Directional control valves are one of the most fundamental parts in hydraulic
machinery as well and pneumatic machinery. They allow fluid flow into
different paths from one or more sources. They usually consist of a spool
inside a cylinder which is mechanically or electrically controlled. The movement
of the spool restricts or permits the flow, thus it controls the fluid flow.
Nomenclature The spool consists of lands and
grooves.The lands block oil flow through the valve body. The grooves allow oil or
gas to flow around the spool and through the valve body. There are two
fundamental positions of directional control valve namely normal position
where valve returns on removal of actuating force and other is working
position which is position of a valve when actuating force is applied. There
is another class of valves with 3 or more position that can be spring
centered with 2 working position and a normal position.
Classification Directional control valves can be
classified according to- number of ports
number of positions actuating methods
type of spool. Example: A 5/2 directional control valve
would have five ports and two spool positions.
= Number of Ports= According to total number of entries or
exits connected to the valve through which fluid can enter the valve or leave
the valve. There are types such as two way, three way, and four way valves.
= Number of Positions= Including the normal and working
positions which a valve spool can take there are types like two position, three
position and proportional valves. = Actuating Methods=
Manual, spring, electrical, pneumatic, and hydraulic.
Manually Operated Manually operated valves work with
simple levers or paddles where the operator applies force to operate the
valve. Spring force is sometimes used to recover the position of valve. Some
manual valves utilize either a lever or an external pneumatic or hydraulic
signal to return the spool. Mechanically Operated
Mechanically operated valves apply forces by using cams, wheels, rollers,
etc., hence these valves are subjected to wear.
Hydraulically Operated A hydraulically operated DCV works at
much higher pressures than its pneumatic equivalent. They must therefore be far
more robust in nature so are precision machined from higher quality and
strength materials. Solenoid Operated
They are widely used in the hydraulics industry. These valves make use of
electromechanical solenoids for sliding of the spool. Because simple application
of electrical power provides control, these valves are used extensively.
However, electrical solenoids cannot generate large forces unless supplied
with large amounts of electrical power. Heat generation poses a threat to
extended use of these valves when energized over time. Many have a limited
duty cycle. This makes their direct acting use commonly limited to low
actuating forces. Often a low power solenoid valve is used
to operate a small hydraulic valve that starts a flow of fluid that drives a
larger hydraulic valve that requires more force.
A bi-stable pneumatic valve is typically a pilot valve that is a 3 ported 2
position detented valve. The valve retains its position during loss of
power, hence the bi-stable name. Bi-stability can be accomplished with a
mechanical detent and 2 opposing solenoids or a "magna-latch" magnetic
latch with a polarity sensitive coil. Positive opens and negative closes or
vice versa. The coil is held in position magnetically when actuated.
= Type of Spool= Spool is of two types namely sliding and
rotary. Sliding spool is cylindrical in cross section, and the lands and grooves
are also cylindrical. Rotary valves have sphere-like lands and grooves in the
form of holes drilled through the. Specification
They are generally specified using the number of ports and the number of
switching positions. It can be represented in general form as np/ns,
where np is the number of ports connected to the direction control valve
and ns the number of switching positions.
In addition, the method of actuation and the return method can also be specified.
A hypothetical valve could be specified as 4-way, 3-position direction control
valve or 4/3 DCV since there are four ports and three switching positions for
the valve. In this example, one port is called the pressure port which is
connected to the pump; one port is the tank port and is connected to the tank;
and the two remaining ports are called working ports and are connected to the
actuator. Apart from characteristics of valve the fluid suitable for
valve,working temperature and viscosity also thought upon before selecting a
particular type of valve. Symbolic representation
While working with layouts of hydraulic machinery it is cumbersome to draw
actual picture of every valve and other components. Instead of pictures, symbols
are used for variety of components in the hydraulic system to highlight the
functional aspects. symbol for directional control valve is made of
number of square boxes adjacent to each other depending on the number of
positions.connections to the valve are shown on these squares by capital
letters.usually they are named only in their normal position and not repeated
in other positions.actuation system of the valve is also designated in its
symbol. = Two way two position directional
control valve= Gate valve is example of 2W/2P
directional control valve which either turns on or off the flow in normal or
working position depending on need of application. Here arrow indicates that
fluid flow is taking place whereas other position shows cut-off position.
= Four way two position directional control valve=
4 way 2 position valve has four connections to it and two valve
positions. One of them is normally open. See also
Hydraulic machinery External links
ttp:www.omega.compdf/SimpValvesguide.pdf wstuffworks.comstructural/wave-pool6.htm
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