Tech Briefs Magazine - August 2021 - MD-8

GAU02
PA
SUPPLY
REG02
VLV01
R
P
R
A
GAU01
VLV02
TO CONTROL
CIRCUIT
FIL01
REG01
LUB01
AIR TOOL
Figure 1. Air prep circuit diagram. A good design practice is to include an air preparation circuit on any automated
machine.
Filter FIL01 is used to remove particulates
and separate moisture from the air
and it is mounted just downstream of
the shutoff valve VLV01. This filter has a
liquid drain, indicated by the triangle at
the bottom of the symbol, that can be
either manual, semiautomatic, or automatic.
The symbol does not show detail
of the type of drain bowl such as allmetal
or guarded. A schematic with a
pneumatic panel layout drawing should
detail this information, along with additional
information such as mounting
brackets.
Downstream of the filter is a regulator
(REG01) or it may be an integral part of
the filter, which would be indicated by a
dashed-line box around both filter and
regulator. While not shown in the circuit
diagram, it is good practice to note the
working pressure of the machine and
the maximum pressure allowed. A phenolic
tag with this information is often
attached near the regulator.
A pressure gauge (GAU01) should
always be included with a regulator,
either built-in or threaded into the pressure
port, typically included with the
regulator. Often, a pressure switch (not
shown in Figure 1) is installed just downstream
of the regulator for monitoring
pressure okay status. This switch's output
is typically a discrete input to the
machine controller such as a programmable
logic controller (PLC).
8
Cov
ToC
Regulators have input and output
ports to ensure proper airflow, in some
cases with a relieving feature. The relieving
function reduces output air when the
regulator is adjusted to a lower pressure.
This function also ensures downstream
pressure is removed when upstream air
is exhausted. The regulator symbols
(REG01 and REG02) show they are
relieving type, indicated by the triangle
at the upper left corner of each device.
The air exiting regulator REG01 produces
clean, dry, and filtered air that is
then split through a tee fitting or a pneumatic
distribution block (not shown).
One branch provides a lubricated air supply
and the other a non-lubricated supply.
The non-lubricated supply feeds a
second regulator and then the electrically
operated soft-start/dump valve VLV02.
This valve typically removes air to motioncausing
pneumatic devices, such as cylinders
and actuators, for safety. These
devices typically don't need lubricating.
When an emergency stop is pressed,
power is removed from the valve, which
causes it to dump the motion-causing air
pressure from the system.
The air prep circuit diagram shown provides
lubricated and non-lubricated air.
Most pneumatic devices today do not need
lubricated air but if lubrication is needed
- such as with pneumatic air tools and
motors - it should be adjusted for light
oiling because too much oil can clog up
the pneumatic system components.
An all-in-one unit (such as the one
shown in Figure 2) can be used for air
preparation. It combines all the devices
discussed in Figure 1, less the lubricator,
in one unit. It can be purchased from a
vendor with a single part number, saving
purchasing, receiving, assembly, and
installation time. It also includes a
clogged filter indicator, adjustable pressure
switch with indicator LEDs, and
adjustable port sizes to match the flow
rate needed for an application.
Double-Acting Cylinder Circuit
Automation to extend and retract an
air cylinder is common in many ma -
chines. Figure 3 shows a pneumatic circuit
consisting of a 4-way solenoid valve
(SOL01) operating a double-acting cyl -
inder (CYL01). Filtered air from the air
preparation unit feeds a solenoid valve
controlled by a PLC.
The solenoid valve symbol SOL01
indicates it is a single-acting, springreturn
valve. This valve is pilot activated,
indicated by the triangles at each side of
the symbol. These pilot valves are efficient,
using a small amount of air to
move a large valve spool. However, a
minimum amount of air pressure is
required to move the spool. This minimum
operating air pressure is noted in
the valve specification - typically, about
20 psi is needed to ensure the valve operates
as designed.
Figure 2. NITRA Total Air Prep (TAP) all-in-one air
prep unit.
As depicted in the symbol of SOL01, a
spring on the left side pushes the valve
spool to the right when in its normal resting
state (OFF). With the valve off, air is
supplied out of port A and flows through
an adjustable flow control to the left side
of cylinder CYL01, retracting the cylinder.
While the cylinder is retracting, air on the
right side of the cylinder exits through a
flow control device. The check valve
around the adjustable flow control device
Motion Design, August 2021

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Tech Briefs Magazine - August 2021

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