Transcript Document

Unit Twelve
Pilot Operated Pressure Control
Valves
At first glance the diagram would appear to show three different
valves but the box surrounding the two schematic symbols is a
component enclosure meaning that there is only one component and
it includes everything seen.
Direct vs. Pilot Operated
A direct acting pressure control valve is one whose
sealing member is forced to open solely by the
pressure of fluid. Direct acting pressure control
valves are common in hydraulics as a relief valve.
Pilot operated pressure control valves are actually two
valves made into one. Sometimes called a “two stage”
pressure control, pilot operated pressure control valves
have a primary and secondary stage. When the
primary stage activates
Override-Direct Operated
Pressure Control Valves
Override is a characteristic of a valve where the valve will
momentarily create a pressure beyond its setting. This is
caused by the compression of a spring. High override can
damage sensitive components as well as waste energy.
Override-Pilot Operated Pressure
Control Valves
Pilot operated valves don’t have as much override because they use a
light spring and fluid pressure to hold the valve in its normal state.
The moment the fluid is released, the valve opens very fast because
the light spring has little resistance.
Direct Acting Pressure Control
In the illustration above, a only spring tension holds the
ball against the seat. The adjustment knob, shown at the
bottom, is for changing tension on the spring.
Pilot Operated Pressure Control
In the illustration above both fluid and spring force hold the main
poppet(main stage) closed. After the pilot relief poppet opens from
system pressure, oil begins to vent from the spring side creating a
pressure differential and reducing the total force holding down on
the main poppet. When the pressure differential is great enough the
main poppet opens quickly. It is this rapid opening capability that
makes this design more efficient than the direct acting type.
Pilot Operated Pressure Control Valve
Operation
Initially at start up the hydraulic system would be at low pressure,
for a couple of seconds, and the relief valve shown above would be
closed. The orifice ensures that system pressure is “felt” on both
sides of the spool. Study the illustration to become familiar with the
individual components.
Pilot Operated Pressure Control
Valve Operation
In the illustration, the port subjected to 1000psi is the
inlet port and is directed attached to the system. The
port down below is connected to tank.
Pilot Operated Pressure Control Valve
Operation
Since the dart is biased by a stiff spring, it will take more pressure to
overcome it than the spring of the main spool which is light by
comparison. The action of the dart opening starts the venting
process by creating a pressure differential across the main spool.
Pilot Operated Pressure Control Valve
Operation
Once the dart has begun to open, it will take an additional pressure
build up of 25psi before the main spool opens fully. It should be
noted that the main spool “throttles” in a relief valve. Main spool
positioning is based on the demand for flow.
Pilot Operated Pressure Control
Valve Operation
While venting, the dart is completely unseated and passing oil at
high pressure. With the dart open, a pressure differential exists
because the oil leaving the area above the spool is greater than the oil
that can flow in through the tiny orifice. When pressure drops the
dart closes and oil pressure builds on the spool, closing it.
Sequence Valve
Simply put, the purpose of a sequence valve is to cause a series of
operations to occur in order. This normally not passing valve blocks
oil to the “secondary” actuator until a preset limit of pressure is
reached. When pressure is correct, the sequence valve opens and
allows flow to reach the secondary actuator for movement. Unlike
the relief valve, the sequence valve will open completely. Usually, a
bypass check valve is included for reverse flow.
Counterbalance or Unloading
Valve
This valve is remotely operated. Remember that the valve is
watching the sense line for pressure, not its inlet, so we can control a
tremendous load but allow it to move at relatively low pressure by
use of the remote pilot.
Pressure Reducing Valve
The pressure reducing valve is the only normally passing pressure
control used in hydraulics. Just like the pneumatic pressure
regulator, the reducing valve controls pressure in hydraulic branch
line circuits. Recall that we can control the clamping force of a
cylinder with this valve.
Methods of Remote Control
Any time the oil on top of the main spool is vented, the
main spool will open. All methods of remote control
work on this principle.
Remotely Operated Pressure Control
Valve
In the example above, the remote pilot valve serves to control the
relief valve by venting the oil on top of its main spool. The remote
pilot might be located some distance from the relief valve, maybe on
a control panel. The other dart on the relief valve could be used for
maximum pressure control of the valve.
Remotely Control of Relief Valve
Schematic Representation
There is no component enclosure symbol because these two
valves are not physically together.
Alternative Remote Control
When examine the above illustration, please remember that any
time the oil is released from above the main spool, this type of
valve will actuate. You could even use two way hand valve to do
the job.
Differential Unloading Relief Valve
This valve, unlike the others, gives us a “cut in” and “cut out”
range for unloading a pump. This valve is usually used with an
accumulator so that the pump may be unloaded during periods
where the system is being powered by the stored volume of the
accumulator.
Differential Unloading Relief Valve
In a Circuit
Notice the two sense lines. This valve is watching the pressure at the
pump outlet as well as the accumulator. The check valve isolates the
system from the pump during the unloaded periods.
Pilot Controlled Pressure
Regulator
Some of the control methods used in hydraulics are also used in
pneumatics. Here, pilot pressure, in this case air pressure, is used to
push the piston down while pressure from the secondary port pushes
up on the piston. Whenever the air on top is exhausted, the valve will
close.