7-DirectionalControlValves

8/11/2019 7-DirectionalControlValves

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Directional Control Valves

Directional control valves fall into one of two categories:

Shear Action Poppet

Lapped Spool

Packed Spool

Packed Bore

Bonded Spool

Metal Face

Bonded


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DCV Flow Path Terminology

2-Position

3-Position

4-Position

The positions refer to which direction the fluid happens to be

going when a valve is shifted. The position corresponds to a

envelope in the schematic symbol.


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DCV Schematic Symbols

The basic valve symbol is a blank envelope.

DCVs have at least two envelops which

corresponds with a 2-position valve. A 3-positionvalve will have three envelopes and so forth.


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Arrows are added to indicate a direction that fluid can travel

through the valve at that point when the valve is in that

position. Each arrow crossing represents two ports. The

example above indicates four ports but not necessarily wherthey are located on the actual valve.


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This is an example of a 3-way valve. Note the little

T emblem which denotes a blocked port.


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Relating Symbols to Components

The cutaway diagram shows how a spool can

connect and disconnect passages.


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If shifted to the envelope on the left, ports A is connected to

the port which is being fed by the pump, port P. At the same

time port B is connected to the tank port T. A DCV used in

pneumatics would use EX which is short for exhaust port.


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If shifted to the envelope on the left, the spool

connects port P with port B, and port A with port T

which gives a reversal of the previous flow paths.


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3-Way Valves in a Circuit

In the circuits above, each cylinder has something to

push it back. The ram on the left has the weight of the

ram and gravity to return it to the starting position.On the left, a spring returns the rod to its starting

point. 3-way valves have other functions as well.


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2-Way Valves in a Circuit

The 2-way DCV is used here to isolate fluid flow from the

4-way DCV until needed. The current condition shows no

flow going to the 4-way DCVs.


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DCV Operators

An operator is a device that physically moves a spool or

poppet. These are examples of the kinds of operators

that might be found on DCVs.


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Valve Operators

Manual operators such as the push button, hand lever, andfoot pedal, use human power for actuation. Mechanical

operators use a machine component to contact a roller or

ball bearing. Pilot operators use a fluid to move the spool.A solenoid uses electro-mechanical force to move a spool or

poppet.


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Solenoid Operation

When the solenoid is energized, the air gap is closedquickly and a force is developed in the direction of the

valve spool. The force developed is determined by the

size of the coil. If the air gap is not closed, the coil willburn up due to inrush current.


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Spring Operator

In DCVs and other valves, springs are used to return oroff-setthe spool or poppet to ensure ahome orrest position. When reading

a schematic symbol, remember that the envelop to which the spring

is attached is thehome position. The terms normally and normallyclosed relate to whether or not the valve will permit flow in a given

position. If the valve will not allow flow in the normal position we

say it is normally closed or not passing. The opposite is said for the

normally open position and we say normally passing.


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Other Operators

Detents

Detents are used to hold a valve spool in place. They serve a safety

function in that the spool is held positively in place to keep it from

moving due to vibration or possibly from gravity where the valve ismounted in an up right position. Shown above is a ball and spring

detent but there are other designs as well.


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Sub Plate mounted 4-Way

Valves

Sub plate orsub base mounting is just a convenient way of attaching

a DCV to a system and reducing the time required to replace it

should the need arise. The DCV is held into place by four fastenersand a seal is made with O-rings. The time savings is in to having to

remove pipe connections.


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Solenoid Controlled, Pilot

Operated DCV

Although a valve could be shifted directly by the force of a solenoid,

large flow DCVs are most often shifted using fluid at systempressure. Large flow valves demand larger shifting forces and it no

longer practical to use a solenoid. Illustrated on the left is a typical

mounting arrangement for such a valve. On the left is the schematicsymbol. Can you tell what the normal position is?


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Piggy-Back DCV

Piggy-backing is a term used to describe a two valves connected as a

unit where a small valve controls the operation of a larger valve.This arrangement makes used of the powerful force of fluids in

order to actuate a large flow DCV.


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Center Conditions

Open Center

This spool is sometimes called a motor spool because it

is often used to control a hydraulic motor and minimize

shock that may be encountered as the valve returns to the

center position. It is usually not recommended for

cylinders because of the possibility of drift while in thecenter position.


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Center Conditions

Closed Center

While in the center position the closed center valve allows no

flow to the system because all ports are blocked off from one

another. This spool is used where multiple stop positions are

required.


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Center Conditions

Float Center

The chief advantage of a float center valve is theability of the actuator move freely. It is used to

control cylinders on mobile equipment where a

blade cylinder must be permitted to move freelyin order to scrape or level.


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Center Conditions

Tandem Center

The tandem center spool is an energy conservative design used on

low power equipment. When in the center position this valve allowsthe flow of the pump to return to tank at low pressure thus reducing

system pressure.

C l d DCV S b l


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Completed DCV Symbols

3-position, Blocked Center 4-way valve, Direct Solenoid Operationand Spring Centered

3-position, Open Center 4-way valve, Direct Solenoid Operation and

Spring Centered

3-position, Tandem Center 4-way valve, Direct Solenoid Operation and

Spring centered

O h DCV S b l


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Other DCV Symbols

Four Position DCV Flow Through Center

DCV

These two valves are commonly found in mobile

hydraulics and each has advantages in the applicationwhere it is found.


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Deceleration Valve

Although considered a DCV, the deceleration valve

operates by shutting down flow as an actuator

approaches the point of operation. This would

prevent a tool crash.


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Pneumatic Valves

The operating principles of pneumatic valves are identical to theones used in hydraulics but there does exist a difference in the

schematic symbology. First, you may notice that the arrows are

canted or at an angle. This serves only to Distinguish them fromhydraulic DCVs and nothing more. Second, these valves are often

referred to as 4-way valves although they have 5 ports. The reason is

that there is an additional exhaust port, usually not found in

hydraulic valves.


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Review

1. What design of spool valve is most commonly used in hydraulics?

2. Name one advantage and one disadvantage of a poppet valve over a shear

action valve.

3. At least how many ports must a three way valve have in order to perform a

three way function?

4. Describe how a four way valve could be used as a two way?

5. Describe the principle of electro-magnetic induction.

6. What type of operation is indicated when a fluid is used to move a spool?

7. What type of operation requires human power?

8. A mounting method for quickly replacing a directional control valve.

9. When a small DCV is placed on top of a large DCV to control large flows.

10. A center condition where the pressure port is connected to the tank passage.

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