Pneumatic circuits and pneumatic circuit diagrams - fundamentals of fluid engineering

Basic knowledge Pneumatics

This article covers the fundamentals of pneumatics, pneumatic circuits and pneumatic circuit diagrams. Pneumatics is an area of fluid engineering that deals with compressed air and its application in various systems. Pneumatic control technology is used in many industries and sectors, such as material handling, robotics, or transportation.

What is pneumatics?

Pneumatics is a sub-area of mechanics that deals with the behavior of gases. In particular, it is fluid engineering that makes use of compressed air or air-operated systems to generate motion and drive power.

In typical compressed air systems, the air has a positive pressure of 6 bar. The pressure level is up to 18 bar in high pressure applications, such as pneumatic applications with high power input requirements. In special cases, the pressure can even be up to 40 bar.

There are many uses for compressed air that can be used as needed and required. For example, it can act as active air to convey substances and materials. It is also used as process air and supports drying processes and other processes. In addition, compressed air may be used in potentially explosive or wet locations, for example to operate motors or the like. A typical application example is a compressed air an eccentric orbital sander in a painting booth.

Applications for pneumatic controls

Pneumatic controls and systems find a wide range of applications in mechanical engineering, custom machine construction and mass production. In addition to their simplicity and reliability, the advantages of pneumatic systems also include fast response times and relatively cost-effective implementation.

Typical application examples of pneumatic controls include:

Welding machines: The control of welding heads and clamping devices.
Machine tools: For clamping or loosening and for tool changes.
Foundry Machines: For opening or closing molds and removing castings - for example in injection molding machines.
Conveyors and Hoists: For moving, lifting and positioning materials.
Printing and paper machines: To control printing processes and paper positioning.

What are the benefits of pneumatics?

Pneumatic systems have numerous advantages. The material used, i.e. air, has an infinite supply, is available virtually anywhere, and can be transported over long distances.

Ability to be stored: It is possible to store compressed air in corresponding compressed air tanks. These compressed air tanks can also be transported.
Temperature resistance: Compressed air is substantially unaffected by temperature fluctuations. It is therefore suited for operation in more extreme conditions compared to fluids, such as hydraulic fluid.
Environmental compatibility: Leaking compressed air does not cause pollution or damage.
Simplicity: The pneumatic components are easy to assemble. They can shift or control the speeds and forces of cylinders in a continuously adjustable manner.
High speed: Compressed air is a fast process medium, therefore allowing relatively high speeds and short switching times to be achieved.
Portability: Compressed air can be easily transported in lines over long distances. Compressed air therefore generally only needs to be conditioned.
Overload protection: Pneumatic circuits and elements can absorb loads even at rest and are thus overload-proof against compressed air peaks.

Various mechanical processes can be operated efficiently by using compressed air as a power source, making it a cost-effective alternative to other power systems.

Design and Operating Principles of Pneumatic Controls

Compressed air is supplied to the desired location using valves. The energy stored in the compressed air is used to generate motion energy. An example for this is the use of compressed air to control a cylinder piston in a particular direction.

Each pneumatic control system basically consists of the following sub-components:

Compressed air generation (compressors)
Compressed air conditioning (pneumatic filter / air filter)
Compressed air accumulator (air tank)
Compressed air regulation (pressure regulator)
Main valves (way valves)
Process elements (cylinders)
Sensors and Switches (solenoid valves, push buttons)
Hoses and fittings

Compressed air generation in pneumatic controls

One or more compressors are used to generate the required process pressure. They draw in and compress the air as needed to a pressure between 6 and 40 bar.

The mechanical and thermodynamic processes in use to compress the air generate a large quantity of heat that must be evacuated from the compressed air. The compressed air is therefore routed through an air chiller to lower the temperature.

Compressed air conditioning

However, cooling down the air also reduces the ability of the air to absorb water. As the air cools it often releases water, which can damage the system. The air is passed through an air dryer to prevent this. There are several types of air dryers, such as refrigeration dryers and adsorption dryers, that remove moisture from the air. It is equally important to remove contaminants from compressed air to ensure optimum compressed air quality and long service life of compressed air systems. This is accomplished by passing the air through filters to remove contaminants, such as dust, particles, and oil. However, because oil is required to lubricate drives, compressed air is enriched with oil by using specialized oilers.

Storing compressed air

The conditioned air is stored in compressed air tanks. These tanks simultaneously compensate for pressure fluctuations when compressed air is removed from the system. The air accumulator is refilled when the pressure drops below a certain value.

Regulating and distributing compressed air

The air pressure is adjusted with a pressure regulator before the compressed air is used in the pneumatic circuit. The air is then distributed in the system over a network of piping and hoses. The compressed air system must be planned by taking into account various requirements, such as the diameter of the lines. The smaller the diameter of a pipe, the higher the flow resistance. The diameter must be selected such that the flow resistances remain as low as possible.

Leaks are yet another risk in compressed air systems. These are common in unions or manifolds. Such leaks lead to a continuous loss of compressed air, resulting in increased power consumption and reduced system performance. In addition to such direct losses, the system can also exhibit indirect losses. Over-sized compressors, overly restrictive or excessively long lines, unfavorable tank placement - all of which leads to performance degradation and inefficiencies in the system. Purposeful planning of the compressed air distribution is therefore a condition for optimizing the compressed air system for durability and performance.

Motion and Power Transmission

Various components in pneumatic circuits wok together to create motion and to transfer force. Valves control the direction, pressure and flow of compressed air. Pneumatic drives, such as cylinders or air motors, perform the actual work in a pneumatic circuit. They convert the energy contained in the compressed air into mechanical motion. Compressed air moves the piston inside the cylinder, transferring force, usually in a linear direction.

The mechanical work is performed by specialized working elements, which mainly come in the form of pneumatic cylinders - for example as pneumatic grippers.

Industrial conveyors move or transport material to various destinations in shop floors or warehouses. Pneumatic conveyors use compressed air to transport materials or components, such as granules, powders, or bulk materials through piping for further processing or disposal. These systems are used at different stages of production since they simplify and facilitate material handling.

General structure of a pneumatic circuit diagram

Pneumatic circuit diagrams are graphical representations of pneumatic controls. They show the function and connection of the individual components of a pneumatic system.

Pneumatic circuit diagrams include supply elements, actuators and process elements. Supply elements are responsible for supplying compressed air, and for processing, storing and distributing compressed air. Actuators are the control elements in a pneumatic circuit diagram. These includes, for example, directional valves, pressure valves, or check valves. They determine the flow and direction of the compressed air. Process elements are the components that perform the physical work in the circuit. They convert the energy stored in the compressed air into mechanical motion. Cylinders, motors or actuators are process elements.

In general, the circuits are arranged so that the power flows from bottom to top, i.e. from the power supply of the compressed air source to the process element. The compressed air source is therefore the first or lowest element and the power element is the uppermost or last element.

Application example with pneumatic circuit diagram

The following practical application example shows a piston rod to be extended (fully-deployed position) and retracted after a defined time (home position). For safety reasons, practitioners usually use 2 hand buttons to prevent unintentional piston deployment.

The application basically consists of the following components:

1 x dual-action pneumatic cylinder with piston rod (1 A)
2 x manual push buttons with directional control valves (1S1 and 1S2)
1 x accumulator with time delay valve (1V3) with throttle valve
1 x dual pressure valve (1V1)
1 x shuttle valve (1V2)
Pulse valves and directional valves

Example illustration - Extension is highlighted in blue in the illustration, retraction in green.

The dual pressure valve 1V1 acts like a logical ‘And’ circuit: compressed air can only pass to the pulse valve 1V4 if both manual buttons 1S1 and 1S2 are actuated simultaneously.
The 1V4 pulse valve is energized by the incoming air and is pressurized with compressed air.
The 1V4 pulse valve energizes the 1V5 directional control valve.
Due to the shift position of the directional control valve 1V5, the compressed air now enters the dual-action pneumatic cylinder 1 A and allows the piston rod to extend there (fully-deployed position). The piston rod initially remains in the fully-deployed position.

The operating principle of the planned circuit causes several things to happen at the same time during the switching action.

By initially actuating the hand buttons, compressed air simultaneously enters the shuttle valve 1V2 - the shuttle valve simultaneously acts as a non-return valve.
Compressed air fills the accumulator 1V3 - the accumulator has a time delay valve.
As soon as the pressure accumulator 1V3 is filled, the released compressed air energizes the pulse valve 1V4, thus causing the directional control valve 1V5 to return to its home position.
Due to the shift position of the directional valve 1V5, the compressed air now enters the dual-action cylinder 1 A and allows the piston rod to retract there (home position).
To extend the piston again, both hand buttons must be “released” and re-actuated.

The elements of the circuit diagram are labeled according to the labeling key specified in DIN ISO 1219-2. Depending on the application, the symbols contained in the standard can be combined accordingly. The following overview shows some examples.

Naming conventions for Directional Valves

The description of the directional valves is based on the number of ports, the number of switch positions and the flow path. Directional valves are assigned two numbers. The first indicates how many ports the valve has, and the second number indicates the number of switch positions. A 3/2-way valve, for example, has three ports and two switch positions. In practice, 2/2, 3/2, 5/2, and 5/3 directional valves are most commonly used.

3/2-way valve (with 3 ports and 2 switching positions, open)

5/3-way valve (with 5 ports and 3 switching positions, center position closed)

Grouping and design of directional control valves

Pneumatic actuators (for example, cylinders, etc.) are controlled by pneumatic valves. The function of the valves is to control the direction of action, the speed (via the flow rate) and the force.

Directional valves are one of the most important elements of pneumatic controls. They are used to determine the flow direction and to open or block the path for the medium. For example, they are used to actuate and control cylinders, valves, or pneumatic tools. Directional valves can be grouped according to different criteria:

By basic structure: Based on their design, one distinguishes between piston spool valves and seat valves.

By type of operation: Directional valves can be operated mechanically, manually, pneumatically or electrically.

By number of positions: There are monostable, bistable, three or multi-position valves. As the term implies, the valve has one stable position for monostable designs and two stable positions for bistable design (home position of the valve).

Based on the number of ports and positions: In terms of ports and positions, one distinguishes between 2/2, 3/2, 3/3, 4/2, 5/2, 4/3 and 5/3 way valves

By shift position in the home position: Depending on the number of ports and positions, 2/2 and 3/2 directional valves are differentiated based on whether they are open or closed in the home position. 3/3, 4/3, and 5/3 directional valves are distinguished into closed, open, and vented center positions.

3/2-way valve, in default position closed

What are your requirements for a pneumatic control?

MISUMI is a strong partner when it comes to offering high-quality, reliable pneumatic products. Explore our range of highly configurable pneumatic elements.

For example:

Pneumatic cylinders, rotary drives, and grippers
Electromagnetic pneumatic valves and pneumatic pressure valves
Pneumatic pipe fittings
Maintenance units, filters, regulators and lubricators for monitoring and conditioning air

Tensile strength classes of stainless steel screws and nuts

The tensile strength of steel and stainless steel is a critical parameter when it comes to the load-bearing capacity and safety of threaded joints. It is made up of two key parameters for steel screws based on which the tensile strength and the yield strength can be easily determined. But starting at a certain size, stainless steel screws and stainless steel nuts are marked with a letter-number combination (e.g. A4-80). The letter-number combination arranged in front of the hyphen assigns the screw so marked to a material group and steel group and thus provides information on the basic material properties of the screw. The number 80 to the right of the hyphen indicates the strength class. This article shows how to use the strength class of stainless steel bolts to determine their limits of use and provide an overview of the load-bearing capacity of typical strength classes.

Basic knowledge Joining

8 Minutes read

Tutorial: Coefficients of expansion of materials - Materials sciences theory

The coefficients of expansion of materials play a crucial role in materials science. Coefficients of expansion describe how the dimensions of a material change under the influence of temperature fluctuations. This article provides a comprehensive introduction to the topic, including the basics of thermal expansion, influencing factors, and various measurement methods. We also compare typical expansion coefficients of material classes and explain their uses in various industries. Lastly, we address thermal stresses and ways to prevent damage from temperature differences in materials.

Basic knowledge

11 Minutes read

Calculating the center of mass - with examples

Calculating the center of mass is an important step in many tasks in mechanical engineering and in the design of machines and components. The center of mass indicates where the weight of a body is concentrated and thus permits determining the forces and moments in the system. This article explores the basics of calculating the center of mass and provides some real-world examples.

Basic knowledge Construction

7 Minutes read

Spring design calculations

Springs are mechanical components that can store and deliver tension energy generated by the work used to tension, elongate or compress the spring in the form of deformation or tension energy. They are used in many applications and can be found in small electronic components as well as in large industrial machines. To ensure that a spring performs its function, it must be correctly designed. For this purpose, parameters such as the spring constant and the spring travel are calculated.