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Directional control valves are one of the most fundamental parts in hydraulic machinery. They allow fluid flow into different paths from one or more sources. They usually consist of a piston inside a cylinder which is mechanically or electrically controlled. The movement of the cylinder restricts or permits the flow, thus it controls the fluid flow.

nomenclature
The spool (sliding type) consists of lands and grooves.The lands block oil flow through the valve body. The grooves allow oil 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.

Classification
Directional control valves can be classified according to-
 * number of ports
 * number of positions
 * actuating methods
 * type of spool.

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 like two way,three way,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 valves.

manually operated
Manually operated valves work with simple levers or paddles where the operator applies force to operate the valve. Spring force is used to recover the position of valve.

mechanically operated
Mechanically operated valve apply forces by using cams,wheels,rollers etc.hence these valves are subjected to wear.

Hydraulically operated
These valves operate on hydraulic pressures which causes spool to slide or rotate.these valves can provide large operating force.these valves can be operated safely under high temperatures to make environment explosion proof.

Solenoid operated
These valves make use of electromechanical Solenoid for sliding of the spool.electrical power is readily available and these valves are extensively used but electrical solenoid cannot generate large forces unless supplied with larger electrical energy which makes there use limited to low actuating forces.heat generation poses the threat to use of these valves when held in one position longer.

In practice combination of these actuators.such as low power solenoid valve is used to operate the other hydraulic valve.here solenoid valve starts the flow of fluid which in turn operates hydraulic valve.

Type of spool
spool is of two types namely sliding and rotary.Sliding spool is cylindrical in cross section lands and grooves are also cylindrical where as rotary valve has sphere like lands and grooves in the form of holes drilled through sphere.

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 (or reservoir); 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 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 where as 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.