Page 225 - Root Cause Failure Analysis

Control Valves 213

actuating unit in one direction. It is returned to its original position either by a spring
or the load on the actuating unit.

Four Way
Most actuating devices require system pressure in order to operate in two directions.
The four-way directional control valve, which contains four ports, is used to control
the operation of such devices (see Figure 17-12). The four-way valve also is used in
some systems to control the operation of other valves. It is one of the most widely
used directional-control valves in fluid-power systems.

The typical four-way directional control valve has four ports: a pressure port, a return
port, and two cylinder or work (output) ports. The pressure port is connected to the
main system-pressure line and the return port is connected to the reservoir return line.
The two outputs are connected to the actuating unit.

Performance

The criteria that determines performance of fluid-power valves are similar to those for
process-control valves as discussed previously. As with process-control valves, fluid-
power valves must be selected based on their intended application and function.

Installation
When installing fluid power control valves, piping connections are made either
directly to the valve body or to a manifold attached to the valve’s base. Care should be
taken to ensure that the piping is connected to the proper valve port. The schematic
diagram affixed to the valve body will indicate the proper piping arrangement, as well

AIR INTRODUCED THROUGH CENTERING SPRINGS PUSH AGAINST
n
THIS PASSAGE PUSHES WASHERS CENTERING WASHERS TO
AGAINST THE PISTON CENTER THE SPOOL WHEN
WHICH SHIFTS THE NO AIR IS APPLIED
SPOOL TO THE RIGHT
\
\

\

PISTONS SEAL THE
AIR CHAMBER FROM
THE HYDRAULIC CHAMBER
Figure 17-12 Four-way,jiuid-power valves.