basic operating principles of all regulators are
similar to the one illustrated in figure 6-14.
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
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.