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DIRECTIONAL CONTROL VALVES
Directional control valves are designed to
direct
the flow of fluid, at the desired time, to the point
in a fluid power system where it will do work.
The driving of a ram back and forth in its cylinder
is an example of when a directional control
valve is used. Various other terms are used to
identify directional valves, such as selector valve,
transfer valve, and control valve. This manual
will use the term directional control valve to
identify these valves.
Directional control valves for hydraulic
and
pneumatic systems are similar in design and
operation. However, there is one major difference.
The return port of a hydraulic valve is
ported through a return line to the reservoir, while
the similar port of a pneumatic valve, commonly
referred to as the exhaust port, is usually
vented to the atmosphere. Any other differences
are pointed out in the discussion of the
valves.
Directional control valves may be operated by differences
in pressure acting on opposite sides
of
the valving element, or they maybe positioned manually,
mechanically, or electrically. Often two or
more methods of operating the same valve will be
used in different phases of its action.
CLASSIFICATION
Directional control valves may be classified in
several
ways. Some of the different ways are by the
type of control, the number of ports in the valve
housing, and the specific function of the valve.
The most common method is by the type of
valving element used in the construction of the valve.
The most common types of valving elements
are the ball, cone or sleeve, poppet, rotary
spool, and sliding spool. The basic operating
principles of the poppet, rotary spool, and
sliding spool valving elements are discussed in
this text.
Poppet
The poppet fits into the center bore of the seat
(fig.
6-21). The seating surfaces of the poppet and the
seat are lapped or closely machined so that the
center bore will be sealed when the poppet is
Figure 6-21.—Operation of a simple poppet valve.
seated (shut). The action of the poppet is similar
to
that of the valves in an automobile engine. In most
valves the poppet is held in the seated position
by a spring.
The valve consists primarily of a movable
poppet
which closes against the valve seat. In the closed
position, fluid pressure on the inlet side tends
to hold the valve tightly closed. A small amount
of movement from a force applied to the top
of the poppet stem opens the poppet and allows
fluid to flow through the valve.
The use of the poppet as a-valving element is
not
limited to directional control valves.
Rotary Spool
The rotary spool directional control valve
(fig.
6-22) has a round core with one or more passages
or recesses in it. The core is mounted within
a stationary sleeve. As the core is rotated within
the stationary sleeve, the passages or recesses
connect or block the ports in the sleeve. The
ports in the sleeve are connected to the appropriate
lines of the fluid system.
Sliding spool
The operation of a simple sliding spool
directional
control valve is shown in figure 6-23. The
valve is so-named because of the shape of the valving
element that slides back and forth to block and
uncover ports in the housing. (The sliding element
is also referred to as a piston.) The inner piston
areas (lands) are equal. Thus fluid under pressure
which enters the valve from the inlet ports
Figure 6-22.—Parts of a rotary spool directional control
valve.
CHECK VALVE
Figure 6-23.—Two-way, sliding spool directional control
valve.
acts equally on both inner piston areas
regardless of the position of the
spool. Sealing is usually accomplished
by a very closely machined fit between
the spool and the valve body or sleeve. For
valves with more ports, the spool is designed with
more pistons or lands on a common shaft. The
sliding spool is the most commonly used type of
valving element used in directional control valves.
Check valves are used in fluid systems to permit
flow in one direction and to prevent flow in
the other direction. They are classified as one-way
directional control valves. The
check valve may be installed independently in
a line to allow flow in one direction only,
or it may be used as an integral part of globe,
sequence, counterbalance, and pressure-reducing valves.
Check valves are available in various
designs. They are opened by the
force of fluid in motion flowing in
one direction, and are closed by fluid attempting
to flow in the opposite direction. The force
of gravity or the action of a spring aids in closing
the valve.
Figure 6-24.—Swing check valve.
Figure 6-24 shows a swing check valve. In the
open
position, the flow of fluid forces the hinged disk
up and allows free flow through the valve. Flow
in the opposite direction with the aid of gravity,
forces the hinged disk to close the passage and
blocks the flow. This type of valve is sometimes
designed with a spring to assist in closing
the valve.
The most common type of check valve,
installed
in fluid-power systems, uses either a ball or
cone for the sealing element (fig. 6-25). As fluid pressure
is applied in the direction of the arrow, the
cone (view A) or ball (view B) is forced off
Figure 6-25.—Spring-loaded check valves.
its seat, allowing fluid to flow freely through the
valve.
This valve is known as a spring-loaded check
valve.
The spring is installed in the valve to hold the
cone
or ball on its seat whenever fluid is not flowing.
The spring also helps to force the cone or
ball on its seat when the fluid attempts to flow in
the opposite direction. Since the opening and closing
of this type of valve is not dependent on gravity,
its location in a system is not limited to the
vertical position.
A modification of the spring-loaded check
valve
is the orifice check valve (fig. 6-26). This
Figure 6-26.—Typical orifice check valves.
valve allows normal flow in one direction and
restricted
flow in the other. It is often referred to
as a one-way restrictor.
Figure 6-26, view A, shows a cone-type orifice
check
valve. When sufficient fluid pressure is applied
at the inlet port, it overcomes spring tension
and moves the cone off of its seat. The two
orifices (2) in the illustration represent several openings
located around the slanted circumference of
the cone. These orifices allow free flow of fluid through
the valve while the cone is off of its seat. When
fluid pressure is applied through the outlet port,
the force of the fluid and spring tension move
the cone to the left and onto its seat. This action
blocks the flow of fluid through the valve, except
through the orifice (1) in the center of the cone.
The size of the orifice (in the center of the cone)
determines the rate of flow through the valve
as the fluid flows from right to left. Figure
6-26, view B, shows a ball-type orifice check
valve. Fluid flow through the valve from left
to right forces the ball off of its seat and allows
normal flow. Fluid flow through the valve in
the opposite direction forces the ball onto its seat.
Thus, the flow is restricted by the size of the orifice
located in the housing of the valve.
NOTE: The
direction of free flow through the orifice
check valve is indicated by an arrow stamped
on the housing.
