This dual exhaust arrangement allows the actuator plunger
to be moved by pressure applied by either port A or port B.
Use your brain’s simulation engine to position the
control valve spool to allow the control valve’s P input to
flow out of the control valve’s B port. Now the actuator’s
plunger is forced in the opposite direction and the exhaust
exits the actuator and flows into port A of the control
valve. The top T port is used to direct the flow of the
exhaust back to the source tank.
There’s one more position that the control valve can
take. Position the control valve spool to block the P input
and the T exhaust ports. I/O ports A and B are isolated
and hydraulic fluid cannot enter or leave the actuator.
We’ve just walked through a simple hydraulic system
built around a 5/3 directional control valve. The 5
represents the number of control valve ports and the 3
tells us how many positions the control valve can assume.
Now that we know what a 5/3 control valve is and how it
works, let’s put together a pneumatic schematic symbol for
the 5/3 valve being used in Figure 1.
Each position the directional control valve can assume
is represented by a box. So, our 5/3 symbol you see in
Figure 2 consists of three boxes that represent the three
possible valve positions. Let’s get up on our donkey and
declare that our 5/3 control valve’s normal unactuated
position is the center box, which is pneumatically
symbolized as totally closed. Closed in pneumatic speak
means that the flow channel is blocked. Thus, the “T”
shaped symbols inside of each box indicate a blocked port.
Note that our T tank return ports have been redefined as
exhaust ports EA and EB. We can’t return compressed air to
an open tank for reuse. So, in most pneumatic circuits, the
exhausted air is vented to the atmosphere. Depending on
the stance of the design engineer, you will also see the
hydraulic-based T replaced with an R, which is the first
letter of the word RETURN. Somewhere down the line,
a group of pneumatic engineers decided that a set of
standardized port identification numbers would work
better than the hydraulic-based alpha symbols. Some folks
adopted the number scheme and some didn’t. The
compromise is to display both as I have done in Figure 2.
So, from now on we will think and speak only in
alphanumeric pneumatic terms.
In Figure 1, the pressurized flow from the tank at
input P is being directed to the control valve’s A I/O port.
This is represented schematically by the left box of the
three-box 5/3 symbol shown in Figure 2. To decode the
left box, simply move the port symbols as a group from
the center box to their respective positions on the left box.
We can then resolve EA as blocked, P feeding I/O port A,
and I/O port B exhausted at EB.
Figure 3 converts our verbal description of the flow
into a graphical depiction of the flow. The “bull’s eye”
pneumatic symbol represents the main pneumatic air
source. The main air symbol hanging from the left box
corresponds to the center position P input port. The
triangular symbol denotes an exhaust port. As you can see,
the left box B port is exhausting into the EB port. Ports A
and B are accessible to equipment outside of the valve. So,
to indicate this availability we add a short line outside of
the box associated with each externally accessible port.
The third position of the control valve in Figure 1 will
direct flow out of I/O port B. Let’s see if our decoding
techniques work on the right box as well as they did on the
A( 4) -> I/O PORT A
B( 2) -> I/O PORT B
EA( 5) -> EXHAUST A
EB( 3) -> EXHAUST B
P(1) -> PRESSURE PORT
FIGURE 2. All you have to do is move the center group of
pneumatic symbols left and right to see the direction of
flow. Note the use of both numeric and alpha characters
to identify the ports of the valve. This is the most common
way you’ll see a pneumatic symbol drawn in the datasheets.
FIGURE 3. This is an etiquette view. Although this is the
proper way to convey the flow information, don’t expect to
see the P and EB ports graphically identified in this manner
all of the time. Also, you won’t see the external access lines
on every pneumatic port symbol you encounter.
FIGURE 4. The direction of flow is now P to B with A
exhausting to EA. Hey! This pneumatic schematic
stuff is almost as logical as electrical schematics.
SERVO 11.2009 43