Chapter 2 Fluid Power Systems 45
The pressure increase causes the pressure control
valve to open, returning excess oil to the reservoir.
The flow control valve continues to supply a set
volume of oil to the actuator until the actuator is
fully extended or the load on the actuator slows or
stops movement.
During retraction of the actuator the oil return-
ing to the reservoir must pass through the flow
control valve. Oil movement through the valve
orifice is reversed, but continues to be at a reduced
flow rate, which slows the retraction speed of the
actuator. A one-way check valve is often included
in a flow control valve to allow free flow of oil
around the restrictive orifice. This design allows
the actuator to be retracted at maximum speed.
This system illustrates the structure and oper-
ation of a typical basic hydraulic system. The size,
arrangement, and complexity of components var-
ies from system to system, but the basic system
functions remain the same.
Basic Pneumatic System
A basic pneumatic system is illustrated in
Figure 2-23. The system includes an electric motor
as the prime mover, which provides the energy to
compress the air used for system operation. The
system also contains basic components from each
of the other component groups to allow operation
and control of a cylinder.
High-pressure oil flow
Low-pressure oil flow
Figure 2-20. Oil fl ow through a basic hydraulic system during cylinder
extension.
High-pressure oil flow
Low-pressure oil flow
Figure 2-21. Oil fl ow through a basic hydraulic system during cylinder
retraction.