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Mechanically Operated Sequence Valve
The mechanically operated sequence valve (fig.
6-17) is operated by a plunger that extends through
the body of the valve. The valve is mounted
so that the plunger will be operated by the
primary unit.
A check valve, either a ball or a poppet, is installed
between the fluid ports in the body. It can
be unseated by either the plunger or fluid pressure.
Port A (fig. 6-17) and the actuator of the primary
unit are connected by a common line. Port
B is connected by a line to the actuator of the
secondary unit. When fluid under pressure flows
to the primary unit, it also flows into the sequence
valve through port A to the seated check valve
in the sequence valve. In order to operate the
secondary unit, the fluid must flow through the
sequence valve. The valve is located so that the
primary unit depresses the plunger as it completes
its operation. The plunger unseats the
check valve and allows the fluid to flow
Figure 6-17.—Mechanically operated sequence valve.
through the valve, out port B, and to the secondary
unit.
This type of sequence valve permits flow in the
opposite direction. Fluid enters port B and flows
to the check valve. Although this is return flow
from the actuating unit, the fluid overcomes spring
tension, unseats the check valve, and flows out
through port A.
PRESSURE-REDUCING VALVES
Pressure-reducing valves provide a steady pressure
into a system that operates at a lower pressure
than the supply system. A reducing valve can
normally be set for any desired downstream pressure
within the design limits of the valve. Once the
valve is set, the reduced pressure will be maintained
regardless of changes in supply pressure
(as long as the supply pressure is at least as
high as the reduced pressure desired) and regardless
of the system load, providing the load does
not exceed the design capacity of the reducer.
Figure 6-18.—Spring-loaded pressure-reducing valve.
There are various designs and types of pressure-reducing
valves. The spring-loaded reducer
and the pilot-controlled valve are discussed
in this text.
Spring-Loaded Reducer
The spring-loaded pressure-reducing valve (fig.
6-18) is commonly used in pneumatic systems.
It is often referred to as a pressure regulator.
The valve simply uses spring pressure against a
diaphragm to open the valve. On the bottom of
the diaphragm, the outlet pressure (the pressure in
the reduced-pressure system) of the valve forces the
diaphragm upward to shut the valve. When the
outlet pressure drops below the set point of the
valve, the spring pressure overcomes the outlet pressure
and forces the valve stem downward, opening
the valve. As the outlet pressure increases, approaching
the desired value, the pressure under
the diaphragm begins to overcome spring pressure,
forcing the valve stem upwards, shutting the
valve. You can adjust the downstream pressure
by turning the adjusting screw, which varies
the spring pressure against the diaphragm. This
particular spring-loaded valve will fail in the open
position if a diaphragm rupture occurs.
Pilot-Controlled Pressure-Reducing Valve
Figure 6-19 illustrates the operation of a pilot-controlled
pressure-reducing valve. This valve
consists of an adjustable pilot valve, which controls
the operating pressure of the valve, and a
spool valve, which reacts to the action of the pilot
valve.
The pilot valve consists of a poppet (1), a spring
(2), and an adjusting screw (3). The valve
Figure 6-19.—Pilot-controlled pressure-reducing valve.
spool assembly consists of a valve spool (10) and a
spring (4). Fluid under main
pressure enters the inlet port (11)
and under all conditions is free to flow through
the valve and the outlet port (5). (Either port
5 or port 11 maybe used as the high-pressure port.)
Figure 6-19, view A, shows the valve in
the open position. In this
position, the pressure in the reduced-pressure
outlet port (6) has not reached the
preset operating pressure of the valve. The fluid
also flows through passage 8, through smaller passage
9 in the center of the valve spool, and into chamber
12. The fluid pressure at outlet port 6 is
therefore distributed to both ends of the spool. When
these pressures are equal the spool is hydraulically balanced.
Spring 4 is a low-tension spring and
applies only a slight downward force on the spool.
Its main purpose is to position the spool and
to maintain opening 7 at its maximum size. As
the pressure increases in outlet port 6 (fig. 16,
view B), this pressure is transmitted through passages
8 and 9 to chamber 12. This pressure also acts
on the pilot valve poppet (1). When this pressure
increases above the preset operating pressure
of the valve, it overcomes the force of pilot
valve spring 2 and unseats the poppet. This allows
fluid to flow through the drain port (15). Because
the small passage (9) restricts flow into chamber
12, the fluid pressure in the chamber drops.
This causes a momentary difference in pressure
across the valve spool (10) which allows fluid
pressure acting against the bottom area of the
valve spool to overcome the downward force of
spring 4. The spool is then forced upward until the
pressures across its ends are equalized. As the spool
moves upward, it restricts the flow through opening
7 and causes the pressure to decrease in the
reduced pressure outlet port 6. If the pressure in
the outlet port continues to increase to a value above
the preset pressure, the pilot valve will open again
and the cycle will repeat. This allows the spool
valve to move up higher into chamber 12; thus
further reducing the size of opening 7. These
cycles repeat until the desired pressure is maintained
in outlet 6.
When the pressure in outlet 6 decreases to a value
below the preset pressure, spring 4 forces the
spool downward, allowing more fluid to flow through
opening 7.
COUNTERBALANCE VALVE
The counterbalance valve is normally located in
the line between a directional control valve and the
outlet of a vertically mounted actuating cylinder
which supports weight or must be held in
position for a period of time. This valve serves as
a hydraulic resistance to the actuating cylinder. For
example, counterbalance valves are used in some
hydraulically operated forklifts. The valve offers
a resistance to the flow from the actuating cylinder
when the fork is lowered. It also helps to
support the fork in the UP position.
Counterbalance valves are also used in air-launched weapons
loaders. In this case the valve is
located in the top of the lift cylinder. The valve requires
a specific pressure to lower the load. If adequate
pressure is not available, the load cannot be
lowered. This prevents collapse of the load due to
any malfunction of the hydraulic system. One
type of counterbalance valve is illustrated in
figure 6-20. The valve element is a balanced spool
(4). The spool consists of two pistons permanently
fixed on either end of a shaft. The inner
surface areas of the pistons are equal; therefore,
pressure acts equally on both areas regardless
of the position of the valve and has no effect
on the movement of the valve—hence, the term
balanced. The
shaft area between the two pistons
provides the area for the fluid to flow
Figure 6-20. —Counterbalance valve.
when the valve is open. A small piston (9) is attached
to the bottom of the spool valve. When
the valve is in the closed position, the top
piston of the spool valve blocks the discharge port
(8). With the valve in this position, fluid flowing
from the actuating unit enters the inlet port
(5). The fluid cannot flow through the valve because
discharge port 8 is blocked. However, fluid
will flow through the pilot passage (6) to the small
pilot piston. As the pressure increases, it acts on
the pilot piston until it overcomes the preset pressure
of spring 3. This forces the valve spool (4)
up and allows the fluid to flow around the shaft
of the valve spool and out discharge port 8.
Figure 6-20 shows the valve in this position. During
reverse flow, the fluid enters port 8. The spring
(3) forces valve spool 4 to the closed position.
The fluid pressure overcomes the spring tension
of the check valve (7). The check valve opens
and allows free flow around the shaft of the
valve spool and out through port 5. The
operating pressure of the valve can be adjusted
by turning the adjustment screw (1), which
increases or decreases the tension of the spring.
This adjustment depends on the weight that
the valve must support.
It is normal for a small amount of fluid to leak around
the top piston of the spool valve and into the
area around the spring. An accumulation would
cause additional pressure on top of the spool
valve. This would require additional pressure
to open the valve. The drain (2) provides a
passage for this fluid to flow to port 8.
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