Chapter 2 Fluid Power Systems 41
Directional control valves regulate the direc-
tion of actuator movement by creating flow paths
to and from the actuator. A basic control valve con-
sists of a valve body, internal elements that open
and close fluid flow paths through the valve, an
external device for shifting the internal elements,
ports to connect the valve to the system, and seal-
ing devices to prevent fluid leakage, Figure 2-13.
When a directional control valve is used in a
fluid power system, it directs fluid to one side of
the internal elements of an actuator, causing the
actuator to move in one direction. Fluid from the
other side of the actuator’s internal elements flows
through the valve and is returned to the system
(or atmosphere). Shifting the valve creates a flow
path to the opposite side of the internal elements
of the actuator, causing the actuator to move in the
opposite direction.
Pressure control valves regulate pressure in
the system or a part of the system. These compo-
nents are used to control maximum system pres-
sure, limit pressure in a part of the system, or
delay the movement of an actuator until a desired
pressure is reached. A basic pressure control valve
consists of a valve body, internal elements that
control fluid flow through the valve, an external
device to allow adjustment of the valve, ports to
connect the valve to the system, and appropriate
sealing devices.
In a hydraulic system, the basic pressure con-
trol device is a closed valve that does not open to
allow fluid to flow through it until a desired pres-
sure is reached, Figure 2-14. When the system
reaches the desired pressure, the internal elements
of the valve move opening a passageway through
the valve. Fluid that is not needed to maintain the
desired system pressure then passes through the
valve to a lower-pressure section of the system.
The amount of fluid that passes through the valve
varies according to the amount of fluid needed to
maintain the desired system pressure. The smaller
the amount of fluid needed to maintain the desired
system pressure, the higher the flow through the
valve to the low-pressure section of the system.
In a pneumatic system, the basic pressure
control device is called a pressure regulator,
Figure 2-15. Regulators are located at each work-
station in a pneumatic system. A regulator is an
open valve that allows air to move through with-
out resistance until a desired system pressure is
reached. Once the desired pressure is reached,
the internal elements of the regulator begin to
close, allowing only enough air to pass through
to maintain the desired pressure.
Flow control valves regulate fluid flow in the
system or a part of the system. These valves con-
trol the volume of flow by varying the size of an
orifice in the passageway through which the sys-
tem fluid flows. A basic flow control valve consists
of a valve body, internal elements to control flow
through the valve, an external mechanism to allow
adjustment of the valve, ports to connect the valve
into the system, and appropriate seals to prevent
fluid leakage, Figure 2-16.
The passageway in a basic flow control valve
provides an open route for fluid flow through the
valve. The volume of fluid flow through the valve
To reservoir
To cylinder
port 1
From
pump
From cylinder
port 2
Lands Body
Spool
valve
Control
lever
Figure 2-13. Cutaway of a two position directional
control valve showing the fl ow passageways that
allow cylinder extension and retraction.
From
pump
To reservoir
Coil spring
Pressure
adjustment
Figure 2-14. The internal construction of a hydraulic
pressure control valve used to set the maximum
operating pressure of a system.