44 Fluid Power
As a result, oil moves from the reservoir into the
intake line of the pump and through a system fil-
ter where impurities are removed. Continued rota-
tion of the pump forces oil into the system lines
(conductors) where it is distributed to other sys-
tem components.
The first system component the oil encoun-
ters as it is forced through the system lines is the
pressure control valve, which is used to set sys-
tem pressure. Next is the directional control valve.
This valve directs the oil to the actuator (a cylin-
der in this example). Forcing oil into the actuator
causes the actuator piston and rod to move (either
extend or retract). Oil already in the actuator on
the other side of the piston is moved out into sys-
tem lines and returned to the reservoir through
the directional control valve, Figure 2-20. Shifting
the directional control valve directs the oil flow to
the other side of the actuator piston, which forces
the piston and rod to move in the opposite direc-
tion, Figure 2-21. Oil from the other side of the pis-
ton is returned to the reservoir. The extension and
retraction of the actuator does the work that the
system was designed to perform.
When the actuator is fully extended or retracted,
or when it encounters a load heavier than it can
move, there is no place for the oil that the pump
continues to force into the system. This condition
could cause pressure in the system to rise to a level
System
pressure
gauge Filter
Pump
Directional
control
valve Flow
control
valve
Pressure
control
valve
Power
unit
Prime
mover
Cylinder
Figure 2-19. The components of a basic hydraulic fl uid power system that can provide
directional, fl ow, and pressure control for a cylinder.
high enough to damage system components. A
pressure control valve located between the pump
and the directional control valve protects the sys-
tem from this potentially damaging high pressure.
The pressure control valve is set at a pressure that
is safe and appropriate for the system. The inter-
nal elements of the pressure control valve begin to
open when system pressure approaches the valve
setting. Oil that cannot be moved into the actuator
passes through the pressure control valve and is
returned to the reservoir. See Figure 2-22.
The speed at which an actuator moves depends
on the rate of oil flow into the actuator. In a basic
system, actuator extension speed is controlled by
placing a flow control valve in the line between the
directional control valve and the actuator. Clos-
ing the valve reduces the size of the orifice, which
restricts oil flow though the valve. This restricted
flow reduces the rate of actuator movement.
Opening the valve enlarges the size of the orifice,
which increases oil flow and the rate of actuator
movement.
Restricting flow using a flow control valve
effects the system in ways other than controlling
actuator speed. The pump moves more oil into
the system in front of the flow control valve than
the lines and valves in that part of the system can
accommodate. Attempting to force the oil into an
inadequate space causes system pressure to rise.