to the compressed air consuming
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device.
The following recommendations regarding air quality are given for standard pneumatic components:
Humidity: Pressure dew point up to 10 °C
Temperature: Higher than the dew point of the compressed air, lower than the maximum
permitted ambient temperature.
Solids: Particle size up to 40 mm
Lubrication: Festo pneumatic cylinders have been already given a basic lubrication from the
factory, thus they can be operated with unlubricated as well as lubricated air.
Directional control valves
Directional control valves influence the path taken by an air stream. The directional control valve is characterised by its number of controlled ports and by the number of switching positions. Additional information is given to define the methods of actuation to achieve the different switching positions. The construction of the valve is important when analysing the flow characteristics like flow rate, maximum pressure and switching times for a particular application.
The following design principles are utilised:
Poppet valves:
- Ball valve
- Disc seat valve
Slide valves:
- Plate slide valve
- Longitudinal slide valve
2/2-way valve
A 2/2-way valve has two ports, an inlet port (1) and an outlet port (2). In one switching position the valve is closed and no air can flow from inlet to outlet port. In the other switching position the valve is open, allowing a flow of air through the valve.
2/2-way valves are simple in design and mostly used as on/off valves.
On the right side a hand lever operated 2/2-way ball valve is shown.
3/2-way valve
A 3/2-way valve has three ports and two switching positions. The valve is available in normally closed and normally open configuration.
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Normally closed 3/2-way ball seat valve
In the not actuated position the valve exhaust the working port (2) to the atmosphere, while the supply port (1) is closed. When the valve is actuated, the supply air is connected to the output port (2) and the exhaust port is closed.
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Normally open 3/2-way disc seat valve
In the not actuated position the air supply at port (1) is connected to the working outlet port (2). Once actuated the air supply (1) is closed and the air from working port (2) empties through exhaust port (3).
Application for 3/2-way valves: - Pneumatic push button
- Pneumatic limit switch
- Control of a single acting cylinder
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A 4/2-way valve has 4 ports: One supply port (1), one exhaust port (3) and two working ports (4) and (2). Furthermore the valve has two switching positions.
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The 4/2-way disc seat valve is not actuated. The supply air is connected to the working port (2) while the air from working port (4) is exhausting to the atmosphere trough port (3).
When the two plungers are actuated simultaneously, the connections (1) to (2) and (4) to (3) are closed by the first movement. By pressing the valve plungers further against the discs, the passages between (1) and (4) and (2) to (3) are opened.
The valve has no overlapping between the two switching positions and is returned to its start position by the springs. 4/2-way valves are used to control double acting cylinders.
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5/2-way valve
5/2-way valves have 5 connecting ports and two working positions. Basically a 5/2-way valve has the same functionality as a 4/2-way valve. The difference between the two valves is that the 5/2-way valve has a separat exhaust port for each working port. Working port (4) exhausts through port (5) while working port (2) exhausts through port (3). Today 4/2-way valves are generally replaced by 5/2-way valves.
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5/2-way suspended disc seat valve, pilot air operated
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The 5/2-way valve is not actuated. The supply air is connected to the working port (2) while the air from working port (4) is exhausting to the atmosphere trough port (5).
When the valve stem is switch to the working position, the connections (1) to (2) and (4) to (5) are closed and the passages between (1) and (4) and (2) to (3) are opened. The valve has small overlapping between the two switching positions where all five ports are shortly connected together. In normal operation this overlapping is not recognized.
The ports (12) and (14) are pilot air signals, which switch the valve stem. When pilot signal (12) is applied, the valve switches to the position where (1) is connected to (2), while pilot signal (14) would switch the valve to the position where (1) is connected to (4). When both pilots are applied, the valve remains in its current switching position.
5/2-way valve with applied pilot signal (14) 5/2-way valve with applied pilot signal (12)
5/3-way valve
A 5/3-way valve has 5 ports and three switching positions. Beside the two switching positions of a standard 5/2-way valve, the 5/3-way valve has an additional mid-position. This position is reached when both pilot signals (12) and (14) are not present.
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5/3-way valves come in three different designs for the mid-position:
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5/3-way longitudinal slide valve mid-position closed
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Mid-position: All valve ports are closed
Pilot (12) applied: Port (1) is connected to port (2) while port (4) exhausts through port (5)
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Pilot (14) applied: Port (1) is connected to port (4) while port (2) exhausts through port (3)
Pilot operated valves
To avoid high actuating force, mechanically controlled valves can be equipped with an internal pilot valve to assist valve switching. This allows for example large bore valves to be operated with small actuating forces. This increases the sensitivity of the system.
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3/2-way roller lever valve, internal pilot, normally closed How does it work?
A small hole connects the pressure connection (1) and the pilot valve. If the roller lever is operated, the pilot valve opens. Compressed air flows to the servo piston and actuates the main valve disc. First the connection from port (2) to port (3) is closed. Second the disc seat of the main valve opens, allowing the air to flow from pressure supply port (1) to working port (2).
The air supply for the pilot valve is either internally connected to the supply port (1) or supplied through a separate port (mostly numbered 12 or 14). External pilot air supply has to be chosen in the following cases:
- Valve is operated with a pressure less than approximately 3 bar
- Valve is operated with vacuum
- Flow path is used bi-directional
External pilot is recommended in the following cases:
- System incorporates a slow pressure build-up valve (soft-start)
- Compressed air is heavily lubricated (tap-off of dry air before lubricator for control purpose)
For proper valve operation the pilot air has to be correctly exhausted. Pilot exhaust ports use the numbers (82) or (84).
Symbols and description of components
The following characteristics have to be displayed:
· Function
· number of connections
· number of switching positions
· Actuation and return actuation
· simplified representation of the flow path
A symbol does not present the following characteristics:
· Size or dimension of component
· construction and design
· orientation of ports
· any physical detail of the element
The symbols used in pneumatics are detailed in the standard DIN ISO 1219 ‘’Circuit symbols for fluidic equipment and systems’’.
Valve symbols are comprised out of the following basics
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Examples:
Actuation methods
The method of actuation of pneumatic directional control valves is dependent upon requirements of the task. The types of actuation vary in mechanical, pneumatic, electrical and combined actuation. The symbols for the methods of actuation are specified in ISO 1219.
Beside the method of actuation, the method of return actuation must also be taken in consideration. Both are shown in the symbol on either side of the position boxes.
Supply and exhaust ports
Supply of compressed air is indicated by a triangle. The exhaust of a valve can be ducted (exhaust port with thread) or unducted (exhaust is just a bore)
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| Port or Connection | ISO 5599 | Lettering System (old) |
| Pressure port | 1 | P |
| Exhaust ports | 5, 3 | R, S |
| Working ports | 4, 2 | A, B |
| Pilot line opens flow 1 to 2 | 12 | Z |
| Pilot line opens flow 1 to 4 | 14 | Y |
| Pilot line closes flow | 10 | Z, Y |
| Auxiliary pilot air | 81, 82 | Pz |
| Pilot air exhaust | 83, 84 |
Examples:
Logic and flow control valves
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Non return valve (Check valve)
Non return valves can stop the flow completely in one direction. In the opposite direction the flow is free with a minimal pressure drop due to the resistance of the valve. The one-way blocking action can be effected by cones, balls, plates or diaphragms.
Pilot operated non return valve
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Unlike the standard non return valve, which allows flow in only one direction, the pilot operate non return valve allows an flow opposite to the blocking direction when a pilot signal (21) is applied.
If the entering pressure at connection P is higher that the outgoing pressure at A, the check valve allows the flow to pass freely. Otherwise, the valve stops the flow. Additionally, the check valve can be opened via the control line (21). This action allows the flow to pass freely in both directions.
Two pressure valve (AND)
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Compressed air flows through the valve only if signals are applied at both inlets (X) and (Y).
The two pressure valve is switched based on the compressed air entering into both input connections (X) and (Y). Only one input signal blocks the flow through the valve. If signals are applied to both (X) an (Y), the signal which is last applied passes on to the outlet (A).
Shuttle valve (OR)
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Compressed air flows through the valve if a signal is applied at one of its inlets (X) and (Y).
If compressed air is applied to the first inlet, the ball inside the valve seals the opposing inlet and a signal is generated at the outlet (A). Should both input connections begin receiving compressed air, the connection with the higher pressure takes precedence passes on to the outlet. When the air flow is reversed (e.g. cylinder is exhausting), the ball remains in its previously assumed position.
Quick exhaust valve
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Quick exhaust valves are used to increase piston speed of cylinders. The valve allows a cylinder to extend or retract at its maximum speed by reducing the low resistance in the exhaust path. To reduce flow resistance the air is expelled to atmosphere close to the cylinder via a large orifice.
The valve has supply connection (P), an outlet (A) and an exhaust port (R). When flow direction through the valve is from (P) to (A) the air passes freely to (A) while the disc blocks exhaust port (R). However if air flow is in the opposite direction the disc seals port (P) and the air is quickly expelled to atmosphere.
Flow control valve
Flow control valves influence the volumetric flow of compressed air. The valves are normally adjustable and the setting can be locked in position. Flow control valves are used to regualte the speed of actuators. They should be fitted as close as possible to the cylinder.
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A throttle valve regulates the speed equally in both flow directions.
One way flow control valve
The one-way flow control valve is made up of a throttle valve and a check valve. With this type of valve the air flow is throttled in only one direction. The check valve stops the flow of air in the bypass an air can flow only through the throttle orifice. In the opposite direction, air can pass freely through the opened check valve.
Throttling circuits for double acting cylinders
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For double acting cylinders, two throttling principles can be distinguished:
Supply air throttling
The air entering the cylinder is throttled. Exhaust air from the cylinder by-passes the throttle and can escape freely. Main disadvantage of this throttling system is that the slightest fluctuations in load on the piston rod lead to irregularities in the speed (stick-slip effect). Supply air throttling is only useful for single acting or small volume cylinders.
Exhaust air throttling
With exhaust air throttling, the supply air flows freely to the cylinder and the exhaust air from the cylinder is throttled. This creates a state where the piston is loaded between two cushion of air - on one side the supply pressure and on the other side the exhaust air being restricted by the throttle. The exhaust air has to pass the throttling orifice since the check valve closes the bypass.
Exhaust air throttling improves the motion behaviour of cylinders at low speeds. It is the only recommended way of throttling for double acting cylinders.
Pneumatic actuators
A pneumatic actuator is designed for converting pressure energy into useful work (motion). They can be divided into the following groups:
Single acting cylinder
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With single acting cylinders compressed air is applied on only one side of the piston face. Therefore the cylinder can generate work in only one direction. The return movement of the piston is effected by a built-in spring or by the application of an external force.
For single acting cylinders with built-in spring, the stroke is limited by the natural length of the spring. The construction and simplicity of operation makes single acting cylinders particular suitable for compact, short stroke cylinders in the following types of applications:
- Clamping of work-pieces
- Cutting operation
- Ejecting parts
- Pressing operation
- Feeding and lifting
Advantages of single acting cylinders
- simple design
- less air consumption than double acting cylinder
- no lip seal at piston rod required
Disadvantages of single acting cylinders
- limited stroke due to natural length of the spring
- work force only in one direction
- work force gets less the more the rod is advancing (problem for long strokes)
Double acting
A double acting cylinder is operated by the reciprocal input of compressed air. When compressed air is applied to the rear port of the cylinder while the other side is open to the atmosphere, the cylinder starts to advance. To return the piston to its initial position the air supply has to be connected to the front port while the rear chamber of the cylinder has to be exhausted. The switching of air is done by means of a directional control valve.
Pneumatic end cushioning
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If large masses are moved by a cylinder, cushioning is used in the end positions. Before reaching the end position, a cushioning piston interrupts the direct flow of air to the exhaust port. The remaining air is forced to flow through a flow control valve. Therefore the speed of the piston is slowed down for the last part of the stroke to reduce impact on the cylinder cap.
The amount of cushioning has to be adjusted by means of the flow control valve according to the maximum load the cylinder will carry.
Position sensing
Cylinders can be fitted with a magnet in the piston. This allows a contactless sensing of the position of the piston with proximity switches.
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Further special designs of double acting cylinders includes:
- Square piston rod
- Double ended piston rod
- Heat resistant seals
- Flat cylinder (square piston)
- Stainless steel cylinder
- Multi position cylinder
- Tandem cylinder
- Rodless cylinder
What is force?
Law of Newton:Force = Mass x Acceleration ![clip_image004[1]](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh5H4HvIqMWhGUxZkCTGxy3jdEJc_Dsh1YyXV0bwxs8_HDhK3OGtpEfnfU6INht68Lpvv4TgkXU4IOblJhPFrj2h-WmYGApwRDCenq8sOmzgoJOy2B4NTsuYyxp159PHuqE5SSXuOyfeYM/s1600-h/clip_image004%25255B1%25255D%25255B2%25255D.gif "clip_image004[1]"){kind=small} |
m Mass (Kg)
a Acceleration (m/s2)
The acceleration on earth due to the gravity is g = 9.81 m/s2 » 10 m/s2.
Example: On a box with a mass of 50 Kg acts a force of 490.5 N.
Cylinder piston force
The theoretically available force is derived from the working pressure and the respective effective piston surface area. This available force is reduced by the proportion of system friction; generally, system friction amounts to between 5 and 10 % of the theoretically available force. The system friction is expressed through the efficiency factor.The theoretical piston force is calculated as the following:
Force = Effective piston area x operating pressure x Efficiency  |
p Operation pressure (Pa)
h Efficiency factor
Example:
Cylinder diameter d1 = 100 mm
Piston rod diameter d2 = 25 mm
Pressure pe = 6 bar
Efficiency factor h = 0.85
1. Effective area for advance stroke:
Aa = d12 x p /4 = (100 mm)2 x p /4 = 7853 mm2 = 78.5 cm2
2. Force for advance stroke:
Fa = p x Aa x h = 60 N/cm2 x 78.5 cm2 x 0.85 = 4003.5 N
3. Effective area for return stroke:
Ar = (d12 x p/4) - (d22 x p/4) = ((100 mm)2 x p/4) - ((25mm)2 x p/4) = 7363.1 mm2 = 73.6 cm2
4. Force for return stroke
Fr = p x Ar x h = 60 N/cm2 x 73.6 cm2 x 0.85 = 3753.6 N
Note: A double acting cylinder has on its advance stroke a greater force than on its return stroke. This is due to different effective piston areas for. For the return stroke the area of the rod has to be subtracted form the area of the piston to get the effective area.
Exercise:
Cylinder diameter d1 = 63 mm
Piston rod diameter d2 = 20 mm
Pressure pe = 6 bar
Efficiency factor h = 0.9
What is the piston force for advance and return stroke?
Designation of symbols in control diagrams:
The design of pneumatic control circuits is defined in VDI 3226. All elements are drawn in their initial positions. The actuating devices with their final control elements are located on top of the drawing. The processing elements and the input elements are located below. All Elements are designated from the top to the bottom.
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| Cylinder | 1.0 | 2.0 | 3.0 |
| Final Control Element | 1.1 | 2.1 | 3.1 |
| Input Element even numbering for extracting* | 1.2 | 2.2 | 3.2 |
| odd numbering for retracting* | 1.3 | 2.3 | 3.3 |
| *Internal Festo designation, not according to VDI 3226 |
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Note: The position of limit switches is marked in the control diagram.
Introduction to the STEP-Diagram
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Example I
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STEP-Diagram
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The extraction of the cylinder can be started by the actuation of the push button 2.2 or the foot lever valve 2.4. The movement can only be started, when the cylinder is in its initial position, which is sensed by the roller lever valve 2.6. After reaching its front position the roller leaver valve 2.3 switches the final control valve 2.1 back to its initial position and the cylinder is retracting
STEP-Diagram
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