8
For convenience A.S.A. symbols (Y32. 10-1958) are
also shown with the following descriptions of each spool.
a. “B”- Motor Spool - “B” spools are used when
flow is directed to the operation of a hydraulic motor instead
of a cylinder. These spools are double acting in character so
that the motor may be rotated in either direction. The cylin-
der ports are left partially open in the neutral position to allow
free flow of oil between the motor and reservoir. See Figure
3 for spool position vs. flow characteristics.
“B” Motor Spool
BA
P
T
MANMAN
b. “C” Float Spools - “C* spools are double acting
with an additional float position. The spool is retained in the
float position by a detent, and it is spring centered to neutral
from the “in” and “out” positions. Both cylinder ports are open
to the tank in the float position to permit free flow of oil in
either direction. See Figure 4 for spool position versus flow.
“C” Float Spool
BA
P
T
MANMAN
c. “D” Double Acting Spool - “D* spools are used
for applications where pump flow must be directed to either
end of a cylinder, depending on the direction of movement
required. The end of the cylinder not under pressure has its
return flow directed to tank via internal coring of the valve
sections. See Figure 5 for spool position versus flow.
“D” Double Acting Spool
B
A
P
T
MANMAN
Figure 3.
Neutral
Out
Pressure
Tank Return Flow
Pressure port
A
“B” spool in
“OUT” position.
Pressure to “B”
port. “A” port to
tank. By-pass
blocked.
In
“B” Spool
B
A
B
A
B
Tank
port
By-
pass
carry
over
port
“B” spool in
“neutral” position.
“A“ and “B” ports
open to tank pres-
sure is blocked.
By-pass open.
“B” spool in “IN”
position. Pressure
to “A”port . “B”
port to tank. By-
pass blocked.
Figure 4.
Neutral
Float
Pressure
Tank Return Flow
Pressure port
A
“C” spool in
“OUT” posi-
tion. Pressure
to “B” port.
“A” port to
tank. By-pass
blocked.
In
“C” Spool
Tank
port
By-
pass
carry
over
port
“C” spool in
“neutral”
position.
“A“ and “B”
ports blocked.
By-pass open.
“C” spool in
“IN” position.
Pressure to
“A”port “B”
port to tank.
By-pass
blocked.
Out
“C” spool in
“FLOAT”
position. “A”
& “B” port to
tank. By-
pass open.
A
AA
B
BB
B
Figure 5.
Neutral
Out
Pressure
Tank Return Flow
A
“D” spool in
“OUT” position.
Pressure to “B”
port . “A” port to
tank. By-pass
blocked.
In
“D” Spool
B
A
B
A
B
“D” spool in
“neutral” position.
“A“ and “B” ports
are blocked.
By-pass open.
“D” spool in “IN”
position. Pres-
sure to “A”port.
“B” port to tank.
By-pass
blocked.
Pressure
port
Tank
port
By-
pass
carry
over
port
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