valve, around the main valve spring, and down
to the return line.
There is also a narrow passage (piston passage)
through the main valve piston. This passage
connects the high-pressure line to the valve
chamber.
The pilot valve is a small, ball-type, spring-
loaded check valve, which connects the top of the
passage from the valve chamber with the passage
through the main valve stem. The pilot valve is
the control unit of the relief valve because the
pressure at which the relief valve will open
depends on the tension of the pilot valve spring.
The pilot valve spring tension is adjusted by
turning the adjusting screw so that the ball will
unseat when system pressure reaches the preset
limit.
Fluid at line pressure flows through the
narrow piston passage to fill the chamber.
Because the line and the chamber are connected,
the pressure in both are equal. The top and
bottom of the main piston have equal areas;
therefore, the hydraulic forces acting upward
and downward are equal, and there is no tendency
for the piston to move in either direction.
The only other force acting on the main valve
is that of the main valve spring, which holds it
closed.
When the pressure in the high-pressure line
increases to the point at which the pilot valve
is set, the ball unseats (fig. 6-13, view B).
This opens the valve chamber through the
valve stem passage to the low-pressure return
line. Fluid immediately begins to flow out of the
chamber, much faster than it can flow through
the narrow piston passage. As a result the
chamber pressure immediately drops, and the
pilot valve begins to close again, restricting
the outward flow of fluid. Chamber pressure
therefore increases, the valve opens, and the cycle
repeats.
So far, the only part of the valve that has
moved appreciably is the pilot, which functions
just like any other simple spring-loaded relief
valve. Because of the small size of the piston
passage, there is a severe limit on the amount
of overpressure protection the pilot can provide
the system. All the pilot valve can do is limit
fluid pressure in the valve chamber above the
main piston to a preset maximum pressure,
by allowing excess fluid to flow through the
piston passage, through the stem passage, and
into the return line. When pressure in the system
increases to a value that is above the flow capacity
of the pilot valve, the main valve opens,
permitting excess fluid to flow directly to the
return line. This is accomplished in the following
manner.
As system pressure increases, the upward force
on the main piston overcomes the downward
force, which consists of the tension of the main
piston spring and the pressure of the fluid in the
valve chamber (fig. 6-13, view C). The piston then
rises, unseating the stem, and allows the fluid to
flow from the system pressure line directly into
the return line. This causes system pressure to
decrease rapidly, since the main valve is designed
to handle the complete output of the pump. When
the pressure returns to normal, the pilot spring
forces the ball onto the seat. Pressures are equal
above and below the main piston, and the main
spring forces the valve to seat.
As you can see, the compound valve over-
comes the greatest limitation of a simple relief
valve by limiting the flow through the pilot valve
to the quantity it can satisfactorily handle. This
limits the pressure above the main valve and
enables the main line pressure to open the main
valve. In this way, the system is relieved when an
overload exists.
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
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