Unit%20Eleven

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Unit Eleven Simple Pressure Control Valves
Fluids that are under pressure are not safe or
useful unless they are controlled. The focus of
this unit is to explain how and where pressure
control valves are used. It should be noted that
the location of the valve is critical in
determining its function as related in a
schematic diagram.
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Basic Valve Symbol
In
Out
The above symbol is representative of both
hydraulic and pneumatic pressure controls. The
system power source determines the fluid nature
of the valve. All pressure controls are
interpreted the same way. These valves have two
ports, one an inlet and the other an outlet, as
indicated by the directional arrow. The dashed
line represents a pressure sensing capability.
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Hydraulic and Pneumatic Pressure Control Valves
Pictured above are two pressure controls most
often discussed in fluid power. On the left is a
relief valve used in hydraulics systems and on
the left, a pressure regulator used in
pneumatics. Their schematic symbols are similar .
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Hydraulic and Pneumatic Pressure Control Valves
At a glance the two valve symbols above look the
same but they are very different. The one on the
left is normally not passing and the one on the
right normally passing. Also, the sense lines
are located in different ports. You can see that
failure to identify these small differences can
cause confusion.
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Pressure Adjustment
In general pressure control valves that can be
adjusted have the arrow through the spring
diagonally as shown but if the arrow is not
present then the valve is considered not
adjustable. Usually, not adjustable valves have
a lock nut or some other such device to prevent
hand adjustment.
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Uses of a Normally Closed (not passing)Pressure
Control Valve
In the circuit above, the pressure control
ensures that fluid reaches the actuator at
maximum pump flow until a resistance is
encountered that exceeds the tension of the
spring. When the tension of the spring, due to
pressure build up, has been reached, oil will
move over the relief valve back to tank. System
pressure is equal to the tension of the spring.
This is a relief valve function.
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Sequence Valve
In the circuit pictured above, a second pressure
control valve appears in series between the DCV
and one of the actuators. This arrangement
ensures that the drill cylinder cannot move until
the clamp has fully extended.
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Counterbalance Valve
In the circuit above, gravity acts on the platen
to generate a pressure on the rod side of the
cylinder. If there was nothing to resist the
movement of oil from the rod side, the platen
could speed the piston away from incoming oil and
slam into the work surface below. By using the
incompressible nature of oil we can slow a heavy
platen by the use of back pressure.
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Counterbalance Valve
Back pressure can also be used to slow a motor
whose shaft is being driven by the inertia of a
flywheel. The back pressure generated is
relative to spring tension as is the stopping
speed.
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Pressure Reducing Valve
In this circuit the clamp cylinder must hold a
part that could be damaged by full system
pressure. The pressure reducing valve senses
pressure downstream and closes when the limit of
the spring is reached. Fluid must be able to
enter a volume in order for pressure to increase
so by cutting off flow to the volume, a cylinder
in this case, the pressure and clamping force are
controlled.
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Drains
Drains are only found in hydraulic pressure
control valves. Since hydraulic pressure control
valves use an internal member that allows some
bypass leakage, a way of releasing that leakage
oil is needed. As the diagrams above indicate,
there exists an area above the spool that could
fill with oil and lock up the valve. To
prevent the lock up we provide one of two ways
for leakage oil to escape the valve. If the
valve does not have pressure on the secondary
port, it may be drained internally to the
secondary port. In the case of all pressure
control valves whose secondary port is under
pressure, the valve must be drained externally
through an additional port.
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Direct and Remote Operation
A valve that senses pressure only from its inlet
or out port is said to be directly operated as
opposed to one that senses pressure from another
location which would be called remotely operated.
Usually, remote operated valves are referred to
as unloading valves.
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Unloading Valve
Unloading valves are used in hydraulic circuits
to momentarily dump the flow of the pump back
to tank during periods of machine idle time.
This reduces heat and saves energy since the load
on the prime mover is reduced.
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Hi-Lo System
Note position of unloading valve.
The purpose of the Hi-Lo system is to provide
momentary high flow for traverse action but then
reduce it for feed rate. A ram, for example,
with a long stroke could be moved at a high rate
of speed to approach a work piece and then slowed
to a crawl before contact thus increasing the
cycle time of the machine. Both pumps are used
for high flow and the large one dumped for low
flow.
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Remotely Operated Counterbalance Valve
In a previous slide a counterbalance valve was
shown as directly operated. In actual practice a
counterbalance valve might be remote operated as
above. The reason for remote operation is reduce
lost energy as a result of back pressure.
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Brake Valve
A real brake valve is more complicated than a
simple pressure control. It has two sense lines
that compare inlet and outlet pressures. It is
assumed by design that when a pressure is present
at the inlet port that the motor should turn.
The pressure at the inlet causes a piston to
move against a spool and to hold the valve open
for free movement of the motor. The moment the
pressure drops at the inlet port the piston
allows the spool with its spring to take over and
begin to apply back pressure to slow down the
motor shaft and its load.
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Reverse Flow
As you may have noticed, the sequence valve, like
many others, can only sense pressure from one
port. For fluid to get around the valve on
return, a check valve is added. The check valve
allows normal operation to take place as fluid is
moving toward the cap end of the actuator.
However, when fluid is leaving the cap end the
cylinder is in retraction and the function of the
sequence valve is not needed. Without the check
valve, fluid would not be able to get around the
valve and the cylinder would lock up.
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Pressure Control Valves in General
1. If a pressure control valve has pressure on
its secondary port, it must be externally drained.
2. If the secondary port of a pressure control
valve is not under pressure, it will usually be
internally drained.
3. Check valves are used to bypass flow around
normally not passing pressure controls.
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Completed Valve Symbols
In the following slides, schematic valve symbols
appear in their completed form. Their simple
form was used to introduce them in a
non-intimidating manner. In the industry, this
is how they really appear. Study each in detail.
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Relief Valve
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Unloading Valve
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Sequence Valve
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Counterbalance Valve
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Brake Valve
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Pressure Reducing Valve
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Pressure Regulator
Pressure regulators are normally found at the
beginning of branch line circuits. Typically,
the pressures required by branch line circuits
are much lower than the air pressure at the
receiver tank. A venting type regulator is one
that can exhaust air from its secondary port if
pressure exceeds the spring setting.