Pneumatic Circuits
Page (1,
2, 3, 4) Quick
Exhaust Valves
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There are several special purpose valves that are used in
pneumatic circuitry. A quick exhaust valve is an example of
one. It is most commonly used in conjunction with a three or
four-way directional control valve to increase the exhaust
flow (and thus generally increase the rod speed of the
cylinder).
The quick exhaust valve typically consists of a resilient disc
in a body. The resilient disc “shuttles” side to side,
changing the free flow paths through ports in the body. |
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How a Quick Exhaust Valve Works
The quick exhaust valve is typically
used in conjunction with three or four-way directional control
valves. When its inlet port (1) is connected to the supply
pressure, it automatically seals its exhaust port (3) and allows
pressure to enter the cylinder (2). If the valve is of the “floppy
disc” design, a control valve is shifted so that it can exhaust
the supply to the cylinder. The resilient disc moves away from the
exhaust port and allows the trapped air in the cylinder to quickly
exhaust directly to the atmosphere. This eliminates the need for
exhaust air from the cylinder to flow through long, often somewhat
restrictive, lines to a main control valve. Quick exhaust devices
should be connected (in juxtaposition) to the cylinder port to
give rapid cylinder velocities.
A Quick Exhaust Valve in a Circuit
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One application for a quick exhaust valve is using single
acting spring return cylinders. With a standard circuit, many
times the return speed may be too slow. This may be due to
inadequate spring force, high frictional resistance due to the
load or the restriction of exhaust flow or a combination of
several of these factors.
Shown in Figure 8-1
is a single acting cylinder with a quick exhaust valve closely
coupled to the cylinder. This permits greater cylinder speeds
than would exist if the air from the cylinder were exhausted
through the directional control valve, but without increasing
its size. During extension of the cylinder, the quick exhaust
is part of the supply system that feeds air to the cylinder
(Figure 8-2). However, during the return stroke, the quick
exhaust shifts as soon as the pressure between it and the
directional control valve drops below the pressure at port C.
This permits free exhausting of the air from the cylinder
directly to the atmosphere, bypassing the directional control
valve and all the interconnecting lines.
Quick exhaust valve may also be added to double acting
cylinder applications. Shown in Figure 8-3 is an application
that will deliver an impact blow. In this circuit, a quick
exhaust was incorporated at the cap end for high return speed.
Note that the volume of air under pressure in our example will
act as an air spring. |
Figure 8-1
Figure 8-2
Figure 8-3 |
Quick Exhaust
Used as Shuttles
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Many quick exhaust valves
may also be used as shuttle valves. Shuttle valves are
automatic flow path selectors, which allow the higher of two
pressures to be directed into a flow path. The use of such
devices is common in control circuits to “select” the higher
sources of pressure within the circuit. However, when
exhausting a large valve, especially through a quick exhaust
valve directly to the atmosphere, unwanted air noise may
become a problem. |
Shuttle Valve |
Sample Circuit
– Controlling Two Movements with One Valve
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A single valve can
control two movements simultaneously while delaying one
actuator in either direction (Figure 9). This is accomplished
by using two remote operated 4-way valves and a 3-way valve.
In this circuit, the main 3-way valve (3), when actuated, will
direct pressure to the pilot ports of the 4-way valves (1 &
2). The flow control (FC1) will prevent a rapid rod extension
allowing the other actuator to complete the stroke first. When
the 3-way valve is returned to the normal position, the pilot
pressure is exhausted from both pilot ports. The flow control
(FC2) will restrict the removal of pilot pressure from valve
(1) and will delay the retraction of the actuator. A small
volume of air between the pilot port of the valve (1) and the
flow control will help increase the time of delay. |
Figure 9 |
Sample Circuit
- Manual Advance with an Automatic Return
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The circuit in
Figure 10 will provide a manual advance with an automatic
return, regardless where the cylinder piston rod is physically
stopped during the extension. The advance is accomplished by
momentary pressure to the pilot of the 4-way directional
control valve. This will advance the cylinder. Back pressure
in the rod end of the cylinder is present until the piston rod
comes to rest. At this point, the threshold sensor senses the
absence of pressure (6 to 9 PSIG) on the rod end of the
cylinder and sends a momentary pilot signal to the other pilot
port of the 4-way valve. The valve will shift and return the
cylinder to the normally retracted position. A second
threshold sensor could be used for a reciprocating system. |
Figure 10 |
Page
(1,
2, 3, 4) |
