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CHAPTER 6
It is all but impossible to design a practical fluid
power system without some means of controlling
the volume and pressure of the fluid and
directing the flow of fluid to the operating units.
This is accomplished by the incorporation of
different types of valves. A valve is defined as any
device by which the flow of fluid may be started,
stopped, or regulated by a movable part that
opens or obstructs passage. As applied in
fluid power systems, valves are used for controlling
the flow, the pressure, and the direction
of the fluid flow.
Valves must be accurate in the control of fluid flow
and pressure and the sequence of operation. Leakage
between the valve element and the valve seat
is reduced to a negligible quantity by precision-machined
surfaces, resulting in carefully controlled
clearances. This is one of the very important
reasons for minimizing contamination in
fluid power systems. Contamination causes valves
to stick, plugs small orifices, and causes abrasions
of the valve seating surfaces, which results
in leakage between the valve element and valve
seat when the valve is in the closed position. Any
of these can result in inefficient operation or
complete stoppage of the equipment. Valves
may be controlled manually, electrically, pneumatically,
mechanically, hydraulically, or by
combinations of two or more of these methods.
Factors that determine the method of control
include the purpose of the valve, the design
and purpose of the system, the location of the
valve within the system, and the availability of
the source of power. The different
types of valves used in fluid power
systems, their classification, and their application
are discussed in this chapter.
CLASSIFICATIONS
Valves are classified according to their use: flow
control, pressure control, and directional control.
Some valves have multiple functions that fall
into more than one classification.
FLOW CONTROL VALVES
Flow control valves are used to regulate the flow
of fluids in fluid-power systems. Control of flow
in fluid-power systems is important because the
rate of movement of fluid-powered machines depends
on the rate of flow of the pressurized fluid.
These valves may be manually, hydraulically, electrically,
or pneumatically operated. Some of
the different types of flow control valves
are discussed in the following paragraphs.
BALL VALVES
Ball valves, as the name implies, are stop valves
that use a ball to stop or start a flow of fluid.
The ball, shown in figure 6-1, performs the
Figure 6-1.—Typical ball valve.
same function as the disk in other valves. As the valve
handle is turned to open the valve, the ball rotates
to a point where part or all of the hole through
the ball is in line with the valve body inlet and
outlet, allowing fluid to flow through the valve.
When the ball is rotated so the hole is perpendicular
to the flow openings of the valve body,
the flow of fluid stops.
Most ball valves are the quick-acting type. They
require only a 90-degree turn to either completely
open or close the valve. However, many
are operated by planetary gears. This type of
gearing allows the use of a relatively small handwheel
and operating force to operate a fairly large
valve. The gearing does, however, increase the
operating time for the valve. Some ball valves also
contain a swing check located within the ball to
give the valve a check valve feature. Figure 6-2 shows
a ball-stop, swing-check valve with a planetary
gear operation.
In addition to the ball valves shown in figures 6-1
and 6-2, there are three-way ball valves that are
used to supply fluid from a single source to one
component or the other in a two-component system
(fig. 6-3).
Figure 6-2.—Typical ball-stop, swing-check valve.
Figure 6-3.—Three-way ball valve.
GATE VALVES
Gate valves are used when a straight-line flow of
fluid and minimum flow restriction are needed. Gate
valves are so-named because the part that either
stops or allows flow through the valve acts
somewhat like a gate. The gate is usually wedge-shaped.
When the valve is wide open the gate
is fully drawn up into the valve bonnet. This leaves
an opening for flow through the valve the same
size as the pipe in which the valve is installed
Figure 6-4.—Operation of a gate
valve.
(fig. 6-4). Therefore, there is little pressure drop or
flow restriction through the valve. Gate
valves are not suitable for throttling purposes.
The control of flow is difficult because of
the valve’s design, and the flow of fluid slapping
against a partially open gate can cause
extensive damage to the valve. Except as specifically
authorized, gate valves should not be used
for throttling.
Gate valves are classified as either rising-stem or
nonrising-stem valves. The nonrising-stem valve
is shown in figure 6-4. The stem is threaded into
the gate. As the handwheel on the stem is rotated,
the gate travels up or down the stem on the
threads while the stem remains vertically stationary.
This type of valve will almost always have
a pointer indicator threaded onto the upper end
of the stem to indicate the position of the gate. Valves
with rising stems (fig. 6-5) are used when
it is important to know by immediate inspection
whether the valve is open or closed and when
the threads (stem and gate) exposed to the fluid
could become damaged by fluid contaminants. In
this valve, the stem rises out of the valve when
the valve is opened.
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