Circuit & Symbols
Basic Rules For Drawing Pneumatic Circuits & Symbols
Pneumatic circuits are represented by common symbols and should be drawn following several basic guidelines. By following these guidelines engineers can quickly and simply construct circuits that can be easily and safely interpreted by all those involved in system design, specification, manufacturing and maintenance.
The symbols used for constructing fluid power systems and for defining individual components are detailed in the standard ISO 1219, which covers both pneumatic and hydraulic devices.
This states that: ‘A symbol consists of the lines, letters and abbreviations which identify the purpose and method of operation of the component represented. The symbols show connections, flowpaths and the functions of the components represented. The symbols do not indicate the physical location of ports or the position of control elements on actual components. With certain obvious exceptions symbols may be reversed and/or rotated without altering their meaning’.
Standard ISO symbol conventions are therefore intended to represent graphically the function, not the construction or appearance, of each device.
It should be noted, however, that the ISO standards do not always keep abreast of developments in pneumatics technology, so that new products often fail to be clearly defined until some time after their introduction. As a result, product manufacturers often use proprietary symbols until such time as a standard unit is defined.
The situation can be further confused by the fact that the Japanese Industry Standard (JIS) and the American Standards Authority (ASA) also use differing sets of symbols.
All three standards authorities use common symbols for air preparation equipment and, with a few exceptions, valves; the JIS, however, reflecting the greater diversity of equipment originating in Japan, often uses different symbols for special purpose or newly developed actuators.
For most pneumatic circuits, however, the standard ISO nomenclature is adequate, with the majority of air preparation, air treatment, actuators, valves and ancillary components being defined by common symbols.
A complete list of symbols and guidance on their construction is available from SMC.
When constructing a circuit diagram, there are always basic rules that should be followed.
• The flow of working energy in a circuit diagram is drawn from the bottom to the top of the diagram
• The sequence of the working cycle is from left to right of the circuit diagram; the air supply is therefore shown at the lower left, with the cylinder that performs the first stroke of the cycle being situated in the upper left.
• Power valves are drawn directly below the cylinders they operate.
• All circuit components are shown in their rest position, with the supply under pressure.
• Electrical connections are not generally shown.
• Mechanically operated valves, controlling the rest position of cylinder driven parts, are operated in the rest position and must be drawn accordingly; external connections are drawn to the valve symbol square on the operator side