Page 226 - Root Cause Failure Analysis

214 Root Cause Failure Analysis
as the designed operation of the valve. In addition, the ports on most fluid-power
valves generally are clearly marked to indicate their intended function.

In hydraulic circuits, the return or common ports should be connected to a return line
that directly connects the valve to the reservoir tank. This return line should not need
a pressure-control device but should have a check valve to prevent reverse flow of the
hydraulic fluid.
Pneumatic circuits may be vented directly to atmosphere. A return line can be used to
reduce noise or any adverse effect that locally vented compressed air might have on
the area.

Operating Methods

The function and proper operation of a fluid-power valve are relatively simple. Most
of these valves have a schematic diagram affixed to the body that clearly explains how
to operate the valve.

Backup Valves
Figure 17-13 is a schematic of a two-position, cam-operated valve. The primary actu-
ator, or cam, is positioned on the left of the schematic and any secondary actuators are
on the right. In this example, the secondary actuator consists of a spring-return and a
spring-compensated limit switch. The schematic indicates that, when the valve is in
the neutral position (right box), flow is directed from the inlet (Pj to work port A.
When the cam is depressed, the flow momentarily shifts to work port B. The second-
ary actuator, or spring, automatically returns the valve to its neutral position when the
cam returns to its extended position. In these schematics, T indicates the return con-
nection to the reservoir.

Figure 17-14 illustrates a typical schematic of a two-position and three-position
directional control valve. The boxes contain flow direction arrows that indicate the
flow path in each position. The schematics do not include the actuators used to acti-
vate or shift the valves between positions.

In a two-position valve, the flow path is always directed to one of the work ports (A or
B). In a three-position valve, a third or neutral position is added. In this figure, a Type
2 center position is used. In the neutral position, all ports are blocked and no flow
through the valve is possible.

Figure 17-15 is the schematic for the center or neutral position of three-position
directional control valves. Special attention should be given to the type of center posi-
tion that is used in a hydraulic control valve. When Type 2, 3, and 6 (see
Figure 17-15) are used, the upstream side of the valve must have a relief or bypass
valve installed. Since the pressure port is blocked, the valve cannot relieve pressure on
the upstream side of the valve. The Type 4 center position, called a motor spool, per-