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
DCV Flow Path Terminology
Valve type
2-way
Valve use
On-Off functions for fluid supply
3-way
Single acting cylinders, uni-directional motors
4-way
Double acting cylinders, bi-directional motors
5-way
Pneumatic systems with dual air pressure
The above descriptions refer to the total number of directions in
which a fluid can travel through a directional control valve.
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.
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-position
valve will have three envelopes and so forth.
DCV Schematic Symbols
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 wher
they are located on the actual valve.
DCV Schematic Symbols
This is an example of a 3-way valve. Note the little
“T” emblem which denotes a blocked port.
DCV Schematic Symbols
This is an example of a 2-way valve. The single
arrow indicates one inlet port and one outlet port.
Relating Symbols to Components
The cutaway diagram shows how a “spool” can
connect and disconnect passages.
Relating Symbols to Components
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.
Relating Symbols to Components
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.
4-Way Valves in a Circuit
Whether in a hydraulic circuit or a pneumatic circuit, 4way DCVs function the same. The two triangles at the
bottom represent the supply source, hydraulic on the left
and pneumatic on the right. The direction of the
cylinder in both cases is determined by the envelope
chosen. Envelopes are shifted by an “operator.”
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.
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.
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.
Valve Operators
Manual operators such as the push button, hand lever, and
foot 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.
Solenoid Operation
Air Gap
Push Pin
Spool
A solenoid is an electrical-mechanical device that takes
electrical energy and produces a linear force by use of
magnetism. It is basically a winding of wire around a
metal core. Many times when a valve fails to shift, the
problem can be traced to a faulty solenoid. A spring, not
shown, holds the spool against the solenoid push pin when
the solenoid is de-energized.
Solenoid Operation
When the solenoid is energized, the air gap is closed
quickly 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 will
burn up due to “inrush” current.
Spring Operator
In DCVs and other valves, springs are used to “return” or”off-set”
the spool or poppet to ensure a home or rest position. When reading
a schematic symbol, remember that the envelop to which the spring
is attached is the home position. The terms normally and normally
closed 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.
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 is
mounted in an up right position. Shown above is a ball and spring
detent but there are other designs as well.
Sub Plate mounted 4-Way
Valves
Sub plate or sub 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 fasteners
and a seal is made with O-rings. The time savings is in to having to
remove pipe connections.
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 system
pressure. 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 schematic
symbol. Can you tell what the normal position is?
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.
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 the
center position.
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.
Center Conditions
Float Center
The chief advantage of a float center valve is the
ability of the actuator move freely. It is used to
control cylinders on mobile equipment where a
blade cylinder must be permitted to move freely
in order to scrape or level.
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 allows
the flow of the pump to return to tank at low pressure thus reducing
system pressure.
Completed DCV Symbols
3-position, Blocked Center 4-way valve, Direct Solenoid Operation
and 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
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 application
where it is found.
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.
Pneumatic Valves
The operating principles of pneumatic valves are identical to the
ones 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 from
hydraulic 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.
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.