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Section III. - Valve Operation
A. General
Directional valve operation is determined by four factors:
spool type, spool positioning, method of control, and special
features. Proper selection of the above factors establish and
regulate desired flow paths through the internal ports of the
valve. The following information discusses those factors with
respect to valve operation.
B. Spool Types - Main Stage Section
Operation of spools are governed by their design as well as
the means of control. The most common designs are open
center, closed center and tandem. During the following
discussion, basic spool design is related to valve port
openings, with the spool in center position. Port openings
are stated as: P-Pressure Port, A&B-Actuator Ports, and
T-Tank or Reservoir Port.
1. Open Center Spools (0, 1, 9 and 11 types): Open center
valves are used in single operations where no other
operation is performed by the same source of power and
where cylinders do not have to be held by pressure. Open
center spools are also used to minimize shock in a system.
Shock develops when a valve spool is shifted from one
position to another across center position. The smoothest
possible minimum shock condition is obtained when fluid
under pressure is allowed to discharge to tank as the spool
passes center condition.
Open center with (A) or (B) ports blocked. A spool of this
type is generally used to operate a cylinder. When the spool
is centered, a cylinder port is blocked and the cylinder is held
in a definite position. In some circuits, flow from the tank port
is piped into the pressure port of another valve. This allows
the same source of power to operate two different cylinders.
This type of arrangement may be used in a system
containing a number of operations. However, each operation
must be performed in a certain sequence, with only one
operation taking place at any one time.
A. Type ‘0’ spool is designed with ports (P), (B), (A) and
(T) interconnected when the spool is in center
position.These ports are momentarily interconnected
during spool crossover when the pilot valve solenoid is
activated. This permits smooth rapid cycle operation.
A B
P T
T
A
P
B
Open Center
B. Type ‘1’ spool is designed with ports (P), (A) and (T)
interconnected. Port (B) is blocked in center position.
A B
P T
‘‘B’’ Closed - Pressure Open
to Tank through ‘‘A’’
T
A
P
B
C. Type ‘9’ spool is similar to type ‘0’ spool except all
ports are partially open in center position.
Open Center, Partial, All Ports
A B
P T
T
A
P
B
D. Type ‘11’ spool is a type ‘1’ spool reversed in the
bore. The type ‘11’ spool interconnects (P), (B) and (T) in
center position with (A) blocked.
A B
P T
‘‘A’’ Closed, Pressure Open to
Tank through ‘‘B’’
T
A
P
B
2. Closed Center Spools (2, 3, 6, 31, 33, 52 and 521 types):
Closed center spools are used where two or more operations
are performed by a single pump or an accumulator. Closed
center valves prevent the loss of fluid from the pump or
accumulator when the spool crosses center.
A. Type ‘2’ spool blocks ports (P), (A), (B) and (T) from
one another in the center position. The ports are
momentarily blocked during spool crossover.
A B
P T
Closed Center - All Ports Closed
T
A
P
B
