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PRESSURE REGULATORS
Pressure regulators, often referred to as unloading
valves, are used in fluid power systems to
regulate pressure. In pneumatic systems, the valve,
commonly referred to as a pressure regulator,
simply reduces pressure. This type of valve
is discussed later in this chapter under pressure-reducing
valves. In hydraulic systems the pressure
regulator is used to unload the pump and to
maintain and regulate system pressure at the desired
values. All hydraulic systems do not require
pressure regulators. The open-center system
(discussed in chapter 12) does not require a
pressure regulator. Many systems are equipped with
variable-displacement pumps (discussed in chapter
4), which contain a pressure-regulating device.
Pressure regulators are made in a variety of types
and by various manufacturers; however, the
A
regulator is open when it is directing fluid under
pressure into the system (fig. 6-14, view A). In
the closed position (fig. 6-14, view B), the fluid in
the part of the system beyond the regulator is trapped
at the desired pressure, and the fluid from the
pump is bypassed into the return line and back to
the reservoir. To prevent constant opening and closing
(chatter), the regulator is designed to open at
a pressure somewhat lower than the closing pressure.
This difference is known as differential or
operating range. For example, assume that a pressure
regulator is set to open when the system pressure
drops below 600 psi, and close when the pressure
rises above 800 psi. The differential or operating
range is 200 psi.
Referring to figure 6-14, assume that the piston
has an area of 1 square inch, the pilot valve has
a cross-sectional area of one-fourth square inch,
and the piston spring provides 600 pounds of
force pushing the piston down. When the pressure
in the system is less than 600 psi, fluid from
the pump will enter the inlet port, flow to the
top of the regulator, and then to the pilot valve.
When the pressure of the fluid at the inlet increases
to the point where the force it creates against
the front of the check valve exceeds the force
created against the back of the check valve by
system pressure and the check valve spring, the check
valve opens. This allows fluid to flow into the
system and to the bottom of the regulator against
the piston. When the force created by the system
pressure exceeds the force exerted by the spring,
the piston moves up, causing the pilot valve
to unseat. Since the fluid will take the path of
least resistance, it will pass through the regulator
and back to the reservoir through the return
line.
When the fluid from the pump is suddenly allowed
a free path to return, the pressure on the input
side of the check valve drops and the check valve
closes. The fluid in the system is then trapped
under pressure. This fluid will remain pressurized
until a power unit is actuated, or until pressure
is slowly lost through normal internal leakage
within the system.
When the system pressure decreases to a point slightly
below 600 psi, the spring forces the piston down
and closes the pilot valve. When the pilot valve
is closed, the fluid cannot flow directly to the
return line. This causes the pressure to increase in
the line between the pump and the regulator. This
pressure opens the check valve, causing the fluid
to enter the system.
In summary, when the system pressure decreases
a certain amount, the pressure regulator will
open, sending fluid to the system. When the system
pressure increases sufficiently, the regulator
will close, allowing the fluid from the pump
to flow through the regulator and back to the
reservoir. The pressure regulator takes the load off
of the pump and regulates system pressure.
Figure 6-14.—Hydraulic pressure regulator.
Figure 6-15.—Installation of sequence valves.
SEQUENCE VALVES
Sequence valves control the
sequence of operation between two
branches in a circuit; that is,
they enable one unit to automatically set another
unit into motion. An example of the use of
a sequence valve is in an aircraft landing gear actuating
system.
In a landing gear actuating system, the landing gear
doors must open before the landing gear starts
to extend. Conversely, the landing gear must be
completely retracted before the doors close. A sequence
valve installed in each landing gear actuating
line performs this function. A
sequence valve is somewhat similar to a relief
valve except that, after the set pressure has been
reached, the sequence valve diverts the fluid to
a second actuator or motor to do work in another
part of the system. Figure 6-15 shows an installation
of two sequence valves that control the
sequence of operation of three actuating cylinders.
Fluid is free to flow into cylinder A. The
first sequence valve (1) blocks the passage of fluid
until the piston in cylinder A moves to the end
of its stroke. At this time, sequence valve 1 opens,
allowing fluid to enter cylinder B. This action
continues until all three pistons complete their
strokes.
There are various types of sequence valves. Some
are controlled by pressure and some are controlled
mechanically.
Pressure-Controlled Sequence Valve
The operation of a typical pressure-controlled sequence
valve is illustrated in figure 6-16. The opening
pressure is obtained by adjusting the tension
of the spring that normally holds the piston
in the closed position. (Note that the top part
of the piston has a larger diameter than the lower
part.) Fluid enters the valve through the inlet
port, flows around the lower part of the piston
and exits the outlet port, where it flows to the
primary (first) unit to be operated (fig. 6-16, view
A). This fluid pressure also acts against the lower
surface of the piston.
Figure 6-16.—Operation of a pressure-controlled sequence valve.
When the primary actuating unit completes its operation,
pressure in the line to the actuating unit increases
sufficiently to overcome the force of the spring,
and the piston rises. The valve is then in the
open position (fig. 6-16, view B). The fluid entering
the valve takes the path of least resistance and
flows to the secondary unit.
A drain passage is provided to allow any fluid leaking
past the piston to flow from the top of the
valve. In hydraulic systems, this drain line is usually
connected to the main return line.
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