Directional air control valves are the building blocks of pneumatic control. Symbols representing these valves provide a wealth of information about the valve they represent. Symbols show the methods of actuation, the number of positions, the flow paths and the number of ports. Here is a brief breakdown of how to read a symbol:
Every valve symbol has multiple parts (see figure below). The Actuators are the mechanisms which cause the valve to shift from one position to another. The Position and Flow Boxes indicate how the valve functions. Every valve has at least two positions (learn more) and each position has one or more flow paths, thus every valve symbol has at least two Flow Boxes to describe those paths.
The number of position and flow boxes that make up a valve symbol indicate the number of positions the valve has. Flow is indicated by the arrows in each box. These arrows represent the flow paths the valve has when it is in that position.
2 Position, Lever Actuated, Spring Return Valve
The box next to the 'active' actuator always shows the current
flow path(s) of the valve. In the example above, when the lever is not activated, the spring actuator (right side) is controlling the valve, and the box adjacent to the spring shows the flow path. When the lever is actuated, the box next to the lever shows the flow path of the valve.
A valve can only be in one "Position" at a given time.
In the example below (a three position valve), the "spring return" actuators on both sides return the valve to the center position but only IF neither of the solenoids is active:
3 Position, Double Solenoid Actuated, Spring Return Valve
With this three position valve, the center position shows the flow path when neither actuator is active, and the springs are holding the valve in the center position). In this fairly common example, the center box indicates that there will be no air flow (i.e. the cylinder will 'hold position') unless one of the two actuators is active. This can be used to "bump" or "inch" a cylinder incrementally along its extension or retraction stroke for various purposes.
The number of ports is shown by the number of end points in a given box. Count only the ports in one flow box per symbol (the other boxes just show different states of the same valve). In the example at right, there are a total of 5 ports. Learn more about the number of ports here. NOTE: Sometimes a port (usually an exhaust port) goes directly to atmosphere and there is no mechanical means for attachment of silencers, flow control valves, or any other accessories. To indicate this (in some flow diagrams), ports with attachment capability will have a short line extending beyond the box (as shown on ports 1, 2, & 4), while the ports you cannot attach to will not have the external line segment (ports 3 & 5 in this example).
Valves are also refered to by the number of "ways" that air can enter or exit the valve. In most situations the number of ports and ways are the same for a given valve, but take a look at the 3 position example above. It has five ports, but it is considered a 4 way valve because two of the ports share the same exhaust function. This is a holdover from hydraulics - where the two exhaust paths are joined (internally to the valve), so that only one exhaust port is required, and only one return line is required to get the hydraulic oil back to the storage tank for re-use. The single return port (exhaust) is only counted as a single "way". In the case of our pneumatic valve (above) with similar functionality, the separate exhaust ports are created for mechanical simplicity (and as a cost saving measure), but they are not considered distinct "ways". The symbols above detail many of the ports, ways, and positions of common pneumatic valves. The specification for "ways" can be somewhat tricky; analyzing the circuit symbols is a better method for verifying that a given valve offers the required functionality.
|Valve Symbols, Flow Paths and Ports||Actuator Symbols|
|2 position, 2 way, 2 ported||Manual|
|2 position, 3 way, 3 ported||Push Button|
|2 position, 4 way, 4 ported||Lever|
|3 position, 4 way, 4 ported, Center Closed||Foot Operated|
|2 position, 4 way, 5 ported||Mechanical|
|3 position, 4 way, 5 ported||Detent|
|Simple Pneumatic Valves||Spring|
|Flow Control, 1 direction||Internal Pilot|
|Relief Valve||External Pilot|
|Lines||Piloted Solenoid with Manual Override|
|Main Line||Piloted Solenoid and Manual Override|
|Pilot Line||Lever with Spring|
|Exhaust Line or Control Line||Solenoid with Spring Return|
|Air Dryer||Direction of Flow|
|Air Motor (One Direction Flow)||Lubricator|
|Air Motor (Two Direction Flow)||Filter|
|Check Valve (Spring Loaded)||Filter (Automatic Drain)|
|Compressor||Filter (Manual Drain)|
|Cylinder (Spring Return)||Fixed Restriction|
|Cylinder Double Acting||Flow Meter|
|Cylinder Double Acting
|Cylinder Double Acting (Single Fixed Cushion)|
|Cylinder Double Acting (Two Adjustable Cushions)|