11
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 operations.
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. Refer to Figure 3.
1.
Open Center Spools (0, 1, 9 and 11)
: Open center
valves are used in single operations where no other opera-
tion 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.
b.
Type “1”
spool is designed with ports (P), (A), and
(T) interconnected. Port (B) is blocked in center position.
c.
Type “9”
spool is similar to type“0” spool except all
ports are partially open in center position.
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.
2.
Closed Center Spools
(2, 3, 6, 31 and 33 types):
Closed center spools are used where two or more opera-
tions are performed by a single pump or by an accumulator.
Closed center valves prevent the loss of fluid from the pump
or accumulator when the spool crosses center. The “P” port
is blocked as 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.
b.
Type “3”
spool is designed with ports (A) and (T)
interconnected and ports (P) and (B) blocked in the center
position.
c.
Type “6”
spool is interconnected at ports (A), (B)
and (T). Port (P) is blocked in the center position.
d.
Type “31”
spool is a type “3” spool reversed in the
bore. A type “31” spool is interconnected at ports (B) and (T),
but blocked at ports (P) and (A) in the center position.
e.
Type “33”
spool provides controlled leakage from port
(A) and (B) to port (T). Port (P) is blocked in the center position.
3.
Tandem Spools (4 and 8 types)
: Tandem spool
valves are used in hydraulic circuits where two or more
hydraulic cylinders or motors are controlled from a single
source of power. The valve’s spool is designed so that in
center position, all cylinder connections are blocked and full
pump delivery is connected to tank. The tank connection of
one valve may be connected to the pressure connection of
another valve and both valves operated simultaneously as
long as another valve and both valves operated simulta-
neously as long as the combined pressures developed by
the two loads are within the capabilities of the power source.
a. Type “4” spool allows oil to circulate freely from port
(P) to port (T) in the center position. Ports (A) and (B) are
blocked to the work load.
b. Type “8” spool is designed similar to type “4” spool.
Port (P) and (T) are interconnected in the center position.
However, ports (A) and (B) are momentarily open during
spool crossover.
Open Center
“B” Closed - Pressure Open to Tank Thru “A”
Closed Center - All Ports Closed
A
P T
B
Type
0
A
P T
B
Type
1
A
P T
B
Type
2
